A method for estimating the mean bed load flux in braided rivers

Abstract

Prediction of bed load flux remains a significant problem in understanding braided river morphodynamics for geomorphic and engineering applications. Two sets of data from laboratory experiments on braided networks performed at the University of Trento (Italy) and at the University of Alberta (Canada) provide the basis for development of a dimensionless bed load function and for testing a simple predictive model. Measured total sediment transport rates (time-averaged) at equilibrium channel configuration collapse to a single dimensionless relationship based on dimensionless stream power. Bed load fluxes predicted by the Parker and Bagnold functions and cross-section average hydraulic parameters under-predict the bed load flux, particularly at low shear stress. This is consistent with previous observation and theory demonstrating the significant influence of transverse variability of the hydraulic parameters in controlling and predicting bed load flux. A simple method for adjusting for this lateral variation is proposed for computing the sediment transport rate using topographic cross-sections of braided rivers. Results show good agreement with the measured values, suggesting that simple assumptions combined with the mean morphology of the channel may be sufficient to estimate mean bed load flux. Model predictions also highlight the significance of active bed width, rather than bed shear stress, as a fundamental ingredient in the prediction of the bed load rate and therefore, as an important morphological property to be modelled. The model also predicts similar behaviour in the at-a-station variability of bed load and in the importance of variation of active width, relative to bed shear stress, in the transport process.