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BERWICK UPON TWEED ESTUARY STUDY Tweed Estuary Modelling Study

PREPARED BY MARTIN WRIGHT ASSOCIATES FOR NORTHUMBERLAND COUNTY COUNCIL PAGE 1

MARTIN WRIGHT ASSOCIATES

Berwick upon Tweed Estuary Study Stage 2 – Estuary Modelling Study Report

September 2011

U N I T 1 , L E A H A L L F A R M , L E A L A N E , A L D F O R D



DOCUMENT HISTORY

Issue Number Status Originator Checked Issued Date

01 DRAFT MWA/CC/JBA GMJ GMJ 04/06/11

02 DRAFT MWA/CC/JBA TI GMJ 13/08/11

03 DRAFT MWA GMJ GMJ 14/09/11

04 FINAL MWA GMJ GMJ 14/09/11

Copyright Martin Wright Associates. All rights reserved

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Front cover: Aerial view of the study area. Photography courtesy of the Shoreline Management Plan.



EXECUTIVE SUMMARY

CONTEXT

The Tweed Estuary is dynamic and has experienced significant variation in morphology during the 20th

century. The volatile nature of the estuary was identified in the recent Northumberland and North Tyneside Shoreline Management Plan 2, which highlighted several key issues that required further investigation. One key concern is that the defences around the estuary were noticed to be sensitive to changes in the morphology at the mouth of the estuary. The spit and the Sandstell end of the frontage remains vulnerable to sudden change in beach volume. With potential loss of sediment to the frontage with sea-level rise and possible increased spate flows in the river, this area could become increasingly vulnerable. Predicted sea-level rise is also of key concern with regards to the flood risk experienced by Berwick. Existing defence heights may need to be increased to afford protection against this projected increase in flood risk.

HYDRODYNAMIC AND MORPHOLOGICAL INVESTIGATION

This study examines key issues and concerns with regards to the hydrodynamic and morphologic processes within the Tweed Estuary. Complex and detailed modelling techniques are used to investigate the typical behaviour of these processes within the estuary. A modelling exercise reveals how these processes are characterised during extreme behaviour in river flow, sea level and offshore wave activity. The exercise is extended to include varying erosion scenarios of Sandstell Point spit morphology. The predicted responses of the estuary morphology to these hydrodynamic processes and spit morphology scenarios are modelled in order to predict the effects on geomorphology.

With regards to hydrodynamic processes, the fastest currents predicted within the estuary are associated with extreme river spates. However currents predicted for extreme sea-level and offshore wave behaviour are also large enough to lead to considerable morphology change at Sandstell Point spit and within the inner estuary.

It would appear from the model results that significant morphological change at the sub-bar scale is likely under all of the scenarios. Extreme fluvial events cause dramatic coarsening of all morphologies in the estuary and exposure of extensive areas of bedrock to the east of Calot Shad. It would appear that extreme fluvial events are critical in maintaining the bedrock morphologies in the estuary and prevent loss through siltation. Extreme sea-level conditions tend to coarsen morphologies along the western edge of Calot Shad and over southern bar features; this effect increases as spit volume decreases. Siltation of bedrock morphologies is predicted regardless of spit state. Extreme wave events result in a complex tidal/fluvial interaction and minor coarsening across Calot Shad. There appears to be a general propensity for morphologies to coarsen west of a line running north – south through Carr rock under any of the extreme event scenarios. Extreme fluvial events cause the greatest change. Following the predicted morphological response of an extreme event recovery of the Tweed Estuary morphology will be governed by normal MHWS conditions. The results from the MHWS modelling show that this will be most rapid over Calot Shad, reducing up the estuary.

The condition of the spit plays a sub-dominant role in influencing estuary morphology with broadly similar patterns of change predicted regardless of spit state. There is, however, a propensity for the magnitude of change to increase with increasing spit erosion. Periodic stripping of accumulated silts and sands by extreme events is important to reset the system, which otherwise has a propensity to silt up under the normal tide and wave conditions.



WAVE OVERTOPPING EVALUATION

The SMP2 identified that defences in the Tweed Estuary may require significant effort to bring them up to an adequate standard of protection, both for present day climate conditions and future climate conditions. Therefore an assessment of the risk of wave overtopping at the present day defences around the estuary was performed. This assessment consisted of calculating wave overtopping rates at three separate defence locations within the estuary for present day conditions, as well as for future projections of sea-level rise and a reduced-volume spit morphology scenario. Finally, the need to increase flood defence levels to afford protection from rising sea-levels was evaluated. The results show that, for present day conditions, the defences at Sandstell Point, south bank and north bank provide good, adequate and poor protection against wave overtopping flood risk respectively. Results for a catastrophic spit loss scenario show significantly increased wave overtopping flood risk within the inner estuary. These results highlight the significant defence role the spit plays by depth-limiting incoming waves for the inner Tweed Estuary.

RECOMMENDATIONS

The results and conclusions of the present study lead to the following recommendations:

The current standard of protection provided by the rock armour/embankment sea defence at Sandstell Point should be maintained. An assessment should be made at a later date in order to upgrade the defence in approximately 50 years time to counter the increased overtopping risk presented by predicted sea-level rise. To protect against the DEFRA +100 years sea-level rise prediction the crest level should be raised by 0.8m, or further wave-dissipating alterations to the defence should be made.

The south bank collapsed gabions/embankment sea defence should be upgraded within 50 years to offset the increased overtopping risk posed by 50 years of predicted sea-level rise. To reduce the overtopping rate expected after 100 years of predicted sea-level rise the crest height should be raised by 2.8 m, or a redesign of the defence should be undertaken to provide a more efficient solution.

The north bank vertical wall sea defence requires immediate upgrading in order to provide an adequate level of protection for the affected infrastructure behind the defence. This upgrade will need to account for predicted rises in sea-level. An elevation of the defence crest level is likely to be impractical, given the large height involved. Therefore options for defence improvements should be considered within a Project Apprasial Report, or similar.

Significant erosion and lowering of Sandstell Point spit should be prevented, in order to maintain its significant role in mitigating wave overtopping flood risk in the inner estuary.



TABLE OF CONTENTS

EXECUTIVE SUMMARY 3 TABLE OF CONTENTS 5 GLOSSARY 10 1 INTRODUCTION 11

1.1 Introduction 11 1.2 Context 11 1.3 Key issues 12 1.4 Objectives 13 1.5 Approach 13 1.6 Structure of report 13

2 HYDRODYNAMIC AND MORPHOLOGY MODELLING METHODOLOGY 15 2.1 Introduction 15 2.2 A hydrodynamic model of the Tweed Estuary 15

2.2.1 Mean High Water Springs 15 2.2.2 Extreme river flow 15 2.2.3 Extreme sea-level 16 2.2.4 Extreme offshore wave event 17

2.3 Sandstell Point morphological variation 17 2.4 A morphology response model of the Tweed Estuary 19

2.4.1 Morphology mapping exercise 19 2.4.2 Morphology model development 19 2.4.3 Morphology response model application 22

2.5 Summary 22 3 HYDRODYNAMIC AND MORPHOLOGY MODELLING RESULTS 23

3.1 Introduction 23 3.2 Tweed Estuary baseline regime 23 3.3 Tweed Estuary response to extreme river flow 29

3.3.1 Hydrodynamic response 29 3.3.2 Morphology response 31

3.4 Tweed Estuary response to extreme sea-levels 35 3.4.1 Hydrodynamic response 35 3.4.2 Morphology response 36

3.5 Tweed Estuary response to extreme wave action 40 3.5.1 Hydrodynamic response 40 3.5.2 Morphology response 41

3.6 Summary 45 4 WAVE OVERTOPPING EVALUATION 47

4.1 Introduction 47 4.2 Wave overtopping assessment methodology 47

4.2.1 Wave modelling 47 4.2.2 Wave overtopping calculations 48 4.2.3 Assessment locations 49 4.2.4 Sandstell Point spit loss scenario 51 4.2.5 Accounting for climate change projections 51

4.3 Wave overtopping results 52 4.3.1 South bank defence 53 4.3.2 Sandstell Point defence 55 4.3.3 North bank defence 56

4.4 Summary 58 5 ENVIRONMENTAL IMPLICATIONS OF NATURAL MORPHOLOGICAL VARIATION AND STORM EVENTS 59

5.1 Introduction and Context 59 5.2 Environmental Constraints and Opportunities 59



5.3 Effects of Scenarios 62 5.3.1 Scenario 1 – MHWS Partial Erosion 63 5.3.2 Scenario 2 – MHWS, Acute Erosion 64 5.3.3 Scenario 3 – Extreme Sea-Level, No Erosion 65 5.3.4 Scenario 4 – Extreme Sea-Level, Partial Erosion 66 5.3.5 Scenario 5 – Extreme Sea-Level, Acute Erosion 67 5.3.6 Scenario 6 – Extreme Fluvial, No Erosion 68 5.3.7 Scenario 7 – Extreme Fluvial, Partial Erosion 69 5.3.8 Scenario 8 – Extreme Fluvial, Acute Erosion 70 5.3.9 Scenario 9 – Extreme Wave, No Erosion 71 5.3.10 Scenario 10 – Extreme Wave, Partial Erosion 72 5.3.11 Scenario 11 – Extreme Wave, Acute Erosion 73

5.4 Summary of Environmental Implications 74 6 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS 77

6.1 Introduction 77 6.2 Assessment methodology 77 6.3 Hydrodynamics of the estuary 78 6.4 Morphological responses within the estuary 78 6.5 Environmental Implications of Natural Morphological Variation and Storm Events 80 6.6 Wave overtopping flood risk assessment 80 6.7 Recommendations 81

7 TWEED ESTUARY FURTHER STUDY 83 7.1 Introduction 83 7.2 Context 83 7.3 Objectives OF a further modelling study 84 7.4 Methodology for a further study 84

APPENDICES

Appendix A – Hydrodynamic Model Design

Appendix B – River Flow Hydrograph Derivation

Appendix C – Full List of Wave Overtopping Rates

Appendix D: Study Plan

TABLES

Table 1: Hydrodynamic extremes at Berwick .............................................................................................. 11

Table 2: Combinations of sea-level and wave parameters constituting a joint probability 1:200-year storm off of Berwick 48

Table 3: Overtopping limits for pedestrians ................................................................................................ 48

Table 4: Overtopping limits for vehicles ...................................................................................................... 48

Table 5: Overtopping limits for property ..................................................................................................... 49

Table 6: Overtopping limits for damage to defences ................................................................................... 49

Table 7: Predicted increases in sea-level with climate change at Berwick .................................................. 52

Table 8: Overtopping discharges for 1:200 year event at south bank defence (present day spit; DEFRA 2006 projections) .................................................................................................................................................. 53

Table 9: Overtopping discharges for 1:200 year event at south bank defence (present day spit; UKCP09



projections) .................................................................................................................................................. 53

Table 10: Overtopping discharges for 1:200 year event at south bank defence (catastrophic spit loss; DEFRA 2006 projections) ......................................................................................................................................... 53

Table 11: Overtopping discharges for 1:200 year event at south bank defence (catastrophic spit loss; UKCP09 projections) .................................................................................................................................................. 54

Table 12: Overtopping discharges for 1:200 year event at Sandstell Point defence (present day spit; DEFRA 2006 projections) .................................................................................................................................................. 55

Table 13: Overtopping discharges for 1:200 year event at Sandstell Point defence (present day spit; UKCP09 projections) .................................................................................................................................................. 55

Table 14: Overtopping discharges for 1:200 year event at north bank defence (present day spit; DEFRA 2006 projections) .................................................................................................................................................. 56

Table 15: Overtopping discharges for 1:200 year event at north bank defence (present day spit; UKCP09 projections) .................................................................................................................................................. 56

Table 16: Overtopping discharges for 1:200 year event at north bank defence (catastrophic spit loss; DEFRA 2006 projections) ......................................................................................................................................... 56

Table 17: Overtopping discharges for 1:200 year event at north bank defence (catastrophic spit loss; UKCP09 projections) .................................................................................................................................................. 57

Table 18: Summary of Environmental Implications Significance ................................................................. 75

Table 19: Deriving the values of Manning's n .............................................................................................. 90

Table 20: Catchment statistics for the Whiteadder river at Hutton Castle .................................................. 94

Table 21: Catchment statistics for the River Tweed at Norham .................................................................. 94

Table 22: Overtopping Discharges at South Bank under DEFRA (2006) Spit Intact .................................... 99

Table 23: Overtopping Discharges at South Bank under DEFRA (2006) Spit Removed .............................. 100

Table 24: Overtopping Discharges at Sandstell Point under DEFRA (2006) Spit Intact ............................... 101

Table 25: Overtopping Discharges at Sandstell Point under DEFRA (2006) Spit Removed .......................... 102

Table 26: Overtopping Discharges at North Bank under DEFRA (2006) Spit Intact ..................................... 103

Table 27: Overtopping Discharges at North Bank under DEFRA (2006) Spit Removed ............................... 104

Table 28: Overtopping Discharges at South Bank under UKCP09 Spit Intact .............................................. 105

Table 29: Overtopping Discharges at South Bank under UKCP09 Spit Removed ......................................... 106

Table 30: Overtopping Discharges at Sandstell Point under UKCP09 Spit Intact ......................................... 107

Table 31: Overtopping Discharges at Sandstell Point under UKCP09 Spit Removed ................................... 108

Table 32: Overtopping Discharges at North Bank under UKCP09 Spit Intact .............................................. 109

Table 33: Overtopping Discharges at North Bank under UKCP09 Spit Removed......................................... 110

FIGURES

Figure 1: Tweed Estuary mouth ................................................................................................................... 12

Figure 2: 0.5% AEP flow rate hydrograph .................................................................................................... 16

Figure 3: 0.5% AEP still water level event hydrograph ................................................................................ 17

Figure 4: Spit morphology: no erosion ......................................................................................................... 18



Figure 5: Spit morphology: partial erosion .................................................................................................. 18

Figure 6: Spit morphology: acute erosion .................................................................................................... 18

Figure 7: Sub-bar morphologic units across the Tweed Estuary at Berwick-upon-Tweed .......................... 19

Figure 8: Energy index values for fluvial and tidally influenced flow conditions for typical flow conditions across the Tweed Estuary ....................................................................................................................................... 20

Figure 9: Determination of fluvial and tidal energy levels and relative influence across the Tweed Estuary 21

Figure 10: Relationship between present fluvial/tidal energy levels and estuary morphology for use in the morphology response model ....................................................................................................................... 21

Figure 11: Assumed extent of spit and bedrock influence on estuary morphology at Berwick-upon-Tweed 22

Figure 12: Conceptual model of hydrodynamic flow in Tweed Estuary....................................................... 23

Figure 13: Hydrodynamic model showing ebbing tide depth-averaged currents........................................ 24

Figure 14: Hydrodynamic model showing flooding tide depth-averaged currents ..................................... 24

Figure 15: Comparison of actual (top) and predicted (bottom) morphology for the Tweed Estuary based on present day conditions ................................................................................................................................. 26

Figure 16: Predicted morphology impact of partial spit erosion for MHWS conditions (top) and difference from MHWS no erosion scenario baseline (bottom) ............................................................................................ 27

Figure 17: Predicted morphology impact of acute spit erosion for MHWS conditions (top) and difference from MHWS no erosion scenario baseline (bottom) ............................................................................................ 28

Figure 18: Model currents for 1:200 year river flow: ebbing tide ................................................................ 29

Figure 19: Model currents for 1:200 year river flow: flooding tide ............................................................. 30

Figure 20: Model currents for 1:200 year river flow: high water ................................................................. 30

Figure 21: Predicted morphology impact of extreme fluvial flow on no spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) ................................................................................... 32

Figure 22: Predicted morphology impact of extreme fluvial flow on partial spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 33

Figure 23: Predicted morphology impact of extreme fluvial flow on acute spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 34

Figure 24: Model currents for 1:200 year sea-level: flooding tide ............................................................... 35

Figure 25: Model currents for 1:200 year sea-level: ebbing tide ................................................................. 36

Figure 26: Predicted morphology impact of extreme sea-levels on no spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) ................................................................................... 37

Figure 27: Predicted morphology impact of extreme sea-levels on partial spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 38

Figure 28: Predicted morphology impact of extreme sea-levels on acute spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 39

Figure 29: Model currents for a 1:200 year extreme wave storm: flooding tide ......................................... 40

Figure 30: Model currents for a 1:200 year extreme wave storm: ebbing tide ........................................... 41

Figure 31: Predicted morphology impact of extreme wave action on no spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 42

Figure 32: Predicted morphology impact of extreme wave action on partial spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 43



Figure 33: Predicted morphology impact of extreme wave action on acute spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom) .................................................................. 44

Figure 34: Locations of defences for which wave overtopping calculations were performed .................... 49

Figure 35: View across Calot Shad Sand to the south bank defences .......................................................... 50

Figure 36: Sandstell Point defences ............................................................................................................. 50

Figure 37: Defence wall at the north bank ................................................................................................... 51

Figure 38: Environmental Constraints .......................................................................................................... 61

Figure 39: Summary of morphological responses to various hydrodynamic and spit condition scenarios for the Tweed Estuary .............................................................................................................................................. 79

Figure 40: CMS model grid showing bathymetry and model boundaries (brown cells are non-computational) 88

Figure 41: Catchments on the Lower Tweed ............................................................................................... 93



GLOSSARY

Acute Erosion Scenario of general coarsening, with sands replacing silts.

