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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 (<6 g/sm). Depletion of material in the patches occurred rapidly, with bed load rates and particle sizes decreasing after only 5 minutes. Combining flume and Helley‐Smith data for the study reach, a breakpoint in the relation between shear stress and bed load rate was calculated to be around 30 N/m2. This represents the transition between bed load transport phases: below the breakpoint, transport occurs at a low rate and is composed predominantly of fine sediment from patches, but above it, much higher rates occur from across the reach as a whole. Hydraulic conditions at the threshold are those which occur during small, frequent floods (25% bankfull, flow equaled or exceeded 15% of time). This indicates that sediment entrainment from patches of fine material is a frequent process and the threshold change between bed load phases occurs regularly.