Multi-Benefit Treatment System Sizing Procedure for Full-Capture Trash Compliance
The State Water Board (SWB) maintains a list of stormwater treatment systems that are certified for compliance with the Statewide Trash Policy. One of the classes of certified systems is multi-benefit treatment systems. These can be designed to address both trash and other pollutants. Volume-based designs are common in existing sizing procedures, such as those found in both local and statewide stormwater handbooks and in some statewide stormwater permits. The SWB has provided information sheets, but these only include flow-based design standards for these systems. Developing a simple procedure to check volume-based designs for compliance with the flow-based trash sizing standard would reduce the needed amount of resources for treatment system design and regulatory review.
The goal of this sizing procedure is to enhance the use of multi-benefit systems for the full-capture of trash by minimizing the design effort for practitioners and review burden for municipal staff plan-checkers and regulators.
This talk will present the outcome of developing this procedure with input from the CASQA trash guidance project, the State Water Board, and other stakeholders.
The following objectives inform the proposed sizing procedure:
• Develop a procedure to check the compliance of a volume-based design with the SWB’s flow-based standard in their multi-benefit treatment system information sheets, then offer ways to adjust the system design to achieve compliance
• Develop a sizing procedure that takes full advantage of filter-based treatment systems (e.g., bioretention) so that the addition of screens is not required
• Develop a procedure that can be integrated into the Phase II LID Sizing Tool
• Use simplified design assumptions to avoid the need to evaluate varying flow throughout a storm event
The proposed approach, as of the writing of this abstract, is to make a few conservative design assumptions that will result in a somewhat overdesigned system compared to a system that would be designed via a more resource-intensive model. This is done by capturing the entire volume associated with the flow-based trash sizing requirement, thus eliminating the need to model flows throughout the storm event. The associated assumptions that must be checked in using this method are:
1. Is the flow of water into the treatment system restricted below the trash sizing flow rate or the capacity of existing drainage (whichever is less)?
2. Does the volume identified for capturing the volume associated with the trash flow have any restrictions that would prevent the rapid filling of that volume during a trash sizing storm event (1 hour)?
3. Do all the treatment mechanisms used to discharge the volume associated with the trash sizing flow rate comply with the requirement to prevent the discharge of material greater than 5 millimeters (mm)?
The benefit of a simple procedure is its ease of use and can be added to the Phase II LID Sizing Tool. Using a more complicated model will likely result in a more efficient (smaller) design result, but guidance for such an analysis is outside the purpose of this memorandum. If a more efficient design is needed, check CASQA, Caltrans, and other California-based programs for their most recent guidance.