HYDRAULIC STUDY OF A 2-STAGE RIVER CHANNEL

Many regulated rivers have compound or two-stage cross-sections consis ting of a deep main channel flanked by one or two floodplains. Existin g methods of assessing discharge capacity in such channels can lead to serious errors, and hence there is a need for the acquisition of flow data to validate models suggested as improvements to existing methods . Stage and discharge data, under both steady and unsteady conditions, were collected from a reach of the River Main in Northern Ireland, wh ich has been reconstructed for flood defence purposes to form a two-st age channel. The data were analysed using a steady flow computational model to yield water surface profiles and values of Manning's resistan ce coefficient at various flow depths. The complex nature of flow resi stance in two-stage channels is demonstrated. Unsteady stage hydrograp hs were linked to a computational model which illustrates the magnitud e of errors incurred in applying conventional methods of discharge est imation in compound rivers.