baile mhic ire final engineering report

BAILE MHIC IRE FLOOD RELIEF STUDY

ENGINEERING REPORT (FINAL REPORT)

2254/RP/001/I Sept. 2013

Design Section, Engineering Services

Office of Public Works

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Intentionally Blank

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EXECUTIVE SUMMARY Baile Bhuirne and Baile Mhic Ire, on the River Sullane in County Cork, have a history of flooding (see Figure A). The Lee CFRAM study highlighted this as an area where a Flood Relief Scheme was justifiable based on social, environmental and costs beneficial analyses. A review of that study concluded that a standalone investigation was required to prepare an outline scheme design.

The main events caused by the River Sullane, itself, occurred in April 1962, August 1986, December 2001, December 2006, November 2009 and November 2011: four of these are in the last twelve years. However, a number of properties have been damaged or been close to flooding when there was no significant flow in the river; implying other flooding mechanisms at play.

This study has found that four tributaries that join the Sullane from the hills to the north cause secondary flooding. These are the River Bohill, the Industrial stream (this has two main tributaries, Industrial North and South), the sports grounds river (Grotto) and the surface water sewer entering from the north west of the town (Coffin). Overland flow coming off the high ground has impacted some properties; of particular note is the local primary school. Finally, localized problems result from any one of a number of minor tributaries that enter the Sullane from the north through the town and by their piecemeal culverting under roads and properties. The main affected areas are upstream of Baile Bhuirne Bridge, through the middle of the town (at the garage yard), upstream of Baile Mhic Ire Bridge and just west of the Post Office; where surface water is backed up from the river.

Information gathered in December 2006 gives a reasonable flood extent, so, this flood has been used for calibration purposes in this study and the November 2011 flood for verification: as a gauged flow was measured almost at the peak of the event and a number of flood points along the town were also recorded. The OPW have a Hydrometric Recording Station just upstream of Baile Bhuirne Bridge since October 2011. This means that there is a lack of locally recorded hydrometric data. The nearest gauged river station is Macroom, 16.6km downstream.

Primarily due to its small catchment size and high slope, the response time of the Sullane is short and, as a result, intense rainfall upstream of Baile Mhic Ire produces a major reactive flood. Further downstream in the catchment, for example, at Macroom, that same intense event is likely to have had only an insignificant effect on its flows.

Figure A - Study Area Showing the Sullane River and Tributary Channels

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A review of the available river and rain data was carried out. The ESB station at Macroom (19031) has recorded data from the early 1980’s, though there are many years where large gaps occurred in the data. Both Hourly and Daily ESB and Met Eireann rain gauged data was used in the catchment design event flow analysis, though prior to 2002 only hourly rainfall data from Met Eireann’s Cork airport station was available, since 2002, ESB installed a number of hourly rain stations.

As part of the Lee CFRAM, a Flow Rating Review was undertaken for the Macroom River Station (19031). That flow rating has been re-evaluated within this study, through re-modelling and a review of the flow gauging data. A review has also been carried out on the flow rating and gauged data for Dromcarra (19014) and the Laney 19027.

The median flood is the 2-year event, i.e. a larger flood is expected to occur, on average, every second year. Gauged river data at Macroom, coupled with a catchment transfer method, provided the Baile Mhic Ire’s median flood estimate. The Institute of Hydrology formula (IH 124) and OPW’s FSU formula were used to provide estimates of the median flood for the smaller tributaries. At Macroom, there are only nine years of reliable Annual Maximum data (i.e. recorded highest yearly flow data). The FSR (Flood Studies Report, 1975) and FSU (Flood Studies Update, 2012) recommends that that number of Annual Maxima to estimate the median flow give more confident results than applying ungauged techniques. This resulted in a median flow to Macroom of 160m3/s.

To transfer flow estimates from Macroom to Baile Mhic Ire (i.e. to define the flow-ratio), four different ungauged flow estimation techniques were employed; these were the FSU, the FSR 5 and 6 variable equations and its Unit Hydrograph method. An average was considered suitable (in terms of factorial standard error), as no bias could be justified to using one technique over the others. This provided a flow-ratio relationship of 0.53 between the catchments.

As Irish catchments were used to develop its formula, the FSU OPW formula for small catchments (<50km2) was employed to determine the final design median flow estimations for the tributaries entering the Sullane River, through the study area. The General Extreme Value (GEV) distribution is recommended for Macroom and the catchment upstream of Baile Mhic Ire, based on the Macroom, Dromcarra and the Laney regional pooled analysis. Flow growth curves were generated for this region using frequency analysis at these three stations. These regional growth curves (or factors) are considered applicable both to Macroom and Baile Mhic Ire. It should be noted, that the 100-year growth factor (relative to the average annual flood) recommended by the FSR for the island of Ireland was 1.96. In this study, growth factors were derived by a weighted average (based on years) from the Annual Maximum frequency analyses for 19031 (9 years) 19014 (47 years) and 19027 (22 years). This was considered better than extrapolating the Macroom single-site frequency analysis, due to the number of invalid years, i.e. out of 19 years of records only 9 are considered reliable enough to be used in an analysis. In particular, if many of the missing years are due to problems in recording particularly large flood events, the resulting reduced record will have a bias that would give the mistaken opinion of a benign flow regime.

As no river data was available to Baile Mhic Ire, and rainfall data was available throughout the catchment, the calibration of a unit hydrograph to Macroom, with the transfer of its parameters to Baile Mhic Ire, was considered the most suitable way to define event flows. This technique required the estimation of an events ‘effective rainfall’ percentage runoff, i.e. how much of an event’s rainfall contributes to runoff-flow; as opposed to entering the soil and adding to the background, groundwater (base-flow) in the catchment. This resulted in the generation of a bespoke technique, based on data from a real-time flood forecasting hydrological model for the Munster Blackwater catchment. The percentage runoff from the soil in Macroom’s contributing catchment and that in the Munster Blackwater were taken as being the same, based on their geographical closeness and from their soil types similarities, (soil survey maps, 1978).

The calibrated unit hydrograph parameters were then transposed to Baile Mhic Ire. The December 2006 event was modelled along with its rainfall data and calculated percentage runoff. This estimated its flood peak flow as 143.5 m3/s and its return period as being between 5 and 10-year.

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There are three main sources of error in determining design flows to Macroom, namely, development of flow ratings and estimation of the median flood and growth curve. Macroom’s rating for the upper extrapolated flows (developed during the CFRAM) gives grounds for concern. This study’s review showed agreement up to flows of 230 m3/s but, above this, there is a lack of confidence in the CFRAM upper rating due to a lack of high flow gauging (i.e. the largest recorded is just 118 m3/s) and this could not be properly made up due to difficulties arising from significant out-of-bank Manning roughness and the existence of ineffective flow areas.

In relation to the median flood event at Macroom, ten of the nineteen years of annual maxima have been rejected due to significant uncertainty; the resulting nine years (N) is a short record. If Macroom’s reliable annual maxima are used to generate regional growth curves, the FSR advises that estimates of floods with return periods greater than 2N need to be treated with caution, i.e. beyond the 18-year event. Confidence in the developed regional growth curve is higher, as it includes the 47 years at Dromcarra and Laney data.

These sources of error lead to an uncertainty in the Baile Mhic Ire Design Flow, however, this is countered by recommending an additional Freeboard allowance of 0.2m on defences for all scheme design options. Published guidance, for fluvial flooding problems recommends that 0.3m freeboard is added to hard defences (such as retaining walls) and a minimum of 0.5m to soft defences, such as embankments. The recommended additional 0.2m means that the median flood flow at Macroom could increase from 160 to 200 m3/s without leading to failure of the Baile Mhic Ire scheme.

To cater for bridges and culverts on the tributaries, their Design Flow is calculated at the upper 66-percentile level, there is only a probability of one in six that the actual Design Flow is greater than it. It is preferred that the design event (with an allowance for climate change), where possible, is capable of free flowing through pipes, culverts and bridges. If not practicable, the resulting afflux at the upstream face will be designed to remain below 0.4m.

An ISIS hydraulic model was used to determine flood profiles through the study area. The December 2006 event was used to calibrate it and the smaller event in November 2011 validated it. The 100-year flood extent map, including the impact of the tributaries, is presented in Figure B.

Figure B – This Study – 100-year Flood Extent Map

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Forecasted land use change due to urbanization and agricultural practices have little effect on the Design Flow. The impacts of Climate Change have also been examined. OPW policy allows for a standard flow increase of 20% for the midrange scenario (Sweeney and Fealy, 2006 and Defra FCDPAG3, 2006) and 30% for the upper scenario. In this Climate Change scenario, the 5-Year flood is about the present-day 10-Year flood or, when interpreted in reverse, the present-day 10-Year flood will become the 5-Year flood. Similarly, the present-day 50-Year flood will be exceeded as frequently as once in 20 years. This change is sufficient to increase flood levels by 0.3m at Baile Bhuirne Town Bridge, 0.2m at the middle one and 0.12m at the third. Flood relief measures have been chosen that can be easily altered to accommodate this effect, should it occur.

Under present-day conditions (i.e. no Scheme), significant flooding can be expected to occur, on average, once in 5 to 10 years, so damages can be assessed for each property for the 5, 10, 20, 30, 50 and 100-Year events (Appendix A). The economic damage caused by flooding of property is estimated using the ‘The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment Techniques -2010 (the ‘Multi-Coloured Manual’: MCM). This recommends that, in built up areas, total property damage should be multiplied by a factor of 1.056 to allow for emergency and recovery costs. The Total Damage for each of these standard flood events, combined with its probability of occurring in one year, allows an estimate to be made of the Average Annual Damage. The Average Annual Damage, discounted at a rate of 4% per annum, is then calculated over a time-horizon of 50 years to produce a Net Present Value of the potential flood damage up to the 100-year. This is estimated as €12.97m.

Many engineering measures can be applied to the flooding problem (refer to Section 7). This study has identified those that are not suitable to Baile Mhic Ire, such as diverting flood flows and upstream storage of the peak. It would take significant dredging of the river to lower flood levels sufficiently to eliminate the need for defensive structures through the centre of the village and minimising the bypass flow along the right bank at Baile Mhic Ire. However, at the beginning of this project, the Environmental Assessment highlighted that large scale dredging was not viable (see Appendix A1). Despite this, along with the ‘Do Nothing’ solution (Scheme Option A), several flood relief schemes that combine a number of engineering measures have been found that fully protect the area against the Design Flood.

The safety factor for flood defences is a freeboard added to the estimated floodwater height. For freshwater flooding, 0.3m is added to hard defences (such as walls) and a minimum of 0.5m to soft defences (such as embankments). To counter uncertainty in the hydrology of the Sullane due to data issues, these values are increased by 0.2m to 0.5 and 0.7m, respectively.

A significant river bend is situated along the middle of Baile Mhic Ire Village. Floods have considerable energy and this ‘village bend’ is severe enough to produce a complex hydraulic issue called super-elevation, where floodwaters rise higher on the village side of the river than on the other. The maximum effect is estimated at 0.22m. Hydraulic complexities and uncertainty in estimation due to insufficient data, etc., mean that a conservative approach must be taken in the area affected by this bend. While it remains, 0.22m is added to the freeboard.

Three alternative Engineering Measures have been examined to either fully alleviate or significantly reduce this problem, namely, rock armouring, a Flood Flow Bypass channel and a Full Bypass Channel along with Infilling of the ‘Village Bend’. All scheme options involve an engineering measure at this bend to either fully alleviate or significantly reduce this problem.

River modelling shows that Baile Mhic Ire cannot be practically defended without flood containment measures. It also highlights that the three bridges within the study area are restrictive. For example, under the existing 100-year flood condition, the afflux (influence on upstream water levels) at Baile Mhic Ire Bridge is 0.9m. With flood containment in place, however, this increases to 1.2m, and would rise further to 1.4m by 2100, if the expected Climate Change occurs. Particularly under Climate Change conditions, this seriously impacts flood defence heights. Such conditions mean that a bridge would need to be removed or have its flow conveyance capacity

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upsizing through additional openings or by localised dredging. The cost associated with bridge removal or providing additional openings means that localized dredging is necessary. The Environmental Assessment scored this as viable in its Multi Criteria Analysis (see Appendix A1). Modelling shows that localised dredging at the bridges reduces afflux to 0.5m. As such, two Engineering Measures are needed for all solutions, namely, flood containment and localised dredging at the bridges.

Options 1, 2, 3 contain these two measures. The difference between them just relates to the choice of Engineering Measure at the ‘village bend’. In the naming of scheme options, there are two variations (A or B). Variation ‘A’ represents the original option along with one additional engineering measure, namely, channel widening upstream of Baile Bhuirne Bridge to eliminate high velocities and resulting erosion, for example Option 1A. Variation ‘B’ is the original Option along with a different additional engineering measure, namely, dredging the river both upstream and downstream of Baile Mhic Ire Bridge, for example Option 1B. While these changes may seem small, their impacts on the sizing of other measures within an option make it worthwhile forming separate options. One option represents a significantly different approach to solving the Sullane flooding problem by employing two Compound Channels linked by river dredged.

Secondary flooding in the study area is primarily caused by four tributaries entering the Sullane from the north (see Map A). These are the:

• Bohill River

• Industrial Channel – (two main streams Industrial North and South)

• Grotto Channel

• Coffin Channel

One flood relief option (combining several engineering measures) has been found for each of these, which has the ability to protect the study area up to its design event. As such, these may be rolled into one global option that caters for the full set of flood risks generated by these tributaries.

Figure C – Protected Scenario – 100-year Flood Extent Map

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All main channel options are working in tandem with this tributaries option. There are 13 combinations of measures that have been found capable of providing relief from flooding caused by the River Sullane. To some extent, these engineering based solutions all involve the provision of walls and embankments, but they differ due to the range of additional measures they employ to protect the town to the design standard. Figure C presents the post-scheme 100-year flood extent.

The viable options analysed are as follows:

� Options 1 to 3 - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:

� At Baile Mhic Íre Bridge:

� Raise the existing road on both sides of the bridge

� Install culverts beneath the raised right side to accommodate bypassing flow

� Install culverts beneath the raised left side to remove damming-up effect caused by the bridge and roadway

� Localised channel widening, upstream right bank, to improve flow through the bridge


� Significant dredging of the channel from upstream of Baile Bhuire Bridge to downstream of Baile Mhic Ire Bridge




� Option 7 - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:

� Construction of two compound channels

� Between the compound channels, the river is dredged by about a metre




� Options 1A to 3A - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:






� At Baile Bhuirne Bridge:


� Options 1B to 3B - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:




� Localised channel dredging upstream and downstream of the Bridges

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Schemes provide the benefit of preventing flood damages that would otherwise occur up to and including their design standard. Along with this, some have added benefits such as reduced damages in the ‘Natural Failure’ condition (for floods greater than the design standard) or enhanced environmental or amenity value, while other schemes have lower benefits such as increased flood or environmental or amenity damage.

To assess this effect, damages were calculated using floods up to the 1000-Year event. The estimated Net Present Value of potential damage using floods between the 1-year and 1000-Year is €15.69M under present day conditions. This is a €2.72M greater than the potential damage using floods between the 1-year and 100-year (the design standard) events.

This added benefit or damage could affect the estimated Costs Benefit Ratios for each of the Flood Relief Schemes Options, though this has not been carried out here, due to the large number of options needing evaluation and the considerable consequential effort. Table A presents these schemes along with a brief description of the individual engineering measures that combine to produce them, along with its estimated All-In Cost and Benefit/Cost Ratio.

Scheme Description of Engineering Measures

All In Costs

(€M)

Benefit / Cost Ratio

Option A Do Nothing (equals flood damage over 50 years) 0.0 -

Option 1 Flood Defences (Walls & Embankments) & Bridge Underpinning, with Bypass & Infilling of ‘village bend’

5.14 2.52

Option 2 Flood Defences & Bridge Underpinning, with Rock Armour at ‘village bend’ 4.82 2.69

Option 3 Flood Defences, Bridge Underpinning, with Bypass of ‘village bend’ 5.10 2.54

Option 4 Flood Defences, Bypass & Infilling of ‘village bend’, significant Dredging 7.05 1.83

Option 5 Flood Defences, Bypass of ‘village bend’ and significant Dredging 6.69 1.93

Option 6 Flood Defences, Rock Armour at ‘village bend’ and significant Dredging 6.68 1.94

Option 7 Two compound channels linked by River Dredging 7.17 1.81

Option 1A Option 1 with channel widening upstream of Baile Bhuirne Bridge 5.25 2.47



Option 1B Option 1 with some dredging U/s & D/s of Baile Mhic Ire Bridge 4.80 2.70



Table A: Estimated Costs and Benefit/Cost Ratios of the Flood Relief Schemes

The Options identified as being potentially feasible in terms of providing protection against the 100-year flood and cost effectiveness were appraised against Multi-Criteria Analysis objectives (see Appendix A1 and B). For a set of given objective, this involved scoring each of these potential schemes in relation to specified minimum requirements that should be meet in order to be acceptable under an objective, and aspirational targets (i.e. targets that an option should seek to achieve to be assigned a maximum score), making use of defined indicators for each objective.

For each option, appropriate scores were determined against each objective based on an assessment of benefits and impacts. The assignment of scores was, where possible, quantitative but otherwise qualitative: with the assignment of a score based on a description or category. To this end, the value to the overall area at risk (Global Weighting) and to the area of potential significant risk: APSR (Local Weighting) were applied to reflect the importance of an objective for the APSR in the town.

For each objective, the indicators (minimum requirements and aspirational targets) along with the Global and Local Weightings were agreed within the OPW to ensure consistency with the appraisal

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of Options in other Schemes nationally. The four criteria were Technical, Social, Environmental and Economic. For each option, the Technical and Social individual local weighting score was largely based relative to the minimum requirement and aspirational targets. The Economic individual local weighting score was also based on these, though its cost benefit ratio was also taken into account. The Environmental Consultants commissioned for the Environmental Impact Assessment (RPS) carried out the Environmental scoring. The results of this Technical, Social, Environmental and Economic Criteria appraisal are included in Appendix A1 and those for Option 1 are presented in Table 2, while the summary for all options in order of ranking is given in Table 3.

CRITERIA Factored Weighted Score

1. Technical 11.67

2. Economic 18.62

3. Social 27.5

4. Environmental -15.38

TOTAL SCORE 42.41

Table 2 – Option 1 - Technical, Environmental, Social and Economic Appraisal

Rank OPTION SCORE

1 1 42.41

2 2 41.17

3 3A 41.09

4 3 40.96

5 2A 39.84

6 1A 27.94

7 A 10.00

8 1B -583.78

9 2B -1513.59

10 3B -1516.38

11 4 -1987.22

11 5 -1987.22

13 7 -2125.09

14 6 -2601.28

Table 3 - Summary for All Options in Order of Ranking

This process has identifies Option 1 as the preferred flood relief scheme for Baile Mhic Íre and Baile Bhuirne. Its Engineering Measures are flood containment through a combination of walls and embankments, channel infilling, bypass channel cutting, along with lowering the invert (bed) of the three bridges by underpinning. On the upstream right bank (to improve the approach of floodwaters) localised channel widening at Baile Mhic Íre Bridge, and, in addition, raising the level of the existing road on both sides of the bridge, installation of culverts beneath this raised stretch to accommodate bypassing flow and to remove the damming-up effect caused by the bridge and roadway.

