wallingford development site e - south oxfordshire · ranges in the sfra analysis. re-analysis...

May 2012EX 6774 R1

Wallingford Development Site EBradford Brook Hydraulic and Hydrological analysis, and FRA evaluation

Wallingford Development Site E Bradford Brook Hydraulic and Hydrological analysis, and FRA evaluation

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Document information Project Wallingford Development Site E

Report title Bradford Brook Hydraulic and Hydrological analysis, and FRA evaluation

Client Mr R Jameson

Client Representative -

Project No. MAM6864

Report No. EX 6774

Project Manager R Kellagher

Project Director A Tagg

Document history

Date Release Prepared Approved Authorised Notes 15/05/12 1.0 RBBK

Prepared

Approved

Authorised

© HR Wallingford Limited HR Wallingford accepts no liability for the use by third parties of results or methods presented in this report. The Company also stresses that various sections of this report rely on data supplied by or drawn from third party sources. HR Wallingford accepts no liability for loss or damage suffered by the client or third parties as a result of errors or inaccuracies in such third party data.


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Summary Wallingford Development Site E Bradford Brook Hydraulic and Hydrological analysis, and FRA evaluation Report EX 6774 May 2012 This statement has been produced by HR Wallingford for Richard and Vanessa Jameson who are challenging the proposed Site E development in Wallingford. This work follows on from a short study on the same topic carried out by HR Wallingford in 2010. The report reference is Berkeley Homes FRA Wallingford development: FRA document evaluation, EX 6380, September 2010. This discussed the modelling of the Bradford Brook in the SFRA produced by JBA, and the outline planning proposal supported by a report from Glanville consultants. The objective of this study is to review any new information, along with making more in-depth investigation regarding the hydrology and river modelling. Since then the debate about development across ‘Site E’ has progressed incorporating land adjoining the Bradford’s Brook proposed to be developed by Wates and Berkeley Homes. We understand that, to date, flooding concerns have not been registered by the Core Strategy Inspector in connection with Site E because of its classification as being in Flood Zone 1. This statement is in two parts after an initial position over-view: A review of the catchment hydrology; Assessment of the development FRA submissions. A review of the river modelling is planned if time allows, and will therefore be included, if appropriate, in a second version of this report. Hydrological analysis The catchment of the Bradford Brook is ungauged. This is a key fact and means that there must be caution applied to any hydrological analysis and its implications for the flood impact for a catchment. Added to this is the fact that the Mill Brook catchment is relatively unusual (hydrologically) and the values derived for the growth curves are at the limits of their accepted ranges in the SFRA analysis. Re-analysis using a number of methods provided support for suggesting a more conservative set of flow rate values should have been used for the flood flow analysis for the river modelling. The original flow rates used were 5.39m3/s and 6.29m3/s for the 1:100 and 1:1000 years respectively, and it is suggested that values of at least 6.0m3/s and 8.0m3/s should have been used to assess the Flood Zone category for the area. FRA submissions Assessment of the FRAs for the proposed developments of Glanville and Wates generally indicate compliance with good practice. The main area of concern is the lack of groundwater levels. The Environment Agency stipulates a minimum height of at least 1.0m (usually 1.5m) of unsaturated material between the base of any infiltration unit and the maximum groundwater level. It is highly unlikely that compliance will be possible with this requirement during wet winter months. Water levels are only available for late summer 2009 when water levels will have been low.


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Summary continued Although Site E is not a classified groundwater source protection zone, an unsaturated zone above chalk is highly desirable in that the fissured nature of chalk makes the migration of pollution much more serious than other soil types.


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Contents

Title page i Document Information ii Summary iii Contents v

1. Introduction....................................................................................................................... 1

2. Current situation and context ........................................................................................... 1

3. SFRA hydrological and hydraulic assessment................................................................. 2 3.1 Hydrological analysis ........................................................................................... 2 3.2 ReFH.................................................................................................................... 4 3.3 IH124.................................................................................................................... 4 3.4 Future development in the Mill Brook catchment ................................................ 5 3.5 Climate change .................................................................................................... 5 3.6 Conclusions ......................................................................................................... 6

4. Hydraulic modelling of Bradford Brook............................................................................. 6

5. Outline development FRA submissions – Wates and Glanville ....................................... 7 5.1 Drainage design approach................................................................................... 7 5.2 The groundwater levels........................................................................................ 7 5.3 The Wates final pond ........................................................................................... 7

Table Table 2.1 Summary of flow rates analysis for Bradford Brook .................................................. 4 Figure Figure 2.1 Flow rates for Bradford Brook.................................................................................... 5


