Size an $80-$ft$-$long circular culvert to convey a $50-$year peak discharge

of $90$ cfs$,$ subject to a maximum allowable head water depth of $6\mathrm{.}5$ ft$.$ The culvert has one barrel $($f $=0\mathrm{.}018)$ laid on a slope of $0\mathrm{.}01$ with an inlet of square edge w$/$headwall$.$ The $50-$year

TW is $2\mathrm{.}5$ ft$.($Answer: $3\mathrm{.}32$ ft$)$