Loss Avoidance Study: Sonoma County California Structure Elevation Mitigation

Following the 2006 winter floods in Northern California, FEMA conducted a Loss Avoidance Study (LAS) of 197 elevation projects. The Sonoma County, CA study focuses on the elevation of structures along the Russian River and its tributaries. Following extensive flooding in 1995, including two Presidential Disaster declarations (FEMA-1044-DR-CA and FEMA-1155-DR-CA), Sonoma County established the Sonoma County Flood Elevation Program to assist homeowners with mitigation for flood-damaged homes. The structures elevated as a result of this initiative provided the basis for the Sonoma County elevation study. The construction completion dates for the elevation projects range from April 17, 1998 to January 23, 2006. Since 1998, five peak flood events have occurred that would have impacted structures. Of those events, the most recent peak flood event occurred on January 1, 2006. All but one of the structures (the one for which construction was completed on January 23, 2006) had a construction completion date prior to the last storm event analyzed.

The purpose of this LAS is to assess the effectiveness of elevation projects in areas impacted by the 2006 floods. Initially, 205 structures were to be included in the Sonoma County LAS; however, five structures were determined to be outside the project area and were eliminated from the final analysis. Additionally, discrepancies between the recorded pre- and post-project first floor elevations for three structures led to their elimination. The final structure count was 197 buildings. Figure 1 shows the locations of these structures. Building replacement values and actual costs for elevation construction were adjusted to 2007 values.

Graphic

Storm recurrence data were determined based on gage records. A mix of recorded and calculated information was used to determine the peak flows for these events. Existing modeling data was updated to the HEC-RAS 4.0 format and used for the hydraulic analysis to determine the depth of flooding at the structures included in this study for each of the historical events.

Loss calculations included structure damage, contents damage, displacement time, and loss of public services. All structures had been elevated to the 100-year flood elevation and received no damages during the five historical events that were used for this LAS.

The losses avoided totaled $13,546,870. The aggregate cost of the mitigation projects was $14,186,135, resulting in a ROI of 0.96.