A WIND-TUNNEL STUDY OF THE INFLUENCE OF PORE-WATER ON AEOLIAN SEDIMENT TRANSPORT

Experiments conducted in a straight-line suction wind tunnel confirm n umerous field observations of sand particles from an upwind source sal tating over wet surfaces. Comparison of the mass transport rate on a w et surface (q(w)) with its dry surface equivalent (q(d)) yields a stro ng linear relationship (r(2) > 0.99) over a broad range in freestream velocity. Slope coefficients vary between 0.75 and 1.1. These results indicate that over short distances in the order of 5 m, the mass trans port rate is minimally affected by the presence of pore water. However , the boundary-layer structure is substantially modified by grain salt ation on a wet surface as compared to the dry condition. Inner region friction velocities associated with dry sand transport (u star(d)) gen erally exceed those for clean air (u star(o)), while u star(w) for mob ile wet surfaces drops below u star(o). A model is proposed which rela tes the ratio q(w)/q(d) to several other dimensionless composite varia bles which address the aerodynamic drag on a wet surface relative to ( 1) that for an untreated surface and (2) capillary force magnitude. Th e product of these emulates the variation observed in the slope coeffi cients from the empirical regressions, thereby quantifying several of the underlying physical processes which govern transport in wet condit ions. (C)1998 Academic Press Limited.