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

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The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC EXECUTIVE SUMMARY The Aquatic Services Unit (ASU) with technical assistance from the School of Biological and Earth Sciences (BEES) and the Hydraulics and Maritime Research Centre (HMRC), all part of UCC, were commissioned by An Taisce, the National Trust for Ireland, to carry out this review of the role of wetlands in flood attenuation in Ireland. As required under the EU Floods Directive (2007/60/EC), Ireland is currently developing a catchment based approach to flood risk management. An integral part of this process, as directed by European best practise guidance, is the identification of strategies to improve water retention within the catchment. The following review examines relevant wetlands types and their potential for flood attenuation in an Irish context, with an aim to their inclusion in future flood risk management in Ireland. Part I examined technical aspects and provides examples of wetlands in a flood attenuation role. Cost-effectiveness and economic values that can be attached to wetlands in a flood attenuation role are examined, through both market and ‘ecosystem services’ views. Part II identifies national legislation and policy affecting wetland habitat that may influence the potential for wetlands to become accepted as part of a national strategy for flood management. The report makes recommendations and is intended to inform national discussion on the future consideration of the use of wetlands for flood attenuation in Ireland. The importance of wetlands for flood attenuation Continuing urban and agricultural expansion in recent decades, often onto historic floodplains, has accentuated the need for cost effective flood prevention measures, particularly given future climate change scenarios of increasing frequency of extreme rainfall and storm surge events. At the same time, there has been a shift in focus in Europe and North America away from ‘hard’ engineering solutions, such as channel alteration and river embankment construction, towards encouraging more natural flood management (NFM) solutions within catchments. The UK’s ‘Making Space for Water’ and Netherland’s ‘Room for Rivers’ approaches, for example, promote spatial rather than purely technical flood management solutions by the provision of more room for peak river discharges. In this context, wetlands are increasingly seen as providing a potential valuable ecosystem service of flood attenuation. This is additional to their purported role as ‘buffers’ to prevent excess sediment and nutrient inputs into waterways and as conservation and biodiversity hotspots within intensively-used landscapes. The value of wetlands for flood attenuation is, however, often exaggerated and many wetlands in fact play only a very weak role, if at all, in attenuating floods. This is largely due to the very heterogeneous nature of wetlands in terms of location within a catchment, hydrological function and physical dimension. The role of different types of wetland with respect to flood control The main natural wetland types can be divided, hydrologically, into alluvial mineral soil floodplains (‘washlands’), which can temporarily store water which spills over the channel banks and headwater peatsoil wetlands (chiefly bogs and fens) which can slow the movement of water from hillslope into channels. Coastal wetlands / estuarine FINAL REPORT, February, 2012 6
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC floodplains can be considered a type of washland, which also attenuate waves and storm surges. The two broad types of wetland differ both in their position in the catchment and in the hydrological properties of their soils. Floodplain wetlands are typically inundated by river floods during major storm events and, by allowing floodwater to move over the banks and ‘spill’ over onto low-gradient land adjacent to the river, can mitigate the peak water volume in the channel. Headwater peatsoil wetlands, in contrast, typically are not inundated by overbank flow to any great extent, but instead receive water from rainwater and hillslopes after precipitation events and can mitigate the speed with which water moves from land into headwater drainage channels. The peat soils of headwater wetlands are saturated for much of the time and therefore possess little soil storage capacity compared with the drier mineral soils of the lowland floodplains. Taken together, the flood attenuation potential for alluvial soil floodplains is typically much greater than that of peatsoil headwater wetlands, and this tends to be supported by most recent empirical evidence. Wetland hydrological function and effectiveness in flood control Within a wetland type, the physical nature of individual wetlands will play a large role in determining its flood attenuation value. Floodplains receive water from four sources: river, rainwater, groundwater and hillslope. 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 rough floodplain surface, consisting of a complex of depressions, pools and ancient channels, 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. Rough topography - both of land form and woody, coarse vegetation – and unsaturated soils will also aid temporary water storage, and the high water evaporation and evapotranspiration of intact wetlands can also potentially reduce catchment runoff. The flood attenuation properties of individual floodplains will vary considerably, depending on their physical and hydrological characteristics. Floodplains with small surface area, high gradient, high hillslope flow and high groundwater levels will store water less effectively than large flat floodplains with low hillslope flow and low groundwater levels. The complex interaction between a given flood level and all these physical and biological characteristics of a floodplain makes it difficult to ascribe a quantitative flood protection value for particular wetlands without some kind of individual assessment. The hydrology of headwater peatlands is very different to alluvial floodplains and they generally receive much of their water either as rainwater (particularly bogs) or groundwater and hillslope (fens), rather than overbank flood water from channels. Bogs and fens can attenuate floods by delaying the runoff of these sources of water into headwater channels. As their peat soils are saturated for much of the time, particularly for blanket bogs, they generally have little effective soil storage capacity and their attenuation value comes from their rough surface topography slowing the movement of surface and sub-surface water, and their capacity to reduce catchment runoff through enhanced evaporation. Peatlands possessing tall woody or coarse vegetation, many small surface depressions and pools and with little surface channel connectivity will tend to hold surface water for longer, so delaying water discharge to downstream channels. Whilst water retention by peatland surfaces may attenuate and delay runoff events, the FINAL REPORT, February, 2012 7
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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