Evolution of bed load grain size distribution with increasing flow strength and the effect of flow duration on the caliber of bed load sediment yield in ephemeral gravel bed rivers

Abstract

The grain size distributions of bed load collected during flows that range from the threshold of motion to near bankfull in Nahal Eshtemoa, a coarse‐grained ephemeral stream of southern Israel, give an unequivocal quantification of the change from unequal to near‐equal mobility of grains of all available sizes. This second threshold occurs at ∼4.5 times the shear stress associated with initial motion (4.5тc). The level determined in Eshtemoa is much higher than previously defined by laboratory flume studies. The difference probably reflects the tighter packing and the greater interlock of natural streambeds and carries implications for a number of issues, including the modeling of sediment transport, especially in the context of dryland reservoir sedimentation. The proportion of time that flow exceeds 4.5тc determines the size distribution of cumulative bed load. Using a flow duration/sediment transport approach, in which fractional transport rates by grain size class are predicted with Parker's [1979] bed load function, it can be shown that the size distribution of bed load coarsens as the flow duration curve of individual flash floods incorporates progressively higher flows of variable duration. The method has utility because simple flood parameters such as peak discharge are imperfect predictors of sediment flux, particularly in desert streams where the shape of the flood hydrograph varies considerably from event to event. It is also shown that over a longer period (annual or decadal) the caliber of cumulative bed load depends upon the spectrum of flows in incident floods. The 4‐year hydrograph record in Eshtemoa provides a model‐derived bed load size distribution just finer than the unarmored bed material. The Eshtemoa data set can also be used to show clearly that water discharge in this type of ephemeral stream is “effective” or “dominant” over a wide range of flow stage ranging from 0.4 to 1.1 times bankfull and that bankfull discharge is no more significant than a wide range of other states of flow in transporting bed load.