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Galva Estate SuDS Retrofit - light case study


Galva Estate SuDS Retrofit and the surrounding highway works is situated in Cockfosters, this area is in the north west of Enfield and part of the Salmons Brook Catchment.

Salmons Brook is a highly urbanised catchment with a large proportion of impermeable areas generating runoff and hence a rapid or ‘flashy’ response to rainfall. Due to the highly urbanised nature of the catchment, pollutants from vehicles build up on the roads and are washed
into the surface water network. 

The Majority of Enfield has separate foul and surface water drainage system; however most surface water sewers discharge directly into the nearest watercourse. The implications of the discharge mean local rivers suffer from poor water quality as a result of urban diffused pollution and increasing the risk of flooding to high.  Cockfosters is known to suffer some localised surface water flooding, particularly in connecting roads. The increased expanse of hard surfaces, a lack of green spaces and inadequate capacity in the existing drainage system exacerbate the flood risk. This is an increasingly common occurrence on many roads in Enfield.

The project was made up of 3 Phases:

Phase 1: Retrofitting SuDS in an existing housing site was Phase 1. The main purpose was to work with our Senior Housing Horticultural officer who was planning on a shrub bed in this location, by working with him we adapted the design to take the down pipes form the surrounding sheltered housing flats into swales that lead to the shrub bed. We adapted the shrub bed to become an under-drained rain garden instead. There is also a domed gully inside the rain garden for exceedance events.

Phase 2: Our Highways Maintenance team made us aware that gullies on Cockfosters road could not deal with heavy rainfall and ponding frequently occurs as a result. Working with our Senior Housing Horticultural officer, we identified an opportunity to take highway runoff into the Galva estate, extending and enhancing the Phase 1 measures. The officer was looking at regenerating an existing shrub area, we used this site to remove the shrub bed and turn it into a rain garden. Side offlet kerbs were installed to take the road runoff under the footway and into swales which connect into the new rain garden area.

Phase3: Mount Pleasant road suffered badly from ponding on the kerbside, this was due to the fact there are no gullies on this section of road. To solve this issue, we installed 2 roadside raingardens in the large existing grass verges. We also removed a tree which was in decline and replaced this with 2 new trees in its place.

All 3 Phases help reduce the pressure on the existing drainage system.

Key Objectives:

  1. Reduce surface water flood risk and increase source storage
  2. Reduce the pressure on the existing drainage system
  3. Improve the public realm
  4. Increase biodiversity

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