Orange County Branch Newsletter

May 2019


OC Public Works Dam Breach Mapping Efforts Villa Park Dam Case Study

By Ben Smith | Orange County Water District

The failures of the Oroville Dam spillways in early 2017 aroused public awareness about dam safety.  In the summer of 2017, Governor Brown signed into law new requirements focused on dam safety, including mandates for dam owners across the state to produce updated inundation maps and Emergency Action Plans.  The Orange County Flood Control District (OCFCD) owns 16 dams, including the Villa Park Dam which has been classified as extremely high hazard.  On April 4, EWRI facilitated a presentation about the new mandates and, more specifically, the Villa Park Dam analysis.  EWRI was pleased to welcome two presenters at the event: Penny Lew, Senior Civil Engineer at Orange County Public Works, and Steve Parker, GIS Manager and Hydraulic Analyst at Tetra Tech.

Ms. Lew discussed the mandates, dams owned by OCFCD, and their respective hazard classifications.  The hazard classification is based solely on downstream considerations, not the current condition of the dam or its appurtenant structures.  Of the 16 dams, 1 is classified as extremely high hazard, 9 are high hazard, 1 is a significant hazard, and 5 are low hazard.  When the County was notified about the need for the inundation updates, the timeline to report for extremely high hazard classification was imminent.  The County was able to reallocate funds from other purposes in order to perform the necessary studies to comply with the mandate.  Analysis of the extremely high hazard and high hazard classifications have been completed, with the high hazard analyses currently under review with the state’s Division of Safety of Dams.  The analysis of the significant hazard dam is currently underway.  Tetra Tech was hired to assist with the inundation study for the Villa Park Dam. 

Villa Park Dam Schematic

Tetra Tech undertook a multi-pronged approach to establish the worst-case “sunny day” failure of the dam.  A “sunny day” failure assumes that the dam is holding water up to its spillway crest elevation, hydrologic inflow conditions are normal, and the dam catastrophically fails.  Tetra Tech utilized HEC-RAS 2D software to develop three dam failure scenarios.  The worst-case scenario, based upon a combination of total volume, peak flow rate and formation time, was selected for the inundation analysis.  HEC-RAS 2D was used further to model the flow released by the worst-case dam failure.  In the creation of the model, Tetra Tech utilized the County’s lidar topographic data to generate the ground surface.  The topographic data was scrutinized to correct areas of conflict, such as bridge overpasses or sub-surface drainage channels.  Roughness values based upon land use were assigned across the ground surface and grade breaks were defined.  The modeling results were submitted to the state’s Division of Water Resources and have been accepted.  Tetra Tech also used GIS software to create maps of the inundation.  An updated Emergency Action Plan is currently in development.

The presenters were very gracious and allowed a robust question and answer period with attendees after the presentation.

Flow across the Villa Park Dam spillway in 1969

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