An equilibrium relationship for shear velocity and apparent roughness lenght in aeolian saltation

Abstract

An equilibrium relationship between shear velocity and apparent surface roughness is established for aeolian saltation. Velocity profiles of wind over blowing sand were digitized from eleven published data sets. The 106 profiles were analyzed to obtain estimates of shear velocity and apparent surface roughness lenghts. These values were used to test Charnock's relationship for suitability in aeolian environments. The results indicate that z 0 = 0.0091 u ∗ 2/g . Modifying Charnock's model to describe wind over an equilibrium saltation field yields z 0 − (2D 50/30) = 0.0252 (u ∗−u ∗t) 2/g . The latter result is used to demonstrate that the focal point concept proposed by Bagnold stems from the increase in apparent roughness with increased shear velocity. The results demonstrate an excellent example of threshold-controlled equilibrium in dynamic systems.