Draft Post-Construction Stormwater Manual
Posted: September 4th, 2024
Authors: Benjamin H.
On January 28, 2023, the Pennsylvania Department of Environmental Protection (PADEP) released the draft Pennsylvania Post-Construction Stormwater Management (PCSM) Manual (Draft). The Draft is an update of the December 2006 Pennsylvania (PA) stormwater BMP Manual released in. This update provides new regulatory language and engineering updates to reflect the current understanding of stormwater management and physics, while still emphasizing the natural landscapes. The focus of the new PCSM Manual is the same as the 2006 Stormwater BMP Manual, which is, according to the Draft, “to protect, maintain, reclaim and restore water quality and the existing and designated uses of waters of the Commonwealth.”
What are the Changes?
The Draft includes changing the widely used term best management practice (BMP) with the new term stormwater control measure (SCM). The term BMP was found to be vague and poorly defined throughout the nation. The new term, SCM, provides new clarity on the subject matter of what the manual tries to target. SCM is defined, as “Physical, structural, and/or managerial measures that, when used singly or in combination, reduce the downstream quality and quantity impacts of stormwater.”
More significant revisions include new criteria for wetland protection, new SCMs, new methods of infiltration tests and other pre-development activities, and new guidance on structural designs regarding water flow. Some new SCMs include new managed release concept (MRC) SCMs which involve the slow release of stormwater to imitate shallow subsurface flow. This involves the MRCs ability to detain a 1.2 inch/2-hour event using vegetation, which is then filtered through a soil media or any other approved media, where the remaining runoff is slowly released. The Draft also provides new design capacity values and the new idea of including dual-path designs where surface flows are routed both overland (using swales, street sags, etc.) and underground (using sewer infrastructure).
The Draft recommends a new method of utilizing hydrological data. According to “PA DEP Stormwater Manual Updates,” climate projections for PA shows that the frequency of storm events with precipitation greater than three inches will increase 52 percent by 2050, and 93 percent by the year 2100. With models predicting continual increase of temperature, the intensity of rainfall events will also be expected to increase. Therefore, the Draft recommends that the 90th percentile of rainfall intensity to be used with data from NOAA-14 or older intensity duration frequency curve sources that do not already consider climate change effects.
What effect does the update have on the industry?
Like the PA Stormwater BMP Manual, the PCSM Manual is non-regulatory in nature, and therefore has no direct effect on Facilities. However, the PCSM Manual provides guidance on staying in compliance with erosion and sediment control requirements found in 25 Pennsylvania Code (Pa Code) §102, including the PCSM requirements of §102.8. The PCSM Manual also provides additional SCMs as alternatives to the PA Stormwater BMP Manual, giving facilities greater flexibility in ways to comply with state regulations.
Despite greater flexibility, facilities may need to accommodate the PCSM Manual’s recommendation of using the 90th percentile rainfall values. A 10-year, 24-hour storm’s rainfall using the 90th percentile will increase the rainfall value from an average of 3.90 inches to the upper 90th percentile value of 4.21 inches. Thus, the storage capacity required for an SCM may increase by approximately 7.36 percent.
It is important to note that the PCSM Manual will be constantly changing as newly available research and data provide more insight into how to manage stormwater. Regulatory language, although less frequent, may change depending on the policies set forth in the nation. Technical values such as land use factors or land cover-based stormwater pollutant concentration, may change with newer published data.
What should you do next?
Facilities that are affected by erosion and sediment control requirements promulgated in the PA Code should review the draft PCSM Manual. The manual may provide your facility with further flexibility in complying with state regulations.
ALL4 is here to assist you in evaluating how the changes to PCSM Manual will affect your operations. We can facilitate meeting environmental compliance with those changes. If there are any questions about the PCSM Manual and how your Facility may be affected, please reach out to me at bhsieh@all4inc.com or your ALL4 project manager for more information.
