Exwick Heights Primary School, Exwick Lane, Exeter EX4 2FB
The SuDS scheme was incorporated as part of a new build of the school building and surrounding landscape.
The Exwick School, including the school building, car park, access road and associated education spaces occupy a plateau that is located at the top of a sloping site with views across the City of Exeter. A second plateau has been created to accommodate a sports pitch with the remainder of the site sloping quickly to the north.
Source control was an important part of the design to provide as much storage as possible at the top of the site as the rest of the site is sloping. To prevent pollution and provide clean water for amenity and biodiversity water flows to surface SuDS features in the school landscape.
Figure 1 Exwick School playing fields (Bob Bray)
Permeable pavement, green roof, detention basin, roadside swale, filter drain, weltand and pond. The sports area is also used as an extra storage space for surface run-off.
Figure 2 Permeable paving in school car park (Bob Bray)
Figure 3 SuDS components as part of the landscaping of the school playground (Bob Bray)
It is often difficult to manage runoff on severely sloping sites so the opportunity to control runoff within the hard surface construction profile at the top of the site guided the design process.
The SuDS design for the School collects, treats and stores runoff as soon as possible in a SuDS management train that deals with the following discreet areas.
All hard surfaces collect and store at least the first flush volume of runoff to control flows and pollution at source from day to day rainfall.
A flood route is demonstrated for each side of the school that flow to the sports pitch. The pitch has a reverse fall towards the school to retain the 1 in 100 return period storm, with a 20% allowance for climate change, in extreme storm conditions to protect properties below the lower site boundary.
The site was underdrained in the past and land drains flowed to a 300mm pipe outfall. All surface water runoff generated by the school is cleaned, attenuated to greenfield rate of runoff and finally leaves the site through this existing outfall.
Wherever possible clean water flows on the surface to provide a demonstration to the school of how SuDS work and how water can contribute to education, appearance and wildlife value. The design will allow simple everyday maintenance by site staff or landscape contractors.
Figure 4 Low flow channel to pond and outfall (Bob Bray)
The scheme was designed to ensure that the 100 year +20% climate change event did not exacerbate flooding immediately down slope of the school.
All hard surfaces collect and store at least the first flush volume of runoff to control flows and pollution at source from day to day rainfall. Elements such as car parks which produce more polluted runoff are treated separately from other cleaner elements ensuring high overall water quality.
The use of permeable tarmac was unfamiliar to the client and the contractor who required assurance that this type of surface was effective. The use of a protected filter strip, although acceptable at design stage was perceived as a hazard when reviewed by a Health and Safety officer. The surface was replaced by permeable block paving.
The following benefits were highlighted:
Direct runoff from hard surfaces is managed fully in accordance with the SuDS Manual, (CIRIA C697), and demonstrates pollution and flow control in the upper levels of the site.
Recent experience by the school has confirmed that volumes and flow rate from the site are being managed effectively, and the visual and biodiversity benefits anticipated are being realised.
Runoff is controlled effectively using construction profiles to provide source control storage and to reduce costs
Runoff from sloping sites can be controlled ‘at source’ to avoid problems at the bottom of the site
The use of playing fields can provide ‘long term’ storage to meet new storage objectives in the guidance
SuDS component can contribute to the visual character and biodiversity of sites, particularly schools where the SuDS can be an educational resource
SuDS can be maintained easily by normal school landscape contractors.
The following challenges were highlighted:
It is often difficult to manage runoff on severely sloping sites so the opportunity to control runoff within the hard surface construction profile at the top of the site guided the design process. Source control was a key element in ensuring successful performance of the scheme on the steep terrain. Where flows cross the contour, they drop down in stone filled baskets to prevent erosion before entering the biodiversity pond at the bottom of the site.
The day to day flows from land drainage within and above the development pass through the storage site and bypass the SuDS system to avoid filling the lower pond
Early SuDS design ensures flow routes are identified
SuDS use existing construction to collect, clean and store runoff
SuDS can reduce costs by reducing the need for dedicated storage features.
Information provided by Bob Bray, Robert Bray Associates.