A concrete box storm sewer 4 feet high by 8 feet wide discharges into an earth channel with a bottom width of 10 feet and side slopes of 3 horizontal to 1 vertical. The storm sewer has a slope of 0.5...

A concrete box storm sewer 4 feet high by 8 feet wide discharges into an earth channel with a bottom width of 10 feet and side slopes of 3 horizontal to 1 vertical. The storm sewer has a slope of 0.5 percent and an n-value of 0.015 and carries a design discharge of 215 cfs. Design a riprap apron for the outfall.