Régis Xhardé

Accuracy and Limitations of Airborne LiDAR Surveys in Coastal Environments

The results of this work enable specification of the vertical and horizontal accuracy of LiDAR systems. The horizontal error is mainly the result of aircraft positioning and LiDAR spatial resolution limitations and is assumed to be smaller than 30 cm. LiDAR vertical errors were found to be slope-dependent, the greatest errors being observed on cliffs. Sea cliffs are in any case the most challenging features to survey and wave-cut notches constitute a major limitation to airborne surveys. The portion of the cliff that can be surveyed with an airborne LiDAR has been determined according to notch dimensions and laser beam incidence angle. Keywords-LiDAR; remote sensing; accuracy; coastal zone; erosion monitoring; sea cliffs

Short-Term Beach and Shoreface Evolution on a Cuspate Foreland Observed with Airborne Topographic and Bathymetric LIDAR

Abstract This paper examines the short-term evolution of a cuspate foreland with diminished sediment supply in the western Gulf of St. Lawrence. Topographic light detection and ranging and airborne light detection and ranging bathymetry are used to provide an overall analysis of the foreland system at Paspébiac, Quebec, including high-resolution digital morphology of combined subaerial and subaqueous components, between 2003 and 2006. Results indicate large differences in coastal stability around the foreland. The western barrier exhibits a stable shoreline (net change  =  +0.3 m·y−1), a moderate beach slope (0.12–0.18), and no subtidal bar system. The morphodynamic response in this sector is influenced by jetties and alongshore variability and is related to beach planform readjustments to varying wave conditions. The eastern barrier has higher wave exposure, high erosion rates (<6.7 m·y−1), wave washover, and an intermediate barred-beach profile, with higher beach slopes (β  =  0.16–0.24). The alongshore variability is controlled, at length scales of about 500 to 700 m, by differences in relaxation time between distal and proximal sections of the foreland. At shorter length scales (∼100–500 m), alongshore variation is related to inner bar morphology, higher erosion rates being observed near rip channels or in the absence of an inner bar. Sand transport patterns reflect wave energy and approach and include a reversal in transport direction along the eastern barrier under storm waves from the SW. We show that, under declining sediment supply, sediment is being lost to a shoal and deep water off the tip of the foreland and erosion on the eastern barrier is not compensated by the slow accretion on the western side of the point.

Significance of LiDAR Return Signal Intensities in Coastal Zone Mapping Applications

This paper presents an attempt at calibrating airborne LiDAR return intensities by correlating them with several geotechnical parameters. The influence of four factors on the backscattered LiDAR intensity has been illustrated. The most influential one is sediment water content, which produces an exponential decrease of return signal intensity. Laser beam incidence angle, sediment grain size, and sediment density are also important factors. Using these results, a preliminary equation is proposed to describe the relationship between return intensity and the four key factors. Keywords-LiDAR; remote sensing; coastal zone; morphology; sedimentology

Large Scale Experimental Storm Impact on Nourished Beach Using Cobble-Gravel-Sand Mix

We have investigated beach stability against storm waves. The studies are done in relation to eroded beaches. We are testing a cobble-sand-gravel mixture as a means of using a soft method for coastal protection on nourished beaches. A physical model of an existing beach was built at scale 1:3. The cobble/sand grain size is in 1:3 scale while the gravel is 1:1.5 scale. The large scale experimental flume tests have been set-up in the new outdoor 120 m long flume in Quebec city, Canada. The tests were conducted over two test seasons (2013–14). While we in the first test season studied impact on the beach due to incoming regular plunging breakers, the last season contained tests with incoming irregular plunging breakers on the beach with/without tidal variation.Herein, we primarily report on the wave impact due to irregular plunging breakers on constant and tidal varying water depths. The wave-tide interactions were conducted with a tidal range of 1 m in relation to beaches with steep beach slopes (1:10, 1:5, 1:1). The model inlet significant wave height was 1.1–1.5 m corresponding to equivalent prototype waves in the range of max. wave heights of 6–8.5 m with dominant periods of 12 s in water depth of about 15 m and tidal range of 3 m.In general, the Equilibrium Beach Profile (EBP) was reached after exposure to about 10,000 plunging breakers or the equivalent of five storms assuming each lasting 3 hours. A cobble berm was formed rapidly on the top of the beach, protecting the backshore against wave action and flooding while finer sediment was transported “offshore”. Beach width reduction was observed when the initial slope of the beach fill material exceeded the equilibrium beach slope. Sediment grain size sorting along the beach profile is discussed and compared to existing beach models, and EBP was compared to several EBP equations.From a coastal management perspective, in terms of durability, the mixed cobble-sand-gravel material is showing promise as a material to use for coastal protection. It is highly absorbent and the beach tends to maintain its shape over long time when exposed to several storms. However, storm surges in the combination with high tides can results in excessive run-up and potential flood risks. The stabilized beach typically had slopes of 1:7–1:9 independent of the initial slope. We found that irregular seas result in a less pronounced trough in the beach profile in the swash zone than incoming regular plunging breakers. The tidal interaction was further advantageous, naturally shifting the material back and forth. However, other materials and other sensitivity studies are necessary in order to provide firm conclusions about the usage of the cobble-gravel-sand mixture for coastal protection.Copyright