Browsing by Author "Florence, Matthew"

Now showing 1 - 4 of 4
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    Coastal topography and hydrogeology control critical groundwater gradients and potential beach surface instability during storm surges
    Paldor, Anner; Stark, Nina; Florence, Matthew; Raubenheimer, Britt; Elgar, Steve; Housego, Rachel; Frederiks, Ryan S.; Michael, Holly A. A. (Copernicus, 2022-12)
    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.
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    Hurricane Michael in the Area of Mexico Beach, Florida
    Kennedy, Andrew; Copp, Andrew; Florence, Matthew; Gradel, Anderson; Gurley, Kurtis; Janssen, Matt; Kaihatu, James; Krafft, Douglas; Lynett, Patrick; Owensby, Margaret; Pinelli, Jean-Paul; Prevatt, David O.; Rogers, Spencer; Roueche, David; Silver, Zachariah (2020-09-01)
    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. This work is made available under the terms of the Creative Commons Attribution 4.0 International license, https://creativecommons.org/licenses/by/4.0/
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    Scour Development and Possible Effects of Momentary Liquefaction in Inundated Coastal Areas During Hurricane Michael
    Florence, Matthew; Stark, Nina; Kennedy, Andrew (ASCE, 2022-03)
    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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    Vertical pore pressure variations and geotechnical sediment properties at a sandy beach
    Stark, Nina; Mewis, Peter; Reeve, Bridgit; Florence, Matthew; Piller, Jan; Simon, Jessica (Elsevier, 2022-03)
    The role of geotechnical properties and soil behavior for beach dynamics has been recognized before, but geotechnical field measurements in energetic beach environments are still rare. This study focused on two days of field measurements along a cross-shore transect reaching from the foot of the dunes to the upper subtidal zone at the western sandy beach of the island of Sylt, Germany, just south of the city of Westerland. Sediment properties and geotechnical parameters were obtained from sediment sampling and limited in-situ testing. Pore pressure measurements were conducted along a vertical array in the upper 55 cm of the beach surface in the lower intertidal zone. Pore pressure recordings were then analyzed using a one-dimensional-vertical (1DV) model based on Biot (1956) and Mei and Foda (1981). Laboratory testing results demonstrated slight trends of increasing grain size and friction angles from the subaerial to the lower intertidal zone. In-situ sediment strength testing using a portable free fall penetrometer supported the trends in friction angles for the subaerial and intertidal zone. Additionally, a significant increase in sediment resistance was observed in the swash zone and upper subtidal zone. Pore pressure recordings showed a consistent trend associated with the tidal water elevations. However, data collected during low tide suggested a decoupling of surface water effects and groundwater, possibly associated with gas content and negative pore pressures in the vadose zone. Pore pressure recordings also suggested a more pronounced wave damping in the upper sediment layers and a minor phase lag. The 1DV pore pressure model succeeded to simulate the observed pressures at all sediment depths well, and suggested no liquefaction events during the measurement period, but a reduction of effective weight that may affect sediment dynamics.