The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC
channel straightening, provides increased capacity and reduces the depth of local
flooding, but will tend to increase flood risk downstream. Temporary storage of water,
either on the catchment surface before it reaches a channel, or on floodplains once it
has spilled over channel banks will reduce channel discharges and attenuate the flood.
Those parts of the catchment that can retain surface water and retard its movement
into channels, thus, provide a valuable function in flood mitigation.
1.2 Wetlands and flood attenuation
In response to climate change predictions, the understanding of how weather patterns
create flood events has risen in prominence in recent years. However, there is still
considerable interest in predicting the interactive effect of catchment land use and
cover on flood generation. In particular, the potential for parts of the catchment to
delay run-off during high risk, low frequency, precipitation events is of high importance
for statutory bodies charged with managing floods and flood risk, e.g., UK Environment
Agency (EA) and Irelands Office of Public Works (OPW) as these may offer a relatively
low cost alternative to flood management compared with traditional, hard engineering
solutions. In many places (e.g. UK, Netherlands) natural flood management (NFM)
measures are now considered as an important part of sustainable flood management.
Wetlands are widely held to be important components of a natural landscape, through
their value to local and regional biodiversity and particularly their functional hydrological
role. Inland wetlands are often said to mediate groundwater recharge and discharge; to
‘buffer’ excess sediment and nutrient inputs, regulate base flow and, critical to this
review, attenuate flooding (Maltby, 1991, MA, 2005).
Most prominent of possible, inland, sustainable flood management solutions is the use
of catchment floodplains. The flooding of riparian land, leading to temporary storage of
flood water, attenuation of the peak discharge of a flood event and reduction of flooding
likelihood downstream has been well documented (e.g., Acreman, 2003; Bullock &
Acreman, 2003; Morris et al., 2004, 2010).
Less well known is the ability of peatlands (bogs and fens) to attenuate flooding,
although this is the focus of current and ongoing field studies (Holden et al., 2009,
Grayson et al., 2010) and catchment based flood risk management simulations such as
the Ripon Land Management Study, UK (JBA Consulting, 2007). Whilst some studies
suggest that peatlands within a catchment can reduce floods, others imply that since
peatland soils are normally saturated, they instead can act more as flood generating
areas (Bullock & Acreman, 2003).
Attenuation and hydraulic performance of certain types of constructed wetland are
more readily recognised since these are constructed to a design standard specific to the
role. The ability of carefully designed wetlands or detention basins to slow the flood
peak and alter downstream hydrographs is demonstrated by Sustainable Drainage
Systems (SuDS) facilities, but less so by Integrated Constructed Wetlands (ICWs).
Many, if not all natural wetlands thus have the potential to alter downstream peak flows
which gives them considerable economic and political value within a regional planning
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