Burn Area Bridge Risk Assessment

In November 2003, a report by the Ventura County Fire Department and the California Department of Forestry and Fire Protection indicated a significant postfire concern of possible massive landslides. The California Department of Transportation (Caltrans) recognized that some bridges might impede channel flow of runoff that contains concentrated and, sometimes, heavy debris. The Caltrans Office of Structure Maintenance needed to deploy crews to those bridges that were potentially at risk from heavy runoff.

Caltrans Division of Structures Hydraulics was assigned the task of identifying state-owned and locally owned bridges at risk in each of the seven major burn areas. The report timeline was short. Structures Hydraulics staff members performed a preliminary study identifying possibly more than 1,300 bridges and then requested Engineering GIS Support to identify a two-mile boundary from each fire to create a list of affected bridges. The list was fulfilled the next day using buffer zones, a standard GIS function, but the method would not guarantee accurate results as runoff and debris flow do not follow buffer zones.

Engineering GIS Support provided a unique solution similar to watershed studies. For each burn area, the topology was mosaicked in ArcGIS Spatial Analyst as a continuous digital elevation model. Digital water droplet tests were performed without having to define watershed boundaries for each bridge. Aerial photos were acquired to confirm the existence of facilities without deploying personnel. The results from the contrasting methodologies—buffer zones versus topographic analysis—were dramatic.

For the San Bernardino fire alone, using the generalized two-mile buffer zone method, 202 bridges were identified as being potentially affected by the fires. Of these, 126 were erroneously identified as at risk. Seventy-six of the bridges were identified correctly, but many were only considered low risk.

Using the topographic analysis method, only 24 state-owned bridges were positively identified as high risk from the burn area. Structures Maintenance confirmed the accuracy of the final bridge selections by deploying crews before and after the rains, noting the debris flow. With the use of more in-depth hydrologic tools and GIS methodologies, maintenance inspection and monitoring work were reduced by more than 80 percent on this project. Similar results were obtained in other fire areas.