Swales and conveyance channels overview
The transfer of surface water runoff (conveyance) across the site, between components is essential. There are a variety of approaches that can be used; underground through pipes with little control or water quality treatment, or through vegetated channels on the surface providing some treatment and attenuation and through more engineered canals or rills. The preference in terms of delivering sustainable drainage objectives is the conveyance of water through vegetated channels or swales. Uncontrolled conveyance to a point of discharge into the environment is discouraged.
Overland flow routes will also be required to convey and control floodwater safely during extreme events. In general, the greater the number of components used in series, the better the performance is likely to be, and the lower the risk of overall system failure.
The components covered in the section include:
• Channels and rills
Swales (figure 1) are usually shallow grassed or vegetated channels used to collect and/or move water. The shallow side slopes and flat bottom means that for most of the time water flows in a thin layer, some of them can be under drained with the use of perforated pipe. Swales can also provide some storage and filtration as well.
Canals and rills are open surface water features with hard edges, they can be commonly found in some of existing cities, towns and villages. They can have a variety of cross sections to suit the urban landscape and can also be planted to provide water treatment. In dense urban developments, or retrofit situations they can be an effective way of providing SuDS and if appropriately designed can also act as pre-treatment to remove silt before water is conveyed into other SuDS components.
Treatment channels collect water, slow it down and can provide storage for silt and oil that is captured. The outlet is designed to act as a mini oil separator and enables the channel to be very effective at treating pollution.
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