The Effects of Sediment Size on Rill Detachment Capacity in Steep Slopes of (Saravan, Rasht)

The maximum value of soil detachment due to overland flow is the soil detachment capacity. The present study aimed to evaluate the effect of particle size classes on the rill detachment capacity (Dc) in Saravan watershed (North of Iran). For this purpose, the variations of Dc have been measured on soil samples with three particle size classes (0-1, 1-2 and 2-3 mm) under laboratory conditions through a flume experiment at five slope gradients (4.1 to 38.3%) and five flow rates (0.26 to 0.67 Lm-1 s-1) with five replications. The results showed that the Dc in 2-3 mm class was significantly higher than the other particle size classes (p < 0.05). The unit stream power was found to be very accurate predictor of the Dc in forestland of Saravan watershed. Rill erodibility, which calculated by relationship of the Dc and the water flow shear stress, had the lowest value in 0-1 mm class. Overall, this study has confirmed that the particles with size larger than 2 mm on steep slopes are more susceptible to erosion and separation by surface runoff in delicate environment ecosystems, such as the Saravan Forest Park in Northern part of Iran.