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  1. Abstract. Ocean surges pose a global threat for coastal stability.These hazardous events alter flow conditions and pore pressures in floodedbeach areas during both inundation and subsequent retreat stages, which canmobilize beach material, potentially enhancing erosion significantly. Inthis 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 formationof beach materials under groundwater seepage. Simulations of surge eventsshow that, during the run-up stage, head gradients can rise to the calculated critical level landward of the advancing inundation line. During thereceding stage, critical gradients were simulated seaward of the retreatinginundation line. These gradients reach maximum magnitudes just as sea levelreturns to pre-surge levels and are most accentuated beneath the still-water shoreline, where the model surface changes slope. The gradients vary alongthe shore owing to variable beach morphology, with the largest gradientsseaward of intermediate-scale (1–3 m elevation) topographic elements (dunes)in the flood zone. These findings suggest that the common practices inmonitoring 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. 
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  2. Abstract

    Coastal aquifers supply freshwater to nearly half the global population, yet they are threatened by salinization. Salinities are typically estimated assuming steady‐state, neglecting the effect of cyclical forcings on average salinity distributions. Here, numerical modeling is used to test this assumption. Multi‐scale fluctuations in sea level (SL) are simulated, from tides to glacial cycles. Results show that high‐frequency fluctuations alter average salinities compared with the steady‐state distribution produced by average SL. Low‐frequency forcing generates discrepancies between present‐day salinities estimated with and without considering the cyclical forcing due to overshoot effects. This implies that salinities in coastal aquifers may be erroneously estimated when assuming steady‐state conditions, since present distributions are likely part of a dynamic steady state that includes forcing on multiple timescales. Further, typically neglected aquifer storage characteristics can strongly control average salinity distributions. This has important implications for managing vulnerable coastal groundwater resources and for calibration of hydrogeological models.

     
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