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 slopes with impeded drainage, and a greater contribution from within the near-surface layers of blanket peat on steeper slopes (Holden & Burt, 2003). Topography and preferential flow paths are important controls on the spatial production of runoff. No significant discharge emerges from the lower layers of peat except from preferential flow pathways (‘soil pipes’), which contribute around 10% of the discharge to the catchment. Most flood storage arises, therefore, from a rough micro-topography and minimised sub-surface flow (Holden & Burt, 2003). The run-off hydrograph of a bog can thus be characterised by a small but perennial baseflow with superimposed storm peaks. Storage effects are unlikely to contribute significantly to attenuation of winter floods, although they may affect runoff arising from storms following long dry periods (Bragg, 2002). Stream flow downstream of a bog peatland is a function of the amount and intensity of precipitation, antecedent conditions, the nature of the peat profile, the location of the wetland within the landscape and the topographic forms within the wetland (Bay, 1969; Verry et al., 1988; Branfireun & Roulet, 1998). The variation in these parameters can lead to very different conclusions about the flood storage potential of bog peatlands. In a study of a Minnesota raised bog peatland, Verry et al. (1988) found that effluent streamflow responded to large storms almost the same way as streamflow from a level, unregulated, reservoir, and that thehe peat, hummock-hollow topography, and tree boles reduced the streamflow rate slightly. Flow rates from the peatland following a very high storm event were actually higher than a reservoir, when wedge storage and a channel-like flow system developed in the lagg area of the peatland. Similar raised bogs in the same location were found earlier to have relatively little impact on streamflow, but that they did store storm runoff, particularly after summer dry periods when bog water tables were low (Bay 1969). In contrast to these findings, Bragg (2002) found that the effluent discharge response of a Scottish raised bog to individual storm events was delayed by up to 22 h relative to that of streams draining nearby, non-bog catchments, after a long dry spell in summer. The time lag amounted to 3–6 h even under conditions of zero storage deficits, indicating that the bog was more effective than the surrounding mineral slopes in delaying runoff, even in wet weather. 3.2.2 Fens Few studies have been carried out on the flood attenuation properties of groundwater (fen) peatlands, and there is no great consensus as to their flood attenuation properties. The high groundwater tables prevalent in many fens will reduce their storage capacity and so limit their ability in many cases to reduce storm-flow volumes (Paavilainen & Päivänen, 1995). In flatter and larger fens, temporary surface water storage may however potentially regulate and attenuate peak runoff. In a study of the flood attenuation potential of Norwegian flat fen, runoff peaks following precipitation events were found to be delayed and attenuated by the fen through the temporary storage of water in surface depressions and to surface topography-induced friction to overland flow (Kværner & Kløve, 2008). Peak outflow was retarded more strongly during small runoff events, a function of the greater friction to overland flow at lower water-table levels. During large events, peak flow retardation occurred because of water storage provided by flooding and filling of local depressions. Further, water-table observations revealed that increasing areas of fen became saturated (leading to greater spatial extent FINAL REPORT, February, 2012 26
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC of temporary surface storage) at increasing runoff, indicating the importance of the size of the storage basin and the narrowness of the fen water outlet (Kværner & Kløve, 2008). The importance of temporary water storage in surface depressions of fens has been supported by recent work by Frei et al. (2010) who showed, using virtual models of wetlands, that the complex micro-topography efficiently buffered rainfall inputs and produced a hydrograph that was characterized by subsurface flow during most of the year and only temporarily shifted to surface flow dominance (> 80% of total discharge) during intense rainstorms. These recent studies demonstrate that intact peatlands with a well-developed micro-topography, (i.e., small pools and hollows) can attenuate storm flows by allowing greater surface water storage. In fens with extensive areas of peat diggings, the increase in depressions and hollows is likely to increase flood water storage. North American ‘Prairie potholes’ and European fens are both groundwater-dominated depression peatlands and thus functionally similar hydrologically. As in Ireland, wetland drainage for agriculture has significantly decreased wetland storage volume (Gleason & Tangen, 2008) and this reduction has been linked to increased frequency of flooding in the Prairie Pothole Region. In an effort to mitigate wetland losses, wetland restoration has been widely advocated, leading to increased research to estimate the flood attenuation properties of such wetlands. Gleason and Tangen (2008) used morphometry data from multiple wetlands in the Prairie Pothole Region to estimate maximum water-storage capacity and interception area of wetlands on the studied lands. They found that pothole wetlands had significant potential to intercept and store precipitation that otherwise might contribute to downstream flooding. A similar pattern of the flood attenuation potential of depression wetlands was found by Lindsay et al. (2004) in a study of Canadian Shield groundwater wetlands. In their study, catchments containing extensive wetlands were marked by a significant decrease in maximum peak discharge and increase in duration of flow during wet periods. The value of such wetlands to attenuate large flood events may, however, be overstated. In a case study on the flood attenuation value of wetlands within the Red River Basin, Manitoba, Canada, Simonovic et al. (2001) showed that although an increase in wetland area could potentially reduce total flood volume, the capacity of wetlands to attenuate large flood events was limited. Similarly, the results of simulation modelling in a US study found that Prairie Pothole wetlands reduced flooding of an annual event by 9–23%, compared with only 5–10% for a 100-year event (SAST, 1994, cited in Leibowitz, 2003). Bengtson and Padmanabahn (1999; cited Schultz & Leitch, 2001) found that restoration of 2,700ha of upland depression wetlands in the North Dakota’s Maple River Watershed (US) resulted in flood peak reductions of 3.8% for small events, 2.2% for medium events, 1.7% for large events and 1.6% for very large events, with the assumption of 1 foot of storage ‘bounce’ 10 . When storage ‘bounce’ was increased to 2 foot the figures increased to 5.4, 3.2, 2.4 and 2.4 percent, respectively. 10 ‘Bounce’ refers to the available storage potential of a wetland. This changes depending on the level of saturation. Wetlands can be at full capacity after heavy and/or prolonged rainfall, at which stage they would have no ‘bounce’. FINAL REPORT, February, 2012 27
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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