A canal cut in rock $($n$=0\mathrm{.}030)$ is trapezoidal in section with a bottom width of $6\mathrm{.}10$m and side slopes of $1$ on $1.$ The allowable average velocity is $0\mathrm{.}76$ m$/$s$.$ What slope will produce $5\mathrm{.}66$ cms$?$