Hazard of coastal cliff top failure through time

Twenty five repeat airborne LiDAR and UAV surveys were used to quantify coastal cliff retreat along 2.5 km of coast in Del Mar, CA from 1998 to 2023. Over the 25 year study period, the cliff face and top retreated by a mean of 1.55 m (0.06 m yr −1 ) and 1.8 m (0.07 m yr −1 ), respectively. Retreat rates varied through time, with faster cliff top retreat in 1998–2012, followed by faster cliff face retreat in 2012–2023. The monitoring period immediately followed the strong 1997–1998 El Niño, and potentially captured a lagged cliff top response following lower elevation wave driven cliff face erosion associated with the El Niño. The difference between cliff face and cliff top retreat may point to locations with an increased hazard of coastal cliff top failure, and consequently, land/property at risk of loss and cliff top infrastructure damage. 70 % of cliff face erosion events in 1998–2017 that did not include retreat of the cliff top were followed by a cliff top failure in later time periods (2002−2023). The timing of delayed cliff top collapse after such cliff-face-only failure followed a power function (R 2 = 0.44). Half of the locations with a positive cliff top hazard metric experienced cliff top failure within the first six years, while after 10 and 20 years the cliff top failed at 61 and 81 % of locations, respectively. • Repeat surveying allows separating short-term variability from longer term trends. • The difference between cliff face and cliff top retreat, can point to locations at risk of cliff top failure and land loss. • 70% of cliff face erosion events in 1998–2017 were followed by a cliff top failure in later time periods. • At 50% of locations with a positive cliff top hazard metric, a cliff top failure occurred within 6 years.