Examining the Effects of Water Surface Slope and Water Stage on Sediment Transport During Short-Lived Hydrographs: An Experimental Study

Flash floods in ephemeral streams are unpredictable, rare, and generally short-lived. Since bed load sediment flux in ephemeral streams is as much as ۱۷ and ۳۵ times that of perennial streams under comparable flows, the prediction of sediment transport is important. Developing a predictive relation between sediment transport/shear stress requires estimates of both the water-surface slope (as a surrogate for the energy slope) and the water stage. Here, ۶ triangular hydrographs having peak flows varying from ۰.۰۱۴۷ to ۰.۰۲ m^۳ s^(-۱)and durations ranging from ۱۵۰ to ۴۰۰ seconds were run over a screeded bed of poorly sorted sediment of ۴.۱ mm median grain size to assess the effect of changing water stage on water-surface slope and, hence, bottom shear stress and consequent bedload flux. Results clearly indicate that the water-surface slope (S_w) and bedload sediment discharge (Q_s) relate directly to water discharge (Qw) and stage. But each relation manifests a clockwise hysteresis: S_w and Q_sthey are both significantly larger during the rising limb of the hydrograph than during the falling limb. This indicates the value of treating water-surface slope as a variable when predicting bedload flux during flash floods in ephemeral channels.

کلیدواژه ها:

Water-surface slope ، Unsteady Flow ، Shear stress ، Flash flood