PARTICLE-TRANSPORT AND ADJUSTMENTS OF THE BOUNDARY-LAYER OVER ROUGH SURFACES WITH AN UNRESTRICTED, UPWIND SUPPLY OF SEDIMENT

Most natural surfaces containing non-erodible roughness elements an co nsiderably more complex than those studied in modelling exercises and wind tunnel simulations. Unlike idealized roughness elements, which ar e uniform in size, shape (i.e., spheres or cylinders) and spacing, nat ural elements are challenging to measure in 3-dimensional space. Simil arly, most deflation lag surfaces, such those as found on sandar and b eaches, are spatially heterogeneous open systems in which sediment tra nsport from an external supply is very likely. The development of irre gular deflation lag surfaces, and the transport of sediment over these surfaces from an upwind source of sediment, was studied in a series o f wind tunnel simulations. Surfaces prepared with crushed gravel and n atural beach shingle respond conservatively in terms of the adjustment to the deflation and deposition of sediment. Deflation lag surfaces, prepared with no spacing between the roughness elements (i.e., close p acked), demonstrate little to no change in coverage with the introduct ion of particles from an upwind source. Neither the element type nor t he friction velocity affect this outcome. As the center-to-center elem ent spacing increases to 60 mm, infilling of the lag surface eventuall y is observed, with the element coverage reduced by a factor between 2 and 4. For a given threshold ratio (R-t), the roughness density (lamb da) is smaller than observed in previous simulation studies based on i dealized roughness elements. (C) 1998 Elsevier Science B.V. All rights reserved.