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 the width and depth of the channel to pre-engineered dimensions increased water levels on the floodplain considerably and led to a 10-15% reduction in downstream flood peak estimates. This indicates that restoration of overbank flooding can alleviate downstream flood risk and points to the potentially beneficial role of restoring more frequent flooding to floodplains. However, it is important to recognise that restoring floodplain hydrology may not alleviate cumulative flood risk if floodplain water tables are maintained at a high level. In a subsequent study, Acreman et al. (2007) simulated the effect of wetland ‘restoration’ (raising water levels by ditch-blocking and allowing greater water residence time) in a SW English catchment with floodplains dominated by low-lying peatsoil wetlands. They showed that restoring riparian wetlands by raising water levels, particularly in winter, actually reduced potential flood storage which could have an impact on downstream flooding. Restoring or recreating wetlands by encouraging land to flood, particularly in winter, may therefore not bring about the expected flood attenuation benefits. This issue is explored further in section 5 below. Given the hydrological complexities of floodplains, constructing artificial or ‘recreating’ wetlands to mimic the natural flood attenuation of natural wetlands is an uncertain exercise. Cole and Brooks (2000) compared the hydrological function of natural and artificial wetlands in Pennsylvania, USA. They found that natural wetlands had lower water tables, shorter periods of soil saturation and inundation and greater soil infiltration, and hence greater flood storage potential, than artificial wetlands. Restoring or enhancing the natural floodplain vegetation may be feasible in some catchments or locations, particularly in areas of marginal agricultural value. Woody riparian or floodplain vegetation can provide a rougher channel profile during floods, slowing down flood flows and enhancing flood storage, so attenuating peak discharges and providing some measure of downstream flood protection (Thomas & Nisbet, 2007; Anderson et al., 2006). Natural wetland vegetation, such as reed swamp, rushes and carr, may need elevated water tables, however, thus reducing the benefit of reduced runoff rates. It is clear from these examples that the flood storage potential created by riparian and floodplain vegetation management needs to be assessed from a scale-dependent view, both in terms of the flood attenuation benefit at larger, downstream scales and the capacity of such management measures to operate effectively during more extreme flood events. 4.2 Peatland management Peatsoil wetlands have been intensively managed in recent decades, to facilitate grazing and forestry. In upland peaty catchments subject to intensification, the prevailing management practice is to cut drains (‘grips’ in the UK) through the peat in order to facilitate the flow of water from the saturated peat soils to create a drier soil surface, allowing for grass and tree growth. These upland drain networks can drain large areas of peatsoil wetland. During rainfall events, flow velocities within drains have been shown to be much greater than over the hillslope surface, increasing flood generation. On the other hand, the drier soils of FINAL REPORT, February, 2012 38
The Use of Wetlands for Flood Attenuation Aquatic Services Unit, UCC drained peatsoil wetlands can increase soil moisture deficit and thereby increase available water storage, leading to reduced hillslope discharge to channels (O’Connell et al., 2004). The effects of an individual drainage network, therefore, depends upon its location within the landscape; slope, local rainfall and peat depth all having an important role (Lane et al., 2003). Further complicating the picture is that drains cut in the peatland, whilst increasing throughflow in soils, can significantly reduce overland flows by increasing the ability of water to flow through the lower peat layers (Holden et al. 2006). Conversely, the Sphagnum moss on intact peat bogs provides greater hydraulic resistance to overland flow on improved, drained peatlands (Holden, 2008). Grayson (2009) showed that, whilst relative surface flow velocities depend on hillslope and water depth, flows over Sphagnum covered peat were slowest and flows over bare peat were significantly faster. Surface flows, where vegetation such as Eriophorum spp. (bog cotton) or Juncus spp. dominated, were intermediate. Flow velocity was slower within vegetated drains, even ones without dams and pools, compared with bare peat drains by at least 10-fold (Holden et al., 2008a, in Holden, 2009). There has been no work published on the effects of drain-blocking on catchment-scale flooding, but modeling work is ongoing in the UK at Newcastle University and Imperial College (Holden, 2009). Wilson et al. (2010) demonstrated the complexity in predicting the effect of drainage management in peatsoil uplands. They showed that drains cut in an upland Welsh blanket bog led to the creation of drier peat soils around drains, particularly downslope. Blocking the drains reversed this effect and led to a re-wetting of the near-drain soils, with much greater surface water occurrence. Drain blocking increased the ability of surrounding peat to hold rainwater, whereas previously it would have flushed through more rapidly. It was found that, at this site, restoration increased water retention within the peat. This was further supported by observed declines in average and peak flow rates in streams draining the peat. An increased buffering between rainfall and discharge led to a decline in the occurrence and magnitude of local scale peak flows. The apparent contradiction between the increased water retention of peat during dry periods and reduction in discharge during rainfall events was explained by the diminished connectivity of the drainage network. By blocking drains, the importance of overland flow increased, which was significantly slower that flow through drainage channels, therefore leading to reduced peak flows. The rougher Sphagnum-dominated vegetation that is likely to recover over ‘smoother’ vegetation on drier peat soils will further enhance this effect. The Ripon Land Management Study (JBA Consulting, 2007) modelled peatland management impact on flood flows in northern England. The study involved a catchment area of 120 km 2 comprised of about 22% moorland and wet blanket bog, located in the headwaters. Simulated changes to drainage of these areas produced discernable effects on hydrograph response at Alma Weir, lower in the catchment. Maintaining drainage channels on bogs and moors (‘grip maintenance’), in association with other catchment wide soil degradation did not change the timing of flood peaks at a downstream point, but increased their magnitude (3-9%). Interestingly, within the main sub-catchment comprised of bog and moorland, a ‘grip maintenance’ scenario produced a 7-21% flood peak increase within that sub-catchment, despite floodplain attenuation effects there. In contrast, grip-blocking (simulated as a 1 hour delay in peak FINAL REPORT, February, 2012 39
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The Use of Wetlands for Flood Attenuation FINAL REPORT - An Taisce

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