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 4. Management of wetlands In northern temperate regions, waterlogged or flooded land is rarely considered useful for agriculture or urban settlement. Exceptions (almost all commercially extinct) include harvesting of wetland products such as summer hay, thatching reed, wildfowl and fish and the ancient practice of allowing water meadows to flood in early spring to provide a spring flush of new grass. Since the onset of agricultural intensification and increased urban settlements in the 19 th and 20 th centuries, intensive efforts have been made to drain wet land so as to increase yields. Many river systems have been deepened, widened and straightened to as to lower local water tables and to speed the drainage of water from catchments (Blann et al., 2009). For many lowland catchments, this has effectively completely removed overbank flooding onto floodplains or when it does occur, rapid flow back into the channel is facilitated (Acreman et al., 2007). Floodplain changes typically include hedgerow loss, increases in field size, the installation of land drains connecting hilltop to river channel, and channelised rivers with no riparian zone. These landscape changes have been accompanied by increasing intensity of land use (Wheater & Evans, 2009). For headwater catchments, many peat soil wetlands have been drained, particularly shallow fens, and converted to more productive pasture (Burt, 1995). It is estimated that 78% of Irish fens have been drained and reclaimed 18 . Large areas of blanket and raised bog in Ireland have been planted with exotic conifer, which require the bog peat surface to be drained. Bogs have also been drained for peat extraction and, for shallow peats, for conversion to pasture. In upland catchments in Ireland, often dominated by peaty podsols, sheep densities increased (driven by grant aid) between 1970 and the 1990s (Coulter et al., 1998). Whilst there has been a more recent stock density decline (around the magnitude of a 15% reduction between 2000 and 2010 (Behan & McQuinn, 2004)) there has been considerable pasture improvement in upland areas involving drainage, ploughing, and reseeding. This process is continuing in Ireland with the potential for more land being converted to intensive grazing as a result of new CAP payments to support expansion of the dairy sector (DAFF, 2010; also see Part II, section 8.2). The impact on flood attenuation of these types of land use changes to wetlands is explored in sections 4.1 - 4.3. 4.1 Floodplain management The impact of more intensive agricultural land management practices on flooding in Europe, is thought to be largely a result of their impact on soil structure (Wheater, 2006). Land management can significantly affect the local generation of surface and subsurface runoff by influencing the soil structural conditions that determine both the inherent storage capacity, macropore structure and flow pathways within the upper soil layers and their saturated hydraulic conductivity (O’Connell et al., 2004). In lowland catchments, changes in crop type and land cultivation practices resulting from the more intensive use of floodplain land can increase runoff due to lower infiltration capacity of 18 http://www.ipcc.ie/ FINAL REPORT, February, 2012 36
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC intensively managed, compacted soils (O’Connell et al., 2004; Wheater & Evans, 2009). In the uplands, increased stock densities, exacerbated by removal of hedgerows and woodland buffer strips, can also lead to soil compaction and a greater amount of bare, eroded soils. The hydrological effects of such changes include reduced infiltration, increased overland flow and potentially higher flood peaks (Wheater, 2006). From an extensive meta-survey of published studies, O’Connell et al. (2004) found substantial evidence that land management practices affect local surface runoff and the timing and magnitude of field drain responses. Runoff was found to be more rapid and acute on intensively managed soils due to lower infiltration capacity. Old established grassland and woodland has the highest infiltration capacity (O’Connell et al., 2004). Overgrazing and trampling by stock can markedly decrease surface infiltration and can double surface runoff at the field and hillslope scale. They also found that both afforestation and field drainage can affect flows in the surface water network but the impacts depend on the local soil type and specific management practices used. Although it is generally accepted that such land use changes can lead to greater flood risk at local scales, the complex interactions of soil type, land use, landscape configuration and local climate at a local sub-catchment level make it difficult to scale these processes up to large catchment scales (Wheater & Evans, 2009). There is very little direct evidence that land management practices can affect flooding at larger scales (O’Connell et al., 2004). Further studies aimed at detecting such effects are needed, with the particular aim of detecting the impact of changing land management with other confounding factors. In some cases, in fact, agricultural intensification of wetlands may, in fact, increase flood storage potential. Floodplain drainage can lower groundwater levels, reduce or divert hillslope flow and increase soil moisture deficit during dry periods. If hydrologic connectivity with the river channel is maintained, such that overbank flooding can still occur, the greater water uptake capacity of such dry floodplain may result in enhanced flood attenuation potential. The realised impact on flood attenuation from land use intensification of wetlands is then a sum of the various positive and negative impacts on water storage and runoff. The concern over the loss of floodplains and the perceived reduction in floodplain water storage is leading to increased interest in reversing large-scale engineering of river corridors and restoring floodplain function. Reinstating more natural infiltration rates on floodplains can reduce runoff at the field scale. Allowing more natural wetland vegetation growth can also help to reduce post-inundation runoff rates, although may be accompanied by higher groundwater levels which may act to reduce soil storage capacity. Such restoration and rehabilitation efforts thus needs careful evaluation about their potential benefits. Although restoration of infiltration rates should reduce surface runoff at the field scale, greater subsurface runoff could also increase as a result, so mitigating the net impact (O’Connell et al., 2004). Some floodplain management measures can have more straightforward and concrete impacts on flood attenuation. Acreman et al. (2003) simulated the effects of embankment on the River Cherwell, (UK). Modelling showed that, at high flows, not allowing water to spill onto the floodplain could potentially increase downstream flood peaks by 150%. Conversely, restoration of more natural overbank flooding by reducing FINAL REPORT, February, 2012 37
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