Case study:River Wandle: Silt & SuDs
Project overview
| Status | Complete |
|---|---|
| Project web site | |
| Themes | Flood risk management, Hydromorphology, Social benefits, Water quality, Urban |
| Country | England |
| Main contact forename | Bella |
| Main contact surname | Davies |
| Main contact user ID | |
| Contact organisation | Wandle Trust |
| Contact organisation web site | http://www.wandletrust.org |
| Partner organisations | Environment Agency, London Borough of Sutton, Queen Mary University of London, Thames Water, Defra |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
This Project is part of a larger scheme with the overall aim of establishing a population of brown trout in the Carshalton arm of the River Wandle for the first time in over 80 years. For more background on this larger scheme, please refer to the parent project "River Wandle Restoration Project".
The overall aim of this Project was to trial and evaluate innovative control measures for mitigation of urban diffuse pollution in an urban environment.
The River Wandle turned black with road runoff every time it rained and whilst Environment Agency data did not detect a problem (due to spot sampling rather than continuous monitoring methods), the Trust suspected river sediments were contaminated and the first flush after rain caused fluctuating water quality, likely limiting the successful recruitment of trout.
The Trust worked with an MSc student from Queen Mary University and confirmed that copper and 15 types of PAH were at a level that would have a severe effect on aquatic biota.
To reduce urban diffuse pollution, five innovative measures were installed and trialled to determine their effectiveness for mitigation of urban diffuse pollution, particularly road runoff:
To reduce urban diffuse pollution, five innovative measures were installed and trialled to determine their effectiveness for mitigation of urban diffuse pollution, particularly road runoff:
1. Rain Gardens installed to intercept surface water and aesthetically enhance an area in Hackbridge.
2. Downstream Defenders (DD) intercept surface water drains and remove sediment by centrifugal force.
3. Mycofilters are bags full of a substrate as a feedstock for the native oyster mushroom which accumulate toxins such as heavy metals from the passing water. A total of 180 mycofilters were made and grown by a team of volunteers, installed in 4 locations on the lower Wandle.
4. Siltex is a natural chalk-like substance which helps to increase the speed of silt breakdown by stimulating natural processes. Working with local volunteers, Siltex was added to Carshalton Ponds in November 2014 and samples taken for the proceeding months to determine the effectiveness.
5. Smart Sponges are a commercial product specially designed to absorb oils mixed in water. When placed in gully pots, Smart Sponges can permanently remove oils from the drainage system before they enter the rivers. A total of 23 Smart Sponges were installed in gully pots along the catchment of one surface water drain linking to a DD.
Monitoring surveys and results
For wider project benefits as part of the larger "River Wandle Restoration Scheme" please see the parent project.
Monitoring was completed for Mycofilters, Siltex, Smart Sponges and Downstream Defenders.
Downstream Defenders: A Downstream Defender (DD) was monitored using upstream and downstream flow meters and water samples were taken using autosamplers during heavy rainfall events. Samples were sent to a UKAS accredited lab to be analysed, and contaminant load was calculated. The surface water pipe along which the DD was positioned had unexpectedly high flows (up to 50 l/s). When flows were within the operating range of the DD it was effective at removing around 47% of Total Suspended Solids (TSS), however, over the a whole data period it removed 9%. These numbers are within the range found by Dr Thomas Curwell, who investigated multiple rainfall events in DDs and found that they were effective in removing 9-49% TSS. Despite the low removal inefficiencies, the units remain the most effective measure if only a few square meters are available to SERT's knowledge, and are likely to have a cumulatively positive effect. The Trust are currently working on a sediment trap-wetland treatment train in an area where there is more space as an alternative method, following promising results from the scientific literature.
Mycofilters: Upstream and downstream water samples were taken for Mycofilter sites and control sites (using straw bags with no oyster mushroom).
Smart Sponges: The concentration of hydrocarbons collected in the gully pots with and without the sponges is being monitored.
Siltex: Silt and water are being monitored.
Lessons learnt
Challenges:
- Identify spaces for DDs underground amongst utilities.
- Each defender had to be custom made to match the angle of the various outfall pipes allowing them to integrate with the established system.
- Getting commitment from parties to empty the DDs.
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Catchment and subcatchmentSelect a catchment/subcatchment
Catchment
Subcatchment
Other case studies in this subcatchment: Butter Hill Phase 2, River Wandle Restoration Project, River Wandle: Butter Hill Phase 1, River Wandle: Silt & SuDs, Wandle River Restoration Project: Hackbridge
Site
Project background
Cost for project phases
Reasons for river restoration
Measures
MonitoringHydromorphological quality elements
Biological quality elements
Physico-chemical quality elements
Any other monitoring, e.g. social, economic
Monitoring documents
Additional documents and videos
Additional links and references
Supplementary InformationEdit Supplementary Information
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