STORM WATER MANAGEMENT EVALUATION & REPORT SNAPSHOT Storm Water Management Overview Bowman Consulting Group (“BCG”) was retained by Vision McMillan Partners (“VMP”) to evaluate and analyze the Storm Water Management (“SWM”) aspects and impacts of the proposed redevelopment of the historic McMillan Reservoir. In preparation of this report, BCG met with key agencies that regulate SWM and Water Quality Control (“BMP”) in the District (DC Water & DDOE). Information such as: Discharge rates (either calculated or measured in the field), drainage areas, WSEL’s, etc., aided in the development of Hydrologic and Hydraulic Models that are consistent with and build upon the information already developed by those agencies over the past decade. In developing a solution for SWM and BMP on the site, the surrounding receiving (downstream) infrastructure evaluated for its adequacy.
Summary of Water Quantity (SWM) Analysis DDOE regulations for SWM require the 15year frequency post-development storm discharge rate to be reduced a level equal to the rate under pre-meadow conditions with a 2-year frequency storm. This creates the need for a significant water detention system to be designed in to the planned redevelopment. Typically, since the system discharges into a “separated” system in 1 Street NW, detention of this magnitude would not be required. However, since the system converts into a combined system approximately 1,500 st
linear feet downstream, these more stringent requirements will be enforced. The SWM/BMP concept plan included on the reverse page depicts the size of the proposed facility, and its relationship to existing and proposed features.
Water Quality (BMP) Analysis BMP will be provided in McMillan through a variety of accepted Low Impact Development (“LID”) practices. These practices are state of the art and in keeping with the Districts requirements for sustainable design. It is envisioned that certain components from the menu of various techniques included in the report will be selected by the development team depending on the type of facilities being designed and how these practices will relate to the buildings from an aesthetic and functional standpoint.
Overall Analysis Conclusion BCG’s analysis indicates that the proposed runoff generated by the redevelopment of McMillan will NOT impact the downstream drainage system. The redevelopment will “over-detain” the additional water discharge created by the new uses, thanks to DDOE’s restriction of maintaining 2-year pre-meadow levels of water discharge. Due to these requirements and the SWM/BMP practices that will be implemented by the redevelopment project in order to meet them, the amount of runoff leaving the site post-development will be at levels LOWER than the current existing conditions. The proposed SWM system will reduce the discharges into the existing storm system and thereby reduce the Hydraulic Grade Line within DC Water’s system.
STORM WATER MANAGEMENT EVALUATION & REPORT SNAPSHOT An exhibit depicting the proposed SWM and BMP solutions is included on the reverse page.
STORM WATER MANAGEMENT EVALUATION & REPORT SNAPSHOT