Hydromodification Controls

Available Reports and Work Products Fact Sheet

The Program's NPDES Permit requires Co-permittees to manage increases in runoff peak flows, volumes, and durations from Group 1 development projects (i.e., projects that create more than one acre of impervious surface) that may cause downstream erosion, through the implementation of a Hydromodification Management Plan (HMP). The Final Hydromodification Management Plan Report, which was adopted by the Water Board on July 20, 2005, delineates areas where increases in runoff are most likely to impact channel health and water quality and provides management options for maintaining pre-project runoff patterns. The HMP is not designed to correct existing erosion problems, but to prevent worsening of creek channel erosion problems from new or redevelopment projects.

Bay Area Hydrology Model

The Program, along with the San Mateo and Alameda County stormwater programs jointly funded the development of the Bay Area Hydrology Model (BAHM), a tool for simulating pre- and post-project hydrology and automatically sizing flow control facilities to meet HMP requirements.  Clear Creek Solutions (CCS), the consulting firm who developed the Western Washington Hydrology Model, was contracted to develop a Bay Area version, based on the HSPF watershed hydrology model.  The Program contracted separately with CCS to perform local calibration of the model to two watersheds in Santa Clara Valley.  BAHM-related information and work products are available at