$7\mathrm{.}21.$ The $450-$mm$-$diameter concrete culvert shown in

Figure $7\mathrm{.}57$ passes surface runoff from a drainage channel

to an open area. The culvert has an allowable headwater

depth of $0\mathrm{.}6$ m$,$ a headwall with square edges at

the entrance, a longitudinal slope of $1\%,$ and a length

of $3\mathrm{.}6$ m$.($a$)$ Determine the flow capacity of the culvert;

and $($b$)$ design the riprap apron at the exit of the culvert.

Consider all applicable equations in determining

the flow capacity of the culvert, and the apron design

should include the length, width, thickness, and stone

size.