A New Tool for LID and Hydromodification for the City of Sacramento
The Sacramento Stormwater Quality Partnership (Partnership), includes the County of Sacramento and the Cities of Sacramento, Elk Grove, Citrus Heights, Folsom, Rancho Cordova, and Galt. They received their third round MS4 NPDES Permit (Permit) in September, 2008. The Permit requires that the development and implementation of a hydromodification mitigation program. The purpose of the HMP is to manage the increases in the magnitude, frequency, volume and duration of runoff from development projects to protect receiving waters from the increased potential for erosion and other adverse impacts, with the goal of maintaining (or reproducing) the pre-development hydrology. The Permit also requires the permittees to adopt low impact development (LID) standards for new development and redevelopment projects.
The Partnership recognized the opportunity to reduce the complexity of implementation guidelines for the HMP and LID standards by combining the guidance in a single design manual, and creating a software tool to assist engineers and Partnership staff in performing the analysis to demonstrate compliance. The Partnership’s LID standards require a project to achieve 100 LID ‘points,’ awarded for the use of LID approaches that achieve runoff volume reduction or use/retention of stormwater runoff. The HMP includes the use of continuous simulation modeling to demonstrate that the post-project flow duration curve matches the pre-project flow duration curve for the range of discharges determined to be significant for alluvial channel stability.
The Partnership modified the Western Washington Hydrology Model (WWHM) to assist designers in analyzing proposed stormwater management measures to demonstrate compliance with the LID standards and the HMP. The modified version of the WWHM is called the Sacramento Area Hydrology Model (SAHM). The model uses continuous simulation to develop flow duration curves for a range of geomorphically significant flows, and also quantifies volume reduction of the HMP and LID measures used in the site design. The volume reduction attributed to the stormwater management measures is converted to LID ‘points’ by the SAHM. This allows the site designer to integrate modeling of LID and HMP measures, and to easily demonstrate compliance and optimize the size and placement of the measures using the SAHM design tools.
This paper describes the Partnership’s LID standards, HMP requirements and the integrated SAHM tool. The approach taken by the Partnership is an important step in integrating stormwater management for site development and simplifying design and analysis of stormwater management measures.