Cuckmere Estuary Options
Modelling Study
Cuckmere Pathfinder Project
Scott 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?
Cuckmere Pathfinder Project
Capita Symonds
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
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
Modelling
• 2-D numerical modelling – how does it work?
Cuckmere Pathfinder Project
Capita Symonds
Modelling
• 2-D numerical modelling – how does it work?
Every cell has:
v Velocity
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
ZV
ZH
ZC
ZU
u Velocity
Water level
calculation point
Capita Symonds
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
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
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
Cuckmere Pathfinder Project
Capita Symonds
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
Cuckmere Pathfinder Project
Capita Symonds
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
Cuckmere Pathfinder Project
Capita Symonds
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)
Cuckmere Pathfinder Project
Capita Symonds
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)
Cuckmere Pathfinder Project
Capita Symonds
Results
Three 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
Cuckmere Pathfinder Project
Capita Symonds
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 landscape
2. Assessing the potential impact on visitor
numbers in future
3. Assessing economic impact
4. Assessing the effect on assets such as
heritage assets.
Cuckmere Pathfinder Project
Capita Symonds
Final Key Points
1. 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 A259
4. Importance of estuary option in determining
requirements for the beach management
5. Importance of beach in determining the
future of the estuary
Cuckmere Pathfinder Project
Capita Symonds