Environmental Fluid Mechanics

Permanent URI for this collectionhttps://digital.lib.washington.edu/handle/1773/34906

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Dataset for: Short-term Arsenic Cycling in a Shallow, Polymictic Lake
(2023) Fung, Samantha; Horner-Devine, Alexander R.; Neumann, Rebecca; Gawel, James; Hull, Erin
We observed repeated diel oscillations in arsenic (As) concentrations in the bottom waters of a shallow, temperate lake during a weeklong measurement period. Arsenic concentrations were highest during the morning or midday and lowest in the evening. In this work, we explore four mechanistic hypotheses to explain the diel As cycles based on the physical and biogeochemical processes that were investigated during the study. Despite pH being known to control As cycles in rivers, we determined that this mechanism was inconsistent with As dynamics observed in Lake Killarney. Instead, we found that iron and manganese concentrations oscillated simultaneously with As concentrations and, thus, concluded that redox conditions adjacent to the lakebed controlled the near-bed availability of these three elements. However, based on timescale analysis, we determined that biogeochemical processes at the sediment water interface alone could not have led to the daily oscillations in bottom water concentrations. Rather, turbulence from convective mixing was necessary to transport dissolved species from the lakebed into bottom waters. Notably, we saw that the timing and intensity of peaks in convectively-driven turbulence were consistent with observed diel fluctuations in bottom water As. Our results indicate that physical mixing is key in controlling As transport and concentrations on diel timescales within shallow lakes. The daily cycling of redox-sensitive elements in shallow lakes and the potential physical controls on this phenomenon should be considered when designing sampling methods to assess the environmental health and water quality of contaminated sites.
Dataset for: Seasonal patterns of mixing and arsenic distribution in a shallow urban lake
(2022) Fung, Samantha; Horner-Devine, Alexander; Neumann, Rebecca; Hull, Erin; Burkart, Kenneth; Gawel, James
Arsenic, a neurotoxin and carcinogen, is a legacy contaminant in the sediments of many urban lakes and poses health risks to aquatic ecosystems and lake users. Arsenic uptake into the aquatic food web is enhanced in shallow, polymictic lakes compared to deep, seasonally stratified lakes. We present the results of a 17-month field study in Lake Killarney, a shallow, urban lake in Federal Way, Washington, USA, which examines the physical and biogeochemical mechanisms controlling arsenic mobilization and transport from sediment into lake waters, a prerequisite for arsenic uptake into the food web. In Lake Killarney, arsenic mobilization and transport into bottom waters occurred only when stratified conditions and elevated temperatures facilitated deoxygenation of bottom waters. Frequency of lake mixing varied seasonally and controlled the vertical distribution of arsenic in the water column. Convective mixing was the main contributor to elevated vertical turbulent intensity in the water column during periods of high arsenic mobilization, and thus to the upwards transport of arsenic from bottom waters. Maximum near-surface arsenic occurred when the lakebed sediment temperature was elevated and the water column was overturning frequently. This work clarifies the mechanisms that contribute to vertical arsenic transport in shallow lakes and provides a basis for identifying contaminated systems with the physical and biogeochemical conditions that promote transport of arsenic into near-surface water.
Time series data of different sand fraction sediment bed flume experiments
(2021) Han, Zhuochen; Horner-Devine, Alexander R.; Ogston, Andrea; Tian-Jian, Hsu
Data repository: The role of sand fraction in mud-dominant wave-supported gravity flows
(2020-07-18) Han, Zhuochen; Horner-Devine, Alexander; Ogston, Andrea; Hsu, Tian-Jian
We performed laboratory experiments to investigate the influence of sand content on the dynamics of wave-supported gravity flows in mud-dominant environments. The experiments were carried out in an oscillatory water tunnel with a sediment bed of either 1% or 13% sand. Low and high energy regimes are differentiated based on a Stokes Reynolds number Re_Delta ≈ 500. In the low energy regime, the sand fraction influences flow dynamics primarily through ripple formation; no ripples form in 1% experiments, whereas ripples form in the 13% experiments that increase turbulence and the wave boundary layer thickness, delta_m. In the high energy regime, small ripples form in both the 1% and 13% sand experiments and we observe high near-bed suspended sediment concentrations. The influence of stratification on the boundary layer flow is characterized in terms of the gradient Richardson number Ri_g. The flow is weakly stratified inside the boundary layer for all runs and critically stratified at or above the top of the boundary layer. In the lower regime, the sand content reduces the relative influence of stratification in the boundary layer, shifting the elevation of critical stratification, L_B, from approximately 1.3delta_m to 2.5delta_m in the 1% and 13% experiments, respectively. In both sets of experiments L_B ≈ delta_m at the strongest wave energy, indicating a transition to strongly stratified dynamics.
