Channel adjustments in a gravel-sand bed reach owing to upstream damming

Upstream damming can greatly alter the flow and sediment conditions entering downstream reaches, leading to remarkable channel adjustments. Especially in a gravel-sand bed reach, the adjustments are more complicated owing to bed-material coarsening process. In order to quantify the adjustment characteristics of a total reach, a reach-averaged method was adopted to calculate the reach-scale bankfull channel dimensions firstly. Then the effects were investigated respectively of the altered flow-sediment regime and the bed-material coarsening degree on the variation in bankfull channel geometry. The results show that the effect of the altered flow-sediment regime on channel adjustments was lower in a coarsening bed reach than a less-coarsening one, because bed-material coarsening limited channel incision and led to a weaker response of channel adjustments to the variation in flow and sediment regime. Moreover, bed-material coarsening also had a significant influence on the adjustments in bankfull channel dimensions, with greater correlations being obtained in the reach suffering higher degree of bed-material coarsening. Therefore, a comprehensive hydraulic geometry relation was proposed to describe the channel adjustments in a gravel-sand bed reach, with these two influencing factors being considered. The obtained correlations were higher than those obtained from the simple relation that solely considered a single influencing factor.