Automating coastal cliff erosion measurements from large-area LiDAR datasets in California, USA

Quantifying coastal cliff erosion is critical for improved predictions of coastal change and coastal management. However, few studies have been conducted at a scale (>100 km) and resolution (similar to 1 m) sufficient to constrain regional change. Here, we quantified cliff erosion for 866 km of the California coastline using airborne LiDAR data collected in 2009-2011 and 2016. A semi-automated method was used to map cliff faces. Negative (volume loss) and positive (volume gain) change objects were created by grouping adjacent cells using vertical and areal change thresholds and surface optical signatures. We assessed the performance of five machine learning algorithms to separate erosion and deposition from other changes within the cliff face, notably vegetation loss and growth, and found that discriminant analysis performed best. After applying the classification method to the entire cliff change dataset, the results were visually inspected for quality control, producing a final dataset comprised of 45,699 erosion and 1728 deposition objects. The net volume loss from 2009-2011 to 2016 was 1.24 x 10(7) m(3), equivalent to an erosion rate of 2.47 m(3) yr(-1) per meter of coastline, and an average cliff retreat rate of 0.06 m yr(-1). Eroded volumes ranged from 6.43 m(3) to 7.52 x 10(5) m(3) and fit a power-law frequency distribution (beta = 0.80; r(2) = 0.99). Over this study period, 7% of eroded material remained on the cliff face. Cliff retreat rates varied spatially with the highest rates in Humboldt County (0.18 m yr(-1)) and the lowest in Orange County (0.003 m yr(-1)). (C) 2021 The Author(s). Published by Elsevier B.V.

Automated summary

This study quantified coastal cliff erosion along 866 km of the California coastline using airborne LiDAR data, revealing a net volume loss of 1.24 x 10(7) m(3) and an average erosion rate of 2.47 m(3) yr(-1) per meter of coastline, with a mean cliff retreat rate of 0.06 m yr(-1). Spatial variations in cliff retreat rates were observed, with the highest rates in Humboldt County (0.18 m yr(-1)) and the lowest in Orange County (0.003 m yr(-1)).