A Watershed-Scale Planning Tool for Optimal Selection and Placement of Green Infrastructure in Urban Watersheds
Addressing stormwater runoff and pollution challenges associated with urbanization is complex and relies on costly engineering, especially in highly-developed urban environments. Increasingly, distributed management of stormwater runoff using Green infrastructure (GI) is emerging as a multi-benefit solution that can address both stormwater quality and quantity concerns. Consistent with this trend and under the NPDES Stormwater Municipal Regional Permit (MRP), many local governments are required to develop and implement watershed-scale green infrastructure plans to cost effectively achieve quantitative water quality improvements and provide reasonable assurance that GI will achieve the desired load reductions. Despite the recognized effectiveness of GI and the MRP requirement, until such plans are in place, GI implementation will likely continue slowly and opportunistically with suboptimal flow and load reduction effectiveness.
GreenPlan-IT is a planning level tool that is designed to support the cost-effective selection and placement of GI in urban watersheds through GIS analysis, watershed modeling and optimization techniques. The GreenPlan-IT is comprised of three tools: (a) a GIS-based Site Locator Tool to identify potential GI sites; (b) a Modeling Tool that quantifies anticipated watershed-scale runoff and pollutant load reduction from GI sites; and (c) an Optimization Tool that uses a cost-benefit analysis to identify the best combinations of GI types and number of sites within a watershed for achieving flow and/or load reduction goals. Tool outputs can be used to develop quantitatively-derived watershed master plans to guide future GI implementation for addressing water quality and quantity targets.
The GreenPlan-IT Modeling Tool is built on a spatially distributed hydrologic and water quality model, EPA Storm Water Management Model (SWMM) version 5.0. Within the GreenPlan-IT, the Modeling Tool was used to establish baseline conditions; identify high-yield runoff and pollution areas; and quantify any reduction made from GI implementation across a watershed.
The GreenPlan-IT Optimization Tool, based on an evolutionary optimization technique (Non-dominated Sorting Genetic Algorithm II), was developed to determine GI scenarios (locations, types, and design configurations) that minimize the total implementation cost while satisfying water quality and quantity objectives and constraints. The Optimization Tool utilizes the site information generated from the GIS Site Locator tool to form its search space, and repeatedly calls up and runs the Modeling Tool as a subroutine during the search process in an iterative and evolutionary fashion to arrive at the optimized GI scenarios.
The GreenPlan-IT toolkit was applied in the City of San Jose to support a cost-benefit evaluation of stormwater runoff control. The focus area was a 4300 acre proposed development area within the lower part of the Guadalupe River Watershed. The results of the application included the cost/benefit associated with a range of flow reduction targets, ranking of sites for specific optimal solutions, and maps showing the distribution of GI within the study area under the chosen optimal solution. These results will be used to identify specific green infrastructure projects; support the City’s current and future planning efforts, such as the development of the San Jose Storm Drain Master Plan; and help comply with future Stormwater Permit requirements.
The GreenPlan-IT can be used to comply with NPDES stormwater permit requirements including the development of an alternative compliance program, as well as addressing loads reduction needs identified in TMDLs. The tool is planned to be utilized by the cities of Sunnyvale, Oakland, Richmond as well as Contra Costa County. Although designed for Bay Area stormwater agencies, the tool has broad applicability and could be used by stormwater agencies across the State.