Bathymetric Underwater plotted contours

Batt Manmade structure to aid salmon netting

BAP habitats Biodiversity Action Plan Habitats

DEFRA Department for Environment, Food and Rural Affairs

Dunes Sand formation caused by the effects of winds

Extreme sea-levels Expected sea level in storm conditions / climate change

Extreme wave height Expected wave height in storm condition

FEP Flood Estimate point / location

FEH Flood Estimation Handbook

Fluvial River Flows

Geomorphological Study of landforms and the processes that formed them

GIS Geographical Information System

Hydrodynamic Flow of water over weirs, through openings, and down pipes and channels

Inter-tidal The foreshore and seashore and sometimes referred to as the littoral zone

LIDAR Light Detection and Ranging

Littoral The exposed area of the foreshore between high water and low water

mBOD meters Below Ordance Datum

mODN meters Above Ordance Datum Newlyn

MHWS Mean high water spring

Morphology Study of forms especially animals and plants

Partial erosion Erosion casued by wave action

SAC Special Area of Conservation

S.E.P.A Scotish Enviroment Protection Agency

Sediment transport Fine materials swept along by rivers and waves

SMP2 Shoreline Management Plan 2

Spit Long sand bar, sometimes referred to as a tombolo (Italian)

SSSI Site of Special Scientific Interest

Turbidity Measure of cloudyness of the water column

UKCP09 projections UK Climate Projections 2009

QMED Median of the annual maximum flow series – the flow that has an annual exceedance probability of 50% or a return period of two years

QMEDurban Urban adjustment factor for QMED

QMEDrural Rural adjustment factor for QMED



1 INTRODUCTION

1.1 INTRODUCTION

This study examines key issues and concerns with regards to the hydrodynamic and morphologic processes within the Tweed Estuary. Complex and detailed modelling techniques are used to investigate the typical behaviour of these processes within the estuary. Further investigation reveals how these processes are characterised during extreme behaviour in river flow, sea level and offshore wave activity. This chapter presents the context for the present study including key areas of concern raised by the recent Shoreline Management Plan 2. An overview of the methodology used to perform the study is given. Finally the structure of the report is described.

1.2 CONTEXT

The Tweed Estuary is a complex estuary, which discharges into the North Sea. It is a long narrow estuary which is still largely natural and undisturbed, supporting a wide range of habitats. Water quality has been classified as excellent throughout. The estuary is macrotidal with a mean spring tidal range of 4.1m. Dominant wave direction along the local coastline is from the north east. The breakwater that extends east from the northern bank therefore protects the inner estuary from this dominant wave climate, leading to the preservation of the sand spit, Sandstell Point, to the immediate south.

Figure 1 reveals the geography of the estuary. The estuary is characterised by a variety of inter-tidal sand and mudflats, substantial sandbanks and rocky shore areas. The River Tweed flows from the west into Berwick, setting against the southern bank before being forced north by the presence of Sandstell Point spit. The river is subsequently diverted out to sea by the northern breakwater.

The coastline within the estuary is characterised by various types of defences such as concrete walls along the north bank, embankments reinforced with rock armour to the south of Sandstell Point, and gabions in a state of disrepair along the south bank. Though the inner estuary is largely protected by wave attack by the breakwater, significant wave activity can still occur from the south-east and propagate into the estuary during high tide. The presence of Sandstell Point mitigates this wave-induced flood risk by depth-limitation effects leading to the reduction of wave heights of inbound waves.

The recent Northumberland and North Tyneside Shoreline Management Plan 2 (SMP2)1 identified the typical

hydrodynamic climate of Berwick. Table 1 shows the return periods for extreme still water levels and offshore wave heights. The Mean High Water Springs (MHWS) level is stated as 2.2mODN.

Table 1: Hydrodynamic extremes at Berwick

Return period (years) Sea level (mODN) Offshore wave height (m)

1 2.84 4.62

10 3.08 6.37

100 3.38 7.46

1000 3.54 8.12

1 Royal Haskoning, 2009: Northumberland and North Tyneside Shoreline Management Plan 2, Northumbrian Coastal Authority Group



Figure 1: Tweed Estuary mouth

1.3 KEY ISSUES

The Tweed Estuary is dynamic and has experienced significant variation in morphology during the 20th

century. A report by Babtie

2 noted that in recent times due to erosion of the sand bar and embankments the high water

mark had moved farther inland, leading to the abandonment of a factory building. The volatile nature of the estuary was further identified in the SMP2, which highlighted several key issues that required further investigation. A key concern is that the defences around the estuary were noticed to be sensitive to changes in the morphology at the mouth of the estuary. The continuing existence of Sandstell Point spit at the estuary mouth is due to the presence of the northern breakwater. Should the breakwater experience catastrophic failure the Spittal frontage would be exposed to increased wave energy. This would most likely lead to significant erosion of Sandstell point and exposure of the inner estuary to increased flood risk. Moreover the Tweed would tend to flow out to the north-east with little opportunity to develop an ebb tide delta and sediment would be removed offshore by the power of the river.

Historically, river flows have played a significant role in the distribution of sediments around the estuary. This role can be great during times of large flows. For example recent monitoring results have shown a link in

behaviour between the northern section of the Spittal frontage and the development of the Sandstell spit. The spit had tended to be lower with a slight channel developing across the spit. This is thought to be as a consequence of high flows in the river during 2003. This decrease in spit elevation coincided with an increase in the volume of sediment along the Spittal frontage. Subsequently, the volume of the Spittal beach reduced and growth occurred in the spit. The spit and the Sandstell end of the frontage remains vulnerable to sudden change in beach volume. With potential loss of sediment to the frontage with sea-level rise and possible increased spate flows in the river, this area could become increasingly vulnerable. Sea-level rise is also of key concern with regards to the flood risk experienced by Berwick. An increase in the average sea-levels experienced at Berwick

2 Babtie, Shaw & Morton, 1991: Coastal erosion at Spittal Point.



would increase the flood risk in two ways: the probability of occurrence of specific extreme sea-levels would increase, and large waves would more regularly propagate into the estuary as depth-limitation effects would be reduced. Therefore existing defence heights may need to be increased to afford protection against this projected increase in flood risk.

1.4 OBJECTIVES

The purpose of this study is to provide information on the linkages between hydrodynamics and morphological changes that can be used to inform future coastal management policies for the Tweed Estuary. The objectives include examining predicted morphological response due to extreme hydrodynamic behaviour, comparing and contrasting this with the typical response to average conditions. The extreme hydrodynamic behaviour to be investigated is that of severe river flow, extreme tidal levels, and massive offshore waves. Furthermore an objective of this study is to examine the flood risk within the estuary due to wave overtopping. This examination is to include any potential increases due to predicted rises in sea-level as well as estuary morphological variation.

1.5 APPROACH

The objectives of this study have been met by the use of a range of suitable tools to investigate the appropriate hydrodynamic and sedimentary processes. The hydrodynamics of the estuary have been simulated using a hydrodynamic circulation model that models estuarine water levels and currents driven by external forces. The model simulates varying water levels and currents due to tidal variations from the North Sea and fluvial flow from the River Tweed. This circulation model is coupled to a nearshore wave transformation model, which simulates the transformation processes that offshore waves undergo as they propagate towards the coastline. The predicted response of the estuary morphology is modelled using a morphological response model. This model uses the hydrodynamic flows determined by the circulation model as inputs, and predicts how these are likely to alter the prevailing geomorphology. Finally the flood risk at the estuary defences due to wave overtopping is investigated using wave overtopping calculation methods.

1.6 STRUCTURE OF REPORT

The context for the present study has been presented in the current chapter, along with a summary of the objectives and approach taken by the study to fulfil those objectives. The rest of the report is organised in the following chapters:

Chapter 2: Hydrodynamic and Morphology Modelling Methodology. The methodologies for simulating hydrodynamic processes and assessing the impacts on morphology are described.

Chapter 3: Hydrodynamic and Morphology Modelling Results. The results from a variety of modelling simulations are presented.

Chapter 4: Wave Overtopping Evaluation. An assessment of the susceptibility of defences within the Tweed estuary to wave overtopping flood risk is presented.

Chapter 5: Environmental Implications of Natural Morphological Variation and Storm events. The ecological and wider environmental requirements around the Tweed Estuary requiring consideration.

Chapter 6:. Summary, Conclusions and Recommendations. The results are summarised and conclusions drawn. Recommendations are presented.

Chapter 7: Tweed Estuary Further Study. The requirements and proposed methodology for a further study to investigate outstanding issues are described

Hydrodynamic Model Design. This appendix provides details of the hydrodynamic modelling system used.

River Flow Hydrograph Derivation. Details of the derivation of the river flow hydrograph are presented.

Full List of Wave Overtopping Rates. The full list of wave overtopping rates from the overtopping risk assessment is provided.



(THIS PAGE IS INTENTIONALLY BLANK)



2 HYDRODYNAMIC AND MORPHOLOGY MODELLING METHODOLOGY

2.1 INTRODUCTION

An aim of this study is to investigate the hydrodynamic behaviour of the Tweed Estuary and assess how this may affect the underlying morphology. Furthermore an assessment of the wave overtopping flood risk within the estuary is to be performed, including an examination of how this may evolve with morphological and sea-level change. This chapter provides an overview of the methodology used to make these assessments. More detailed information concerning the methodology is presented in the appendices.

2.2 A HYDRODYNAMIC MODEL OF THE TWEED ESTUARY

The simulation of hydrodynamic processes in the Tweed Estuary was performed using a numerical model of the coastal ocean. The circulation component of this model simulates variations in the sea-level due to tidal influence from the North Sea and fluvial influence from the River Tweed. This model also calculates depth-averaged currents throughout the model domain. The wave transformation component of this model simulates the transformations that waves undergo as they approach the coastline. These include refraction and depth-limitation of wave heights. The effects of wave set-up are simulated by the modelling system due to the enabled communication between the wave transformation model and the circulation model. A detailed explanation of the construction of the hydrodynamic modelling system is provided in Appendix A.

Simulations of typical and extreme hydrodynamic conditions within the estuary were required. Therefore a number of model scenarios were developed to provide these simulations. These are described herein.

2.2.1 MEAN HIGH WATER SPRINGS

A model run was performed that simulated a MHWS tidal cycle in the estuary peaking at 2.2mODN, with an average flow rate specified for the River Tweed. This model run represents a ‘control’ run describing typical conditions within the estuary.

2.2.2 EXTREME RIVER FLOW

This model run simulated a 1:200-year fluvial event occurring over a 3-day storm period. The derivation of the fluvial event is described in Appendix B. Figure 2 shows the appropriate hydrograph for the event. The extreme fluvial flow was combined with neap tide and average offshore wave conditions

3 in order to maximise

the signature of the river flow on the estuary hydrodynamics.

3 The neap tidal range used peaked at 1.46 mODN. The offshore wave height used was ascertained by averaging wave buoy data supplied by the CEFAS WaveNet network. No wave data were available for the coast directly off Berwick-upon-Tweed. Data were, however, available from the offshore areas of the Forth to the north and the Tyne to the south. To produce a measure of the average wave climate offshore of Berwick averages were calculated from the Forth and Tyne recorded wave parameters. The average offshore wave climate was found to be characterised by a significant wave height of 1.2m and a peak period of 7.1s.



Figure 2: 0.5% AEP flow rate hydrograph

2.2.3 EXTREME SEA-LEVEL

The SMP2 reports that the 1 in 200-year extreme still water level event reaches a height of 3.43 mODN at Berwick. In order to run a model simulation of such an event a hydrograph describing the appropriate tidal conditions was designed. Figure 3 shows the hydrograph for the extreme still water level event. The hydrograph was formed from a summation of the deterministic astronomical tide component and the surge residual component. This latter component accounts for the rise in still water level due to meteorological effects (i.e. strong winds and low pressure). A Highest Astronomical Tide, computed using TotalTide

4

predictions, was used as the underlying tide component. The shape of the surge residual used for the tidal graph was created using the average of observed surge residual data for storm surges recorded at nearby North Shields. The peak of the surge residual was arranged so that it occurred on the rising tide, consistent with known storm surge behaviour along the east coast of the UK in the North Sea

5. The surge residual

component was scaled so that, when added to the tide, the resultant total still water level peaked at 3.43 mODN. The tidal event was combined with an average river flow (described in Appendix A) and the average wave conditions used in the fluvial event.

4 Admiralty TotalTide software, UK Hydrographic Office

5 Horsburgh, K., C. Wilson, 2007: Tide-surge interaction and its role in the distribution of surge residuals in the North Sea. J. GEO. RES., 112, C08003, 13 PP. doi:10.1029/2006JC004033



Figure 3: 0.5% AEP still water level event hydrograph

2.2.4 EXTREME OFFSHORE WAVE EVENT

The SMP2 reports offshore extreme wave heights, as shown in Table 1. The 1 in 200-year wave height was deduced from interpolation to be 7.7m. The wave period was determined to be 13.5s by averaging the periods of waves with wave heights in the range 7.5-8.0m in the CEFAS WaveNet wave buoy datasets obtained for the Forth and Tyne. A storm period of 30 hours was chosen, during which time the offshore significant wave heights were set to gradually increase to the 1 in 200-year height and then decrease. The waves were specified as originating from the east, which represents a worst-case scenario as the spit is not protected from this direction of wave attack by the breakwater.

2.3 SANDSTELL POINT MORPHOLOGICAL VARIATION

One aim of this present study is to understand how the hydrodynamic and sediment transport regimes may change with various scenarios of Sandstell Point spit evolution. This is prompted by anecdotal evidence derived from historical mapping of significant evolution of the spit over time, revealing periods when the volume of sediment on the spit has significantly decreased. In order to investigate the effects of different spit morphologies on estuarine processes the bathymetry data is manually edited. Three scenarios of spit morphology were created. A present day ‘no erosion’ scenario was used, which uses the original 2010 survey data of the spit (Figure 4; bathymetric depths are shown to metres Below Ordnance Datum (mBOD)). A ‘partial erosion’ scenario was created by reducing elevation levels on the spit down to between -0.5 and 0.5 mBOD (Figure 5). This scenario includes a breached channel through the spit, added to represent anecdotal evidence of a history of breach channel creation. An ‘acute erosion’ scenario was created, representing catastrophic loss of spit sediment, by reducing the spit levels down to 1 mBOD (Figure 6).



Figure 4: Spit morphology: no erosion

Figure 5: Spit morphology: partial erosion

Figure 6: Spit morphology: acute erosion



The four hydrodynamic model scenarios (MHWS, extreme river, sea-level and wave events) were simulated for each of the three Sandstell Point spit scenarios (no, partial and acute erosion), leading to a total of twelve model simulations.

2.4 A MORPHOLOGY RESPONSE MODEL OF THE TWEED ESTUARY

In order to make predictions about how the Tweed Estuary morphology might respond to specific hydrodynamic conditions a suitable modelling technique was developed. The local hydrodynamic conditions define the ability of the estuarine flow to influence morphology development through the erosion and deposition of sediment around the estuary. This technique therefore used the model output from the hydrodynamic simulations as input data. A description of the modelling technique is presented herein.

2.4.1 MORPHOLOGY MAPPING EXERCISE

A morphology mapping exercise was carried out across the bar and spit features on the north and south banks of the Tweed downstream of the Royal Border Bridge. Following this exercise a number of sub-bar units were identified, distinguished on the basis of their surface sedimentology (Figure 7). These units were geo-referenced and added to a GIS database for comparison with the present day hydrodynamic conditions influencing their development.

Figure 7: Sub-bar morphologic units across the Tweed Estuary at Berwick-upon-Tweed

2.4.2 MORPHOLOGY MODEL DEVELOPMENT

The estuarine morphology is influenced by both tidal and fluvial processes. This influence is spatially variable across the estuary and temporally variable as flow and tide levels change. River flows are variable with flood flows having the most influence on estuary morphology. These flows interact with the tide, which is a process that needs to be viewed across the tidal cycle between low and high water levels. This is performed by integrating the relative hydrodynamic influence across a complete tidal cycle, in order to determine the relative spatial influence.



First, the numerical model was used to simulate local hydrodynamics across a single tidal cycle representing MHWS conditions, applying a mean flow rate at the upstream boundary. Then, the current magnitude and direction and water depth were extracted from the model for each hour across a full tidal cycle. The overall energy available to transport sediment and cause morphologic change was approximated by the product of the depth of water multiplied by the average velocity for each hour. Relative influences due to tidal and fluvial processes were determined by categorisation based on the local flow direction. The relative influence of fluvial or tidal processes was then determined across the complete tidal cycle by differencing the combined fluvial hourly energy index values and the combined tidal hourly energy index values. Figure 8 shows the differenced energy index values for the MHWS tide range simulation.

Figure 8: Energy index values for fluvial and tidally influenced flow conditions for typical flow conditions across the Tweed Estuary

These data represented a geo-referenced spatial picture of the energy levels across the estuary under MHWS tidal and average river flow conditions. The individual data point values were then linked directly to the geo-referenced current morphology map (Figure 9) and a database of morphologic unit based local hydrodynamics was created. This database was then used to define characteristic flow energy ranges associated with each morphology type (Figure 10).



Figure 9: Determination of fluvial and tidal energy levels and relative influence across the Tweed Estuary

Figure 10: Relationship between present fluvial/tidal energy levels and estuary morphology for use in the morphology response model

It is clear that there is a relationship between local tidal cycle energy levels and morphology type. Finer grained morphologies exhibit generally low fluvial and tidal energy levels, whereas rippled silts and sands are linked with increasing energy levels. Gravels and cobbles display the highest energy classes.

Local conditions across the estuary mean that these categories need to be modified to account for:

1. The differing spit morphology where an abundance of sand and fine gravels does not lead to a coarsening up of the surface to create cobble based morphologies;

2. The influence of bedrock which is close to and at the surface to the east and north of the estuary creating rocky areas rather than cobble zones under high energy conditions.



The extent of these underlying influences is shown in Figure 11.

Figure 11: Assumed extent of spit and bedrock influence on estuary morphology at Berwick-upon-Tweed

2.4.3 MORPHOLOGY RESPONSE MODEL APPLICATION

The methodology of the morphology response modelling was applied to the output from the twelve hydrodynamic model simulations described in section 2.3. The model predictions based on the output from the MHWS no spit erosion scenario was designated as the baseline, which was compared with the present-day observed morphology map. The results of the various modelling scenarios then provided predictions of morphology change from this baseline.

2.5 SUMMARY

The methodology used for performing an assessment of the hydrodynamic and morphology processes within the Tweed Estuary has been described. Hydrodynamic processes were simulated using a coupled tide-wave modelling system that simulates tide levels and currents, flow rates at the upstream boundary and wave processes. The model was used to simulate typical MHWS conditions within the estuary, as well as extreme conditions in river flow, sea-level and offshore wave activity. A technique was developed to predict the morphological responses of the estuary to these hydrodynamic scenarios. This technique was applied to these hydrodynamic simulations for various scenarios of Sandstell Point spit erosion. The results are presented in the next chapter.



3 HYDRODYNAMIC AND MORPHOLOGY MODELLING RESULTS

3.1 INTRODUCTION

The methodology for the assessment of linkages between the hydrodynamic and sedimentary processes in the Tweed Estuary has been described in the previous chapter. The results of the hydrodynamic and morphology response modelling tasks are presented in this chapter. First the baseline conditions are described and the modelling techniques verified against observations. Then the predicted behaviour due to hydrodynamic extremes in the Tweed Estuary is detailed.

