$5\mathrm{.}2\mathrm{.}2$ Compute the critical flow depth in a trapezoidal channel with a bottom width of $20ft,$ slope of $0\mathrm{.}0016,$ Manning's $n$ of $0\mathrm{.}025,$ and side slopes of $2$:$1($h:v$)$ for a discharge of $400cfs.$