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 mitigation role of peatsoil wetlands is often overstated and it has been recognised for many years that not all peatlands reduce storm flows, particularly in winter, nor provide higher flows in summer. Wetlands in Irish karstic landscapes (turloughs) are temporary in nature. Turloughs hold water in winter, when groundwater levels are high and fill valley basins and smaller depressions within the landscape. The seasonal inundation of these depressions in a function of complex surface-groundwater interactions, in which water levels are both drained and recharged through sinkholes. Their flood attenuation property is similar to that of fens and likely is determined by the rate at which they can temporarily store surface water, via soil storage and also in their somewhat greater evapotranspiration of water relative to non-wetlands. The complex nature of each catchment area and discharge route makes prediction of the flood attenuation of turloughs problematic, and the understanding of turlough hydrology and drainage is far from complete. Coastal wetlands can attenuate seaward flooding, resulting from storm surges, high waves and high tides and landward flooding resulting from rivers spilling over banks onto estuarine floodplains. Salt marshes are effective dissipators of wave energy and provide a first line of defence against tides and waves, particularly during stormy conditions. Highly resilient emergent and near emergent salt marsh vegetation creates roughness that reduces wave height and speed as it travels across the intertidal surface, so attenuating waves. The large water storage capacity of the large expanse of flat estuarine wetlands will also be extremely important in attenuating water spilling over channel banks either due to high river levels or high tidal levels. Tidal flood attenuation is particularly important in areas where sea water is confined in narrow inlets, bays and natural harbours, such that the tidal flow cannot be displaced along the coast. The important function of riverine floodplains of delaying floodwater runoff, via a rough surface topography, is probably less important in coastal or estuarine wetlands than available storage volume capacity, as downstream flooding is rarely an issue, except in cases where urban settlements are located in the lower estuary. The flood attenuation potential of function-specific constructed wetlands and artificial wetlands will depend largely on their location within a landscape and design and be governed by the same constraints and factors as for floodplains and headwater peatlands. It is important to note that their flood attenuation function may not necessarily be complementary with their primary function. For example, a wetland with saturated soils may have high denitrification, but low flood attenuation potential. Similarly, wetlands that are designed to store particulate phosphorus and sediment on their surface and shallow sub-soil may shed these to downstream receiving waters, with negative consequences for water quality, in the event of flood waters spilling onto the wetland. Flood attenuation and land use changes Human management can potentially increase or decrease the capacity of a given wetland to attenuate floods. Encouraging extensive surface water for long periods on natural floodplains (for example, to benefit wetland plant and animal communities, particularly wading birds) can raise groundwater levels, reduce soil moisture deficits and FINAL REPORT, February, 2012 8
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC infiltration rates and speed runoff, so negatively affecting flood attenuation potential. As for artificial or constructed wetlands, enhancing wetland biodiversity is often conflated with enhancing flood attenuation potential, whereas in fact the two may not be necessarily wholly compatible. Agricultural intensification of floodplains can have positive and negative effects on flood attenuation potential. Increased soil compaction and surface drainage may act negatively by reducing the infiltration of flood waters into floodplain soils, and speeding the runoff of surface water to the river channel. Greater surface drainage to dry out floodplains can, on the other hand, reduce groundwater levels and soil moisture, potentially leading to enhanced flood water storage. Removal of hedgerows, woody vegetation and surface depressions (including relict channels and pools) will all tend to reduce surface storage of flood waters. For peatsoil wetlands, the drainage channels cut in the past to drain bogs and fens can increase the soil moisture deficit of the peat surface, enhancing soil water storage capacity, but also speed water flow to channels. Similarly, rapidly removing floodwaters in wetlands with seasonally high groundwater levels, via runoff drainage channels, will lessen the overland water storage capacity of the wetland, although may enhance soil moisture storage capacity. Blocking drainage channels – a practice sometimes undertaken to ‘restore’ disturbed bogs and fens - may retard water flow to downstream streams but can also elevate soil moisture levels, so reducing soil water storage capacity. Although many wetlands have the potential to attenuate flooding, the planning process requires more definite and detailed information on the impact of individual wetlands and also on the likely impact of landuse changes, such as agricultural intensification, afforestation and urbanisation. However, the complex interactions between soil type, land use, landscape configuration and climate at a local sub-catchment level make it difficult to scale these processes up to large catchment scales. ‘Hydrological’ floods vs ‘economic’ floods Where a particular wetland has the potential to attenuate downstream flooding, the realised attenuation will depend strongly on the rainfall and flood event. In general the influence of wetlands in reducing flood peaks is greatest for high frequency, low to medium rainfall events that occur when wetlands have a large capacity for storage. It is least for large events, particularly following a long period of prior rainfall, when soil and wetland storage are saturated. In this regard, a distinction can be made between ‘hydrological’ floods (high frequency, low to medium rainfall events that occur commonly without economic damage) and “economic” floods (low frequency events following high intensity rainfall, potentially causing economic damage). Wetlands may readily attenuate ‘hydrological’ floods but are much less likely to attenuate ‘economic’ floods. As flood height may be a critical determinant of the economic cost of a flood, the management of alluvial floodplains upstream of sensitive areas so as to maximise flood storage and thus to reduce flood height (although not duration) is likely to be the most cost-effective use of wetlands for flood attenuation. Wetlands and Flood Management Policy Irelands present Catchment Flood Risk Assessment and Management (CFRAM) approach indicates that policy is in place that recognises catchment scale processes in flood generation and management. However, there are significant hurdles that need to be overcome in order to achieve sustainable flood management, such as conflicts with FINAL REPORT, February, 2012 9
Page 1 and 2: The Use of Wetlands for Flood Atten
Page 7: The Use of Wetlands for Flood Atten
Page 59 and 60:
The Use of Wetlands for Flood Atten
Page 61 and 62:
Page 63 and 64:
Page 65 and 66:
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

Create successful ePaper yourself

Delete template?

Save as template?