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

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-g rained ephemeral stream of southern Israel, give an unequivocal quantificat ion of the change from unequal to near-equal mobility of grains of all avai lable sizes. This second threshold occurs at similar to4.5 times the shear stress associated with initial motion (4.57 tau (C)). The level determined in Eshtemoa is much higher than previously defined by laboratory flume stud ies. The difference probably reflects the tighter packing and the greater i nterlock of natural streambeds and carries implications for a number of iss ues, including the modeling of sediment transport, especially in the contex t of dryland reservoir sedimentation. The proportion of time that flow exce eds 4.57 tau (C) determines the size distribution of cumulative bed load. U sing a flow duration/sediment transport approach, in which fractional trans port 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 a s the flow duration curve of individual flash floods incorporates progressi vely higher flows of variable duration. The method has utility because simp le flood parameters such as peak discharge are imperfect predictors of sedi ment flux, particularly in desert streams where the shape of the flood hydr ograph varies considerably from event to event. It is also shown that over a longer period (annual or decadal) the caliber of cumulative bed load depe nds upon the spectrum of flows in incident floods. The 4-year hydrograph re cord in Eshtemoa provides a model-derived bed load size distribution just f iner than the unarmored bed material. The Eshtemoa data set can also be use d 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 th an a wide range of ether states of how in transporting bed load.