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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 theR_2%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.

 

Abstract Type Ia supernovae (SNe Ia) may originate from a wide variety of explosion scenarios and progenitor channels. They exhibit a factor of ≈10 difference in brightness and thus a differentiation in the mass of 56 Ni → 56 Co → 56 Fe. We present a study on the fate of positrons within SNe Ia in order to evaluate their escape fractions and energy spectra. Our detailed Monte Carlo transport simulations for positrons and γ -rays include both β + decay of 56 Co and pair production. We simulate a wide variety of explosion scenarios, including the explosion of white dwarfs (WDs) close to the Chandrasekhar mass ( M Ch ), He-triggered explosions of sub- M Ch WDs, and dynamical mergers of two WDs. For each model, we study the influence of the size and morphology of the progenitor magnetic field between 1 and 10 13 G. Population synthesis based on the observed brightness distribution of SNe Ia was used to estimate the overall contributions to Galactic positrons due to escape from SNe Ia. We find that this is dominated by SNe Ia of normal brightness, where variations in the distribution of emitted positrons are small. We estimate a total SNe Ia contribution to Galactic positrons of <2% and, depending on the magnetic field morphology, <6–20% for M Ch and sub- M Ch , respectively. 

Erosion of coastal dunes during storm events is an increasingly common problem in the face of global climate change and sea-level rise. To investigate the efficacy of bio-mediated ground improvement for reducing the impact of extreme events such as hurricanes, a near-prototype-scale experiment was performed. In the experiment, a model sand dune was constructed in a large wave flume and divided into treated and untreated zones which were instrumented with pressure and moisture sensors. One of the treated sections was subjected to a surface-spray technique to apply bio-cementation. Afterward, the dune was subjected to a discretized severe storm event (a scaled Hurricane Sandy) consisting of 25 trials. Surge runup and drawdown cause surface erosion and also internal instability due to liquefaction. Pore pressure sensors were embedded in different depths of the dune to study the pressure fluctuations during the wave action and the consequent momentary liquefaction phenomenon. Momentary liquefaction leads to detachment of fine sand particles and the initiation of internal erosion and sediment transport. In this project, remote assessment technology (lidar) was used between each trial to evaluate the performance of the dune under the surge flow by detecting the eroded volume of the sand. To better quantify material properties in-situ, a series of triaxial experiments were conducted on bio-cemented cores taken from the formed crust. The strength and stiffness of the cemented sand were measured under different drainage conditions. Element test results indicate a significant increase in critical bed shear stress (τc) due to cementation.