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INDEX OF CONTENTS

SECTION 1. INTRODUCTION

1.1 REASON FOR STUDY 1.2 CATCHMENT DISCRIPTION

1.2.1 General Description the Sullane Catchment 1.2.2 Available Survey Datasets 2. BAILE MHIC IRE AND ITS FLOODING HISTORY

2.1 EXTENT OF THE FLOODING AREA COVERED BY THIS STUDY 2.2 NOTABLE FLOOD EVENTS

2.2.1 Historical Flood Frequency 3. PRESENT DAY CONDITION - RIVER CORRIDOR AND TRIBUTARIES

3.1 FLOODING FROM THE RIVER 3.2 FLOODING FROM THE TRIBUTAIRES

4. HYDROLOGY 4.1 PROBLEMS OF FLOW ESTIMATION IN BAILE MHIC IRE 4.2 HYDROMETRIC STATIONS IN THE REGION

4.2.1 Rating Review of the Hydrometric Stations 4.2.2 Flow Records at the Hydrometric Stations

4.3 ANALYSIS OF THE ANNUAL MAXIMUM FLOOD PEAK FLOWS 4.3.1 Flood Studies Update - Estimate of the Median Flood at Baile Mhic Ire 4.3.2 Macroom (19032) Annual Maxima 4.3.3 Rainfall Data

4.4 ESTIMATION OF DESIGN EVENT FLOWS 4.4.1 Estimation of the Calibration Events 4.4.2 Validation of the calibrated Unit Hydrograph Parameters

4.5 STATISTICAL PROCEDURE FOR ESTIMATING DESIGN FLOOD FLOWS 4.5.1 Development of the Catchment Growth Factors 4.5.2 The Median Flow to Macroom 4.5.3 Development of Return Period Flood Estimates for Baile Mhic Ire 4.5.4 Development of the Median Flow and 100-year Flood for the Tributaries 4.5.5 Main Channel and Tributary Design Flow Uncertainty and Probability of

Occurrence 5. HYDRAULIC MODELLING FLOOD PROFILES AND CLIMATE CHANGE

5.1 PROFILE OF A RETURN PERIOD FLOOD 5.2 SURVEY OF THE SULLANE RIVER, ITS STRUCTURES AND TRIBUTARIES 5.3 THE NUMERICAL (COMPUTER) HYDRAULIC MODEL 5.4 CALIBRATING THE NUMERICAL MODEL - CHANNEL RESISTANCE

VALUES 5.5 MODELLING THE 2006 CALIBRATION EVENT AND FLOOD

CHARACTERISTICS 5.6 FINAL MODELLED FLOOD LEVELS 5.7 VALIDATION OF THE HYDRAULIC MODEL - THE NOVEMBER 2011

EVENT 5.8 MAIN CHANNEL - PRESENT DAY RETURN PERIOD FLOOD DEPTHS 5.9 FLOW VELOCITIES AND SEDIMENT TRANSPORT DURING FLOODS

AND IN GENERAL FLOW CONDITIONS 5.10 THE 100-YEAR FLOOD ENVELOPE 5.11 EXPECTED CLIMATE CHANGE CONDITION

5.11.1 Accounting for Climate Change 5.11.2 The 2100 Climate Change Scenario 5.11.3 Climate Change Return Period Profiles and Velocity Changes

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6. FLOOD DAMAGE AND SCHEME BENEFIT ESTIMATES 6.1 INTRODUCTION 6.2 BENEFITS OF A FLOOD RELIEF SCHEME 6.3 DISCOUNT RATE AND PROJECT HORIZON 6.4 FLOOD DAMAGE DATA 6.5 EURO/STERLING CONVERSION RATE 6.6 TANGIBLE BENEFITS – DENEFIT CALCULATION

6.6.1 Property Categorisation Assumptions 6.6.2 Design Floods and Water Levels 6.6.3 Flood Depth Calculation and Duration 6.6.4 Property Capping Assumptions 6.6.5 Indirect Damages 6.6.6 Traffic Disruption 6.6.7 Potential Losses in Emergency Services Sector 6.6.8 Total Tangible Damages 6.6.9 Intangible Benefits – Benefit Calculation 6.6.10 Total Tangible Damages 6.6.11 Intangible Benefits – Benefit Calculation

6.7 BENEFIT/DAMAGE ASSESSMENT RATIO 6.7.1 Natural Failure Scenario

7. ENGINEERING MEASURES FOR FLOOD ALLEVIATION 7.1 DESIGN FLOOD STANDARD FOR BAILE MHIC IRE

7.2 ENGINEERING MEASURES FOR FLOOD ALLEVIATION 7.3 DO NOTHING 7.4 MINIMAL MEASURES 7.5 FREEBOARD – THE SAFETY FACTOR FOR DEFENCE WALLS AND

EMBANKMENTS 7.6 TYPES OF FLOOD WALL DEFENCES

7.7 TECHNICAL DECISION CRITERIA TO SELECT SUITABLE ENGINEERING MEASURES

7.8 A FLOOD FORECASTING AND FLOOD WARNING SERVICE 7.9 INDIVIDUAL PROPERTY PROTECTION 7.10 RELOCATION OF AFFECTED RESIDENTS OR BUSINESS 7.11 RECONSTRUCTION OF LOW LYING PROPERTIES AT A HIGHER LEVEL 7.12 UPSTREAM STORAGE OF THE FLOOD RISK 7.13 FLOW DIVERSION MEASURES 7.14 FLOOD CONTAINMENT (Floodwalls and Embankments)

7.15 INCREASE CONVEYANCE 7.15.1 Bridge Underpinning 7.15.2 Channel Widening Upstream of Baile Bhuirne Bridge 7.15.3 Straightening the Existing River Bend Just Upstream of Baile Mhic Ire Bridge

7.15.4 Removal of Weirs 7.15.5 Channel Re-Grading Through Dredging

7.16 THE VILLAGE BEND AREA 7.16.1 Rock Armouring 7.16.2 Flood Flow Bypass Channel 7.16.3 Full Bypass Channel Along With Infilling of the ‘Village Bend’

7.17 DREDGING 7.17.1 Significant Dredging 7.17.2 Partial Dredging at Baile Mhic Ire Bridge 7.17.3 Compound Channel Downstream of Baile Bhuirne Bridge 7.17.4 Compound Channel Downstream of Baile Mhic Ire Bridge 7.17.5 Dredging with Compound Channel

7.18 ROAD RAISNG AT BAILE MHIC ÍRE BRIDGE

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7.18.1 Road Raising at Baile Mhic Ire Bridge 7.19 GRAVEL DEPOSITION AND GRAVEL TRAPS 7.20 DEBRIS TRAPS 8. POSSIBLE RELIEF SCHEMES FOR THE SULLANE RIVER

8.1 VIABLE SCHEME OPTIONS 8.2 OPTIONS 1, 2 AND 3

8.2.1 Description of Options

8.3 OPTIONS 1A, 2A AND 3A 8.3.1 Description of Options

8.4 OPTIONS 4, 5 AND 6 8.4.1 Description of Options 8.4.2 Justification for the Significant Dredge Depths

8.5 OPTIONS 1B, 2B AND 3B – Effects of Partial Dredging at Baile Mhic Ire Bridge 8.5.1 Description of Options 8.5.2 Effects of this Partial Dredging Measure

8.6 OPTION 7 8.6.1 Description of the Option 8.6.2 Effects of a Compound Channel and Localized Dredging

8.7 ROAD RAISNG - BAILE MHIC ÍRE BRIDGE 8.7.1 Road Raising Baile Mhic Ire Bridge – Right Bank 8.7.2 Dealing with Flow Bypassing Baile Mhic Íre Bridge (along the Right Bank)

8.7.3 Road Raising Baile Mhic Ire Bridge – Left Bank 9. POSSIBLE RELIEF MEASURES FOR THE TRIBUTARIES

9.1 INTRODUCTION TO THE TRIBUTARIES 9.2 THE COFFIN CHANNEL 9.3 PROPOSED DESIGN FOR THE COFFIN CHANNEL 9.4 EXISITING GROTTO CHNNEL CONDITIONS 9.5 PROPOSED DESIGN FOR GROTTO 9.6 INDUSTRIAL NORTH CHANNEL - Existing Conditions 9.7 PROPOSED DESIGN 9.8 INDUSTRIAL SOUTH CHANNEL - Existing Conditions 9.9 PROPOSED DESIGN 9.10 BOHILL RIVER

10. COSTS OF THE POSSIBLE FLOOD RELIEF SCHEMES 11. BENEFIT TO COST RATIO AND MULTI CRITERIA ANALYSIS 12. Multi Criteria Analysis 13. CONCLUSIONS AND RECOMMENDATIONS REFERENCES

2254/RP/001/I (Sept. 2013) – BAILE MHIC IRE FEASIBILITY FLOOD RELIEF STUDY (DRAFT FINAL REPORT) 1

1. INTRODUCTION

1.1 REASON FOR STUDY From the Lee CFRAM study, Baile Mhic Ire was highlighted as an area where a Flood Relief Scheme was justifiable based on social, environmental and costs beneficial analyses, and should be undertaken. A subsequent review of the study’s report concluded that a standalone detailed study was required to develop a design standard flood alleviation preferred scheme option. The Baile Mhic Ire study area is highlighted on Figure 1.

Baile Mhic Ire, on the River Sullane in County Cork, has a history of flooding. The main sources are the river itself and three tributaries from the north of the village (with two of these also producing overland flow due, in part, to their piecemeal culverting under roads and properties that affect properties a considerable distance away). In addition, overland flow coming off the high ground or from backing up of the drainage system runs along roads and has resulted in a number of properties being damaged; it is understood to have flooded the Primary School in 2008. Finally, the storm network can independently cause flooding through gullies leading to damage to a number of residential houses opposite the Co-operative, even when there is no significant flow in the river.

The main areas affected are upstream of Baile Bhuirne Bridge, through the middle of the town (at the garage yard), upstream of Baile Mhic Ire Bridge and just west of the Post Office; where surface water is backed up from the river.

The two worst river related floods occurred in April 1962 and August 1986. Along with these, significant inundation occurred in December 2001, December 2006 and to a lesser degree November 2009 and November 2011.

Figure 1 - Study Area – The Sullane River and

Tributary Channels


This project comprised of the following tasks:

� Review of the Lee CFRAM for Baile Mhic Ire

� Hydrology Analysis

� Hydraulics Modelling

� Flood Damage and Cost Benefit Estimation

� Development of a River Option for Flood Alleviation

� Flood Mapping

� Multi Criteria Analysis on Viable Options

� Recommend a Preferred Option

Figure 2: River Sullane Catchment

1.2 CATCHMENT DISCRIPTION

1.2.1 General Description the Sullane Catchment The Sullane rises in the Derrynasaggart Mountains in West Cork and flows southwest to Macroom before it enters the River Lee. By Baile Mhic Ire, it has travelled 15.2km and amassed a catchment of 75.2km2, by Macroom, it has stretched 31.8km and has a catchment of 216.1 km2 (see Figure 2).

Figure 1 presented the four tributaries that flow through the village from the north. The largest of these, the Bohill, drains 12.7 km2 and enters the main river 500m upstream of Baile Bhuirne Bridge. This study has assigned names to the three smaller ones, the Industrial River (1.47 km2) joins the Sullane 200m upstream of Baile Bhuirne Bridge - this has also been split into two streams; Industrial North (0.64 km2) and Industrial South (0.83 km2), the Grotto Stream (0.96 km2) enters 60m upstream of the Co-operative. In addition, the very minor Coffin River (0.46 km2) is culverted under the Coffin Makers Yard and the Pharmacy, after which it flows into the Sullane.

1.2.2 Available Survey Datasets The quality and availability of topographical data sets has a direct impact on outputs, such as those from the hydraulic model. The Lidar available for this study has an accuracy of +/- 0.2m in the vertical and has two points recorded in every square metre. For the majority of the River Sullane in the study, the in-bank and out-of-bank survey area was carried out during 2007 for the Lee CFRAM, this used Lidar for the out-of-bank extents and merged it with the respective in-bank survey data to form cross-sections for the CFRAM models. For this study, additional, more detailed, ‘on the ground’ cross sections of the Sullane and its tributaries were surveyed in 2010. Both these data sets were merged and used to generate the hydraulic model.


2. BAILE MHIC IRE AND ITS FLOODING HISTORY While records of past floods may be anecdotal and incomplete, they give insight on flooding sources, frequency, intensity and seasonality. Such historical information may come from accounts of personal experience, newspapers or post event reports and surveys. A review of the flood history of Baile Bhuirne and Baile Mhic Íre is detailed in this section.

Flooding in Baile Mhic Ire and Baile Bhuirne generally occurs when the capacity of the river channel is exceeded. Upstream of each of the three bridges, the rising up of floodwater due to the restriction imposed by the structure (afflux) leads to increased flood levels. The two most extreme events in recent memory occurred in 1962 and 1986. In 1986, the depth was over 0.6m at the garage on the main street, and some houses were flooded to 1.5m. A significant section of the upstream right-bank wall failed (collapsed) alongside Baile Bhuirne Town Bridge. The resulting impact was described as a surge of floodwaters rising and falling in a short space of time. During such large river-flood events, waters can flow along the N22 (left bank) from Baile Bhuirne Bridge towards Baile Mhic Ire.

The OPW commissioned Halcrow to carry out the Lee CFRAM. Their investigation of flood risks within the village led to their conclusion that it was at significant risk. The subsequent OPW review resulted in a more detailed flood risk assessment being undertaken. The assessment lead to a revision of the hydrology, hydraulics and the preferred flood relief scheme options, likely to be viable on technical, economic, social and environmental grounds.

A Strategic Environmental Assessment was carried out within the Lee CFRAM study. This provided some base-line data for this present study, it has subsequently been expanded upon, through a site specific Environmental Study for the Baile Mhic Ire study area, carried out by RPS. This feed into the overall assessment to find the most suitable flood relief option for the area.

2.1 EXTENT OF THE FLOODING AREA COVERED BY THIS STUDY The subject area covers all properties that have flooded in the past and also reflect the area at risk from the 100-year design event. It incorporates the Sullane River from 500m upstream of Baile Bhuirne Town Bridge to 1km downstream of Baile Mhic Ire Bridge, the downstream 0.4km stretch of the Bohill, 0.6km of both the Industrial North and South, 1.2 km of the Grotto and also 0.35km of the Coffin Channel.

Figure 3: www.floodmaps.ie Extract


2.2 NOTABLE FLOOD EVENTS The OPW provides a National Flood Hazard website (www.floodmaps.ie) that makes available information on areas potentially at risk from flooding. As shown in Figure 3, this website shows several historical floods that have affected Baile Bhuirne and Baile Mhic Íre. Through examination of these events, it is clear that there is a history of frequent, severe flooding within Baile Mhic Íre village, as summarized in Table 1.

Date of Flood Comments

April 1962 Flood waters in properties along the main street of Baile Mhic Íre to a depth of 5 feet (1.5m), 2 feet (0.6m) of water along the N22

5th August 1986 N22 flooded, over 200 people given shelter in the local Irish College. Collapse of bridge upstream of the town, apparently releasing a large volume of water suddenly. Properties flooded in both villages.

3rd of December 2001 Floor damage to houses within Baile Mhic Íre village.

2nd December 2006 Flooding was worse than 2001 but not as bad as 1986. A number of local residents described how floodwaters rose to within millimetres of flooding their properties, but other properties were inundated.

19th November 2009 Properties flooded in Baile Mhic Íre. N22 and L3405 flooded.

17th November 2011 Houses in the village of Baile Mhic Íre came close to flooding. Road was flooded from village to Baile Mhic Íre Bridge.

Table 1: Historical Flood Chronology

2.2.1 Historical Flood Frequency

1962 Flood Event The village of Baile Mhic Íre was badly flooded in April 1962, though very little information exists about it. Local accounts suggest that it was of similar magnitude and severity to that in 1986. It may even be possible that the 1962 event was the larger of the two, however due to changes to watercourses in the intervening years, and the poor information, it is not possible to decide the case. During the first Public Information Day (PID) held on the 23rd of November 2011, one resident (though as only a child at the time) described it and clearly remembers having to be freed from the rising flood levels through a first floor window. There was about 1.5m of water in the ground floor of his home and more that 0.6m on the main Cork-Kerry road.

1986 Flood Event As discussed, the 1986 flood may well be the worst in living memory. It occurred on the 5th of August during the rainstorm that preceded Hurricane Charlie by three weeks. Two properties flooded upstream of Baile Bhuire Town Bridge, but have not flooded since. A section of the right-bank wall at that bridge collapsed and this may have created a sudden release of waters towards Baile Mhic Ire.

There is also evidence that parts of Pol Na Bro Bridge (about 2km downstream of Baile Mhic Ire Bridge) failed and collapsed. Though, as will be seen later in Section 5, the increased water level at the bridge (afflux) does not seem to have directly caused flooding within Baile Mhic Ire, primarily due to the steepness of the intervening catchment, this is contrary to some local opinion.

For the duration of the event, the main road from Cork to Killarney (N22) was closed, as it was inundated in parts with 1.2m to 1.5m of water. Traffic from Kerry was halted ten miles outside the village of Baile Bhuirne and, as a result, over 200 people were given shelter in the local Irish College. About 1.2m of water was reported in properties along the main road through Baile Mhic Íre and a local resident remembers an unoccupied car being swept into the River Sullane.


2001 Flood Event A flood occurred on the 3rd of December 2001. While this is the smallest of the noted events for the area, it still caused damage to the floors of properties along the main street in Baile Mhic Íre.

2006 Flood Event Post the 2nd of December 2006 event, a survey was carried out during the Lee CFRAM, so a reasonable amount of detail is available. There is a general consensus that this caused worse flooding than occurred in 2001 but not as bad as in 1986.

One property at the eastern side of the village was inundated with 0.05m of water. A local farmer described it as unusual to have such extensive flooding in Baile Mhic Íre and no flooding in Baile Bhuirne. He described the flows in the Bohill River, a tributary of the River Sullane between Baile Bhuirne and Baile Mhic Íre, as some of the highest he had ever witnessed. He was of the opinion that this tributary was the major contributor to the flooding in the town and described the rainfall in its upper catchment as heavy over a number of days.

A number of local residents described how water rose to within millimetres of flooding their properties. To the west of the village, downstream of Baile Bhuirne Bridge, waters surrounded a number of properties, but did not reach floor levels. They also described how water flowed down the main street and poured out of a gateway just downstream of the bridge.

The owner of one property on the main street (NGR 120701 76801) described how floodwaters inundated her home to a depth of approximately 0.075m, having entered through both the front and back doors. Neighbouring properties were not affected with the exception of the property directly across the road (NGR 120747 76816).

The Lee CFRAM flood map presented in Figure 4 provides an indication of the flood extent and number of inundated properties. The map extends between Baile Bhuirne Bridge and Baile Mhic Íre Bridge and is based on information provided by the local residents. The Flood Extent Envelope along the river’s right bank was not investigated, as properties there were not affected. Information on flood level was obtained both from trash lines and discussion with local residents. Recorded flood levels are indicated by red dots and inundated properties with a red/dark brown property outline. This figure, though, does not show the full numbers of inundated properties, as not all property owners could be contacted at the time of the survey.

Figure 4: December 2006 Flood Extent Envelope for the Left-Bank (West Side)


2008 Flood Event While no significant flooding took place from the main channel in 2008, overland flow from the Grotto Stream resulted in the flooding of Colaiste Ghobhnata to 0.05m.

2009 Flood Event A rainstorm on the 19th of November 2009 caused widespread flooding of Cork City and County, including Baile Mhic Íre. This is considered to be smaller than the 2006 and 1986 events.

Twelve residential properties, one community building, three shops and one garage were affected. In addition, 100m of the N22 flooded and 180m of the L3405 also flooded. The L3405 is a local road that connects Baile Mhic Íre to Baile Mhic Íre Bridge. A pumping station located next to the bridge was also flooded and, as a result, water supply was affected for 4 days.

2011 Flood Event Minor flooding was recorded on the November 17th 2011. The L3405 road from the village to Baile Mhic Íre Bridge flooded but residents reported that, while properties did not flood, some houses were under threat.

The river gauging station installed in September 2011 to aid this study and, during the peak of this event, a flow gauging was taken by OPW Hydrometric Section, this measured 110m3/s.

Other Events Flooding can occur of low-lying properties adjacent to Scannal’s pub from the piped, local drainage system backing up, as water pours out of the low point in the network and enters the respective properties. This can occur during a moderate to significant rainfall event over its drainage catchment (i.e. the hills to the north of Scannal’s pub). Such flooding can occur when the piped outfall of its sewer network is surcharged by a median annual event on the Sullane and just a 5-year event on the Coffin channel.


3. PRESENT DAY CONDITION - RIVER CORRIDOR AND TRIBUTARIES

Figure 5: Reference point map

3.1 FLOODING FROM THE RIVER The flooding mechanisms of Baile Mhic Íre have been analysed from a combination of hydraulic modelling and witness accounts of recent events. At Baile Bhuirne Town Bridge, flooding can begin along the left bank upstream of the Bridge. Between op_15 and op_13 in Figure 4, floodwater can spill from gate access points through run down the N22.

Just upstream of Baile Mhic Ire, at Baile Bhuirne Bridge, floodwaters first spill from the gate at op_13, (on the left bank upstream side of Baile Bhuirne Bridge) and flows down the N22 towards the village of Baile Mhic Ire.


Just downstream of the bridge, floodwaters first spill from the river between op_11 and op_10, however, higher flows can spill from the gate opposite the Abbey Hotel. In addition, floodwaters can reach the properties in the reach between op_10 and op_9 both from the floodwater travelling along the N22 and also directly from the river.

At op1_9, where there is a high right bank ground level relative to the left bank, the river begins to elbow (bend). This creates a complex hydraulic issue where the right bank flood level is similar to the results from hydraulic modelling but super-elevated river levels (higher than might otherwise be expected) rise up along the left bank. This means that floodwaters spill from this ‘bend’ prior to flooding along the N22. As a result these super elevated levels are included to the hydraulic modelled level, so providing the design flow levels.

