Gully Control and Drop Spillways

For a parabolic grassed waterway, the hydraulic radius $R$ for shallow depths ($t \gg d$) is closely approximated by:

Options:

• (A) $R = \frac{2}{3} d$
• (B) $R = d$
• (C) $R = \frac{1}{2} d$
• (D) $R = \frac{1}{3} d$