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Ventura County, CA Flood Control, Watershed Modeling, and Watershed Management Studies

Client: Ventura County Watershed Protection District, Public Works Agency, Ventura, CA

The Ventura County Watershed Protection District (VCWPD), as owner and operator of the receiving watercourses throughout the County, is responsible for operation and maintenance of flood control facilities and, under the county-wide NPDES permit, administration of the stormwater quality program. To improve and expand its technical procedures, VCWPD is exploring the use of continuous watershed simulation as a tool for fulfilling its flood control, water resource management, and water quality management obligations. Since June 2000 AQUA TERRA has been assisting the Ventura County Watershed Protection District in its efforts to improve and expand its watershed assessment procedures, through the use of continuous watershed simulation as a tool for fulfilling its water resource management and water quality management obligations. This assistance has occurred through a series of individual projects described below.

Arroyo Simi Pilot Study

This project was a pilot study to evaluate the use of the U.S. EPA Hydrologic Simulation Program FORTRAN (HSPF) as a management tool for comprehensive watershed assessment within the climatic, physiographic, and topographic conditions of Ventura County.

In the initial phase of the pilot study, HSPF was set up on the Arroyo Simi watershed, and a preliminary hydrology calibration and validation was performed on an eleven-year period of observed flows at two sites within the watershed. The model results were presented in a Final Report in 2001, and a presentation was made to Ventura County FCD, along with a series of recommendations for future refinement of the Arroyo Simi model calibration.

HSPF-VCRAT Comparison Study

In a follow-up effort to the Arroyo Simi model, the ‘preliminary’ calibration of the Arroyo Simi was used to compare HSPF model predictions for flood peaks and volumes for various flood frequencies, to those generated by the VCRAT method, a modified rational formula approach previously used by VCWPD. Given that the HSPF model calibration was not complete, the objective was not to assess how close the two procedures agree, but rather to evaluate and compare the types of flood assessment information generated by each approach. The flood frequency comparisons of HSPF, VCRAT and the historic data show reasonable agreement (see probability plot on following page). We also explored and demonstrated the additional types of information available from the HSPF watershed model that would further assist

USEP and Hydromodification

To address water quality issues, VCWPD has developed a Stormwater Quality Urban Impact Mitigation Plan (SQUIMP) at the request of the Los Angeles Regional Water Quality Control Board. The SQUIMP addresses storm water pollution from new development and redevelopment in the private sector. One listed requirement of the SQUIMP (as defined in NPDES Permit No CAS004002) is that “The discharger shall control the post-development peak storm water runoff discharge rates to maintain or reduce predevelopment downstream erosion, and to protect stream habitat.”

To respond to this requirement, VCWPD developed, in conjunction with the LARWQCB, the Urban Stream Erosion Prevention Model (USEP) work plan to address the issues of urbanization impacts on stream erosion and habitat alteration. This effort extended the initial HSPF model setup on the Arroyo Simi to include the specific habitat sites monitored in USEP, finalized the preliminary hydrology calibration/validation, and used the final model to assess stream erosive conditions under current, natural and alternative scenarios for mitigation of urbanization impacts. The objective was to develop and demonstrate procedures within the Arroyo Simi watershed that might be applicable throughout the County. The approach was to analyze the model-generated information on flow and bed shear stress to determine how often (i.e., percent of time) channel scour/erosion conditions occur. Specifically, the calculated shear stress timeseries values were analyzed to determine how often the values exceeded critical values for channel scour (tau ratios of 1.5 and 2.5, see figure); this analysis established the potential for channel scour conditions at any point within the watershed.

Both current/baseline and scenario results were analyzed in a similar fashion, so that urbanization impacts could be identified and evaluated. The methodology assessed how often (i.e. what percent of time) scour/deposition conditions occurred under each scenario, with the differences from natural conditions representing the urbanization impacts, and the differences for detention scenarios representing mitigation impacts. Duration (cumulative frequency) curves were used to analyze results for flow rate, velocity, and shear stress; shear stress values were compared to ‘critical shear values’ for scour and deposition to assess the frequency, duration, and ‘percent of time’ for scour/deposition conditions. Also, flood statistics (e.g. 100-year, 50-year, 25-year, 10-year peak flows) were generated for each scenario using accepted standard statistical procedures for flood analysis (e.g. Log Pearson III), to assess scenario impacts on flood events along with changes in the time duration of erosive conditions.

In January 2005 the California Regional Water Quality Control Board (Los Angeles Region), recognizing the value of the approach, adopted Resolution No. 2005-002 to provide a regional policy for hydromodification analyses:

“…..The Regional Board and local agencies have undertaken or sponsored hydro- modification field assessments and studies to develop peak flow design criteria to minimize or eliminate adverse impacts form urbanization for watercourses. These studies include the ‘Urbanization and Channel Stability Assessment in the Arroyo Simi Watershed of Ventura County, CA…. The results will be used to develop objective criteria to reduce or eliminate the adverse impacts of hydromodification in the Los Angeles Region from new development and redevelopment.”

Calleguas Creek Watershed Study

The Arroyo Simi work efforts were extended downstream in this study to develop a comprehensive watershed hydrologic model of the entire Calleguas Creek Watershed, from its origins in the Arroyo Simi to its discharge to Mugu Lagoon and the Pacific Ocean. This effort was jointly funded by the Calleguas Creek Watershed Management Plan and the Ventura County Watershed Protection District (VCWPD). HSPF was set up and calibrated to available flow records for recent hydrologic conditions, and customized to include consideration of localized groundwater pumping impacts and lawn/landscape and agricultural irrigation practices on surface water flow levels.

The Calleguas Creek Watershed covers an area of 340 sq. mi. and is surrounded to the north, east, and south by largely undeveloped hills and canyons, while the main stem flows through flat valleys consisting of a mixture of urban and agricultural land. The goal was to develop a watershed-wide hydrologic assessment tool. The watershed is subject to flooding and erosion, resulting in sediment deposition downstream in Mugu Lagoon. Topographic, soils, land use, and agricultural cropping information were used to develop the model segmentation and input, and detailed streamflow data were selected to allow calibration over a 9 year period (WY 1994 – WY 2002) and validation over a separate 6 year period (WY 1988 – WY1993). Both quantitative and qualitative comparisons were performed to support the model performance evaluation effort.

Santa Clara River Feasibility Study and HSPF Model

AQUA TERRA has recently completed a major effort for Ventura County that extends their HSPF modeling to the Santa Clara River watershed, the neighboring watershed to Calleguas Creek. The new project is part of the ongoing Santa Clara River (SCR) Watershed Feasibility Study being undertaken jointly by VCWPD, Los Angeles Department of Public Works, and the LA District of the U.S Army COE (USACE). In this effort a ‘Baseline’ hydrologic model was developed with HSPF through model setup, calibration, and validation with existing climatic data, soil/watershed information, and hydrologic data for the SCR Watershed. In addition, the model is being used to simulate extreme (i.e. 100-year events), and long-term flow records at various locations in both counties under Baseline, Natural, and Future Conditions. In the future, it will be used for water quality simulations as described in the 2003 Project Management Plan.