sTORMWATER MANAGEMENT DESIGN STANDARDS
Stormwater management design is one of the most critical factors that drives site design and layout for subdivision and land development plans. Location of stormwater management facilities, such as infiltration basins, rain gardens, and others, can significantly alter the layout of proposed lots, buildings, and associated site features depending on how much volume is needed. To accurately size stormwater facilities, the infiltration characteristics of the soils below stormwater management facilities must meet a certain threshold for infiltration to be viable. Subsurface investigation and geotechnical evaluation must be performed to determine the soils’ ability to infiltrate runoff.
Since its release in 2006, the PA DEP Stormwater Management Best Management Practices (BMP) Manual has outlined standards for infiltration testing, which includes types of testing, the number of tests required, and associated guidance for designers. The standards outlined in the original BMP manual have, and still do, govern current infiltration testing practices. That is all about to change, however, as PA DEP has been working on an updated BMP (or SCM – Stormwater Control Measures as they now refer to them) over the last several years.
Beginning in 2028, infiltration testing procedures will begin to follow ASTM standards and the minimum tests per SCM will increase. The table below outlines the updated number of tests required, which is based upon SCM footprint and type of test. Ultimately, this will likely result in substantial increases in geotechnical evaluations for stormwater management and developers and consultants will need to adjust proposals accordingly.
SCM INFILTRATION AREA
≤ 2,500 SF
> 2,500 to 5,000 SF
> 5,000 SF to 10,000 SF
> 10,000 SF
Max Test Per Pit
DOUBLE RING
3
4
5
1 per 2,000 SF (max 15)
2
SINGLE RING/CASED BOREHOLE
5
6
8
1 per 1,250 SF (max 20)
3 single ring: 2 cased borehole
BASIN FLOOD TEST
1
1
2
1 per 5,000 SF (max 3)
1
The changes proposed by PA DEP for infiltration testing practices aligns with their goal to spread out stormwater management facilities on project sites instead of concentrating all runoff at singular locations and combining runoff rate, volume and water quality management into a single, larger SCM. To accommodate this, consultants and developers must adapt to the updated standards by further analyzing project sites during due diligence stages and manage expectations of how much additional area may be necessary for stormwater management.