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

The Performance Approach is available for projects with unique circumstances that require analysis that go beyond the capabilities of the Simplified and Presumptive Approaches. It may be used to address specific site conditions or project needs, or to apply a new or emerging design technology. Detailed engineering calculations must be provided as evidence of the proposed design’s performance with respect to achievement of the SWMM’s stormwater requirements.

Infiltration facility design

After reviewing the three methodologies, it appeared that a vegetated swale constructed over an infiltration trench, using the Presumptive Approach, would be the most appropriate for use on this project. Field infiltration tests conducted prior to design indicated that the soils near the 158th Avenue entrance are conducive to infiltration (Figure 6). A falling head infiltration test was conducted, and the documented infiltration rate was 99 feet per hour in a layer of well-graded gravelly sand at a depth of 9 to 10-feet below the existing ground surface at the location of the proposed infiltration facility. The Presumptive Approach Calculator (PAC) limits the infiltration rate to a maximum of 20 inches per hour. As such, this is the infiltration rate used in the design of the infiltration facility. Elsewhere, in the outfall and swale areas, a conservative infiltration rate of two inches per hour was used. This results in a composite maximum infiltration rate of 13.1 inches per hour.

Figure 6

The Manual includes the PAC, a spreadsheet that can be used to perform sizing calculations (based on impervious area) for vegetated surface facilities with impervious catchment areas up to 1 acre in size. If the impervious catchment area exceeds one acre, appropriate alternative software that follows the same principles as the PAC must be used.

The Santa Barbara Urban Hydrograph (SBUH) methodology is a common software used to estimate runoff generated from storm events. The computer program “HYD,” developed by King County, Washington Department of Public Works, was used for the SBUH calculations for this project.