Ryan K. Walter

Stratified turbulence in the nearshore coastal ocean: Dynamics and evolution in the presence of internal bores

High-frequency measurements of stratified turbulence throughout the water column were collected over a 2 week period in the nearshore environment of southern Monterey Bay, CA, using a cabled observatory system and an underwater turbulence flux tower. The tower contained a vertical array of acoustic Doppler velocimeters and fast-response conductivity-temperature sensors, providing a nearly continuous data set of turbulent velocity and density fluctuations and a unique look into the stratified turbulence field. The evolution of various turbulence quantities and direct measurements of the vertical turbulent diffusivity is examined in the presence of nearshore internal bores, both in the near-bed region and in the stratified interior. We show that individual bores can drive substantial changes in local turbulence and mixing dynamics, with considerable differences between the leading and trailing edges of the bores. Using direct observations of the flux Richardson number, our measurements confirm previous observations that show the highest mixing efficiencies (Γ) occurring in regions of buoyancy-controlled turbulence. Parameterizations of the flux Richardson number as a function of the turbulence activity number are also presented. Finally, we demonstrate that the commonly used assumption of a constant mixing efficiency (Γ = 0.2) for calculating turbulent diffusivities leads to significant overestimates compared to diffusivity values calculated using the directly measured mixing efficiency. Implications of the results are discussed.

Investigation of factors affecting the accumulation of vinyl chloride in polyvinyl chloride piping used in drinking water distribution systems

Plastic piping made of polyvinyl chloride (PVC), and chlorinated PVC (CPVC), is being increasingly used for drinking water distribution lines. Given the formulation of the material from vinyl chloride (VC), there has been concern that the VC (a confirmed human carcinogen) can leach from the plastic piping into drinking water. PVC/CPVC pipe reactors in the laboratory and tap samples collected from consumers homes (n = 15) revealed vinyl chloride accumulation in the tens of ng/L range after a few days and hundreds of ng/L after two years. While these levels did not exceed the EPAs maximum contaminant level (MCL) of 2 μg/L, many readings that simulated stagnation times in homes (overnight) exceeded the MCL-Goal of 0 μg/L. Considerable differences in VC levels were seen across different manufacturers, while aging and biofilm effects were generally small. Preliminary evidence suggests that VC may accumulate not only via chemical leaching from the plastic piping, but also as a disinfection byproduct (DBP) via a chlorine-dependent reaction. This is supported from studies with CPVC pipe reactors where chlorinated reactors accumulated more VC than dechlorinated reactors, copper pipe reactors that accumulated VC in chlorinated reactors and not in dechlorinated reactors, and field samples where VC levels were the same before and after flushing the lines where PVC/CPVC fittings were contributing. Free chlorine residual tests suggest that VC may be formed as a secondary, rather than primary, DBP. Further research and additional studies need to be conducted in order to elucidate reaction mechanisms and tease apart relative contributions of VC accumulation from PVC/CPVC piping and chlorine-dependent reactions.

Local diurnal wind‐driven variability and upwelling in a small coastal embayment

The oceanic response to high-frequency local diurnal wind forcing is examined in a small coastal embayment located along an understudied stretch of the central California coast. We show that local diurnal wind forcing is the dominant control on nearshore temperature variability and circulation patterns. A complex empirical orthogonal function (CEOF) analysis of velocities in San Luis Obispo Bay reveals that the first-mode CEOF amplitude time series, which accounts for 47.9% of the variance, is significantly coherent with the local wind signal at the diurnal frequency and aligns with periods of weak and strong wind forcing. The diurnal evolution of the hydrographic structure and circulation in the bay is examined using both individual events and composite-day averages. During the late afternoon, the local wind strengthens and results in a sheared flow with near-surface warm waters directed out of the bay and a compensating flow of colder waters into the bay over the bottom portion of the water column. This cold water intrusion into the bay causes isotherms to shoal toward the surface and delivers subthermocline waters to shallow reaches of the bay, representing a mechanism for small-scale upwelling. When the local winds relax, the warm water mass advects back into the bay in the form of a buoyant plume front. Local diurnal winds are expected to play an important role in nearshore dynamics and local upwelling in other small coastal embayments with important implications for various biological and ecological processes.

Transcritical generation of nonlinear internal waves in the presence of background shear flow

While the occurrence of large amplitude internal waves in the Earths natural bodies of water is widely documented, the generation of these waves remains an active area of exploration. We discuss numerical simulations of transcritical flows of a density stratified fluid with a dual focus on the role of a background shear current and transitions of the background current from super to subcritical. We demonstrate that the presence of a background shear can lead to the formation of large quasi-trapped regions of high vorticity over the downstream slope of the topography, but that this vorticity leads to only moderate perturbations of the underlying pycnocline, and hence that a wave and instability can co-exist for long times. Subsequently, we demonstrate the existence of hysteresis in the wave amplitude when the current is accelerated to supercritical then decelerated to subcritical, as opposed to accelerated to the subcritical value from rest. Finally, we explore situations in which the background shear is ...

Nesting nonhydrostatic GCCOM within hydrostatic ROMS for multiscale Coastal Ocean Modeling

The Regional Ocean Modeling System (ROMS) is a hydrostatic free-surface ocean model ideally suited to simulate mesoscale to basin-scale (10 km - 10000 km) ocean processes. The General Curvilinear Coastal Ocean Model (GCCOM) is a nonhydrostatic large eddy simulation (LES) model designed specifically for high-resolution (meters) simulations. In this research, a hybrid model is developed that nests a fine-grid GCCOM model within a coarse-grid ROMS. The nested GCCOM-ROMS model is tested in an idealized flow over a seamount.