A composed cross$-$section channel consists of a deep main channel and two shallow floodplains. The floodplains $(n=0\mathrm{.}04)$ are rougher than the main channel $(n=0\mathrm{.}02)$; so the mean velocity in the main channel is greater than the mean velocities in the floodplains. If the channel has the same slope everywhere $S_0=6\mathrm{.}6,$ estimate the total water discharge $).$

Answer: $$