Simplicity and complexity of bed load response during flash floods in a gravel bed ephemeral river: A 10 year field study

Abstract

A decade of data for the Nahal Eshtemoa, an unarmored, gravel bed, ephemeral river, reveals that bed load flux during rain‐fed flash floods is a simple function of channel average boundary shear stress. However, the relation is inadequately described by a power function of the type commonly used in predictive bed load equations, and a linear function gives a much better fit. The success of a linear function is related to the unarmored nature of the bed material, typical of ephemeral gravel bed rivers, and the ready availability of sediment at all levels of transport stage. Bed load response to changing shear stress is largely undifferentiated as flash floods wax or wane. However, while most data conform to a simple relation between bed load and hydraulic stress, some for individual floods or flood segments do not, reflecting variability in sediment supply and texture. Three type behaviors are identified that assist in explaining both the general, simple pattern and the limited scatter of the bed load–shear stress plot. Type 1 provides the majority of the data, giving the general linear relation between bed load flux and boundary shear stress. Type 2 is rare, given the flashy nature of the hydrograph, but represents comparatively steady flows well above the entrainment threshold, during which bed load sheets or the breakup of grain clusters lead to spasmodic changes in bed load flux. Type 3 shows a weak relation between shear stress and bed load flux, representing flow conditions just above the entrainment threshold.