The Ashby Grove raingarden in Islington was designed and constructed as a practical example of what can be done in small landscape spaces as suggested in the Islington SuDS Design Guide.
This raingarden was conceived to demonstrate how a small raingarden could manage the roof water from a typical house.
Although there are other examples of raingardens in the UK as far as we know this is the first to be designed to collect return period volumes with drain down outfall based on a ‘greenfield rate of run-off’.
The Raingarden has be planted in a gardenesque style using ornamental herbaceous plants that can tolerate occasional inundation with water and which provide seasonal variation and interest. It is also important that the planting provides a continuous cover to encourage infiltration into the soil. This style of planting may require occasional lifting and splitting of herbaceous plants on a three to five year cycle as is normal for garden planting.
The Raingarden has been welcomed by the residents and is looked after by the local housing group. However even after very heavy rain, water is rarely seen in the basin as it soaks into the ground as quickly as it falls from the sky.
The Raingarden manages run-off from approx. 30m2 of roof and is large enough to hold the 1 in 100 year return period with a 30% allowance for climate change. This is larger than is typical for raingardens which originally evolved in the US to manage day-to-day rainfall events and which should accommodate up to approximately the 1 in 2 year event.
In practice the ‘raingarden’ is located over urban debris due to human activity and Second World War damage. Therefore although originally designed for a clay sub-soil water penetrates rapidly into this free draining urban soil.
Roof run-off, which is relatively clean, is collected by an existing down pipe at ground level and fitted with a ‘shoe’ to direct water onto a sett channel and then into the Raingarden.
The Islington Raingarden is also planted using commonly available garden plants that can tolerate being submerged for 24-48 hours.
The topsoil was specified using a root zone mix together with organic matter to allow water to filter through the soil profile.