Cuckmere Estuary Options Modelling Study

Cuckmere Pathfinder ProjectScott Ferguson

Landscape, Visitors, Heritage and Economy of the Cuckmere:Evidence Presentation Stage 2 (Public Event)

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?

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What is it we’re trying to achieve from this study?

• Flows and water levels• Changes in habitats• Existing assets• Flood risk• Knock on effects or secondary impacts

Understanding of how future management of the river will affect:

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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.

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Modelling• 2-D numerical modelling – how does it work?

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Modelling

• 2-D numerical modelling – how does it work?

W ater leve lca lcu la tion po in t

ZU

ZHZV

ZC

u Velocity

v Velocity

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)

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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?

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What it is we’ve modelled?

Base case: Do nothing – no further maintenance or improvements of existing embankments or structures.

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Baseline

•Key Features

•Representation in model

• Likely outcome of ‘do nothing’• Current land levels (Cells A, B and C)

• Role of the beach

What it is we’ve modelled?

Option A: Partial breach managed realignment

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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

What it is we’ve modelled?

Option B: Full breach managed realignment

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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

What it is we’ve modelled?

Option C: Engineered reactivation of meanders and saltmarsh creeks

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Option C

• Key Features: – Engineered diversion,– Excavate meanders ,– Fill cut– Engineered tidal creeks

• Representation in model

• Likely long term outcome

• Beach and river mouth

What it is we’ve modelled?

Options D, E and F: Maintain or improve the defences

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

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)

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Baseline – 2060 Spring Tide Velocity Contours

Baseline – 2060 100 year Fluvial flood event

Baseline – 2060 100 year Tidal flood event

Baseline – 2060 100 year Combined Tidal & Fluvial flood event

Baseline – 2060 100 year flood event

Baseline – 2060 Potential Habitat

River Mouth Flows – Defended & Undefended

(Q - Cumecs)

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ResultsThree main outcomes:

1. 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.

2. Fully natural where much of the estuary becomes an active inter tidal zone - options B and C initially, with do nothing eventually

3. 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

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Land form type 1

Land form type 2

Land form type 3

Secondary Results

Results of the modelling and visualisation project also used for:

1. Assessing the effect on the landscape2. Assessing the potential impact on visitor

numbers in future3. Assessing economic impact4. Assessing the effect on assets such as

heritage assets.

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Final Key Points1. None of the Estuary options will impact on

flood risk upstream (as long as the river mouth stays clear)

2. The tight bend downstream of Exceat Bridge for option C

3. Flooding of the A2594. Importance of estuary option in determining

requirements for the beach management5. Importance of beach in determining the

future of the estuary

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