Observations of coastal cliff landslide runout in southern California from 21 years of data

Abstract Coastal landslides threaten communities, cultural and recreational lands, infrastructure and public safety. To better understand the landslide hazard zone on the beach, this study used historical failures to measure the landslide runout distance from the cliff toe. Landslide debris on the beach was mapped for a 30 km study area in northern San Diego County, California using over 700 LiDAR and photogrammetric surveys collected between 2002 and 2023. The results provide a unique statistical database of coastal landslide runout observations. The maximum, mean and median runout distances were 36.2 m, 6.0 m and 4.9 m, respectively. Most runout was constrained relatively close to the cliff, with 90% of all runout distances less than 13 m. Results indicate that taller cliffs generated larger maximum runout distances and that a maximum runout of one half the cliff height accounted for 98.5% of the observations. The observed runout distances and statistics can help inform mapping of coastal hazard zones, improve public safety measures, and also improve geomorphic models of cliff retreat.