Evaluation of the effect of slope gradient on threshold of erosion (gully) in flood spreading projects

Improper and inadequate usage of not-renewable resources and drought has prepared condition for degrading of vegetation cover and desertification. Herein control and flood spreading project that could be applied in more deserts is an appropriate solution to this problem. On the other hand, different parameters including slope gradient, soil texture, vegetation cover and flow resistance affect on flow hydraulic characteristics. In this study, seasonal floods are spreading with spreading channels which is constructed on contour lines. This study carried out in the Gareh Baygan plan, in the south east of Fars province to investigate the effect of slope steepness on critical distance for scouring. An open hydraulic flume with 100 meter length, 40 centimeter width and 25 centimeter depth was used to simulate overland flow with 10 centimeter depth. Five treatments including 0.002, 0.004, 0,006, 0.008 and 0.010 for slope were examined, and for each treatment, three to four experiments were used and some hydraulic parameter including discharge and flow depth were measured directly, and others such as flow area and flow velocity were calculated. After each experiment erosion and sedimentation depth were measured individually. Results of this study show that threshold value will be increasing with increase slope gradient. Average distance of erosion threshold obtained for different treatments 66.5, 56, 45, 36 and 28, respectively. Likewise, flow velocity was increased with flow length. Also, this increasing trend was linear and uniform and changed according to bed slope and roughness and reached the maximum value in each experiment.