THE VARIATION OF WATER-SURFACE SLOPE AND ITS SIGNIFICANCE FOR BEDLOADTRANSPORT DURING FLOODS IN GRAVEL-BED STREAMS

Water-surface slope is usually assumed to be constant when predicting bedload sediment transport in rivers despite its significance as a det erminant of shear stress and the impact that variability would have on calculated sediment flux. This is pragmatic. It recognises that confi rmatory data are unlikely to be available, especially during flood flo ws, and it is an appropriate assumption where discharge is steady. Whe re discharge is unsteady, water-surface slope varies and an expected p attern of hysteresis in the relation between water-surface slope and f low depth emerges from datasets collected in four gravel-bed streams, two ephemeral, one seasonal and one perennial. When water-surface slop e is treated as a variable in applying a bedload equation, if is shown that flood bedload yields are about 8 percent higher than those deriv ed with the same equation but with water-surface slope held constant a nd approximating the slope of the channel bed. It is concluded that, i n engineering design, accounting for the variation in water-surface sl ope in arid-zone ephemeral streams, where bedload yield is high, is mo re significant than in perennial streams, where event frequency may be high but transport rates are low and highly variable.