Experimental Study on Anisotropic Attributes of Surface Roughness in Watersheds

The spatial variability of surface roughness can affect the overland flow processes. To accurately simulate the overland flow processes of watersheds, many theories have been developed which consider the spatial variability in surface roughness. Several theories claim that the surface roughness has anisotropic attributes, but there is little experimental evidence to support this claim. The authors developed a hydraulic laboratory model to simulate land surfaces, and replicated water flows in different directions over the exact same overland surface. Two typical strip overland surfaces were simulated, and the Manning’s roughness coefficient is calibrated. The results show that in a completely turbulent regime when the angle $\theta$ of the flow direction and the; overland strips direction are 90° and 45°, respectively; the average Manning’s roughness coefficient values $n$ are 0.0438 and 0.0492 over a $60\times 60\mathrm{mm}$ simulation overland surface, and 0.0276 and 0.0382 over a $120\times 60\mathrm{mm}$ simulation overland surface, respectively. Over the exact same strips of surface, there are different Manning’s roughness coefficient values in different flow directions. This proves that the surface roughness of watersheds has anisotropic attributes, which will have effects on overland runoff path and distribution.