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

The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC
The widespread view that wetlands, as a single habitat, are valuable for flood
attenuation has arisen in the absence of synthesis of empirical data. The most
comprehensive literature review on the hydrological properties of wetlands to date
(Bullock & Acreman, 2003) found that:
• The large majority of wetlands studied were found to have significant impacts on
the hydrological cycle;
• 82% of lowland floodplains (‘washlands’) but only 45% of headwater wetlands
studied were found to attenuate flooding. Almost half of headwater wetlands
studied (up to 2003) were found to increase either flood peaks or generate
higher flood volumes.
• Evaporation rates of all wetland types were generally higher than other land use
types with the result that 66% of wetlands reduced downstream flows during dry
periods, and only 20% of wetlands increased river flows during dry periods.
Although the majority of wetlands reduce flood peaks, a significant number of
those studied either had little effect or may enhance flood flows (Bullock and
Acreman 2003).
For the purpose of this review, we have assigned 6 broad categories of wetland types:
(1) alluvial floodplains; (2) peatlands; (3) karstic landscapes; (4) coastal wetlands; and,
(5) function-specific constructed wetlands. A process of literature review and
consultation was undertaken to investigate basic hydrological principles and flood
attenuation potential of these, presented in sections 3.1 to 3.5.
3.1 Alluvial floodplains
Floodplains are alluvial soil, fringing wetlands that receive water from four origins: river,
rainwater, groundwater and hillslope (Burt, 2001) (Fig. 1). River water will inundate
floodplains intermittently resulting from over-bank flow during periods of high fluvial
discharge. The water that moves from channel to floodplain is stored temporarily on the
floodplain surface and in floodplain soils, delaying the flood peak, before being released
later as channel water drops to below that of the water level in the bank (Hunt, 1990;
Whiting & Pomeranets, 1997).
The flood attenuation properties of individual floodplains will vary considerably,
however, depending on their physical and hydrological characteristics. The ability of a
floodplain to receive and discharge surface- and groundwater, as well as its ability to
store flood water, will be dictated largely by local climate (particularly rainfall pattern),
topography, geomorphology and soil, both of the entire catchment and the wetland
area itself (Gleason & Tangen, 2008; Shane & Regan, 2011). This complexity would make
it difficult to ascribe a quantitative flood protection value for particular wetlands
without some kind of individual assessment (Acreman & Miller, 2006; Acreman, 2011).
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