| Abstract: | Geomorphological, engineering and ecological surveys were carried out at 18 flood alleviation schemes in the UK to evaluate their environmental performance. As no pre-scheme river corridor surveys had been carried out at any of the sites to enable the environmental impact to be directly assessed, controls were established in natural sections adjacent to five of the engineered reaches. A space–time substitution enables the pre-scheme conditions to be estimated for comparison with the engineered reach. The physical characteristics of the channel at bankfull conditions exert a strong influence on plant community composition. Paired assessments were carried out for engineered/control reaches on five rivers and this revealed which species and habitats were most affected by various engineering treatments. The general conclusion was that dredging, widening and straightening rivers reduces the number of desirable species, whereas two-stage channels and schemes involving the construction of flood banks at the edge of the meander belt maintained the richness and preserved and enhanced the occurrence of key species. Data from every cross-section, engineered and control reaches, enabled a model to be developed relating species occurrence to physical habitat features. This can be used to predict, at the stage of appraisal or design of a project, the likely response of the river to any particular engineering works. Given information on the existing condition in the river, it enables the environmental impact of the scheme, in terms of its effect on river bed plant species, to be assessed. |
