The Effect of Plan and Slope Profile on Runoff Initiation Threshold

Knowledge of initiation of soil runoff is important in estimating surface runoff, volume of runoff and losses from a rainfall event over soil surfaces. Many parameters in the beginning of runoff are effective which topography is a major influencing parameter. The plan shape is an important parameter which affects the run-off initiation which has been investigated in this research. In order to investigate the effect of the plan shape, a three dimensional geometric model is required. Calculating the run-off appearance in hillslopes leads to measuring the time that the run-off starts, as well as surface water flow and the amount of runoff and sediment movement. A laboratory research is conducted to investigate the effects of plan shape which are divergent, parallel, and convergent on time of run-off initiation. The results showed that the nine complex hillslopes have a different runoff initiation threshold. As well, when the plan shape changes from convergent to divergent, the time of runoff initiation of surface and subsurface flow increases. This means that in the convergent hillslopes, runoff starts later. Also, the effect of plan shape is more important than profile curvature for starting runoff. Thus, in the divergent and concave hillslopes, runoff is started posterior than convergent and convex hillslopes.