Probabilistic coastal cliff collapse hazard from repeated terrestrial laser surveys: case study from Mesnil Val (Normandy, northern France)

Along cliff edges, coastal managers dealing with life-safety matters often wonder "how much time have I got left before I need to expropriate this house?" Here, we present a case study where repeated terrestrial laser scanner surveys (TLS) were performed to monitor a chalk cliff section in Normandy. The Mesnil Val cliff, cut in Upper Cretaceous chalk known as the Upper Lewes Nodular Chalk, is a ca. 750-m-long, 20-80m-high cliff section. It was surveyed 6 times between December 2005 and March 2008 at a 1 -point-per-5cm resolution. Successive Digital Surface Models (DSM) of the cliff surface were subtracted to reveal the location and shape of erosion scars. Scars detection relies on a robust indicator of observed noise distribution. The resulting scar inventory contains more than 8500 objects with volumes spanning 8 orders of magnitude (10(-4) to 10(4)) cubic meters. Probability Distribution Functions (PDF) of erosion scar thickness, area and volume scale as power laws. The scaling relationships between area (0.77) and volume (0.53) power law exponents demonstrate that rockfall are simply scaled (area/volume exponent ratio = 3/2). This finding legitimates the inference of rockfall volume distribution from easy-to-measure area PDF. PDF can also be turned into complementary cumulative distribution function (CCDF) which then gives the annual rate of exceeding a given scar dimension. Our statistical estimates have been empirically validated with five complete cliff collapse events occurring on the same cliff section in 13 years. Repeated TLS surveys provide invaluable information to help coastal managers make informed decision.