ARkStorm II: A Hydraulic Modeling and Flood Inundation Mapping Effort on Santa Clara River for Emergency Planning Exercises by Local Responders in Ventura County, CA

The ARKStorm II Emergency Planning and Response Exercise was a joint effort of Ventura County Public Works Agency (VCPWA), Federal Emergency Management Agency (FEMA), and United States Geological Survey (USGS). It is a FEMA RiskMAP pilot project designed to showcase how scientifically based watershed modeling of a large and plausible storm event can enhance disaster response and mitigation planning and foster public awareness. The inundation mapping for this project was developed using hydrological and hydraulic modeling based on data from two severe and rare storms on record for the Santa Clara River (SCR) watershed, namely the 1969 and 1986 extreme precipitation events. The rainfall data was provided by the USGS; these data represent the last two catastrophic flooding events that occurred in Ventura County, California. Using the rainfall data, hydrologic time series of flow (hydrographs) for each of the major tributaries of SCR were developed and used for hydraulic modeling. Real-time storm progression results from the hydraulic modeling were developed to mimic realistic expectations of where flooding is most likely to occur, the resulting flood depth, and what critical community facilities and public and private infrastructure along the Santa Clara River are projected to be adversely impacted at specific time intervals during the storm event. For example, which transportation evacuation routes, bridges and underpasses, and designated community gathering places could become inaccessible due to flooding and flood-induced landslides at a predicted time interval (e.g., 30 hours into the storm). With such information in hand, alternate routes and public gathering locations would need to be identified through coordinated emergency planning and response exercises by the participating responsible officials of the County of Ventura and Cities along the SCR, FEMA, and USGS. Flood plain maps showing the extent of inundated areas with simulated water depth, critical facilities and utilities flooded, and real photographs of the destroyed areas along the SCR in the 1969 and 1983 floods were prepared to dramatize the events and make it as realistic as possible during the course of the workshop exercise. The main objective of this exercise was to determine which course of actions the county and city emergency planners and engineers would need to take to be better prepared for responding to such a disastrous event. This paper will focus on the hydraulic modeling, flood plain mapping, and the day-long emergency planning and response exercise for this joint effort.