San Diego Hydromodification Management Plan
San Diego County is developing a Hydrograph Modification or "Hydromodification" Management Plan (HMP) to define runoff control requirements for new development and prevent excess erosion in creek channels. Within the HMP, the County wanted to identify the range of flows which cause the most creek erosion throughout the County and test the effects of controlling these flows to pre-development levels.
ESA provided guidance to the County and other NPDES co-permittees for setting runoff control requirements based on hydrologic and geomorphic analyses. Because of the wide range of natural variability in geomorphic processes, we conducted a large sensitivity analysis of potential watershed and channel conditions to test potential flow control ranges and their effects on channel erosion. Our work involved continuous rainfall-runoff modeling of a range of representative watersheds and running the resulting flows through a series of channel sediment transport models to test for differences in erosion. By simulating a wide range of conditions and potential controls over a 40-year modeling period, we were able to assess the cumulative effects of small erosion events relative to larger, more infrequent events and identify a critical range of events for flow control.
As part of the San Diego County Hydromodification Management Plan, ESA provided guidance to the County and other NPDES co-permittees for setting runoff control requirements, based on hydrologic and geomorphic analyses.