Spatial pattern of flow competence and bed load transport in a divided gravel bed river

A 14-month continuous record of the spatial variation in the incidence and magnitude of bed load transport was provided by nine pit traps sunk into the bed of a 21-m-wide upland gravel bed river. The bed load data are related to a simultaneous record of average bed shear stress derived from the depth-slope product. The study reach is straight and includes a small, low-relief mid-channel bar which represents the divide between two zones of markedly different bed structure and stability. he bed of the left channel and bar tail is loose with an open framework, while that of the bar head and right channel is tightly interlocking and well imbricated. Bed surface grain size is similar throughout the reach. The two zones of contrasting bed structure have a strong influence on the threshold of sediment movement (τ c) and consequently the spatial pattern of bed load transport. Entrainment thresholds for the tightly structured bed are up to seven times higher than those for the loosely structured bed. Sediment transport is therefore most frequent in the left channel and over the bar tail, where flows with excess shear stresses (τ > τc) peaking at 8τc constitute 19% of the flow record. This is in marked contrast to the right channel and bar head, where excess shear stresses of maximum 1.4τc total only 1.3% of the flow record. However, despite the marked difference in the magnitude and duration of competent flows between the two contrasting bed areas, little difference in bed load yield is observed between the nine traps when averaged over the 14-month study period. This study shows that different zones of bed structure and stability in a channel will control the incidence and spatial pattern of bed load transport but not necessarily the medium and long-term bed load yield. This has important implications for local and short-term bed load sampling programs and the numerical modeling of sediment budgets and zone-to-zone bed load transfer.