Loss Avoidance Study: Northern California Flood Control Mitigation

Study Summary

Study Area – Northern California (five counties)
Hazard Type – Flood
Project Type – This study evaluated six flood mitigation projects, each with several sub-projects, completed using a combination of various FEMA grant program funds and local funds, in northern California counties that experienced repeated flooding:

  • Broadway Culvert Replacement: Project completed in 1998 and included enlarging the culverts, installing rock slope protection at the inlet and outlet, and canal dredging.
  • Humboldt Road Culvert at Malloy Creek: Project completed in 1998 and included enlarging culverts, realigning the creek channel, placing large rocks along the creek channel walls, and reconstructing the roadway using aggregate material.
  • Soscol Avenue Area Drainage Interceptor: Project completed in 1998 and included drainage improvements to existing storm drains and pipes.
  • Hilltop Green Flood Mitigation Project: Project completed in 1999 and included the relocation of the pump station electric controls and emergency generator, construction of a new building to house the relocated controls (19 feet above original) that included a waterproof enclosure and watertight exterior door, and new curbing.
  • Alhambra Creek Channel Improvements: Project completed in 2001 and included bank stabilization measures, creek widening and straightening, realignment of a creek section, creek bank terracing, and restoration of riparian vegetation.
  • Petaluma River Payran Reach Flood Control and Floodways: Project completed in 2004 and included the construction of floodwalls, channel excavation and widening, restoration of riparian vegetation, bridge replacements and relocations, and storm drain installation.

Total Aggregate Project Cost – $48,028,996 (six projects)
Total Aggregate Losses Avoided – $46,905,204 (in 2008 dollars)
Return on Investment – 0.98

Pre- and Post-Mitigation Events

After experiencing repeated flooding, each of five communities in northern California implemented flood mitigation projects. All of the project areas had experienced several flooding events over the previous 10 to 20 years. FEMA undertook this study to evaluate the effectiveness of the flood mitigation projects in the different communities.

Loss Avoidance Methodology

Six projects located in the five northern California counties were used to analyze losses avoided as a direct result of flood mitigation projects. This study included several different project types across a large geographic region of Northern California. The six project locations are shown in Figure 1.

Project Locations. Loss Avoidance Study: Northern California Flood Control Mitigation Projects.
Figure 1. Map of Project Locations

This study looked at the actual performance of the mitigation projects by comparing the losses from flooding events before the mitigation measures were in place to flooding that each location experienced after the mitigation projects were completed. FEMA calculated physical losses (i.e., damage to structures and content), loss of function (i.e., lost business income, wages, or public services), and the cost of emergency services required to deal
with flooding.

All of the projects resulted in losses avoided as a result of flooding. Some resulted in higher losses avoided than others, but the project types and locations varied widely. Because this study is based on only one post-project flood event, the value of the losses avoided will likely increase if additional flooding events occur during the projected lifespan of the projects.

Return on Investment

Table 1 lists the losses avoided, the project cost, and the ROI of each of the projects. All values were calculated in 2008 dollars.

Table 1. Northern California flood control mitigation projects return on investment 

ProjectLosses AvoidedProject CostROI
Petaluma River Payran Reach Flood Control and Floodways$44,170,317$44,907,8020.98
Soscol Avenue Area Drainage Interceptor$ 400,361$ 766,9140.52
Humboldt Road Culvert at Malloy Creek$ 67,924$ 257,1060.26
Alhambra Creek Channel Improvements$ 470,044$ 1,709,6930.27
Hilltop Green Flood Mitigation Project$ 192,709$ 248,5200.78
Broadway Culvert Replacement$ 1,603,849$ 138,96111.5
Total$46,905,204 $48,028,9960.981
1 aggregate ROI
New pump station controls that were moved to a new structure 19 feet above original floor.
Photo: Hilltop Green Flood Mitigation Project – pump station controls were moved to a new structure 19 feet above the original floor.
Dual 96-inch diameter, 95-foot long corrugated metal pipe culvert.
Photo: Broadway Culvert Replacement – Project improvements included replacement of the existing 72-inch diameter pipe with dual 96-inch-diameter, 95-foot-long corrugated metal pipes. Rock slope protection was also built at the inlet and outlet.
Last updated February 11, 2021