Downstream of the ‘bend’ floodwaters spill from the river and first access the N22 at op_7 and then through low points between properties location from op_6 to op_4.

A bypassing flow occurs along the right bank at Baile Mhic Ire Bridge (i.e. across the L3405 road between op_22 and op_23). This can be very significant, under the present conditions, up to 80 m3/s during the 100-year event. These floodwaters then overtop the L3405 wall and return to the river.

3.3 FLOODING FROM THE TRIBUTAIRES The four main tributaries enter the Sullane from the North (also see Section 9 for details of the existing flooding mechanisms associated with these tributaries).

Flooding from the Bohill River follows the contours of the land, however, even at very high flows, this river has been shown to be of little risk to Baile Mhic Ire.

Including the one underneath the N22, culverts along both the Industrial North and South channels have insufficient capacity to take their extreme event. This can cause afflux at high flows and results in flooding along both riverbanks.

The fast flowing Grotto Stream has produced overland flow in the direction of the primary school and is made up of two channels that join at the back of the Cluain Reidh Housing Estate. At high flows, channel capacity is inadequate, so out-of-bank flooding can occur, especially from the channel that drains rounds the GAA grounds. Also, the Grotto Stream’s N22 culvert has insufficient capacity, this has been reduced further by two 450mm diameter sewer-mains connected to its soffit; at high flows, these cause an increase in afflux along its upstream face, thereby worsening flooding conditions.

The surface water pipe network that connects into the Coffin Channel carries runoff from the catchment to the northeast of the village and this combines with a surface-water piped sewer from the east (that takes runoff drainage from that direction), i.e. opposite the Pharmacy along the centre of the N22. During high flows in the catchment to the north, the piped sewer from the east can become ‘backed up’, resulting in water flowing out of the low-lying drains. In the past, this had resulted in the flooding of three to five properties opposite the Co-operative. Also, water can flow from the back of the Pharmacy, where an old gravel trap exists, toward the N22.


4. HYDROLOGY The estimation of Design Flows is arguably the most important part of a Flood Study, in that it can have the largest influence on the final flood outline. It can also be the greatest source of uncertainty and impact viability of possible solutions to the flooding problems. This chapter details the availability of flow data and reviews the quality of the existing data sets. All flood estimation is based, however indirectly, on measurements of river flow and/or rainfall data. The availability of gauges that attribute to the catchment is also considered along with an appraisal of the data.

4.1 PROBLEMS OF FLOW ESTIMATION IN BAILE MHIC IRE No record of river level or flow data exists in the local vicinity of Baile Mhic Ire. Two river gauges have been installed during 2011. Due to this, a number of flow estimation techniques were used, ranging from the updated FSU methodology (i.e. the Flood Studies Update for Ireland, an OPW lead review of hydrological flow estimations, etc.) to bespoke rainfall runoff site specific Unit Hydrograph techniques; this drew upon the knowledge gained in estimating catchment effective rainfall runoff from the Munster Blackwater Flood Forecasting System.

4.2 HYDROMETRIC STATIONS IN THE REGION The ESB have a gauging station within the River Sullane catchment in Macroom Town (19031). A assessment was undertaken of both its hydrometric data and the Lee CFRAM rating review (refer below). The River Laney is the eastern neighbour of the Sullane and the Dromcarra catchment is located to the south. Data for their respective gauging stations (19027 and 19014) was reviewed in help both the hydrological assessment and development of regional growth curves for the Sullane. The locations of these two gauging stations are shown in Figure 6.

During the course of this study (October 2011), the OPW installed two gauging stations, one at Baile Mhic Íre Bridge and the other at Baile Bhuirne Bridge. The records for these are short and, to date, have not been employed to calculate Design Flows for the area. Since their installation, however, a number of flow gaugings have been taken and these have helped verify the methodology used to calculate event flows (refer below).

Figure 6: Gauging Stations

Red - Met Eireann Rain Gauges Blue – ESB Rain Gauges, Pink – ESB River Gauges

Macroom

Dromcarra

Laney


4.2.1 Rating Review of the Hydrometric Stations Gauging stations record water level at a particular location and a rating curve is used to produce the relationship between its water level and flow, to estimate, for example, peak flood values. The curve is established by plotting measured flows against the water level during the measurement. Extrapolation of the curve is often necessary, as its data tends not to cover the full range of levels.

Three stations were used to estimate its Median Flood and in the analysis of Baile Mhic Ire’s Growth Factors. The Lee CFRAM study carried out a Rating Review for 19014 Dromcarra and 19031 Macroom. This study examined the Lee CFRAM Rating Review for Macroom and found that, for low to mid flows (below a staff gauge reading of 2.85m), the rating corresponded well (i.e. gave a very good match). This assessment was based on the CFRAM ISIS model through Macroom and the ESB flow gauging at 19031 pre 1995. Above 2.85m, the match was not good.

Some uncertainly exists about the precise date when the Macroom gauge was relocated, best information suggests it occurred in the late 1990’s. Also, the level (relative to Malin) of zero on its staff is uncertain. It is being taken here that pre-1998 flow measurements belong to the old gauge location (Refer to Section 4.5.2 and Tables 6 and 7).

While this investigation of the Macroom Rating Review provided a median flow of 164 m3/s that is different to the CFRAM rated median flow of 159.65 m3/s (Refer to Section 4.5.2), this is essentially caused by varying in-bank and out-of-bank Manning Resistance Values by an amount less than 0.005, which is subjective and arguments can be put forward for either case. In the main, as the difference is less than 3%, these are being taken as essentially the same. This study, though, did find more of a divergence with the CFRAM rating for the bigger flows (i.e. those with levels greater than 2.85m and flow estimates above 220 m3/s). For example, at a level of 3.0m, the CFRAM rating gives 248 m3/s while the revised rating found a value of 277m3/s, i.e. about 12% bigger.

Despite this, both the CFRAM low to mid flow rating and its upper one have been used here (Refer to Section 4.5.3), as the improved upper rating generated in this study only changes peak flow estimates that are above 2.85m, and this does no affect the Median Flood estimate for Macroom. In addition, there are no out of bank flow measurements to validate either of these high flow ratings.

For the Laney (19027), the existing OPW rating was reviewed and then applied. At Dromcarra (19014), the CFRAM rating was assessed and then applied. Both reviews largely agreed with the initial ratings; as they were only based on validating using spot flow measurements.

4.2.2 Flow Records at the Hydrometric Stations Macroom gauging station (19031) is located at the Macroom Sewage Treatment Works, approximately 1.3km downstream of Macroom Bridge. While it is on the same catchment as Baile Mhic Ire, it is a further 17.3km downstream; with a catchment size of 216 km2, i.e. almost three times that of the 75 km2 at Baile Mhic Ire. Over its 19 years of recording life, though, there are only 9 useable years, i.e. 10 years were incomplete and were, therefore, deemed unreliable; especially where the missing period occurred when nearby catchments had their largest flood in the year.

The Laney station (19027) has a contributing catchment of 82 km2 and 22 years of recorded data while Dromcarra station (19014) has a catchment of 170 km2 and 47 years of recorded data.

4.3 ANALYSIS OF THE ANNUAL MAXIMUM FLOOD PEAK FLOWS

4.3.1 Flood Studies Update - Estimate of the Median Flood at Baile Mhic Ire The Flood Studies Update (FSU) recommends the use of a pooling group analysis for ungauged catchments. It determines the most similar gauged site to the area of interest, based on the contributing catchment Area (A), Standard Average Annual Rainfall (SAAR) and Base Flow Index (BFI, a measure of the amount of flow that enters the ground and, therefore, does not directly run off). For Baile Mhic Ire, this method gave a median flood (Q Med, i.e. the middle flood from the recorded series of Annual Maximum Floods; this is the 1 in 2 year flood) of 45 m3/s.


4.3.2 Macroom (19032) Annual Maxima Over the period of record, the Annual Maxima Series contains the maximum flood event from each valid year. The Annual Maxima Series (Amax) from Macroom Station gives a Median Flow (Q Med) of 160 m3/s. As there are only 9 years (N) of full data, the FSR advises that only return period floods up to 2N may be reliability estimated, i.e. an 18 year return period.

4.3.3 Rainfall Data The rainfall analysis used a combination of daily and hourly ESB and Met Eireann rain gauges in and around the Baile Mhic Ire and Macroom catchment, as shown in Figure 6.

4.4 ESTIMATION OF DESIGN EVENT FLOWS

Due to the lack of gauged Hydrometric Data to Baile Mhic Ire, the following ‘ungauged catchment’ Flow Estimation techniques were carried out:

� FSU equation and pivotal site method

� FSR 5 and 6 Variable methods

� FSR Ungauged Unit Hydrograph method

Using 0.9477 to convert the Mean Annual Flood (Qbar) to the Median Flow (Q Med), this gave the following Median Flow values for Baile Mhic Ire (i.e. to Baile Bhic Ire Bridge):

FSU 5 Variable 6 Variable Unit Hydrograph

m3/s m3/s m3/s m3/s Baile Mhic Ire 45.50 13.63 22.98 57.21

Table 2: Median Flows from ungauged Flow Estimation Techniques

FSU 5 Variable 6 Variable Unit Hydrograph AVERAGE

0.51 0.49 0.54 0.55 0.53*

Table 3: Ratios from Macroom to Baile Mhic Ire

On first inspection, these estimates all appeared low compared to the gauged value to Macroom of 160

m3/s (see Section 4.5.2). To investigate whether these Q Med values are realistic, they were first simulated through the ISIS hydraulic model for Baile Mhic Ire, along with site inspected Manning Channel Resistance Values (see Section 5). It is reasonable to assume that the Median Flow will have river levels that are ‘there about’ bank-full. For the FSU and FSR 5 & 6 methods, in particular, the model outputs showed that these required extremely high and unrealistic resistance values to reach bank full levels. This implies that the ‘ungauged catchment’ flow estimation techniques are likely to provide underestimates of the design flood event to Baile Mhic Ire. In addition, the FSR recommends the use of gauged data when there is 2 years or greater of recorded data over ungauged data techniques.

This resulted in estimation of the design event flow to Baile Mhic Ire by ‘gauged data’ from Macroom and the transfer of its Median Flood (through scaling by 0.51) to Baile Mhic Ire (see Table 3). The overall average from the above four ungauged catchment flow estimation techniques is 0.53, i.e. very similar.

It is noted the Area scale factor from Baile Mhic Ire to Macroom is almost three, yet the catchments respective S1085 (i.e. the slope of the main stream (m/km)) increases from 4.7 at Macroom up to 13 for Baile Mhic Ire. This lend support for the ratio of 0.51.

4.4.1 Estimation of the Calibration Events As event rainfall data was available through the Sullane and adjoining catchments (see Figure 6), best practice would recommend its use in defining event flow estimates. This has resulted in an very thorough Rainfall Runoff Analysis and the development of a site specific Unit Hydrograph for both the gauged Macroom catchment and at Baile Mhic Ire.


Initially ten events were selected, as they coincided with floods that were recorded at Macroom. The effective percentage runoff (i.e. percentage rainfall contributing to the river flow) was evaluated for each event through the use of the Munster Blackwater Flood Forecast System, based on preceding rainfall and the time of year. This helped estimate how the catchment dried out and to what degree it did so. The rainfall-soil potential runoff response for both the Munster Blackwater and Upper Sullane was taken to be the same, based on their geographical closeness and similarities in their soil types (as obtained from Soil Survey Maps 1978). This allowed the Unit Hyrodrgraphs (UH) general parameters of 220 and 2.52 (defined by the FSR for an ungauged catchment) to be changed to an average calibrated factor based on the ten events, (see Table 4).

Table 4: Unit Hydrograph Calibration Events

The ungauged Unit Hydrograph factor value of 220 changed to 270, by comparing to the volume of calibrated unit hydrograph versus the actual hydrograph. As a result, the volume ratio (i.e. the averaged comparison between the volume of water from calibrated and actual events) became 0.995. It was found that changing the 2.52 factor produced very little change in results.

A sensitivity analysis was carried out on the above data. This omitted the years from late 1999, prompted, in part, to uncertainty associated with the timing of the change in staff gauge location, (refer to Section 4.5.5). This gave a volume ratio of 1.05. As only daily rainfall data was available, it was necessary to convert this to hourly values. This was estimated by employing the Cork Airport hourly rainfall data.

Figure 7 presents the 29th Oct. 1996 event as an example of the resulting rainfall profile versus water level at Macroom, and the calibrated Unit Hydrograph flow (titled FSR Rainfall) versus the actual flow is given in Figure 8.

Figure 7: Example 29/10/96 Event

0

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25

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41

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57

65

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105

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129

137

145

153

161

169

177

W ater Level

R ainfa ll

X-axis is in 15-minute time steps Y-axis river gauge level (m) and rainfall intensity (mm per hour)


Figure 8: 29/10/96 Example Event

Some recorded data exists for Baile Mhic Ire. The November 2006 event has seven calibration points that stretch from the Abbey Hotel to Baile Mhic Ire Bridge (see Figure 4), however, there is no recorded flood level data upstream of Baile Bhuirne Bridge or for the tributaries.

To estimate the flow for that calibration event, the Unit Hydrograph, as discussed, was calibrated using the 270 and 2.52 factors along with an ANSF of 0.2. A Base flow of 15.04 m3/s along with rainfall data and its effective percentage runoff of 80% (as estimated from the Munster Blackwater Flood Forecast System) were then applied to it. Figure 9 presents the resulting hydrograph for Baile Mhic Ire that has a Peak Flow of 145.9 m3/s.

Figure 9: November 2006 Event, based on the calibrated Unit Hydrograph

4.4.2 Validation of the calibrated Unit Hydrograph Parameters In September 2011, a River Gauging Station was installed at the upstream right-bank of Baile Mhic Ire Bridge. On the 17th of November, a minor flood was recorded and a flow gauging was taken during its peak, this estimated it to be 110 m3/s, though there was some bypassing flow on the right-bank of the bridge that was not measured, possibly 10 m3/s. When the event’s rainfall data was applied to the calibrated Unit Hydrograph it gave a flow of 110 m3/s, a surprising match.

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4.5 STATISTICAL PROCEDURE FOR ESTIMATING DESIGN FLOOD FLOWS Design flow were estimated for Baile Mhic Ire through the following three phases:

a) Determine the catchment Growth Factors.

b) Determine the Q Med to Macroom.

c) Event Flow Transfer from Macroom to Baile Mhic Ire (see Section 4.4).

a) The General Extreme Value (GEV) distribution has being recommended for the island of Ireland. In addition, the FSR recommended a 100-year growth factor of 1.96 for the whole region.

The FSU highlighted that, in general, growth factors get progressively bigger the further upstream one goes in a catchment, as such, its growth curve can not be applied to small, hilly catchments such as that draining through Baile Mhic Ire.

The annual maximum analysis was carried out for Macroom 19031 (9 years), Dromcarra 19014 (47 years) and the Laney 19027 (22 years). Due to the lack of validated years (out of 19 years of records only 9 are valid), the Macroom analysis of its median flow to the 100-year was not considered a suitable means to extrapolate to Baile Mhic Ire.

A local-region growth curve was, therefore, developed using a weighted average (based on years) annual maximum analyses from these locations (see Section 4.5.1). This provides a 100-year growth factor of 2.64 (see Table 5). Scaling up these factors to Baile Mhic Ire is not practical, as there is little recorded data at the town. As a result, the local-region growth curve factors are considered applicable and may be used for both the Macroom and Baile Mhic Ire catchments.

b) The median flow (derived from the annual maxima) is the 2-year event flow. The FSR and FSU recommend that, at Macroom, even with just 9 valid annual maxima, its results give greater confidence over ungauged techniques. This resulted in a median flow to Macroom of 160 m3/s (see Section 4.2 and for more detail Section 4.5.2).

c) An average of the Ungauged catchment flow estimate techniques is presented in Table 3 and these were used to transfer flow between Baile Mhic Ire and Macroom (see Section 4.4).

4.5.1 Development of the Catchment Growth Factors The best method for “fitting” the data in the frequency analysis carried out on Macroom, Dromcara and Laney was assessed in terms of Skewness and Kurtosis, Figures 10 to 15 present results.

Figure 10: Macroom 19031 Extreme value analysis

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Flood Frequency Curves fitting

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Figure 11: Macroom 19031 best fit diagram

Figure 12: Dromcarra - Extreme value analysis Figure 13: Dromcarra – Best-fit diagram

Figure 14: Laney - Extreme value analysis Figure 15: Laney – Best-fit diagram

From the analysis, it was determined, as expected, that the General Extreme Value (GEV) analysis produced the Best Fit for each site and this resulted in the Growth factors presented the Weighted Average column in Table 5. The FSU pooled frequency analysis was carried out for Macroom to attempt to best match that pivotal site to other similar sites throughout Ireland. This, coupled with the GEV, produced the ‘Best-Fit’ for the data. This analysis, however, gave a Q100/QMed Growth Factor of 1.78.

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urt

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Data GEV GLO LN3 Pooled EV1 LO LN2

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Two and Three Parameter Moment Ratio Diagram

0.00

0.05

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0.30

0.35

0.40

-0.5

5

-0.5

0

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5

-0.4

0

-0.3

5

-0.3

0

-0.2

5

-0.2

0

-0.1

5

-0.1

0

-0.0

5

0.0

0

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5

0.1

0

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5

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0

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5

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0

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5

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0.5

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5

L - Skewness

EV1LO

LN2

GEV

LN3

GLO

Station L-Moments - Red Dot

Two and Three Parameter M oment Ratio D iagram

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-0.5

5

-0.5

0

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5

-0.4

0

-0.3

5

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5

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5

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-0.0

5

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0

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0

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5

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Table. 5: Growth Factors

4.5.2 The Median Flow to Macroom As opposed to the above ungauged catchment flow techniques, it is considered best practice to use gauged records when more than two years of recorded data are available. The nine years of record at Macroom are being used to estimate its flows and the resultant flow ratios to Baile Mhic Ire (see Table 3). Tables 6 and 7 illustrate the Macroom Annual Maximum data, and the validated Annual Maxima, i.e. those that may be used with confidence because they are the true maximum for the year. Defining the period of missing data and assessing the likelihood of the year’s biggest flood occurring during it by comparing it against the Laney and Dromcarra gauging stations achieved this.

Table 6: Unchecked Annual Maximum Series

Table 7: Valid Annual Maxima Series

It should be noted that the above assumes that the 2001 flow gauging are to the pre 1998 staff gauge zero (see Section 4.2.1).


The nine valid maxima give a Mean Annual flow of 159.65 m3/s; the 19 years would have given 138 m3/s. It was noted that a number of significant observed events (i.e. in excess of 200 m3/s) occurred during years where there was a large break in the data recorded. While greater that the validated median flow, these significant events cannot be included in the above Valid Annual Maximum Data, as they would skew this statistical process because smaller ones would be left out.

To provide additional confidence in the accepted Median Flow, the Laney’s Annual Maximum Series was compared against it for the years that had valid Annual Maxima overlapping each other, a total of eight years. This resulted in a median flow of 50.17 m3/s for the Laney, compared to 49.45 m3/s, when its full 22 years were used.

4.5.3 Development of Return Period Flood Estimates for Baile Mhic Ire Applying the Weighted Average Growth Factors in Table 5 to the Annual Median Flow for Macroom, and then using the ratio from Table 3 that establishes the link between Baile Mhic Ire and Macroom gives the Return Period Flood estimates for Baile Mhic Ire in Table 8.

Return Period Flow (m3/s) (Years) Growth Factor Macroom Baile Mhic Ire

2 1.00 159.7 87.81 5 1.40 223.9 123.12

10 1.68 268.4 147.64 20 1.96 313.0 172.15 30 2.13 339.5 186.75 50 2.34 373.7 205.55

100 2.64 421.8 232.01 200 2.96 472.3 259.77 500 3.40 543.4 298.89

1000 3.77 601.1 330.63

Table. 8: Design Flows to Baile Mhic Ire

4.5.4 Development of the Median Flow and 100-year Flood for the Tributaries The following two methods were used to determine, and validate, Median Flows for the tributaries:

� The FSU OPW equation; from the FSU update, is based on regression studies on catchments (2.8 km2 to 28.6 km2), with an FSE of 1.68

� The IH 124 equation, from the Institute of Hydrology UK, is based on regression studies on small catchments (0.9 to 25 km2), with an FSE of 1.65, applicable for catchments >0.5 km2

As Irish catchments were used to develop it, the FSU OPW formula for small catchments (<50 km2) was chosen for the estimation of return period flows for the tributaries entering the Sullane River through the study area, and the IH 124 equation helped validate its results.

To cater for bridges and culverts on the tributaries, Design Flow is calculated at the upper 66-percentile level; i.e. only a probability of one in six that the actual Design Flow is greater than it. This is found by multiplying the Factor Standard Error (FSE) by the Flow estimate, the FSU OPW formula for small catchments has an FSE of 1.65.

