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

Recommendations

Info

The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC flood protection to protect the newly built houses, thus eliminating any flood protection function (Wheater & Evans, 2006). 1.4 Policy implications In response to recent flooding events, and in anticipation of more frequent and perhaps more severe events in the future owing to climate change, restoring the natural functioning of wetlands (floodplains in particular) to accommodate more frequent and severe flooding may provide a range of benefits including flood defence for urban areas, biodiversity enhancement and improved water quality (English Nature Joint Statement, 2003). Murphy & Charlton (2006) report predictions of rainfall increases of 17% in western areas of Ireland; possibly as much as 25% in places under climate change scenarios. The likelihood of increased frequency of storms and rising sea levels could threaten to overwhelm sea defences and increase the risk of coastal flooding to lowlying towns and cities. Increasing national and international attention is therefore being paid to wetlands for their potential in flood attenuation and planning authorities will be expected to give much greater emphasis to this role in future. Questions remain, however, about the ability of individual wetlands to attenuate floods and the relative value of the various types of wetland. FINAL REPORT, February, 2012 14
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC 2. How do wetlands attenuate flooding? 2.1 Overview Wetlands are transitional habitats between dry land and deep water. They are characterised by land periodically or permanently inundated by relatively shallow water, with plant communities adapted to anaerobic soils and flooding (Keddy, 2000). The hydrology of wetlands is complex and varies widely both between wetland types and between individual wetlands of the same type, which is a function of the relative complexity of their physical habitat (Acreman, 2011). It is thus difficult to make generalisations about flood reduction services of wetlands. Notwithstanding, there is potential for wetlands of various sizes and at different locations within a catchment to play complementary roles in flood attenuation and prevention. Wetlands in the upper catchment can, theoretically, reduce and delay flood peaks by temporarily storing water before it enters stream channels, whilst large floodplains downstream can store water that has flooded over channel banks. Since this review primarily examines flood attenuation, it must be based on consideration of the impact that wetlands may have on downstream flood peaks. This depends largely on the wetland’s available water storage capacity and the intensity of the flooding at a particular time. Flood attenuation estimates need to be made on a case-by-case basis, preferably using continuous hydrological modelling that can take into account site specific and local factors. Factors to consider are spatial and temporal variations in precipitation and soil moisture, available storage capacity, outflow rate and flood route. Past and present disturbances, drainage patterns and local soil type differences also influence the hydrological processes of individual wetlands (Bullock & Acreman, 2003, JBA Consulting, 2005, Ramchunder et al., 2009). It is, therefore, difficult to generalise in terms of which wetland habitat types are most effective for flood attenuation, since each wetland differs in direct relation to climatic, topographic and geological variation within Ireland. The only general statement that can be made is that the influence of wetlands in reducing flood peaks is probably greatest for high frequency, low to medium rainfall events that occur when wetlands have a large capacity for water storage. It is least for low frequency, large rainfall events, particularly when soils are saturated and wetland storage capacity has been reduced by preceding high rainfall - as is common in Ireland. This review examines types of wetlands that may have a role in flood attenuation in Ireland. This is done on the basis of empirical data and demonstrated principles of wetlands in a flood attenuation role, and also through consideration of existing Irish and NFM projects. 2.2 The process of flood attenuation “Attenuation” refers to loss of intensity of flux through any type of medium. Flood attenuation is achieved where there is a measurable change to the downstream FINAL REPORT, February, 2012 15
Page 1 and 2: The Use of Wetlands for Flood Atten
Page 13: The Use of Wetlands for Flood Atten
Page 65 and 66:
The Use of Wetlands for Flood Atten
Page 67 and 68:
Page 69 and 70:
Page 71 and 72:
Page 73 and 74:
Page 75 and 76:
Page 77 and 78:
Page 79 and 80:
Page 81 and 82:
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?