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 su nk into the bed of a 21-m-wide upland gravel bed river. The bed load d ata are related to a simultaneous record of average bed shear stress d erived from the depth-slope product. The study reach is straight and i ncludes a small, low-relief mid-channel bar which represents the divid e between two zones of markedly different bed structure and stability. The bed of the left channel and bar tail is loose with an open framew ork, while that of the bar head and right channel is tightly interlock ing and well imbricated. Bed surface grain size is similar throughout the reach. The two zones of contrasting bed structure have a strong in fluence on the threshold of sediment movement (tau(c)) and consequentl y the spatial pattern of bed load transport. Entrainment thresholds fo r the tightly structured bed are up to seven times higher than those f or the loosely structured bed. Sediment transport is therefore most fr equent in the left channel and over the bar tail, where flows with exc ess shear stresses (tau > tau(c)) peaking at 8 tau(c) constitute 19% o f the flow record. This is in marked contrast to the right channel and bar head, where excess shear stresses of maximum 1.4 tau(c) total onl y 1.3% of the flow record. However, despite the marked difference in t he magnitude and duration of competent flows between the two contrasti ng bed areas, little difference in bed load yield is observed between the nine traps when averaged over the 14-month study period. This stud y shows that different zones of bed structure and stability in a chann el will control the incidence and spatial pattern of bed load transpor t but not necessarily the medium and long-term bed load yield. This ha s important implications for local and short-term bed load sampling pr ograms and the numerical modeling of sediment budgets and zone-to-zone bed load transfer.