Journal of Waterway, Port, Coastal, and Ocean Engineering

Geotechnical sediment properties, morphological change, and hydrodynamics were measured as part of the During Nearshore Event Experiment (DUNEX) in October 2021 at the sandy Atlantic side beach in Duck, North Carolina. In this study, direct relationships were explored between in situ soil properties and direct erodibility measurements in the context of morphological change. Moisture content, grain size, total density, relative density, void ratio, and sediment strength were compared to bed-level change using ground-based LiDAR and erodibility parameters from laboratory jet erosion tests (JETs) conducted along a cross-shore transect stretching from the dune toe to the lower intertidal zone. Directly relating changes in sediment properties to changes in morphology from LiDAR proved inconclusive due to the complex interactions between sediments, hydrodynamics, and morphology, even on a local scale, but initial observations and possibly impacting factors were discussed. Void ratio and total unit weight correlated well with the detachment rate coefficient from JETs, with denser sediments testing as less erodible. In situ sediment strength measurements related—as expected—to total unit weight, void ratio, and water content, with increases in firmness factor associated with increases in total unit weight, void ratio, and water content. These strength measurements were also found to have a correlation with the detachment coefficient from the JET, with stronger/firmer sediments being less erodible than weaker ones.

Extending an earlier two-dimensional (2D) implementation, a novel method is introduced for both detecting the onset of wave breaking and simulating the resulting energy dissipation in limited water depth, in a three-dimensional (3D) fully nonlinear potential flow (FNPF) model. Breaking onset is identified using a universal criterion, based on the ratio of the horizontal particle velocity at the crest to the crest phase velocity. The breaking-induced energy dissipation is based on the nondimensional breaking strength parameter and is implemented in the model as an absorbing surface pressure. The 3D-FNPF solves Laplace’s equation using a higher-order boundary element method based on Green’s second identity and marches the solution forward in time. The implementation of wave dissipation due to breaking is carried out in three steps: (i) a nondimensional breaking strength parameter is calculated based on a previous 2D unified depth-limited dissipation model; (ii) the instantaneous power to be dissipated is computed using this parameter and energy dissipation is modeled as a damping pressure specified in a region around the breaking crest; and (iii) the dissipation process of each breaking wave is terminated using a criterion calibrated through a comparison of the free surface elevation with experimental data from the literature. The new 3D model is experimentally validated for regular spilling and plunging breaking waves propagating over a 3D submerged bar and an elliptical shoal. Future work will extend this model to irregular 3D breaking waves.

Estuarine intertidal flats are exposed to a wide range of changing environmental conditions, but despite many years of field studies, little is known about their geotechnical characteristics. This study investigates (1) the variability of sediment grain size distributions, water contents, and Atterberg limits of three distinct exposed tidal flats; and (2) the potential pathways for remote characterization of those sites. Sediment samples were collected from three flats in the Great Bay Estuary located in the Gulf of Maine at the border of New Hampshire and Maine, United States: two sites composed of primarily fine-grained soils (Adams Point and Woody) and one composed of predominately coarse-grained soils (Mast Cove). Additionally, high-resolution X-band synthetic aperture radar (SAR) images were collected concurrently to field measurements from various satellites and over different incidence angles. In each image, the footprint area corresponding to the three exposed tidal flats was extracted, and statistical properties of the backscatter intensity histograms for each region were computed and compared. Moisture contents, fines contents, and grain size distributions varied between the three sites, with the USCS classifications ranging from elastic silts (Woody) and low-plasticity silts (Adams Point) with high water contents (83.2%–172.7%) and fines contents (83.4%–99.5%) to silty sand and well-graded sands with gravels (Mast Cove) with lower water contents (17.7%–48.3%) and fines contents (4.8%–44.3%). Across the sites, the statistical properties of the SAR backscatter exhibited distinct trends regarding mean backscatter, entropy, and uniformity in relation to the Unified Soil Classification Scheme (USCS) classification and fines content. These trends were consistent across all images collected. A potential sediment classification scheme to derive the USCS classification from the X-band SAR imagery is suggested utilizing the mean, entropy, and uniformity of the backscatter coefficient.