MECHANICS OF JET SCOUR DOWNSTREAM OF A HEADCUT

Scour immediately downstream of a headcut is analyzed by considering the effect of jet diffusion on sediment detachment. The analytical development results in equations for: 1. the equilibrium scour depth; 2. the rate of scour hole development. These equations are applicable for a variety of jet configurations and any bed material including cohesive soils. The equations are tested in laboratory simulated headcuts on bed materials consisting of two non-cohesive uniform sands and one cohesive natural soil. The agreement between measured and predicted rates of scour depth increase shows that scour does not increase as a semi-logarithmic function of time. The rate of scour depth increase is very rapid for depths less than 95% of the equilibrium scour depth. Graphical solutions for the rate of scour depth increase are in agreement with the experimental measurements.