Conveyance of a managed vegetated two-stage river channel

Abstract

This paper describes the results obtained from a five-year study of a two-stage river channel with a managed flood-plain, in the south of England. Water levels at five sections, as well as the discharge, have been measured and recorded at 15 min intervals. Analysis of these data reveals that the resistance coefficient for this reach varies both annually and with the seasons. These variations are shown to be due to the annual cycle of vegetation growth both in the channel and on the floodplain. Some 100 storm events have been recorded during this period. Resistance coefficient values, based on Manning's n, are evaluated for overbank, bankfull and low flows in the channel. A qualitative and quantitative assessment of the channel's behaviour is given, demonstrating the relationship between conveyance and channel conditions for seasonal variation and at different flow depths. A proposal to incorporate this information into a two-stage design method is outlined in connection with river restoration projects and channel maintenance programmes.