Sediment entrainment and depletion from patches of fine material in a gravel‐bed river

This paper presents the results of experiments in which a portable flume was used to manipulate hydraulic conditions and create bed load transport in a gravel bed river. Flume data are coupled with those from Helley‐Smith samples to assess bed load characteristics at shear stresses ranging from 5 to 60 N/m2. Experiments demonstrate that patches of fine sediment control both the intensity and duration of bed load under hydraulic conditions characteristic of the early stages of floods. The experiments allow quantifying bed load at the entrainment threshold, providing the first empirical evidence that marginal bed load transport can be attributed to the mobilization of sediments from patches. Bed load transport was recorded consistently once shear stress exceeded 5 N/m2. The experiments produced low bed load rates (