$[17$ pts$]$ Rational Method: A storm drain system is shown in Figure below

For the flow conditions indicated,

a$.$ determine the time of concentration using Figure in page $7$ showing average velocity of overland flow as a function of slope and land cover $($from U$.$S$.$SCS $1975$b$)$

b$.$ determine the peak flow at the outlet ditch $($Point C$)$ by the rational method, assuming the $5-$year rainfall intensity is given by $\frac{250}{(t_c+30)},$ where $t_c$ is in minutes and $i$ is in$/$hr$.$

c$.$ If the duration of runoff is $2$ times the time of concentration, estimate the runoff volume $($ac$-$ft$)$ and depth of runoff $($in$)$ assuming a triangular hydrograph. $1$ acre $=43560f{t}^2$