Cuckmere Estuary Options Modelling Study

1. Cuckmere Estuary Options Modelling Study Cuckmere Pathfinder Project Scott Ferguson Landscape, Visitors, Heritage and Economy of the Cuckmere: Evidence Presentation Stage 2 (Public Event)

2. Introduction • Objectives - what is it we’re trying to achieve? • Approach - how have gone about it? • Options - what it is we’ve modelled? • Outcome - what are the results and how can we use them? Cuckmere Pathfinder Project Capita Symonds

3. What is it we’re trying to achieve from this study? Understanding of how future management of the river will affect: • Flows and water levels • Changes in habitats • Existing assets • Flood risk • Knock on effects or secondary impacts Cuckmere Pathfinder Project Capita Symonds

4. How have gone about ? • Defined options (including do nothing), what they mean and how they will be implemented and over what time span • Built a hydrodynamic computer model to allow iterative model runs to assess change • Using the model output to prepare visual interpretations of the estuary behaviour and how key features may change. Cuckmere Pathfinder Project Capita Symonds

5. Modelling • 2-D numerical modelling – how does it work? Cuckmere Pathfinder Project Capita Symonds

6. Modelling • 2-D numerical modelling – how does it work? • Every cell has: • ZC = Elevation at cell centre for storage; • ZU = Right Edge Elevation for flows to right; • ZV = Top Edge Elevation for flows to top; • ZH = Corner elevations (no role • hydraulically) Cuckmere Pathfinder Project Capita Symonds

7. Modelling • Why a 2-D model? • What is included and how? • What’s not included and does it matter? • What outputs do we get from the model? • How do we decide on the future land form? Cuckmere Pathfinder Project Capita Symonds

8. What it is we’ve modelled? Base case: Do nothing – no further maintenance or improvements of existing embankments or structures. Cuckmere Pathfinder Project Capita Symonds

9. Baseline • Key Features • Representation in model • Likely outcome of ‘do nothing’ • Current land levels (Cells A, B and C) • Role of the beach

10. What it is we’ve modelled? Option A: Partial breach managed realignment Cuckmere Pathfinder Project Capita Symonds

11. Option A • Key Features: • Existing embankments • Maintained embankments • River channel • Breaches • Representation in model • Likely long term outcome • Current land levels (Cells A, B and C) • Role of the beach

12. What it is we’ve modelled? Option B: Full breach managed realignment Cuckmere Pathfinder Project Capita Symonds

13. Option B • Key Features: • Existing embankments • New embankments • Controlled breaches, • Additional embankments • Excavated tidal lagoon • Representation in model • Likely long term outcome • Beach and river mouth

14. What it is we’ve modelled? Option C: Engineered reactivation of meanders and saltmarsh creeks Cuckmere Pathfinder Project Capita Symonds

15. Option C • Key Features: • Engineered diversion, • Excavate meanders , • Fill cut • Engineered tidal creeks • Representation in model • Likely long term outcome • Beach and river mouth

16. What it is we’ve modelled? Options D, E and F: Maintain or improve the defences

17. Option D, E and F • Key Features: • Existing embankments • D is maintain • E is maintain and raise • F is maintain and partially raise • Representation in model • Likely long term outcome • Beach and river mouth

19. What are the results and how can we use them? Series of flood maps showing depth and extent of inundation Detailed velocity maps Time series data showing how water depth and velocities vary throughout a flood event Use of simple rules to define land form Use of simple rules to determine land cover – i.e. habitat (mud or saltmarsh or tidal grazing) Cuckmere Pathfinder Project Capita Symonds

20. Baseline – 2060 Spring Tide Velocity Contours

21. Baseline – 2060 100 year Fluvial flood event

22. Baseline – 2060 100 year Tidal flood event

23. Baseline – 2060 100 year Combined Tidal & Fluvial flood event

24. Baseline – 2060 100 year flood event

25. Baseline – 2060 Potential Habitat

26. River Mouth Flows – Defended & Undefended (Q - Cumecs) Cuckmere Pathfinder Project Capita Symonds

27. Results Three main outcomes: • Fully defended where the estuary will remain very much as it is at present - options D, E and F and do nothing for the short term. • Fully natural where much of the estuary becomes an active inter tidal zone - options B and C initially, with do nothing eventually • Partially defended with a mix of habitats – Options A, and in the medium term for ‘do nothing’ and long term for option D, once the embankments start to fail Cuckmere Pathfinder Project Capita Symonds

28. Land form type 1

29. Land form type 2

30. Land form type 3

31. Secondary Results Results of the modelling and visualisation project also used for: • Assessing the effect on the landscape • Assessing the potential impact on visitor numbers in future • Assessing economic impact • Assessing the effect on assets such as heritage assets. Cuckmere Pathfinder Project Capita Symonds

32. Final Key Points • None of the Estuary options will impact on flood risk upstream (as long as the river mouth stays clear) • The tight bend downstream of Exceat Bridge for option C • Flooding of the A259 • Importance of estuary option in determining requirements for the beach management • Importance of beach in determining the future of the estuary Cuckmere Pathfinder Project Capita Symonds