Geology topics

Allison Penko

Publications and source records attributed to Allison Penko.

3 recordsLinked to original sources

USGS2023

Vortex trapping of sand grains over ripples under oscillatory flow

Sand ripples significantly impact morphodynamics in the nearshore by generating coherent vortices, which can transport suspended sediment to greater heights in the water column than above flat beds. Coherent vortices can trap sediment grains if the settling velocity of the grain is smaller than the maximum vertical fluid velocity in the vortex (Nielsen 1992). Particle image and tracking velocimetry were used to measure small-scale fluid-sediment interactions over sand ripples in a small oscillatory flow tunnel. Here we present some of the first measurements of vortex-trapped sediment grains under oscillatory flows. Results showed that the vortex-trapped sand grain traversed an orbit offcenter of the vortex near the ripple slope. Some grains then spiralled outward and settled to the bed; others were transported by the flow as the vortex was shed from the crest. Vortex trapping can delay settling and increase settling times, potentially causing inaccurate sediment transport predictions by large-scale numerical models, which do not typically account for this non-linear small-scale process.

Conference Paper↗

USGS2022

Modeling morphodynamics of coastal response to extreme events: What shape are we in?

This review focuses on recent advances in process-based numerical models of the impact of extreme storms on sandy coasts. Driven by larger-scale models of meteorology and hydrodynamics, these models simulate morphodynamics across the Sallenger storm-impact scale, including swash, collision, overwash, and inundation. Models are becoming both wider (as more processes are added) and deeper (as detailed physics replaces earlier parameterizations). Algorithms for wave-induced flows and sediment transport under shoaling waves are among the recent developments. Community and open-source models have become the norm. Observations of initial conditions (topography, land cover, and sediment characteristics) have become more detailed, and improvements in tropical cyclone and wave models provide forcing (winds, waves, surge, and upland flow) that is better resolved and more accurate, yielding commensurate improvements in model skill. We foresee that future storm-impact models will increasingly resolve individual waves, apply data assimilation, and be used in ensemble modeling modes to predict uncertainties.

Annual Review of Marine Science↗

USGS2018

Laboratory observations of artificial sand and oil agglomerates

Sand and oil agglomerates (SOAs) form when weathered oil reaches the surf zone and combines with suspended sediments. The presence of large SOAs in the form of thick mats (up to 10 centimeters [cm] in height and up to 10 square meters [m 2 ] in area) and smaller SOAs, sometimes referred to as surface residual balls (SRBs), may lead to the re-oiling of beaches previously affected by an oil spill. A limited number of numerical modeling and field studies exist on the transport and dynamics of centimeter-scale SOAs and their interaction with the sea floor. Numerical models used to study SOAs have relied on shear-stress formulations to predict incipient motion. However, uncertainty exists as to the accuracy of applying these formulations, originally developed for sand grains in a uniformly sorted sediment bed, to larger, nonspherical SOAs. In the current effort, artificial sand and oil agglomerates (aSOAs) created with the size, density, and shape characteristics of SOAs were studied in a small-oscillatory flow tunnel. These experiments expanded the available data on SOA motion and interaction with the sea floor and were used to examine the applicability of shear-stress formulations to predict SOA mobility. Data collected during these two sets of experiments, including photographs, video, and flow velocity, are presented in this report, along with an analysis of shear-stress-based formulations for incipient motion. The results showed that shear-stress thresholds for typical quartz sand predicted the incipient motion of aSOAs with 0.5–1.0-cm diameters, but were inaccurate for aSOAs with larger diameters (>2.5 cm). This finding implies that modified parameterizations of incipient motion may be necessary under certain combinations of aSOA characteristics and environmental conditions.

Open-File Report↗