Coastal topography and hydrogeology control critical groundwater gradients and potential beach surface instability during storm surges

Abstract. Ocean surges pose a global threat for coastal stability.
These hazardous events alter flow conditions and pore pressures in flooded
beach areas during both inundation and subsequent retreat stages, which can
mobilize beach material, potentially enhancing erosion significantly. In
this study, the evolution of surge-induced pore-pressure gradients is studied through numerical hydrologic simulations of storm surges. The spatiotemporal variability of critically high gradients is analyzed in three dimensions. The analysis is based on a threshold value obtained for quicksand formation
of beach materials under groundwater seepage. Simulations of surge events
show that, during the run-up stage, head gradients can rise to the calculated critical level landward of the advancing inundation line. During the
receding stage, critical gradients were simulated seaward of the retreating
inundation line. These gradients reach maximum magnitudes just as sea level
returns to pre-surge levels and are most accentuated beneath the still-water shoreline, where the model surface changes slope. The gradients vary along
the shore owing to variable beach morphology, with the largest gradients
seaward of intermediate-scale (1–3 m elevation) topographic elements (dunes)
in the flood zone. These findings suggest that the common practices in
monitoring and mitigating surge-induced failures and erosion, which typically focus on the flattest areas of beaches, might need to be revised to include other topographic features.
