Solve Prob. $6\mathrm{.}8($Ans: $0\mathrm{.}492$ acre; $465ft$;$10\mathrm{.}16f\frac{t^{\frac{1}{3}}}{sec\text{;}5\mathrm{.}483sec\text{;}3\mathrm{.}375cfs}).$

Figure P$6-8$

$6\mathrm{.}8$ A planned $5\mathrm{.}68-$ac subdivision is sketched in Fig. P$6-8.$ The soils are generally

sandy, and the only runoff will occur from the directly connected $($i$.$e$.,$ hy$-$

draulically effective$)$ impervious street and driveway surfaces shown in the fig$-$

ure. $($Only the $2030ft$ portion of each driveway that drains to the street is

shown.$)$ The street is $30ft$ wide and the cul$-$de$-$sac has a radius of $30ft.$ For storm

drainage, the plan is to let the stormwater run along the street gutters in lieu of

installing a pipe. For purposes of this problem, the entire drainage system can

be treated as overland flow. The street slopes from an elevation of $165ft$ at the

center of the cul$-$de$-$sac to $160ft$ at the entrance to the subdivision and has a

Manning roughness of $0\mathrm{.}016.$

Drainage regulations specify a $5-$yr return period design. Local IDF curves

can be approximated functionally as

$i=\frac{a}{b+t_r},$

where

$i=$ rainfall intensity $(in\frac{.}{h}r),$

$t_r=$ duration $(min),$

$a,b=$ constants for different return periods.

For this hypothetical location, $a=160$ and $b=18$ for a return period of $5yr.$

$($a$)$ Estimate the directly connected impervious area $($ac$).$

$($b$)$ Estimate the maximum drainage length along the DCIA $($ft$).$