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

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??? http://bacoastal.co.uk

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 of 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 optimization 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