The variation of water-surface slope and its significance for bedload transport 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 determinant of shear stress and the impact that variability would have on calculated sediment flux. This is pragmatic. It recognises that confirmatory data are unlikely to be available, especially during flood flows, and it is an appropriate assumption where discharge is steady. Where discharge is unsteady, water-surface slope varies and an expected pattern of hysteresis in the relation between water-surface slope and flow depth emerges from datasets collected in four gravel-bed streams, two ephemeral, one seasonal and one perennial. When water-surface slope is treated as a variable in applying a bedload equation, it is shown that flood bedload yields are about 8 percent higher than those derived with the same equation but with water-surface slope held constant and approximating the slope of the channel bed. It is concluded that, in engineering design, accounting for the variation in water-surface slope in arid-zone ephemeral streams, where bedload yield is high, is more significant than in perennial streams, where event frequency may be high but transport rates are low and highly variable.