Reimagining Green Infrastructure in Urban Redevelopment: Engineering Out Catch Basins

Many jurisdictions across the US incentivize or require private developments to implement green infrastructure (GI) as part of their stormwater management. More recently, municipalities themselves are beginning to explore how GI can be integrated into public stormwater systems, and are seeking to understand the implications and benefits of GI in the context of their distributed, predominantly 'gray' subsurface stormwater assets. In communities where the 'low-hanging fruit' opportunities (such as integrating GI into public parks and other discrete open spaces) have been largely seized, the value of GI integration into the broader public streetscape is being recognized as a potentially significant source of further benefits. Advantages of utilizing GI include enhanced pollutant removal, reduction of stormwater runoff, climate resiliency, recreational and health benefits, and the provision of ecosystem services. The implementation of GI to achieve these outcomes for discrete project sites, be they public or private, is commonplace in many urban centers. However, there exist challenges to the effective implementation of GI practices more broadly throughout the network of stormwater collection and conveyance infrastructure that cities rely on to keep their streets functioning during significant rainfall events. These include concerns about space constraints, provision of appropriate pre-treatment, maintenance requirements, jurisdictional issues between municipal departments, treatment capacity and performance during large storms. The objective of this presentation is to provide the audience with a real-world example of a practical approach to fully integrating GI components within an urban stormwater system that addresses many of the challenges noted above. The case study at the center of this presentation is part of a roadway utility and stormwater system design (see Figure 1) that WSP developed and is overseeing the construction of in Boston MA. Construction of the design is partially complete, with the urban redevelopment project currently securing final permit approvals for the remaining phases of construction. Like many urban redevelopment sites, the case study project involved overcoming some site-specific challenges, such as a relatively high groundwater table (which can make infiltration challenging), and the desire to minimize excavation given the need to treat all in-situ material as potentially contaminated. Along with these considerations, the development of the GI-integrated stormwater design to service the proposed roadways focused on three principles: 1.Treat inflows early and don't rely on the subsurface infiltration capacity of the underlying soils; 2.Provide high-flow treatment and bypass capacity in a small footprint; and 3.Make maintenance simple! A key feature of the proposed design is that catch basins are eliminated from the new streets in favor of high-flow biofiltration cells (BFCs) which are supported by pretreatment inlets that screen out trash, debris and other coarse sediments and floatables. The BFCs were intentionally utilized as inlets so as to remove upwards of 90% of the sediment from runoff before it reaches the proposed subsurface infiltration trenches, enhancing and prolonging their functionality, and thereby minimizing maintenance and the potential need for future renewal. The treatment train proposed for the roadways achieves 98% total suspended solids (TSS) removal and 95% total phosphorous (TP) removal for a 1.25-inch 'first flush' rainfall event. This is a significant runoff water quality improvement compared to the 25% TSS and 0% TP removal typically credited to standard deep sump catch basins which are commonly the only form of treatment present in municipal stormwater systems (see schematic comparison in Figure 2). Utilizing the project case study, this presentation will provide a GI implementation template and lessons learned for integrating GI components into typical urban streetscapes with approaches that maximize benefits and minimize long-term maintenance costs.