Sustainable Drainage System, Rednock School, Dursley

Rednock Secondary is a Science Demonstrator School that secured funding under the Building Schools for the Future (BSF) One School Pathfinder scheme, enabling it to undertake a major redevelopment of its site and building stock, updating its current poor quality and disjointed range of buildings in a unified manner. The challenge of the development was to enable the school to function whilst the new building was constructed, to deal with the issues of contamination that arose from the previous land use and the significant level change across the site.

The project was taken through from landscape visual and character assessment and masterplanning, to comprehensive detailed design drawings and site implementation. The school had previously experienced incidental flooding on site due to the ineffectiveness of the existing drainage system and as the school was also aiming for the highest levels of sustainability and BREEAM excellent rating, from the inception of the project the surface water on site was to be dealt with by means of a Sustainable Drainage System.

The aim of the SuDS system was to maximise the existing potential of the site, to attenuate and clean water, while providing a valuable amenity. The SuDS were designed to provide many opportunities for the pupils to learn about the natural water cycle and habitats and for the staff to use it as a resource within the curriculum, by creating and integrating well designed landscaped features and promoting a greater diversity of flora and fauna.

Source control forms the first line of defence within the SuDS on site and a number of different features were included. The new building was fitted with a green roof system which acts as interception storage for the smaller more regular rain events (less than 5mm) and the first stage in treatment. This overflows into rainwater harvesting tanks, where water is reused for the toilet system in the school. Permeable paving in the carparks allows water to be attenuated and treated before overflowing into the next feature within the SuDS management train. Rain gardens were installed along the main entrance path to collect and infiltrate water from the surrounding hard surfaces.

A series of conveyance features including grassed swales and rocky channels were located around the new building to collect excess flow from the green roof, rainwater harvesting tanks and the car park, as well as surface runoff from adjacent hard surfaces. These convey and treat the water before discharging into the final ‘site control’ features.

The site control features of infiltration basin and wetland are located at the lowest part of the site and were partially lined to encourage some water to stay within the basin to support marginal plant communities and provide a teaching resource for the school. Water from an existing car park discharges directly into the wetland area via a reed bed, which has made the use of an oil interceptor redundant.

During and following construction and subsequent reviews a number of ‘lessons learnt’ emerged. The location of the new building and other site constraints did not allow sufficient space for full continuation of the swales around the building, and therefore sections of the channel had to be placed underground. This was not only more expensive but also did not allow a visual connection of the SuDS features to the front of the building with the features at the back, creating a lack of legibility through the scheme.

Two additional underground storage tanks were added along the management train during the construction process to increase storage volume due to late concerns expressed by the engineers. The wetland area now very rarely has water in it even after prolonged rain events, due to these tanks, which has reduced the effectiveness of this feature both as an educational resource and in terms of habitat creation.

Despite early risk assessments and discussion, the school added timber fencing around the swales at a later stage to prevent pupils from walking across the swales.