Aerodynamic roughness of gravel surfaces

The interactions between surface winds and gravel surfaces that can be characterized by aerodynamic roughness length have important implications for sediment mobilization, transport, sedimentation and the development of desert pavements (‘gobi’ deserts). Wind tunnel results of systematic tests carried out to determine the aerodynamic roughness length of gravel surfaces of different size and coverage at a range of free-stream wind velocities are reported. The aerodynamic roughness of gravel surfaces is a function of gravel size, gravel coverage and free-stream wind velocity. The 1/30 law proposed by Bagnold, based on Nikuradseʹs results of water pipe flow, or taking the aerodynamic roughness length as a fixed percentage of roughness element height is not supported for gravel surfaces. The aerodynamic roughness of gravel surfaces decreases with free-stream wind velocity. The variation of aerodynamic roughness with gravel coverage can be described by quadratic curves. Maximum aerodynamic roughness length occurs at gravel coverage range above 40–75%, increasing with gravel size. Multivariate models based on free-stream wind velocity and gravel coverage have been developed for predicting the aerodynamic roughness of different gravel-sized surfaces. The maturity of the roughness effects of gravel surfaces increases with gravel coverage. In general, the roughness effects are fully developed when the gravel coverage is over 15%.