Sediment Transport Modelling - River Thames Scheme, England
- oliverashton3
- 4 days ago
- 3 min read
What our clients say:
“Flood Modeller was a great tool for this task. The outputs greatly enhanced our understanding of the impact of the River Thames Scheme on sediment transport. Using the same software as the main flood model made the setup easier. The flexibility in input data enabled extensive sensitivity testing of key parameters that gave greater confidence in the results.”
Rob Fraser, Chief Engineer, Binnies
Key Facts
Flood alleviation scheme for the largest area of undefended developed floodplain in England, which has suffered major floods in recent years.
A new river channel over 5 miles (8.5km) long, to be built in two sections, together with other downstream capacity improvements.
Sediment transport modelling undertaken using Flood Modeller.
Flood modelling also undertaken in a coupled 1D-2D Flood Modeller/ TUFLOW model.
Binnies were commissioned by the Environment Agency to develop the River Thames Scheme (RTS) design. The RTS will reduce the risk of flooding to thousands of homes, business and vital infrastructure while unlocking the economic, health and environmental benefits of the river between Egham and Teddington. It is an integrated scheme which will deliver both blue and green infrastructure – reducing flood risk, improving access to quality green open space, supporting a more sustainable travel network and delivering high-quality habitat to achieve a biodiversity net gain.
The RTS study has involved extensive modelling work covering many topics including:
Fluvial flood risk and option development in 1D, 1D-2D and 2D model versions (using Flood Modeller);
Integrated groundwater and water quality modelling;
Computational Fluid Dynamics (CFD) models of structures;
Low flow modelling; and
Sediment transport modelling.
Sediment transport modelling was required to assess the long-term impact of the scheme on both the existing River Thames and within the new flood channel. A thorough review of potential approaches and software was undertaken, with Flood Modeller selected as the best option.
Representation of the river channels was exported directly from the overall flood model to create separate simplified models of the River Thames and each flood channel section.
Runnymede channel sediment Spelthorne channel sediment transport model
Flood Modeller allows a choice of sediment transport equations and input parameters. For this study:
The Westrich-Jurashek equation was used to represent smaller cohesive sediments (clay/silt).
The revised Ackers-White equation was used for the larger non-cohesive sediments (sand/gravel).
Critical bed shear stress and settling velocities were defined for cohesive sediments, based on a literature review.
Ten bed material profiles were developed to represent the spatial variation in sediment along the flood channel route, based on borehole samples.
Sediment load was defined from River Thames sampling data, to relate load to incoming flow.
A 37 year simulation period was used, with inflows based on the available river flow gauging station record.
The Flood Modeller results were used to analyse several different aspects:
Bed levels in the River Thames. Cases with and without the RTS included were tested. This showed that the predicted impact on bed levels in the Thames is very small – an average change of only 0.1mm per year.
Deposition and erosion within the RTS flood channel. Cumulative changes over the simulation showed most deposition occurs during flood conditions, near the upstream end of the channel sections.
Sediment balance by sediment size and location. Most clays and smaller silts remain in transport through the flood channel, whereas larger silts and sands are mainly deposited. Most deposition occurs within the lakes that form part of the flood channel.
Overall sediment balance. There is an increase in deposition with the RTS but most sediment (over 90%) remains in transport and passes downstream through the system.
Channel maintenance. There will be some need for silt removal by clearing or flushing near the channel intake structures but the volumes are small.
Scour protection. The RTS design already included erosion protection for some parts of the flood channel bed. The need for this was confirmed by the modelling, with some additional areas identified as experiencing erosion without protection.
Overall, the sediment transport modelling helped to understand how the river system will behave with the scheme in place. It also led to some refinements and improvements to the design.
Rob Fraser
Rob is a chief engineer leading the Binnies UK river modelling team. He has over 20 years of experience as a hydrologist and river modeller and has led all the River Thames Scheme flood modelling for the last decade.