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1. Introduction This statement has been produced by HR Wallingford for Richard and Vanessa Jameson who are challenging the proposed Site E development in Wallingford. This work follows on from a short study on the same topic carried out by HR Wallingford in 2010. The report reference is Berkeley Homes FRA Wallingford development: FRA document evaluation, EX 6380, September 2010. This discussed the modelling of the Bradford Brook in the SFRA produced by JBA, and the outline planning proposal supported by a report from Glanville consultants. The objective of this study is to review any new information, along with making more in-depth investigation regarding the hydrology and river modelling. Since then the debate about development across ‘Site E’ has progressed incorporating land adjoining the Bradford’s Brook proposed to be developed by Wates and Berkeley Homes. We understand that, to date, flooding concerns have not been registered by the Core Strategy Inspector in connection with Site E because of its classification as being in Flood Zone 1. It is understood that although a hearing has already taken place, it still has yet to have a ruling. As there have been a number of additional aspects which have come to light a second hearing is planned for the 22nd May. This deadline means that a number of things are still outstanding from this report, but which will become available before that date. As part of this study, a meeting with the Environment Agency (Jack Moeran) has taken place to discuss the hydrological analysis of Bradford Brook which was carried out for the recent SFRA and the reclassification of the area through Site E as Flood Zone 1 from Flood Zone 2. This statement is in two parts after an initial position over-view: a summary of the hydrological analysis and a discussion on the differences

between this analysis and the SFRA results; development related FRA points. A review of the river modelling is planned if time allows, and will therefore be included, if appropriate, in a second version of this report.

2. Current situation and context The current situation is that the hearing has been held on the proposed development site and a decision is expected soon by the Inspector. The two planning applications for the area have not been approved, and if the developments are approved, there is opportunity to influence detail design aspects of the drainage and other elements. There a number of general things worth noting regarding proposed developments in the context of current national changes underway. It is probably useful that you understand the current national context on planning and drainage. The recent Floods and Water Management Act now requires the local authority (upper tier - County Council in the case of Oxfordshire) to assess drainage proposals for compliance to the proposed SuDS Standards, and then take ownership of them. This has implications for what drainage structures might be considered to be desirable by the local authority. This assessment / approval aspect is yet to be implemented into law (probably Christmas or early next year). This is because the SuDS Standards, although


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drafted, are still receiving a lot of scrutiny as a result of the Consultation and may undergo changes. A second major piece of government policy that has already been enacted (March 2012) is the National Planning Policy Framework. This has removed a large number of guidance documents used previous in planning and therefore introduces more flexibility on behalf of all stakeholders in the planning process.

3. SFRA hydrological and hydraulic assessment The main element of this review is focused on the predicted flood flows of the Bradford Brook. The reason for this is that the section of the stream inside the Wallingford bypass has been changed from having a category of flood Zone 2 to all of it being in Flood Zone 1 (no flooding for events more frequent than the 1:1000 years). It is very unusual for any natural stream or river to remain in-bank for this scale of event. This is particularly relevant as developments have to carry out what is referred to as the sequential test to prefer developments taking place in Zone 1 compared to Zones 2 or 3. JBA was the consulting engineer which carried out the SFRA in 2009. Additional technical information on the SFRA analysis was provided directly by JBA upon request which enables the following comparison to be made to the hydrological results that have been assessed by HR Wallingford.

3.1 HYDROLOGICAL ANALYSIS

This section focuses on the hydrological analysis and a flow rates derived by JBA. The following section deals exclusively with the issue of predicting river flow rates and the relative merits of different values obtained. As it will be seen, there is definitely an argument that can be made to say that the flow rates have been under predicted. As with all hydrological analysis, although the analysis produces a specific value, the results must be in the context of the degree of certainty that can be placed on the analysis. In this case, where there is no recorded flow data, and a stream which should be classified as ‘unusual’ in its characteristics, must be regarded as having a wide range of uncertainty. The JBA work applies the correct approach that was in place at the time. Advice has changed a little since then, but the differences established really illustrate the uncertainties of the science of hydrology rather imply a failure of method applied by JBA. This work emphasises the need for judgement and the need to take a precautionary approach rather than to accept a single value answer for a flow rate. JBA used the FEH tool (Flood Estimation Handbook) to predict a peak flow rate and passed this through a model using the surveyed profile of the brook. The use of FEH is correct; it is the accepted tool for doing such work. They used version 2 (current at the time) whereas version 3 is now the current version. The differences found in the results are not necessarily associated directly with this upgrade of the tool. For ungauged catchments there are basically two methods in FEH used for predicting flow rates for specific return periods. These are: The ReFH method; The Statistical pooling method. There is also a simple correlation equation based on the predecessor to FEH (FSR – Flood Studies Report) generally referred to as the IH124 method. This is still used in providing runoff estimates for development sites, but it is not generally used in hydrological studies like this. However recent papers have shown that it should not be


