Effects of Hydraulic Parameter, Bed Slope and Rock Fragment Cover on Soil Detachment Rates.
Ali Jafari Ardakani, Soil Conservation and Watershed Managment Institute, Iran- Tehran - Tarbiat Modares Univ- Soil Science Dept., Tehran, Iran and Ali Reza Zahirnia, Soil and Watershed Managment Research Institute, Agriculture Faculty of Tehran University, Karaj, Tehran, Iran.
A precise understanding of soil detachment rates is necessary to establish a basic understanding of soil erosion. This study was conduct to investigate the relationship between soil detachment rates and flow discharge, slope gradient, rock fragment cover, flow depth, mean flow velocity, unit stream power, stream power and shear stress. A 6 m long × 1.5 m wide hydraulic flume was used in this study. Flow discharge ranged from 2 to 10 L S-1; slope gradient varied from 1-3% and rock fragment cover varied from 0-30%. The exponential rates indicated that detachment rates increased with greater flow discharge, slope gradient and rock fragment cover. Detachment rates was affected more by discharge in 2, 4 and 6 L S-1 than by slope gradient and in high discharges, effect of flow rate on soil detachment was decreased. The effect of flow depth on soil detachment was also dependent on slope gradient and rock fragment cover. The exponential results indicated that rock fragment cover reduces concentrated flow erosion rates (E) in an exponential way (i.e. E=e-bRC), which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet-rill erosion measured on runoff plots. The decay rate (b) of this exponential relationship increased throughout the experiments because of scour-hole development and bed armoring. In plots that have no rock fragment cover, linear correlation, mean stream power and the Reynolds number were more closely correlated to detachment rate (r2= 0.946); and in non linear (power function) correlation, Water depth was more closely correlated to detachment rate that was any other hydraulic parameters (r2= 0.9758 ). In plots that are rock fragment cover (10, 20 and 30% rock fragment), linear correlation, mean stream power and Reynolds number were more closely correlated to detachment rate (r2= 0.9478); and in non linear (power function) correlation, shear stress and water depth were more closely correlated to detachment rate that was any other hydraulic parameters (r2= 0.9312).