Storm Drainage Practices of Thirty-Two Cities

Wisconsin cities with populations of 7,500 to over 60,000 are rapidly developing comprehensive plans independent of community size with all storm sewer designs done by registered professional engineers. Storms with 5 to 10 year frequency are used in design. Two-foot inlets depressed one in., 12-in. minimum pipe size and pipe velocities of 2 to 15 fps are common. Storm water pollution is recognized but only 1/3 of cities have ideas on its alleviation. Urban drainage designers who provided current practice, policy, procedure, and cost information for a typical 15-acre, 6-block area indicated wide diversity in results when using the Rational Method to compute flows. Only six of twenty-three cities which contributed sample designs used variable intensities correctly in the Rational Method. Errors in the use of the runoff coefficient C was common. The resulting designs gave flows of 68 to 100 cfs at outlet; total project costs which varied from $8,000 to $65,000 for this sample area. The need for narrower guide lines for design is suggested.