Brief Description of the Performance of Highway 1 System during the January 27 to 28, 2021, Precipitation Event

This chapter describes the system-level assessment of Highway 1 performance. This assessment was conducted to understand the reasons that the failure happened specifically at Rat Creek as opposed to elsewhere along the highway and to determine the effectiveness of sixteen of the other seventy-three drainage crossings of Highway 1 between Half Moon Bay and San Luis Obispo. Precipitation runoff transported rock, mud, burned trees, and other debris into Rat Creek and down toward Highway 1. Based on eyewitness accounts, Highway 1 was overtopped and still intact on the morning of January 28, 2021, so erosion and failure occurred shortly thereafter. Unmanned aerial vehicle (UAV) imagery was conducted using a Mavic 2 Pro UAV, with the intent to complement terrestrial LiDAR collection and provide overview imagery and three-dimensional models of the debris dam and breach area in the upper portion of Rat Creek.