AEOLIAN SEDIMENT FLUX DECAY - NONLINEAR BEHAVIOR ON DEVELOPING DEFLATION LAG SURFACES

Wind tunnel simulations of the effect of non-erodible roughness elemen ts;on sediment transport show that the flux ratio q/q(s), shear veloci ty U and roughness density lambda are co-dependent variables. Initial ly, the sediment flux is enhanced by kinetic energy retention in relat ively elastic collisions that occur at the roughness element surfaces, but at the same time, the rising surface coverage of the immobile ele ments reduces the probability of grain ejection. A zone of strong shea ring stress develops within 0.03 to 0.04 m of the rough bed because of a relative straightening of velocity profiles which are normally conv ex with saltation drag. This positive influence on fluid entrainment i s opposed by declining shear stress partitioned to the sand bed. Simil arly, because the free stream velocity U-f is fixed while U increases , velocity at height z and particle momentum gain from the airstream d ecline, leading eventually to lower numbers of particles ejected on av erage at each impact. When the ratio of the element basal area to fron tal area sigma is approximately equal to 3.5, secondary flow effects a ppear to become significant, so that the dimensionless aerodynamic rou ghness parameter Z(0)/h and shear stress on the exposed sand bed tau(s ) decrease. It is at this point that grain supply to the airstream and saltation drag appear to be significantly reduced, thereby intensifyi ng the reduction in U-. The zone of strong fluid shear near the bed d issipates.