3.2 TWEED ESTUARY BASELINE REGIME

The Tweed Estuary is a largely undisturbed habitat of intertidal mudflats, sandflats and gravelly littoral areas where the gross morphology is strongly controlled by intertidal processes. A conceptual model has been constructed that describes these processes

6, informed by on-site observations of bed forms and

hydrodynamics. This model links recorded changes with estuary processes operating at the morphologic unit (spit, channels, banks and dunes) upwards. Its design facilitates the evaluation of morphologic and physical habitat response to changing processes in the estuary system. Figure 12 shows the conceptual model of the general tidal and fluvial flows observed within the Estuary. The model shows that fluvial flows travel downstream and set against the south bank, then curve to the north past Sandstell Point before being driven into the sea towards the south-east by the north bank breakwater. River flows are weakest over the inter-tidal area of the north bank (e.g. Calot Shad Sands). Tidal flows propagate in from the south-east and flood the inter-tidal areas of Sandstell Point and Calot Shad, travelling upstream to the tidal limit. The effect of the flooding tide could be seen in the nature of bed forms in inter-tidal areas. Evidence of strong tidal currents is also visible over the exposed bedrock behind the breakwater at Innerstell Battery.

The hydrodynamic model results for typical Tweed Estuary processes (i.e. the MHWS simulation) were verified against this conceptual model. Figure 13 shows the current vectors from the 2D depth-averaged model for a MHWS ebbing tide. The fluvial influence can be seen clearly, with increased flow speeds to the south of Calot Shad and greatest currents out to sea along the southern face of the breakwater. Fluvial flow is minimal over the inter-tidal areas of Calot Shad Sands and Sandstell Point spit. Figure 14 shows the model currents for a MHWS flooding tide. Tidal flow can be seen to travel in from the south-east across Sandstell Point, flooding the north bank inter-tidal area and propagating upstream. These general ebbing and flooding tide model characteristics agree with those proposed by the conceptual model, providing verification for the hydrodynamic model.

Figure 12: Conceptual model of hydrodynamic flow in Tweed Estuary

6 Martin Wright Associates, 2010: Stage 1 ‘Conceptual Understanding’ Report”. Ref CH055/01



Figure 13: Hydrodynamic model showing ebbing tide depth-averaged currents

Figure 14: Hydrodynamic model showing flooding tide depth-averaged currents



A verification process was performed for the output of the morphology response model also. The model predictions based on the MHWS hydrodynamics were validated against the present morphology map (Figure 15). It is clear that the gross morphologic units are well predicted with coarse material seen across the southern bar surface of Calot Shad overlain by finer silts to the west. Silts and sands across the inner and central bar area are also well predicted. The extent of exposed bedrock is under-predicted across the Shad but the small area exposed on the southern bar downstream of Spittal Quay is predicted. The generally finer nature of the right bank bar between Crows Batt and Carr Rock is well picked up, as is the ribbon pattern of coarser and finer morphologies across Bailiffs Batt downstream of the multi-arched Berwick Bridge.

The morphology response of the Tweed Estuary to both partial and acute erosion of the spit during typical (MHWS) tidal flow was calculated. Under a scenario where the spit is partially breached a new channel has been simulated to the east of Spittal Quay as the palaeo-evidence preserved on the present day spit indicates that this is the point most vulnerable to incision. Flow occurs both around the spit to the north and through the breach to the south. Figure 16 shows the predicted morphological response of the Tweed Estuary in the top panel, with the difference from the MHWS no erosion baseline scenario (see bottom panel of Figure 15) given in the bottom panel. The major morphology change is one of a coarsening of the spit along the breach. This is accompanied by an increase of more silty units across the north of the spit associated with a loss of flow energy over present day due to the flow split. Siltation is also predicted across the south eastern margin of Calot Shad leading to a loss of bedrock dominated morphologies. Elsewhere, upstream of a line running north-south through Spittal Quay change is minimal, indicating that a breach would have little impact on estuary morphology upstream.

In the modelling scenario representing acute loss of the spit during MHWS conditions the pattern of change is similar to that for the partial breaching, with siltation over the east of Calot Shad and general stability elsewhere (Figure 17). In both of these scenarios increased tidal influence close to the mouth of the Tweed appears to be influencing the increased alleviation. These scenarios are useful indicators of the loci of recovery to the estuary should the more extreme scenarios described below occur. Recovery is likely to be fastest over Calot Shad, with finer alluvium only returning to a finer alluvial morphology more slowly.



Figure 15: Comparison of actual (top) and predicted (bottom) morphology for the Tweed Estuary based on present day conditions



Figure 16: Predicted morphology impact of partial spit erosion for MHWS conditions (top) and difference from MHWS no

erosion scenario baseline (bottom)



Figure 17: Predicted morphology impact of acute spit erosion for MHWS conditions (top) and difference from MHWS no erosion scenario baseline (bottom)



3.3 TWEED ESTUARY RESPONSE TO EXTREME RIVER FLOW

3.3.1 HYDRODYNAMIC RESPONSE

The model was run for an extreme river flow event lasting 3 days, during average offshore wave and neap tide conditions. The resulting flow speeds within the mouth of the estuary change significantly depending on the state of the tide. During the ebbing tide there is the least resistance to fluvial flow, leading to current magnitudes in the estuary reaching 2.5-3.0 m/s along the southern face of the breakwater (Figure 18). Flow speeds are typically 1.5-2.0 m/s along the river curve to the west of Sandstell Point

7. Such fast currents will

inevitably lead to significant erosion of susceptible areas along the estuary bed.

During a flooding tide the fast river flow still dictates the depth-averaged current direction in the estuary, with continued downstream flow, shown in Figure 19. The flooding tidal currents greatly reduce the downstream current magnitude however to 0.4-0.8 m/s. Areas of circulating water are evident due to competing river and tidal flows. At high water, when the contribution to currents from the tide is zero, river flow dictates the current magnitudes in the estuary. Figure 20 shows these currents which can be 1.0-1.2 m/s over inter-tidal areas. Crucially such magnitudes are significantly greater than critical values

8 of current speed required to

erode the non-cohesive sand characteristic of Sandstell Point. Therefore significant erosion of the spit is likely during a river spate at this time of the tide.

Figure 18: Model currents for 1:200 year river flow: ebbing tide

7 Due to the lack of an observation period of detailed current measurements it has not been possible to calibrate and validate the model currents. The model current results therefore contain a degree of uncertainty that should be considered.

8 Brown, E., A. Colling, D. Park, J. Phillips, D. Rothery, J. Wright, 2005: Waves, tides and shallow-water processes. Open University



Figure 19: Model currents for 1:200 year river flow: flooding tide

Figure 20: Model currents for 1:200 year river flow: high water



3.3.2 MORPHOLOGY RESPONSE

No erosion

Under an extreme river flood flow with Sandstell Point spit present changes are likely to be dramatic. Widespread stripping of fine sediments is predicted from all bar surfaces apart from a tidally influenced patch on the eastern side of Calot Shad (Figure 21). Marginal gravels and sands remain to the north on this bar. Elsewhere across this bar and over the entire southern bar surfaces cobbles dominate.

The northern margin of the spit also displays coarser sub-bar morphology. Bedrock is exposed on the southern bank east of Spittal Quay. It is clear from these results that flood flows along the Tweed have a major role to play in influencing the morphology of the estuary.

Partial erosion

The pattern of change predicted for an extreme river flood coupled with a spit breach (a more realistic scenario than the previous one given the energy of the flood flow) is very similar to that of the ‘no erosion’ scenario. This would indicate that the state of the spit has little influence on estuarine morphological change at the sub-bar level (Figure 22).

Acute erosion

The story is even more extreme for a major flood that removes the spit completely (Figure 23). All bar surfaces are completely stripped of fine sediment leaving extensive cobble areas across the estuary. The loss of the spit fails to concentrate tidal flows through the north of the estuary mouth and as a result fluvial forces dominate across the eastern margins of Calot Shad. This results in the complete exposure of bedrock across the area rather than the usual tidal induced siltation. This prediction is interesting in that it suggests that extreme fluvial flows are responsible for maintaining the bedrock morphologies seen across the eastern margin of Calot Shad, with progressive alluviation likely during other events.



Figure 21: Predicted morphology impact of extreme fluvial flow on no spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



Figure 22: Predicted morphology impact of extreme fluvial flow on partial spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



Figure 23: Predicted morphology impact of extreme fluvial flow on acute spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



3.4 TWEED ESTUARY RESPONSE TO EXTREME SEA-LEVELS


A model simulation of an extreme sea-level event was performed, with average river flow and wave conditions. The 1:200 year return period for a still water level is 3.43 mODN. This constitutes a massive transport of water in to and out of the estuary on the rising and falling tides respectively, leading to significant current speeds. Figure 24 shows the maximum current magnitudes during the flooding tide. Large currents propagating in to the estuary occur over a wide area, reaching 0.6-0.7 m/s over Sandstell Point. The ebbing currents, shown in Figure 25, are of a similar magnitude and spatial distribution to the flooding currents. Magnitudes are slightly larger over the spit at 0.8-0.9 m/s, but not as great as those shown in the extreme river flow event. The large spatial extent of significant current magnitudes is likely to have ramifications for wide-spread geomorphological effects during an extreme sea-level event.

Figure 24: Model currents for 1:200 year sea-level: flooding tide



Figure 25: Model currents for 1:200 year sea-level: ebbing tide


No erosion

Under conditions of extreme sea-level and present day spit levels there is a significant change predicted to the morphology across Calot Shad with sands and gravels extending across the western side of the bar (Figure 26). Fining on the eastern margin leads to a loss of significant areas of exposed bedrock. Farther upstream on the south bank the bar between Crows Batt and Carr Rock displays a variable response with bank edge siltation and gravels exposed closer to the main channel. The sub-bar morphologic diversity seen across Bailiffs Batt bar is lost as fine sediments are removed by stronger tidal flows exposing a more uniform coarse sediment bar.

Partial erosion

An extreme sea-level event combined with a breached spit results in a coarser breach channel through the spit to the south (Figure 27). Across Calot Shad the coarsening is more extensive, grading from cobbles at the western bar edge through gravels and sands extending farther east. Siltation across the eastern margin persists under this scenario covering bedrock dominated morphologies. Coarsening across the southern bank bars is more general.

Acute erosion

Sands extend across large areas of Calot Shad under extreme sea-level conditions with the loss of the spit, restricting silty morphologies to northern areas (Figure 28). Sands and silts also cover the bedrock to the east. Again the bars on the southern bank show significant and widespread coarsening with most silty morphologies lost.



Figure 26: Predicted morphology impact of extreme sea-levels on no spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



Figure 27: Predicted morphology impact of extreme sea-levels on partial spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



Figure 28: Predicted morphology impact of extreme sea-levels on acute spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



3.5 TWEED ESTUARY RESPONSE TO EXTREME WAVE ACTION


The coupled tide-wave model system was run for a simulation of a storm characterised by extreme offshore wave heights of 7.7m. As these waves propagate to the shoreline they break due to depth-limiting effects. A transfer of momentum towards the shoreline occurs due to this wave-breaking, which is manifested as a force known as wave radiation stress. This force leads to a build-up of water at the shoreline referred to as ‘wave set-up’. This simulation examines the contribution of extreme wave radiation stress and wave set-up to the hydrodynamics of the estuary.

Figure 29 shows the current magnitudes during the flooding tide with extreme offshore waves. Overall the currents are much reduced from those of the extreme sea-level flooding tide; however those over the spit are greater at 1.0-1.1 m/s. This is due to the significant wave breaking here leading to wave radiation stress forcing significant volumes of water into the estuary. Combined with large bed sheer stress due to the significant wave action in this area, these currents are likely to cause significant erosion of the non-cohesive sediment on the spit.

The currents during the ebbing tide are shown in Figure 30. The effects of the wave radiation stress lead to a circulation pattern on the spit as it forces water into the estuary, which in turn flows out with the tide along the southern face of the breakwater. Significant currents are forced up the Spittal frontage at both phases of the tide, due to the constant breaking of large waves along the face of the beach.

Figure 29: Model currents for a 1:200 year extreme wave storm: flooding tide



Figure 30: Model currents for a 1:200 year extreme wave storm: ebbing tide


No erosion

Under an extreme wave scenario with a full spit siltation extends more widely across the south eastern margin of Calot Shad. Some coarsening is predicted around Berwick Bridge (Figure 31).

Partial erosion

Where wave action is allowed to extend farther into the estuary, through the breaching of Sandstell Spit, silty morphologies develop along the southern edge of Calot Shad and across the diverse bar surface associated with Berwick Bridge (Figure 32).

Acute erosion

In this scenario there is a general coarsening across much of Calot Shad with sands replacing silts (Figure 33). The increased current energy impacting on the margin of Calot Shad causes some coarsening of the morphology effectively creating a gravely fringe. The bedrock to the east is covered by a mixture of gravels, sands and silts. On the southern bank the bar downstream of Berwick Old Bridge exhibits dramatic coarsening indicating that heightened current energy extends upstream to the bridge to remove finer sediments from all but the extreme inner margin of the bar. The bar between Crows Batt and Carr Rock also coarsens along its riverside edge but retains a more diverse morphology elsewhere.



Figure 31: Predicted morphology impact of extreme wave action on no spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



Figure 32: Predicted morphology impact of extreme wave action on partial spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



Figure 33: Predicted morphology impact of extreme wave action on acute spit erosion scenario (top) and difference from MHWS no erosion scenario baseline (bottom)



3.6 SUMMARY

This chapter has presented the results from the hydrodynamic modelling and morphological response modelling exercises. These results describe how the hydrodynamics in the Tweed Estuary behave under typical (MHWS) and extreme storm conditions in river flows, sea-levels and offshore wave action. The hydrodynamics exhibit significant differences which are dependent on the dominant forcing mechanism. During river spates this dominant forcing mechanism is river flow, during storm surges it is tidal currents and during large wave storms it is wave radiation stress. All modelled simulations reveal current magnitudes that are large enough to impact the estuary’s geomorphology. Indeed the morphology response modelling results predict wide-ranging and extensive changes to the estuary during the occurrence of extreme storm events. The nature of this response is different for each of the three storm types examined (river flow, sea-level and wave action). Conclusions derived from these results are presented in Chapter 6.



4 WAVE OVERTOPPING EVALUATION

4.1 INTRODUCTION

The Northumberland and North Tyneside SMP2 identified that defences in the Tweed Estuary may require significant effort to bring them up to an adequate standard of protection, both for present day climate conditions and future climate conditions. The SMP2 document also indicates that the performance of these defences may be sensitive to the potential changes in estuary morphology discussed in the previous chapters. Therefore an assessment of the risk of wave overtopping at the present day defences around the estuary was performed. This assessment consisted of calculating wave overtopping rates at three separate defence locations within the estuary for present day conditions, as well as for future projections of sea-level rise and a reduced-volume spit morphology scenario. Finally, the need to increase flood defence levels to afford protection from rising sea-levels was evaluated. The assessment methodology and results are presented within this chapter.

4.2 WAVE OVERTOPPING ASSESSMENT METHODOLOGY

4.2.1 WAVE MODELLING

The hydrodynamic model described in section 2.2 (and in detail in Appendix A) was used to simulate wave action in the Tweed Estuary. The model is a coupled tide-wave model, which simulates tidal elevations and currents as well as the transformation of waves from offshore to the coastline. This type of model is particularly of relevance for the Tweed Estuary due to its ability to simulate wave set-up: a process that increases the sea-level along coastlines during times of significant wave action.

The model was used to simulate a 1:200-year offshore wave/extreme sea-level storm event, thereby calculating the sea-levels and wave conditions that would prevail within the estuary during such an event. The parameters of offshore wave height and sea-level for a 1:200-year storm were provided by the SMP2. The simultaneous occurrence of a 1:200-year wave height and a 1:200-year sea-level does not equate to a 1:200-year combined sea-level/wave storm. Therefore a joint probability analysis

9 was undertaken to produce four

equally plausible combinations of water level and offshore wave heights which constituted a wave/sea-level storm event with a 1:200-year return period. These combinations are presented in Table 2. The first permutation in these tables combines a higher wave with a lower sea level, while the last permutation constitutes a lower wave with a higher sea level. The remaining two permutations represent intermediate wave height and sea levels.

The model was run for all storm scenarios by specifying the appropriate sea-level and wave parameters at the model ocean boundary, with the effects of an appropriate storm wind speed also included

10. Wave direction

was specified as originating from the east, so that the performance of the defences within the inner estuary due to waves not blocked by the breakwater could be assessed. The output from the model simulations provided characteristic nearshore wave properties at the bases of the defence assessment locations for use in the wave overtopping calculations.

9 Use of Joint Probability Methods in Flood Management: A guide to best practice. R&D Technical Report FD2308/TR2, produced by DEFRA/Environment Agency Flood and Coastal Defence R&D Programme, 2005

10 The effect of wind is also accounted for in the wave model. Wind velocity and direction is supplied to the model which computes the effect of this on wave heights. The wind was forced from the same direction as the waves. The magnitude of the wind was determined using return periods. A close approximation of the return period of a given wind speed is the return period of the wave created by it. The return periods of the waves used in the 1:200-year joint probability permutations are known. Using the method described in McConnel (1998) the wind speeds for each return period were calculated.



Table 2: Combinations of sea-level and wave parameters constituting a joint probability 1:200-year storm off of Berwick

1:200-year combination Sea-level (mODN) Offshore significant wave height (m)

Wave peak period (s)

Permutation 1 2.44 7.31 12.9

Permutation 2 2.76 5.60 11.1

Permutation 3 3.08 3.24 8.7

Permutation 4 3.38 1.44 6.8

4.2.2 WAVE OVERTOPPING CALCULATIONS

The rate at which water is likely to discharge over a defence due to wave overtopping processes was calculated using the latest tools and methods provided by the EurOtop project

11. In order to calculate overtopping

discharges at a sea defence these tools require wave characteristics at the base of the defence (provided by the wave modelling) and a profile of the defence structure. The tools used in this study are PC Overtopping and Empirical Method

12.