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considered as having been significantly improved upon by the existence of the newer FEH tools, and has been shown to be slightly less accurate. I have therefore provided this in the mix to add to the information. Both FEH methods produce the annual median flood flow rate, which is therefore equal to the 2 year return period. A growth factor is then separately derived and applied to this figure to give peak flow rates for more extreme events (100 and 1000 year events). Similarly the IH124 method provides a mean annual figure for the peak flow rate (approximately 1:2.33 years) and then has a regional growth factor applied. I will not go too deeply into the details of either and their merits, but I have provided sufficient information on the various issues associated with the analysis that has been carried out. The ReFH method is regarded as being quite poor in predicting runoff for pervious catchments, and also is inappropriate for urbanised catchments. The cut-off value is based on a parameter SPRHOST being less than 20. The catchment value is just above this at 20.81. JBA therefore have not used ReFH to provide an estimate of peak flow. (It is worth noting that the catchment is not an urbanised one to any significant measure). For completeness I have provided an estimate using ReFH, though we must not give this information too much weight. However I add it knowing that assessments of the accuracy of ReFH method has shown that it often significantly under-predicts flows and therefore one can use it in the context of this knowledge. The estimate made by JBA using the pooling method gives a Qmed value of 2.56m3/s at the confluence of the Mill Brook with the Bradford Brook. JBA also provide an assessment of the flow rate at the confluence with the Thames. However for clarity I will focus on the upstream point (as the findings are no different). The output for the lower point is reported in Table 2.1 for completeness at the end of this section. JBA used a pooling group of just over 1000 years. This means that they have aggregated a data set of gauged catchments with the most similar catchment characteristics to that of the Mill Brook (the stream discharging into the Bradford Brook). The length of data set used is a trade-off between the added accuracy of a long data set when predicting a rare event, against using some catchments which are not really very similar. The list of catchments used and the length of their respective recorded data sets is available in the technical supporting report produced by JBA. The growth curve derived gives figures of 5.39m3/s and 6.29m3/s for the 1:100 and 1:1000 years respectively. The growth factors are therefore 2.1 and 2.46. Current advice is that a pooling group should be at least 500 years long. When HR Wallingford did this work, two sets were produced, one of 490 years and one of 234 years. The reason for this was that the parameters, which indicate how representative the other catchments are, show that the 500 years of information draws on some catchments which are not very representative (H1 = 1.9 and H2 = 4.4), while the 200 year data set however shows a better fit of (H1 = 1.4 and H2 = 2.4). The H1 and H2 values for the JBA data are not reported. However, seeing as how different their predictions are for both Qmed and the growth curve factors, it would suggest that the pooling group is really quite different in character. The Qmed values derived by me are 1.90 for both pooling groups. The 1:100 year and 1:1000 year flow rates for the 500 year series are predicted as 4.70m3/s and 5.67m3/s respectively with growth curve factors of 2.47 and 2.97. However for the 200 year series the values are 5.90m3/s and 8.10m3/s with growth curve factors of 3.1 and 4.25. The growth curve values are noted in the JBA technical report as being particularly low, and I would suggest that they are unusually low. However as the Qmed value starts off being so high in the JBA assessment, this results in the 1:100 year flow rate being


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relatively similar to the results I have obtained. Unfortunately as the 500 year series would generally be regarded as a safer set of data, this would suggest that the JBA data was a reasonable set of flow rates to use. The final point that must be understood about these predictions; there is a general expectation that the answers are “correct” and can be trusted. In practice, any ungauged catchment has a very wide range of possible Qmed value. This is usually expressed in terms of 2 standard deviations which means that Qmed has a 5% chance of being even lower that the low estimate of 0.42Qmed and 5% chance of being above the 2.4Qmed estimate. Although this uncertainty range is high, extrapolating a few years of data from a range of different river gauges to obtain the 1:1000 year event is an even greater leap of faith with uncertainties which are even greater. Further work on uncertainties could be done to illustrate this aspect, but it is getting into ‘deep waters’. The key point to make is that a wide range of possible flows should be taken into account in making judgements on the consequences of possible flood flows in the Bradford Brook.

3.2 REFH

The ReFH analysis gives figures of Qmed 2.1 m3/s, 1:100 year of 6.4 m3/s which gives a growth factor of 3.05. The 1:1000 year value is not usually predicted when using ReFH. Although the ReFH method is not considered to be as accurate a tool, nevertheless this result adds weight to the possible range of possible flow rates.

