The concept of dominant discharge applied to two gravel‐bed streams in relation to channel stability thresholds

Abstract Dominant discharge may be defined as that discharge which transports most bed sediment in a stream that is close to steady‐state conditions. The concept is examined in relation to two single thread gravel‐bedded streams. One stream is alluvial and free to adjust its geometry whilst in the other, channel capacity and form are partially constrained by cohesive till‐banks and a heavily compacted bed. The total quantity of bedload and its calibre were measured for every flood over a six year period, so that the relationship between the grain‐size of bedload and the most effective discharge could be examined in the context of thresholds for channel change. The dominant discharge concept was applicable to the alluvial stream in that the bankfull value is an effective discharge for maintaining channel capacity. The concept applied less well to the ‘non‐alluvial’ stream. Although in both streams the bankfull value was exceeded for less than 0.34 per cent of the time, overbank flows are important in instigating channel change. It is only during overbank flows that the largest bed material is entrained in quantity. For within‐channel flows a threshold separates flows which winnow fine matrix from those which entrain the finer bed gravels. This threshold occurred at 60 per cent bankfull. It was concluded that the dominant discharge concept can be applied to streams close to steady‐state which are alluvial, competent, and free to adjust their boundaries. An important proviso is that two channel‐stability domains can be recognized. These domains represent channel maintenance and channel adjustment and are defined by intrinsic thresholds in the bed material entrainment function.