Each campus has a different stormwater management regulatory hurdle to meet. According to the Wissahickon Watershed Ordinance, impervious surface on Sugarloaf cannot exceed 20% of the entire parcel. There is no such restriction on impervious surface on Main Campus. However, the presence of both the 100-year flood plain and the floodway confine development.
Stormwater management is regulated primarily by the Philadelphia Water Department (PWD) and the Pennsylvania Department of Environmental Protection (DEP). The basic intent of PWD’s stormwater management regulations is to negate the impact of development on local streams and hydrology. This is accomplished by encouraging new development or re-development to capture and infiltrate a fairly common volume (2.64”) of water for a given rain storm, also known as the “one-year, twenty-four hour storm.” For larger storms, a site should contribute the same rate and volume of water as the predevelopment condition. Typically, the largest storm that an undeveloped or “natural” site can capture and infiltrate is between the one and ten year storm event. This depends on the storage volume of soils, depth of soils, vegetation types, and severity of slopes. For master planning purposes, we have used the five year storm event volume of 5.28” as our target storage volume for capture and infiltration.
Each campus has a similar conceptual stormwater design: capture as much stormwater from rooftops as possible (the cleanest means of collecting rainwater), and direct it to a centralized cistern located in the foundation of the new development. The cistern is intended to infiltrate captured water back into the ground. Recent geotechnical investigations show ample soil horizons above the water table to accommodate infiltration on both campuses. This technique of storage and slow release reduces or eliminates the large pulse of water (usually somewhat dirty with salts and oils and too warm to carry much oxygen) from new development to the Wissahickon Creek. Another means of disseminating captured stormwater in a way that does not negatively impact local hydrology is irrigation of lawns, green roofs, and other formal landscapes; but due to its seasonal nature irrigation is not as effective a stormwater management tool as direct infiltration. Nonetheless, irrigation using stormwater offsets the ever increasing expense of using potable water. In a large storm event, the cisterns will fill and stormwater will make its way overland to the Wissahickon Creek via vegetated swales known as bioswales.
Not all stormwater will pass through a cistern. Paved surfaces may contribute their runoff to bioswales where water can infiltrate through the soil and be transpired by plants, thus allowing any suspended solids and nutrients to be filtered through the soil before reaching the Wissahickon. Bioswales parallel most roadways on both campuses. For this reason, vegetation within bioswales must be able to withstand periodic inundation, drought, and salt contamination. The swale systems on Sugarloaf make their way to settling ponds, and finally into underground detention facilities that slowly meter out their release as groundwater.