Suppose that the trapezoidal channel of Fig. P10.17 contains sand and silt which we wish not to

Question:

Suppose that the trapezoidal channel of Fig. P10.17 contains sand and silt which we wish not to erode. According to an empirical correlation by A. Shields in 1936, the average wall shear stress τcrit required to erode sand particles of diameter dp is approximated by