River response to inter-basin water transfers: Craig Goch feasibility study

Abstract

Catchment development programmes, including river regulation, land drainage, flood alleviation schemes and land use changes, can seriously affect the stability of upland rivers. Changes in flow regimes and sediment transport rates occasioned by such activity can result in systematic erosion and deposition along the river. As part of the Craig Goch reservoir development feasibility study an investigation was carried out on the effect of proposed releases from the reservoir on the stability of the rivers Wye, Dulas and Severn. Dominant discharge concepts indicate that regulation will not affect the natural stability of the channel provided that releases do not increase the frequency of flows above bed material transport and bank erosion thresholds. Any increase in the frequency of such flows will cause unnatural instability. Standard field techniques were used to determine the critical threshold discharge for movement of the surface bed material at twenty-two sites on the three rivers. These data, together with simulated flow information at the proposed outfall points, enabled identification of the sections of channel which would experience erosion or deposition as a result of river regulation. Test releases from the Clywedog reservoir confirmed the results of the investigation on the river Severn.