Search

Erosive beach scarps influence beach vulnerability, yet their formation remains challenging to predict. In this study, a 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive event. Pressure and moisture sensors buried within the dune were used both to monitor the water table and to examine vertical pressure gradients in the upper 0.3 m of sand as the slope of the upper beach developed into a scarp. Concurrently, a line‐scan lidar tracked swash bores and monitored erosion and accretion patterns along a single cross‐shore transect throughout the experiment. As wave conditions intensified, a discontinuity in the slope of the dune formed; the discontinuity grew steeper and progressed landward at the same rate as the R2% runup extent until it was a fully formed scarp with a vertical face. Within the upper 0.15 m of the partially saturated sand, upward pore pressure gradients were detected during backwash, influencing the effective weight of sand and potentially contributing to beachface erosion. The magnitude and frequency of the upward pressure gradients increased with deeper swash depths and with frequency of wave interaction, and decreased with depth into the sand. A simple conceptual model for scarp formation is proposed that incorporates observations of upward‐directed pressure gradients from this study while providing a reference for future studies seeking to integrate additional swash zone sediment transport processes that may impact scarp development. Plain Language Summary: Dunes are important protective features of many sandy coasts around the world. Often, when dune erosion occurs, steep faces known as scarps form. Scarps can erode landward quickly, and it is therefore useful to understand when and where they might form. Here, a large‐scale laboratory experiment was used to study the processes involved in the erosion of a dune. The water pressure and moisture content within the dune sediments, and the runup extent and changing elevation of the sand were observed simultaneously. During the experiment, a scarp developed as the wave heights, periods, and water levels increased. As waves passed over the sand surface, the water pressure within the sand changed rapidly, sometimes causing differences (gradients) in pressure between sand at different depths. If the pressure gradients became high enough and were directed upwards, they put an upward force on the sand grains, potentially making them prone to erosion. Upward‐directed pressure gradients were observed as wave conditions grew more energetic. To show how the processes observed in this study may have contributed to the formation of the scarp in the experiment, a conceptual model of scarp formation is proposed. Key Points: A 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive eventDestabilizing pressure gradients were identified within the sand surface, and were impacted by swash conditions and depth within the sandA conceptual model for scarp formation is proposed that incorporates observations from this study and could guide future work [ABSTRACT FROM AUTHOR]

Intense sediment transport experiments were performed in a gravity driven open-channel flow with two sizes of non-spherical acrylic particles of diameter dp=1.0mm and dp=3.0 mm, and a maximum packing volumetric concentration of 0.55 m3/m3. Two flow conditions were adapted to ensure the same sediment transport regime for particle sizes as sheet flows (Shields numbers above unity) with an equi-repartition of the total net sediment transport rate between the suspended load and the bedload (Suspension number around unity). An acoustic scattering-based system, the Acoustic Concentration and Velocity Profiler (ACVP) and electric conductivity probes, the Conductivity Concentration Profiler (CCP) with two different vertical resolutions of 1mm (CCP1mm) and 2 mm (CCP2mm),were used to measure time-resolved and averaged concentration profiles across the bed-load and suspension layers. A detailed comparative analysis of concentration and sheet flow layer thickness measurements obtained with the two systems across both the suspension and the bedload layers is presented. The capabilities and limitations of the two flow measurement technologies are outlined. Average sediment concentration profiles were overestimated by 10% with the ACVP in the dense sheet layer when ⟨φ(z)⟩ ≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨φ(z)⟩ ≲ 0.015 and ⟨φ(z)⟩ ≲ 0.20 for CCP1mm and CCP2mm, respectively. Good agreement is found elsewhere between the three systems in terms of average and time-resolved concentration as well as bed level position and sheet flow layer thickness.

An approach to estimating sediment transport along the Delaware--Maryland coast is described. Wave information study (second generation model, WISWAVE 2.0) hindcast frequency-direction spectra from 1980 to 1999 are used to generate a radiation stress-conserving angle for bringing offshore waves to breaking to estimate alongshore sediment transport. Using standard values for the coefficients in the United States Army Corps of Engineers' Coastal Engineering Research Center (CERC) equation yields 20-year mean alongshore sediment transport rates between 470,942 and -517,088 [m.sup.3]/year depending on alongshore location after shoaling waves to shore. Interannual variability is large with SDs between 95,078 and 241,313 [m.sup.3]/year again depending on alongshore location. Alongshore sediment transport rate estimates are commensurate with past studies using WIS data but overpredict the typically cited value of ~100,000 [m.sup.3]/year near Indian River Inlet, Delaware based on sediment budget data by a factor of 3-4. This overestimation could be compensated for by altering the constant K in the CERC transport equation or slight variations in the shoreline normal. The yearly estimates of the nodal point location where the sediment transport rate reverses direction were just south of the Delaware-Maryland border. The nodal point experienced interannual variability on the order of several thousand meters and is mainly attributed to the interannual variability in wave forcing. It is found that estimating alongshore sediment transport rates from offshore wave data are dependent on proper preservation of the radiation stress (using a radiation stress-conserving wave angle) and accurate determination of the shoreline normal. DOI: 10.1061/(ASCE)WW.1943-5460.0000034 CE Database subject headings: Beaches; Water waves; Sediment transport; Delaware; Maryland. Author keywords: Beach.

Six one-dimensional-vertical wave bottom boundary layer models are analyzed based on different methods for estimating the turbulent eddy viscosity: Laminar, linear, parabolic, k--one equation turbulence closure, k--[epsilon]--two equation turbulence closure, and k--[omega]--two equation turbulence closure. Resultant velocity profiles, bed shear stresses, and turbulent kinetic energy are compared to laboratory data of oscillatory flow over smooth and rough beds. Bed shear stress estimates for the smooth bed case were most closely predicted by the k--[epsilon] model. Normalized errors between model predictions and measurements of velocity profiles over the entire computational domain collected at 15[degrees] intervals for one-half a wave cycle show that overall the linear model was most accurate. The least accurate were the laminar and k--[epsilon] models. Normalized errors between model predictions and turbulence kinetic energy profiles showed that the k--[omega] model was most accurate. Based on these findings, when the smallest overall velocity profile prediction error is required, the processing requirements and error analysis suggest that the linear eddy viscosity model is adequate. However, if accurate estimates of bed shear stress and TKE are required then, of the models tested, the k--[omega] model should be used. CE Database subject headings: Shear stress; Boundary layer; Kinetics; Hydrologic models; Eddy viscosity.