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 statutory drainage maintenance and, critically, socio-economic obstacles that prevent land use and/or land management changes required to achieve NFM solutions. Dedicated agri-environmental schemes will be essential in Ireland to encourage the use of wetlands for flood attenuation, particularly in the most effective parts of the catchment, i.e., alluvial floodplains. The currently available mechanisms under CAP and national agri-environmental schemes are insufficient to encourage such changes. The opportunity should be explored for incorporating flood risk management at the farm level, perhaps under new ‘greening’ measures proposed for CAP 2014-2020. Measures such as 7% set aside of ‘ecological’ areas and use of ‘innovative practices’ could be applied to the concept of floodplain management. Two measures to enhance the flood attenuation potential of floodplains are: (1) restoring the natural hydrological connectivity between river and floodplain so allowing land to inundate more frequently; and, (2) retaining or restoring ‘rough’ floodplain surfaces, in the form of walls, hedges, coarse and woody vegetation, relict channels and depressions. Both of these measures can clearly conflict with the needs of intensive agriculture (with its emphasis on large uninterrupted field systems) and their implementation will require financial incentives or compensation. International experience has shown the importance of agri-environment schemes to allow for NFM and successful catchment based flood management solutions. Effective, widely supported adoption of the types of land use changes required needs alteration of existing, or the development of new, mechanisms capable of providing long term support to co-operating farmers. The UK’s ‘Farming Floodplains for the Future’ initiative provides a useful model by examining new incentives tailored to the delivery of flood management objectives through land use change. Using a template similar to agrienvironment grants, it was suggested that a one-off capital payment to cover initial outlay, plus regular incentive payments, could be made to farmers who participate. The creation, restoration and use of wetlands for flood attenuation (primarily floodplain storage, washland, polder and coastal sites) have become increasingly popular abroad over the past 20 years. Ireland clearly lags behind in this field. By far the most influential shift behind the growth of NFM solutions was the philosophical and practical adoption of an approach that promotes the controlled spreading out of excess water over the landscape as in ‘Making Space for Water’ and ‘Room for Rivers’. The UK, in particular, has implemented a number of floodplain restoration, managed floodplain storage schemes, and coastal realignment schemes. These have realised flood alleviation benefits, and often a range of associated benefits, such as biodiversity enhancement and sediment control. Benefits, however, need to be examined on a caseby-case basis as flood alleviation and biodiversity goals are not always synonymous. A key element in the process of utilising wetlands for flood attenuation abroad has been the involvement of other public and semi-State authorities as well as the general public. Funding for public consultation in particular is a central element of NFM. If the use of wetlands for flood attenuation is to be considered at certain locations in future, public engagement will be essential from the earliest stage of the catchment management planning process. FINAL REPORT, February, 2012 10
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC PART I The role of wetlands in Flood Attenuation 1. Introduction Although the use of wetlands for flood attenuation is receiving much greater attention than previously, both in Ireland and abroad, there remains a degree of uncertainty about the relative value of different wetland types, in different parts of a catchment, for flood storage and attenuation. The commonly held belief that all wetlands always serve to reduce flooding and regulate river flows is not supported by readily available empirical data. The purpose of this review is to investigate literature and reported current practice, in Ireland and abroad, to examine what is known about the effectiveness of wetlands in a flood attenuation role and to assist in the understanding of their potential within future flood risk planning. 1.1 Flooding Flooding is a natural part of the hydrological cycle and occurs whenever the capacity of the drainage system is exceeded by high channel discharges. Flooding is also important for maintaining the ecological functioning of wetlands. Three types of flooding and their associated wetland habitats in Ireland have been considered for the purpose of this review, (1) river flooding, where heavy rainfall causes flow to exceed the capacity of the river channel, overtop the banks and flood the surrounding areas (2) coastal flooding, where combinations of extreme weather conditions, high tides, surges and wave overtopping cause sea water to inundate land (Farrell, 2005), and (3) groundwater flooding, primarily confined to karst landscapes of the western seaboard, occurring when turloughs overflow their bounds (Peach & Wheater, 2009). Floods vary considerably in size and duration. Local intense rainfall events can deliver high water volumes, chiefly via overland flow, into small headwater channels, but also via excess groundwater recharge. In the case of surface flows, smaller channels quickly develop a short acute peak flood discharge, which may overspill banks and lead to local flooding. These flood events are usually short in duration and rarely extend far away from banks owing to the generally steep nature of riparian habitats along headwater streams. Prolonged high rainfall over a wide area, however, can deliver high discharges from multiple tributaries into higher order channels. Peak discharges arrive later and take longer to reduce than those in the headwaters, giving a characteristic attenuated shape to the flood hydrograph. Flood events then may last for longer as it takes longer for water to drain from the system and extend far from the banks owing to the lower gradient of large order river floodplains (Gordon et al., 2004) Flooding continues to pose a threat worldwide to life, property and infrastructure. Recent flooding in Ireland, the UK, Australia and Brazil, have shown that extreme events can overwhelm the capacity of large, populated catchments to shed water in a controlled manner. Flood flows have historically been managed in two ways, by: (1) increasing channel capacity or, (2) temporarily storing excess water. Traditional drainage methods of deepening and widening channels, or increasing flow velocity by FINAL REPORT, February, 2012 11
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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