Alexander Drive Basin Retrofit
The Alexander Drive Basin was constructed in the early 1980s to provide stormwater detention for a 37.81-acre single-family residential area. Located at the southwest corner of Alexander Drive and Burnside Avenue, the detention basin currently provides peak rate stormwater runoff control for storm events. The detention basin provides for minimal treatment of stormwater runoff – providing only a 10% reduction of sediment for stormwater runoff passing through the basin.
Currently, Stony Creek is listed on the Integrated List of Waters as impaired for aquatic life due to siltation fromurban runoff/storm sewers. Stony Creek is classified as a trout stocked stream in the Pennsylvania Chapter 93 water quality standards (WQS). The TSF classification requires that permitted discharges in the basin meet water quality criteria designed to allow for the maintenance of stocked trout from February 15 through July 31, and the maintenance and propagation of warm water fishes and associated flora and fauna for the remainder of the year.
The Stony Creek flows through the northern portion of West Norriton Township towards the south-southeast, where it meets and feeds into the Schuylkill River in the Borough of Norristown. In the Stony Creek Watershed, the conversion of permeable land cover to less permeable surfaces have increased the volume and frequency of runoff and led to several problems. The major conflicts include increased incidence of flooding, impaired water quality, and ecological degradation.
|Preliminary & Final Design||Winter 2020|
|Permits and Approval||Spring 2020|
|Construction Start||Sumner 2020|
|Construction End||Fall 2020|
|Maintenance/Replanting||Fall 2020 - Fall 2021|
Alexander Drive basin is roughly 26,000 square feet with a bottom elevation of 212 feet and a top elevation of 219 feet. Soil infiltration testing shows poorly drained soils due a fragipan layer in the bottom of the basin. Basin cover is mowed lawn with landscaped mature trees around the perimeter. The basin bottom is extremely eroded with deep gullies leading from the two (2) inflows to the riser structure.
Most of the basin banks are bare soil, while the basin bottom is comprised of mowed grass.
There are two (2) sources of inflow to the basin: 1- 30” RCP endwall and 1- 24” RCP endwall. The riser structures are a series of two (2) vertical CMP pipes with a 24” RCP connector pipe. The outflow from the basin consists of a 36” CMP, which connects and discharges under Burnside Avenue into Stony Creek (stormwater outfall #62). This outfall has a collection and drainage area of 38.09 acres; so,nearly the entirety of runoff discharging at this outfall passes through the Alexander Drive Basin.
Environmental Results and Scope of Work
The Alexander Drive Basin will be retrofit as a water quality basin. The proposed project will offer significant water pollution reduction benefits to the Stony Creek watershed. The retrofit basin will improve runoff quality through settling, filtration, uptake, chemical and biological decomposition, volatilization, and adsorption and will be relatively effective at removing sediment pollution plaguing the Stony Creek. Based on modeling completed by the Township stormwater engineer, the proposed project will: reduce sediment load by 18,316.73 lbs/yr – a net increase of 15,263.94 lbs/yr more than the existing condition.
Also, although not typically considered a volume reducing BMP, the proposed new condition will achieve some reduction in the volume of runoff reaching the Stony through infiltration and evapotranspiration. It is estimated approximately 19% of the runoff entering the retrofit facility will be diverted from the municipal storm sewer system.
The proposed water-quality basin is part of the Township’s Pollutant Reduction Plan strategy for reducing sediment to the Stony Creek. The new basin is expected to “green”, i.e. treat, approximately 37.81 acres of tributary area, mostly constituted by residential and institutional uses. The basin will receive overland flow from the immediate surrounding area. In addition to providing water quality benefits to treated runoff, the basins will also provide stormwater volume and peak flow rate control.