Bringing a Stormwater Master Plan to Life – Challenges Faced During Design of Level 2 Wet Pond at USCG Base Portsmouth

Stormwater Master Planning helps to identify where future investments are needed by deficiencies of the existing stormwater system. At the planning stage, many assumptions are made, which are based on the best available information. But what happens when elements of that Master Plan move into design and field discoveries and phasing plans challenge these assumptions?

A Master Plan previously developed for USCG Base Portsmouth identified improvements to reduce recurrent flooding at the Base and provide nutrient and sediment credits to comply with Chesapeake Bay TMDL requirements in the Base’s MS4 Permit. Improvements included installation of a Level 2 Wet Pond, modifications to redirect runoff to the new pond and increasing capacity of an 18-in trunk main.

Our project included investigation, design and construction coordination of the pond and stormwater modifications, with the goal of meeting the intent of the Master Plan. This is where the real fun began, as field investigations, and additional information challenged the original assumptions. In addition, the trunk main capacity improvements were being postponed until a later date. Our design now had to factor in extensive utilities and concrete duct banks that prevented a new pond outlet pipe and an undersized 18-in trunk main to convey pond discharge flow.

We investigated bypassing the undersized trunk main by installing a new pond discharge pipe to daylight into an existing ditch, but utility lines and a high ditch elevation prevented this option from moving forward. Due to these constraints, we developed an alternative approach which included a new box structure in the location of an existing catch basin, with three components: (1) pond discharge to the existing storm sewer 18-in trunk main; (2) overflow pipe discharging into the existing ditch and (3) option to add a future pipe to increase capacity. This alternative provides the USCG an option to upsize the larger stormwater trunk main in the future while using the existing trunk main for smaller flows and it helps lower the hydraulic grade line in the system, which impacted the proposed pond discharge rate, reduced the size of the pond and alleviated the upstream recurrent flooding in the existing system.

Stormwater Master Plans are great tools for budget planning and can provide a preliminary insight to a future design. Bringing these Master Plans to life, however, can take some creating thinking and consideration of future phasing