SuDS in London - a guide

1.3.2 Water quality
Surface water is often polluted. Runoff
from roads, for example, includes
contaminants from tyre abrasion such
as rubber and soot, nickel and chromium
from brake pad linkings and oil, silt and
iron oxide from general traffic use.
During warm, dry periods, these
substances build up on sun-warmed
surfaces and heavy showers can wash
them into the drainage system. This creates
a warm, contaminated, low-oxygen
water mix, which flows into watercourses
and groundwater.
Managing the quality of runoff helps
protect the natural environment from
pollution and SuDS can be crucial in this
respect. The risk of pollution in a SuDS
scheme must be assessed and a mitigation
strategy proposed to determine the
required number of treatment stages to
ensure water is clean enough to flow to a
watercourse.
SuDS can also improve the quality of
water entering combined sewers, reducing
pressure on sewage treatment plants.
Improvements to water quality can also
contribute to amenity and the potential for
for biodiversity. Reed beds, for example,
which naturally slow and treat water,
provide an active edge to water features
and attract a wide range of birds and
insects.
Designing for water quality must take
account of:
• Interception and treatment methods
to meet CIRIA standards
• The quality of surface water and
groundwater receiving run-off
• The extent of existing pollution
control systems in the catchment
• The extent to which risk management
measures for spillages of contaminants,
such as oil, are in place
• The proportion of permeable
surfaces, green roofs, and/or surfaces
discharging to a rainwater harvesting
system or soil-based feature
• The proportion of the surface water
management system that is on or near
the surface to facilitate treatment
Reed beds can contribute to water quality
• The extent to which the design of
the system incorporates sediment
retention, such as forebays or
hydrodynamic separators
• System resilience to cope with future
demand, including allowances for
climate change and urban intensification
13 1 Principles of SuDS