Deploying an Interagency Debris Flow Decision Matrix for the Ferguson Burn Scar across Yosemite National Park and California Highway 140

During a PREPARE meeting (Process for Post-Fire, Rapid Assessment and Actions) for the Ferguson Burn Scar at the California Department of Transportation (CalTrans) District 10, on October 8, 2018, several agencies discussed the simplest way to communicate debris flow impacts. The goal of this partnership was to increase confidence in proactive measures with regards to debris flows along California Highway 140/41. The result was a color-coded decision matrix, Green (Level 0), Yellow (Level 1), Orange (Level 2), Red (Level 3) and Purple (Level 4) that focuses on impacts and applicable to each agency’s responsibilities like: Emergency Operation Center activations, road closures and portable changeable message signs.

The Burned Area Emergency Response Team (BAER) assessed the soil and watershed conditions to determine risk. Soil damage was considered high-severity, and both Yosemite National Park (YNP) and the BAER team agreed that sediment delivery during a debris flow at several locations on Highway 140/41, would increase by a factor of 50, if rainfall rates of 0.25”/15 minutes, or 1”/hour occurred.

Unfortunately, these were very high impact locations: a hotel for Yosemite, an RV park and a one lane bridge with a stoplight. Intense rainfall rates could trigger debris flows that would destroy the RV Park or hotel. These worst case scenarios are stratified by the Decision Matrix.

Initial partner feedback has been overwhelmingly positive. CalTrans is eager to use this format across the state. Here is a direct quote from Lisa Worthington, CalTrans HQ. “The cost savings of the Decision Matrix is calculated to be $3 million during the 2018-19 wet season. This number will be submitted by the CalTrans Director to the legislature. CalTrans must identify $100 million in savings over the next 10 years in exchange for $1 billion in new funds. This is a big deal.”