17TH RRC ANNUAL
NETWORK CONFERENCE
Tweet us
#RRCBlackpool
Planning, delivery and evaluation of our
rivers: challenges and choices
We would like to thank the sponsors of the conference who support discounted places
[email protected]
01234 752979
www.therrc.co.uk
Sediment Management:
Finding the balance
In-channel sediment
Lots of our rivers are : Overwide, impounded and depositional
= Lots of in channel sediment!
The reason we are working on these rivers!
Sediment mobilisation
Most work we do in rivers mobilises sediment.......
..........even just walking in the river!
Sediment mobilisation
• Rivers move sediment
Can be both a good and a bad thing!
• How much sediment movement is an issue?
• When does it become an issue?
Perception vs reality?
• Can we monitor anything to inform us?
• Can we mitigate against sediment movement
adequately......and pragmatically?
Sediment management techniques
• Lots of methods depending on the work carried
out.
• None of the techniques are a one size fits all.
• Look at a couple of main types of methods used.
Sediment management techniques
Simple adjustments
• Minimise silt movement
For example: use a long reach vs in channel work
Sediment management techniques
• Permeable silt curtains – filtering structures
Straw bales
Sediment management techniques
• Permeable silt curtains – filtering structures
Faggot Bundles
Sediment management techniques
• Permeable silt curtains – filtering structures
Sedimats
Sediment management techniques
• Permeable silt curtains – filtering structures
Combination: Faggot bundles and geotextiles.
Sediment management techniques
Do they work?
• ……sometimes! best with low (or no) flows
Issues
• create a weir – impounds.
• some silt drops out , a lot carries on over the
top
• removal of the silt that drops out?
• blow out in high flows or erode
around/underneath
Sediment management techniques
• Isolating the works - Over Pumping
Portable dam and flumes
Sediment management techniques
• Isolating the works - Over Pumping
Sediment management techniques
• Isolating the works - Coffer dam
Sediment management techniques
Isolating the works - Over Pumping
• Working dry – lowers risk
• Tends to be expensive
- valid argument?
- could rule out a lot of projects
• Not always appropriate in some areas – pumps
• Not always pragmatic
– Can’t see how river responds/work with the river
e.g. woody debris, levels for berms, adapting design to
flows etc…
• Still get flush through once stop overpumping
Sediment mobilisation
• Effectiveness of silt mitigation in river
restoration is often the elephant in the room.
Case study-ButterHill
Carshalton Arm of the Wandle
• Weir lowered 1m
• 120m river narrowing
Sediment mitigation
• sedimats
• faggot bundles &
geotextiles
Case study-ButterHill
Carshalton Arm of the Wandle
Alternative Sediment mitigation
• Overpump 120m river
•
•
•
•
Very urban – houses metres away
Noise from pumps
Security
Can’t work with the river for the narrowing/habitat work
• Remove the silt, store it, work wet and replace/take
to landfill?
• Cost – project would not have happened.
• No GEP, trout reproduction etc...
How much silt movement is acceptable – -what is the balance?
• How much silt movement is acceptable?
– Can be a subjective assessment
• Depends where/when it is
– Downstream users/habitats, spawning season/gravels, SSSI etc..
• Crack a few eggs to make an omelette?
• Short term pain for long term gain?
Monitoring
• Can we monitor to ensure we aren’t causing
an issue?
• When should we monitor?
• What should we monitor (SS, DO, turbidity,
observations?)
• What are we going to do with the data?
Monitoring
•
•
•
•
Protocols?
Set levels that trigger actions?
Baseline levels helpful as comparison?
Suitable Baseline: rain event (first flush)?
Monitoring
Sediment after rain
Summary
• Sediment mobilisation occurs with all
activities
• No perfect solution to silt mitigation
• Can we find a balance between risk and
pragmatism?
• Can monitoring play a part?
• Develop protocols?
• Let’s share our experiences – good and bad!
17TH RRC ANNUAL
NETWORK CONFERENCE
Tweet us
#RRCBlackpool
Planning, delivery and evaluation of our
rivers: challenges and choices
We would like to thank the sponsors of the conference who support discounted places
[email protected]
01234 752979
www.therrc.co.uk