Three raingardens, each designed in a different style, provide the students of St Peter's High School, Gloucester, with vibrant, wildlife-friendly landscape features that come alive when it rains, helping to reduce flooding and improve water quality in Whaddon Brook.
The raingardens formed part of a wider retrofit project within the school grounds commissioned by the Environment Agency to help reduce flooding and improve the water quality of the nearby Whaddon Brook.
The ‘raingarden’ is increasingly seen as an effective way of managing rainwater from roofs and pavements in a simple and attractive way. We identified three locations within the heart of the school campus where simple raingardens could easily be installed to store and slowly release roof and pavement runoff.
We wanted the raingardens to offer educational value as well as improving the appearance of the landscape.
The three raingarden styles at the school show how the basic function of this SuDS feature can be expressed in a number of different ways: the classic raingarden, a biodiversity raingarden and a productive ‘potager’ raingarden.
Taking water from adjacent roofs, the raingardens demonstrate how easily raingardens can be integrated into existing developments and reinvigorate under-used spaces.
An opportunity to personalise the raingardens in student construction projects, particularly in the way water is delivered from roof level to the raingardens, has been left to the ingenuity and creativity of students.
Rainfall is intercepted from roofs and adjacent pavements using tactile paving channels, grass channels and, at the moment, temporary rainwater gutters and downpipes that carry water to the raingardens.
The soil in the basin is improved, by incorporating compost and sharp sand, to help water soak into the ground and grow a wide selection of plants. The natural soil conditions at the site are clay so where rain cannot soak away very easily there is a flow control to hold water back for a while and let water seep away very slowly to the existing pipe system with an overflow for exceptional storms.
Most of the rainfall that runs into the rain gardens from the roof will soak into the soil, evaporate or be breathed out by plants. In very heavy rainfall there will be a small amount of water standing in the raingarden for an hour or two. Very rarely there will be up to 350mm of water for a few hours before it slowly flows out of a pipe at the end of the basin back into the sewer.
Each raingarden can store a particular volume of water that is held back for a short period of time. In the case of the Wildlife and Ornamental raingardens this volume is just day to day rainfall, holding the 1 in 2 year return period rain event. The Potager raingarden is able to store up to the 1 in 10 year return period rain event.
The clay soil present on site limits infiltration, so simple protected flow control devices allow a maximum discharge rate of 5L/sec to the surface water sewer with an un-controlled overflow to the sewer once storage capacity is exceeded.
the client: Environment Agency & St Peter's RC School
return period: 1:2 (classic and biodiversity raingarden) and 1:10 (potager)
discharge destination: pipe to the Whaddon Brook
discharge flow rate: max. 5L/sec up to storage limit then overflow to sewer pipe