Guidance on appropriate limits for overtopping rates at sea defences for various conditions are provided by EurOtop. These are reproduced in Table 3 to Table 6 for reference.

Table 3: Overtopping limits for pedestrians

Category Mean Discharge (l/s/m)

Trained staff, well shod and protected, expecting to get wet, with overtopping at lower levels only, no falling jet and low danger of fall from walkway

1-10

Aware pedestrian, clear view of the sea, not easily upset or frightened, able to tolerate getting wet, wider walkway

0.1

Unaware pedestrian, no clear view of sea, easily upset or frightened, not dressed to get wet, on narrow walkway or close to trip or fall hazard

0.031

Table 4: Overtopping limits for vehicles


Driving at low speed, overtopping by pulsating flows at low flow depths, no falling jets, vehicle not immersed

10-50

Driving at moderate or high speed, impulsive overtopping giving falling or high velocity jets 0.01-0.05

11 Pullen, T. Allsop, N.W.H. Bruce, T. Kortenhaus, A. Schuttrumpf, H. Van Der Meer, J. W., 2007: EurOtop Wave Overtopping of Sea Defences and Related Structures: Assessment Manual

12 The wave overtopping methods provided in EurOtop, including the PC Overtopping Method, can be used to calculate either Probabilistic or Deterministic wave overtopping estimates. It is generally accepted that the Probabilistic results are more realistic than the Deterministic results. This stems from the fact that the Deterministic Method outputs are simply the Probabilistic values plus one standard deviation. The addition of a standard deviation is a measure of conservatism. The results presented in this report for all wave overtopping calculations are based on the more realistic Probabilistic Method. Clearly, if and when the defences are re-designed, a margin of safety will need to be included. The results from Deterministic outputs could help to inform this.



Table 5: Overtopping limits for property


Significant damage or sinking of larger yachts 50

Sinking of small boats set 5-10m from wall/damage to larger yachts 10

Building structure elements 1

Damage to equipment set 5-10m back from wall 0.4

Table 6: Overtopping limits for damage to defences


Damage to paved or armoured promenade behind seawall 200

Damage to grassed or lightly protected promenade or reclamation cover 50

4.2.3 ASSESSMENT LOCATIONS

Three representative vulnerable defence locations in the estuary were chosen for evaluation, namely those at the south bank, Sandstell Point and the north bank, as shown in Figure 34. These defences were chosen for evaluation because of their exposure to wave action and the potential impact that erosion of Sandstell Point spit could have on them.

Figure 34: Locations of defences for which wave overtopping calculations were performed



South bank defences

The south bank defences takes the form of collapsed gabions fronting a low grass embankment adjacent to Dock Road (Figure 35). The defences are approximately 400m in length and protects the road, a footway and residential and commercial property, including the Berwick Shellfish Company. Because of the nature of these defences, the problem is one of wave run-up and over-washing, which has less of an impact than impulsive overtopping. That is to say that, because the defence has a gentler slope, waves are more likely to wash over the structure than to break upon it. The wave overtopping method used for the south bank defence was the PC Overtopping method that forms part of EurOtop. This method is designed for dikes, embankments and rubble mound structures and is deemed the most appropriate at this location.

Figure 35: View across Calot Shad Sand to the south bank defences

Sandstell Point defences

The defences at Sandstell Point takes the form of an embankment reinforced with rock armour (Figure 36). The embankment, which faces the open coast and is approximately 290m in length, protects a small car park, footpath and a road which leads to Spittal promenade further south. This defence is of particular interest because it may be the subject of increased wave action if Sandstell Point spit erodes. It is also a location where pedestrians may be exposed to wave action during a storm, due to the close proximity of the foorpath.

The nature of wave overtopping here is likely to be impulsive, whereby waves will break upon contact with the structure, producing a high velocity jet which can entrain beach material. As this breaker jet falls, people and property beneath it are at risk from falling debris and potentially large volumes of water. The wave overtopping method used for the Sandstell Point defence was PC Overtopping.

Figure 36: Sandstell Point defences



North bank defences

The defences at the north bank of the Tweed Estuary is a simple wall backed by Pier Road. Behind the wall are seating for pedestrians, car parking spaces and adjoining residential property. The length of the defence is approximately 400m. For the overtopping calculations the north bank defence was approximated as a simple vertical wall. The calculations were performed using the EurOtop Empirical Method, which is designed for the assessment of wave overtopping processes at vertical and steep-fronted coastal structures. It is also a location where pedestrians may be exposed to wave action during a storm event.

Figure 37: Defence wall at the north bank

4.2.4 SANDSTELL POINT SPIT LOSS SCENARIO

The Northumberland and North Tyneside SMP2 identifies that existing defences on the Tweed Estuary play a vital role in protecting local residential and commercial properties. It was deemed that the defences within the estuary are sensitive to changes in the local morphology. At the mouth of the estuary, Sandstell Point spit acts to dissipate waves that propagate into the estuary. The spit is therefore a flood defence itself, in that it reduces the height of waves that reach the defences along the river banks within the estuary. There is concern that, should significant erosion of the spit occur, its dissipating effects will decrease and larger waves will be able to propagate into the estuary. These larger waves could lead to an increase in the flood risk due to wave overtopping. Therefore an additional wave modelling scenario was performed using the ‘acute erosion’ of the spit scenario described in section 2.3. A comparison of the results of the overtopping calculations for this scenario with that of the present day (‘no erosion’) case highlights the degree of wave overtopping risk mitigation that the spit provides.

4.2.5 ACCOUNTING FOR CLIMATE CHANGE PROJECTIONS

Sea-levels are projected to rise with climate change in the near future. As a consequence the susceptibility of the defences in the estuary to wave overtopping will increase in the future. This susceptibility was assessed by calculating the wave overtopping discharges based on predictions of future sea-levels. Calculations were performed using sea-level rise predictions provided by both current DEFRA guidance

13 and UKCP09

14

13 Department for Environment Food and Rural Affairs (2006) Flood and Coastal Defence Appraisal Guidance FCDPAG3 Economic Appraisal, Supplementary Note to Operating Authorities - Climate Change Impacts, October, 2006

14 Lowe, J.A., Howard, T., Pardaens, A., Tinker, J., Holt, J., Wakelin, S., Milne, G., Leake, J., Wolf, J., Horsburgh, K., Reeder, T., Jenkins, G., Ridley, J., Dye, S., Bradley, S. (2009), UK Climate Projections Science Report: Marine and Coastal Projections. Met Office Hadley Centre, Exeter, UK. p30



projections. The DEFRA projections are the current official guidance for the UK, whereas UKCP09 projections are from more recent research that has yet to be assimilated into official guidance. The three climate change horizons of +20, +50 and +100 years were evaluated; the projected increases in mean sea-level in the north-east of England due to these predictions are given in Table 7. It is worth noting that the predictions based on more recent research (UKCP09) forecast much lower increases than current DEFRA guidance.

Table 7: Predicted increases in sea-level with climate change at Berwick

Future period (years) Predicted rise in mean sea-level (m)

DEFRA 2006 UKCP09

+20 0.08 0.06

+50 0.30 0.17

+100 0.88 0.43

Finally, further overtopping calculations are performed in order to assess the likely improvements required to ‘at risk’ defences to mitigate for predicted future increases in sea-level.

4.3 WAVE OVERTOPPING RESULTS

The results of the wave overtopping calculations are presented in this section. These overtopping rates are given for:

3 defence locations within the Tweed Estuary (the south bank, Sandstell Point and the north bank);

2 scenarios of spit morphology (present day and catastrophic loss);

4 sea-level scenarios (present day, +20 years, +50 years and +100 years);

2 sea-level rise projection sets (DEFRA and UKCP09).

The most relevant results are reported in this section with a full list of results presented in Appendix C.

It is important to recognise that wave overtopping calculations are inherently uncertain. This stems from the fact that wave overtopping models are generally empirical in nature, based on a limited set of laboratory and field experiments. In particular, it is very difficult to measure wave overtopping for real life defences, meaning that the calibration of the models for real defences is limited. Whilst the latest techniques for estimating overtopping discharges were used in this study, the EurOtop methods are very sensitive to the assumptions made for a defence in terms of defence profile and variations in defence height along a structure; the method is also very sensitive to the empirical methods chosen for the analysis.



4.3.1 SOUTH BANK DEFENCE

Table 8 to Table 11 show the wave overtopping discharges at the south bank defence for a 1:200-year joint probability wave/water level storm event. Table 8 gives the results for the present day morphology of the spit for DEFRA sea-level projections. Table 9 gives the results for UKCP09 climate projections. Table 10 and Table 11 give the corresponding results for the scenario of catastrophic Sandstell Point spit loss. The tables show the wave parameters at the base of the sea defence as well as the sea-level. It should be noted that these sea-levels are typically higher than those specified in Table 7 as these levels include a contribution due to the process of wave set-up. Overtopping rates are reported in units of litres per second per metre (l/s/m).

Table 8: Overtopping discharges for 1:200 year event at south bank defence (present day spit; DEFRA 2006 projections)

Relative sea-level rise horizon (years)

Wave height/water level permutation (see Table 2)

Incident Wave Height at defence (m)

Peak Period (s)

Still Water Level (mODN)

Overtopping Discharge (l/s/m)

0 3 0.96 8.3 3.12 0.1

20 3 0.99 8.3 3.20 0.9

50 3 1.08 8.3 3.41 3.9

100 2 1.23 11.1 3.89 30.6

Table 9: Overtopping discharges for 1:200 year event at south bank defence (present day spit; UKCP09 projections)




Peak Period (s)



0 3 0.96 8.3 3.12 0.1

20 3 0.98 8.3 3.18 0.8

50 3 1.02 8.3 3.29 1.6

100 3 1.12 8.3 3.54 7.2

Table 10: Overtopping discharges for 1:200 year event at south bank defence (catastrophic spit loss; DEFRA 2006 projections)




Peak Period (s)



0 1 1.23 12.5 3.08 4.1

20 1 1.29 12.5 3.16 7.1

50 2 1.42 11.1 3.32 18.4

100 2 1.83 11.1 3.85 156.6



Table 11: Overtopping discharges for 1:200 year event at south bank defence (catastrophic spit loss; UKCP09 projections)




Peak Period (s)



0 1 1.23 12.5 3.08 4.1

20 1 1.28 12.5 3.14 6.4

50 1 1.36 12.5 3.24 11.8

100 2 1.52 11.1 3.45 33.2

The results show that for the present day scenario, the rate of overtopping expected during a 1:200-year event is 0.1 l/s/m. Whilst this is above the EurOtop maximum recommended tolerance for wave overtopping based on "unaware pedestrians" (refer to Table 3), it is close to the limit recommended for "aware pedestrians" (refer to Table 3). This is considered to be a sensible target limit for this defence.

Not surprisingly, the wave overtopping calculations illustrate that flood risk at the south bank defence will increase steadily in line with climate change projections. These projections illustrate that after just 20 years of sea-level rise, the overtopping discharges are expected to exceed the "aware pedestrian" category. After 50 years, the wave overtopping discharges are expected to disrupt vehicular transport (Table 4) and potentially cause damage to adjacent building structure elements (Table 5). These assessments are valid for both DEFRA and UKCP09 projections, though DEFRA guidance predicts significantly more overtopping at the +100 year horizon.

To reduce the overtopping rate expected after 100 years of sea-level rise (DEFRA guidance predicts 30.6 l/s/m) to the target discharge of 0.1 l/s/m (a limit that would provide protection against all of the risks highlighted above), the south bank crest level would need to be raised to more than 8.2 mODN (the current crest level is approximately 5.4 mODN). This advice is based on a simple increase in the elevation of the crest of the embankment. Clearly, an increase in crest level of 2.8 m is probably impractical and will not be welcomed by the public. It is more likely the case that the more appropriate manner to reduce overtopping for this defence will be to install a rock revetment or similar to dissipate wave action; this could be done in conjunction with some crest raising to provide a sensible compromise. It is beyond the scope of this study to detail potential structure designs; this is something which will merit future investigation as the situation develops.

The results for the catastrophic spit loss scenario highlight the significant defence role the spit plays by depth-limiting incoming waves for the inner Tweed Estuary. For present day sea-levels the loss of the spit would lead to a flood risk at the south bank defences greater than would be the case due to 50 years of sea-level rise with the spit intact. DEFRA projections of sea-level rise at the +100 year horizon for the lost spit scenario lead to predictions of wave overtopping rates (156.6 l/s/m) that represent a significant risk to structures along the south bank. Therefore if the spit does erode defence improvements would be required significantly earlier and to a higher standard given that waves will be able to propagate towards the defences more freely.



4.3.2 SANDSTELL POINT DEFENCE

Table 12 presents the overtopping results for the Sandstell Point defence, including results due to DEFRA projections of sea-level rise. Table 13 gives the results including UKCP09 projections of sea-level rise. Results from the spit loss scenario are not presented as these are effectively the same. This is because the spit is to the north of the defence and does not affect waves propagating towards the coastline directly from the east.

Table 12: Overtopping discharges for 1:200 year event at Sandstell Point defence (present day spit; DEFRA 2006 projections)




Peak Period (s)



0 1 0.84 12.5 3.08 0.03

20 1 0.91 12.5 3.17 0.1

50 1 1.03 12.5 3.32 0.1

100 2 1.48 11.1 3.89 0.3

Table 13: Overtopping discharges for 1:200 year event at Sandstell Point defence (present day spit; UKCP09 projections)




Peak Period (s)



0 1 0.84 12.5 3.08 0.03

20 1 0.89 12.5 3.14 0.1

50 1 0.98 12.5 3.25 0.1

100 1 1.13 12.5 3.45 0.2

The results show that for the present day scenario, the rate of overtopping expected during a 1:200-year event is very small at 0.03 l/s/m. This is below the EurOtop target tolerance for "unaware pedestrians". Whilst climate change is expected to increase wave overtopping at this defence, the maximum overtopping limit after 100 years for both DEFRA and UKCP09 projections is expected to remain below 0.4 l/s/m. To limit overtopping to below the target threshold of 0.1 l/s/m, defence improvements would be required in approximately 50 years time. To upgrade the defence to the DEFRA +100 year climate change horizon, based on the target threshold of 0.1 l/s/m, would require the crest to be raised by approximately 0.8 m to 7.8 mODN; improvements to the revetment may have a similar effect and should be considered if this defence is to be improved.



4.3.3 NORTH BANK DEFENCE

Table 14 and Table 17present the wave overtopping discharges at the north bank defence for a 1:200-year joint probability wave/water level storm event. Table 14 gives the results for the present day morphology of the spit for DEFRA sea-level projections. Table 15 gives the results for UKCP09 climate projections. Table 16 and table 17 give the corresponding results for the scenario of catastrophic Sandstell Point spit loss.

Table 14: Overtopping discharges for 1:200 year event at north bank defence (present day spit; DEFRA 2006 projections)




Peak Period (s)



0 3 1.08 8.3 3.13 62.6

20 3 1.11 8.3 3.21 90.3

50 3 1.21 8.3 3.42 295.3

100 4 0.65 6.7 4.26 Water level above defence height

Table 15: Overtopping discharges for 1:200 year event at north bank defence (present day spit; UKCP09 projections)




Peak Period (s)



0 3 1.08 8.3 3.13 62.6

20 3 1.10 8.3 3.19 81.4

50 3 1.14 8.3 3.30 133.4

100 2 1.21 11.1 3.55 336.0

Table 16: Overtopping discharges for 1:200 year event at north bank defence (catastrophic spit loss; DEFRA 2006 projections)




Peak Period (s)



0 3 1.24 8.3 3.10 102.4

20 3 1.30 8.3 3.18 159.0

50 2 1.42 11.1 3.32 202.4

100 4 0.67 6.7 4.26 Water level above defence height



Table 17: Overtopping discharges for 1:200 year event at north bank defence (catastrophic spit loss; UKCP09 projections)




Peak Period (s)



0 3 1.24 8.3 3.10 102.4

20 3 1.28 8.3 3.16 139.9

50 2 1.32 11.1 3.21 106.9

100 2 1.51 11.1 3.45 430.0

The results show that for the present day scenario, the rate of overtopping expected during a 1:200-year event is very large at 62.6 l/s/m. This rate exceeds almost all of the EurOtop recommended tolerances. After 50 years of sea-level rise the predicted overtopping rate during such a storm is likely to cause significant damage to adjacent structures. After 100 years, for the DEFRA projections, the storm still water level would exceed the level of the defence leading to significant flooding of Pier Road behind. The lower projections of sea-level rise from UKCP09 also show very large increases in overtopping rates at the defences in the future.

The elevation of the north bank defence is presently 4.04 mODN, (1.36m and 1.6m less than the south bank and Sandstell Point defences, respectively) which is low relative to the other defences investigated herein. To reduce the level of overtopping to below the target threshold of 0.1 l/s/m will simply be impractical without a major new flood defence scheme. To reduce the overtopping rate to below 0.1 l/s/m based on present day sea level alone would require an increase in the defence crest level of approximately 6 m. As discussed above with respect to the south bank defence, it will probably be more feasible to improve the defences based on a combined revetment/embankment type approach, or some other design which dissipates wave energy. The development of an appropriate defence would require a more detailed design study, informed by the results presented herein.

Results for the catastrophic spit scenario, as with the south bank defence, highlight the role the spit plays in protecting the inner estuary from large waves

15. Therefore if the spit does erode defence improvements would

be required significantly earlier and to a higher standard given that waves will be able to propagate towards the defence more freely.

15 It is seen that after 50 years of sea-level rise the rate of discharge seems to reduce. The cause of the reduction may be linked to the type of overtopping. According to the Empirical Calculation Tool, the discharge of 295 l/s/m is impulsive. The discharge of 202 l/s/m is non-impulsive. Impulsive overtopping occurs where a wave height and water level combine to create a discharge over the defence crest which is sudden and violent.



4.4 SUMMARY

This chapter has described the methodology and results of a wave overtopping assessment of three representative sea defences within the Tweed Estuary. The defences are at Sandstell Point, the south bank and the north bank of the inner estuary. The assessment method used the coupled tide-wave hydrodynamic model to transform offshore wave heights to the base of the defences for a 1:200-year joint probability wave height/water level storm. This technique benefits from the ability of the modelling system to simulate wave set-up: a significant contributor to still water level along the coastline during times of large offshore waves. Standard wave overtopping calculation routines, provided by EurOtop, were then used to evaluate the predicted overtopping rates due to multiple combinations of wave height and water level that comprise a 1:200-year storm event. The potential increases in these rates due to projected climate change-induced sea-level rise were calculated using both DEFRA guidance and UKCP09 predictions. Furthermore the flood defence properties of Sandstell Point spit were highlighted by assessing overtopping rates within the estuary for a scenario of catastrophic spit loss. The results show that, for present day conditions, the defences at Sandstell Point, south bank and north bank provide good, adequate and poor protection against wave overtopping risk respectively.

