County of Orange Watershed Infiltration and Hydromodification Management Plan (WIHMP)

Orange County, CA


PACE performed a watershed planning analysis of all of the regional watersheds within the County of Orange (including watersheds in both the Santa Ana and San Diego RWQCB areas) to illustrate how to implement new control measure requirements to mitigate stormwater quality and hydromodification for surface runoff. These measures include low impact development (LID), hydromodification conditions of concern (HCOC), and tools to support long-term watershed masterplanning goals. The planning process involved a multi-step process of characterizing and analyzing the watershed and sub-surface conditions utilizing sophisticated GIS mapping tools. This information was used to develop watershed plans for each of the 11 regional watersheds in the county to identify opportunities for sub-regional centralized BMPs focusing on integrated water resources benefits.


The project resulted in a dynamic planning tool that is accessible by County staff and property owners that provides watershed-specific support for integrating water quality, hydromodification, water supply and habitat protection issues as part of selecting and designing BMPs to meet the requirements of the County’s Model Water Quality Management Plan (WQMP). Specifically, this planning tool provides information related to infiltration feasibility and hydromodification susceptibility at the watershed and sub-watershed scale that will help inform BMP selection and design for a proposed project. Infiltration feasibility and hydromodification susceptibility are fundamental factors in the selection and sizing of BMPs to meet the requirements of the WQMP and Technical Guidance Document (TGD).


Key Features:

  • Inventory of existing drainage facilities
  • GIS based mapping of watershed sub-areas
  • Watershed connectivity for intelligent watershed mapping
  • Automated watershed facility calculation tools Watershed information mapping database characteristics and constraints to assist a site- specific user
  • Technical foundation for site assessment screening using infiltration LID
  • Identification of areas with ability to participate in regional / sub-regional BMP opportunities Sub-watershed plans and strategies planning / process

OC Public Works Stormwater Program

GIS Data Management, GIS Water Resource Applications, Stormwater Management / Flood Control, Watershed Analysis / Planning