The trapezoidal channel has a slope of 004 The sides of the

The trapezoidal channel has a slope of .004. The sides of the canal are sealed with a plastic membrane, which is protected by a 1 ft thick layer of 1 in diameter gravel. The bottom is a 6 in slab of rough concrete. If the water is flowing at a uniform depth of 6 ft, do you think the gravel will be scoured by the flowing water? Justify your answer by appropriate calculations. Thanks!

Solution

Yes the gravel will be scoured by following water in canal.   Due to Average velocity of flow is greater than critical velociy of flow. Justification :Velocity of flow by manning formula, V=1/n*R2/3S1/2. Where:n-rugosity constant is 0.023,R-hydraulic mean depth,S-slope of chanel. The bottom width of channel is find B=1500ft,Top width is=3000ft. So R=[1500+250*6]/[1500+2*6((1+250*250))]. R=4, The velocity of flow V=6.9m/s, Critical velocity of flow by kennedy theory Vcritical=0.55*1.33(1.82)^(0.64). Vcritical=1.07m/s. **Here we see that average velocity flow of canal flow is more than the critical velocity of canal flow ,thats why gravel will be scoured. *To prevent this proper slope will be given