Field Evaluation of Channel Erosion Threshold in the Flood Water Spreading Projects

Erosion and sedimentation study and design of the distance between spreading channels is a basic requirement in flood water spreading projects. Spreading channels decrease stream power and their distance should be designed so that it could stop the hydraulic parameters before reaching the threshold value. On the other hand, different environmental parameters including topography and slope, soil texture, vegetation cover and flow resistance affect the hydraulic characteristic of the flood water spreading. So it is necessary to investigate the effect of environmental parameters according to natural condition and simulation of overland flow. This study was done in the Gareh Bygone plain, in the south of Fasa city. An open hydraulic flume with 120 m length, 0.4 m width and 0.25 depth was used to simulate concentrated overland flow with 0.1 m depth. Three experiments were examined and for each experiment, discharge flow and mean flow depth were measured directly and other parameters such as mean velocity and shear stress were calculated. Erosion or sedimentation depths were calculated by comparing flume bed elevations in the cross sections with one meter intervals. Results of this study show that erosion and sedimentation phenomena were changed with increase or decrease in trends of shear stress along the flume and shear stress parameter were also changed with bed slope and roughness coefficient and became maximum at 75 m. from initial of the flume. In the other hand, according to the bed elevations of cross sections in two steps (before and after experiments) maximum erosion depth (3cm) were occurred at 75 m. from initial of the flume, which were confirmed with maximum shear stress. So that we can conclude that in this study channel erosion threshold was established in 75 m. from initial of the flume and it can be advised as channel spreading distance.