It is preferred that the design event (with an allowance for climate change), where possible, is capable of free flowing through pipes, culverts and bridges. If not practicable, the resulting afflux at the upstream face will be designed to remain below 0.4m.

The effects of land use change due to urbanization and agricultural practices show little forecasted change on Design Flows. Flood relief measures are chosen that can be easily altered to accommodate the effects of climate change, should it occur. OPW policy allows for a standard flow increase of 20% for the midrange scenario and 30% for the upper scenario.


Combining the Regional Growth Factors to the FSU OPW formula estimates provides Design Flows for each of the tributaries, the factor of 1.65 is applied to these for the Industrial South, North, Grotto and Coffin tributaries for designing bridges and culverts and sizing pipes (see Table 9).

Channel Name Area (km2) SAAR BFI FARL S1085 Soil 100-Year (m3/s)

Bohill 12.72 1860 0.56 1.0 43.84 0.45 36.67

Industrial South 0.83 1984 0.58 1.0 95.90 0.45 3.69

Industrial North 0.64 1984 0.58 1.0 77.00 0.45 2.88

Grotto 1.03 1984 0.58 1.0 47.40 0.45 3.78

Coffin 0.45 1984 0.58 1.0 85.40 0.45 2.04

TABLE 9: Tributary Characteristics and 100-Year Design Flows

4.5.5 Main Channel and Tributary Design Flow Uncertainty and Probability of Occurrence

Design Flow Uncertainty The Macroom rating for the upper extrapolated flows gives grounds for concern. The CFRAM Rating was reviewed within this Study, this showed an agreement for flows up to 230 m3/s, above this, the out of bank Manning Roughness, Ineffective Flow areas and lack of flow measurements (the highest recorded is 118 m3/s) leads to a lack of confidence in rating extrapolation. Also, some confusion exists regarding the change in its location; the ESB state that this took place during the 1990’s, and also that its staff gauge zero datum has not been validated. However, fortunately, this does not affect the estimation of its median flow (see Section 4.5.2).

The lack of both hydrometric data to Baile Mhic Ire and sufficient Annual Maxima at Macroom leads to uncertainty in design flows to Baile Mhic Ire. This, however, is being catered for by adding an allowance of 0.2m to the freeboard for all defences in every scheme design option. Published guidance for fluvial flooding problems recommends that 0.3m be added to hard defences (such as retaining walls) and a minimum freeboard of 0.5m to soft defences (such as embankments); this does not include an accommodation for settlement. The addition of 0.2m to the freeboard is capable of accommodating a change in the median flood at Macroom from 160 m3/s to about 200 m3/s, i.e. an additional 25%.

For the tributaries entering the study area, where bridges and culverts exist, the Design Flow will cater for the upper 66-percentile flow. At the upper 66-percentile flow level there is only a one in six chance that the probability of the actual Design Flow is greater than it. This flow is simply the Factor Standard Error (FSE) multiplied by the Design Flow. For the FSU OPW formula for small catchments, FSE is 1.65.

Where practicable, afflux at pipes, culverts and bridges will be kept below 0.4m.

Main Channel and Tributary - Probability of Joint Occurrence of Floods The probability that design flows in the tributaries will occur in combination (coincident) with the River Sullane’s has been considered. The chosen scenario sees the Design Flow Event for the Tributary combining with the 66-year event for the main channel.


5. HYDRAULIC MODELLING FLOOD PROFILES AND CLIMATE CHANGE

5.1 PROFILE OF A RETURN PERIOD FLOOD The level a flood reached at different points along a stretch of river must be known to determine the potential damage that can be caused by it, and hence the potential benefits of protecting against it. The relationship between flood level and distance along the channel is called the Flood Profile. This study requires Flood Profiles for a series of Return Period Floods for the Study Area, i.e. from above Baile Bhuirne Town Bridge to a distance of about 500m downstream of Baile Mhic Ire Bridge

Flood Profiles can be generated in a number of ways, depending on the availability of information. The lack of recorded historical flood profiles at Baile Mhic Ire, other than for the November/December 2006 event, necessitates developing profiles through numerical hydraulic modelling. A numerical model requires the following information:

− A physical survey of the river, its flood plains and structures,

− A numerical (computer) hydraulic model developed from the survey,

− Calibrating information: this information enables estimates to be made of the resistances to flow both within the river and on its floodplains.

This study requires input information capable of producing Flood Profile predictions with sufficient accuracy to give an acceptable indication of conditions.

5.2 SURVEY OF THE SULLANE RIVER, ITS STRUCTURES AND TRIBUTARIES As part of the Lee CFRAM study, a survey was carried out for the main channel, including the upstream face of each bridge and a LiDAR survey covered approximately 250m out from the left and right bank of the main channel was carried out. No survey had been carried out along the tributaries. As part of this study, extra sections were carried out for the main channel and, along with their culverts and bridges, the tributaries were surveyed both within and out-of-bank.

5.3 THE NUMERICAL (COMPUTER) HYDRAULIC MODEL As the Lee CFRAM had used it, the ISIS model has been chosen; this is a widely used and accepted numeric hydraulic modelling package developed by the Halcrow Group Limited. It is considered that this particularly suits the requirements for Baile Mhic Ire.

A model of the river has been developed from approximately 2.8 kilometres upstream of Baile Mhic Ire Bridge to about 4 kilometres downstream of the bridge.

The tributaries were modelled separately from the main channel, but their downstream conditions were fixed to its 66-year flood level at their respective boundaries, i.e. the 66-year main channel flood was taken as coinciding with the 100-year event in the tributary (see Section 4.5.5). The tributaries were dealt with separately from the river model to ensure model stability and avoid failure. The 100-year downstream boundary was also looked at, to provide a sensitivity analysis.

5.4 CALIBRATING THE NUMERICAL MODEL - CHANNEL RESISTANCE VALUES The choice of Manning’s Resistance values was found in two phases. First, Chow (1969) recommended setting a base line value for the type of bed material in the channel and then modifying it for vegetation, cross section size and shape (though this is largely taken account of by the model), along with channel surface irregularity, modification due to obstructions and channel curvature, etc. Second, the calibrated model was confirmed against observed data from the 2006 event to check (and modify, where necessary) these Manning’s values.

This resulted in Manning values within the main channel (in-bank) of between 0.042 and 0.06. Downstream of Baile Bhuirne Bridge, its values are from 0.042 to 0.44, however, as might be expected, it increases to 0.06 along the ‘bend’ in the village. While it may be a slight overestimate (leading to conservative results), a Manning’s of 0.045 has been used upstream of Baile Bhuirne


Bridge to help account for the lack of calibration data in that area. Out of bank values were primarily 0.05 through floodplain areas, but ranged between 0.05 and 0.10 to account for vegetation.

As there was little observed data for the tributaries to help calibrate their models, Manning’s resistance values were defined solely using Chow’s recommendations. This lead to in-bank values (within channel) of between 0.025 and 0.045, while out-of-bank values were primarily 0.05 through floodplain areas, but ranged between 0.05 and 0.10 to account for vegetation. These were informed through site visits and those found when calibrating the main channel model.

5.5 MODELLING THE 2006 CALIBRATION EVENT AND FLOOD CHARACTERISTICS When the hydraulic model was run for the 10-year flood, it was in good agreement with observed flood levels and flow gradient produced by the model for the November 2006 observed profile (see Figure 16). This suggests an approximate 10-year return period for the November event; this also gives confidence in the Manning’s ‘n’ values.

Observations of this event showed the following:

� Floodwaters that exited the gate along the left bank upstream face of Baile Bhuirne Bridge then ran down along the N22. Out-of-bank flooding first began from the bend in the village across the Garage Yard.

� A surface-water drain (located along the N22 upstream of the garage) surcharged prior to this area flooding from the river.

� A number of surface-water drains (for locations, see Section 9) opposite the Co-operative (this is point 6 in Figure 4) released water prior the area being inundated by the river. As a result, it was clear that this area required its own separate hydraulic analysis (Refer to Section 9).

� Floodwaters overtopped the road along the left bank of Baile Mhic Bridge, flooded the pump-house (Point 7 Figure 4) and then flowed to the village towards the Co-operative.

� Floodwaters overtopped the road along the right bank of Baile Mhic Ire Bridge and flowed back into the River a distance downstream of it.

Figure 16: Modelled flood profile versus the 2006 Observed Levels

5.6 FINAL MODELLED FLOOD LEVELS As confirmed from the observed levels for the 2006 event, super-elevation occurs at the river bend alongside the village. Its effects have been estimated using Newton Second Law of Motion.

∆y = CV2W/(r.g) Where, ‘∆y’ is the rise in water surface (m) at the outside relative to the centreline of the channel

‘C’ is the curvature coefficient, for example, 1 for trapezoidal channels under supercritical flows

‘W’ is the top width at the design water surface at channel centreline (m)

‘V’ is the mean channel velocity (m/s)

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‘r’ is the radius of curvature at centreline of channel (m)

‘g’ is the gravitational acceleration (m/s2)

To represent the reality of the situation, for each of the Design Flows, the estimated super-elevation was added (as an allowance) to the modelled levels through the bend reach.

In many scenarios, out-of-bank flows will run across vegetation, around structures, etc. in a direction often quite different to the main channel. This can result in out-of-bank velocities falling to zero. There is a water level increase associated with this loss in velocity. Therefore, velocity energy losses were also added to the modelled water levels to give flooding levels within the town.

5.7 VALIDATION OF THE HYDRAULIC MODEL - THE NOVEMBER 2011 EVENT Minor flooding was recorded on the November 17th 2011. The L3405 road from the village to Baile Mhic Íre Bridge was flooded, but residents reported that, while properties did not flood, some houses came close to it during the event. Though the amount of observed data was less than observed for the 2006 event, it still provided beneficial in validating the calibrated hydraulic model. The Hydrometric Station installed in the area in September 2011 recorded the flood hydrograph and a flow of 110m3/s was the measurement taken by OPW Hydrometric Section during its peak.

When this flow was processed through the calibrated hydraulic model, there was a good match between the levels and gradient of the observed flood profile (see to Figure 17). This suggests an approximate 2 to 5-year return period for the November 2011 event; this also helps validate the model and gives confidence in its Manning’s ‘n’ values.

Figure 17: Modelled flood profile versus the 2011 Observed Levels

5.8 MAIN CHANNEL - PRESENT DAY RETURN PERIOD FLOOD DEPTHS Figure 18 presents estimates of the return period Flood Profiles produced for the full length of the town using the calibrated model. These profiles represent the best estimate possible within the constraints of this study. The restrictions placed on the channel’s flow capacity by existing bridges and weirs are evident. This gives a strong indication that these worsen local flooding.

Properties near the garage also show the worsened flooding caused by super-elevation due to the curvature of the adjacent ‘bend’ in the river. This has been accounted for through an empirical relationship between the river curvature dimensions and flow velocity as discussed above.

Appendix A contains the return period flood depths for each surveyed property.

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Figure 18. Estimates of Return Period Profiles Along the Sullane River.

5.9 FLOW VELOCITIES AND SEDIMENT TRANSPORT DURING FLOODS AND IN GENERAL FLOW CONDITIONS Site inspection confirms that the Sullane erodes, transports and deposits sediment and that these visibly contain significant amounts of course gravels and stones. The estimated present-day in-channel velocity profiles under low flow conditions (i.e. 1/20th of the Median Flow), and under the 100-year are presented in Figure 19.

By definition, deposition largely occurs in areas of lower velocity and erosion under higher velocities. Deposition, with individual large stones of up to 0.15m diameter, is significant both up- and downstream of Baile Mhic Ire Bridge and also downstream of Baile Bhuirne Bridge, as well as around the village bend (between OP9 and OP8, see Figure 36). And, as expected, significant erosion occurs downstream of Baile Bhuirne Town Bridge, on the right-bank downstream of Baile Bhuirne Bridge, along with upstream (OP9) and downstream of the village bend (OP8). The model’s velocities tie-in well with what can be observed along the river.

Figure 19: Velocity profiles - Low flow (Green), Median (Red) and 100 Year (Navy)

In the vicinity of Baile Mhic Ire, in the main, the Sullane has the same relative difference in flow velocities during general conditions as during flooding conditions. As expected within the town stretch, flow velocities are highest downstream of Baile Bhuirne Town Bridge: due to the locally high slope of the channel. After that, due to the channel flattening (reducing slope), velocities get smaller towards Baile Bhuirne Bridge. At the Median Flow, upstream of Baile Bhuirne Bridge, flow remains in-bank and its velocities increase locally due to the restriction of the bridge. Through the bend area of


the town, the reduction in velocities is caused by its bed-level being 1.5m lower than that at the next surveyed section at OP8 (60m downstream). While flow velocities drop off upstream of Baile Mhic Ire Bridge, this is again largely due to the reducing slope of the channel.

The size (grading) of deposited material (and their extent) is a result of river velocities. This is also the historic condition, and given no information to the contrary, it is stable in so far as a location will continue eroding or depositing. Should a significant change occur to the size of flow velocities as a result of alterations to the river, this would alter that present-day pattern and that could possibly lead to significant movement of large volumes of river material. It is therefore necessary to take flow velocities into account when considering possible flood relief options for Baile Mhic Ire. Failure to do so could, for example, lead to the continued deposition of gravels and stones upstream of Baile Mhic Ire Bridge, thereby reducing its conveyance efficiency, and generating a rise in upstream flood levels. This scenario of locally elevated levels, therefore, could compromise the scheme and cause flooding from an event smaller than the design standard of the scheme, i.e. failure of the scheme.

In addition, a flood relief scheme that uses defence walls and embankments, etc. to contain the entire flood within a river corridor may lead to an increase in flow velocities in the river along the protected area, and this could destabilise the present sediment condition. If this were to show up in the hydraulic modelling, it would therefore be necessary to carryout sufficient widening and/or deepening in the higher risk areas to ensure, as close as possible, the present day velocity condition.

The Safety, Health and Welfare at Work (Construction) Regulations specifies water as a Hazardous Substance; the possible risks associated with an increase in flow velocities, etc. must therefore be assessed and all reasonable effort made to keep them to a minimum.

5.10 THE 100-YEAR FLOOD ENVELOPE The 100-Year flood envelope has been produced for the study area, this is presented in Figure 20.

Figure 20: 100-Year Flood Map for Baile Mhic Ire (not showing overland flow)


5.11 EXPECTED CLIMATE CHANGE CONDITION

5.11.1 Accounting for Climate Change The Flood Policy Review Report produced by OPW states that, should increased peak flow due to Climate Change actually occur, where possible, a scheme should be capable of easy and cost efficient adaptation to reinstate its stated protection standard. This means that the design flood is based on historic records of flow (such as exists at Macroom) but Climate Change flood flows must also be considered when investigating potential flood alleviation schemes.

5.11.2 The 2100 Climate Change Scenario OPW policy allows for a standard flow increase for a Mid Range Future Scenario (MRFS) of 20% and, as an upper consideration, a High Range Future Scenario (HRFS) of 30%. The MRFS increase of 20% to fluvial flow by 2100 is based on Sweeney and Fealy (2006) [winter precipitation estimated to increase 17% by 2080]. This is supported by the Defra FCDPAG3 (2006) guidance policy, where 20% is used as a sensitivity range to be adopted for peak river flow. As such, the MRFS increase of 20% is used for sensitivity purposes in this study (see Table 10). The table also contains the return periods that these peak flows have under present-day conditions, i.e. if the expected change due to Global Warming does not take place. In this Climate Change scenario, the 5-Year flood is about the present-day 10-Year flood or, when interpreted in reverse, the present-day 10-Year flood will become the 5-Year flood. Similarly, the present-day 50-Year flood will be exceeded as frequently as once in 20 years.

Climate Change Return Period

Baile Mhic Ire Flow (m3/s)

Present-day Return Period

2 105.4 3.5

5 147.7 10

10 177.2 23

20 206.6 50

30 224.1 85

50 246.7 153

100 278.4 343

200 311.7 702

Table 10. Return Period Floods under the Expected Climate Change MRF Scenario

5.11.3 Climate Change Return Period Profiles and Velocity Changes Compared to the 100-Year present-day flood profile, the Climate Change scenario increases flood levels by 0.3m upstream of Baile Bhuirne Town Bridge, 0.2m at the middle one and 0.12m at the third (Baile Mhic Ire Bridge). In the vicinity of the Baile Mhic Ire Village, the level is estimated to rise by between 0.2 and 0.4m.

Under Climate Change conditions, even though a larger percentage of flow is expected to flood out there will still be more flow in the river than at present; that is why channel flow velocities will still be generally higher; the maximum increase is expected to be between 0.2 and 0.4 m/s.


6. FLOOD DAMAGE AND SCHEME BENEFIT ESTIMATES

6.1 INTRODUCTION The economic flood damages for the scheme have been calculated in the form of the Annual Average Damage, based on a range of probabilities and a resulting expected Net Present Value (NBV) of damages.

A survey was conducted of the properties within the flood risk area, in order to record floor levels and areas, along with building usage, etc. Flood damages have been estimated using the methodologies outlined and values presented in ‘The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment Techniques -2010), which is often referred to as the ‘Multi-Coloured Manual’ (MCM). This chapter presents the Damage Assessment for the town.

The underlying philosophy is:

a) To calculate the magnitude of damage expected should no improvements be undertaken.

b) To calculated the damages which would remain after the proposed scheme is completed and to therefore ascertain the benefits to be gained (in this case damage avoided), to convert the benefits over the design life of the scheme to Net Present Value (NPV) and

c) To compare these with the present value of construction costs and therefore derive benefit/cost ratios.

6.2 BENEFITS OF A FLOOD RELIEF SCHEME Tangible benefits are those to which it is possible to assign monetary values. In general, benefit is assigned a valuation equivalent to the monetary loss that would occur if the Flood Alleviation Scheme were not in place, with appropriate allowances made for discount rate and project horizon. Tangible benefits include:

- Direct Damage to buildings and contents averted

- Indirect Property, community and business loss avoided

- Disruption of road traffic avoided

Intangible benefits are those to which it is not possible to assign a monetary value from recognised economic principles. Monetary values placed on these benefits are, therefore, subjective. Intangible benefits include:

- Avoidance of anxiety, inconvenience and ill health

- Avoidance of the inconvenience of post flood Recovery.

For this benefit appraisal the range of benefits comprise of the following:

- Tangible Benefit – Residential Properties

- Tangible Benefit – Non-Residential Properties

- Indirect Costs – Residential (Costs incurred in the aftermath of a flood)

- Traffic Disruption

- Emergency services costs

- Intangible Benefits

6.3 DISCOUNT RATE AND PROJECT HORIZON Given a choice between receiving a specific sum now and the same amount sometime later, most people will express preference for the present sum. The tangible benefits accruing from a flood alleviation scheme will not provide cash sums to the beneficiaries; however, they will prevent a negative cash flow (avoidance associated flooding costs) from the individuals.


The avoidance of fixed negative cash flow now is also preferable to avoidance sometime in the future. The “social time preference” (STP) can be measured by an appropriate Discount Rate (STPDR) and is taken as the compound rate of interest ‘r’ (% per annum) by which ‘y’ Euros in ‘x’ years time is equal to one euro now.

The benefits arising from a flood relief scheme commence on the completion of the scheme and exist for the life of the works put in place to secure the relief. To obtain a measure of the overall benefit in present day monetary values, it is necessary to:

(a) Estimate the benefit arising each year of the project life,

(b) Discount them to present values using the appropriate discount rate.

(c) Total the present values to obtain the overall benefit.

The Department of Finance test discount rate for public investment is 4%. The life of the works over the benefits are discounted is taken as 50 years. For computation purposes, it is assumed that the residual value of the works at the end of the period is nil. This may be regarded as somewhat conservative since the works at the end of this scheme typically have a design life of 100 years. However, it is in accordance with standard methodology for economic appraisal of flood relief schemes developed in other countries

6.4 FLOOD DAMAGE DATA A considerable amount of research has been undertaken by the Middlesex Flood Hazard Research Centre (FHRC) on the costs of flood damage in urban areas in the U.K.

The land use in a flood prone area often referred to as the Benefit Area (see Figure 20), profoundly influences the likely damage characteristics and costs. Houses are affected differently from offices and shops, which in turn, suffer different kinds and costs of damage from those experienced in industrial premises. Various land use sectors have been chosen to assess the impact of different depths of flooding on each. Flood damage data for the residential, retail, distribution, office and manufacturing sectors are provided in the Multi-Coloured Handbook 2010 (CD accompanying handbook). Detailed descriptions of these data sets are provided in Chapters 4 and 5 of the Manual. Additional costs to emergency services in dealing with flooding are also given in Chapter 6. All cost data in the Multi-Coloured Handbook 2010 is in Sterling at 2010 values.

