Hydraulic 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 decrease the hydraulic parameters before reaching the threshold value. In 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 m height was used to simulate concentrated

overland flow with 0.1 m depth. Three experiments were examined and for each experiment, discharge

and mean flow depth were measured directly and other parameters such as mean velocity, shear

stress, Froude number and Reynolds number were calculated. Results of this study show that, hydraulic

parameters such as velocity and shear stress were increased with flow length. These increasing trends were

not linear and uniform. They changed according to the flow resistance. Flow was turbulent and subcritical

moreover and became super critical at the end of flume, that velocity was increased and erosion initiated.

In this study maximum shear stress was measured at the 75m from beginning of the flume and if we

consider that shear stress is stream power for erosion, so that 75 m is suggested as the best distance between

channel spreading in the Kowsar station.