The SR 0322 Potters Mills Gap (PMG) Project was initiated to improve safety and reduce congestion by advancing the limited access portion of SR 0322 from the Centre/Mifflin County line towards State College and by extending the four-lane highway from its current terminus at Decker Valley Road through the Potters Mills Gap. The project is comprised of three design and construction sections.
- Section B04 – new bridge over SR 0322 near Sand Mountain Road
- Section B05 – new full access Sand Mountain Road interchange at Seven Mountains
- Section B06 – four-lane extension and interchange in the valley to the west of Potters Mills Gap
NTM was responsible for the Erosion and Sediment Pollution Control (E&SPC) Plan, Post Construction Stormwater Management (PCSM) Plan, and the NPDES Permit for all three sections. NTM also completed a hydrologic and hydraulic (H&H) analysis of the Potter Run crossings that were part of Section B06.
Site constraints throughout the project corridor were a known challenge to implementing a PCSM design that would satisfy regulatory requirements. Constraints included the steep mountain terrain through Potters Mills Gap; the special protection water quality designation of Potter Run and Laurel Creek; the Penns/Brush Valley Rural Historic District; the PA DCNR Penn Nursery; and commercial and residential properties along existing SR 0322.
The Section B04 and B05 projects were coupled and permitted separately from the Section B06 project. As part of the initial Study Phase of the project, NTM prepared an Individual NPDES Permit for Section B04/B05, which included final ESPC and PCSM plans for Section B04. NTM designed an infiltration basin in the existing SR 0322 median to mitigate stormwater runoff associated with the Section B04 improvements. The NPDES Permit (for Section B04/B05) was approved by PA DEP two months after it was submitted with no formal agency review comments.
Steep terrain, residential properties, and a campground in Section B05 necessitated locating stormwater control measures (SCM) immediately adjacent to the highway. Realizing both safety and PCSM benefits, 3000 feet of rock median was removed and replaced with a vegetated median, which eliminated the need for two stormwater basins. Other SCMs designed for Section B05 included an infiltration basin, biofiltration basin, vegetated filter strip, and numerous vegetated swales. NTM prepared a major modification to the Section B04/B05 Individual NPDES Permit, which was approved by PA DEP in four months.
The submission resulted in only two technical review comments (on the ESPC Plan).
The Section B06 project included many ESPC and PCSM design challenges. The eastern portion of the project is flanked by a two large mountain cuts, the PA DCNR Penn Nursery, state forest lands, and a creek (Potter Run). In this area, stormwater runoff was collected and conveyed to an area that was occupied by a commercial business, where a dual-purpose biofiltration and infiltration basin was designed to reduce runoff volume, attenuate peak flows, and improve water quality.
The center portion of Section B06, a four-lane alignment of SR 0322 through Potters Mills Gap, presented virtually no opportunities to implement SCMs along a one-mile stretch of steep mountain slopes and an adjacent creek. Instead, runoff from the new four-lane highway is collected and conveyed to the valley in the area of the western interchange.
The western portion of Section B06 is located in the valley, which is dominated by karst geology and seasonal high-water tables, and the Penns/Brush Valley Rural Historic District. These conditions required unique designs to ensure stormwater was adequately controlled. Through design refinements and agency coordination, the total number of stormwater basins was reduced from 17 during the study phase of the project to 12 in Final Design. NTM was responsible for preparing the ESPC and PCSM Plans and submitting the NPDES Permit for the Section B06 project. The Section B06 Individual NPDES Permit was approved 8 months after it was submitted. Despite the size of the project and design complexities, the permit submission generated only seven technical review comments (all related to the ESPC Plan); no formal technical deficiencies were received for the PCSM Plan.
NTM also completed the H&H analysis for the proposed Potter Run crossing, which involved the elimination of an existing arch culvert that will be replaced with a two-span overhead mainline structure and a single-span structure for SR 2015. East of the proposed structures, 1,700 feet of Potter Run was analyzed for floodplain impacts due to fill placed to accommodate a shift to the alignment of relocated Crowfield Road and guiderail placement.
