A Proposal for Statewide Knee-of-the-Curve BMP Sizing Criteria
Numeric sizing criteria are a fundamental means for regulating stormwater discharges. In the absence of tools for post-construction projects regulated by the Phase II and Construction General permits, the use of uniform criteria statewide (e.g., the 80% capture criteria and the 85th percentile, 24-hr design storm) have been justifiable. However, such uniform criteria may no longer be appropriate as they were established many years ago under technologies and land use practices that are not reflective of today. They also did not necessarily represent the wide climatic variations of California. New sizing criteria that better represents today’s practices and regional climates are needed. This presentation will: 1) provide an overview concerning the history of numeric sizing criteria, 2) describe why these standards may not be appropriate for current storm management practices (e.g., LID), and 3) propose establishment of new sizing criteria that addresses the different types of BMPs in use today.
The historic overview will include background information regarding how the 80% capture and 85th percentile, 24-hour design storm numbers for sizing BMPs came to be. These design criteria have been useful for sizing treatment-based BMPs. However, they do not necessarily represent true knee-of-the-curve performance for BMPs in California. For example, the 80% capture criteria was based on only six detention basins throughout country and was not statistically valid or fully representative of California. In fact, an earlier version of Caltrans Basin Sizer showed the true knee-of-the-curve is not always 80% for detention basins. Furthermore, in contrast with LID BMPs being used today, the detention basins used for developing the 80% capture criteria had a single discharge mechanism (orifice drawdown); LID BMPs include treatment and retention (multiple discharge mechanisms). These differences influence whether the true knee-of-the-curve volume treated for LID BMPs is smaller than, equal to, or greater than 80%.
For these and other reasons, current sizing standards can result in oversized or undersized BMPs, depending on what method is selected to meet regulatory goals (presumably, MEP use of BMPs). The speaker will therefore propose re-evaluating sizing criteria under new stormwater management approaches, with the additional intent of providing criteria that is easy to adopt into permits, is supported by easy-to-use sizing tools, and allows time-efficient review by municipal and water board plan checkers. The proposal will offer a review of available tools that are the best candidates to support knee-of-the curve BMP sizing.
The intent of the presentation is to get the audience to realize that, while stormwater management has greatly evolved (for the better) over the last decades, further improvements are needed to ensure the latest performance goals (i.e., combining use of stormwater as a resource with the traditional objectives of flood control and water quality protection) are achieved. The audience will be engaged by proposing questions and allowing them to weigh-in on their experiences and perspectives, such as similar issues with uniform flow-based criteria (0.2 in/hr). In line with the conference theme, the presentation will acknowledge that stormwater practitioners are intending to “make a difference” in the positive direction by adopting more sustainable, non-numeric performance goals (e.g., enhance infiltration to provide treatment, prevent pollutant transport and erosion, and recharge groundwater), but numeric sizing criteria need to catch up with current goals so that actual performance of California’s various BMP, soil, and location combinations is optimized at each BMP location.