Innovative Methods to Select and Implement Green Infrastructure Improvements within large watershed areas – A case study of GI planning and design within four sub-watersheds in DC’s Anacostia Watershed.

In accordance with the Municipal Separated Storm Sewer System (MS4) Permit issued to the District of Columbia by the United States Environmental Protection Agency (USEPA), the District Department of Transportation (DDOT) is required to retrofit areas in the city within the transportation public right of way (ROW). In this effort, the District Department of Energy and Environment (DOEE) and DDOT Infrastructure Project Management Division (IPMD) have identified four sub-watersheds located within the Anacostia watershed to be retrofitted with green infrastructure (GI) as part of the long-term stormwater management solution to improve the water quality and overall conditions of the affected tributaries. This presentation will summarize various aspects of one planning and design project implementing over 40 individual GI practices within these sub-watersheds. From planning and site selection through the detailed design process, focusing on innovative tools for site prioritization, neighborhood scale challenges surrounding community impact, and design lessons learned. Notable elements of the project are described below: Planning and Site Selection Process. An innovative GIS-based screening tool was developed using open source and utility GIS data together with system prioritization criteria to identify the optimal locations for GI systems across over 1000-acres in four sub-watersheds. The large analysis area necessitated the use of this type of tool to help rank and prioritize potential locations for GI implementation without having to manually analyze each location. Selection criteria included numerical rankings for street slopes, utility conflicts, parking impacts, street classifications, drainage areas, and tree coverage factors. This presentation will demonstrate how this challenge was overcome by using a GIS-based screening tool to screen, score and rank sites based on design and regulatory contributing factors. Figure 1 shows the scoring/ranking for GI in the subwatershed area. Outreach and Community Engagement. Public outreach efforts were initiated early in the project to share information related to the benefits of GI, potential project impacts to the community during construction and long-term, and to solicit their feedback to incorporate suggestions into the design process. Several innovative approaches were used to share this information and reach all the impacted residents during a time where in-person public meetings were not possible. Enhanced GI Design. GI designs included bioretention, permeable pavement parking lanes, permeable pavement green alleys, expanded green spaces, and other impervious area removal practices. Project implementation focused on a treatment train approach, combining multiple GI practices in series together with impervious area reductions to optimize overall benefit and performance. Several innovate approaches were implemented including incorporating multiple small-scale tree bump-outs in series along several streets with limited existing tree coverage. And providing expanded tree soil space to enhance performance and promote long-term tree health utilizing adjacent spaces under the sidewalks and adjacent areas. The use of project innovations such as a GIS-based screening tool to optimally site GI and a unique process that incorporates the use of soil volume under pavement areas for tree pit/bump-out installations provides an effective and cost-efficient way for communities to retrofit areas and maximize stormwater management options.