A grass verge along the Priory Road, became an ideal place to trial a very simple SuDS approach to cleaning and storing road runoff. Runoff was diverted from a single gully, into a new ‘Rainmeadow’. Once in, this runoff was retained by a low berm. The ground was lightly leveled and seeded with a biodiverse meadow, to ensure that rainwater spread widely, and slowly. The sinuous meadow is delineated by a colourful and inspirational walk of bollards - each highlighting the cause of the nearby River Moselle.
The Rainmeadow is one of three simultaneous SuDS interventions along the ‘Priory Road SuDS Highway’. The Moselle had been identified as having poor water quality, and so these SuDS systems were commissioned to improve water quality, as well as to reduce flood risk, provide amenity and biodiversity.
The scheme also intended to demonstrate the efficacy of introducing SuDS flows to urban tree roots. Extensive discussions with Borough Arboriculturalists were sought, in order to best achieve this, while balancing concerns. Stringent cultivation depths and areas were agreed, in order to protect and enhance the conditions for the trees in question.
The site - The A504, Alexandra Palace - benefits from excellent visibility, and so proposals were designed to utilize the full 85m length, so to offer visual benefits to motorists and pedestrians alike.
The site, as well as bordering a main road, is part of a thriving residential community. Extensive consultation with the community was undertaken, and a positive input and response to the concepts was indication of suitability. Old photography demonstrated.
Facing the entrance to the Rainmeadow there is a community garden, well maintained and productive. The Rainmeadow was designed to create a pleasant compliment to the adjacent gardens – a biodiverse meadow, with the retention of existing cow-parsley as a good shade tolerant wildflower would bring further pollinators. A sweeping array of interpretation bollards,. inscribed with messages to reinforce the positive purpose of SuDS was designed by RBA, with text by Thames 21. The style of these bollards is repeated in the Rainpark, some 750m to the East.
The Rainmeadow is unconventional, in that it does not store a specific volume. It is designed to act as a broad meandering swale, with some light ponding, in order to create a ‘sheet flow’. This has several benefits, regarding water quality and quantity.
1) Infiltration rates are typically much higher below trees. For example, measurements from a fully grown lime tree (crown diameter approx. 14 m) in Malmö in summer 2006 showed that that particular tree consumed around 670 litres of water per day during the month of July(Planting beds in the city of Stockholm - A Handbook, 2009).
There are five fully mature London Planes within the verge – surveys show a canopy average of 15m.
2) Meadow plants, as well as offering visual and biodiversity benefits, will provide excellent cleaning of polluted water, as longer grasses and vegetation provide a better level of cleaning than shorter vegetation.
3) The length of travel is important, and the 85m of travel here provides excellent cleaning. It also prevents ‘warm’ runoff from hot road surfaces (in summer this can reduce the waters capacity to hold oxygen) from entering the watercourse.
A central channel with a weir, slows the movement of water further, while allowing a clear passing point for pedestrians over the SuDS. At the end of the system, a letterbox weir allows for the free passage of any rainwater through a brick wall. The letterbox is designed to admit any remaining flow, and neatly outlet to a kerbside channel next to an existing gully.
Location : Hornsey, North London
Return Period Attenuated : Variable
Techniques : Gully and Rainmeadow
Discharge Destination : Surface Water Network
Client : Haringey Council/Thames21