Case study:Houndsden Road Rain Gardens

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Location: 51° 38' 21.52" N, 0° 6' 25.57" W

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Project overview

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Status	In progress
Project web site	http://http://www.thames21.org.uk/salmons-brook/
Themes	Flood risk management, Habitat and biodiversity, Social benefits, Water quality, Urban
Country	England
Main contact forename	Aimee
Main contact surname	Felus
Main contact user ID	User:AimeeThames21
Contact organisation	Thames 21
Contact organisation web site	http://www.thames21.org.uk
Partner organisations	London Borough of Enfield, Thames Water
Parent multi-site project	Case_study:Rewilding Enfield's Urban Rivers
This is a parent project encompassing the following projects	No

Planted swale April 2015

Project summary

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The Houndsden Gutter is a tributary of the Salmons Brook, a small river running through Enfield which joins the Lea Navigation. Like many urban streams it suffers from pollution. Oils and heavy metals wash off the roads when it rains, flowing into surface water drains which lead straight into our streams and rivers. The plumbing in some homes and businesses is misconnected, sending water from appliances, sinks and sometimes even toilets into watercourses instead of the sewers.

In 2015 we worked with the London Borough of Enfield to create two rain garden systems to improve the water quality in the stream. They have been designed to treat water that runs off Houndsden Road every time it rains, taking run-off from a busy section of Houndsden Road, which previously flowed directly into the Houndsden Gutter. Water is directed along the verge where it infiltrates into the soil or, in high flow conditions, into the large, natural woodland treatment basin.

In the first system a two metre wide swale was created along the verge using a low impact digger. A footbridge was created to allow the swale to continue into the natural basin of the spinney. Kerbs were dropped at two points along the verge to allow water to enter the swales from the road. Finally, the road gully just up from the top of the system was filled so water would be redirected into the swales. Further down same road a second rain garden system was implemented in September 2015, made possible with match funding from LBE. These roadside rain gardens have allowed for another road gully to be filled and more road run-off off to be filtered before entering Houndsden Stream. This scheme consists of a swale and two rain gardens taking water from Houndsden Road. In low rainfall conditions water is temporarily stored in the rain garden basins and slowly infiltrates into the verge. In high rainfall conditions, and when the basins reach a critical limit, water is transferred through a pipe, under the footpath and into the natural woodland basin in the spinney.

The water treatment occurs in two ways. In the natural woodland basin of the Spinney, chemicals which would pollute the river are naturally and safely used by plants to grow or broken down by bacteria in the soil. The swales have been planted with sedges and native grasses to slow water flow and begin the treatment, and the woodland basin planted with suitable native seed mixes. Water will then naturally soak into the soil, recharging the water table. This is particularly important during times of summer drought. These rain gardens prevent oils and heavy metals washing into the Houndsden Gutter. The stream supports a small fish population, and fish are seriously impacted by such pollutants, so reducing the pollutant levels in the stream is of great importance.

In addition through this project we flagged up a highly polluting outfall, which was then investigated immediately and rectified, having an enormously positive effect on the health of the stream.

Interpretation will be created to showcase the benefit of rain gardens for rivers, as well as access improvements in the woodland to allow more people to get close to the stream.

Monitoring surveys and results

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At the Houndsden Spinney it has thus far been impossible to collect road run off, so we have been unable to show an effect on water quality above and below the rain garden at this site.

Through our work alongside Thames Water and the EA to identify and rectify the misconnections blighting this site a significant improvement in water quality has been seen. We will soon be repeating invertebrate surveys to demonstrate this, along with public questionnaires which we hope will show an improvement in understanding of the urban water cycle and household misconnections.

Lessons learnt

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The road inlet designs had to be amended to capture water running along the gully with a road section cut out to allow a deep enough fall into swale. This has been rectified and the majority of run off is now entering the swales as designed.

The SuDS features are currently drier than anticipated. The resilient sedges are thriving, as are unwanted terrestrial plants. This has additional maintenance implications until replanting of edge species next spring.

Originally a much larger wetland was planned at this site to clean water in the stream. However the project had to be significantly scaled back as we were unable to provide data needed to the EA to approve the scheme, as collecting this level data would have been prohibitively expensive. It would be better in future to allow for longer lead time on the project and budget far more for pre-project monitoring, however in this case it is still unlikely that we would have been able to fund the data required. Monitoring of schemes such as the Salmons Brook Healthy River Challenge is imperative to provide proof that SuDS are a positive element in the toolkit of approaches to tackle urban diffuse pollution.

Image gallery

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Volunteers planting the swale

Creating the swales

Misconnection into stream

Sewage fungus

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 Salmons Brook Area category 10 - 100 km² Area (km2) Maximum altitude category 100 - 200 m Maximum altitude (m) 136136 m <br />0.136 km <br />13,600 cm <br /> Dominant geology Calcareous Ecoregion Great Britain Dominant land cover Suburban Waterbody ID GB106038027960 Other case studies in this subcatchment: Alma Road Rain Gardens, Bury Lodge Wetlands, Enfield Town Park Wetlands, Glenbrook Wetlands, Grovelands Park Wetlands, Laymer Road Silt Trap and Recreational Ground, Rewilding Enfield's Urban Rivers, Salmons Brook Flood Alleviation Scheme, Montagu Recreation Ground site, Salmons Brook River Restoration at Laymer Road, Salmons Brook at Grange Park Site Edit site Name Houndsden Spinney WFD water body codes GB106038027960 WFD (national) typology WFD water body name Salmons Brook Pre-project morphology Reference morphology Desired post project morphology Heavily modified water body Yes 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 2012/08/06 Works started 2015/03/02 Works completed 2015/10/09 Project completed Total cost category Total cost (k€) Benefit to cost ratio Funding sources London Borough of Enfield, corporate funder, Defra 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 Urban diffuse pollution from road run off Hydromorphology n/a Biology Pollution limiting biological potential Physico-chemical Pollution from misconnections and road run off Other reasons for the project Community demand, education project, interpretation of urban water cycle Measures Edit Measures Structural measures Bank/bed modifications Floodplain / River corridor Creation of swales and wetland basin Planform / Channel pattern Other Non-structural measures Management interventions Opening of tree canopy to encourage varied ground flora Social measures (incl. engagement) Community consultation, Community involvement, Interpretation boards, Improved public access 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 Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information