Hydraulic Study for Storm Sewer Outlet

A storm sewer is planned to discharge into the Mississippi River from its west bank just below St. Anthony Falls. The sewer pipe is 12 ft in diameter and its centerline will make an angle of about 60° with the river bank at the point of discharge. At that point the river is used by barges approaching a lock at the Falls. The water level is closely controlled at 750 ft, MSL. It is required that sewer discharges should not produce local mean crossflow velocities in the river in excess of 4 fps at a sewer design discharge of 1700 cfs nor in excess of 2 fps at the more likely discharge of 500 cfs. The river bed is sandstone at elevation 739.3 ft MSL and the sewer invert has been designed to intersect the river bank at that elevation. Because of the rook, it is desired to diffuse or spread the flow horizontally at the sewer outlet to achieve the required velocities rather than to dissipate energy in a conventional stilling basin. The sewer pipe is laid on a straight tangent of at least 600 ft length at a uniform slope, of 0.001 before reaching the river.