Geotechnical Stability Analysis of Dune Face Erosion by Waves

Predicting the extent of wave-driven dune erosion under wave impact and elevated water levels will improve our ability to safeguard the livelihood of ecosystems, communities and infrastructure living behind sandy beaches worldwide. However, the geophysical processes leading to time-dependent dune face failures are still not fully understood. Here, physical laboratory experiments are used to inform a coupled groundwater and limit equilibrium slope stability model to explore dune face stability. The model incorporates a spatially time-varying phreatic (or “water table”) surface and the associated changes in pore water pressure due to wave runup to highlight three key physical processes leading to geotechnical dune face failure during wave-driven erosion. First, results from numerical modeling indicate that wave runup impacting the dune face has a destabilizing effect due to excess pore water pressure during downrush. Second, dune face instability during saturated pore water conditions occurs due to excess pore water pressure and the lack of apparent cohesion resulting from the super-elevated water table present inside the dune face. Instability is further exacerbated by wave runup reaching the dune and further elevating the phreatic surface. Third, an important feature in the timing and resiliency of the dune face under wave attack is the location and temporal evolution of the slumped sand post a failure event. The unconsolidated slumped sand acts to temporarily protect the base of the dune from direct wave attack until it is eroded away using swash processes.