Stability of flood alleviation schemes
PublisherUniversity of East Anglia
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The engineering of natural river channels alters their morphology and hydraulic regime. In many cases this can adversely affect the riverine environment as well as cause instability. Several methods designed to predict channel change at engineered sites were evaluated using a range of river engineering schemes across England and Wales, which encompassed conventional and environmentally sensitive schemes in both upland and lowland areas. The methods tested included critical tractive force and permissible velocity theory, Griffiths natural stability index, Brookes stream power index, shear stress analysis, regime theory, sediment transport equations and one dimensional mathematical models. Of these, the use of sediment transport equations linked to rating curves for individual cross-sections, based on frictional characteristics, most accurately predicted the morphological changes recorded along the engineered reaches of simple channels. Their performance on compound channels was impaired due to the poor understanding of the frictional effect of the flood berms. It is recommended that this method be adopted to assess the stability of engineering proposals at the design stage. The impact of engineering schemes on aquatic niacrophytes is also evaluated. Five physical factors were found to influence individual species, namely channel depth, flow area, surface sediment size, bank slope and flow velocity. Tables are presented which allow the prediction of the species assemblage likely to be associated with a given engineering design. It is also clear that species diversity is increased if a range of habitats are incorporated within an engineering scheme, as has occurred on many of the environmentally sensitive flood relief schemes. These approaches to river engineering also appear to enhance the diversity of species above that for the natural channel, whereas conventional methods show the opposite trend. Such methods are therefore to be recommended as options in the design and rehabilitation of flood relief schemes.