Some aspects of flow over coastal dunes

Abstract

The paper discusses limits under which flux-gradient relationships derived for horizontal homogeneous terrain can be applied to aeolian transport over non-uniform terrain with coastal dunes. For flat sandy beaches the roughness change upstream of the beach-dune boundary can be expected to be small. For horizontal beaches that are some hundred metres wide and where the length scale of the horizontal heterogeneities does not exceed some tens of metres, consistent surface stress and aerodynamical roughness values may be deduced from the wind-speed profile.Slope effects will modify the air flow strongly as it crosses a dune ridge or escarpment. Recent developments in the theory of turbulent flow over hills are discussed in relation to common coastal dune terrain. The depth of the layer where the profile is in equilibrium with the underlying surface typically does not exceed a few tens of centimeters. This strongly restricts our ability to derive reliable values for roughness and surface stress from the wind profile.Experimental studies at two coastal dunes in Denmark indicate that it is often difficult to obtain a good upwind reference for the flow. Analysis with a numerical model, set up for one of the sites, suggests that information about the flow and the value of experimental data may be improved when these can be both related to and supported by numerical calculations.