Field evaluation of channel erosion threshold in flood water spreading projects

Erosion and sedimentation study and design of the distance between spreading channels are basic requirement in flood water spreading projects. Different environmental parameters including topography and slope، soil texture، vegetation cover and flow resistance affect the hydraulic characteristics of the flood water spreading area. So it is necessary to investigate the effect of environmental parameters according to natural conditions 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 the increase or the 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 the top end of the flume. On the other hand، according to the bed elevations of cross sections in two steps maximum erosion depth occurred at 75 m from the start of the flume which was confirmed with maximum shear stress. So، we can conclude that channel erosion threshold was established in 75 m from the top end of the flume and it can be considered as channel spreading distance.