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

Recommendations

Info

The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC 2008). As the understanding of turlough hydrology and drainage is far from complete (Zaidman et al., 2010), this is likely to be a costly and difficult exercise. Irish turlough floodplains have been extensively drained to protect rural and urban infrastructure (Ní Bhrion, 2008). Individual basins, drained or undrained, probably have very limited flood mitigation potential. Nevertheless, it is likely that collective turlough floodplains within a catchment, taken as a whole, have a high potential for storage during large flood events. The largest remaining naturally functioning turlough floodplain in Ireland is at Rahasane Turlough, near Craughwell, Co, Galway. A designated cSAC ( 12 Site code 000322) and SPA ( 13 Site code 004089), the area floods annually to an area of approximately 256ha. It is presently the subject of a study to ascertain the hydrological and ecological impact within the turlough of a proposed OPW drainage scheme upstream and downstream of the turlough. 3.4 Coastal wetlands Coastal wetlands are the at the transition zone between land and sea and provide retention areas which can provide both riverine floodwater storage and tidal seawater storage (PWA, 2004). They thus perform an essential flood defence and coastal protection role (MA, 2005). Two types of coastal flooding processes have to be considered: (1) tidal flooding as a result of storm surges, high waves and high tides; and (2) flooding from a river catchment following heavy rainfall (Farrell, 2005). These can often occur together during storms and prolonged rainfall periods present a severe flood threat to many coastal settlements. Field and laboratory studies have shown that salt marsh vegetation attenuates waves (Augustin et al. 2009; Feagin et al., 2011; Möller & Spencer, 2002). Salt marshes are particularly efficient at this because highly resilient emergent and near emergent rough vegetation reduces wave height and speed as it travels across the intertidal surface. The vegetation also binds sediment together, thus, increasing shoreline stability (Augustin et al., 2009). Möller and Spencer (2002) studied wave/tide datasets at 2 sites in the Dengie marshes, eastern England, addressing the effect of (1) marsh edge topography; (2) marsh width; (3) inundation depths; and (4) seasonal changes in marsh surface vegetation cover on wave height and wave energy dissipation. They pooled data from a similar previous study (Möller et al., 1999, cited Möller & Spencer, 2002) and found that on average >40% of wave energy arriving at permanently vegetated marsh edges was attenuated across the first 10m of a marsh; the following 28m attenuated a further 60% of the wave energy. The width and quality of salt marsh habitat clearly had a direct effect on wave attenuation properties. Salt marshes are, thus, effective in dissapating tidal and wave energy and provide a first line of defence, particularly during stormy conditions. Coastal wetlands in Ireland include estuaries, tidal flats and mudflats, coastal lagoons, shallow inlets and bays; many with associated salt marsh, salt meadow and sand dune habitats. The Status of EU Protected Habitats and Species Report (NPWS, 2008) 12 http://www.npws.ie/protectedsites/specialareasofconservationsac/rahasaneturloughsac/ 13 http://www.npws.ie/media/npwsie/content/images/protectedsites/sitesynopsis/SY004089.pdf FINAL REPORT, February, 2012 30
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC recorded infilling, land reclamation, embankment and coastal protection works as significant threats to the extent and quality of these areas. Curtis and Sheehy- Skeffington (1998) recorded 238 salt marshes in the Republic of Ireland and found that reclamation had greatly reduced the national area of salt marsh, primarily through embankment and agriculture (particularly in Waterford, Wexford, Wicklow, Donegal and Clare), and to a lesser extent owing to industrial and port development (Shannon, Cork, Dublin). Any removal or reduction in the functional area of coastal wetland is likely to impact on flood storage / defence capability, as well as affecting habitat and important depositional and hydraulic processes in the coastal zone. The flood relief role of coastal wetlands is widely recognised for coastal protection. The natural resilience and resistance to frequent inundation provided by salt marshes and estuarine floodplains has meant that their use is becoming a recognised alternative to hard engineering approaches for coastal protection. The strategy of ‘managed coastal realignment’ involves setting back the line of defence and allowing an area to become flooded, rather than trying to hold the sea at bay. It almost always involves a reversion of previously reclaimed land, where maintenance of specific design-standard seawalls is not economically viable in the long term. Methods and examples of coastal realignment are reported in sections 6 and 7. The solution has gained increasing international interest as countries recognise the need for long-term cost effectiveness in coastal adaptation strategies in the face of predicted sea level rise. The UK, in particular, have embraced ‘soft’ coastal protection solutions decision making is increasingly based on between 50 and 100 year futures with climate change scenarios in mind. Climate change-driven increases in rainfall and storminess, coupled with sea level rise, would have wide coastal repercussions in Ireland (Devoy, 2008). The combined effects of river floods and marine surges create notable flood events in coastal areas, and these are predicted to increase in severity. Coastal flooding and storm surge damage has become more frequent and widespread in Ireland (Casey, 2009). In Ireland, even with a modest 0.4m rise in mean sea level, a 1 in a 100 year coastal flooding event will occur at least every 5 years, if not more often (IAE, 2007). Storm surge events are predicted to increase along all but southern Irish coasts in the future, with a significant increase in the height of extreme surges along the west and east coasts (over 1m), especially during winter (Wang et. al., 2008). Coastal wetlands are projected to be negatively affected by sea level rise, especially where they are constrained on their landward side (i.e., ‘coastal squeeze’). Devoy (2008) predicts Ireland could lose 30% of its’ coastal wetlands given a 1m sea level rise. With room to encroach landward, salt marsh-mudflat systems would provide some resilience or resistance to climate change generated sea level rise, through accretion of saltmarsh. The OPW are responsible for coastal protection and flood risk management 14 and local authorities are responsible for identifying and implementing strategies. Schemes tend to predominantly involve hard engineering solutions in response to erosion. The Environmentally Friendly Coastal Protection – Code of Practice (ECOPRO, 1996) issued to all Irish local authorities (O’Connor et al., 2009) outlines many coastal protection 14 http://www.opw.ie/en/FloodRiskManagement/WhatWeDo/RolesResponsibilities/ FINAL REPORT, February, 2012 31
Page 1 and 2: The Use of Wetlands for Flood Atten
Page 29: The Use of Wetlands for Flood Atten
Page 81 and 82:
The Use of Wetlands for Flood Atten
Page 83 and 84:
Page 85 and 86:
Page 87 and 88:
Page 89 and 90:
Page 91 and 92:
Page 93 and 94:
Page 95 and 96:
Page 97 and 98:
Page 99 and 100:
Page 101 and 102:
Page 103 and 104:
Page 105 and 106:
Page 107 and 108:
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?