Case study:River Wandle: Silt & SuDs

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Location: 51° 22' 14.47" N, 0° 9' 34.59" W

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Project overview

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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	Case_study:River Wandle Restoration Project
This is a parent project encompassing the following projects	No

Volunteers adding Siltex to Carshalton Ponds in November 2014. Please seek permission from the Wandle Trust to reuse this photograph.

Project summary

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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

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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

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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.

Image gallery

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Catchment and subcatchment Select a catchment/subcatchment Edit the catchment and subcatchment details (affects all case studies in this subcatchment) Catchment River basin district Thames River basin London Subcatchment River name Wandle (Carshalton Branch at Carshalton) Area category 10 - 100 km² Area (km2) Maximum altitude category 100 - 200 m Maximum altitude (m) 160160 m <br />0.16 km <br />16,000 cm <br /> Dominant geology Calcareous Ecoregion Great Britain Dominant land cover Suburban Waterbody ID GB106039017640 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 Edit site Name WFD water body codes GB106039017640 WFD (national) typology WFD water body name Wandle (Carshalton Branch at Carshalton) Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body No National/international site designation Local/regional site designations Protected species present No Invasive species present No Species of interest Dominant hydrology Dominant substrate River corridor land use Average bankfull channel width category Average bankfull channel width (m) Average bankfull channel depth category Average bankfull channel depth (m) Mean discharge category Mean annual discharge (m3/s) Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) Project started 2012/07/01 Works started Works completed Project completed 2015/03/31 Total cost category Total cost (k€) Benefit to cost ratio Funding sources Defra Catchment Restoration Fund Cost for project phases Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname Investigation and design Stakeholder engagement and communication Works and works supervision Post-project management and maintenance Monitoring Reasons for river restoration Edit reasons for restoration Mitigation of a pressure Pollution incident, Flood risk management, Urbanisation Hydromorphology Substrate conditions Biology Physico-chemical Other reasons for the project Measures Edit Measures Structural measures Bank/bed modifications Reduce diffuse pollution Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions Social measures (incl. engagement) Community involvement Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit Physico-chemical quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Any other monitoring, e.g. social, economic Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents RRC Monitoring table - Wandle Additional documents and videos Upload additional documents CRF Project briefing note - Wandle Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information