Herbert, Rowland & Grubic, Inc. (HRG) is responsible for a full range of professional engineering services for the commercial development of an approximate 66-acre site located within Silver Spring Township, Cumberland County, PA. The development proposes a shopping center with approximately 480,000 square feet of retail space and related site improvements.
Silver Spring Square II presented a variety of challenges that had to be overcome. In working with the township, HRG was able to submit a revised final plan and an updated Conditional Use for a plan preliminarily approved with a different layout. Another challenge was the location of the site along Trindle Spring Run; a high quality water shed stream. Working with the local municipality, DEP, Trout Unlimited and the adjacent quarry, we were able to devise an approach that satisfied the required controls for runoff quantity and quality. HRG also designed a “constructed wetland” to treat site runoff before entering Trindle Spring Run. With this design, a large part of the previously untreated drainage area tributary to the Conodoguinet Creek and Trindle Spring Run was now directed through BMP devices.
As part of this complex commercial development project, HRG provided services related to a dam removal. Along the northwest corner of the site and just upstream of the confluence of Trindle Spring Run and the Conodoguinet Creek was the Silver Spring Dam, a 9-foot high dam constructed out of hand-cut stone. The Silver Spring Dam was over 150 years old and was originally built to power a grist mill. However, the dam had not been used for its original purpose in many years and had since accumulated a great amount of sediment, thereby raising the stream bottom. The accumulated sediment was also partially contaminated with polychlorinated biphenyls (PCBs). In conjunction with an adjoining project, the Silver Spring Dam was demolished, much of the contaminated sediment removed, and the stream bottom was re-graded to natural conditions. After removal of the dam, an access drive for the site was built and a 142-foot span bridge was constructed over Trindle Spring Run at the same approximate location of the dam. The final hydraulic calculations showed a decrease of approximately 6 feet in the 100-year water surface elevation in Trindle Spring Run.
