Mapping Coastal Erosion of a Mediterranean Cliff with a Boat-Borne Laser Scanner: Performance, Processing, and Cliff Erosion Rate
Résumé
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 coast-southern France) to quantify cliff erosion in such micro-tidal environments. Three surveys were subcontracted over a period of 17 months to monitor a 3.5-km-long cliff of Carry-le-Rouet (15 km west of Marseille). Data quality was checked independently using man-made planar walls positioned above the cliff face, to retrieve survey precision and change detection thresholds. A 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 points/m 2 . 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 the plane change) on grids of 10 × 10 cm pixels. Integrated over the entire cliff face, the average annual cliff recession rate at Carry-le-Rouet is 1.1 cm/year. In 17 months, erosion was three times more effective in sandstone and marl layers than in calcarenites and conglomerates. Erosion varies vertically with erosion three times more effectively in the lower 25 m of the cliffs than above. Despite imperfections, boat-borne laser scanning systems are capable of delivering meaningful erosion data even in this low erosion context.
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