Appeal 14164

OPSC 1101.5.3.2 Location (of Dry Wells)

No dry well shall be located closer than 5 feet of a property line nor closer than 10 feet to a building unless approved by the building official.

The project is requesting the installation of up to (2) 20-foot deep stormwater drywells within the drive aisle of the covered parking garage to dispose onsite surface stormwater (see attached exhibit). Stormwater treatment, beyond a sediment trap, will not be provided prior to the drywell since only rooftop runoff will be collected by the storm drain system and routed to the drywell.

Based on the BES Early Assistance Appointment Comments (EA 16-115812) from Emma Kohlsmith dated March 24, 2016, there is no storm only sewer available and therefore BES will only allow sanitary connections. Stormwater discharges will be restricted according to the SWMM Hierarchy.

A portion of the roof area will be installed with green roofs or retained planters. The majority will be exposed roof with various HVAC units.

As a result, the project will include a drywell located within the parking garage drive aisle for ease of access and to keep it farthest from the building’s footings. Additionally, there is not room outside the building to meet setback requirements.

The geotechnical report by GeoDesign Inc., dated June 24, 2016, describes infiltration test results of 5.4 inches per hour that support the existing in situ soil’s ability to infiltrate the project’s onsite stormwater. The ability of the soil to infiltrate at depth is unchanged by the proposed structure. The dry well was sized utilizing the SBUH method with a total rainfall of 4.4" in a 24 hour period with a type 1A storm event. The dry well(s) were modeled based on the field measured infiltration rate of 5.4 in/hr times the wetted area of the dry well structure. Any increases in infiltration rate due to the increase in the head pressure as the water level in the dry well built up was not accounted for to ensure our sizing was conservative. A detailed analysis of the modeling is located in the Preliminary Stormwater Drainage Report. Given the approximately 10,700 square feet of roof area, the dry well system was designed as two 48-inch diameter manholes with a depth of 20-feet. The tributary area of the drywell is rooftop and therefore pollution reduction prior to infiltration is not required as described in Chapter 1 of the SWMM.

Stormwater conveyance pipes will be sized to meet the 10-year storm event, using the Rational Method and Manning’s Equation.