P$\mathrm{.}5\mathrm{.}20.$ A very sinuous length segment of an earthen channel carrying $Q=35\frac{m^3}{s}$ frequently erodes. The channel cross$-$section can be approximated as a trapezoid with a bottom width of $b=3m$ and side slopes of $m=3\mathrm{.}0.$ The bottom slope is $S_0=0\mathrm{.}003.$ The channel will be lined with very angular riprap to prevent the erosion problem. Select a proper riprap size. To determine the Manning roughness factor, use Eq$.(5\mathrm{.}28)$ with $C_m=0\mathrm{.}039.$