Fen Management Handbook - Scottish Natural Heritage

12.2 Environmental regulation
12.2.1 Hydrological regulation
Wetlands have been described as ‘the kidneys of the landscape’ because of their
valuable role in the regulation of water quality and flow. Some fens fill rapidly during
floods, and then the floodwater slowly filters back out through the fen plants and
soils. This transient water retention function reduces the magnitude of peak flood
flows and can help reduce erosion caused by flooding, rather than simply displacing
the problem downstream. Fens also provide a resilient source of baseflow for
streams and rivers during drought conditions. The value of the hydrological
regulation function provided by wetlands is likely to increase as rainfall patterns
become more erratic as a result of climate change.
Construction of earth bunds or similar structures to enhance wetland water storage
capacity and retard return flow of flood waters to the main river channel is welcome
in conservation terms where wetland water levels have previously been reduced, for
example by abstraction from an underlying aquifer.
Negative consequences of promoting restoration of the ‘natural’ function of
floodplain wetlands and channels include risk of contamination by surface water
runoff from urban and suburban areas (e.g. hydrocarbons and salt) or by sediments,
dissolved nutrients and chemicals (e.g. biocides) from agricultural runoff. Careful
positioning of Sustainable Urban Drainage Systems can help mitigate such risks.
Doxey and Tillington Marshes SSSI is a good example of the flood
attenuation benefits of wetlands. This 124 ha of floodplain grassland,
marsh, swamp and pools reduces peak flood flows through the town
of Stafford which lies immediately downstream on the River Sow.
Staffordshire Wildlife Trust developed the Staffordshire Washlands
Project with the aim of restoring natural fluvial processes to rivers and
floodplain wetlands within the county.
In carrying out an Environmental Impact Assessment for a flood alleviation scheme,
or a project with flood risk implications, developers have a legal obligation to
investigate all reasonable options to fulfill the required function. The responsibility
of the wetland manager is therefore primarily to make sure that possible flood
alleviation benefits offered by a wetland are included in the cost-benefit analysis.
12.2.2 Attenuation of water-borne contaminants
Wetlands can break down or lock up contaminants, both in surface water
and discharging groundwater, thus improving water quality. An example of
this is denitrification of discharging groundwater, where microbially-mediated
transformation of nitrate to nitrogen gas is supported by the high concentrations of
organic carbon and the anaerobic conditions. Over 1200 wetlands in the UK have
now been purpose-constructed to attenuate a number of contaminants: to provide
a ‘final polish’ for water which has already undergone treatment, to treat industrial
wastewaters, road run-off and landfill leachate, and for domestic sewage treatment
from small communities.
Constructed wetlands are designed to mimic the bio-filtration properties of natural
wetlands but are engineered according to their location and the concentration, type
and volumes of contamination in the water to be treated. Water travels vertically
or horizontally through a filtration medium (sand, gravel or soil). The plants most
commonly used are common reed and bulrushes (Typha). It is important natural
wetlands are not used for this function.
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