connection with British Rail) underpins good sustainable
drainage design. Essentially, it provides a series of interlinked drainage
techniques or features that incrementally
reduce pollution, flow rate and volumes to
acceptable levels. There are
numerous features that can be selected and built in to the drainage scheme at an early stage (which is preferable to bolting on the feature later).
There may be hybrid features, e.g. ponds with shallow wetland
margins or swales, which also infiltrate to ground through
soakaways.
The first step in the process can be very close to the source of surface run- off for drainage, and may involve infiltration methods, notably soakaways or, for buildings and car parks, green roofs and pervious pavements respectively. Having small control features at source removes the need for larger structures further down the chain. Drainage water may also be routed to
soakaways or detention/infiltration basins (which hold water during high rainfall/storm events and releases it at a
controlled rate) which, in turn, may be connected to wetlands and ponds for final clean up prior to discharge.
Typical Sustainable Drainage Scheme Components are:
FILTER STRIPS - wide, gently sloping areas of grass or other dense vegetation that treat run-off from adjacent impermeable areas.
SWALES - broad, shallow channels covered by grass or other suitable vegetation. They are designed to convey and/or store run-off, and can infiltrate the water into the ground (if ground conditions allow).
INFILTRATION BASINS - depressions in the surface that are designed to store run-off and infiltrate the water to the ground. They may also be landscaped to provide aesthetic and amenity value.
WET PONDS - basins that have a permanent pool of water for water quality treatment. They provide temporary storage for additional storm run-off above the permanent water level. Wet ponds may provide amenity and wildlife benefits.
EXTENDED DETENTION BASINS - normally dry, though they may have small permanent pools at the inlet and outlet. They are designed to detain a certain volume of run-off, as well as providing water quality treatment.
CONSTRUCTED WETLANDS - ponds with shallow areas and wetland vegetation to improve pollutant removal and enhance wildlife habitat.
FILTER DRAINS & PERFORATED PIPES - trenches that are filled with
permeable material. Surface water from the edge of paved areas flows into the trenches, is filtered and conveyed to other parts of the site. A slotted or perforated pipe may be built into the base of the trench to collect and convey the water.
INFILTRATION DEVICES - temporarily store run-off from a development and allow it to percolate into the ground.
PERVIOUS SURFACES - allow rainwater to infiltrate through the surface into an underlying storage layer, where water is stored before infiltration to the ground, reuse, or release to surface water.
GREEN ROOFS - systems which cover a building’s roof with vegetation. They are laid over a drainage layer, with layers providing protection, waterproofing and insulation.
What methods are best?
Deciding which components are most appropriate will be dependent on: • a thorough site assessment • the water catchment characteristics
• the quality and quantity performance requirements
• amenity/environmental requirements
For example, on a small compact site there may be little room for ponds and wetlands and, therefore, infiltration techniques (if ground conditions are suitable) or sub-surface storage may be the only viable alternatives. Environmentally sensitive areas can also impose stringent design
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