Reducing Stormwater Runoff Impacts

U-Tech: Underground Technology Cutting Edge Technical Information for Utility Construction and Rehabilitation
By Aaron Eder, P.E., Kennedy/Jenks Consultants | May 2010 Vol. 65 No. 5

In accordance with the surface infiltration design requirements of the Manual, a 10-year storm (3.4 inches of rainfall over 24 hours, NRCS Type 1A rainfall distribution) was used to generate the runoff flow rates presented. A conservative estimate of 5 minutes was used for the time of concentration (Tc). A pervious area Curve Number of 86 was used, based on local soil conditions, and an impervious area Curve Number of 98 was used, which is generally the industry standard for impervious surfaces.

Figure 7

Figure 7 presents the inflow and outflow hydrographs for the 10-year, 24-hour storm event for the drainage basin. The inflow hydrograph, generated from the HYD program, represents the inflow into the stormwater facility from the new storm drainage piping. The outflow hydrograph represents the infiltration (outflow) into the ground.

Figure 8

At the early stages of the inflow hydrograph, the incoming runoff infiltrates into the ground until the incoming runoff exceeds the composite maximum infiltration rate of 13.1 inches per hour. After the incoming stormwater exceeds this amount, runoff accumulates in the infiltration facility until shortly after the peak inflow, 1.07 cubic feet per second, is reached. After this point, the inflow is less than the maximum infiltration rate and the runoff stored gradually infiltrates into the ground.

Figure 9

The area between the inflow and outflow hydrographs represents the volume that must be contained within the infiltration facility. Using the HYD program, it was determined that 1,240 cubic feet of storage would be needed. Using a trench section eight-feet wide by six-feet deep (Figure 8) and a void ratio of 0.30 for the infiltration fill, 86 feet of infiltration trench would be required. As an additional measure of safety, the design length was 100 feet, resulting in an actual storage capacity of 1,440 cubic feet. An eight-foot wide by 13-foot long rip-rap channel was designed at the outfall for energy dissipation, and the remainder of the infiltration trench is covered by a swale with plantings.