In summary the overall findings are tabulated below.

Location

DEFRA UKCP09

With Spit Without Spit With Spit Without Spit

South Bank +2.8 +2.8

Sooner

+1.7 +1.7

Sooner

North Bank +6.0 +6.0

Sooner

+6.0 +6.0

Sooner

Sandstell Point +0.8 +0.8 +0.3 +0.3

All figures are based on a 1:200 return period, allowing for 100 years of climate change.



5 ENVIRONMENTAL IMPLICATIONS OF NATURAL MORPHOLOGICAL VARIATION AND STORM EVENTS

5.1 INTRODUCTION AND CONTEXT

The aims and objectives of any future management of the costal and fluvial defences around the Tweed Estuary should take into consideration ecological and wider environmental requirements. The Stage 1 ‘Conceptual Understanding Report’ included a description of environmental features, including species and habitats and recreational resources that characterise the estuary. In the event of any change in the estuary’s management, it is possible that legislative requirements pertaining to the management of internationally and nationally designated sites would be enacted.

Outputs of hydrodynamic modelling described in chapter 2 were used as inputs to the Morphology Response Model to investigate morphological responses of the estuary to variations in the extent of erosion of Sandstell Point Spit and different fluvial, tidal and sea level regimes. The outputs of the Morphology Response Model are described in section 3. Changes to the current geomorphological baseline are likely to affect habitat distribution across the estuary. This is especially the case for the Tweed Estuary as its intertidal habitats are constrained by existing hard defences and development and are therefore liable to be ‘squeezed’. This chapter will describe the current habitats in terms of the constraints and opportunities they present, and investigate how habitats and environmental resources may be impacted by the modelled changes in morphology described in chapter 2. All modelling was undertaken by Jeremy Benn Associates.

5.2 ENVIRONMENTAL CONSTRAINTS AND OPPORTUNITIES

As shown in Figure 38, the study area constitutes part of the Tweed Estuary Special Area of Conservation (SAC), and part of the Lower Tweed and Whiteadder Site of Special Site of Scientific Interest (SSSI). At its seaward end, the study area borders the Berwickshire and North Northumberland Coast SAC. SACs are protected under the Habitats Directive

16, as implemented in England by the Conservation of Habitats and Species Regulations

201017

(Habitats Regulations), whereby any plan or project which

is likely to have a significant effects on a European site (either alone or in combination with other plans or projects); and

is not directly connected with or necessary to the management of that site,

must be subject to appropriate assessment as to the implications for that site in view of that site’s conservation objectives. In the light of the conclusions of the assessment, consent may only be granted after it has been ascertained that the plan or project will not adversely affect the integrity of the site. SSSIs are also provided protection through domestic law including the Countryside and Rights of Way Act 2000 and the Wildlife and Countryside Act 1981 (as amended).

The qualifying interests of the Tweed Estuary SAC are:

estuaries

mudflats and sandflats not covered by seawater at low tide

sea lamprey (Petromyzon marinus)

16

Habitats Directive (Council Directive 92/43/EEC on the Conservation of Natural Habitats and of Wild fauna and Flora, as amended)

17 Conservation of Habitats and Species Regulations 2010 SI 490. Replaces 1994/2716. Implements the EU

Habitats Directive in respect of conservation of natural habitats and wild flora and fauna. Also partially implement parts of Marine and Coastal Access Act 2009.



river lamprey (Lampetra fluviatillis)

The conservation objective for the Tweed Estuary SAC is to maintain in favourable condition the habitats for which it is designated (estuary, and mudflats and sandflats).

The qualifying interests of the Berwickshire and North Northumberland Coast SAC are:

mudflats and sandflats not covered by seawater at low tide

large shallow inlets and bays

reefs

submerged or partially submerged sea caves

grey seal (Halichoerus grypus)

Advice prepared under Regulation 33(2) of the Conservation (Natural Habitats &c.) Regulations 199418

, in support of, (a) the conservation objectives and (b) determination of any operations which may cause deterioration of natural habitats or the habitats of species, or disturbance of species for the Berwickshire and North Northumberland Coast European marine site, contains conservation objectives, as well as a table describing determinands of ‘favourable condition’ for each of the interest features of the SAC, including mudflats and sandflats not covered by seawater at low tide. In relation to mudflats and sandflats, competent authorities are advised that human activities should not result in deterioration or disturbance as a result of any of the following:

Physical loss through removal or smothering.

Physical damage through siltation and/or abrasion and/or selective extraction.

Synthetic and/or non-synthetic toxic contamination.

Nutrient and/or organic enrichment and/or changes in turbidity.

Biological disturbance through the selective extraction of species.

A significant area of rocky shore can be found just inside the pier on the northern shore. This habitat is not specifically mentioned as a primary reason for the selection of the Tweed Estuary SAC, although ‘estuaries’ are, and rocky reefs may form part of the complex association of habitats characteristic of estuaries

19. This area,

along with areas of sandflats, and muddy and rocky intertidal sediments form a diverse mosaic of habitats which support a diverse assemblage of flora and fauna, in close to a natural state.

Any plan or project which could affect the integrity of either or both of the listed SACs, such as a coastal or flood risk management strategy, would need to be subjected to assessment under the Habitats Regulations. Depending on the outcome of initial screening, a full Appropriate Assessment may be required.

The Phase 1 Habitat Survey described in the Stage 1 ‘Conceptual Understanding Report’ identified habitat distribution across the estuary. Important habitats found by this survey, for example BAP habitats and those that are qualifying interests for the Tweed Estuary SAC, have been identified as constraints on Figure 38 also identifies other constraints such as important recreational resources. No opportunities have been identified to create or enhance habitat within the study area as space is limited by development and existing flood defences.

18 English Nature, Scottish Natural Heritage (2000). Berwickshire and North Northumberland Coast European Marine Site SAC. Advice given in compliance with Regulation 33 (2) and in support of the implementation of The Conservation (Natural Habitats &c.) Regulations 1994. 19 JNCC. Marine, coastal and halophytic habitats – Estuaries. Accessed at http://www.jncc.gov.uk/protectedsites/sacselection/habitat.asp?FeatureIntCode=H1130 on May 5, 2010.

http://www.jncc.gov.uk/protectedsites/sacselection/habitat.asp?FeatureIntCode=H1130



Figure 38: Environmental Constraints



5.3 EFFECTS OF SCENARIOS

11 alternative scenarios (to the current baseline), comprising varied extents of spit development and discrete extreme tidal, fluvial and wave events, were investigated using the Morphology Response Model as described in chapter 3 (the derivation of the extreme events is described in section 1.4.1). This section summarises the morphological changes modelled in response to each scenario, and describes environmental impacts according to a range of relevant ecological and recreational criteria. Assessment of impacts under each scenario will in future be useful to consider the environmental effects of different management options, e.g. no further intervention or maintenance of existing defences.

Based on the environmental baseline audit and consultation responses as reported in sections 4 and 5 of the Stage 1 ‘Conceptual Understanding’ Report, the criteria listed below were derived in order to assess the potential environmental, recreational and heritage impacts of the scenarios. The criteria, and assessment against them, were informed by the constraints shown on Figure 38

1. Are existing habitats, including qualifying interests of the Tweed Estuary SAC and Berwickshire and North Northumberland Coast SAC, and BAP habitats, likely to be compromised?

Both of the SACs list ‘mudflats and sandflats not covered by seawater at low tide’ as qualifying interests. Regulation 33 advice for the Berwickshire and North Northumberland Coast European Marine Site

20 advises

competent authorities that human activities should not result in deterioration or disturbance as a result of physical loss through removal or smothering, physical damage through siltation and/or abrasion and/or selective extractions, synthetic and/or non-synthetic toxic contamination, nutrient and/or organic enrichment and/or changes in turbidity, or biological disturbance through the selective extraction of species.

A significant area of rocky shore can be found just inside the pier on the northern shore. This habitat is not specifically mentioned as a primary reason for the selection of the Tweed Estuary SAC, although ‘estuaries’ are, and rocky reefs may form part of the complex association of habitats characteristic of estuaries

21.

The key factors associated with decline of saltmarsh and mudflat (BAP habitats) are land reclamation, disruption of processes maintaining the habitat due to flood defences, coastal squeeze and erosion and a reduction in vegetation diversity due to invasion of common cord grass (Spartina anglica).

2. Are any important species identified either as Priority species or BAP species likely to be impacted (sea lamprey, river lamprey, Atlantic salmon, allis shad, sea trout, eel, otter, common seal)

3. Are there likely to be any ecosystem benefits, such as new or compensation habitats?

4. Is the integrity of recreational resources such as footpaths, and the use of the estuary for sailing, angling or other pursuits, likely to be compromised?

5. Is the integrity of cultural heritage, such as archaeological monuments, likely to be compromised?

20

English Nature, Scottish Natural Heritage (2000). Berwickshire and North Northumberland Coast European Marine Site SAC. Advice given in compliance with Regulation 33 (2) and in support of the implementation of The Conservation (Natural Habitats &c.) Regulations 1994.

21 JNCC. Marine, coastal and halophytic habitats – Estuaries. Accessed at http://www.jncc.gov.uk/protectedsites/sacselection/habitat.asp?FeatureIntCode=H1130 on May 5, 2010.

http://www.jncc.gov.uk/protectedsites/sacselection/habitat.asp?FeatureIntCode=H1130



5.3.1 SCENARIO 1 – MHWS PARTIAL EROSION

The major morphology change is one of a coarsening of the spit along the breach (Figure 24). This is accompanied by an increase of more silty units across the north of the spit associated with a loss of flow energy over present day due to the flow split. Siltation is also predicted across the south eastern margin of Calot Shad leading to a loss of bedrock dominated morphologies. Elsewhere, upstream of a line running north-south through Spittal Quay change is minimal, indicating that a breach would have little impact on estuary morphology upstream.

Criteria Assessment Response Assessment Rating

1. Are existing habitats, including qualifying interests of the Tweed Estuary SAC, and BAP habitats, likely to be compromised?

The current distribution and variation of mudflats and sandflats would be affected to a limited extent. Overall there would be net deposition of finer silts/sands across the north of the spit and at the upstream margin of Calot Shad. This may cause some smothering of existing coarser habitats, including rocky shore, cobbles and gravels. This may be balanced to an extent by coarsening of habitats through the breach. There would be little impact on habitats upstream of Calot Shad.

Minor impact

2. Are any important species identified either as Priority species or BAP species likely to be impacted (sea lamprey, river lamprey, Atlantic salmon, allis shad, sea trout, eel, otter, common seal)?

Passage of migratory species is unlikely to be affected.

Value of the estuary as seal habitat is also unlikely to be affected – it may actually be slightly improved if access to the spit is restricted and disturbance reduced.

Negligible

3. Are there likely to be any ecosystem benefits, such as opportunities to create new or compensation habitats?

Unlikely, as the estuary is constrained by development on all sides.

No impact


Partial erosion of the spit may restrict access to the estuary for angling and other recreation.

The characteristics of the estuary may be affected, in terms of speed and direction of currents, and nature of winds, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse.

Minor impact


No impacts No impact



5.3.2 SCENARIO 2 – MHWS, ACUTE EROSION

The pattern of change is similar to that for spit breaching (Scenario 1) with siltation over the east of Calot Shad and general stability elsewhere (Figure 25). In this scenario and scenario 1, increased tidal influence close to the mouth of the Tweed appear to be influencing the increased alluviation. The loss of Sandstell Point spit itself is a major morphological change.



Sandstell Point spit, currently comprising fine, clean mobile sand, would be lost as a sandflat habitat.

There would be deposition of finer silts/sands across the upstream margin of Calot Shad, including over currently exposed bedrock habitats. There would be little impact on habitats upstream of Calot Shad.

Significant Impact



Wading bird feeding habitats may be increased by fining of cobbles/gravels to silts.

Negligible


Unlikely, as the estuary is constrained by development on all sides.

No impact


Loss of the spit may restrict access to the estuary for angling and other recreation.

The characteristics of the estuary may be affected, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse. The loss of the spit would dramatically affect the sheltered nature of the inner estuary.

Significant impact


No impacts No impact



5.3.3 SCENARIO 3 – EXTREME SEA-LEVEL, NO EROSION

Under conditions of extreme sea level and present day spit levels there is a significant change to the morphology across Calot Shad with sands and gravels extending across the western side of the bar (Figure 26)Fining on the eastern margin leads to a loss of significant areas of exposed bedrock. Further upstream on the south bank the bar between Crows Batt and Carr Rock displays a variable response with bank edge siltation and gravels exposed closer to the main channel. The sub-bar morphologic diversity seen across Bailiffs Batt bar is lost as fine sediments are removed by stronger tidal flows exposing a more uniform coarse sediment bar.



Coarser substrate habitats are likely to be smothered on Sandstell Point and on the eastern edge of Calot Shad, including smothering of existing exposed bedrock and cobble/boulder habitats. On the western side of Calot Shad, habitats may become coarser and more predominated by gravels. Upstream habitat diversity may also be impacted, for example at Bailiffs Batt bar, where substrate would be coarsened from sands/gravels to cobbles. The intertidal zone at Davies Batt would continue to provide a diverse range of habitats. Distribution of habitats would be changed but limited impact to habitat diversity.

Significant impact



Extent of wading bird feeding grounds is unlikely to be impacted.

Negligible


Unlikely, as the estuary is constrained by development on all sides

No impact


Modelled morphological changes would be unlikely to affect recreational use of the estuary.

The characteristics of the estuary may be affected, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse.

Minor impact


Extreme sea levels cause influence on morphology further upstream, so it is conceivable that the supports of Berwick Bridge could be impacted.

The town’s fortifications, bordering Calot Shad, could also be impacted by extreme sea levels.

Minor impact



5.3.4 SCENARIO 4 – EXTREME SEA-LEVEL, PARTIAL EROSION

An extreme sea-level event combined with a breached spit results in a coarser breach channel through the spit to the south (Figure 27). Across Calot Shad the coarsening is more extensive grading from cobbles at the western bar edge through gravels and sands extending further east. Siltation across the eastern margin persists under this scenario covering bedrock dominated morphologies. Coarsening across the southern bank bars is more general.



Generally substrates become coarser, particularly across the western half of Calot Shad. Across the eastern half there is a net loss in the area of exposed bedrock due to smothering by sand. Sandflat habitats on the southern bank are also generally coarsened under this scenario. Distribution of habitats would be changed but limited impact to habitat diversity.

Significant impact


Passage of migratory species is unlikely to be affected. Unlikely to be impact on other species, although wading bird populations may be adversely affected by change in mudflat/sandflat distribution to coarser substrates.

Minor impact



No impact


Partial erosion of the spit may restrict access to the estuary for angling and other recreation.

The characteristics of the estuary may be affected, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse.

Minor impact




Minor impact



5.3.5 SCENARIO 5 – EXTREME SEA-LEVEL, ACUTE EROSION

Sands extend across large areas of Calot Shad under extreme sea-level conditions with the loss of the spit, restricting silty morphologies to northern areas (Figure 28). Sands and silts also cover the bedrock to the east. Again the bars on the southern bank show significant and widespread coarsening with most silty morphologies lost.



Sandstell Point spit, currently comprising fine, clean mobile sand, would be lost as a sandflat habitat, although there would be some exposure of bedrock at Spittal Point. Habitats on Calot Shad and the southern bank would be impacted by the shift to generally sandier substrates, with loss of gravel, bedrock and cobble habitats. Habitat diversity would be reduced.

Significant impact


Passage of migratory species is unlikely to be affected. Unlikely to be impact on other species, although wading bird populations may be adversely affected by change in mudflat/sandflat distribution to coarser substrates.

Minor impact



No impact



The characteristics of the estuary may be affected, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse. The loss of the spit would dramatically affect the sheltered nature of the inner estuary.

Significant impact




Minor impact



5.3.6 SCENARIO 6 – EXTREME FLUVIAL, NO EROSION

Under an extreme river flood flow with Sandstell Point spit present, changes are likely to be dramatic. Widespread stripping of fine sediments is predicted from all bar surfaces apart from a tidally influenced patch on the eastern side of Calot Shad (Figure 29). Marginal gravels and sands remain to the north on this bar. Elsewhere across this bar and over all of the southern bar surfaces cobbles dominate.

The northern margin of the spit also displays coarser sub-bar morphology. Bedrock is exposed on the southern bank east of Spittal Quay. It is clear from these results that flood flows along the Tweed have a major role to play in influencing the morphology of the estuary.



Much of the diversity characteristic of the mosaic nature of the current habitat distribution would be lost.

Fine silts, sands and rocky shore habitats would all be vastly reduced in area, making way for a more homogenous habitat of widespread cobbles.

Highly significant impact



Wading bird populations are likely to be adversely affected by change in mudflat/sandflat distribution to coarser substrates

Significant impact



No impact


The characteristics of the estuary may be affected by extreme river flows, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse.

Exposure of bedrock along the southern bank may affect recreational access to the river.

Significant impact


Extreme river flows could impact Berwick Bridge.

Minor impact



5.3.7 SCENARIO 7 – EXTREME FLUVIAL, PARTIAL EROSION

The pattern of change predicted for an extreme river flood coupled with a spit breach (a more realistic scenario than the previous one given the energy of the flood flow) is very similar to scenario 6 (full spit). This would indicate that the state of the spit has little influence on estuarine morphological change at the sub-bar level (Figure 30).



Much of the habitat diversity characteristic of the mosaic nature of the current habitat distribution would be lost.

Fine silts, sands and rocky shore habitats would all be vastly reduced in area, making way for a more homogenous habitat of widespread cobbles.




Wading bird populations may be adversely affected by change in mudflat/sandflat distribution to coarser substrates

Significant impact



No impact


The characteristics of the estuary may be affected by extreme river flows and partial erosion of the spit, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse.

Exposure of bedrock along the southern bank may affect recreational access to the river.

