The role of floodplains on the propagation of land management signals in the Vale of York

Abstract

The floodplains of many UK rivers are heavily managed to reduce the risk of flooding. The presence of levées is thought to increase peak flows, decreasing the ability of the floodplains to attenuate flood waters. Understanding the role of floodplains for reducing flood risk is vital in order to create flood management strategies that utilise the natural attenuation properties of floodplains to reduce future flood risk. The impact of flood levées on the propagation of upstream land management signals has been assessed in this thesis. The study utilised the 1-D model HEC-RAS to determine the impact of levées on the transmission of possible land management signals (from moorland grips) in the Ouse catchment, and establish the resulting flood risk for the City of York. A link between rural land management in the upper catchments and flood routing was demonstrated. The flow was scaled in relation to the hypothesised effect of blocking grips to examine the scale of possible downstream consequences. At upstream cross-sections the reduction in peak discharge was higher, suggesting that the positive impact of removing the levées, in terms of decreasing the flood peak, is dissipated as the flood wave propagates downstream. Further downstream the attenuation reduced suggesting that land use signals are dissipated with distance downstream. Solely removing the levées reduced peak flows by only 1.3%. Crucially, by combining the positive effects of levée removal and grip blocking, peak discharge downstream at York could be reduced by 4.2% for the York 2000 flood event. This approach also reduced the peak flow for the York 2000 flood event below the threshold for peak flows associated with a 25 year return period. Peak discharge was more sensitive to flow scaling with the levées, suggesting that levées do make the grip effect more noticeable, although the influence of the levées is not large. The findings suggest that levées do transmit the flood signal further downstream to York due to a reduction in floodplain attenuation. The removal of levée sections combined with grip blocking in the upland catchments could prove an effective and sustainable approach for future flood risk management in the Ouse catchment.