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 Fig. 3 Example of isolated inter-drumlin fens and lakes, south of Bellanode, Co. Monaghan The Drumlin Regions of the north east of Ireland are peppered with small groundwaterfed depression wetlands (fens) (Foss & Crushell, 2007; 2008), and although many have been drained and converted to pasture, numerous small, isolated fens also occur in parts of Munster. The ability of these small, widely dispersed wetlands to decrease flooding at a catchment scale ultimately depends upon the total available water storage capacity relative to the volume of floodwater and the water balance (Potter, 1994). Individual wetlands probably have quite limited storage and attenuation potential on their own, but to illustrate the potential impact that isolated wetlands in a landscape may collectively have on flood attenuation, three examples are used: (1) The increased severity and frequency of flooding at Devils Lake, North Dakota, prompted an investigation into the level of flood storage that could be gained by restoring the wetland resource within the lake’s various sub-catchments. The region is characterised by many isolated groundwater wetlands, many of which have been drained for conversion to agriculture. Modelling estimated the storage potential in; (1) possibly 11 intact/undrained wetlands; and, (2) possibly drained wetlands. A number of scenarios were modelled, such as gains in storage capacity derived from restoring 25%, 50%, 75% or 100% of drained wetlands. The runoff reduction estimates based on possible storage capacity increases indicated that depression wetland restoration could reduce the volume of runoff entering Devils Lake, and contribute to reduction of flood risk (Doeing & Forman, 2001). Despite limitations within the model, the study shows the potential for a collective, isolated-wetland resource to contribute to flood risk management. 11 The prefix ‘possibly’ was used as depressions were located and assessed using remote methods. Lack of ground-truthing is a major limitation to modelling for this purpose. FINAL REPORT, February, 2012 28
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC (2) Lane & D’Amico (2010) used remotely-sensed Light Detection and Ranging (LiDAR) data to estimate potential water storage capacity of isolated wetlands in north central Florida. They also modelled the water storage potential of what they highlighted as >8500 polygons identified as isolated wetlands and found that, collectively; they stored 1619 m 3 /ha on average, with a median value of 876 m 3 /ha. They discussed the difficulty of quantifying wetland ecosystem services, especially the difficulty of scaling the loss of numerous individual wetlands to effects at the catchment scale. Their results could be used to improve watershed models that incorporate isolated wetland recharge, discharge, and flow-through hydrodynamic processes, which may enable estimation of the potential attenuation role of such wetlands. (3) Schultz and Leitch (2001) simulated restoration of depression wetlands in the Red River Catchment, North Dakota. They found that wetland restoration could improve flood attenuation at a catchment scale, but that wetland restoration was not a cost effective solution given the scale of the flooding problem (see section 8.3 for cost effectiveness in relation to this example). The picture that emerges of the flood attenuation properties of peatlands is one of potential, rather than realised flood storage. Increasing flood storage potential is gained where: (1) surface water runoff is delayed and temporarily stored within dips and hollows of a hummocky, uneven surface topography , (2) peatland gradient is low, (3) peatland areal extent is large, (4) vegetation is dense and rough and retards surface flows, and (5) soil saturation is low. Thus, for undisturbed peatlands with a large surface area, low gradient, relatively unsaturated surface soils and rough surface topography, flood storage potential is high. Small, steeper wetlands with a high water table, smoother surface and enhanced drainage runoff (such as occur for grazed peatlands) would likely have lower flood attenuation potential. Ireland has a significant peatland resource (Conaghan, 2001) and better knowledge of the catchment scale effects of peatland management in Ireland is critical to inform flood management policy and strategy, and to inform conservation goals. 3.3 Karstic landscapes Karstified areas of Carboniferous Limestone, common in the west of Ireland, are vulnerable to groundwater flooding and are categorized as either upland or lowland systems (Zaidman et al., 2010). Upland karst systems (such as the Burren, Co. Clare) are characterized by low or poorly-connected storage and a steep hydraulic gradient. These areas are subject to localized flash flooding, with little or no surface storage. Lowland karst systems possess complex surface-groundwater interactions that influence the seasonal inundation of turloughs in which water levels are both drained and recharged through sinkholes (Zaidman et al., 2010). The conservation status of turloughs is currently being investigated through an NPWS project undertaken by the School of Natural Sciences, Trinity College Dublin (TCD), coordinated by Dr Steve Waldren. The work has involved investigations of ecology and hydrology at 22 Irish turloughs. Preliminary indications are that the hydrological processes of each individual turlough are unique and complex (Dr Laurence Gill, pers. comm.). Flood mitigation assessments at each site must, therefore, be evaluated on an individual basis (Ní Bhrion, FINAL REPORT, February, 2012 29
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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