A Total Load Algorithm for Sand Transport by Fast Steady Currents

Abstract

A simple algorithm for total load sand flux is proposed which is suitable for application to high energy environments where fast currents promote significant sand suspension. The algorithm is a power relationship constrained by a threshold condition:q=k[(u1−u1t)/u1t]nwhereqis total sand flux,u1andu1tare mean current velocity and threshold current velocity 1m above the bed, respectively, andkis an appropriate entrainment function. The algorithm has been calibrated for sands (mean grain diameter 190–930μm) over a range of mean current velocities (0·2–1·5ms−1) using the extensive experimental data set of Guyet al. (1966) (Prof Papers U.S. Geology Survey,462,96). It is shown thatkandnare dependent on mean grain diameter; they can be determined from appropriate calibration curves. The exponentnvaries from <3 to >4, increasing with decreasing grain diameter as transport by suspension becomes increasingly important;napproaches a minimum constant value for coarser sands as bed load transport becomes dominant. There is evidence that the exponentndecreases with increasing current velocity in the case of fine sands moved by fast currents. The entrainment functionkdecreases with decreasing grain diameter as bed roughness and entrainment efficiency decrease, and approaches a minimum constant value for fine sands. The algorithm reproduces flume sand transport rates more reliably than the previous formulations of Gaddet al. (1978) (Journal of Sedimentary Petrology,48,239–252) and Hardisty (1983) (Journal of Sedimentary Petrology,53,1007–1010) which ignore suspension and use erroneous calibration coefficients.