For the cross$-$section provided in the figure, if bottom width $(b)=4ft,$ side slope $(z)=1\mathrm{.}5,$ depth $(y)=4ft,$ and the flowrate $(Q)=500\frac{f{t}^3}{s},$ then determine the altern$$te depth for $4$ ft :

$$ a$)5\mathrm{.}64$

$$ b$)7\mathrm{.}11$

$$ c$)0\mathrm{.}55$