Neighborhood-Scale Water Quality Improvements The Broadway Neighborhood Stormwater Greenway Project Presenter: Stacy Luell, Geosyntec Consultants Co-Authors: R. Batchelder, W. Tam, M. Hanna, M. Sadeghi March 30, 2015
Background 2012 Regional and Distributed Stormwater Capture Feasibility Study Funded by Watershed Replenishment District of Southern California (WRD) Completed by Santa Monica Bay Restoration Commission, Council for Watershed Health, and Geosyntec Identified catchments with greatest potential to provide groundwater recharge AND water quality benefit to Central and West Coast Basins
Background Result of the Feasibility Study Selected the Broadway Neighborhood Stormwater Greenway pilot project Developed concept design for pilot project Pilot project selected to: Increase infiltration Improve downstream water quality Reduce flooding Beautify the community
Implementation LA Bureau of Sanitation used Broadway concept design to secure $3 million from Proposition 84 Stormwater Grant Received additional $1 million from Prop O to expand the project and provide positive cash flow LA Bureau of Engineering is managing the project design and implementation
Location South of Downtown LA Tributary to Compton Creek Los Angeles River Economically disadvantaged community Mixed commercial and residential Highly impervious 30 acre tributary area, plus additional 170 acres from storm drain
Purpose Recharge the Central Basin aquifer Assist the City of Los Angeles to be in compliance with the LA River TMDLs (bacteria, nutrients, trash and metals) Reduce pollutant loads by infiltrating stormwater Provide beautification and revitalization to economically disadvantaged community
Design Concept 4 BMP Types: Residential Rain Gardens/Infiltration Trenches Residential Street End Infiltration - Drywells Commercial Green Streets Subregional Infiltration Gallery
Residential Property BMPs Intercept and store/infiltrate water from downspouts to rain gardens or infiltration trenches Sized to capture roof runoff generated from the 3/4-inch design storm event; bypass flows to residential streets Participation is voluntary. Goal was to retrofit 60 lots (43% of all lots), actual participation thus far is 10 (7%) Predicting participation will double within 2 years Proposed Existing Rain Condition Garden
Neighborhood Street Ends Intercept water from residential street and residential properties into proprietary dry wells Two infiltration facilities per street over three streets (6 total) BMPs sized to capture runoff generated from the 3/4-inch design storm event, bypass flows to subregional BMP Plan View Drywell Schematic
Commercial Green Streets Intercept runoff from commercial properties and infiltrate into vegetated swales installed within the sidewalk ROW BMPs originally sized to runoff generated from the 3/4- inch design storm event, bypass flows to subregional BMP 2 blocks will be retrofitted Plan View Proposed Existing Commercial Condition Swale
Subregional BMP Intercept water from storm sewer main beneath Broadway, pass through pretreatment, and into infiltration gallery under church parking lot Multiple configurations considered due to geotechnical constraints and property ownership Sized for 2-inch design storm event from the project area (30 ac), assuming all upstream BMPs are in place (prevents oversizing) Will also handle smaller storms and dry weather flows from entire tributary area (200 acres total)
Modeling Approach (SWMM 5.0) used to model runoff and infiltration 10 years of historic rainfall data Field-verified site conditions Living tool used evaluate different BMP scenarios/ configurations Pollutant removal assumed to be equivalent to the percent of runoff intercepted
Modeling Approach Initial sizing in SWMM Residential properties, street ends, commercial green street: The total BMP footprint was adjusted until zero runoff was produced from BMP-retrofitted areas during ¾-inch storm Subregional BMP: The total BMP footprint was adjusted until zero runoff was produced from BMP-retrofitted areas during 2-inch storm Scenario evaluation Model updated to reflect design changes due to constraints and participation encountered during final design Residential Properties Residential Street Ends Commercial Green Street Subregional Footprint 4,500 sf 80 sf 460 sf 6,100 sf Volume 6,800 cf 5,400 cf 900 cf 55,500 cf
Results On an annual basis, the Project has potential to: Recharge groundwater aquifers by 0.8 ac-ft per acre of tributary area Decrease combined modeled pollutant loadings by approximately 200 pounds per acre BMP type Percent Pollutant Removal (%)* Infiltrated Volume (ac-ft/yr) Residential Properties 55% 6 Residential Street Ends 18% 4 Commercial Green Street 18% 1 Subregional 12% 21 TOTAL 17% 31 *Percent infiltration
Pilot Project Cost Estimate Cost Private Home Lots $70,000 Neighborhood Street Ends $503,000 Commercial Green Streets $97,000 Parking Lot Infiltration $866,000 General Conditions $287,000 Total Indirect Costs $537,000 Total Estimated Construction Costs $2,360,000
Implications Approach can be implemented and replicated throughout the City and, in particular, above unconfined aquifers All BMPs sized for widespread BMP implementation Design adapted to other subwatersheds Because of recent amendment to the Central Basin Adjudication, all infiltrated water (monitored) will count as additional withdrawal credit for the City of Los Angeles
Next Steps Bids due to City by March 24, 2015 Construction to begin in July/August 2015 1 year of post construction water quality and flow monitoring will begin after project construction 3 continuous flow monitors: Representatively placed to monitor flows and losses throughout the project Water quality grab samples: Representatively performed to monitor improvements throughout the project Project to be complete by April 2016 Including all monitoring
Questions? Stacy Luell, PE Sluell@Geosyntec.com