Date of Award

Fall 2025

Project Type

Dissertation

Program or Major

Ocean Engineering

Degree Name

Doctor of Philosophy

First Advisor

Diane Foster

Second Advisor

Tracy Mandel

Third Advisor

Majid Ghayoomi

Abstract

Characterizing the hydrodynamic forces responsible for the initiation of sediment motion is fundamental for improved understanding of large-scale sediment transport processes in nearshore environments. Traditional sediment transport models assume incipient sediment motion is only driven by the force applied by bed stress (Bagnold, 1966; Shields, 1936) while more recent work has shown that incipient motion is influenced by the forcing of the horizontal pressure gradient due to surface gravity waves (Foster et al., 2006; Frank et al., 2015; Sleath, 1999). Wave-induced vertical pressure gradients can also destabilize the sediment bed through the process of momentary liquefaction (Florence et al., 2022b; Mory et al., 2007; Zen and Yamazaki, 1991). Understanding the rapid mobilizations of sediment beds requires improved resolution of the full wave-induced pressure field (i.e., the pressure in both the vertical and horizontal directions).

This dissertation seeks to advance our general understanding of how sediment moves in nearshore environments through several field experiments and observational methods. Observations of vertical pressure gradients are collected in the surf zone of an ocean beach using novel autonomous pressure-profiling instruments. These pressure-profiling instruments, known as Pressure Sticks, are designed and constructed as an integral part of this dissertation. Multiple Pressure Sticks deployed in a cross-shore array are used to directly measure the wave-induced horizontal pressure gradient. Pressure propagation into sediment beds is also investigated with these field observations. Continuous measurements of the location of the sediment-water interface are collected using an optical backscatter instrument called the SediMeter from Lindorm, Inc. These elevation observations are used to track the location of the pressure sensors relative to the sediment-water interface. Additionally, these elevation measurements are able to quantify any occurrence of small-scale net sediment movement.

Observations of vertical pressure gradients show that exceedances of momentary liquefaction thresholds occur within 16 - 20 cm of the sediment-water interface in the surf zone of a moderately-sloping, nearly macro-tidal sandy beach in New Hampshire. These exceedances occur at the onset of the wave crest. As the depth normalized wave height (H/h) and the Sleath parameter increase (specifically H/h > 0.4 and Sleath > 0.1) so does the likelihood of exceedances of momentary liquefaction thresholds. Additionally, the individual wave steepness and nonlinearity are other wave properties that may be necessary to induce large vertical pressure gradients.

Attenuation and phase lags of the pressure signal propagating into the sediment bed are observed. A numerical model for pressure propagation into sediment beds based in Biot Consolidation theory compares fairly well to the field observations of pressure propagation, with a percent difference of less than 6% and root mean square error values of less than 0.03 meters. Deviations between the model and the observations occur at sediment depths that are consistent with the depth and timing of large vertical pressure gradients that deviate from the expected hydrostatic condition. Additionally, model agreement decreased with decreasing water depths (increasing H/h). The sediment elevation measurements show that as the water depth increases with the rising tide, the sediment elevation decreases (losing sediment) while on the falling tide when the water depth is decreasing the sediment elevation increases (gaining sediment). During the high water depth times (lower H/h), there is minimal sediment movement. Exceedances of both the momentary liquefaction thresholds and the horizontal forcing thresholds occur during these times of sediment elevation change.

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