Colocated sediment pore pressures at depths of approximately 0.02 and 0.22 m below the sand surface and near-bed water velocities were measured for approximately 2 weeks in approximately 1 m mean water depth on an ocean beach near Duck, North Carolina. These measurements suggest that storm wave-driven liquefaction processes may enhance local shoreward sediment transport. During the passage of tropical storm Melissa, wave heights in 26-m water depth (NDBC 44100) were 1-4 m, and storm surge (approximately 1 m) and wave-induced setup increased the water depth on the beach. Upward vertical gradients in pressure heads between the sensors increased with the storm approach, with the largest values observed before the maxima in local wave heights, wave periods, and water depths. The large gradients in pore pressure exceeded several liquefaction criteria and usually occurred when near-bed velocities were upward-and shorewarddirected.

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Category 5 Hurricane Michael made landfall near Mexico Beach, Florida on October 9, 2018, with measured high water marks (HWMs) reaching 7.2 m NAVD88. The town itself received great damage, with many areas destroyed down to the foundations. In this study, we document the storm and its effects on the greater Mexico Beach area: hazard, structural damage, and their relationships. Wave and surge damage was nearly total for low-lying properties, but damage decreased greatly with increasing elevation. Major wave and surge damage was noted in Federal Emergency Management Agency (FEMA) X zones, which are out of the 100-year floodplain, and it is suggested that the 100-year storm is a deficient measure for categorizing flood risk.

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.

Scour holes around slender piles were measured in areas inundated during Hurricane Michael and were compared with scour hole depths estimated from existing scour prediction equations. Despite testing a wide range of feasible input parameters, some measured scour depths could not be predicted by five common scour prediction equations (one wave only, three current only, one wave and current equation). Current only equations yielded the best prediction rate despite the site being in a wave-dominated environment. The scour depths that were not accurately predicted by the equations tended to be underpredictions despite the range of input values. A range of factors were considered that might have caused these differences. Momentary liquefaction was investigated as one possible explanation to some of the discrepancies between observed and predicted scour depths using laboratory tests and field measurements. The results suggested that momentary liquefaction of the top layer of sediment is possible for wave heights of approximately 0.83 m in 1.3 m of water depth, indicating that momentary liquefaction of sediments was possible during Hurricane Michael with 2 m waves in 3.5 m of water and therefore presents one possible explanation for the observed mismatch between the scour predictions and observations.

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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 3D. The analysis is based on a threshold value obtained for momentary liquefaction 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 level, 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.