$5\mathrm{.}3\mathrm{.}18$ A $5-$m wide rectangular channel with two reaches, each

with a different slope, conveys $40\frac{m^3}{s}$ of water. The channel slope

for the first reach is $0\mathrm{.}0005$ and then a sudden change to a slope of

$0\mathrm{.}015$ so that critical flow occurs at the transition. Determine the

depths of flow at locations $10m,20m$ and $30m$ upstream of the

critical depth. The Manning's $n$ for the channel is $0\mathrm{.}015.$