Wave-Induced Horizontal and Vertical Pressure Gradients Within Nearshore Sediment Beds

Melissa Marry

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. Recent work has shown that incipient motion is influenced by the forcing of the horizontal pressure gradient due to surface gravity waves. Additionally, wave-induced vertical pressure gradients can also destabilize the sediment bed through the process of momentary liquefaction. Understanding the movement of sediment will require resolution of the full wave-induced pressure field (i.e., the pressure in both the vertical and horizontal directions). This work aims to advance our general understanding of how sections of sediment move in nearshore environments through field observations. Novel autonomous pressure-profiling instruments, known as Pressure Sticks, were deployed in the inner surf zone of an ocean beach in Rye, New Hampshire. The Pressure Sticks contain time-synced absolute pressure sensors distributed over 80 cm distance to measure absolute pressure in the water column and in the sediment. These field observations of wave-induced pressure within the sediment bed are compared to theoretical models for wave-induced pressure responses within poroelastic sediment beds. These field observations are analyzed to determine under what conditions wave-induced pressure gradients will destabilize and potentially mobilize the sediment bed.

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A Physics-Informed Geoacoustic Bayesian Inversion for Seabed Properties Using Calibrated Single-Beam Echosounder Data

Andrew Niedbala

Abstract: Seafloor sediment characterization is of interest to many, including those studying benthic environments, pursuing offshore development, and providing for national defense. Discerning seafloor properties at scale needs reliable remote sensing data and effective modeling. Acoustics offers a reliable remote sensing option; however, it requires both efficient inversions and reliable relationships between geoacoustic and geophysical properties. Additionally, high quality acoustic data is expensive and time-consuming to obtain. The National Oceanic and Atmospheric Administration (NOAA) maintains a database for calibrated single beam echosounder (SBES) water column data collected at several frequencies and covering large swaths of the US continental shelf for fisheries and other research. These data also capture bottom return pulse envelopes. A physics-based time-domain model is coadapted with more established geoacoustic and geophysical relationships and a seafloor gradient model to parameterize seafloor properties at the location of these bottom returns. Parameters include density, porosity, mean grain size, interface roughness, and volume scattering parameters. Geoacoustic properties are inverted for using a Bayesian framework utilizing parallel tempering and Metropolis-Hastings algorithm. This approach has the advantage of providing a probability distribution for each parameter, and this data set allows the inversion to leverage the frequency dependence of some parameters. This combined model and inversion process is planned to be used to process NOAA’s bulk SBES data at scale and characterize seafloor sediments with quantified uncertainty.

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Characterization of Sediment-Flow Interactions in Tidally Driven Salt Marsh Environments

Elijah Rousseau

Abstract: Salt marshes are effective sediment traps and under static sea level are known to increase in elevation over time. However, current rates of sedimentation are not high enough to keep up with estimated sea level rise. Strategies such as the mud motor, which proposes the injection of dredged sediment into incoming flood tided, require a deeper understanding of the how tidal flows evolve in complex estuarine systems and how suspended sediment is dispersed and deposited in vegetated environments. Two field deployments occurred in the summer of 2023 to study the mechanisms of sediment transport and accretion along a horizontal marsh transect in the tidally driven Hampton-Seabrook Estuary salt marsh in Southern New Hampshire. Three measurement stations were established in the high marsh, low marsh, and mudflat, each with two vertically separated YSI turbidity sensors and one Nortek ADV. Results showed that marsh vegetation serves the purpose of attenuating flows, increasing vertical turbulence, homogenizing turbidity, and creating an environment prone to the settling of suspended sediment. Additionally, an overall increase of vertically homogenized turbidity in the low marsh relative to the mudflat was observed.