Unincorporated Los Angeles County Green Streets Master Plan: Screening Process Development, Considerations, and Results
The County of Los Angeles, Department of Public Works (County) is in the process of developing a Green Streets Master Plan for the unincorporated County areas. The objective of the project is to identify 10 half-mile roadway segments (sites) that are conducive to green infrastructure retrofit in each of the 11 watersheds that the County is a permittee of, for a total of 110 sites. From these 110 sites, 5 locations will be developed as “signature projects” as a example project in each of the 5 supervisor districts. Because there are 5,336 miles of roadway within the unincorporated area that could be considered as potential sites, a screening process was developed to identify those sites with the greatest potential through geographic information system (GIS), and also to quantify water quality benefits through hydrologic and hydraulic modeling to rank sites against one another.
Green streets are an important strategy to meet water quality compliance in areas that are fully built out, such as Los Angeles County. This reliance is clearly depicted in the enhanced watershed management plans and watershed management plans (E/WMP) presented by the permittees of the Los Angeles national pollution discharge elimination system (NPDES) municipal separate storm sewer systems (MS4) permit to the Los Angeles Regional Water Quality Control Board (LARWQCB). Based on the E/WMPs, the storage capacity within the unincorporated County areas alone that would require green streets would need to provide 495 acre-feet (AF) out of the total 1,760 AF (28% of the total). Low impact development (338 AF) and regional facilities (927 AF) would comprise the rest.
Another primary reason for a screening process, is that costs for retrofitting green infrastructure into existing streets is expensive and many municipalities are struggling to develop funding mechanisms. Grant funding or capital improvement funds will not cover all the retrofit projects required to meet water quality objectives as presented in the E/WMPS. Other funding sources will be necessary and may include additional property taxes specific to stormwater water quality improvements. This makes it imperative that the sites selected have the greatest potential for benefits with the strongest return on investment. As an indication, the unincorporated County green streets 495 AF would cost around one-billion dollars based on E/WMP capital costs assumptions.
The screening process used for the development of the Green Streets Master Plan primarily was completed within GIS by creating screening coverages and intersecting those coverages with the roads coverage. The roads that intersected with the screening coverages were excluded from further consideration. Modeling and ranking of the remaining roads was then completed and a final visual check of the highest ranked sites for placement areas such as wide sidewalks and medians followed. The process included:
Phase 1: Desktop GIS screening out of roads with steep slopes and roads in areas of known soil and/or groundwater contamination.
Phase 2: Desktop GIS screening out of roads in areas of poor infiltration, high liquefaction potential, and high groundwater table.
Phase 3: Hydrologic and hydraulic modeling to quantify and rank the remaining road segments.
Phase 4: Desktop screening of the highest ranked road segments for feasibility from aerial photography.
The presentation will be delivered in the form of an interactive back and forth with questions such as, What screening information is readily available and the most important? The audience will also be encouraged to ask questions and provide comments throughout the presentation.
The content will address the conference theme by providing an example for other municipalities to follow when developing their green streets program with the added insight into what the key elements and considerations were for the County program.