Figure 20: Benefit Area – 100-Year Flood Envelope


In the FHRC 2010 publication, for a particular property, the damage due to flooding is a function of the flooding depth and also of the flood duration. The flood depths considered in the residential dwellings sector range from -0.3m to + 3.0m in relation to the ground floor of the buildings. Information is tabulated for flood durations less than and greater than 12 hours.

The Multi-Coloured Manual gives detailed data for five house types, seven building ages and four different social classes of the dwellings’ occupants. The flood damage/depth data sheets contained on a CD that accompanies the report is further broken down into the cost of damage occurring at each flood depth, to plasterwork, floors, joinery, decorations, plumbing, etc. Thus, one can go into the calculations of flood damage in an area in very great detail.

The Multi-Coloured CD 2010 provides a set of databases for retail, commercial and industrial flood damage. The depth/damage data sets were derived by the FHRC based on data collections and discussions with representatives from a range of non-residential properties.

6.5 EURO/STERLING CONVERSION RATE Flood damage data in the FHRC publication applies to U.K. properties and the value is given in 2010 Pounds Sterling. It is both reasonable and acceptable to assume that flood damage costs in Ireland are proportional to those in the U.K.

For the purpose of flood relief schemes, Purchasing Power Parity (PPP) rates are used as the conversion mechanism from Sterling to Euro. PPP rates are currency conversion rates that both convert to a common currency and equalise the purchasing power of different currencies. In other words, they eliminate the differences in price levels between countries in the process of conversion. PPP rates are calculated on an annual basis by Eurostat through the European Comparison Programme, which involves National Statistical Institutes in the collection of data for the calculations. On a wider basis the OECD undertakes the International Comparison Programme involving the OECD member states. Once all data has been submitted Eurostat can perform its calculations and produce the parities for the given year.

The 2010 PPP value is the most recent, this uses a factor of 1.30374 to convert from Sterling to 2010 Euro. The 2010 Euro now needs to be brought to 2012 figures using the CPI index; this figure is 1.027393. The 2010 PPP rate now needs to be combined with the 2010-2012 CPI, this gives a conversion factor of 1.33945.

Some figures used in the MCM are 2005 Pounds Sterling. In the same manner, 2005 PPP uses a factor of 1.5716 to convert from Sterling to Euro. This 2005 Euro is then brought to 2012 prices by using a CPI of 1.0977 to give a combined conversion factor of 1.7532.

6.6 TANGIBLE BENEFITS – DENEFIT CALCULATION

6.6.1 Property Categorisation Assumptions A list of properties affected by flooding was compiled. The Medium Flood Risk Future Scenario (1 in 1000 year flood event) output of the Lee CFRAM (see Map1 in Appendix A.1) was used to select the properties likely to be affected and the expected benefit area. Each Building was assigned a reference number, as indicated in Appendix A.2.

Prior to a site visit, the properties were geographically linked to An Post Geo-database. Where multiple An Post points existed within the same building polygon it was assumed the building footprint was divided equally between points. The An Post directory assigns one of four codes to each of the property points to indicate the property type. These are R- residential, C-commercial, B-Both and U-Unknown. Cork County Council conducted a survey of properties within the flood risk area, in order to record floor areas and floor levels.

Residential Properties were categorised using the full suite of options available in the MCM data sets (Type, Age and Income Group). A site visit, along with Google Street View, helped to apply the appropriate property classification and correct residential category. The Income Group data


set is subjective and was based on a visual inspection of the properties. The age of the property was confirmed using OS Maps produced at different points of time.

The appropriate MCM code was applied by completing a visual inspection of the Non Residential properties. The business function of each unit in the two industrial estates was individually assessed in order to ensure the correct MCM code. Once a code had been applied the susceptibility level was also determined. For example, a charity shop, local grocery shop and electronic store would all be assigned MCM code 211-High Street Shop, however, they would have a susceptibility level of low, indicative and high respectability based on the damage each could be assumed to incur should they become inundated by flood waters.

Where residential and commercial property types are within the same building, these were reviewed; these generally consisted of commercial at ground floor level and residential above. Damage to such residential properties was removed unless flooding depths reached ceiling height of the ground floor property. Appendix A.3 provides details of property classifications.

6.6.2 Design Floods and Water Levels Extreme flooding in Baile Mhic Íre is fluvial in origin. The Sullane catchment is mountainous and the nature of its flood is therefore fast and flashy. The flood recedes quickly and for purposes of damage assessment can be considered to be less than 12 hours in duration. Damage to property was assessed from the relevant building-type flood-damage data (in the MCM manual) for flood durations less than 12 hours.

Design floods and water levels along the Sullane, and its tributaries that enter along the town, were determined using an ISIS 1-D model. The final modelled flood levels combine the effects of increased flood flow levels due to a loss of its velocity energy gradient and the effects of super-elevated flows through the bend in the village (see Section 5.5). A good understanding of the flood mechanisms is needed to apply the correct chainage. The presence and condition of defence assets must be taken under consideration to ensure that benefit is not overstated.

These effects were respectively applied throughout the town except for the Grotto Channel up to 125m upstream of its intersection with the N22 where flood waters on exiting the channel are considered to freely flow overland and, as a result, do not loose their velocity energy gradient. Here the water surface level alone was taken as the flooding level.

It is these levels that were used in the analysis of Cost Benefit.

6.6.3 Flood Depth Calculation and Duration The depth of flooding in a property from a particular return period is calculated as follows:

a) Each property is assigned a chainage point along the River from which it is deemed to flood from. Interpolation between cross sections for some properties was necessary to find the appropriate chainage water levels.

b) For each design flood event, the predicted level at the chainage was obtained.

c) The threshold level of each property was subtracted from the water level to give a level of flooding at the property for each flood event.

6.6.4 Property Capping Assumptions Average residential property values were obtained from the Quarterly House Prices Bulletin produced by the Department of Environment, Heritage and Local Government. For 2010, this gave second-hand house prices in the Cork area of €248,304, these were chosen to correspond to 2010 FHRC prices.

Average commercial property values have proven to be difficult to pinpoint. The high level approach outlined within the MCM is to estimate values as a factor of 10 greater than the rateable value of the property. Rateable incomes are available from Cork County Council. However the Ireland Valuation Office is currently going through a revaluation process owing to the poor


correlation between rental value of properties and the rateable income due to the significant changes in property prices in recent times. Given the limited data available, the sensitivity of the capping valuation of commercial property damages has been reviewed against a number of scenarios. These are a factor of 10 greater than rental rates of €100, €150, €200 and €250 per m2 per year. Calculated damage values for commercial properties are lower than even the lowest (€100) of these scenarios and so capping was not necessary.

6.6.5 Indirect Damages Indirect damages include property rental and additional electricity and heating costs associated with clean-up operations. Property rental values have been estimated based on current rental prices for the Baile Mhic Íre are and are assumed to be €175 per week. Electricity and additional heating costs are available in MCM (2005) and have been adjusted using CPI indices. It is noted that clean costs and dehumidifier rental costs have already been incorporated into domestic damage figures, so are not included here to avoid double counting.

6.6.6 Traffic Disruption Two traffic routes were identified as being affected during significant flooding. These are:

- The main Cork to Killarney road (N22) through the town. Historical data and hydraulic analysis confirm it becomes inundated and impassable. This national primary route has large daily traffic volumes, so disruption is significant; as road users are forced to detour.

- The road L3405 road, which connects the two subject towns and continues on to Cahercarney, has in the past flooded and the hydraulic model predicts that this will continue to happen. This results in traffic needing to enter the town or pass over the Sullane having to divert around the flooded route.

No allowance has been made at this time for disruption to traffic. While this should have a significant contribution, particularly due to the disruption to the N22, the damage estimated without its inclusion leads to robust benefit to cost ratios.

6.6.7 Potential Losses in Emergency Services Sector The MCM recommends that the total property damage calculated in project appraisals of flood alleviation schemes should be multiplied by a factor ranging between 1.107 and 1.056 to allow for emergency and recovery costs that can be justified as real economic costs, not counted elsewhere in the benefit assessments. This figure should be applied for flood at all annual probabilities and for all scales of flood alleviation scheme, in the absence of better information. It is recommended that the lower factor should be applied in urban areas to reflect economy of scale in emergency services. Therefore, a factor of 1.056 was applied at all probabilities to account for emergency costs.

6.6.10 Total Tangible Damages The tangible benefit in each of the sectors described above was estimated for each of the design return periods. The calculations required were carried out using an Excel based program, utilising the MCM data. Table 11 summarises the tangible benefits for each design flood. Further details of the calculation of tangible benefits are detailed in Appendix A.5.

The total tangible benefit for each return period is plotted in Figure 21 against the exceedance probability: P (= 1/T), where T is return period on years. The annual average tangible benefit from a flood relief scheme providing protection against a flood of return period T is the area under the curve to the right of 1/T in Figure 21.

The value for a scheme providing protection from the 100-year flood (P = 0.01) is €577,514.


Tangible Benefits Direct Indirect 5.6% Return

Period (years)

Non - Residential Residential

Total Property

Rental Costs

Clean Up Costs

Emergency Costs

Total Tangible Benefit

1 €0 €0 €0 €0 €0 €0 €0 2 €18,149 €38,600 €56,749 €2,464 €1,359 €3,178 €63,751 5 €25,587 €225,227 €250,814 €17,248 €10,578 €14,046 €292,686

10 €558,784 €604,571 €1,163,355 €36,960 €27,822 €65,148 €1,293,286 20 €2,647,266 €1,521,714 €4,168,980 €64,064 €51,269 €233,463 €4,517,776 30 €3,451,960 €2,354,312 €5,806,272 €64,064 €62,946 €325,151 €6,258,433 50 €4,651,563 €3,117,940 €7,769,503 €76,384 €69,743 €435,092 €8,350,723

100 €5,982,813 €4,164,962 €10,147,775 €108,416 €92,723 €568,275 €10,917,190 200 €7,303,527 €5,340,411 €12,643,937 €145,376 €126,915 €708,060 €13,351,998 500 €9,052,059 €6,633,863 €15,685,922 €170,016 €156,433 €878,412 €16,564,334

1000 €10,420,073 €7,453,840 €17,873,913 €194,656 €180,643 €1,000,939 €18,874,852

Table 11: Summary of Tangible Benefits for Stated Return Periods

Damage Versus Probability of Occurrence

€0

€2,000,000

€4,000,000

€6,000,000

€8,000,000

€10,000,000

€12,000,000

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Probability of Occurrence 1/T

Da

ma

ge

(€

20

12

pri

ce

s)

Total Damage (Excluding Disruption to Traffic)

Figure 21: Annual Average Tangible Flood Losses

6.6.11 Intangible Benefits – Benefit Calculation Intangible damages have been developed using the methodology outlined within in the MCM, in particular Table 4.19. Recent research into the valuation of intangible health benefits concludes that the potential value of avoiding such impacts is, on average £200 per household per year. In addition, research concluded that the most important factor when calculating potential intangible impacts is the flood risk. As a result it is necessary to consider how the level of exposure to household flood risk varies with, and without, the proposed scheme. Once the standard of protection before and after the proposed scheme intervention has been established, it is then required to apply the risk redistribution matrix to establish the values of the potential intangible benefits (Table 4.19). It should be noted that these figures are not subjected to a distributional impact analysis because the values are already account for such impacts. Details of Intangible Benefits are shown in Table 12.

Table 12: Intangible Benefits Calculation

From FHRC Table 4.19 Intangible Health Benefits Return Period (Years) 1 2 5 10 20 30 50 100

Number of Properties 0 1 6 9 17 9 22 22 Value / Event / Property £200 £199.5 £197.5 £195.0 £188.0 £175.0 £127.0 £0.0 Total Value per Event £0.0 £200 £1,185 £1,755 £3,196 £1,575 £2,794 £0.0

£10,705 Sterling 2005 €18,767 Euro 2012


6.7 BENEFIT/DAMAGE ASSESSMENT RATIO The Average Annual Damage is estimated using the Total Tangible Damage for each of the standard return period flood events up to the 100-year combined with its probability of occurring in one year. This alone generates a figure of €577,514 per annum. Intangible Damages are estimated based on the difference between the present-day flood risk and the design standard of the scheme; this adds €18,767 to give a total of €596,281 per annum.

The Average Annual Damage, discounted at a rate of 4% per annum, is then calculated over a time-horizon of 50 years to produce a Net Present Value of the potential flood damage. This is estimated as €12.97M (see Table 13).

Table 13 – Estimated Scheme Benefit Up to 100-Year Event Table 14: Estimated Flood Damage up to the 1000-Year Event

6.7.1 Natural Failure Scenario

Schemes provide the benefit of preventing damages that would otherwise occur up to and including their design standard. Along with this, some schemes have added benefits, such as enhanced environmental or amenity value or lower water levels during ‘Natural Failure’ (i.e. during floods greater than the design standard) than those that would occur if a scheme were not in place (thereby reducing damages). To assess this effect, damages were calculated using floods up to the 1000-Year event. The estimated Net Present Value of potential damage using floods between the 1-year and 1000-Year is €15.69M under present day conditions (see Table 14).

This is a €2.72M greater than the potential damage using floods between the 1-year and 100-year (the design standard of the scheme) events.

Once the preferred scheme has been determined this €2.72M difference could be compared with damages generated up to the 1000 year event with the scheme in place (i.e. Damages between the 100-year and 1000-year as there will be no damage up to the 100-year, the protection standard of the scheme) to determine the additional benefit attributed to the scheme under ‘Natural Failure’ circumstances. The inclusion of benefit due to ‘Natural Failure’ will further enhance the Cost-Benefit Ratio.


Figure 23 – Alignment of Walls and Embankments Along the Sullane River

Red: Walls Green: New Embankments

Maroon: Upgrade existing Embankments Light Blue: Underpinning and Re-grading


7. ENGINEERING MEASURES FOR FLOOD ALLEVIATION

7.1 Design Flood Standard for Baile Mhic Ire The 100-Year flood is being taken as the Design Flood for the purposes of this study.

7.2 Engineering Measures for Flood Alleviation It is necessary to find solutions that provide protection against flooding for flows up to the 100-Year Design Standard. One engineering measure over the full area, or a combination of locally effective engineering measures can provide the basis of a flood relief scheme.

The range of engineering measures typically considered in viable options for possible flood alleviation schemes in an Engineering Study include, but are not limited to, the following: a) Do Nothing (i.e. implement no new flood alleviation measures) b) Do Minimal (local trimming of vegetation along the riverbank, etc.) c) Non-Structural Measures

i. Installation of a Flood Forecasting System and Flood Warning Service ii. Individual property protection

d) Relocation of Properties and/or infrastructure e) Reconstruction of Properties and/or infrastructure to a higher level f) Flow Reduction

i. Upstream catchment management (i.e. reduce runoff) ii. Upstream flood storage (single site or multiple sites)

g) Flood Containment through Construction of Flood Defences. There are two types: i. Permanent Walls or embankments ii. Demountable Walls

h) Increase Conveyance (upstream, through and / or downstream of the area) i. Remove/ reduce constraints, e.g. bridges, bends, throttles, material on a floodplain. ii. Reduce the roughness of the channel / floodplain (remove vegetation, lining, etc.) iii. Specify ongoing channel / floodplain maintenance iv. Change channel shape (dredge to re-grade – excavate to widen the channel) v. Change the floodplain section and / or grade by excavation

i) Flow Diversion. There are two types: i. Diversion of the entire river ii. Flood flow bypass channel

j) Sediment Deposition and Possible Sediment Traps k) Tidal Barrage l) Pump storm waters from behind flood defences m) Measures Specific to the Study Location

7.3 Do Nothing ‘Do Nothing’ is the decision not to implement a flood relief scheme, i.e. leave the situation as it currently stands. This may be the preferred outcome and, therefore, this is Scheme A. This could occur due to unacceptable environmental effects or if the cost of alleviation is prohibitive or would result in a poor overall Benefit-Cost Ratio, i.e. no cost-effective solution is found.

The cost of Doing Nothing is the net present value of the flood damages expected over a 50-years time-horizon, if no scheme were to be implemented. Up to the 100-year event, 72 residential and 18 commercial properties are affected. This flooding damage has been estimated at €12.97M.

7.4 Minimal Measures Minimal measures could include annual local dredging of the Sullane River and local trimming of vegetation along the riverbank. Due to the severity of flooding, the reduction in risk from this measure would be negligible. Overall, this option is not considered acceptable on its own, as residual risk would remain almost unaltered and therefore very high.


7.5 Freeboard - The Safety Factor for Defence Walls and Embankments The safety factor for flood defence works is a freeboard added to the estimated height of the floodwaters. In general, for freshwater flooding, 0.3m is added to hard defences (such as retaining walls) and a minimum of 0.5m to soft defences (such as embankments). To counter uncertainty in hydrology of the Sullane River, these values are increased by 0.2m to 0.5 and 0.7m, respectively.

The larger freeboard for soft defences is due to increased risks associated with overtopping, breaching and rapid flooding of the protected area. Constructed embankments are likely to be even higher to account for possible settlement; this is dependant on the materials used and is not part of the freeboard.

7.6 Types of Flood Wall Defences There are two types of containment walls: � Permanent Walls. All permanent walls must be brought to the acceptable Minimum Safe

Height of 1.15m (Building Regulations 1991, Technical Guidance Document K, Section 2). This is also, in general, an appropriate and acceptable height from an environmental viewpoint. Where no visual or other adverse effect would result, or where there is Local Authority agreement, higher wall heights may be employed. Localised raising of road or ground levels can also form part of this measure. To improve the appearance, where appropriate, all structural defence walls, etc. should be provided with stone facing and stone capping (possibly pre-cast). Where permanent walls are employed with a height of 1.2m, and this incorporates a freeboard of 0.3m, floodwaters may rise 0.9m above adjacent ground and road levels.

� Demountable Walls. These are non-permanent walls that may be several metres in height and are capable of being erected within a short period of time. The requisite machinery needs sufficient space to both access and construct these temporary walls and a Flood Forecasting System is required to give an advance warning to mobilise response units and erect these walls (see Section 7.8). A Local Authority and Emergency Services need, at least, a six-hour forecast.

7.7 Technical Decision Criteria to Select Suitable Engineering Measures The ability of an ‘Engineering Measure’ to solve, or aid the solution of, the flooding problem is examined, within the scope determined in the Environmental Consultant’s Constraints Report, to define its most efficient, acceptable hydraulic form, i.e. its basic solution.

Particular technical criteria and appropriate policies that are considered include: � ‘Natural Failure’, this is where a flood greater than the design standard overtops the defences. A

measure that leads to an increase in flood levels worsens the damage caused by the event relative to the ‘Present Condition’ while, conversely, a measure that decreases levels reduces damages.

� Flow velocity. Flow velocity changes produced by a measure impact Health and Safety (through possible increased risk) and sediment erosion, transport and deposition.

� Climate Change. What possible future changes are required to the measure should the present expectation of Climate Change occur? The Flood Policy Review Report states that design floods should be based on historic records of flow, but it is also necessary to consider Climate Change (Global Warming) flood flows throughout the investigation. A flood alleviation scheme must be capable of easy and cost efficient adaptation to reinstate its stated protection standard. Such changes would be carried out as a separate scheme at that time.

7.8 A Flood Forecasting System and Flood Warning Service A Flood Forecasting System and Flood Warning Service have two possible functions:

�� In the absence of a flood alleviation scheme, flood warning reduces the amount of flood damage.

�� Construction of Demountable Walls is dependent on having a reliable flood warning. This warning is not issued in relation to the peak of a flood; rather it is issued in relation to the onset of flooding of the permanent wall foundations on which the walls are to be built. And, in addition, staffing levels, and other resources, must be capable of transporting and erecting the demountable walls within this forecast constraint. As such, warning time is the forcing parameter for the construction of those defences; a large number of units puts considerable strain on limited resource.


To Baile Mhic Ire the Sullane can produce a peak in less than three hours due to the fast flashy nature of its catchment, however, as may be expected, flooding starts ahead of the peak, so that, in fact, there is only about one and a half hours warning time. The available time makes it impossible to provide such a service.

7.9 Individual Property Protection The protection of properties on an individual basis by erecting barriers at doors, windows and air-vents, etc. can be a viable option for reducing flood damages where flood levels rise slowly and reach levels not significantly greater than one metre above floor levels.

Sullane flood levels, however, rise quickly, and in extreme events, up to 2m above low-lying floor levels. Retaining these water heights by domestic walls may not be feasible on structural grounds and practicality of application is dependant on underlying geology, and on foul-pipe systems, etc.

