IMPACT OF FLOOD ALLEVIATION SCHEMES ON AQUATIC MACROPHYTES

Geomorphological, engineering and ecological surveys were carried out at 18 flood alleviation schemes in the UK to evaluate their environmen tal performance. As no pre-scheme river corridor surveys had been carr ied out at any of the sites to enable the environmental impact to be d irectly assessed, controls were established in natural sections adjace nt to five of the engineered reaches. A space-time substitution enable s the pre-scheme conditions to be estimated for comparison with the en gineered reach. The physical characteristics of the channel at bankful l 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 aff ected by various engineering treatments. The general conclusion was th at dredging, widening and straightening rivers reduces the number of d esirable species, whereas two-stage channels and schemes involving the construction of flood banks at the edge of the meander belt maintaine d the richness and preserved and enhanced the occurrence of key specie s. Data from every cross-section, engineered and control reaches, enab led a model to be developed relating species occurrence to physical ha bitat 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 parti cular engineering works. Given information on the existing condition i n the river, it enables the environmental impact of the scheme, in ter ms of its effect on river bed plant species, to be assessed.