MODELING STAGE-DISCHARGE RELATIONSHIPS IN ANASTOMOSED BEDROCK-INFLUENCED SECTIONS OF THE SABIE RIVER SYSTEM

Flow dynamics in a bedrock-influenced river system, the Sable River, S outh Africa, have been found to be significantly different from those in temperate alluvial systems. The lack of lateral water connectivity leads to multiple bedrock distributaries with varying water surface el evations across a cross-section. Distributary activation is dependent on upstream breaching of bedrock barriers between distributaries by ri sing discharge. Where measurement of individual stage-discharge relati onships in each distributary was not possible, a 'Multiple Stage' mode l was developed to predict hydraulic conditions in each distributary, using a single measured rating curve and knowledge of individual distr ibutary water surface elevations at a low flow. Use of the 'Multiple S tage' model has enabled realistic prediction of channel geometry and h ydraulic variables, that accounts for the different stages found in be drock-influenced sections, yet is not prohibitively data intensive. Pr edicted 'Multiple Stage' results for maximum depth and velocity demons trate the vast improvement on modelling flow dynamics, when compared t o the conventional assumption of a single stage representing the whole cross-section. (C) 1998 John Wiley & Sons, Ltd.