Orange County Watershed Hydromodification GIS Mapping

Orange County, CA

PACE provided the technical basis of policy related to the implementation of “hydromodification” regulation associated with the North Orange County MS4 NPDES Stormwater Permit which covers approximately 550 square miles.  This effort involved developing county-wide digital watershed mapping of hydromodification susceptibility.  A comprehensive GIS database of digital information for the entire county was accumulated and organized. The data was used to perform an assessment of hydromodification susceptibility utilizing specialized GIS mapping tools and policy oriented screening procedures developed specifically for this study.

The enormous GIS mapping database was uniquely organized and designed with a specialized data management system so that it provided inherent flexibility for potential future policy development and watershed assessments.  The major data categories included county-wide information on drainage facilities, topography, landuse, biological resources in the channels, floodplains, soils information, groundwater data, precipitation, and aerial imagery. The GIS tools were utilized with the multiple layers of digital mapping data to evaluate or test alternative policy rules for determining hydromodification susceptibility.  The objective of the watershed plans was to define land areas which would be susceptible to hydromodification through development based on the potential change in stream stability downstream within the watershed or damage to in-stream biological resources.