Two demonstration raingardens store and treat polluted rainwater that previously flowed to the Moselle river to reduce flooding and river pollution. Each takes a different approach to providing water quality treatment.
To enhance typical underused municipal greenspace by integrating raingardens that demonstrate different techniques and styles. The client, Thames21’s, primary objective for the raingardens was to reduce pollution reaching the Moselle river, however flood attenuation was also desired.
The raingardens were carefully designed into the existing landscape with the flow of water being evident from the kerb, throughout the installation to the final outlet. Two planting themes were chosen – one being ornamental herbaceous planting and the other laid out as a wildflower meadow. The upper, ornamental, raingarden also features a dramatic Miscanthus grass channel traversing the slope from road to raingarden enlivening what was previously an undistinctive area of mown grass.
Road runoff was intercepted by blocking off existing gullies and building new kerb inlets with channels to carry water under the pavement to the landscape areas containing the raingardens.
The upper raingarden employs a traditional free draining topsoil approach in its build-up. This approach is suitable for roof runoff which is less polluted but in this case the runoff is from a residential road which carries more pollutants. To provide additional treatment and to take out the more visible pollutants as soon as possible, a meandering channel was introduced between the road outlet and the raingarden basin. The Miscanthus grass planted channel also manages the significant level change between the road and the raingarden. This raingarden has a flow controlled drain-down and an exceedance channel that both flow to a wildflower meadow swale that leads to the lower raingarden.
The lower raingarden is a bioretention raingarden incorporating filtration layers beneath a free-draining topsoil in order to provide adequate treatment. A Miscanthus grass planted forebay area removes the more visible pollution before the main perennial meadow basin fills.
Discharge from the drainage layers passes through a Controflow flow control chamber before final discharge back to the surface water sewer.
Location : Boyton Rd, Haringey, London
Catchment area : 320m2
Return period attenuated : 1-in-10yr
Techniques used : Kerb inlets, planted channels, raingarden, bioretention raingarden, swale, flow controls.
Discharge destination : Surface water sewer to Moselle River
Discharge flow rate : 5 l/sec/ha
Client : Thames21 & Haringey Council