Unincorporated Los Angeles County Green Streets Master Plan: Evaluating Green Street Alternatives and Lessons Learned From Preliminary Design
The Los Angeles County (County) Public Works is currently developing a Green Streets Master Plan (GSMP) for the unincorporated County areas. The objective of the GSMP is to identify 10 half-mile roadway segments (sites) that are conducive to green infrastructure retrofit in each of the 11 watersheds where the County is implementing Municipal Separate Storm Sewer System (MS4) permit requirements, for a total of 110 sites. Because there are 5,336 miles of roadway within the unincorporated area that could be considered as potential sites, an extensive screening process was performed to identify those sites with the greatest potential through geographic information system (GIS), and also to quantify water quality benefits through hydrologic and water quality modeling to rank sites against one another. Based on the results of the screening analysis performed for the 110 sites, 5 locations were identified as “signature projects” to serve as examples for GSMP implementation in each of the 5 supervisor districts.
Green streets are an important strategy to achieve water quality compliance in dense urbanized settings, 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) MS4 Permit to the Los Angeles Regional Water Quality Control Board (LARWQCB). Based on the E/WMPs, a 495 acre-feet (AF) storage capacity is required within the unincorporated County Green Streets areas out of the total 1,760 AF (28% of the total) being targeted.
To begin the preliminary design, an alternatives analysis was performed for each of the 5 signature projects that identified which Best Management Practices (BMPs) would be most feasible and effective for maximizing water quality and water supply benefits. The types of BMPs evaluated during the alternatives analysis process included small dispersed surface features such as bioretention, biofiltration, and pervious pavement as well as larger underground facilities such as deep recharge drywells, infiltration vaults, treat and release systems, and capture and reuse systems. A variety of BMP types was desired across the 5 sites to illustrate how green streets can be implemented in areas with differing geological conditions and site constraints.
The results of the alternatives analysis provided the basis for completing additional preliminary design tasks, including basemap development and geotechnical investigations to determine soil types, groundwater depths, and permeability rates. The results of the geotechnical investigations were used to confirm feasibility of BMPs identified during the alternatives analysis and to calculate the required size and number of BMPs required to achieve water quality targets as either the 85th percentile water quality design storm event or the 80% primary pollutant load reduction. The preliminary design was then progressed to the 30% level and Project Concept Reports (PCRs) were prepared to assist with pursuit of funding, including grants and Safe Clean Water Program Regional Program funds.
By the end of the presentation, the audience will gain an understanding of green street preliminary design approaches and will be able to apply lessons learned when designing similar projects in different locations and jurisdictions. The presentation will cover an introduction with results from the screening process, the alternatives analysis approach and results, geotechnical investigation processes and results, and development of 30% design drawings.
The content will address the conference theme by describing how the County’s green streets program applies sustainability and advanced technology solutions to solve complex water quality challenges. This program will help ensure that current and future generations can enjoy clean surface waters and increased resiliency of water resources.
Associate Civil Engineer, Los Angeles County Public Works