Significant impact



Minor impact



5.3.8 SCENARIO 8 – EXTREME FLUVIAL, ACUTE EROSION

The story is even more extreme for a major flood that removes the spit completely (Figure 31). All bar surfaces are completely stripped of fine sediment leaving extensive cobble areas across the estuary. The loss of the spit fails to concentrate tidal flows through the north of the estuary mouth and as a result fluvial forces dominate across the eastern margins of Calot Shad. This results in the complete exposure of bedrock across the area rather than the usual tidal induced siltation. This prediction is interesting in that it suggests that extreme fluvial flows are responsible for maintaining the bedrock morphologies seen across the eastern margin of Calot Shad, with progressive alluviation likely during other events.



Much of the habitat diversity characteristic of the mosaic nature of the current habitat distribution would be lost.

Mudflats and sandflat habitats would disappear almost entirely, to make way for cobbles and bedrock.

Sandstell Point spit would be lost as a sandflat habitat.




Wading bird populations likely to be adversely affected by change in mudflat/sandflat distribution to coarser substrates throughout study area..

Significant impact



No impact


Loss of the spit may restrict access to the estuary for angling and other recreation. The characteristics of the estuary would be affected by extreme river flows and erosion of the spit, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse. The loss of the spit would dramatically affect the sheltered nature of the inner estuary. Exposure of bedrock and shift to coarser cobble substrates along the southern bank may affect recreational access to the river.

Significant impact



There may be impacts on the town fortifications that border Calot Shad.

Minor impact



5.3.9 SCENARIO 9 – EXTREME WAVE, NO EROSION

Under an extreme wave scenario with a full spit, siltation extends more widely across the south eastern margin of Calot Shad. Some coarsening is predicted around Berwick Bridge (Figure 32).



Rocky shore habitat at the eastern end of Calot Shad would be smothered by sand, and there would be a net loss of gravel substrate habitats. There would be coarsening of habitats around the fringe of Calot Shad and upstream towards Berwick Bridge. Little overall reduction in habitat diversity.

Significant impact



Unlikely to be impact on other species.

No impact



No impact


The characteristics of the estuary may be affected by extreme wave conditions.

Modelled changes in morphology are not expected to affect the recreation use of the estuary.

Minor impact


No impacts

No impact



5.3.10 SCENARIO 10 – EXTREME WAVE, PARTIAL EROSION

Where wave action is allowed to extend further into the estuary through the breaching of Sandstell Spit, silty morphologies develop along the southern edge of Calot Shad and across the diverse bar surface associated with Berwick Bridge.



Habitat distribution would be altered significantly, and much of the silt habitat on Calot Shad would be lost making way for sand/gravel habitats, and cobble around the fringe.

The area of rocky shore habitat to the east of Calot Shad would be lost, as would some of the cobble habitat across the bar.

There would be some fining of the substrate upstream on Bailiffs Batt bar




Wading bird populations may be adversely affected by loss of silty habitats.

Significant impact



No impact


The characteristics of the estuary may be affected by extreme wave conditions and partial erosion of the spit, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse.

Minor impact


Extreme waves could impact Berwick Bridge.


Minor impact



5.3.11 SCENARIO 11 – EXTREME WAVE, ACUTE EROSION

In this scenario there is a general coarsening across much of Calot Shad with sands replacing silts. The increased current energy impacting on the margin of Calot Shad causes some coarsening of the morphology effectively creating a gravely fringe. The bedrock to the east is covered by a mixture of gravels, sands and silts. On the southern bank the bar downstream of Berwick Bridge exhibits dramatic coarsening indicating that heightened current energy extends upstream to the bridge to remove finer sediments from all but the extreme inner margin of the bar. The bar between Crows Batt and Carr Rock also coarsens along its riverside edge but retains a more diverse morphology elsewhere.



Habitat distribution would be altered, and some of the silt habitat and much of the gravel on Calot Shad would be lost. The area of rocky shore habitat to the east of Calot Shad would be lost.

Sandstell Point spit would be lost as a sandflat habitat. There would be some exposure of bedrock at Spittal Point.

Habitats on the southern bank would be coarsened.




Wading bird populations may be adversely affected by general coarsening of habitats.

Significant impact



No impact



The characteristics of the estuary may be affected by extreme wave conditions and complete erosion of the spit, in terms of speed and direction of currents, which could in turn affect the use of the estuary for sailing and other recreation. It is not known whether effects would be beneficial or adverse. The loss of the spit would dramatically affect the sheltered nature of the inner estuary.

Significant impact


Extreme waves could impact Berwick Bridge.


Minor impact



5.4 SUMMARY OF ENVIRONMENTAL IMPLICATIONS

Effects of the various scenarios on the selected criteria are summarised in Table 18. In its existing state the estuary is a mosaic of sandy, muddy, and rocky intertidal sediments supporting diverse flora and fauna that in the absence of discernible pollution exist in a close to natural state. Modelling has indicated that significant morphological change, and therefore change to the mosaic distribution of habitats, is likely under all of the scenarios investigated. Under some scenarios, for example extreme sea level with the spit retained, changes are limited to the distribution of habitats, and there is little or no net loss of biotope diversity. In other cases, for example under extreme fluvial conditions, a significant loss of habitat diversity is predicted. Effects appear to be magnified with increasing extent of spit erosion. The erosion and loss of Sandstell Point would in itself be a significant change as it provides sandflat habitat.

Under normal sea level, wave and fluvial conditions (Scenarios 1 and 2), impacts are mainly limited to shifts in the distribution of habitats, with the most change seen when the spit is completely eroded. However, siltation is predicted on the eastern side of Calot Shad, which would result in smothering of the rocky shore habitat.

More significant changes to habitat distribution through the estuary are predicted under conditions of extreme sea level. Again, siltation is predicted towards the east of Calot Shad, including smothering of the rocky shore, with coarsening of substrates upstream.

With progressive spit erosion, extreme waves would cause fining of substrates to the east of the estuary and coarsening of substrates to the west and upstream towards Berwick Bridge. The rocky shore habitat on the east of Calot Shad would be smothered.

The most significant changes to habitat diversity are predicted to occur under extreme fluvial conditions, which cause widespread coarsening of substrate. When these conditions are combined with acute erosion of the spit, habitat would be almost entirely limited to cobble and exposed bedrock to the east. The loss of finer substrates would affect feeding requirements of wading birds such as turnstones (Arenaria interpres) and oystercatchers (Haematopus ostralegus), both of which were observed during the walkover survey reported in the Stage 1 report.

The predicted changes to morphology and associated habitat indicate that management options, particularly those that would reduce intervention compared to the current baseline, potentially resulting in failure of the breakwater, would be likely to affect the habitats for which the Tweed Estuary SAC is designated. These options would therefore be liable to assessment under the Habitats Regulations. In order to maintain the habitats in favourable condition, the present level of management, including maintenance of the breakwater would need to be continued. It should be noted that as the events featured by each of the scenarios were extremes, the morphology of the estuary would tend to return to that characteristic of MHWS conditions after extreme events. For example, after a fluvial event of 3 days duration as was modelled, deposition and siltation processes would start to re-establish finer substrates over the exposed cobbles and bedrock across Calot Shad.

Modelled morphological changes would also cause impacts to the use of the estuary as a recreational resource. Again the loss of Sandstell Point spit would affect recreational use of the estuary directly, as it currently provides a route of access for anglers and walkers. Its erosion would dramatically alter the sheltered nature of the inner estuary with consequences for sailing and other waterborne recreation. Coarsening of substrates would affect recreational use of the estuary in other ways, such as by reducing ease of boat access. Changes to bottom substrates will impact fauna, including fish populations.

The scheduled monuments of Berwick Bridge and fortifications that border Calot Shad may become increasingly exposed to erosive forces with progressive erosion of the spit. It is conceivable that extreme fluvial conditions may impact the supports of Berwick Bridge.

Due to the constrained nature of the estuary, which is bordered on all sides by development as far upstream as the Royal Border Bridge, there are no obvious opportunities for estuarine habitat creation within the study area. There may be opportunities further upstream or along the coast, for example in conjunction with the Northumberland Shores Project.



Table 18: Summary of Environmental Implications Significance

1 –

MH

WS,

Part

ial

2 –

MH

WS,

Acut

e

3 –

Ext

rem

e

Sea

Leve

l, N

o

eros

ion

4 –

Extr

eme

Sea

Leve

l,

Part

ial

5 –

Extr

eme

Sea

Leve

l,

Acut

e

6 –

Extr

eme

Fluv

ial,

No

eros

ion

7 –

Extr

eme

Fluv

ial,

Part

ial

8 –

Extr

eme

Fluv

ial,

Acut

e

9 –

Extr

eme

Wav

e, N

o

Eros

ion

10 –

Ext

rem

e

Wav

e, P

artia

l

11 –

Ext

rem

e

Wav

e, E

ntire

1.

Ar

e ex

istin

g ha

bita

ts,

incl

udin

g qu

alify

ing

inte

rest

s of

the

Twee

d Es

tuar

y SA

C, a

nd

BAP

habi

tats

, lik

ely

to b

e

com

prom

ised

?

Min

or

impa

ct

Sign

ific

ant

Impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Hig

hly

sign

ific

ant

impa

ct

Hig

hly

sign

ific

ant

impa

ct

Hig

hly

sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Hig

hly

sign

ific

ant

impa

ct

Hig

hly

sign

ific

ant

impa

ct

2.

Ar

e an

y im

port

ant

spec

ies

iden

tifie

d ei

ther

as

Prio

rity

spec

ies

or B

AP s

peci

es

likel

y to

be

impa

cted

(sea

lam

prey

, riv

er la

mpr

ey,

Atla

ntic

sal

mon

, alli

s sh

ad, s

ea

trou

t, ee

l, ot

ter,

com

mon

sea

l)?

Neg

ligi

ble

Neg

ligi

ble

Neg

ligi

ble

Min

or

impa

ct

Min

or

impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

No

impa

ctSi

gnif

ican

t

impa

ct

Sign

ific

ant

impa

ct

3.

Ar

e th

ere

likel

y to

be

any

ecos

yste

m b

enef

its, s

uch

as o

ppor

tuni

ties

to c

reat

e ne

w

or c

ompe

nsat

ion

habi

tats

?

No

impa

ctN

o im

pact

No

impa

ctN

o im

pact

No

impa

ctN

o im

pact

No

impa

ctN

o im

pact

No

impa

ctN

o im

pact

No

impa

ct

4.

Is

the

inte

grity

of

recr

eatio

nal r

esou

rces

suc

h as

foot

path

s, a

nd th

e us

e of

the

estu

ary

for s

ailin

g, a

nglin

g or

othe

r pur

suits

, lik

ely

to b

e

com

prom

ised

?

Min

or

impa

ct

Sign

ific

ant

impa

ct

Min

or

impa

ct

Min

or

impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Sign

ific

ant

impa

ct

Min

or

impa

ct

Min

or

impa

ct

Sign

ific

ant

impa

ct

5.

Is

the

inte

grity

of

cultu

ral h

erita

ge, s

uch

as

arch

aeol

ogic

al m

onum

ents

,

likel

y to

be

com

prom

ised

?

No

impa

ctN

o im

pact

Min

or

impa

ct

Adve

rse

Adve

rse

Min

or

impa

ct

Min

or

impa

ct

Min

or

impa

ct

No

impa

ctM

inor

impa

ct

Min

or

impa

ct



6 SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

6.1 INTRODUCTION

The requirement for this present study arose from the findings of the SMP2 for Northumberland and North Tyneside, which found that within the Tweed Estuary:

1. Some of the defences on both banks of the estuary mouth require significant effort to bring them up to a good condition standard;

2. The defences are sensitive to changes in the morphology at the mouth of the estuary, particularly the sandbanks, sand spit, channel, dunes and beaches around Sandstell Point and at Spittal shore;

3. In the future there may be a need to increase existing defence levels to afford protection against rising sea levels.

As a result the present study involves a numerical modelling study of relevant processes in order to examine these important issues. This report details the development and application of a methodology for examining the potential hydrodynamic and morphological changes in the Tweed Estuary due to natural morphological variation and storm events. It also details an assessment of the current flood risk at vulnerable points within the estuary, predicting how this risk changes with estuarine morphological change and projections in sea-level rise. General advice is provided on the time-scales needed to raise defence levels in order to counter increased flood risk due to these projections.

6.2 ASSESSMENT METHODOLOGY

The estuary is particularly susceptible to change driven by storm events, which can lead to considerable river flows, extreme sea-levels and massive offshore wave heights. Therefore a numerical model of the estuary was developed that incorporated these separate hydrodynamic processes. This model took the form of a coupled tide-wave 2D depth-averaged model, using a hydrodynamic circulation model and a nearshore wave transformation model. The modelling system simulates tide levels and currents, upstream flows and varying wave conditions within the Tweed Estuary. Model scenarios were developed that characterised typical conditions within the estuary, as well as extreme conditions in river flow, sea-level and offshore wave heights, representing the 1:200-year return period event. Scenarios of Sandstell spit morphology were also developed in order to assess the impact on hydrodynamics and potential morphology response of variations in estuary morphology. These corresponded to situations of no erosion (present day), partial spit erosion and acute spit erosion; the latter representing a worst-case scenario of massive spit erosion.

A modelling technique was developed in order to assess the likely morphology response within the estuary to the hydrodynamic and morphological scenarios described above. The model was based on a calculation of the available energy for sediment transport, derived from the output from the hydrodynamic model simulations. Available energy was calculated at each location within the estuary and separated into fluvial and tidal influences. This energy map then corresponded to a morphology map. The potential impact to morphology at a location due to a model scenario was then assessed by comparison with a baseline morphology map. This baseline case was specified as that due to typical MHWS conditions within the estuary. The model predictions of this case were validated against a morphology map constructed from on-site observations, revealing that the gross morphologic units were well predicted.



6.3 HYDRODYNAMICS OF THE ESTUARY

The 2D numerical model was run for the various permutations of the hydrodynamic and morphological scenarios. The simulation of typical conditions revealed good agreement with the dynamics proposed by the previously-developed conceptual model of the estuary.

In the simulation of extreme river flow the resulting flow speeds within the mouth of the estuary change significantly depending on the state of the tide. During the ebbing tide there is the least resistance to fluvial flow, leading to current magnitudes in the estuary reaching 2.5-3.0 m/s along the southern face of the breakwater. Flow speeds are typically 1.5-2.0 m/s along the river curve to the west of Sandstell Point. At high water, when the contribution to currents from the tide is zero, river flow dictates the current magnitudes in the estuary which can be 1.0-1.2 m/s over inter-tidal areas and therefore likely to lead to significant erosion.

During extreme sea-level events the tidal currents are far greater than during typical tidal conditions, but not as large as those produced during the extreme river flow scenario. The model simulation of a 1:200-year level of 3.43 mODN reveals currents over 0.6 m/s over a wide area on the flooding and ebbing tide, hinting at the potential for large-scale morphology change during such an event.

The effects of extreme offshore waves are manifested in large wave radiation stress forcing water into the estuary. The model simulation shows significant current speeds as a result of this force over the spit of 1.0-1.1 m/s. This force can oppose the directions of river flow and ebbing tide, leading to a circulation pattern developing immediately to the north of the spit on the ebbing tide.

6.4 MORPHOLOGICAL RESPONSES WITHIN THE ESTUARY

It would appear from the model results that significant morphological change at the sub-bar scale is likely under all of the scenarios. Figure 39 summarises these morphological responses for all scenarios investigated. Breaching or loss of the spit under normal MHWS conditions has the greatest impact over the eastern margin of Calot Shad with siltation leading to a loss of bedrock morphology; impacts elsewhere are minimal.

Extreme fluvial events cause dramatic coarsening of all morphologies in the estuary and exposure of extensive areas of bedrock to the east of Calot Shad. Tidal influences are minor, especially when the spit is removed which otherwise acts to concentrate tidal flow along the northern breakwater. It would appear that extreme fluvial events are critical in maintaining the bedrock morphologies in the estuary and prevent loss through siltation.

Extreme sea-level conditions tend to coarsen morphologies along the western edge of Calot Shad and over southern bar features; this effect increases as spit volume decreases. Siltation of bedrock morphologies is predicted regardless of spit state.

Extreme wave events result in a complex tidal/fluvial interaction and minor coarsening across Calot Shad. Wave influence appears to extend further into the estuary following loss of the spit causing the morphology around Berwick Old Bridge to coarsen.

There appears to be a general propensity for morphologies to coarsen west of a line running north – south through Carr rock under any of the extreme event scenarios. Extreme fluvial events cause the greatest change. Following the predicted morphological response of an extreme event recovery of the Tweed Estuary morphology will be governed by normal MHWS conditions. The results from the MHWS modelling show that this will be most rapid over Calot Shad, reducing up the estuary.



Figure 39: Summary of morphological responses to various hydrodynamic and spit condition scenarios for the Tweed Estuary

The condition of the spit plays a sub-dominant role in influencing estuary morphology with broadly similar patterns of change predicted regardless of spit state. There is, however, a propensity for the magnitude of change to increase with increasing spit erosion. The presence of extensive areas of gravel and cobble and the exposure of bedrock across eastern areas of Calot Shad, point to the importance of extreme fluvial flows in maintaining the present estuary morphology (and in supplying new coarse material to the estuary bars from upstream). Periodic stripping of accumulated silts and sands by extreme events continues to be important to reset the system, which otherwise would have a propensity to silt up under the normal tide and wave conditions.

The predictions of morphological response to extreme hydrodynamic behaviour within the estuary are likely to have ramifications for navigational considerations. Significant changes are predicted to the morphology along the banks of the river channel and across the large inter-tidal area on the north bank. Consideration needs to be given to these changes where they overlap with navigational routes. Extreme fluvial flows are predicted to strip the morphology down to bedrock over large areas. Given that occasional dredging is performed in the estuary it may be the case that such extreme flows will reduce the amount of dredging that is required.



6.5 ENVIRONMENTAL IMPLICATIONS OF NATURAL MORPHOLOGICAL VARIATION AND STORM EVENTS

Modelling of changes to estuary morphology in response to the various scenarios indicates that the current mosaic of intertidal habitats of varying substrates, which exists in a close to natural state, would be significantly affected under extreme fluvial, wave and sea level conditions, such that habitat distribution and diversity would be reduced. Modelling showed that effects would be exacerbated were Sandstell Point spit to be partially or completely eroded. Furthermore, the erosion and loss of Sandstell Point spit would in itself be a significant change as it provides sandflat habitat.

