Scour Development and Possible Effects of Momentary Liquefaction in Inundated Coastal Areas During Hurricane Michael

Abstract

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.

Field Observations of Hydrostatic Pressure Deviations in a Nearshore Sediment Bed

“…This implies that the pressure gradient is directed downward within the sediment from sensor 6 to sensor 8. The downward pressure gradient will increase the effective stress within this lower sediment section, which may increase the strength of this sediment section (Florence et al., 2022a; Sumer & Fredsøe, 2002). Neither Florence et al.…”

“…These plug flow studies suggest that the passing waves may, at times, be the force that initiates sediment mobility and should be considered as a force that has the potential to transport liquefied sediment during and after the occurrence of ML. Furthermore, previous studies have discussed that the magnitude of forces needed to mobilize a section of liquefied sediment would be smaller than for a section of non-liquefied sediment, though the magnitude of this difference in the mobilizing forces has not been quantified (Florence et al, 2022a(Florence et al, , 2022bGuest & Hay, 2017;Mory et al, 2007;Sakai et al, 1992;Shields, 1936;Yeh & Mason, 2014).…”

“…The field campaigns of Florence et al. (2022a, 2022b), Mory et al. (2007), and Zen and Yamazaki (1991) have collectively shown that thresholding‐exceeding vertical pressure gradients exist and if ML had occurred, it could have liquefied a 20–32 cm section of sediment based on the observed penetration depth of the large, threshold‐exceeding vertical pressure gradients for each experiment.…”

“…There are limited discussions or observations of the potential extent or frequency of surf zone ML. The field campaigns of Florence et al (2022aFlorence et al ( , 2022b, Mory et al (2007), and Zen and Yamazaki (1991) have collectively shown that thresholding-exceeding vertical pressure gradients exist and if ML had occurred, it could have liquefied a 20-32 cm section of sediment based on the observed penetration depth of the large, threshold-exceeding vertical pressure gradients for each experiment. Additionally, Florence et al (2022b) showed that their observed threshold-exceeding pressure differences often occurred during shoreward and upward directed fluid velocities, indicating that any suspended liquefied sediments would be transported onshore.…”