7.10 Relocation of Affected Residents or Business Relocation of residents does reduce flood damages. It is generally only viable in rural areas with a low-density of population or urban areas with extreme flood risk and/or no technically, economically or environmentally viable engineering solution. This option, however, incurs additional social costs due to disruption to the community; and an associated increase in intangible costs.

The relocation of those in particularly low-lying areas could form a part of the overall solution for Baile Mhic Ire.

7.11 Reconstruction of Low Lying Properties at a Higher Level It may be more cost effective to demolish and reconstruct some properties to a higher level in the same location than defend them as presently constructed. This avoids the additional social impacts and increased intangible costs often associated with relocation. This study, though, shows that it is possible to provide a flood attenuation scheme for Baile Mhic Ire without the need to reconstruct residences.

7.12 Upstream Storage of the Flood Peak Upstream storage solutions deliberately flood a designated area of land to reduce downstream river flow to a maximum safe value. Once the storage area is full, however, no additional benefit results, so in the Natural Failure condition, i.e. when a flood greater than the 100-Year Design Flood occurs, substantial flooding may result in the town as the peak river flows are no longer reduced.

During large floods, a river’s floodplains are (by definition) flooded, so the search for an upstream storage area involves locating potential new storage areas or places where it is possible to increase the depth of flooding without incurring too much environmental or structural costs. Areas already heavily inundated are generally not suitable as choosing these could constitute a significant health and safety hazard (in the instance of a breach or collapse of the impoundment structure, i.e. a ‘dam-break’).

Prolonged storage of waters on agricultural land may result in another problem. The Dead River (Mulkear Catchment, County Limerick) is very flat and prone to flooding, and so an embankment scheme was built to give a degree of flood relief to its lowlands. In August 1997, a flood occurred which overtopped the embankments and the river took about two weeks to recede. By then, the stored waters had become eutrophied and a devastating fish kill occurred; many adult salmon were killed but juvenile stocks were decimated. The full effects will not be known for several decades.

Storage solutions fall into two broad categories:

� In-line storage solutions use an impounding embankment (dam) constructed across the floodplain and a control structure within the river. These allow the river flood the storage area as the flood level increases, i.e. the area is not exclusively reserved for peak flows.

� Off-line storage solutions use embankments and high ground to surround a storage area and, by employing a control structure within the river, only allow entry to higher flood flows. By excluding flooding during minor events, it trades off the increased size of their downstream floods in exchange for significantly less impoundment than In-line storage solutions.


Off-line storage makes more effective use of an area than an In-line solution, but its structures and significant lengths of embankments running along the riverbank have higher economic and environmental costs. An In-line solution for flooding from the Sullane would require the creation of a dam across the plain upstream of Baile Mhic Ire Village, is required to store the 100-year floodwaters that are in excess of the Safe Flow (the 2-year flood) , i.e. a volume of 729,797m3. A site visit and examination of the OS maps confirms that the topography of the area does not lend itself to providing suitable storage for such a volume of water. In addition, while unlikely, this stored water presents a considerable residual risk in the event of a dam-break and major Environmental issues are likely.

As such, these options are not considered acceptable due to the storage required and residual risks.

7.13 Flow Diversion Measures Diverting floodwaters away from an affected area can mitigate a flooding problem. This can be achieved by excavating a new channel as either a re-alignment of the existing river (a full river diversion), or as an additional relief channel designed only to carry excess flood flows.

In the case of Baile Mhic Ire, these solutions must deal with either the full 100-Year flow of 232 m3/s, or the difference of about 145 m3/s between the 100-Year and the 2-Year peak flow of 87 m3/s (the Maximum Safe Flow) that will pass without flooding the town. Rehabilitation, i.e. environmental integration and aesthetic works, would form part of such a solution.

A site visit and examination of OS maps confirms that the topography of the area does not lend itself to providing an alternative route for floodwater without the requirement for very significant pumping or deep-channel, rock cuts.

This combination of conditions, on physical impact and economic grounds, precludes the construction of any form of diversion option that could allow flow bypass the town.

7.14 FLOOD CONTAINMENT (Floodwalls and Embankments) Containing floodwaters within designated floodable areas through the use of floodwalls and embankments is a commonly considered option in flood relief.

As discussed in Section 7.8, it is not feasible to use Demountable Walls. In general, a minimum forecast of six-hours is needed for resources to transport and construct the walls. Based on real time data, a warning of about 1.5 hours is possible. This is clearly not enough.

Figure 23 shows embankments and walls from op_20 to op_19 protecting properties on the left bank upstream of Baile Bhuirne Town Bridge. From op_16 to Baile Bhuirne Bridge embankments prevent flooding of the N22. Downstream of the bridge, from op_11 to op_4, a mix of walls and embankment protect Baile Mhic Ire. And, downstream of Baile Mhic Ire Bridge, embankments on the right bank from op_3 to op_2 and on the left bank from op_23 to op_24 protect the remaining properties in the study area. Road raising (from op_22 to op_23) closes out protection for the study area.

This has been modelled and its resulting flood profile compared against the present-day. Changes to flood heights (additional afflux of more than 0.5m at each of the bridges) and changes to flow velocities mean that containment alone is not an acceptable solution.

However, for example, Section 7.15.1 shows that bridge underpinning helps reduces the afflux head at each bridge. In combination, they can protect the area and achieve other flood risk management objectives to the design standard. This is, therefore, being carried forward for consideration as a potential solution.

7.15 INCREASE CONVEYANCE An increase in conveyance results from removal of local constraints to flood flows and also from deepening or widening the existing channel. All have the ability to reduce the surface level of a 100-year flood event and, as a result, bring down the physical size and costs of flood defence measures.


Local obstructions (such as bridges, natural rock weirs or restricted sections) can constrict flow in the river or on the floodplain, thereby increasing upstream flooding. Removal of, or alterations to, such obstructions often provides a partial (or complete) cost effective solution.

Several of these are technically viable and will be carried forward to form part of full scheme options. Of necessity, assessment begins with Flood Containment measures in place (see Section 7.14).

Each individual measure is assessed using a numeric hydraulic model that may (or may not) maintain the preceding measures; in such cases, the potential benefits are taken as the added benefits compared to the unaltered ‘present day’. The combined effects, including those on velocities, are then evaluated and, if acceptable, the combination may be brought forward as a possible Flood Alleviation Scheme.

7.15.1 Bridge Underpinning Section 7.14 modelled the impacts of containing flood flows by comparing the flood profile and flow velocity conditions against the present-day. Increased afflux of more than 0.5m at each of the bridges and changed velocities mean that it is not acceptable as a solution in its own right.

This section examines underpinning the bridges to counter these effects. The underpin-depth for each bridge had to reverse the increase in flood height caused by containment. From upstream, these are 1.2m, 1m and 1.2m, respectively. Dredge lengths tie into the existing bed level 30m away from the nearest bridge face. Figure 24 presents the resulting defence heights.

7.15.2 Channel Widening Upstream of Baile Bhuirne Bridge Directly upstream of Baile Bhuirne Bridge, during flows in the range of the median event, high velocities are causing excessive bank erosion and the transport of sediment further downstream. Channel widening can counter this.

Hydraulic modelling showed that, directly upstream of the bridge, widened by 7m is needed for 115m along the right bank and 100m along the left (see Figure 25). This dredging is to the existing channel bed level and has side slopes of 2:1. Defence heights are not altered by this measure, i.e. the same as ‘Flood Containment and Bridge Underpinning’.

Figure 24 - Heights of Flood Defences

Figure 25


Maroon: Upgrade existing Embankments Light Blue: Underpinning and Re-grading


7.15.3 Straightening the Existing River Bend Just Upstream of Baile Mhic Ire Bridge Straightening the existing bend upstream of Baile Mhic Ire Bridge can help prevent the build up of river sediment that is currently taking place along the right-bank both upstream and downstream of its location (see Figure 26) and it could also aid the distribution of materials further downstream. This would provide less resistance to flow through this area by reducing the effect of the present-day bend on velocities, as its realigning of flows is the key to the effective movement of the materials.

This engineering measure can be incorporated into any design option.

7.15.4 Removal of Weirs This involves removal of the weirs at Baile Bhuirne and Baile Mhic Ire Bridge. This measure’s value lies in its ability to aid an overall solution comprising several measures.

7.15.5 Channel Re-Grading Through Dredging A full dredging solution, i.e. that removes the need for walls and embankments, is not viable. It is in the region of 1.5 to 2 times the costs of other effective options.

The general, existing bed-slope from Baile Bhuirne to Baile Mhic Ire is 1:240. Re-grading the riverbed through excavating from 0.5km upstream of Baile Bhuirne Bridge to 1.7km downstream will improve flow conveyance.

A second conveyance option is also possible. This starts with the existing riverbed levels 240m upstream of Baile Mhic Ire and finishes 280m downstream of it.

7.16 THE VILLAGE BEND AREA The significant bend situated along the middle of Baile Mhic Ire (‘village bend’) creates a complex hydraulic issue called super-elevation, where river level rises higher along the right bank than on the other. The maximum effect is estimated at 0.22m. Hydraulic complexities and uncertainty in estimation due to insufficient data, etc., (and 1-D model limitations) mean that a conservative approach must be taken for flood levels affected by this bend. Super elevation is calculated as starting 30m upstream of op1_9, and, while the bend remains, its estimated 0.22m is added to the freeboard. Three alternative measures have been examined.

7.16.1 Rock Armouring Rock armouring could be placed along both banks of the bend to reduce super-elevation effects, beginning near op1_9 and running 180m downstream to beyond op_8 (see Figure 27). Up to and including the design flow, this would remove water energy and, so, reduce velocities. The new energy profile would not cause super-elevation but upstream flood levels would increase.

7.16.2 Flood Flow Bypass Channel Beginning at op1_9 and rejoining the main channel at op_8, a 25m wide bypass channel (similar to the river), with a maximum bed level of 2.9m and an average depth of 2.2m, can be cut to take flood flows (see Figure 28). The right bank is about 145m long and the left 70m. This bypass channel will reduce the flow’s centripetal force. However, it cannot entirely remove this super-elevated effect, as the bend in the river will still be operating.

Figure 26 – Bend Easement upstream

of Baile Mhic Ire

Figure 27 – Rock Armouring

Figure 28 – Flood Flow

Bypass Channel


7.16.3 Full Bypass Channel Along With Infilling of the ‘Village Bend’ At the bend, a new short channel (maroon in Figure 29) is cut to take over from the river, which will be infilled (shown in green). This infilling eliminates the super elevated levels and associated hydraulic uncertainties. As such, flooding levels through this area can be determined with more confidence from modelled flow profiles. And, in addition, the conservative approach of adding 0.22m to the Freeboard is not needed; thereby further reducing design flow levels and the costs of flood defences.

7.17 DREDGING The following river works have been considered both individually and in useful combinations:

� Significant Dredging - From 500m upstream Baile Bhuirne Bridge to approximately 500m downstream of Baile Mhic Ire Bridge, a total length of 2.2km.

� Partial Dredging - From about 240m upstream of Baile Mhic Ire Bridge to 280m downstream. This is to minimise the bypass flow along the right bank at the bridge and locally reduce upstream embankment heights.

� A Compound Channel Downstream from Baile Bhuirne Bridge for 700m, then dredging to just beyond the downstream face of Baile Mhic Ire Bridge. And finally a Compound Channel from Baile Mhic Ire Bridge Downstream face for 425m.

7.17.1 Significant Dredging This dredging may be broken into two parts. The first stretch begins 500m upstream of Baile Bhuirne Bridge and runs to the bridge (see mustard line in Figure 30) at an average depth of 0.74m and a dredge-slope of 1:242, this is similar to the average channel grade through this reach. This will remove 8,067 m3 of gravel and rock. The second stretch continues 1,716m from the bridge and finishes 525m downstream of Baile Mhic Ire Bridge, where it will tie into the existing bed level (see navy line). This will have an average depth of 1.46m and a slope of 1:242, again similar to the existing channel grade. The will remove about 57,702 m3.

This dredging aims to eliminate the need for walls through the centre of the village (i.e. op_10 to op_8) and minimising flow bypassing along the right bank at Baile Mhic Ire Bridge. Between the two bridges, this also provides an average reduction in flood defence heights of 1m, and 0.4m locally upstream of Baile Bhuire Bridge.

Figure 30 – Significant Dredging with Infill and Bypass Channel

Figure 29 – Bypass Channel and Infilling


7.17.2 Partial Dredging at Baile Mhic Ire Bridge The primary reason for dredging the area surrounding Baile Mhic Ire Bridge is to minimise the bypass flow along the right bank at the bridge to reduce residual risk to downstream properties, op_23. Secondly, it will locally reduce upstream embankment heights.

This measure consists of dredging 520m, from 240m upstream of the bridge to 280m downstream of it, and tying into the existing bed level at both dredge extents (see Figure 31), with an average dredge depth of 0.75m. A slope of 1:190 is required to merge the upstream existing bed levels to the downstream one. Of necessity, this means that Baile Mhic Ire Bridge will also need to be underpinned to a depth of 2.6m. The volume of gravel and rock to be removed is 19,000m3.

This measure will eliminate the need for walls and embankments through the centre of the village and reduce the heights of the remaining embankments and wall through the study area.

While it effectively removes all of the bypassing flow, it is necessary to accommodate the uncertainty in design flow estimation (see Chapter 4). As a result, this still needs culverts to cater for a bypassing flow of 10 m3/s.

7.17.3 Compound Channel Downstream of Baile Bhuirne Bridge The primary functions of this compound channel beginning at Baile Bhuirne Bridge and running 800m downstream (i.e. to just beyond op1_7) is to increase conveyance through this reach and, thereby, reduce embankment heights in Baile Mhic Ire village and eliminate walls along the main channel between op1_8 to op1_9 (see Figure 32). Its bed level is about 0.5m higher than the existing bed (i.e. the river becomes an Inset Channel). The excavation volume (mainly gravel and rock) is 40,000m3.

Running along the right bank, this is a 20m wide, except near the ‘town bend’, where it increases to 30m at its widest. This reduces the radius of the bend and, as a consequence, reduces the effects of super-elevation.

Of necessity, Baile Bhuirne Bridge must be underpinned and the weir at the bridge removed.

Figure 31 – Partial Dredging

Figure 32 – Compound Channel Downstream of Baile

Bhuirne Bridge


7.17.4 Compound Channel Downstream of Baile Mhic Ire Bridge The primary functions of this compound channel beginning at Baile Mhic Ire Bridge and running 425m downstream are to increase conveyance and, thereby, reduce embankment heights in Baile Mhic Ire Village and the bypassing flow along the right bank of Baile Mhic Ire Bridge (see Figure 32). Its bed level is about 0.5m higher than the existing bed (i.e. the river becomes an Inset Channel). The excavation volume (mainly gravel and rock) is 17,000m3.

Of necessity, Baile Mhic Ire Bridge must be underpinned and the weir at the bridge removed.

This channel (along with localised dredging) reduces defence heights through the centre of the village and effectively eliminates all bypassing flow on the right bank at Baile Mhic Ire Bridge. However, it is necessary to accommodate the uncertainty in design flow estimation (see Chapter 4). As a result, this still needs culverts to cater for a bypassing flow of 10 m3/s.

7.17.5 Dredging with Compound Channel A dredge of average depth 0.95m carried out for approximately 500m from just beyond op1_7 to just downstream of Baile Mhic Ire Bridge, while maintaining a river grade of 1:300 (i.e. maintaining an approximate same channel grade as the existing channel through this reach). Dredging will create approximately 9,375m3 of material will be moved from the river.

7.18 ROAD RAISNG AT BAILE MHIC ÍRE BRIDGE

7.18.1 Road Raising at Baile Mhic Ire Bridge The resurfaced road will run out on both sides from the bridge deck as shown in Figure 34. This, in combination with culverts underneath the stretch (as shown) will stop floodwater crossing it. In addition, raising the part of the road near the village will prevent water flowing off in that direction.

Road Raising is required along the maroon line, while levels are graded down to the existing road level along the pink line.

7.19 GRAVEL DEPOSITION AND GRAVEL TRAPS River gravel is a feature of the area and, in particular, of the tributaries. This needs to be taken into account. Widening a river reduces flood velocities, while this can reduce localised erosion issues, it can induce the depositing out of sediment that would otherwise be transported past the town: leading to unwanted deposition of gravel during flood times. This gravel could cause a significant decrease in the protection afforded by the scheme and possibly its failure and flooding in the town.

This has shown to be the case especially on the Grotto channel, where a gravel trap is required approximately 500 metres upstream of its confluence with the River Sullane.

7.20 DEBRIS TRAPS Trees and other woody vegetation are a feature of this area. Large branches have been observed flowing down the river and getting trapped at a bridge. The build up of such vegetation

Figure 34 – Road Raising

Figure 32 – Compound Channel Downstream of Baile

Mhic Ire Bridge

Figure 35 – Debris Trap Alignment


leads to a reduced flow area through the bridge that causes increased pressure on the structure and reduced velocities, leading to afflux (rising up of the water surface). By preventing such debris from flowing into the bridges, a Debris Trap alleviates such negative impacts.

It is recommended that traps are installed 200m upstream of each of the three bridges. Each trap shall consist of a number of columns, approximately 0.3m in width, installed from the riverbed up to two meters above the 100-year flood profile, as shown in Figure 35.


Figure 35: Reference point maps


8. POSSIBLE RELIEF SCHEMES FOR THE SULLANE RIVER

For the River Sullane, the Design Standard for a Flood Alleviation Scheme is the 100-Year event, and its estimated peak flow is 232m3/s.

It is necessary to find solutions that provide protection against flooding up to this standard. One engineering measure over the full area or a combination of locally effective measures can provide the basis for an option of a Flood Relief Scheme. This study has considered a range of engineering measures (as introduced in Chapter 7) and in combining different measures has identified viable options to fully protect the town against the 100-Year Design Flood. In this chapter, measures will be combined to provide further solutions that fully protect the town against the Design Flood.

8.1 VIABLE SCHEME OPTIONS Viable scheme options are considered in two parts, main river solutions, and those for the tributaries.

The secondary flooding in the study area is primarily caused by the four tributaries entering the Sullane from the north (see Figure 37). These are the:

• Bohill River

• Industrial Channel, its two primary streams have been separated out and individually named as the Industrial North and Industrial South Channels

• Grotto Channel

• Coffin Channel

It is necessary to provide protection from these channels; that is the subject of Section 9. In fact, one flood relief option (combining several engineering measures) has been found for each of these, which has the ability to protect the study area up to its design event. As such, these may be rolled into one global option that caters for the full set of flood risks generated by these tributaries. The remainder of this section will discuss main channel options that will be understood to be working in tandem with this tributaries option.

Figure 37 – The River Sullane and its Tributaries through the Study Area


There are 13 combinations of measures that have been found capable of providing relief from flooding caused by the River Sullane. To some extent, these engineering based solutions all involve the provision of walls and embankments, but they differ due to the range of additional measures they employ to protect the town to the design standard.

The viable options analysed are as follows:








� Significant dredging of the channel from upstream of Baile Bhuire Bridge to downstream of Baile Mhic Ire Bridge




� Option 7 - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:

� Construction of two compound channels

� Between the compound channels, the river is dredged by about a metre




� Options 1A to 3A - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:






� At Baile Bhuirne Bridge:


� Options 1B to 3B - Flood containment through a combination of walls and embankments, along with lowering the invert (bed) of the three bridges by underpinning. And, in addition:




� Localised channel dredging upstream and downstream of Baile Mhic Ire Bridge


The Village Bend is located alongside the centre of Baile Mhic Íre Village. Flood flows have considerable energy and this bend is severe enough to produce super-elevation of floodwaters on the village side of the river. Excluding the compound channel option, all options involve engineering measures to either fully alleviate this or significantly reduce it. The three engineering variations are:

� A highly localised Bypass Channel and Infilling of the bend

� Rock Armouring

� Bypass Channel

There three variations provide the basis of Option naming (1 to 3). Options 4 to 6 represent a repeat with the one addition of significant dredging, i.e. from 500m upstream of Baile Bhuirne Bridge to approximately 500m downstream of Baile Mhic Ire Bridge, a total length of 2.2km.

There are two variations to Option names (A or B). Variation ‘A’ represents the original Option along with one additional engineering measure, namely, widening 115m along the right bank and 100m along the left upstream of Baile Bhuirne Bridge, for example Option 1A. Variation ‘B’ is the original Option along with a different additional engineering measure, namely, dredging the river from about 240m upstream of Baile Mhic Ire Bridge to 280m downstream, for example Option 1B. While these changes seem small, their impact on the sizing of other measures within an option means that it is worthwhile forming separate options.

Finally, Option 7 represents a significantly different approach to solving the Sullane flooding problem. This employs two Compound Channels; the first starts at Baile Bhuirne Bridge and runs downstream for 700m. Directly below this, the river is dredged by an average of 0.95m for a length of 500m. And, then the second Compound Channel covers the 425m directly downstream of Baile Mhic Ire Bridge.

