In January 2007, the California Regional Water Quality Control Board for the San Diego Region (Regional Water Board) reissued a municipal stormwater NPDES permit to San Diego area municipal Copermittees. The reissued permit updates and expands stormwater requirements for new developments and redevelopments. Stormwater treatment requirements have been made more widely applicable and more stringent; minimum standards for Low Impact Development (LID) have been added, and the Copermittees are required to develop and implement criteria for the control of runoff peaks and durations from development sites.
Low Impact Development is an integrated site design methodology that uses small-scale detention and retention to minimize pollutants conveyed by runoff and to mimic pre-project site hydrological conditions. As required by the reissued permit, the Copermittees have prepared an updated Countywide Model SUSMP to replace the current countywide model SUSMP, which has been in effect since 2002. Each municipality will update its local SUSMP to implement the requirements.
To assist the land development community, to streamline project reviews, and to maximize cost-effective environmental benefits, the updated Countywide Model SUSMP incorporates a unified LID design procedure. This design procedure integrates site planning and design measures with engineered, small-scale Integrated Management Practices (IMPs) such as bioretention. By following the procedure, applicants can develop a single integrated design which complies with the complex and overlapping NPDES permit LID requirements, stormwater treatment requirements, and runoff peak-and-duration-control (hydromodification management) requirements.
Countywide Model SUSMP
The Stormwater Copermittee Land Development Workgroup is responsible for the development and update of the Countywide Model Standard Urban Stormwater Management Plan (SUSMP) and Hydromodification Management Plan (HMP), as well as related tools and resources. Workgroup information can be found here.
Final Countywide Model SUSMP (March 2011)
Countywide Hydromodification Plan
Regional Water Quality Control Board Order R9-2007-0001 Provision D.1.g (6) (Board Order) requires the County of San Diego and its NPDES Co-permittees to develop a Hydromodification Plan to manage the increases in runoff discharge rates and durations from all Priority Development Projects, where such increased rates and durations are likely to cause erosion of channel beds and banks, sediment pollutant generation, or other impacts to beneficial uses and stream habitat due to increased erosive force.
A key element of the creation of the San Diego HMP is the involvement of a Technical Advisory Committee (TAC). The TAC members consist of respected individuals from academia, technical resource agencies, the development community, consulting engineers, and environmental organizations. The TAC was tasked with providing technical input to the scientific approach and interpretation of results integral to the establishment of numerical flow control standards for the HMP.
The HMP can be found as an appendix to the Model SUSMP and below. Any priority development project subject to SUSMP requirements is also subject to HMP requirements.
The following report provides an evaluation of the two alternative methods, HSPF watershed model and San Diego Hydrology Model, for satisfying the HMP, and is intended to provide the San Diego Copermittees with a clear understanding of the applicability, functions, and results of these tools. Both software tools, are reviewed for compliance with the HMP and standard stormwater management methodology.
The San Diego BMP Sizing calculator provides a simplified method for sizing BMPs to comply with treatment control and hydromodification management requirements without project specific continuous simulation. The following links provide the background for the methodology, the spreadsheet calculator, and the user guide.
The San Diego Hydromodification Management Plan (HMP) has been implemented over the past 5 years. Detailed results of the monitoring program have been presented in a series of annual monitoring reports between 2011 and 2016. This Final HMP Report compiles and summarizes the results and findings of the five year field monitoring and desk-top analyses. This Final HMP Report includes the following monitoring and technical reports as exhibits that provide discussions that are more detailed and data from which the conclusions in this report are based.
Low impact development (LID) is a term used to describe a land planning and engineering design approach to managing stormwater runoff. LID emphasizes conservation and use of on-site natural features to protect water quality. This approach implements engineered small-scale hydrologic controls to replicate the pre-development hydrologic regime of watersheds through infiltrating, filtering, storing, evaporating, and detaining runoff close to its source.
Excellant information is available at the following resources: