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 experimental evidence that shows all three functions coexist. ICWs are a prime example (see Section 3.5.1), where high levels of particulate nutrients and sediment stored within the wetland are vulnerable to erosive flood flows, which may export such pollutants to the downstream channel, with negative consequences for water quality. During overbank flooding, water velocities over extensive, low gradient lowland floodplains are, however, likely to be low, allowing particulate material to be deposited on the floodplain from the floodwater’s suspended load. Though for floodplains constrained between narrow valley sides (which are relatively common in Ireland), overbank flood flows may attain relatively high velocities and erode, rather than deposit fine particulate material. The nutrient and sediment retention function of such floodplain wetlands may therefore conflict with their flood attenuation function when located in these valley forms. A preliminary study showed that drain blocking on upland peatlands in Britain reduced the production (through biotic processes) of dissolved organic carbon (DOC) with the potential to reduce DOC export to watercourses (Bonnett et al., 2008). Blocking of eroding gullies on upland peat was effective in reducing both stream flow and DOC flux in another UK study (O’Brien et al., 2008). Such potential for DOC export reduction in conjunction with increased water retention function of peatlands is positive for downstream water quality. Elevated DOC can impact negatively on water quality and aquatic ecology through its influence on acidity, trace metal flux, light penetration and energy supply (Evans et al., 2005). FINAL REPORT, February, 2012 44
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC 6. Methods to enhance and mimic natural drainage processes 6.1 Overview The concept of allowing space for water to inundate tidal wetland and alluvial floodplains has been crystalised in policies such as the UKs “Making Space For Water” and the Netherlands “Room for Rivers” (see Part II, section 9). Such policies have been driven by increased frequency of severe flooding in Europe and the need to find costeffective long-term strategies for flood defence that take future climate change scenarios into account. This has led to an array of solutions that protect, restore and emulate the natural regulating function of catchments, rivers, floodplains and coasts. A recent UK Environment Agency report – Working with Natural Processes to Manage Flood and Coastal Erosion Risk (2010) 19 provides a comprehensive overview of a range of techniques for working with ‘natural’ drainage methods in all areas of a catchment – upland, lowland, urban, rural, and coastal. The report includes a comprehensive list of some UK and international schemes, and a detailed description of some of these. There are further international examples of similar projects in ECOFLOOD (2006), Morris et al. (2004) and Rose et al. (2005). Table 3 summarises a range of different techniques and projects with demonstrable flood mitigation properties. The examples were chosen to include schemes within different parts of a catchment, from headwater to coastal, with an international scope to demonstrate that such schemes have broad acceptance. Within Ireland, other than small SuDS-type facilities (retention ponds and basins) and existing dams and resevoirs, only one example of specific use of wetlands for flood attenuation was found (Corkagh Park). Instead, methods that have been considered for floodplain storage options during design and feasibility stages for large OPW Drainage Schemes are reported on in section 7.3. 6.2 Restoring alluvial floodplain function By far the most common strategy of employing wetlands for flood attenuation is the utilisation of floodplain function. This almost always involves a combination of engineered and non-engineered strategies. At one end of the spectrum is allowing water to spill over natural (or artificially raised) banks - spread out over the floodplain and drain, by gravity, back to the channel. At the other end of the spectrum is the use of engineered inflow/outflow and containment structures to hold water within floodplain storage areas (‘washlands’). The solutions at a particular location will always be determined in relation to specific site attributes, such as, storage requirements, available area, topography, geography, biodiversity and economic constraints. Section 8 examines cost effectiveness issues. In general, strategies employed to restore river connectivity to floodplains are: removal of bank fixation; allowing/increasing lateral channel migration or river mobility; remeandering of water courses; shallowing of water courses; lowering river banks or floodplains to enlarge area for inundation; removal of hard engineering structures that impede lateral connectivity and setting back of embankments. Strategies employed to 19 http://publications.environment-agency.gov.uk/pdf/GEHO0310BSFI-e-e.pdf FINAL REPORT, February, 2012 45
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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