Deep infiltration through the implementation of drywells provides an opportunity to capture stormwater for the purpose of groundwater recharge and pollutant reduction in downstream receiving waterbodies.
The small footprint of drywell systems provide flexibility for projects in the public right-of-way, where construction in the built environment presents challenges of constructing around existing infrastructure. Due to vertical installation and small footprint of drywells, spatial constraints are more manageable and can be installed in the roadway, sidewalks, or parkways. They can also typically be installed within a few days, and maintenance can be done using a vacuum truck without the need to enter the chamber. All of this makes drywells an enticing option for green streets stormwater infrastructure projects.
Los Angeles Bureau of Sanitation (LASAN) partnered with Los Angeles Department of Water and Power (LADWP) and the State Coastal Conservancy (SCC) to increase water supply through groundwater replenishment, improve receiving water quality of the Los Angeles River, reduce localized flooding, and aid overwhelmed existing stormwater infrastructure with the San Fernando Valley Green Streets Projects. This project consisted of seven locations in the San Fernando Valley and Pacoima and was designed to capture and infiltrate an average of over 473 acre-feet per year of stormwater runoff from 594 acres of tributary drainage areas consisting of mostly residential and commercial land use through implementation of drywell systems, storm drain diversions, rain gardens, and catch basins. The project utilized a comprehensive alternative project delivery (design-build), allowing the prime contractor to design, construct, monitor, and maintain the proposed green street infrastructure over the course of the project.
A risk register was also developed for the project to identify and mitigate various risks from design to construction associated with the project, including potential impacts to groundwater quality. Stormwater is conveyed through drop inlet catch basins with screens to prevent large debris from entering the drywell system. All drywells systems also included a screening and pretreatment settling chamber to screen out debris and further reduce pollutant load before stormwater enters the groundwater basin. Preliminary design activities included a drainage analysis to delineate drainage areas and determine total flows, a geotechnical investigation to estimate the infiltration capacity of the drywell systems, and a utility investigation to identify potential for utility conflicts and identify finalized locations.
This presentation will describe the benefits of incorporating drywells into green streets infrastructure projects and provide the audience insight on the process to do so. It will touch on aspects from all phases of the project, including design, permitting, construction, and monitoring. Discussion regarding the lessons learned, including construction during the COVID-19 pandemic, will also serve to streamline future projects and further reduce risks.
This project completed design in August 2020 and completed construction installing over 63 drywell systems with pretreatment chambers and catch basins, 6 storm drain diversions, 18 rain gardens, and 3 landscaped stormwater medians in April 2021 with a total cost of $16.8 million with zero change orders.
- Geosyntec Authors: Jose Avina
- All Authors: Esther Woo, Jose Avina
- Title: San Fernando Valley Green Streets Project: The Potential for Drywell System Implementation for Groundwater Recharge
- Event or Publication: CASQA 2021
- Practice Areas: Water and Natural Resources
- Citation: Geosyntec practitioners will present at California Stormwater Quality Association Virtual Annual Conference on October 26-27, 2021.
- Date: Wednesday, Oct. 27 at 8:30-8:55 a.m. PST
- Location: California
- Publication Type: Platform Presentation