Extreme fluvial conditions were predicted to cause the most significant changes to habitat diversity. High river flows were shown to remove finer sediments resulting in restricted habitat diversity over Calot Shad. When these conditions are combined with acute erosion of the spit, habitat would be almost entirely limited to cobble and exposed bedrock to the east. The loss of finer substrates would affect feeding requirements of wading birds such as turnstones (Arenaria interpres) and oystercatchers (Haematopus ostralegus), both of which were observed during the walkover survey reported in the Stage 1 report. In practice it is likely that extreme fluvial conditions, characteristic of a storm, would be combined with extreme wave and sea level conditions. After events, morphologies would start to return to those typical of non-extreme conditions.

The predictions show that certain management options, particularly those that would reduce intervention compared to the current baseline, potentially resulting in failure of the breakwater, would be likely to influence the habitats for which the Tweed Estuary SAC is designated. Such management options would therefore be liable to assessment under the Habitats Regulations.

Modelled morphological changes would also cause impacts to the use of the estuary as a recreational resource. Again the loss of Sandstell Point spit would affect recreational use of the estuary directly, as it currently provides a route of access for anglers and walkers. Its erosion would dramatically alter the sheltered nature of the inner estuary with consequences for sailing and other waterborne recreation. Coarsening of substrates would affect recreational use of the estuary in other ways, such as by reducing ease of boat access. Changes to bottom substrates will impact fauna, including fish populations.

The scheduled monuments of Berwick Bridge and fortifications that border Calot Shad may become increasingly exposed to erosive forces with progressive erosion of the spit. It is also conceivable that extreme fluvial conditions may impact the supports of Berwick Bridge.

6.6 WAVE OVERTOPPING FLOOD RISK ASSESSMENT

This study has investigated these issues at three locations within the estuary that represent vulnerable areas, namely South Bank, Sandstell Point and North Bank. These defences were chosen for evaluation because of their exposure to wave action and the potential impact that erosion of Sandstell Point spit could have on them.

The wave overtopping assessment method used the coupled tide-wave hydrodynamic model to transform offshore wave heights to the base of the defences for a 1:200-year joint probability wave height/water level storm. Standard wave overtopping calculation routines, provided by EurOtop, were then used to evaluate the predicted overtopping rates due to multiple combinations of wave height and water level that comprise a 1:200-year storm event. The potential increases in these rates due to projected climate change-induced sea-level rise were calculated using both DEFRA guidance and UKCP09 predictions. Furthermore the flood defence properties of Sandstell Point spit were highlighted by assessing overtopping rates within the estuary for a scenario of catastrophic spit loss.

The results confirm that the estuary is indeed vulnerable (in terms of flood risk) to both climate change-induced sea-level rise and the morphology of Sandstell Point spit. The defence at Sandstell Point provides good protection against present day wave overtopping flood risk, when compared with the suggested safety thresholds supplied by EurOtop. Improvements to the defence would be required in approximately 50 years time in order to offset the increased overtopping risk posed by predicted sea-level rise. To upgrade the defence to the DEFRA +100 year climate change horizon, based on a suitable target threshold of 0.1 l/s/m,



would require the crest to be raised by approximately 0.8 m to 7.8 mODN. However improvements to the revetment may have a similar effect and should be considered if this defence is to be improved.

The defence at the south bank provides adequate protection against present day wave overtopping flood risk. However predicted increases in sea-level after 50 years will lead to a decrease in this level of protection to levels of risk that could potentially cause structural damage during a 1:200-year storm. To reduce the overtopping rate expected after 100 years of sea-level rise to the target discharge of 0.1 l/s/m the south bank crest level would need to be raised by 2.8 m to 8.2 mODN. This advice is based on a simple increase in the elevation of the crest of the embankment. It is more likely the case that the more appropriate manner to reduce overtopping for this defence will be to install a rock revetment or similar to dissipate wave action; this could be done in conjunction with some crest raising to provide a sensible compromise.

The defence at the north bank of the Tweed Estuary currently provides poor protection against present day rates of predicted wave overtopping during a 1:200-year storm. This defence consists of an approximate vertical wall backed by a road and property, which are therefore currently at risk of flooding. The degree of risk is predicted to increase significantly so that after 100 years of sea-level rise a 1:200-year storm water level will overtop the defence crest height. To reduce the overtopping rate to below 0.1 l/s/m based on present day sea level alone would require an increase in the defence crest level of approximately 6 m. It will probably be more feasible to improve the defences based on a combined revetment/embankment type approach, or some other design which dissipates wave energy.

The modelling results show that the spit plays a major role in the protection of the inner estuary defences from wave attack. It acts to depth-limit waves propagating into the estuary from the east. If the spit were allowed to erode, larger waves would be able to travel into the estuary during storms and lead to a greater risk of wave overtopping.

It is important to stress that the guidance recommended in EurOtop is indicative only. Furthermore, the recommendations provided herein with respect to defence improvements are general and indicative only. This assessment does not represent a full appraisal of flood risk for Berwick. This would require the commissioning of a more encompassing Strategic Flood Risk Assessment. This assessment only considers three representative defences in Berwick that are considered to be particularly vulnerable to sea-level rise and morphological changes in the estuary.

6.7 RECOMMENDATIONS

The results and conclusions of the present study lead to the following recommendations:

The current standard of protection provided by the rock armour/embankment sea defence at Sandstell Point should be maintained. An assessment should be made at a later date in order to upgrade the defence in approximately 50 years time to counter the increased overtopping risk presented by predicted sea-level rise. To protect against the DEFRA +100 years sea-level rise prediction the crest level should be raised by 0.8m, or further wave-dissipating alterations to the defence should be made.

The south bank collapsed gabions/embankment sea defence should be upgraded within 50 years to offset the increased overtopping risk posed by 50 years of predicted sea-level rise. To reduce the overtopping rate expected after 100 years of predicted sea-level rise the crest height should be raised by 2.8 m, or a redesign of the defence should be undertaken to provide a more fficient solution.

The north bank vertical wall sea defence requires immediate upgrading in order to provide an adequate level of protection for the affected infrastructure behind the defence. This upgrade will need to account for predicted rises in sea-level. An elevation of the defence crest level is likely to be unfeasible, given the large height involved. Therefore options for defence improvements should be considered within a Project Apprasial Report, or similar.

Significant erosion and lowering of Sandstell Point spit should be prevented, in order to maintain its significant role in mitigating wave overtopping flood risk in the inner estuary.



7 TWEED ESTUARY FURTHER STUDY

7.1 INTRODUCTION

The present study examines the changes to hydrodynamics and sediment transport regime in the mouth of the Tweed Estuary due to extreme events and natural variations in the development of Sandstell Point spit. The SMP2 recommends that a further detailed study be carried out to consider the modification of defences around Spittal and Sandstell Point, thereby changing flow patterns around the head, to create more sustainable conditions for dune development. The modelling technique developed here, which predicts morphological response due to hydrodynamic and morphological changes, is ideally suited for performing such a study. This chapter describes suggested objectives of this future study, along with details regarding how the modelling technique can be adapted to fulfil these objectives.

7.2 CONTEXT

The SMP2 for Northumberland and North Tyneside proposes two management approaches for Berwick-upon-Tweed's estuary and coastline. These are to adopt a policy of "No Active Intervention" or to continue "present management". The preferred practice in coastal management is to allow the coastline to realign itself, where possible, to its natural state, rather than employing hard-engineering defences. An advantage of re-alignment inland is that intertidal habitats are not drowned as sea-levels rise, but may be able to migrate inland. Maintaining hard-engineered structures in the coastal zone is also expensive. To determine which, if either, of these management approaches should be adopted a further study of Spittal Point is required. According to the SMP2, this investigation must be a:

"Detailed study to modify defences around Sandstell Point, thereby changing flow patterns around the head, creating more stable conditions for dune development and possibly limited areas of saltmarsh or mud flat".

To fully assess how the morphology response modelling technique developed for the present study could be used in future investigations, it is first necessary to understand the modifications which may be required to Spittal Point and the surrounding defences. In the discussion of the "No Active Intervention" management approach, the SMP2 reports that, were the breakwater to the north of the estuary mouth to be allowed to fail, Spittal Point would be eroded. The result of this would be that Spittal Beach will erode and the defences behind the beach will be undermined. Furthermore the course of the River Tweed may change so that its mouth is situated further north. The SMP2 suggests that the south bank of the estuary will become increasingly exposed and that use of the harbour will not be possible. To the north of the estuary, the northern dunes may be eroded and defensive walls may be undermined and fail. As a result, Pier House and Pier Road may be lost and further properties may be at risk.

In the discussion of the “present management” scenario SMP2 states that all defences would continue to be maintained. The document also reports that Berwick-upon-Tweed is reliant principally on the breakwater for its coastal protection. The breakwater maintains the shape of the Tweed Estuary and stabilises its habitats, according to the report. Therefore it is the breakwater which is responsible for stabilising the spit and fixing in place the estuary mouth. The report indicates that the topography and construction around the harbour leave little scope for the recreation of intertidal habitats, which may be squeezed against hard defences as sea levels rise.

Finally, SMP2 reports that even if the present defences are fully maintained, increases in sea level, and potential increases in river spates as a result of climate change, may cause higher erosion rates on Spittal Point and the beach frontage.

With regards to flood risk at Berwick the present study provides general advice on the time-scales for increasing defence crest elevations as sea levels rise. This advice is made in relation to the tolerable wave overtopping discharges recommended in EurOtop for different types of land use and pedestrian risk. Again, it is important to stress that the guidance recommended in EurOtop is indicative only.



7.3 OBJECTIVES OF A FURTHER MODELLING STUDY

As stated above a further modelling study is required in order to inform the decision of a future coastal management policy for Berwick. It is recommended that the objectives of this study include:

1. The determination of the potential changes to hydrodynamics in the Tweed Estuary due to modifications of defences around Spittal and Sandstell Point. This should include an examination of both average and extreme conditions in river flow, sea-level and wave action. A scenario of complete failure of the northern breakwater should be included in order to examine potential changes to direction of river flow and exposure to wave action.

2. The assessment of potential morphological changes in the estuary and along Spittal beach as a result of these defence modifications for average and extreme hydrodynamic conditions. Again, a scenario of complete failure of the breakwater should be examined.

3. An assessment of how the potential hydrodynamic and morphological changes for the scenarios specified above may impact on issues relating to estuary navigation and habitat and species management.

4. A determination of the change in potential flood risk along the coastal defences within and around the estuary due to the scenarios of defence modification, including complete failure of the breakwater. It is recommended that this take the form of a Strategic Flood Risk Assessment (SFRA), which will comprehensively look at flood risk around the estuary and potential increases due to climate change projections.

7.4 METHODOLOGY FOR A FURTHER STUDY

The methodology developed in the present study is ideally suited for use in the proposed future study. It can be adapted easily in order to perform relevant tasks to fulfil the objectives described above. The use of a coupled tide-wave hydrodynamic model to investigate various scenarios of hydrodynamic conditions would remain the same. The accuracy of the model would benefit from the collection of current measurements within the estuary, which would allow for better model calibration. The morphological response modelling method for examining morphological change and locations of impact should also be used.

The different scenarios that should be investigated by the study would easily be produced. These scenarios which represent modifications to defences would be generated by editing the numerical model grid. For example, a simulation of a scenario characterised by the complete failure of the breakwater would be performed by setting the elevation of the model grid cells that encompass the area of the breakwater to an appropriately decreased level.

Outputs from the morphological response model can be used to examine the potential impacts on estuary morphology and ramifications for sensitive habitats and estuary usage. The hydrodynamic model output can be used to ascertain likely impacts on navigational issues within the estuary.



APPENDIX A – HYDRODYNAMIC MODEL DESIGN



HYDRODYNAMIC MODEL DESIGN

INTRODUCTION

This appendix provides details of the construction of the hydrodynamic modelling system used to simulate circulation and wave processes within the Tweed Estuary.

MODELLING SYSTEM

The modelling system used, CMS (Coastal Modelling System), consists of a hydrodynamic circulation model, CMS-Flow, linked to a spectral nearshore wave transformation model, CMS-Wave.

The hydrodynamic flow within the Tweed Estuary was modelled using CMS-Flow22

. CMS-Flow is developed and maintained by the US Army Corps of Engineers' Coastal Inlets Research Program (CIRP). It is a finite-volume numerical representation of the two-dimensional (2D) depth-integrated continuity and momentum equations of water motion. Water elevation and depth-integrated current flow is computed at each of the calculation cells, which are defined on a staggered, rectilinear grid and can have constant or variable side lengths. The model solves equations of motion implicitly, allowing for a relatively large model time step and therefore fast computation time. Boundary conditions can be specified as flow-rate forcing (for upstream river input) or water-surface-elevation forcing (for downstream tidal variations).

Simulation of waves within the Tweed Estuary is performed using the spectral wave model CMS-Wave23

. CMS-Wave is developed and maintained by CIRP. It is a phase-averaged, 2D wave spectral transformation model. The term 'phase-averaged' means that the model neglects changes in the wave phase in calculating wave and other nearshore processes. The model calculates the shallow water wave transformation processes of depth-induced wave refraction and shoaling, current induced refraction and shoaling, depth and steepness-induced wave breaking, wind-wave growth, wave-wave interaction, and white-capping. Diffraction and wave reflection processes are approximated by the model, as opposed to being explicitly simulated.

The effects of waves on coastal circulation are included in CMS-Flow through coupling with CMS-Wave. In the CMS, radiation stresses from surface waves are communicated by CMS-Wave to CMS-Flow, thereby forcing currents and changing water levels in the hydrodynamic model. The coupling is 'two-way', in that currents and changing water levels calculated by CMS-Flow are input to the wave model to transform propagating waves. Coupling was performed every 3 hours of model time.

The model grids were constructed using the Surface-water Modelling System (SMS)24

. SMS is a graphical user interface and analysis tool that allows engineers and scientists to visualize, manipulate, analyze and understand numerical data and associated measurements. Many of the tools in SMS are generic, and they are designed to facilitate the establishment and operation of numerical models of rivers, coasts, inlets, bays, estuaries, and lakes.

22

http://cirp.usace.army.mil/wiki/CMS-Flow

23 http://cirp.usace.army.mil/wiki/CMS-Wave

24 http://www.xmswiki.com/xms/SMS:SMS



MODEL BATHYMETRY

The estuary model grid was constructed using various sources of bathymetry and elevation data, provided from:

A bathymetric survey, undertaken by Survey Operations in February 2010;

EA-supplied LiDAR data, covering the Spittal beach frontage and upstream of the Tweed railway bridge to Horncliffe;

Digitised Admiralty Chart data, covering depths from the estuary mouth to 15km offshore, supplied by Seazone Solutions Ltd.

All bathymetry and elevation data were converted (where necessary) to metres Above Ordnance Datum Newlyn (mODN).

MODEL GRID

Figure 40 shows the model grid for the CMS model. The horizontal resolution of the grid cells is 20m by 20m. There is a river boundary upstream, where a flow rate is specified as an input condition. Along the ocean boundary a varying water surface elevation condition is provided in order to simulate the propagation of tide waves into the mouth of the estuary. The River Tweed is included in the model up to the tidal limit near to Horncliffe. This is to allow for the accurate propagation of tidal waters up the estuary. The river boundary does not allow water to leave the model domain. If this boundary was farther downstream and closer to the estuary, the model flooding tide water may reflect off this boundary and propagate back into the model domain, which is clearly not a realistic process.

Figure 40: CMS model grid showing bathymetry and model boundaries (brown cells are non-computational)



BED FRICTION

The effect of friction at the channel margins serves to transform waves and slow tidal and river flow. In this modelling study, friction is represented by a Manning's n value. Derivation of n is very subjective. While coefficients of roughness were assigned to sections of the channel by the consultants, guidance was sought from literature sources.

According to Chow (1959)25

the total value of n is given by:

n

0+n

1+n

2+n

3+n

4+m

5

Where n

0 = roughness value of straight, uniform channel for given material, n

1 = value of surface irregularities,

n2 = value for variations in cross-sectional area and shape, n

3 = value to include effect of obstructions, n

4 =

effect of vegetation and flow type and m5 = correction factor for meanders.

The model domain can be divided into three sections. The first section, upstream of the Royal Tweed Bridge, is reasonably regular with little obstruction to the flow. Channel width here is around 125m to 160m. The reach of the channel from just north of the Royal Tweed Bridge to the pier at the downstream side of the entrance to the docks is less regular, with constrictions and obstructions caused by the bridges and the Bailiffs Battery sand deposit. At its narrowest, this reach of the channel is around 82m across. Beyond the Dock Pier, the channel meanders more. The channel cross-sectional area also becomes much greater, particularly during periods of high tide and flow. While meanders present resistance to flow, a greater cross-sectional area allows flow to pass more freely.

Beyond the estuary mouth, in the open sea, the absence of channel walls made computation of a value for "n",

using the method proposed by Chow, impossible. As a result, a formula was used to produce a roughness value for the model's offshore area (see below):

The parameter "k" is a roughness value accounting for the effects of bed forms and grain diameter. In the formula above, a value of 0.08 was specified for k. The parameter "R" represents the hydraulic radius of the channel. For the purposes of this calculation, to determine a roughness value for the open sea, the average water depth in the offshore zone of the model was used. A value of R = 22.6m was used. Using these values, f = 0.0191.

The Manning's n value is related to f in the above formula via the Darcy-Weisbach friction coefficient, Cf. The

equation below relates Cf to f:

Based on the above equation, Cf = 0.0024. The equation below relates n to Cf:

Where "h" is the water depth (22.6m) and "g" is the gravitational constant at 9.81m/s. The roughness value for

use in the model is n = 0.026. The Manning's n values used in the model are presented in Table 19.

25

Open Channel Hydraulics (1959) by Chow, V. T. Published by The Blackburn Press, New Jersey pp106-109



Table 19: Deriving the values of Manning's n

Reach Manning's n Components Total Value of n

1. Upstream boundary to Royal Tweed

Bridge

Straight, smooth uniform channel of fine gravel (n

0) =

0.025

Slight surface irregularity (n

1) = 0.005

0.03

2. Royal Tweed Bridge to pier

downstream of dock

Straight, smooth uniform channel of fine gravel (n

0) =

0.025

Surface irregularity(n1) = 0.005

Change in cross-section size (n

2) = 0.015

Effect of bridge obstructions (n

3) = 0.02

0.065

3. Dock to Estuary Mouth

As for reach 1. Effect of meandering countered by increase in cross-sectional area.

