Case study:River Wandle: Silt & SuDs

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Location: 51° 22' 14", -0° 9' 35"
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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.


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Catchment and 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) 160
160 m
0.16 km
16,000 cm
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

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

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

Mitigation of a pressure Pollution incident, Flood risk management, Urbanisation
Hydromorphology Substrate conditions
Biology
Physico-chemical
Other reasons for the project


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

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

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

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents




Additional documents and videos


Additional links and references

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Supplementary Information

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