Scheme Name

Description of Engineering Measures All In Costs (€M)




5.14 2.52













Table 15: Brief description of the Option Measures


8.2 OPTIONS 1, 2 AND 3 8.2.1 Description of Options Options 1, 2 and 3 lower the invert (bed) of the three bridges by underpinning and use flood defence structures. These three are the same except for the measure chosen to deal with hydraulic issues through the ‘village bend’ (see Section 7.16).

Figure 23 presents defence heights (including freeboard). These may be broken into four groups, protection of properties on the left bank upstream of Baile Bhuirne Town Bridge (op_20 to op_19). Between that bridge (op_16) and Baile Bhuirne Bridge, protecting the N22. Below that bridge (op_11), down to Baile Mhic Ire Bridge (op_4) protecting the village. And, finally, below the bridge, on the right bank from op_3 to op_2 and on the left bank from op_23 to op_24 protecting the remaining properties . Raising the level of the road closed out the protection for the study area (from op_22 to op_23).

Along the right bank of Baile Mhic Ire Bridge, the model estimates a bypassing flow of 45m3/s. To counter this, a culvert is required under the raised road.

Flood defence heights were examined to optimise underpinning depth for each bridge, from the upstream bridge to downstream, these were determined to be: 1.2m, 1m and 1.2m, respectively: with dredge lengths tying into the existing bed levels 30m away from the nearest bridge face. Figure 38a indicates the Design Flood profiles for the three options.

Upstream of Baile Bhuirne Town Bridge, defence heights generally range from 0.7 to 1m, however, in the middle of the stretch, they reach a maximum of 1.4m. While the defence height is 0.9m directly downstream of the bridge, because it moves back from the river to run along the roadway, it quickly drops to a general value of 0.2m alongside Baile Bhuirne Town; it does have a maximum of 0.4m about half way along.

Its height has risen to 1m by the time it reaches Baile Mhic Ire. It reaches 1.6m before the embankment needs to turn back towards the river and tie into the upstream face of Baile Bhuirne Bridge, where it has rise to 2.3m. Downstream of the bridge it starts low (0.7m) but quickly increases to 1.9m and after that the 1.6 to 1.7m embankment is replaced by a 1.5m high wall half way along to the Village Bend. Option 1 needs a 111.35m high wall to skirt around the bend while Options 2 and 3 only need a 1.25m embankment. This rises to between 1.55 and 1.7m while defences need to stay at the riverbank, but, after stepping into the first of the two meadows upstream of Baile Mhic Ire Bridge, it reduces to about 0.7m. In the second field, though, it moves from a height of 1.1m to 1.75. The natural hollow near the road means the final height is 2.25. This hollow is being designated for infilling by excavated materials and, as a result, the visible defence height will reduce.

Downstream of the road a 1 to 1.15m high embankment runs until it ties into high ground. Across the river, the embankment starts at ground level and rises to 1.55m next to the road.

Figure 38 – Alignment of Walls and

Embankments


Maroon: Upgrade existing Embankments Blue: Underpinning and Regrading

The black grided area outlines the measures that

differentiate between Options 1, 2 and 3


Figure 38a: Design flow flood profiles for Option 1, 2 and 3 (i.e. where there is minimal level difference along the village ‘bend’ between each of the Options)


8.3 OPTIONS 1A, 2A AND 3A 8.3.1 Description of Options These options include channel widening by 7m for 115m along the right bank and 100m along the left directly upstream of Baile Bhuirne Bridge (see Figure 39). This will be dredged to the channel bed level and has side slopes of 1.5:1.

This is primarily to reduce velocities for flows in the range of the median event to reduce the excessive bank erosion and transportation of that sediment downstream.

Defence heights are virtually the same as Options 1 to 3.

8.4 OPTIONS 4, 5 AND 6 8.4.1 Description of Options In addition to walls and embankments, underpinning of the three bridges and removal of the weirs at Baile Bhuire Bridge and Baile Mhic Bridge, Options 4 to 6 investigate the potential of significant dredging over the full length of the study area, i.e. from 500m upstream Baile Bhuirne Bridge to approximately 500m downstream of Baile Mhic Ire Bridge, a total length of 2.2km.

8.4.2 Justification for the Significant Dredge Depths Primarily, the dredging depth was chosen to eliminate the need for defences through the centre of the village. This also provides an average reduction in the remaining defence heights between Baile Bhuire Bridge and Baile Mhic Ire Bridge of 1m, and upstream of Baile Bhuire of 0.4m. The long-section flood profile is presented in Figure 40, along with the profile from Option 1 for comparative purposes, i.e. the difference in level represents the additional lowering in levels.

This also effectively eliminates all of the design bypass flow on the right bank at Baile Mhic Ire Bridge, only 10 m3/s needs to be accommodated by installing culverts.

Figure 40: Difference in flood levels between Option 1 (red) and Option 4

Figure 39 – Channel Widening

Red: Option 1 Blue: Option 4

Note: Dredged bed profile in place


8.5 OPTIONS 1B, 2B AND 3B – Effects of Partial Dredging at Baile Mhic Ire Bridge 8.5.1 Description of Options Option 1B, 2B and 3B employ a partial dredging measure that runs 520m, from 240m upstream of Baile Mhic Ire Bridge to 280m downstream of it (see Figure 41), with an average dredge depth of 0.75m. A slope of 1:190 is required to tie into the existing bed level at both dredge extents. This means that the bridge needs to be underpinned 2.6m. The volume of gravel and rock to be removed is 19,000m3.

8.5.2 Effects of this Partial Dredging Measure The primary reason for this measure is to minimise the bypass flow along the right bank at the bridge and reduce residual risk to downstream properties, op_23.

Secondly, these options do not require upstream embankment heights through the centre of the village and have reduced heights through the remainder of the study area. The Option 1B long-section flood profile is presented in Figure 41, along with the profile from Option 1 for comparative purposes, i.e. the difference in level represents the additional lowering in levels.

While it effectively removes all of the bypassing flow, it is necessary to accommodate the uncertainty in design flow estimation (see Chapter 4). As a result, this still needs culverts to cater for a bypassing flow of 10 m3/s.

Figure 41: Difference in flood levels between Options 1 and 1B at Baile Mhic Ire Bridge

Figure 41 – Partial Dredging


8.6 OPTION 7 8.6.1 Description of the Option Option 7 has a compound channel from Baile Bhuirne Bridge to 700m downstream (i.e. to just beyond op1_7). Directly downstream of this, for a length of 500m (i.e. to just downstream of Baile Mhic Ire Bridge), the river is dredged by an average of 0.95m. There’s a second compound channel in the 425m directly downstream of Baile Mhic Ire Bridge. In addition to walls and embankments, the weirs at the two bridges are removed and the three bridges are underpinned.

The first compound channel is 20m in width and runs along the right bank, except near the ‘town bend’, where it increases to 30m at its widest. This reduces the radius of the bend and, as a consequence, reduces the effects of super-elevation (see Figure 42). The second has the same width and both are cut to within 0.5m of the existing bed level (i.e. leaving the

existing river as an Inset Channel). This will maintain a bed-gradient of 1:300 (i.e. the general channel grade through this reach). The second compound channel starts downstream of the bridge and runs 425m. The excavation volume (mainly gravel and rock) to be removed from the first compound channel is 40,000m3 and 17,000m3 from the second. The river dredging will remove 9,375m3.

8.6.2 Effects of a Compound Channel and Localized Dredging Primarily, its functions are similar to those for Significant Dredging, i.e. the excavation width was chosen to eliminate the need for defences through the centre of the village. This also provides an average reduction in the remaining defence heights between Baile Bhuire Bridge and Baile Mhic Ire Bridge of 1m, and locally upstream of Baile Bhuire Bridge of 0.4m.

This also effectively eliminates all of the design bypass flow on the right bank at Baile Mhic Ire Bridge, only 10 m3/s needs to be accommodated by installing culverts.

Figure 42 – Compound Channel

plus Dredging


8.7 ROAD RAISNG - BAILE MHIC ÍRE BRIDGE Under all the non-dredge options (Option 1, 2, 3, 1A, 2A and 3A), a large flow (up to 45 m3/s) bypasses along the right bank near Baile Mhic Ire Bridge.

8.7.1 Road Raising Baile Mhic Ire Bridge – Right Bank The resurfaced road will run out on both sides from the bridge deck as shown in Figure 43. Road Raising is required along the maroon line, while levels are graded down to the existing road level along the pink line. This, in combination with culverts underneath the stretch will stop floodwater crossing it. In addition, raising the part of the road near the village will prevent water flowing off in that direction.

8.7.2 Dealing with Flow Bypassing Baile Mhic Íre Bridge (along the Right Bank) A bypassing flow of 60 m3/s is 33% more than the estimated out-of-bank flow of 45m3/s and it is being taken to provide a safety factor against the uncertainty in hydrological estimates, etc. In addition, culvert sizing was hydraulically modelled using ISIS and validated against HEC-RAS and by hand calculations (using Excel) using a conservative culvert entry loss coefficient of 0.5.

For Options 1, 2, 3, 1A, 2A and 3A, modelling has shown that, 15 box culverts (3m by 1m) at 2m spacing, and angled 45 degrees to the road to improve their efficiency, are needed to neutralise bypassing flow (pink polygon from RR 6 to RR 8). With these culverts in place, Figure 44 presents three levels, namely, upstream of the culvert, at the upstream culvert face, and just downstream (i.e. tail-water elevation) for out-of-bank flows of 5, 15, 30, 45, 60, 75, 90 m3/s.

With 60 m3/s flowing through the culverts, the HEC-RAS model estimated an upstream level of 114.6m OD, and ISIS gave 114.45m OD. When the required 0.5m road-cover was added to 114.6m OD, this gave a road-surface level of 115.1m OD: this road-cover is also equal to the freeboard for hard defences of 0.5m (a conservative choice to counter uncertainty in hydrology, etc., see Section 7.5). Ground level through this reach is between 113.6 and 113.2m OD, so (allowing the culverts to be bedded by 0.2m) the averaged invert level for these 1m high culverts is taken as 113.4m OD (see Tables 16). For the record, the culverts can take a flow of 80 m3/s without becoming surcharged.

Figure 43 – Road Raising and Culvert Locations


Figure 44: Road Raising and Culvert Installation for the For Options 1, 2, 3, 1A, 2A and 3A

Under the dredge Options 4, 5, 6, 7 and 1B, 2B and 3B, bypass flow is effectively eliminated. However, (again, to counter uncertainty in hydrology, etc.) the addition of 0.2m to the standard freeboard means that a 10 m3/s bypassing flow has to be catered for. As a result, the level of the road leading from Baile Mhic Ire Bridge to the south (i.e. toward op_23) must be raised to no less than 114.4m OD; this is more than 0.7m higher than the existing level (see Tables 17). These options only need three 1m culverts near RR5 to take this small bypass flow.


8.7.3 Road Raising Baile Mhic Ire Bridge – Left Bank All options also need the existing arch under the road that is furthest from the left bank of the bridge (near AA3) to be replaced with two 3m by 1m box culverts, to compensate for any effects that may occur from other local works. Here ground level is 113.1m OD, and as they are to be recessed 0.2m below it, their invert will be 112.9m OD.

Table 16: Existing and proposed Levels - Option 1, 2, 3, 1A, 2A and 3A Ref. Point Existing Road Level m OD Proposed Road Level m OD Height Raise Distance Between (m)

RR1 114.1 114.1 0 30

RR2 114.1 115.1 1 45

RR3 114.05 115.1 1.05 50

RR4 115.1 115.1 0 -

RR5 115.1 115.1 0 25

RR5A 114.4 115.1 0.7 20

RR6 114.1 115.1 1 35

RR7 113.9 115.1 1.2 30

RR8 113.7 115.1 1.4 25

RR9 113.6 114.5 0.9 22.5

RR10 113.5 114.05 0.55 22.5

RR11 113.4 113.4 0

RR5B 114.6 115.1 0.5 35

RR5C 114 114 0

Table 17: Existing and proposed Levels - Option 4, 5, 6, 7, 1B, 2B, and 3B Ref. Point Existing Road Level m OD Proposed Road Level m OD Height Raise Distance Between (m)

RR1 114.1 114.1 0 30

RR2 114.4 114.4 0 45

RR3 114.05 114.4 0.35 50

RR4 115.1 115.1 0 -

RR5 115.1 115.1 0 25

RR5A 114.4 114.4 0 20

RR6 114.1 114.4 0.3 35

RR7 113.9 114.4 0.5 30

RR8 113.7 114.4 0.7 25

RR9 113.6 114 0.4 22.5

RR10 113.5 113.5 0 22.5

RR11 113.4 113.4 0

RR5B 114.6 114.6 0 35

RR5C 114 114 0

Note: A slope of 1:30 is allowed for the fall in road level.


9. POSSIBLE RELIEF MEASURES FOR THE TRIBUTARIES

9.1 Introduction to the Tributaries The area needs to be protected from flooding that originates form the tributaries entering the Sullane from the north (refer to Figure 37). These are:

- Bohill River - Industrial North Channel - Industrial South Channel - Grotto Channel - Coffin Channel

9.2 The Coffin Channel The Coffin channel flows as an open channel from the hills to the north of the village to point A, where an existing gravel trap is in place (see Figure 45). There, it is replaced by two 0.45m and one 0.6m diameter pipes that run to point B. From there, one 0.45m diameter pipe goes southwest into the Grotto stream just upstream of the small bridge near the Grotto. In the meantime, the other 0.45m and the 0.6m diameter pipes run south toward the Pharmacy, but just upstream of it, the stream briefly returns to being an open channel, after this, though, it is piped all the way to its outfall at MH. The Blue line in the figure shows the approximate location of the network.

Pipe Data From Point Diameter (mm) Invert (m OD)

Ground Level (m)

Channel Chainage (m)

ps107 450 114.73 116.11 375.4

ps106 450 113.85 114.93 329.4

ps105 450 113.62 114.72 305.5

ps104 525 113.5 114.78 273.7

ps104a 525 113.31 114.77 224.7

ps103 2 by 525 113.19 115.22 216.3

ps102 900 112.69/112.94 114.49 191.2

ps101 900 112.48 114.37 10D.5

Manhole 900 112.17 114.36 13

Outfall 112.13 114.36 0

stream 115.27 115.84 41

es200 450 114.6 115.69 35

Pipe Data From Point Diameter (mm) Invert (m OD)

Ground Level (m)


es 200 450 114.60 115.69 35

ps104a 450 113.31 114.77 8

ps103 2 by 525 113.19 115.22 25

ps102 900 112.94 114.49 87

ps101 900 112.48 114.37 95

Manhole 900 112.17 114.36 13

Outfall 900 112.13 114.4

Table 18 - Details of Pipe Network

Figure 45 - Stream and Pipe Network Detail


The two properties (in black) adjacent to Scannal’s Pub have a doorstep level approximately 0.5m lower than that of the pub. Those houses (also in black) to the north of the N22 are along a dip in the road and their doorstep levels are similar to the surface water drains next to the footpath (ps 104 and ps 105).

When the main outfall (at MH) is surcharged by the main river (i.e. on average, every 6 months) and a ‘reasonable’ flow arrives from the hills to the north and from local drainage, the volume of water is usually greater than what can be stored in the main pipe network and, as a result, water backs up along the network and surface water drains. This then pours out at the low-point and floods these properties.

The head of water within the pipe network is not able to overcome the friction-losses between ps 104a and the outfall and drive through the surcharged outfall.

Under existing conditions, hydraulic calculations (using Pipe Losses, Moody Diagram and Reynolds), with just the Median Flow in the Main Channel (i.e. an outfall water level of 114.3m OD) and a 5-year event in the main pipe network of the Coffin Stream, a head of 0.5m needed to drive flow into the Sullane through the surcharged outfall, i.e. 114.8m OD: pipe frictional losses along the existing sewer from the road (ps 104a) to the outfall are estimated to be 3,500N/m2. The level of the surface water gullies at ps 104 and ps 105 are 114.77 and 14.72m OD, respectively (see Table 18), i.e. just less that 114.8m OD. As such, this scenario would result in water flowing onto the road and flooding properties. Many variations considered between flows in the Coffin Stream and the Sullane cause flooding in this area.

Any scheme for this channel must accommodate the 100-year stream flow coupled with a 66-year flood event in the main channel, this is in keeping with best joint-probability practice. The above should indicate that significant works are necessary.

9.3 Proposed Design for the Coffin Channel At the upper 66-percentile, the 100-year design flow is 3.4 m3/s, and 33% is added to this flow to cater for blockages in the network.

While maintaining the existing invert levels and gradients, a pipe diameter of 0.85m is required at B, along with a new services-point (manhole) and realignment along the road’s right edge from B to C (see Table 19 and Figure 46). The open channel at the back of the Pharmacy is to be closed off.

A pressurized services-point is needed at C* to allow water build up pressure within the pipe network during times of high Sullane flows. To ensure un-surcharged conditions at Point C while still using the existing invert, a 0.9m diameter pipe is required to run to Point E. With a starting invert of 112.5m OD at E, using a slope of 1/270, an 180m long open channel will be cut past the Sullane embankment (Point F) to a new confluence (Point G), its finishing invert will be 111.85m OD. This shall have an average depth of 1.5m below the existing ground level, a base width of 1.5m and top width of 3m, i.e. side slopes of one-to-one. This channel will need to be fenced off.

Figure 46 - Location of new Pipe Network

F

G

F

E


In combination with Options 1/A, 2/A and 3/A for the Sullane (and allowing for the main channel freeboard), between E and the main channel embankment line at Point F (see Figure 47), a 0.9m high wall (in red) and an 1.1m height embankment (green) are needed. Under Options 4, 5, 6, 7, 1B, 2B and 3B, this defence line is not needed and, in addition, they do not need a pressurised manhole at Point C.

The pipe network arriving from the east (i.e. ps106 to ps104a) is to be separated from the above network (i.e. the network from the north). Where a new service point is required at D, while using the existing pipe size and invert level, next a new pipe alignment is used to divert it from D toward Point E, see Table 19.

Only in combination with Options 1/A, 2/A and 3/A for the Sullane, a non-return value is required at Point E to stop the main channel floods running up this eastern pipe network and flooding out from the low lying gullies.

The design 100-year flow for the eastern pipe network must include the upper 66 percentile. This is 0.18 m3/s, based on the rational method, where;

Flow: Q = 0.0028C.i.A

� Permeable area = 0.16, C = 0.3

� Grassed area = 0.16, C = 0.9

� ‘i‘ for the 100-year event is based on Met Eireann data of 35mm/hr, while the lag time for this catchment is 15minutes, allowing for 20mm/hr

� The Factor standard error = 1.65

Pipe Data From Point Diameter (mm) Invert m OD

Ground Level (m)


B 850* 115.59** 116.95 61

ps104a/C 1000* 113.31 114.77 10

E Open channel 113.0** 115.22

* Upsized pipes ** Design new invert levels

Pipe Data From Point Diameter (mm) Invert m OD

Ground Level (m)


ps107 450 114.73 116.11 46

ps106 450 113.85 114.93 24

ps105 450 113.62 114.72 32

ps104 525 113.50 114.78 49

D 525 113.31 114.77 10

E Open Channel 113.2** 115.22

Table 19 - Upgraded details of Pipe Network

Figure 47 – Walls: Red

Embankments: Green

F

G

E


9.4 Existing Grotto Channel Conditions The Grotto channel flows behind the Bun Glaise Estate, crosses the N22 and enters the main river approximately 150m upstream of Baile Mhic Íre Bridge, Figure 48. There is a subcatchment that joins it from the North (at point 16).

During a period of intense rainfall, a

flow from this subcatchment has entered the Grotto stream at point 16 and lead to overland flow resulting in the flooding of the primary school. It is also expected that at design flow the afflux created at the upstream face of the road bridge (Points 5 and 6) would cause overtopping of the existing defence walls along its upstream left and right banks.

9.5 Proposed Design for Grotto The proposed option is primarily to dredge from the existing bed level at Point 13 to where the Grotto Channel enters the Sullane River, construct walls and embankments (see Table 20 and Figure 49 for these locations and heights). In addition, a culvert is to be installed at the N22 along with a Gravel Trap.

At present, the channel gradient is 1:200. Excavation is required for a length of 400m (from 225m upstream of the road bridge to the main river), this will start by maintaining the existing bed level at Point 13 and use a gradient of 1:150 all the way to the Sullane (where the existing bed level of 113.1m OD is lowered to 112.4m OD). A volume of 920m3 will be removed. This gives an extra 1.1m underneath the existing road bridge (i.e. below its existing invert). As underpinning the bridge may be unsafe due to insufficient working-height, a new 12m long (3 by 1.2m) box culvert is more likely.

