Minimizing Cost and Maximizing Load Reduction By Targeting the First Flush

Date / Time:
Wednesday, Oct 21 9:40am to 10:10am
Big Sur
Track / Session:
Stormwater Treatment / BMP Development and Studies

The recently introduced stormwater retention requirements for new development and redevelopment projects to retain the 85th percentile storm and/or volume produced from a 0.75 inch event create overwhelming challenges for any Low Impact Development, especially retrofit projects. Is this amount of stormwater treatment feasible to improve stormwater quality given constraints present at each already developed site, and at what cost? What if the treatment system design focused on the first flush?

The County of Ventura assessed costs and pollutant load reduction benefits by focusing on first flush capture treatment rather than the new development/redevelopment storm design requirements when designing the County Government Center Parking Lot Green Streets Urban Retrofit project completed in September 2014. Building upon SCCRWP Technical Report 343 and Caltrans’ 2005 First Flush Phenomenon Study, the County came up with an innovative design along with a robust monitoring strategy to quantify stormwater pollution reduction and long term Best Management Practice (BMP) effectiveness. Additional monitoring and testing programs were implemented to evaluate effectiveness of Operations and Maintenance (O&M) procedures and to track long term soil infiltration rates.

The innovative design incorporates pervious concrete gutters atop aggregate filled storage basins supplemented with dry wells to convey impervious surface runoff through the site’s approximate 13 feet of clay to soils capable of accepting it. The system was sized to store and infiltrate at least the first 20 minutes of a 0.5 inch/hour storm, or 0.17 inches determined as first flush. This includes infiltrating all dry weather flows.

During 2014/2015 winter and 2015 spring, monitoring data was collected and compared to pre-construction baseline to evaluate reduction of runoff volume and pollution from the County Government Center parking lots during wet and dry weather conditions. The results support the County’s goal of maximizing load reduction in a cost effective manner. Overall storm event infiltration volumes exceeded design targets even though only about 80% of the first flush was captured and infiltrated on average due to variations in storms such as duration and intensity. The project monitoring data shows that first flush phenomenon does not exist for the majority of pollutants. However, by targeting the first flush volume (23% of new development and redevelopment requirements) over 60% of pollutant loads can be reduced on average throughout an entire rainy season.

The construction and annual maintenance costs are directly proportional to the targeted volume reduction percentage, thereby costing significantly less to build and maintain. By following this methodology, the County estimates a reduction in construction cost of $6 million and annual O&M costs of $190,000. This project is a great example of efficient design and cost effective implementation resulting in significant improvement of stormwater quality.

The proposed presentation will be filled with pictures and videos capturing construction, unique material quality control testing and O&M methods, as well as BMP performance monitoring during rain events. Water quality, flow, and other relevant data will be presented in graphs and charts for a quick understanding of key principles.

Primary Speaker:
David Kirby, Ventura County Watershed Protection District
David Kirby graduated in from the University of Regina in Canada, with a bachelor’s degree in Industrial Systems Engineering. He worked as a plant efficiency specialist and then a water resources engineer in the province of Saskatchewan before starting work with RBF Consulting (now Michael Baker International) in 2005 out of their Camarillo office in Southern California. He worked there for 8 years as a design engineer where he was fortunate to work on various types of projects in several different disciplines. This included water and sewer design and construction, storm drain master plans, agency enterprise GIS, and transportation construction management. He was then introduced to the glorious world of stormwater and it’s over-abundance of acronyms, and was hooked. David is a registered professional civil engineer (PE) in California, is certified as a Qualified SWPPP Developer and Practitioner (QSD/P) as well as a Geographical Information Systems Professional (GISP). He is a Water Quality Engineer with the Ventura County Watershed Protection District where he is the lead engineer on design, construction and implementation of water quality and Low Impact Development (LID) projects. He oversees Ventura County’s MS4 Permit’s stormwater quality protection requirements for new development, redevelopment and construction projects. Additionally, he conducts inspections of private and public post-construction BMPs, training for County employees, contractors, developers, and the general public on NPDES permit requirements.
Supporting Speaker 1:
Ewelina Mutkowska, Ventura County Watershed Protection District