3.3 IH124

A cross-check using the IH124 method gives a Qbar value of 1.99 m3/s using an SPR value of 0.21 (assuming SPRHOST of 20.5 is a comparable measure). Clearly this assumption is a further argument which could be used to avoid use of this method. The reason for using SPRHOST it is that the IH124 method was based on a different (coarse) category of soil types and the HOST categories provide a higher resolution soil information. The growth curve factor for the Thames region for the 1:100 year return period is 3.19. This gives a flow rate of 6.36m3/s. The value for the 1:1000 year event is 5.16 which gives a flow rate in excess of 10m3/s. This illustrates the extreme uncertainty of extreme flood events. In summary the various results can be seen more easily in the following tabular form. Table 2.1 Summary of flow rates analysis for Bradford Brook

JBA FEH Stat’ Upstream / (Downstream)

HRW FEH stat’ Upstream / (Downstream)

HRW ReFH Upstream

HRW IH124 Upstream SPR / (SPRHOST)

Qmed / Qbar 2.56 / 2.86 1.9 / 2.12 - (500) 1.9 / - (200)

2.1 1.99

1:100 year 5.39 / 5.54 4.7 / 4.86 (500) 5.9 / - (200)

6.4 6.36

1:1000 year 6.29 / 6.27!! 5.7 / - (500) 8.1 / - (200)

- 10.27

100 & 1000 yr Growth factors

2.11 / 1.93 2.46 / 2.19

2.47 / 2.29 (500) 3.10 / - (200) 3.00 / - (500) 4.26 / - (200)

3.05 -

3.19 5.16

Pooling accuracy factors H1 & H2

- 4.4 / 1.9 (500) 2.4 / 1.4 (200)

- -

These values exclude the application of the climate change factor


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The results as a whole seem to point to the JBA growth curves definitely being too flat and their Qmed values being too high. This means that their predictions at the 1:100 year, taking the results as a whole, seem to be slightly low, though the counter argument of having longer pooling data strengthens arguments that this is the preferred value to use. As the growth curve will continue to be proportionally more for the 1000 year flow rate, this would suggest that the 1000 year figures are very likely to be too low. With high range of possible flows in this ungauged stream a precautionary approach should be taken in using an upper-bound set of values for assessing its Flood Zone map classification. Figure 2.1 illustrates the broad range of predicted flows and an upper bound value of around 6.0m3/s is suggested as being appropriate for the 1:100 year event, and a value of at least 8.0m3/s for the 1:1000 year event. It is high likely that the Brook will be out-of-bank for the 1:1000 year event and therefore the area beside the stream classified as being in Flood Zone 2.

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7 8

Gumbel reduced variate

Flo

w (

m3/s

)

JBA HRW pool500yr HRW pool200yr HRW ReFH HRW IH124

2 10 20 50 100 200 500 1000

Return Period (years)

95% Confidence interval on JBA Qmed estimate

Figure 2.1 Flow rates for Bradford Brook

3.4 FUTURE DEVELOPMENT IN THE MILL BROOK CATCHMENT

It is worth noting that these figures are based on current catchment characteristics. Thus future development of Didcot or other villages in the catchment are not considered. However I suspect that much of Didcot’s future development is probably not within the catchment, nor will it be sufficiently extensive to make a big difference to the predictions.

3.5 CLIMATE CHANGE

The value for climate change used by JBA is an uplift on the flows of 20%. This is in keeping with standard practice at the time found in PPS25 which requires a 20% uplift for planning horizons beyond 2025. This has been temporarily carried through into the National Planning Policy Framework. However Defra guidance on flood risk is different (it came out before the recent Planning Policy document) and is focused at schemes involving Government expenditure. This would suggest that for the Thames region 25% uplift would apply, along with a consideration of 90% increase as an unlikely, but possible, future scenario to be considered where consequences might be extreme. This


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extreme might be applied in the situation of a potential dam failure and loss of life, and probably not for development planning situations. However this factor does illustrate the trend to recognising and using uncertainty analysis. The following text is taken from the introduction in this document. This advice replaces Defra’s Supplementary Note to Operating Authorities – Climate Change Impacts, October 2006. It is provided as supplementary information to Defra’s policy statement on Appraisal of Flood and Coastal Erosion Risk Management (2009) and the Environment Agency’s Flood and Coastal Erosion Risk Management Appraisal Guidance (FCERM-AG), as well as supporting the FCERM National Strategy for England. This advice is based on Government’s policy for climate change adaptation, and is specifically intended for projects or strategies seeking Government Flood Defence Grant in Aid (FDGiA). However, Risk Management Authorities (RMA) in England may also find this information useful in developing plans and making FCERM investment decisions even if there is no intention of applying for central government funding. Guidance for Wales is still provided by the Defra 2006 supplementary note – Treatment of climate change impacts. The purpose of this advice is to ensure that an economically credible appraisal, taking account of the uncertainties associated with climate change, can be made to support Government investment decisions. This is necessary to ensure that a fair comparison can be made between investment in projects in different locations that compete for central government grant, as well as ensuring that the most appropriate means of reducing risk is investigated in any one place.

