Wind speed and sediment transport recovery in the lee of a vegetated and denuded nebkha within a nebkha dune field

• Recovery of wind speed behind nebkha or vegetation free dome dune occurs 8 element heights downwind.
• Recovery of wind speed behind element occurs more rapidly for dome than nebkha.
• Recovery of wind speed is more rapid for dome and nebkha than for a two-dimensional porous fence.
• Recovery of the saltation flux begins less than two element heights downwind.
• Saltation flux in the lee does not match the recovering wind shear due to supply limitations.

Field observations of scaled wind speed and sand transport recovery in the lee of a nebka within a field of nebkhas and then subsequently for the nebkha denuded of its vegetation cover were collected. The measurements of wind speed at 0.4 times the element height indicate that for both conditions wind speed recovery in the lee is exponential. The porous vegetation cover influences the rate of this recovery being more gradual for the vegetated form. The return to equilibrium wind speed occurs in both cases at approximately eight element heights. For either case the recovery of shear stress and the return to a constant value occurs much closer to the bluff body form than has been described for porous fences. The recovery of sand transport in the lee appears to be more rapid for the un-vegetated condition, which corresponds to the observed faster rate of wind speed increase. The observations did not show a continual increase in saltation flux with increasing downwind distance due to the increasing shear stress downwind and the increase that may be expected due to the fetch effect. The change in saltation flux with downwind distance was controlled by the sediment supply, which diminished with downwind distance. The interaction of a changing shear stress and the zone of influence created by the wind as it interacts with the roughness dimensions, along with the distribution of sediment available for transport bring increased complexity to modeling sand flux for this type of environment over different temporal scales.