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 solutions including consideration of coastal realignment and spatial planning approaches in addition to traditional, hard engineering strategies, but it is unclear to what extent this has been implemented. It was not possible to locate any Irish examples of coastal realignment during this review, although, with regard to spatial planning, the recent Fingal County Development Plan (2011-2017), for example, does specify that “Development should be set-back a sufficient distance from soft defences and erodible coastline to allow for natural processes, such as erosion and flooding, to take place” (p180). Part II, Section 6.2, highlights the lack of a national coastal policy. Although a national policy document on Integrated Coastal Zone Management (ICZM) was published in 1997, it has never been taken forward in any Government department. It should be noted that all foreshore responsibilities were transferred to the DEHLG in February 2010, under the provisions of the Foreshore & Dumping at Sea (Amendment) Act, 2009, which means that for the first time in the history of the State, foreshore responsibilities are housed within the same Government department that is responsible for spatial planning, conservation of species/habitats, river basin planning and management. Given that local authorities are also under the aegis of the DEHLG all of this should, in theory, facilitate a more integrated approach to marine and coastal resource management but this has not, so far, transpired. A lack of national policy means that there is no framework within which to consider approaches to erosion management such as managed realignment. 3.5 Function specific constructed wetlands 3.5.1 Integrated Constructed Wetlands (ICWs) Irelands Flood Risk Guidelines (DEHLG & OPW, 2009) state that “The Department has commenced the preparation of good practice guidance on constructed wetlands which will look at, inter alia, the use and performance of constructed wetlands in the attenuation of flood hazard.” The ensuing ICW Guidelines (DEHLG, 2010) reported that adequately designed, shallow, vegetated wetlands provide cost effective, environmentally sustainable solutions in the treatment of waste water and make generic reference to their potential benefit in a flood management role. The document did not explore technical flood management aspects of ICWs further and, in fact, recommended in that they should not be considered if they can’t be protected from flood damage. Harrington et al. (2007) carried out an ICW demonstration project in the 25 km 2 catchment of the Dunhill-Annestown stream in south county Waterford. A performance assessment of 12 ICWs that intercept dirty farmyard water was conducted finding that ICWs are: (1) capable of treating farmyard dirty water; and, (2) effectively reduce nutrient and contaminant loss from farmyards, whilst providing additional new landscape values (flora, wildlife and aesthetic appeal). The authors argue that ICWs mimic the function of a once widespread wetland resource that has been lost. Whilst hydraulic retention times and design standards for the ICWs were not reported, annual water balance was examined, showing that 23% of water exited the wetland through evapotranspiration, with 73% discharging into the ground. Almost half of the total inflow volume was lost at the fourth pond in the system. Only 4% of the total FINAL REPORT, February, 2012 32
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC inflow discharged to the surface water stream, and this was seasonal, occurring only for a short period in the winter/spring. This data suggests that the ponds have some level of flood storage, but their performances during high rainfall events were not reported. The work principally showed that attenuation of dirty water in ICW’s provides short to medium term P-retention, N-immobilisation and significant reductions in concentrations of organic material, suspended material and faecal bacteria. ICWs (for the treatment of polluted waste water) have been endorsed under the Department’s Statement of Strategy and in the Water Services Investment Programme 2010-2012 (DEHLG, 2010). It is envisaged that ICWs will play an increasingly important role in Ireland’s requirement to reach WFD water quality targets. In relation to flood attenuation, the ICW Guidelines suggest that additional ponds be incorporated into ICW design for a number of reasons including “increased capacity to store water for longer periods before discharge to surface water thus aiding flood control” (p 60). It is important to note, however, that the various proposed functions of ICWs may not necessarily be compatible. The fact that these wetlands store sediment, organic matter and nutrients means there is a risk, during larger flood events, that these are mobilised downstream, with negative water quality implications. Such a situation has been observed by the authors for a constructed wetland south of Cork City, on the River Curraheen. It is likely that, as with any small isolated wetland, an ICW may have limited flood attenuation capacity during smaller events, but very little during large events. In fact, given the potential risk to water quality from flood-driven inputs of these stored materials, it can be argued that flooding of ICWs is highly undesirable and that their flood attenuation potential be strongly downplayed. 3.5.2 Sustainable Drainage Systems (SuDS) Sustainable Drainage Systems (SuDS) have been mandatory in the Greater Dublin Area since 2006. This was introduced on foot of the 2005 Greater Dublin Strategic Drainage Study (GDSDS), which assessed Dublin’s entire drainage system to take account of all potential development that might affect the city and environs up to 2030. SuDS are a management solution that deals with the problem of surface run-off at source as opposed to the traditional approach of rapidly conveying water elsewhere. It relies on strategies such as swales 15 , filter drains, detention basins, porous paving and the use of storm water attenuation ponds, which can be developed into attractive and biologically important wetlands. Macdonald & Jefferies (2003) reported to the Irish Hydrological Seminar on the monitoring of fourteen different SuDS facilities in Scotland. Performance assessments between 1997 and 2003 showed they all provided flow attenuation to varying degrees (Macdonald & Jefferies, 2003). Two attenuation ponds recorded flow lag times 16 of 100 and 130 minutes compared with equivalent areas of hard paving. Peak flows from all the SuDS components studied were at least 50% of the peak flow from the equivalent paved surface. In addition, ponds had become integrated into the urban landscape, were visually pleasing, and had recorded increases in flora and fauna, creating a focus for biodiversity. A survey of public perception showed that the 15 Swale = roadside detention 16 Lag time = the time from the center of mass of excess rainfall to the hydrograph peak. FINAL REPORT, February, 2012 33
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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