Best Hydraulic Section

Now Q is a maximum when A*)P2 is a maximum. If x is some variable, such as a leading dimension, the condition for maximum discharge is (d/dx)(A5/P2) = 0. Thus

For a given sectional area A , dA/dx = 0, and hence the maximum dis­ charge is obtained when

or

(7.25)

(7.26)

that is to say when the wetted perimeter is a minimum. This section is known as the best hydraulic section. Furthermore, it follows that for a

given discharge the sectional area is a minimum, although the excava­ tion volume is not a minimum unless bank-full flow conditions pertain.