Impact of channelization on the mimmshall brook, Hertfordshire, Uk

Abstract Water End, Hertfordshire, has been designated as a Site of Special Scientific Interest due to the presence of some of the most important swallow holes in southern England. However, the swallow holes are being infilled by sediment and rubbish, resulting in severe environmental deterioration. Sedimentation appears to have been accelerated by human disruption of the Mimmshall Brook catchment, within which the swallow holes are located. A study of the sediment dynamics within the Mimmshall catchment was undertaken to identify instabilities and sediment transfer dynamics in the fluvial system. Channelization is shown to be the primary cause of the sedimentation problem. It was found that channelization triggers sediment flux divergences which in turn necessitate channel changes to restore equilibrium. As nickpoints migrate upstream of channelized reaches, bed degradation increases bank height. This decreases the stability of the bank with respect to mass failure. Bank failures occur under ‘worst case’ conditions once the threshold of bank instability is crossed. Bank erosion and mass failures were found to be the major sources of sediment to Water End and it was concluded that bank retreat must be controlled if environmental deterioration at Water End is to be slowed or reversed.