0.03

4. Estuary mouth to model's

downstream boundary

Offshore areas

0.026



APPENDIX B – RIVER FLOW HYDROGRAPH DERIVATION



RIVER FLOW HYDROGRAPH DERIVATION

INTRODUCTION

This appendix presents details on the derivation of hydrographs for both average and extreme fluvial flow at the upstream boundary of the estuary model.

DERIVATION OF RIVER FLOW HYDROGRAPHS

A flow estimate was required at Royal Border Railway Bridge in Berwick, on the River Tweed. This flood estimation point (FEP) is located at 399260, 653230. At this point the river is tidal, and therefore catchment descriptors from the FEH CD-ROM cannot easily be obtained.

In order to derive catchment descriptors for the subject site, others were taken from the tidal limit of the River Tweed, at Union Bridge (393400, 651030) (as shown on the FEH CD-ROM). However, there was a further inflow (Whiteadder Water) to the Tweed downstream of this location, but upstream of the FEP. Catchment descriptors were altered using the area-weighting method, to ensure these are representative of both of the catchments. Remaining areas that do not fall into either catchment have also been included. These three areas are shown in Figure 41.

Figure 41: Catchments on the Lower Tweed

The flow estimates for the FEP have been calculated using the FEH Statistical method. The new catchment descriptors were used to calculate QMED. A donor site was used to improve the estimate of QMED by providing an adjustment factor, calculated using flow data from the HiFlows UK gauge 21009 (Tweed at Norham). The new QMEDrural value was then altered to obtain QMEDurban (using the Urban Adjustment Factor within the JBA FEH spreadsheet).



Growth curve parameters were derived using a pooling group. The stations used in the pooling group were determined within the WINFAP software package. The growth curve parameters were then applied to the final QMED estimates to give peak flows to various return periods.

Hydrographs were produced using ReFH (Revitalised Rainfall Runoff method). This method is suitable for producing hydrograph shapes, but in this case the statistical method is favourable for estimating peak flows. The hydrographs were then scaled to the peak flow for a 1:200-year event.

The average annual river flow was calculated in order to be supplied as upstream boundary conditions for the extreme wave and tide model simulations. There is a gauge on the River Tweed, at Norham and another on Whiteadder Water at Hutton Castle. Table 20 and Table 21 show the catchment statistics used in the calculation.

Table 20: Catchment statistics for the Whiteadder river at Hutton Castle

Grid reference: 36 (NT) 881 550

Operator SEPA

Catchment area 503.0 km2

Level of station 29.0 mOD

Max. altitude 533.0 mOD

Mean flow 6.5 m3s

-1

95% exceedance (Q95) 1.1 m3s

-1


-1

61-90 av. ann. rainfall 813 mm

Table 21: Catchment statistics for the River Tweed at Norham

Grid reference: 36 (NT) 898 477

Operator SEPA

Catchment area 4390.0 km2

Level of station 4.3 mOD

Max. altitude 839.0 mOD

Mean flow 79.7 m3s

-1


-1


-1

61-90 av. ann. rainfall 955 mm



The combined mean flow at the two gauges was calculated:

79.7 + 6.5 = 86.2 m3s

-1

The combined area of the two catchments from which this flow originates is:

4390.0 + 503.0 = 4893.0 km2

From the Flood Estimation Handbook CD-ROM, the total catchment area at the limit of the tidal model is approximately 4439 + 535 km

2, plus approximately 30 km

2 of small watercourses that drain into the tidal reach.

The total catchment area of the River Tweed therefore is 5004 km2.

The mean flow from the gauged 4893 km2 catchment area is 86.2 m

3s

-1.

If the rainfall is assumed constant across the entire Tweed catchment, then the volume of surface run-off entering the watercourse should be proportional to the land area. The un-gauged catchment area is:

5004 - 4893 = 111 km2

The mean annual flow originating in this un-gauged catchment area needed to be determined. As 5004 = 4893 x 1.02, the total mean flow generated from the entire Tweed catchment area is 86.2 x 1.02 = 87.9 m

3s

-1.




APPENDIX C – FULL LIST OF WAVE OVERTOPPING RATES



FULL LIST OF WAVE OVERTOPPING RATES

INTRODUCTION

This appendix presents the full list of results from the wave overtopping assessment described in Chapter 4. The table provide overtopping rates for the following combinations:

4 permutations of wave height and water level that comprise a 1:200-year joint probability wave height/water level storm event;

3 defence locations within the Tweed Estuary (the south bank, Sandstell Point and the north bank);

2 scenarios of spit morphology (present day and catastrophic loss);

4 sea-level scenarios (present day, +20 years, +50 years and +100 years);

2 sea-level rise projection sets (DEFRA and UKCP09).

Table 22: Overtopping Discharges at South Bank under DEFRA (2006) Spit Intact

Permutation of wave and water level + RSLR

Incident Wave Height at Structure (m)

Peak Period (s)

Water Level m AOD(N)


Ru2% Rc (m)

Permutation 1

0yrs 0.86 12.50 3.06 0.06 2.20 2.34

20yrs 0.90 12.50 3.14 0.11 2.25 2.26

50yrs 0.96 12.50 3.30 1.23 2.86 2.10

100yrs 1.16 12.50 3.76 16.93 3.53 1.64

Permutation 2

0yrs 0.85 11.11 3.06 0.05 2.12 2.34

20yrs 0.91 11.11 3.14 0.11 2.21 2.26

50yrs 1.00 11.11 3.34 1.92 2.93 2.06

100yrs 1.23 11.11 3.89 30.62 3.68 1.51

Permutation 3

0yrs 0.96 8.33 3.12 0.11 2.13 2.28

20yrs 0.99 8.33 3.20 0.92 2.68 2.20

50yrs 1.08 8.33 3.41 3.89 3.01 1.99

100yrs 1.10 8.33 3.99 22.18 3.04 1.42

Permutation 4

0yrs 0.58 6.67 3.38 0.00 1.38 2.02

20yrs 0.58 6.67 3.46 0.00 1.37 1.94

50yrs 0.59 6.67 3.68 0.01 1.33 1.72

100yrs 0.60 6.67 4.26 0.23 1.25 1.14



Table 23: Overtopping Discharges at South Bank under DEFRA (2006) Spit Removed



Peak Period (s)



Ru2% Rc (m)

Permutation 1

0yrs 1.23 12.50 3.08 4.08 3.60 2.32

20yrs 1.29 12.50 3.16 7.06 3.81 2.24

50yrs 1.41 12.50 3.31 16.96 4.17 2.09

100yrs 1.76 12.50 3.76 115.49 5.23 1.64

Permutation 2

0yrs 1.21 11.11 3.05 3.27 3.46 2.35

20yrs 1.27 11.11 3.13 5.55 3.64 2.27

50yrs 1.42 11.11 3.32 18.36 4.13 2.08

100yrs 1.83 11.11 3.85 156.63 5.34 1.55

Permutation 3

0yrs 1.20 8.33 3.10 3.16 3.24 2.30

20yrs 1.22 8.33 3.18 4.63 3.33 2.22

50yrs 1.25 8.33 3.40 9.91 3.49 2.00

100yrs 1.30 8.33 3.97 40.11 3.46 1.43

Permutation 4

0yrs 0.62 6.67 3.38 0.00 1.44 2.02

20yrs 0.62 6.67 3.46 0.00 1.42 1.94

50yrs 0.62 6.67 3.68 0.10 1.68 1.72

100yrs 0.62 6.67 4.26 0.31 1.30 1.14



Table 24: Overtopping Discharges at Sandstell Point under DEFRA (2006) Spit Intact



Peak Period (s)



Permutation 1

0yrs 0.84 12.50 3.08 0.03

20yrs 0.91 12.50 3.17 0.06

50yrs 1.03 12.50 3.32 0.13

100yrs 1.39 12.50 3.78 0.21

Permutation 2

0yrs 0.84 11.11 3.08 0.01

20yrs 0.90 11.11 3.15 0.02

50yrs 1.06 11.11 3.35 0.09

100yrs 1.48 11.11 3.89 0.33

Permutation 3

0yrs 0.87 8.33 3.12 0.00

20yrs 0.93 8.33 3.20 0.00

50yrs 1.10 8.33 3.41 0.01

100yrs 1.55 8.33 3.98 0.31

Permutation 4

0yrs 1.08 6.67 3.38 0.00

20yrs 1.14 6.67 3.46 0.00

50yrs 1.27 6.67 3.68 0.03

100yrs 1.26 6.67 4.26 0.09



Table 25: Overtopping Discharges at Sandstell Point under DEFRA (2006) Spit Removed



Peak Period (s)



Permutation 1

0yrs 0.85 12.50 3.09 0.03

20yrs 0.91 12.50 3.16 0.06

50yrs 1.02 12.50 3.31 0.12

100yrs 1.37 12.50 3.76 0.18

Permutation 2

0yrs 0.82 11.11 3.05 0.01

20yrs 0.87 11.11 3.12 0.01

50yrs 1.03 11.11 3.32 0.08

100yrs 1.44 11.11 3.84 0.23

Permutation 3

0yrs 0.86 8.33 3.10 0.00

20yrs 0.92 8.33 3.18 0.00

50yrs 1.09 8.33 3.40 0.01

100yrs 1.54 8.33 3.97 0.28

Permutation 4

0yrs 1.08 6.67 3.38 0.00

20yrs 1.14 6.67 3.46 0.00

50yrs 1.27 6.67 3.68 0.03

100yrs 1.26 6.67 4.26 0.09



Table 26: Overtopping Discharges at North Bank under DEFRA (2006) Spit Intact



Peak Period (s)



Permutation 1

0yrs 1.05 12.50 3.07 37.69

20yrs 1.11 12.50 3.22 72.32

50yrs 1.17 12.50 3.32 123.58

100yrs 1.39 12.50 3.91 23214.31

Permutation 2

0yrs 1.03 11.11 3.14 36.80

20yrs 1.09 11.11 3.22 57.64

50yrs 1.16 11.11 3.42 153.20

100yrs 1.47 11.11 3.95 64360.59

Permutation 3

0yrs 1.08 8.33 3.13 62.58

20yrs 1.11 8.33 3.21 90.29

50yrs 1.21 8.33 3.42 295.33

100yrs 1.38 8.33 3.98 103803.14

Permutation 4

0yrs 0.61 6.67 3.38 8.53

20yrs 0.62 6.67 3.46 13.24

50yrs 0.63 6.67 3.68 57.62

100yrs 0.65 6.67 4.26 Water level exceeds defence crest elevation



Table 27: Overtopping Discharges at North Bank under DEFRA (2006) Spit Removed



Peak Period (s)



Permutation 1

0yrs 1.17 12.50 3.00 44.91

20yrs 1.28 12.50 3.15 93.71

50yrs 1.35 12.50 3.24 150.57

100yrs 1.79 12.50 3.80 10429.05

Permutation 2

0yrs 1.20 11.11 3.05 47.56

20yrs 1.26 11.11 3.12 68.73

50yrs 1.42 11.11 3.32 202.40

100yrs 1.83 11.11 3.85 17950.54

Permutation 3

0yrs 1.24 8.33 3.10 102.44

20yrs 1.30 8.33 3.18 159.03

50yrs 1.46 8.33 3.39 196.66

100yrs 1.49 8.33 3.97 88822.38

Permutation 4

0yrs 0.65 6.67 3.38 11.14

20yrs 0.65 6.67 3.46 16.14

50yrs 0.66 6.67 3.68 70.05

100yrs 0.67 6.67 4.26 Water level exceeds defence crest elevation



Table 28: Overtopping Discharges at South Bank under UKCP09 Spit Intact



Peak Period (s)



Ru2% Rc (m)

Permutation 1

0yrs 0.86 12.50 3.06 0.06 2.20 2.34

20yrs 0.90 12.50 3.12 0.10 2.25 2.28

50yrs 0.93 12.50 3.22 0.72 2.74 2.18

100yrs 1.02 12.50 3.42 2.91 3.06 1.98

Permutation 2

0yrs 0.85 11.11 3.06 0.05 2.12 2.34

20yrs 0.90 11.11 3.12 0.09 2.20 2.28

50yrs 0.96 11.11 3.22 0.85 2.76 2.18

100yrs 1.05 11.11 3.47 4.03 3.10 1.93

Permutation 3

0yrs 0.96 8.33 3.12 0.11 2.13 2.28

20yrs 0.98 8.33 3.18 0.75 2.63 2.22

50yrs 1.02 8.33 3.29 1.62 2.79 2.11

100yrs 1.12 8.33 3.54 7.16 3.14 1.86

Permutation 4

0yrs 0.58 6.67 3.38 0.00 1.38 2.02

20yrs 0.58 6.67 3.44 0.00 1.37 1.96

50yrs 0.59 6.67 3.55 0.00 1.35 1.85

100yrs 0.63 6.67 3.81 0.18 1.64 1.59



Table 29: Overtopping Discharges at South Bank under UKCP09 Spit Removed



Peak Period (s)



Ru2% Rc (m)

Permutation 1

0yrs 1.23 12.50 3.08 4.08 3.60 2.32

20yrs 1.28 12.50 3.14 6.37 3.77 2.26

50yrs 1.36 12.50 3.24 11.78 4.02 2.16

100yrs 1.51 12.50 3.43 31.48 4.46 1.97

Permutation 2

0yrs 1.21 11.11 3.05 3.27 3.46 2.35

20yrs 1.25 11.11 3.11 4.87 3.60 2.29

50yrs 1.33 11.11 3.21 9.57 3.86 2.19

100yrs 1.52 11.11 3.45 33.16 4.41 1.95

Permutation 3

0yrs 1.20 8.33 3.10 3.16 3.24 2.30

20yrs 1.22 8.33 3.16 4.22 3.31 2.24

50yrs 1.24 8.33 3.27 6.72 3.42 2.13

100yrs 1.27 8.33 3.52 14.62 3.55 1.88

Permutation 4

0yrs 0.62 6.67 3.38 0.00 1.44 2.02

20yrs 0.62 6.67 3.44 0.00 1.43 1.96

50yrs 0.62 6.67 3.55 0.01 1.40 1.85

100yrs 0.66 6.67 3.81 0.27 1.70 1.59



Table 30: Overtopping Discharges at Sandstell Point under UKCP09 Spit Intact



Peak Period (s)



Permutation 1

0yrs 0.84 12.50 3.08 0.03

20yrs 0.89 12.50 3.14 0.05

50yrs 0.98 12.50 3.25 0.11

100yrs 1.13 12.50 3.45 0.15

Permutation 2

0yrs 0.84 11.11 3.08 0.01

20yrs 0.88 11.11 3.13 0.02

50yrs 0.96 11.11 3.23 0.04

100yrs 1.15 11.11 3.48 0.08

Permutation 3

0yrs 0.87 8.33 3.12 0.00

20yrs 0.92 8.33 3.18 0.00

50yrs 1.00 8.33 3.29 0.00

100yrs 1.20 8.33 3.54 0.03

Permutation 4

0yrs 1.08 6.67 3.38 0.00

20yrs 1.12 6.67 3.44 0.00

50yrs 1.21 6.67 3.55 0.01

100yrs 1.26 6.67 3.81 0.04



Table 31: Overtopping Discharges at Sandstell Point under UKCP09 Spit Removed



Peak Period (s)



Permutation 1

0yrs 0.85 12.50 3.09 0.03

20yrs 0.90 12.50 3.15 0.05

50yrs 0.97 12.50 3.25 0.10

100yrs 1.12 12.50 3.44 0.15

Permutation 2

0yrs 0.82 11.11 3.05 0.01

20yrs 0.86 11.11 3.10 0.01

50yrs 0.94 11.11 3.20 0.03

100yrs 1.12 11.11 3.44 0.09

Permutation 3

0yrs 0.86 8.33 3.10 0.00

20yrs 0.90 8.33 3.16 0.00

50yrs 0.99 8.33 3.27 0.00

100yrs 1.19 8.33 3.52 0.03

Permutation 4

0yrs 1.08 6.67 3.38 0.00

20yrs 1.12 6.67 3.44 0.00

50yrs 1.21 6.67 3.55 0.01

100yrs 1.27 6.67 3.81 0.04



Table 32: Overtopping Discharges at North Bank under UKCP09 Spit Intact



Peak Period (s)



Permutation 1

0yrs 1.05 12.50 3.07 37.69

20yrs 1.08 12.50 3.12 48.00

50yrs 1.09 12.50 3.21 65.75

100yrs 1.23 12.50 3.45 252.52

Permutation 2

0yrs 1.03 11.11 3.14 36.80

20yrs 1.06 11.11 3.20 49.06

50yrs 1.11 11.11 3.30 81.27

100yrs 1.21 11.11 3.55 336.02

Permutation 3

0yrs 1.08 8.33 3.13 62.58

20yrs 1.10 8.33 3.19 81.41

50yrs 1.14 8.33 3.30 133.37

100yrs 1.29 8.33 3.54 264.18

Permutation 4

0yrs 0.61 6.67 3.38 8.53

20yrs 0.62 6.67 3.44 12.00

50yrs 0.62 6.67 3.55 21.63

100yrs 0.63 6.67 3.81 221.86



Table 33: Overtopping Discharges at North Bank under UKCP09 Spit Removed



Peak Period (s)



Permutation 1

0yrs 1.17 12.50 3.00 44.91

20yrs 1.26 12.50 3.13 83.55

50yrs 1.27 12.50 3.14 88.48

100yrs 1.45 12.50 3.37 312.13

Permutation 2

0yrs 1.20 11.11 3.05 47.56

20yrs 1.24 11.11 3.11 63.18

50yrs 1.32 11.11 3.21 106.89

100yrs 1.51 11.11 3.45 429.99

Permutation 3

0yrs 1.24 8.33 3.10 102.44

20yrs 1.28 8.33 3.16 139.93

50yrs 1.37 8.33 3.26 96.13

100yrs 1.47 8.33 3.52 370.13

Permutation 4

0yrs 0.65 6.67 3.38 11.14

20yrs 0.65 6.67 3.44 14.64

50yrs 0.66 6.67 3.55 28.13

100yrs 0.66 6.67 3.81 269.73



APPENDIX D: STUDY PLAN

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