Recent Advances in Designing Bioretention Facilities to Meet Treatment, Retention, Hydromodification, and Peak Flow Stormwater NPDES Requirements in California
Many California municipal stormwater NPDES permits specify Low Impact Development (LID) for stormwater treatment on new and redevelopment projects. Bioretention is the most common type of LID treatment facility and is also the primary type of facility used in green infrastructure retrofit projects. Most municipal stormwater permits also include additional requirements to retain a specified amount of runoff on-site and/or to control peak flows and flow durations. This presentation reviews the use of routing methods and continuous simulation models to evaluate hydrologic performance of bioretention facilities. It then provides a survey of recent advances to integrate criteria for stormwater treatment facilities and flow-control/retention criteria into unified guidance for bioretention sizing and design: • The statewide Phase II permit for California’s small municipalities specifies a bioretention design and sizing procedure for treatment and also specifies that peak flows not exceed the pre-project flow produced by a specified design storm. Modeling to be conducted during the summer of 2014 will examine whether a bioretention facility meeting the permit-specified configuration and sizing for treatment also meets the hydromodification management requirement for north San Francisco Bay counties. • The Central Coast Region Post-Construction Requirements (PCRs) specify that new developments must provide for retention of the volume produced by the 85th or 95th percentile storm. A routing method, such as the Santa Barbara Unit Hydrograph (SBUH) method, may be used to account for infiltration of treated runoff achieved by the facility during the course of the design storm. Under a Proposition 84 grant, in 2014 the County of Santa Barbara prepared an MS-Excel-based sizing calculator that integrates the SBUH calculation. The calculator facilitates the design of bioretention facilities and direct infiltration facilities. In a separate task, an SBUH calculation was used to show that runoff from roofs and paving may be dispersed to landscape in a 2:1 ratio (impervious:pervious) without producing runoff from a 2.4-inch storm. • The PCRs also specify that post-development peak flows not exceed pre-project peak flows for 2-year through 10-year storm events. For the County of Santa Barbara, Dubin Environmental Consulting conducted a 2014 analysis of whether bioretention facilities designed to retain the 95th percentile storm would be sufficiently large to meet this requirement. • Permittees subject to the Bay Area Municipal Regional Stormwater Permit (MRP) have noted bioretention facilities’ capability to accomplish stormwater infiltration as well as treatment; they advocate changing the current permit’s prioritization of infiltration and harvesting/reuse over bioretention. Contra Costa Permittees monitored five bioretention facilities over a 2-year period to validate their performance in controlling runoff rates and durations as required by the permit. The permittees are currently preparing a proposal to integrate LID and hydromodification management requirements in the reissued permit. In each of these four efforts, Permittee staff and consultants have considered how to optimize the “flow through” treatment capacity of an underdrained bioretention facility while also providing the detention volume needed to achieve retention (via infiltration) and/or flow control. Both objectives can be accomplished by providing enough subsurface storage beneath the underdrain discharge elevation. However, surface storage can also contribute to the needed storage if an orifice is included on the underdrain. In that case, it is necessary to use a model (either event-based on continuous) to characterize how the bioretention facility fills, drains, and overflows during runoff events. The presenters will engage the audience by asking structured questions (show of hands/audience comments) throughout the presentation.
Dan Cloak, P.E., has been assisting California municipalities with stormwater NPDES compliance since 1992. He has been principal of Dan Cloak Environmental Consulting since 2002. He has authored guidance manuals used for implementing stormwater new development requirements in Contra Costa, San Diego, Riverside, Marin, Santa Barbara, and Monterey Counties.