Mapping Coastal Erosion of a Mediterranean Cliff with a Boat‐Borne Laser Scanner: Performance, Processing, and Cliff Erosion Rate

Abstract

Terrestrial laser scanners (TLS) are well known for providing an efficient means to monitor coastal cliff erosion. Cliffs along micro-tidal coasts, however, have often escaped quantification because the narrow or absent coastal platforms do not offer stable and embracing vantage points. To circumvent this issue, mobile laser scanning surveys from a boat can be used. We present a case study from Provence-Alpes-Côte-d'Azur (Mediterranean coastsouthern France) to quantify cliff erosion in such micro-tidal environments. Three surveys were sub-contracted over a period of 17 months to monitor a 3.5-km-long cliff of Carry-le-Rouet (15km west of Marseille). Data quality was check independently using man-made planar walls positioned above the cliff face, to retrieve survey precision and change detection thresholds.Boat-borne mobile lidar system was capable of describing planar features with a precision of 3-4 cm (epoch 1 and 2) and improved to 2.6 cm (epoch 3) with point densities around 100 pts/m². Absolute positioning accuracy varied between 0.1 cm and 0.3 cm. Because the coastline is very sinuous, we describe a method to unfold the point clouds using a continuous analytical surface made of vertical planes joined by arcs of cylinders and perform the analysis in 2.5D.Change was detected using a unique, conservative, threshold of 14 cm (99% quantile estimated on plane change) on grids of 10 x 10 cm pixels. Integrated over the entire cliff face, the average annual cliff recession rate at Carry-le-Rouet is 1.1cm/year. In 17 months, erosion was three times more effective in sandstone and marls layers than in calcarenites and conglomerates.Erosion varies vertically with erosion three times more effective in the lower 25 m of the cliffs than above. Despite imperfections, boat-borne laser scanning system are capable of delivering meaningful erosion data even in this low erosion context.

Smartphone Structure-from-Motion Photogrammetry from a Boat for Coastal Cliff Face Monitoring Compared with Pléiades Tri-Stereoscopic Imagery and Unmanned Aerial System Imagery

“…Whereas boat-based methods are mainly dedicated to submarine surveys, very few boat-based studies proposed mapping emerged areas. Among these rare studies, some used a radar system for 2D mapping [34] or a mobile laser scanner (MLS) for 3D mapping [35][36][37]. With the latter method, the quality of the reconstruction mainly depends on the accuracy of the navigation measurements (positioning, inertial unit, sensor synchronization).…”

Quantifying the contribution of bank erosion to a suspended sediment budget using boat‐mounted LiDAR and high‐frequency suspended sediment monitoring

“…By re‐orienting the vertical riverbank horizontally, the shortcomings of the 2.5D nature of DEM differencing are mitigated, but this technique only lends itself to relatively straight segments before requiring a transformation to ‘unfold’ the banks of a sinuous river. More recently, Giuliano et al (2022) used a boat‐mounted LiDAR scanner to map coastal cliff erosion rates in difficult terrain. They employed a cloud ‘unfolding’ strategy to allow the conversion of the 3D point cloud along a sinuous coastline into a 2.5D surface to overcome the limitations of DEMs of difference (DoDs) of semi‐vertical surfaces along non‐uniform reaches.…”