Much farther downstream, an impermeable sidewall $($see schematic$)$ forces the volume

flow rate of water per unit width to vanish at each x location, i$.$e$.$ Q$/$b $=0.$ Moreover,

the presence of the sidewall causes C$2=0.[$In practice, the free surface is very slightly

tilted, so that the water depth increases slightly as x increases. Although this slight

inclination of the free surface mostly can be ignored to first approximation $($e$.$g$.,$ it$$s

ok to assume v $0),$ the x pressure gradient induced by hydrostatic effects$$where

the water is deeper, the pressure is larger$$cannot be ignored. Hence, C$2=0.]$

$($