As part of our research Arc staff visited Enschede in Holland; a regeneration project that features a swale as its focal point. A swale is a man-made irrigation channel that aleviates surface water.
Enschede is now one of the most popular places to live in Holland and is well known as a centre for contemporary architecture and culture.
Planning authorities require developers to consider the feasibility of sustainable urban drainage systems (SUDs) such as soakaways, swales and green roofs before conventional engineering solutions. SUDs techniques not only help to reduce flood risk, but may also give rise to benefits in relation to water quality, biodiversity and, to some degree, air quality.
Examples of SUDs techniques are described below:
Swales: Swales are linear grassed drainage features in which surface water can be stored or conveyed. They have a significant pollutant removal potential and can be designed to allow infiltration under appropriate conditions. They are particularly suitable for diffuse collection of surface water runoff from small residential or commercial developments, paved areas and roads.
Soakaways and infiltration trenches: Soakaways are drainage structures with a high available storage. Surface water runoff can be directed to the soakaway, where the storage volume provides attenuation of the flows to allow gradual infiltration into the surrounding soil. An overflow can be provided for extreme rainfall events that exceed the storage capacity.
Filter drains: A filter drain comprises a perforated or porous pipe in a trench surrounded with a suitable filter material, granular material or lightweight aggregate fill. Filter drains have been used extensively for road and car park developments, where they have been constructed in the verge and median strip to intercept surface water and convey it to a suitable discharge outlet. The filter material provides biological treatment of any potential contaminants from the road surface.
Infiltration basins: Infiltration basins store surface water runoff and allow it to gradually infiltrate through the soil of the basin floor. An emergency overflow can be provided for extreme rainfall events, when the storage capacity of the basin is exceeded.
Detention basins: Detention basins are vegetated depressions. They are formed below the surrounding ground and are dry except during and immediately following storm events. Detention basins only provide flood storage to attenuate flows. Extending the detention times improves water quality by allowing the settlement of coarse silts.
Retention ponds: Retention ponds are permanently wet ponds with rooted wetland and aquatic vegetation mainly around the edge. These systems offer a degree of biological treatment.
Wetlands: Wetlands comprise relatively shallow ponds and marshland areas which are covered almost entirely in aquatic vegetation. Wetland vegetation is well suited for the biological treatment and removal of dissolved contaminants and nutrients, and the use of wetlands for water and wastewater treatment is well documented.
Permeable surfaces: Permeable surfaces are load-bearing constructions surfaced with materials that allow surface water to enter the underlying construction. Permeable surfaces remove the need for conventional drainage ancillaries such as gully pots and manholes and can be used as attenuation or infiltration devices.
Filter strips: A filter strip is an area of vegetated land through which runoff is directed. It usually lies between a hard-surfaced area and a receiving stream, surface water collector or disposal system. Filter strips can be in any natural vegetated form, from grass verge to shrub areas. Silt is trapped by the vegetation and naturally incorporated into the soil over time.
Rainwater harvesting: Fresh water is a limited and precious resource that is often taken for granted. The conservation of freshwater resources must be improved. A rainwater harvesting system can save up to 50% domestic water consumption and industrial use could save even more. By treating water with UV light, it is possible to make the water safe to drink.
Garden roofs: In Europe, green roof technologies have become very well established. As well as providing rainfall attenuation, green roofs represent opportunities for significant social, economic and environmental benefits. One sqm of green roof can remove 0.2kg of airborne particulates every year; 1.5sqm of uncut grass produces enough oxygen per year to sustain a human for that year.
