Measurement of shear stress and roughness length on a beach

Measurements of surface shearing stress and aerodynamic roughness length on a beach were made by simultaneous temperature- and wind-profile methods in the following three areas of the beach slope on the Gulf of Mexico coast near Fort Walton Beach, Florida: the swash zone, the mid-foreshore, and the area near the berm scarp. Under adiabatic and onshore wind conditions, it was found from the roughness ratios that the swash zone is approximately 100 times smoother than the mid-foreshore and 500 times smoother than the area near the berm scarp; the stress ratios revealed that the shear stress is approximately 2.5 and 3.5 times larger at 10 m and 20 m fetch downwind, respectively, from the swash zone. It is concluded that the stress ratios measured from the transition from smooth to rough on the beach are in fair agreement with those predicted by Panofsky and Townsend [1964].