Tool to Reduce Tidal Propagation in Estuaries Roger Morris Bright - - PowerPoint PPT Presentation

http://bacoastal.co.uk

Managed Realignment as a Tool to Reduce Tidal Propagation in Estuaries

Roger Morris Bright Angel Coastal Consultants

http://bacoastal.co.uk

Realignment is a conservation tool???

But does it need to be

http://bacoastal.co.uk

A German problem?

http://bacoastal.co.uk

What has happened?

• High tide levels have increased - additional strain on flood

defences;

• Low tide levels have dropped - groundwater

drawdown/subsidence;

• Sediment mobilisation has increased referred to as ‘tidal

pumping’;

• SSC have risen – sometimes dramatically, as in the Ems;
• Increased sedimentation in dredged channels and in

berthing pockets; and

• Greater mixing of clean and contaminated sediments.

http://bacoastal.co.uk

And a German solution?

• Big realignments
• Reconnect tributaries

http://bacoastal.co.uk

But they have a few problems

• The best places to re-align are Natura 2000

sites;

• People do not like ‘de-poldering; and
• The estuaries concerned are huge, so

realignments must also be very big.

http://bacoastal.co.uk

Are there lessons from elsewhere?

http://bacoastal.co.uk

Lessons from the Petitcodiac

• Reconnecting tributaries creates more tidal

volume;

• Causes erosion downstream and

sedimentation upstream – estuary seeks to achieve new Regime geometry;

• Plays out the concept of rollover; and
• Provides a model to explain how the coast

works.

http://bacoastal.co.uk

Reverse engineering

• We know that loss of accommodation space

and channel deepening increase tidal propagation;

• We also know that realignments cause

changes in water levels; and

• We know that realignments create sediment

sinks.

http://bacoastal.co.uk

So why make the fuss about nature conservation???

• Coastal communities regard it as retreat;
• The political establishment regards it as loss
• f valuable farmland;
• It is downgraded as a tool to adapt to

climate change; and

• The role of accommodation space as an

ecosystem function gains little traction.

http://bacoastal.co.uk

Re-branding or re-positioning?

• Much less emphasis on nature conservation – we

need an engineering champion for realignment;

• Designing realignments to reduce tidal

propagation is a new engineering challenge; and

• The research agenda needs to focus on
• ptimization of locations to reduce tidal

propagation and maximise sediment absorption.

http://bacoastal.co.uk

And what about the wildlife?

• If realignment is seen as an engineering tool

there will be lots more realignment;

• Realignment may not be exactly where it is

wanted now; but

• It will be in due course.
• We cannot be precious – you have to break

eggs to make an omelette!

http://bacoastal.co.uk

A tactical agenda?

• Analyse estuaries with the greatest flood risk for

the best geometry to REDUCE tidal propagation;

• Prioritise landside assets according to their

suitability for creating accommodation space; and

• Develop a long-term plan that possibly accepts

some loss of Natura 2000 now in return for a more sustainable estuary in due course?

http://bacoastal.co.uk