Wave generation of gravity-driven sediment flows on a predominantly sandy seabed
(2018) Flores Audibert, Raul; Rijnsburger, Sabine; Meirelles, Saulo; Horner-Devine, Alexander; Souza, Alejandro; Pietrzak, Julie; Henriquez, Martijn; Reniers, Ad
Wave-supported gravity flows (WSGF) generate rates of sediment flux far exceeding other cross-shelf transport processes, contributing disproportionately to shelf morphology and net cross-shelf fluxes of sediment in many regions worldwide. However, the conditions deemed necessary for the formation of WSGF limit them to a narrow set of shelf conditions; they have been observed exclusively in regions where the seabed consists of very fine-grained sediment and typically co-occur with nearby river flood events. Here we document the occurrence of a WSGF event on a predominantly sandy seabed and in the absence of a preceding river flood. Our measurements confirm that the dynamics are governed by the friction-buoyancy balance observed in other WSGF, but also reveal how grain size influences the structure and transport rate of WSGF. We observe that, although sufficient concentrations of fine sediment are required for their formation, WSGF can form in mixed grain-size environments and can transport high concentrations of sand. The occurrence of WSGF on a sandy seabed suggests that they may occur under a much wider range of conditions and, given the global prevalence of sandy shelves, they may be a more frequent and more ubiquitous feature of shelf dynamics than previously thought.
Lobe-Cleft Instabilities on a river plume front
(2016-05-12) Horner-Devine, Alexander; Chickadel, Chris
These data accompany a paper submitted to Geophysical Research Letters. The abstract is below. Gravity currents represent a broad class of geophysical flows including turbidity currents, powder avalanches, pyroclastic flows, sea-breeze fronts, haboobs and river plumes. A defining feature in many gravity currents is the formation of three-dimensional lobes and clefts along the front and researchers have sought to understand these ubiquitous geophysical structures for decades. The prevailing explanation is based largely on early laboratory and numerical model experiments at much smaller scales, which concluded that lobes and clefts are generated due to hydrostatic instability exclusively in currents propagating over a non-slip boundary. Recent studies suggest that frontal dynamics change as the flow scale increases, but no measurements have been made that sufficiently resolve the flow structure in full-scale geophysical flows. Here, we use thermal infrared and acoustic imaging of a river plume to reveal the three-dimensional structure of lobes and clefts formed in a geophysical gravity current front. The observed lobes and clefts are generated at the front in the absence of a non-slip boundary, contradicting the prevailing explanation. The observed flow structure is consistent with a alternative formation mechanism, which predicts that the lobe scale is inherited from subsurface vortex structures.
Wave-Supported Gravity Currents Project: Experimental data from UW Sediment-Wave tank. Zero slope
(2015-03-01) Hooshmand, Abbas; Horner-Devine, Alexander
Paper abstract: We present results from laboratory experiments in a wave flume with and without a sediment bed to investigate the turbulent structure and sediment dynamics of wave-supported mud layers. The presence of sediment on the bed significantly alters the structure of the wave boundary layer relative to that observed in the absence of sediment, increasing the TKE by more than a factor of 3 at low wave orbital velocities and suppressing it at the highest velocities. The transition between the low and high-velocity regimes occurs when Re_delta = 450, where Re_delta is the Stokes Reynolds number. In the low-velocity regime (Re_delta < 450) the flow is significantly influenced by the formation of ripples, which enhances the TKE and Reynolds stress and increases the wave boundary layer thickness. In the high-velocity regime (Re_delta > 450) the ripples are significantly smaller, the near-bed sediment concentrations are significantly higher and density stratification due to sediment becomes important. In this regime the TKE and Reynolds stress are lower in the sediment bed runs than in comparable runs with no sediment. The regime transition at Re_delta=450 appears to result from washout of the ripples and increased concentrations of fine sand suspended in the boundary layer, which increases the settling flux and the stratification near the bed. The increased stratification damps turbulence, especially near the top of the high-concentration layer, reducing the layer thickness. We anticipate that these effects will influence the transport capacity of wave-supported gravity currents on the continental shelf.

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