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Managing the site can significantly reduce quality and quantity problems, and can provide improved amenity. Site management includes design and maintenance as well as the education of users.
Runoff increases in proportion to
the impervious area of the site. If the site is similar to its greenfield
state (prior to development), the runoff will also be similar. If less than 5 per cent of a site
is paved or compacted, the impact on the quantity of the surface runoff will
be negligible.
Reducing the amount of runoff also reduces the washoff of pollutants.
Rainwater harvesting can remove runoff from the drainage system altogether.
Hard paving and roofed areas can be drained onto unpaved areas. Driveways
and footpaths can be drained onto surrounding lawns (this is common in Europe).
The amount of pollution in the first flush of a storm will be minimised by keeping paved areas clean particularly around commercial or industrial areas. Maintenance measures such as sweeping hard surfaces regularly will reduce pollution. Planned maintenance regimes should be used for all impermeable areas. Preventing the accumulation of contaminants is even more effective. For example, placing canopies over areas of potentially high contamination removes the risk of surface water runoff becoming polluted.
Informing and educating users of the site about the way the site is drained can help prevent contaminants from entering the drainage system. Typical pollutants may include:
These should be used carefully and disposed of properly, not poured down
surface water drains. Fertilisers, herbicides and pesticides should be used
sparingly, in accordance with the manufacturers’ instructions, and not
used where they can be washed directly to a watercourse.
Litter and animal faeces can be kept out of drainage systems by education
and provision of bins.
Connecting foul sewers to the surface water system causes pollution. The use of swales and permeable surfaces can limit these mis-connections by replacing underground surface water drains because there are no surface water pipes to connect to. If a foul connection is made to a SuDS, the source of pollution will soon become apparent.
Preventative measures include good practice during cleaning, winter maintenance and general maintenance. See Control of pollution from highway drainage discharges (CIRIA, 1994) for more information.
Some substances are so polluting that special measures must be taken to contain them and stop them reaching the drainage system. Risk assessments must be made and appropriate bunds and treatment facilities provided. Further details are given in the Design of containment systems for the prevention of water pollution from industrial incidents (CIRIA, 1997).
Oil separators are discussed in the pollution prevention guidelines PPG3 (The Environment Agency).