Figure 50 presents the long section of the modelled Grotto Channel, the red line shows the existing level for the 100-year flow of 6.36m3/s for a defence only solution while the solid red line indicates the reduced defence level when dredging and defences are employed.

This design takes account of the existing pipe network running underneath the road bridge i.e. allowing for these pipes to remain. Table 20 details the required wall and embankment heights.

Figure 48 – Upper Grotto Channel


Figure 49 illustrates the alignment of walls and embankments for Option 1/A/B, 2/A/B and 3/A/B. With Option 4, 5, 6 and 7, the heights are the same, except between the road bridge and the main channel embankment, i.e. from Points 3, 4 to 1,2, no flood containment measures are needed.

Significant volumes of gravels can be observed along this channel, in particular upstream of the N22. In addition to the above flood relief measures, the existing bed level at Point 15 needs to be locally lowered by 0.3m and a Gravel Trap installed. While having almost no effect on design flow levels, this will locally reduce general flow velocities. For example, the median flow velocity reduces from 1.3 to 0.7m/s, resulting in deposition of sediment and gravels.

Also, the existing access bridge at Point 14 will be replaced by a new one, this will maintain local access and provide a maintenance route to the Gravel Trap. Finally, the existing pipe located between Point 17 and Point 16 will be removed and replaced by an open channel.

Figure 49 - Wall and Embankment Alignment, etc.

along the Grotto Channel

Red: Walls, Green: Embankments

Pink: Possible Wall Upgrade Maroon Circle: Box Culvert

Light Blue: Dredging

Figure 50 – Design Level Profile Along the Grotto

Channel, D/S extent is Point 1,2

Sullane River


Defence Height

Point

Channel Chainage

(m)

Design Flood

Level (m) Description Right Bank Left bank

19 195 120.188 Wall upstream to high ground and wall/embankment downstream to 18 0.32/0.52

18 120 119.172 0.00

17 38 119.126 Wall from 18 to 16 0.16

16 112 119.084

To allow for super elevation (Design Level 119.084, right bank ground level 119.65) from the north channel, a 0.3m wall is required upstream to 17 0.30

15 70 117.749 Embankment from 16 down to 14. Installation of a Gravel Trap 0.30

14 108 117.35 Embankment from 15 to 13 0.37

13 131 116.48

11 88 115.76 From Point 12 to Point 7 - Ensure existing right bank wall can take 0.1m hydrostatic load or build new wall. 0.10

10 to 8 1m embankment for 50m

7,8 43 115.4 Ensure the existing wall can take 0.14m hydrostatic load or build new wall. 0.14

5,6 115.35 Ensure existing walls can take 0.11m and 0.24m, respectively, hydrostatic load or build new wall. 0.11 0.24

3,4 77 115.23 Wall from point 4 to 2 0.22

1,2 41 115 Wall from point 3 to 1 0.06 0.76

Table 20 - Details of Design Wall and Embankment Heights along the Grotto Channel


9.6 Industrial North Channel - Existing Conditions The Industrial North channel flows towards the Industrial Estate along the flow path indicated by the dark blue line in Figure 51.

At present, intense rainfall can cause flow to overtop the road at location A, after which, it runs south / southeast along the road.

9.7 Proposed Design At the upper 66-percentile, the 100-year design flow is 4.84 m3/s.

With an upstream invert level of 124.85m OD, a 0.9m diameter pipe is recommended under the road at Point A (refer to Figure 52) with a fall of 1:20, to take the upstream flow (along with a 33% allowance for blockage). In keeping with the road, this will have a fall of 1:20.

Specifying a surface cover of 0.4m for this 0.9m diameter pipe means that the existing road surface only need to be raised by 0.1m.

In addition, as highlighted by the Green line, a one metre high embankment will cater for any out-of-bank flooding from the existing channel north of A. Cleaning and/or dredging of this stretch of the Industrial North Stream is recommended (as indicated by the light blue line), this will improve the efficiency of the channel to cater for flow and reduce the likelihood of out of bank flooding.

Also, beginning at Point A, it is proposed to excavate a new Flood-flow Diversion all the way to the Bohill River. With an average depth of 1.5m and width 3m, this 175m long open channel (maroon) will accommodate a within-bank flow of 3.63 m3/s (with a 0.3m freeboard). Over the first 50m, the bed-gradient is 1:50 (average ground level is 125.3m OD), but, for the remaining 125m to the Bohill, it changes to 1:200 (average ground level is 124.5m OD) and enters at 122.45m OD, 0.5m above the river’s bed level.

The existing channel downstream of A will cater for the remaining flow, i.e. 1.21 m3/s, this will still enter the Sullane (i.e. south of the diversion). So, as existing channel can cater for this flow, other than future maintenance, no other relief work is required.

Figure 51 - Industrial North & South Catchments

Industrial North

Industrial South

Green: Embankment Pink Circle: Culvert Maroon: Diversion

Light Blue: Dredging Black: Existing Road

Bohill River

Figure 52 - Defence Alignment, etc. along the Industrial North

Catchment Boundary


9.8 Industrial South Channel - Existing Conditions The Industrial South channel flows from the northwest toward the Industrial Estate along the path indicated by the dark blue line in Figure 51.

Intense rainfall can cause flow to overtop the culverts at both A and B in Figure 53, after which, it runs southwest across the Industrial Estate.

9.9 Proposed Design At the upper 66-percentile, the 100-year design flow is 6.22 m3/s.

The existing partial-defence in the 20m between E and F needs to be upgraded (or replaced) by a 0.5m high defence structure. Dredging 0.4m below existing bed level from A to B, and continuing this excavation with a reducing depth until it ties into the existing bed level at C caters for this flow, and includes the standard freeboard.

At Point D, the existing 1m diameter piped culvert (see Figure 54) is upsized to 1.9m. By lowering its invert level 0.4m below the existing, its soffit level is therefore the same as the existing. The upstream existing cross section area at the culvert face needs to be widened by 0.9m to accommodate the larger pipe. Widening by 0.4m on the right bank and 0.5m on the left is recommended and that this widening is tapered back to the existing channel width 35m upstream, i.e. in going from G to H and I to J. The downstream cross section area at the culvert is likewise increased to cater for the upsized culvert, here, it is suggested that the whole 0.9m is taken from the left bank and, again, this widening is tapered back to the existing channel width 40m downstream, i.e. between B and K. Design defence only solution levels (red line) are presented in Figure 55 versus those that would result from the above proposed measures in place (solid blue line).

Red: Walls Orange: Widen Channel Pink Circle: Box Culvert

Light Blue: Dredging

Figure 53 - Defence Alignment, etc. along the Industrial South

Figure 54 - Culvert at D - Existing Upstream Face

Figure 55 - Industrial South Design Flow Profiles - Proposed Measures (blue) Versus Defence Only option (red)

A

B

C


9.10 Bohill River The Bohill River is the final channel needing investigation (see Figure 56). It joins the Sullane 260m upstream of the Industrial confluence. Its 100-year flow of 36.67 m3/s makes it the largest, by far, of the local tributary. This flow is increased by 3.63 to 40.33 m3/s to cater for the Flood-flow Diversion from the Industrial North.

The figure also shows the extent of the study area (highlighted by the red polygon) and the surveyed cross section locations. To ensure confidence, it is recommended that the channel bank between these sections are carefully surveyed to ensure that there is no breach by flood waters at intermediate low points. This should be carried out at the Detailed Design Stage of this project.

The hydraulic model showed that this flow only caused flooding out at Point A, and that was only to a depth of 0.15m. As such, a 30m long, a 0.65m high embankment (green line) can alleviate it. Currently, the model has indicated that the Bohill River does not cause any significant flooding of property. So, as the existing channel can cater for 40.33 m3/s, other than future maintenance and the embankment, no other relief work is required.

Figure 56 - Bohill River


10. COSTS OF THE POSSIBLE FLOOD RELIEF SCHEMES The items of work that make up each identified engineering measure are specified in Appendix B along with their costs.

Table 21 lists the possible Flood Relief Schemes along with a brief description to the individual engineering measures that combine to produce these schemes. The estimated scheme ‘Works Costs’ is the sum of the costs of the individual engineering options that form the Main Channel Flood Relief Scheme including the Tributary Flood Relief Scheme costs, plus 10% for un-measurable items, plus for 10% contingency, plus 7% for site supervision.

“All In Costs”. The estimate of the “All In Costs” is included in Table 21 are Works Costs plus maintenance over 50years at 1% Works Costs, at a discounted rate of 4% per year, plus 10% for Archaeology, plus 6% for Environmental Monitoring, plus Land acquisition and a regulated value for Art of €38K

Scheme Description of Engineering Measures All In

Costs (€M)

Option A Do Nothing (equals flood damage over 50 years) 0.0


5.14

Option 2 Flood Defences & Bridge Underpinning, with Rock Armour at ‘village bend’ 4.82

Option 3 Flood Defences, Bridge Underpinning, with Bypass of ‘village bend’ 5.10

Option 4 Flood Defences, Bypass & Infilling of ‘village bend’, significant Dredging 7.05

Option 5 Flood Defences, Bypass of ‘village bend’ and significant Dredging 6.69

Option 6 Flood Defences, Rock Armour at ‘village bend’ and significant Dredging 6.68

Option 7 Two compound channels linked by River Dredging 7.17

Option 1A Option 1 with channel widening upstream of Baile Bhuirne Bridge 5.25



Option 1B Option 1 with some dredging U/s & D/s of Baile Mhic Ire Bridge 4.80



Table 21 - Estimated Costs of Flood Relief Schemes


11. BENEFIT TO COST RATIO AND MULTI CRITERIA ANALYSIS

Benefit Costs Analysis The estimated Benefit-Cost Ratios for each of the Flood Relief Schemes are presented in Table 22; these are between 2.89 and 1.84.

Scheme Description of Engineering Measures

‘All In’ Costs (€M)




5.14 2.52













Table 22: Cost-Benefit Assessment of the Flood Relief Schemes

Numerical modelling has estimated that, in a 100-year event, 90 properties would have water above their floor level. The flood damage estimate under climate change conditions have not been taken into account, as they will not overturn this very encouraging Benefit to Cost Ratio.


12. Multi Criteria Analysis

The Options identified as being potentially feasible in terms of providing protection against the 100-year flood and cost effectiveness were appraised against Multi-Criteria Analysis objectives (see Appendix A1 and B). For a set of given objective, this involved scoring each of these potential schemes in relation to specified minimum requirements that should be meet in order to be acceptable under an objective, and aspirational targets (i.e. targets that an option should seek to achieve to be assigned a maximum score), making use of defined indicators for each objective.


For each objective, the indicators (minimum requirements and aspirational targets) along with the Global and Local Weightings were agreed within the OPW to ensure consistency with the appraisal of Options in other Schemes nationally. The four criteria were Technical, Social, Environmental and Economic. For each option, the Technical and Social individual local weighting score was largely based relative to the minimum requirement and aspirational targets. The Economic individual local weighting score was also based on these, though its cost benefit ratio was also taken into account. The Environmental Consultants commissioned for the Environmental Impact Assessment (RPS) carried out the Environmental scoring. The results of this Technical, Social, Environmental and Economic Criteria appraisal are included in Appendix A1 and those for Option 1 are presented in Table 23, while the summary for all options in order of ranking is given in Table 24.

CRITERIA Factored Weighted Score

1. Technical 11.67

2. Economic 18.62

3. Social 27.5

4. Environmental -15.38

TOTAL SCORE 42.41

Table 23 – Option 1 - Technical, Environmental, Social and Economic Appraisal

Rank OPTION SCORE

1 1 42.41

2 2 41.17

3 3A 41.09

4 3 40.96

5 2A 39.84

6 1A 27.94

7 A 10.00

8 1B -583.78

9 2B -1513.59

10 3B -1516.38

11 4 -1987.22

11 5 -1987.22

13 7 -2125.09

14 6 -2601.28

Table 24 - Summary for All Options in Order of Ranking

The results from the multi-criteria analysis show that Option 1 is preferred, refer to Appendix A1 for a full description.


13. CONCLUSIONS AND RECOMMENDATIONS From the Lee CFRAM study, Baile Mhic Ire was highlighted as an area where a Flood Relief Scheme was justifiable based on social, environmental and costs beneficial analyses and could be undertaken. A review of that study concluded that a standalone investigation was required to prepare an outline design of a preferred flood relief scheme to reduce the flooding problems in Baile Bhuirne and Baile Mhic Ire due highlighted from the Historical sever floods occurring: April 1962, August 1986 and less significant flooding during December 2001, December 2006.

The April 1962 and August 1986 floods were particularly severe with water levels of up to 1.5m in the village: modelling suggests that they were about the 100-year return period event. Some information was gathered in December 2006 and this has lead to a reasonable flood extent map: this event was between the 5 and 10-year return period event.

The OPW have a Hydrometric Recording Station just upstream of Baile Bhuirne Bridge since October 2011, so there is a lack of locally recorded data. The nearest gauged river station is Macroom,

The 100-Year flood is being taken as the design event in this study. The estimated extent of the 100-Year flood is presented in Figure 20.

Climate Change has been taken into account in this study where the expected increase in flood peaks is estimated to be 20%. This may not seem like a big change, however, it is sufficient to increase levels by between 0.10 and 0.35m and over double the frequency of flooding. Baile Mhic Ire could expect significant flooding every five years compared to about every ten years under ‘present day’ conditions.

Under present-day conditions (i.e. no Scheme), significant flooding can be expected to occur, on average, once in 5 to 10 years, so damages can be assessed for each property for the 5, 10, 20, 30, 50 and 100-Year events (Appendix A). The economic damage caused by flooding of property is estimated using the ‘The Benefits of Flood and Coastal Risk Management: A Handbook of Assessment Techniques - 2010 (the ‘Multi-Coloured Manual’: MCM). The Average Annual Damage, discounted at a rate of 4% per annum, has been calculated over a time-horizon of 50 years to produce a Net Present Value of the potential flood damage up to the 100-year. This is estimated as €12.97m.

Many engineering measures can be applied to the flooding problem (see Section 7). This study has identified those that are not suitable to Baile Mhic Ire, such as diverting flood flows and upstream storage of the peak. It would take significant dredging of the river to lower flood levels sufficiently to eliminate the need for defensive structures through the centre of the village and minimising the bypass flow along the right bank at Baile Mhic Ire. However, at the beginning of this project, the Environmental Assessment highlighted that large scale dredging was not viable (see Appendix A1): costs were also prohibitive (in the region of €10M to €14M). Despite this, along with the ‘Do Nothing’ solution (Scheme Option A), several flood relief schemes that combine a number of engineering measures have been found that fully protect the area against the Design Flood. These are presented in Table 21 along with the estimates of their ‘All In Costs’.

The safety factor for flood defences is a freeboard added to the estimated floodwater height. In general, for freshwater flooding, 0.3m is added to hard defences (such as walls) and 0.5m to soft defences (such as embankments). To counter uncertainty in the hydrology of the Sullane due to data issues, these values are increased by 0.2m to 0.5 and 0.7m, respectively.

A significant river bend is situated along the middle of Baile Mhic Ire village. Floods have considerable energy and this ‘village bend’ is severe enough to produce a complex hydraulic issue called super-elevation, where floodwaters rise higher on the village side of the river than on the other. The maximum effect is estimated at 0.22m. Hydraulic complexities and uncertainty in estimation due to insufficient data, etc., mean that a conservative approach must be taken in the area affected by this bend. While it remains, 0.22m is added to the freeboard.

Three alternative Engineering Measures have been examined to either fully alleviate or significantly reduce this problem, namely, rock armouring, a Flood Flow Bypass channel and a


Full Bypass Channel along with Infilling of the ‘Village Bend’. All scheme options involve an engineering measure at this bend to either fully alleviate or significantly reduce this problem.

River modelling shows that Baile Mhic Ire cannot be practically defended without flood containment measures. It also highlights that the three bridges within the study area are restrictive. For example, under the existing 100-year flood condition, the afflux (influence on upstream water levels) at Baile Mhic Ire Bridge is 0.9m. With flood containment in place, however, this increases to 1.2m, and would rise further to 1.4m by 2100, if the expected Climate Change occurs. Particularly under Climate Change conditions, this seriously impacts flood defence heights. Such conditions mean that a bridge would need to be removed or have its flow conveyance capacity upsizing through additional openings or by localised dredging. The cost associated with bridge removal or providing additional openings means that localized dredging is necessary. The Environmental Assessment scored this as viable in its Multi Criteria Analysis (see Appendix A1). Modelling shows that localised dredging at the bridges reduces afflux to 0.5m. As such, two Engineering Measures are needed for all solutions, namely, flood containment and localised dredging at the bridges.

Options 1, 2, 3 contain these two measures. The difference between them just relates to the choice of Engineering Measure at the ‘village bend’. In the naming of scheme options, there are two variations (A or B). Variation ‘A’ represents the original option along with one additional engineering measure, namely, channel widening upstream of Baile Bhuirne Bridge to eliminate high velocities and resulting erosion, for example Option 1A. Variation ‘B’ is the original Option along with a different additional engineering measure, namely, dredging the river both upstream and downstream of Baile Mhic Ire Bridge, for example Option 1B. While these changes may seem small, their impacts on the sizing of other measures within an option make it worthwhile forming separate options. One option represents a significantly different approach to solving the Sullane flooding problem by employing two Compound Channels linked a dredged river.

There are secondary sources of flooding that originate form the tributaries entering the Sullane from the north (see Figure 57). As a result, the town needs to be protected from the:

� Bohill River

� Industrial North Channel

� Industrial South Channel

� Grotto Channel

� Coffin Channel

One design-standard option (combining several engineering measures) has been found for each of these. As such, they may be rolled into one global option that caters for the full set of flood risks generated by these tributaries. This solution is therefore common to all possible options for the main river. Those, however, produced differences in River Sullane flood levels that alter defence heights along the tributaries between the river and the N22 road.

Thirteen combinations of measures have been found capable of providing relief from flooding caused by the Sullane. These engineering based solutions involve the provision of walls and embankments, underpinning of the three bridges and removal of the weirs at the Baile Bhuire and Baile Mhic Bridges, but they differ in the additional measures they employ to protect the town to the design standard.

Options 4 to 6 investigate the potential of significant dredging over the full length of the study area, i.e. from 500m upstream Baile Bhuirne Bridge to approximately 500m downstream of Baile Mhic Ire Bridge, a total length of 2.2km.

Option 7 is the compound channel option. Its primary function is similar to the significant dredge options, to reduce embankment heights through Baile Mhic Ire, eliminate walls through the

Figure 57 – Location of

Side Channels


center of the village and also reduce right bank bypass flow of Baile Mhic Ire Bridge. It involves forming two compound channels (cut to within 0.5m of the existing bed level, i.e. leaving the existing river as an Inset Channel) and river dredging both sides of Baile Mhic Ire Bridge.

The Options identified as being potentially feasible in terms of providing protection against the 100-year flood and cost effectiveness were appraised against Multi-Criteria Analysis objectives (see Appendix A and B). For a set of given objective, this involved scoring each of these potential schemes in relation to specified minimum requirements that should be meet in order to be acceptable under an objective, and aspirational targets (i.e. targets that an option should seek to achieve to be assigned a maximum score), making use of defined indicators for each objective.


For each objective, the indicators (minimum requirements and aspirational targets) along with the Global and Local Weightings were agreed within the OPW to ensure consistency with the appraisal of Options in other Schemes nationally. The four criteria were Technical, Social, Environmental and Economic. For each option, the Technical and Social individual local weighting score was largely based relative to the minimum requirement and aspirational targets. The Economic individual local weighting score was also based on these, though its cost benefit ratio was also taken into account. The Environmental Consultants commissioned for the Environmental Impact Assessment (RPS) carried out the Environmental scoring. The results of this Technical, Social, Environmental and Economic Criteria appraisal are included in Appendix A1 and the summary for all options in order of ranking is given in Table 24, above.

This process has identifies Option 1 as the preferred flood relief scheme for Baile Mhic Íre and Baile Bhuirne. Its Engineering Measures are flood containment through a combination of walls and embankments, channel infilling, new channel cut, along with lowering the invert (bed) of the three bridges by underpinning and localised deepening of the channel to ensure the full benefit from the bridges. On the upstream right bank (to improve the approach of floodwaters) localised channel widening at Baile Mhic Íre Bridge, and, in addition, raising the level of the L3405 road on both sides of the bridge, installation of culverts beneath this raised stretch to accommodate bypassing flow and to remove the damming-up effect caused by the bridge and roadway. Detailed scheme drawings are provided in Appendix C.

Figure 58 - Sullane Option 1 with Tributaries Solution


REFERENCES

baile mhic ire final engineering report

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