Allison Penko

Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows

ABSTRACT Calantoni, J., Landry, B.J. and Penko, A.M., 2013. Laboratory observations of sand ripple evolution using bimodal grain size distributions under asymmetric oscillatory flows The heterogeneity of sand beds has been suggested to significantly impact the resulting sand ripple morphodynamics. However, the majority of previous experiments for sand ripple morphodynamics were conducted using only unimodal grain size distributions. Here we performed a series of ripple growth and transition experiments in a small oscillatory flow tunnel in the Sediment Dynamics Laboratory at the U.S. Naval Research Laboratory. Sand beds were constructed from mixtures of two unimodal sands median grain sizes of 0.30 mm (blue) and 0.70 mm (white), respectively. Experiments were performed with compositions of bimodal mixtures with percent by mass of 10/90, 25/75, 50/50, 75/25, 90/10. Additionally, similar experiments were performed for each of the unimodal cases (i.e., 100/0, 0/100). For each experiment, starting from a planar bed, three different flow forcing conditions were applied in sequential blocks (with minimum of one-hour duration) until the ripples appeared to be uniform and in equilibrium. We analyzed ripple characteristics such as migration rate, wavelength, height, and steepness as a function of the mobility number. Over a range of nearly identical mobility numbers, we observed opposing trends with migration rates increasing in one block forcing and decreasing in another, where the two blocks were comprised of different combinations of the semiexcursion amplitude and oscillatory frequency. The results suggested that the commonly used mobility number might not be appropriate to characterize ripple migration rates, especially for sediment beds composed of bimodal size mixtures. Overall, wavelength, height, and steepness are consistent with empirical ripple predictors. However, observed subtleties existed among the different forcing blocks across the same range of grain size distributions.

Simulating biogeo-optical dynamics in the bottom boundary layer: northern Gulf of Mexico test case

Interdisciplinary coastal observations over a two-week period in the northern Gulf of Mexico reveal a complex and dynamic bottom boundary layer (BBL) that is characterized by both biological and suspended sediment (biogeo-) optical signals. Much of the BBL optical variance is concealed from remote sensing by the opacity of the nearly omnipresent surface river plume, however, the BBL physical dynamics and resulting optical excitation are indeed responding to surface wind stress forcing and surface gravity wave-induced turbulence. Here we present a series of numerical modeling efforts and approaches aimed towards resolving and simulating these observed biogeo-physical and –optical processes. First, we examine results from the Tactical Ocean Data System (TODS), which combines daily satellite imagery with numerical circulation model results to render a three-dimensional estimate of the optical field and then execute a reduced-order complexity advection-diffusionreaction model to render hourly forecasts. Whereas the TODS system has the advantage of effectively assimilating both glider data and satellite images, the 3D generation algorithms still have difficulty in the northern Gulf’s complex 3-layered system (surface plume, geostrophic interior, BBL). Second, we present results from the Coupled Ocean-Atmosphere Prediction (COAMPS) system that has been modified to include interactive surface-gravity wave simulations. Results from this complex numerical modeling system suggest that Stokes drift current (SDC) has a potentially major role in determining the physical and kinematic characteristics of the BBL, and will substantially impact model-based estimates of sediment resuspension and transport.

Modeling and Observations of Sand Ripple Formation and Evolution During TREX13

Ripples on the seafloor affect acoustic scattering and transmission loss, wave attenuation, and the amount of sediment transported in shallow water. Historically, seafloor roughness (a function of ripples, bedforms, sediment type, and size) is assumed to be spatially homogeneous and temporally static in hydrodynamic and acoustic models despite the often dynamic nature of the seafloor in the nearshore region. We present a spectral ripple model, Navy Seafloor Evolution Archetype (NSEA), which simulates the variations in seafloor roughness given measured or predicted wave conditions in sandy environments. NSEA simulates sand ripple formation and evolution based on bottom velocities either measured or predicted by a wave model. The time dependency is a function of equilibrium ripple geometries and the amount of sediment transport needed to reach an equilibrium state, which is dependent on the relict ripples. Spectral decay due to bioturbation is incorporated as a diffusive process. NSEA was validated with time series observations obtained in water depths of 7.5 and 20 m from April 20, 2013 to May 23, 2013 during the 2013 Target and Reverberation Experiment (TREX13) offshore of Panama City, FL, USA. The model predicted spectral ripple wavelengths that were in good agreement with observed spectral ripple wavelengths obtained using a fixed platform, high-frequency (2.25 MHz) sector scanning sonar. Likewise, the variations in the predicted normalized ripple heights and orientations were similar to the normalized spectral decay and orientations estimated from the sector scanning sonar imagery.

Hyperspectral determination of ocean color as an ocean monitoring tool: example applications in the Gulf of Mexico

The combination of increased spectral resolution for in situ ocean optical instrumentation as well as future ocean remote sensing missions (e.g., PACE) provides an opportunity to examine new methods of analysis and ocean monitoring that were not feasible during the multispectral satellite era. For example, hyperspectral data enables a much more precise determination of the apparent true color for natural waters, one based on the full spectral shape of water-leaving radiance distributions. Herein we provide examples of how specific integrated biogeo-optical and physical processes in the northern Gulf of Mexico have characteristic hyperspectral signatures, and thusly, characteristic true color identifiers. Our emergent hypothesis is that once the characteristic hyperspectral color signature of a specific biophysical process is known, it can be detected and monitored even with multispectral or broad-band response digital imaging systems. To test this hypothesis, we examine archived imagery from MODIS and HICO to identify putative bottom boundary layer ventilation events along divergent shelf-frontal boundaries across the northern Gulf continental margin. Whereas on-demand in situ physical data that provide spatiotemporal correspondence with archived images are not available, we employ the data-assimilative Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) as a physical data surrogate. Preliminary results of this method appear to support the hypothesis, with the caveat that model results must be interpreted with due caution.

Investigation of Sand Ripple Dynamics with Combined Particle Image and Tracking Velocimetry

AbstractAccurately assessing the response of sediments to oscillatory flows requires high-resolution fluid velocity and sediment transport measurements at the fluid–sediment interface. Fluid and se...

Time-dependent seafloor roughness simulations using a coupled spectral ripple-biological decay model

Ever changing hydrodynamic conditions in the nearshore continuously affect biological communities and sediment features on the seafloor. Seafloor sediments have complicated interactions with the co-existing biological flora and fauna on the seabed. For example, ripple formation by waves and ripple degradation from biological activity are opposing forces occurring continuously. Simulating this complex natural environment is challenging due to these dynamic interactions of processes that span across a wide range of length and time scales. Existing seafloor models are often based on equilibrium conditions and neglect any change due to biological effects. Additionally, typical equilibrium ripple models provide only geometric dimensions, not a roughness spectrum necessary for acoustic applications. We present a time-varying spectral seafloor model, NSEA, that predicts the spatial and temporal evolution of seafloor roughness accounting for both the evolution and degradation of ripples due to hydrodynamics and b...