3.6 CONCLUSIONS

In conclusion there is some evidence that the output from JBA might be considered as being a reasonable estimate to use based on current practice. However for classification of an area not to be in Flood Zone 2 when the intrinsic uncertainty in hydrology modelling, together with the evidence that values of Qmed and the low growth factors being just within acceptable limits, would suggest that a precautionary approach using upper bound flow rates should have been used in assessing the potential for flooding. The various estimates found using other supporting hydrological techniques, even if regarded as being less accurate than the FEH statistical method, are sufficiently consistent to suggest that a value of the order of 6.0 or higher should be used for the 1:100 year event and at least 8.0m3/s for the 1:1000 year event. If these were used, it is very likely that the flows in the river, if re-modelled, would be out-of-bank. This would require flood zone re-classification of the area and the requirement to apply the sequential test to prefer Flood Zone 1 areas in development planning applications.

4. Hydraulic modelling of Bradford Brook The river model used for analysing flows in Bradford Brook was ISIS and a steady state analysis was applied. A 1D model of the channel was built from survey data. The model commenced at the confluence of Mill Brook with Bradford Brook and extended down to the R. Thames. Water levels were assessed as remaining in-bank throughout the reach between the bypass and the R. Thames for both the 1:100 and the 1:1000 year flows. This is extremely surprising for any river which is not heavily managed for flood control. The Bradford Brook may operate in its most severe flood state during a summer storm of the order of 6 to 12 hours. The critical duration is estimated as 11 hours, so any rainfall event of this length or a few hours longer will tend to be of the same magnitude.


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Evidence for the recent storm of the 21st July 2007, which had around 90mm fall on Didcot, but only 50mm at Wallingford, and was therefore less than the 100 year event did remain in-bank when it passed under London Rd, though it was reported to be near the top of the bank. The R. Thames Flood Zone 2 area stretches up to the London Rd along the Bradford Brook. Although its critical duration is much longer than the Bradford Brook, (and therefore the peaks of any flooding are extremely unlikely to coincide), it is important to take account of the likely increased water level in the R. Thames when modelling the lower reaches of the Bradford Brook. The river model has yet to be investigated or used to assess the potential for flooding for higher flow rates.

5. Outline development FRA submissions – Wates and Glanville These FRA submissions comply with requirements though there are some issues that could be picked on. There are three points that are worth noting.

5.1 DRAINAGE DESIGN APPROACH

Both development proposals focus on the use of infiltration for stormwater disposal as they are correct in assuming that rainfall on the site effectively infiltrates rather than runs off into Bradford Brook. However their respective proposals are very different and one can expect the proposals to evolve significantly especially if the SuDS owners (the County Council) take an interest in what they will own.

5.2 THE GROUNDWATER LEVELS

Groundwater levels were reported as being monitored for the Glanville development. These were started in summer 2009, so only a few readings are given in the FRA document. A request has been placed with the Environment Agency to see whether data collection had continued through the winter and into 2010 and 2011, and to provide this information if it exists. This has yet to be received. It is known that groundwater levels at Hithercroft, which is close by, is often very high in winter. The relevance of this is that the drainage design based on infiltration has to take account of groundwater levels and must not contravene Environment Agency guidelines of infiltration. They specifically require at least 1.0m unsaturated zone at all times between the base of any infiltration system and the maximum water level of groundwater. I suspect that this will be very difficult to comply with in wet winter months. Although Site E is not a classified groundwater source protection zone, an unsaturated layer above chalk is highly desirable in that the fissured nature of chalk makes the migration of pollution much more serious than other soil types.

5.3 THE WATES FINAL POND

This development has a compensation storage pond where there is an existing pond already. I think it may have an outfall to the stream, though the drawing does not show this. As we discussed, there is a tendency to use ponds and basins as amenity features. The Environment Agency is sympathetic to maximising the privacy of existing residents and in this regard I would like to suggest the boundary border is maximised using the


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argument of the Wallingford biodiversity action plan. This should be based on planting up the border and excluding amenity use (targeting dog walkers especially).

Certificate No. EMS 558310

Certificate No. FS 516431

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