1,720,989 research outputs found
IMPACTS OF LATERAL SPREADING AND UPSTREAM CONDITIONS ON BUOYANT RIVER PLUMES: MIXING, STRUCTURE AND PLUME DYNAMICS
No full textThesis (Ph.D.)--University of Washington, 2012This dissertation investigates the nature of physical processes associated with buoyant river plumes under the impacts of rotation, lateral spreading and upstream conditions. Two sets of laboratory experiments, one focused on the whole rotating buoyant plume, and the other zoomed into the near field plume, are studied. When lighter fluid is released into denser water, buoyancy causes the lighter intruding flow to run on top of the ambient water. This baroclinic flow propagates both in the offshore direction due to strong momentum from the inflow source and laterally in the alongshore direction because of the horizontal pressure gradient. This energetic region is the so-called near-field region where the plume behaves like a buoyant jet and is characterized by high momentum and strong stratification. Under the influence of the Coriolis force, the density driven flow is then guided along the coast, forming an anti-cyclonic bulge (in northern hemisphere). In this large scale region, called far-field region, the flow is more geophysical and less energetic. The transition between the two regions is the mid-field region in which the fluid transfers from an energetic flow into a geophysical current. The first laboratory experiment is designed to simulate the dynamics of two ad- jacent coastal river plumes in a rotating reference frame. The plumes are generated on a rotating table using two identical fresh water inlets, with blue and red dye indicating upstream and downstream river flows, respectively. We successfully calculate the depth field for the combined two-plume system and differentiate between the two plumes using a two-dimensional calibration map. With the upstream coastal current acting as the ambient condition, the downstream plume bulge does not reach a steady condition. The downstream bulge is pulled into the upstream bulge, forming a larger re-circulating bulge which then becomes unstable. The coastal current transport can be calculated by assuming a geostrophic cross-balance balance with an empirical coefficient &alpha = 0.6. The impact of lateral spreading on mixing was investigated for laboratory scale non-rotating stratified-shear plumes. The experiment is begun with the release of a vertical wall of freshwater and the simultaneous activation of a pump which supplies freshwater to the filling basin. Velocity and density fields are obtained using the combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) method, while the lateral spreading rate is determined using the optical thickness method (OTM). The vertical mixing is parameterized by the turbulent buoyancy flux, which is calculated using a control volume approach. Both the lateral spreading rate and mixing are related to inflow Fr, where the lateral spreading rate decreases with Fri while mixing increases with Fri. By comparing the mixing in the laterally confined and unconfined cases, we observe that although the lateral spreading significantly modifies the plume vertical structure, it does not change the local turbulent buoyancy flux. On the other hand, the lateral spreading increases the horizontal area over which mixing occurs and as a result it increases the net dilution of river water at a fixed distance from the river mouth. Unlike the local mixing process, we observe that the plume structure is signif- icantly different in the laterally confined and unconfined plumes. We hypothesize that the laterally spreading river plume might be a source of non-linear internal solitary waves with trapped cores. Such waves are commonly observed in fjords, straits and coastal ocean, where a strong shear-stratified flow meets dramatical topography changes. A series of small scale Kelvin-Helmholtz instability billows are generated along the edge of the large-scale waves, propagating downstream and finally breaking at the wave trough. This phenomenon highly increases the mixing and entrainment at the edge and trailing edges of the wave while it inhibits the mixing at the frontal side of the wave
Hydrodynamics and sediment transport in the Duwamish River Estuary
No full textThesis (Ph.D.)--University of Washington, 2020Nearly two-thirds of the world’s major cities have been built around estuaries. As populations have grown, many estuaries have been modified, impacting their hydrodynamic regime. Urbanized estuaries, which are typically among the most highly modified, are also often contaminated due to the proximity of intense industrial activity, motivating the need to better understand their hydrodynamic and transport processes in order to better inform current management and future modifications. This study focuses on hydrodynamics and sediment transport in the Duwamish River estuary, a highly engineered and contaminated tidal salt wedge estuary located next to downtown Seattle, WA. Hydrographic measurements of salt wedge structure and dynamics spanning a 20-fold range of river discharges are used to investigate the seasonal variability in the strongly stratified Duwamish River estuary. The effect of river discharge on salt wedge length, stratification, pycnocline thickness, and intratidal differences in salinity structure are evaluated. The salt wedge decreases in length and becomes more stratified as river discharge increases. The ebb and flood responses to increasing discharge are markedly different; the flood phase structure shows little seasonal dependence, while the ebb structure changes due to an internal hydraulic response that is strongest at two severe lateral constrictions. This response varies with the net barotropic forcing, which is controlled by river discharge. The asymmetry between the flood and ebb structure is explained using a two-layer hydraulic framework, although spatially, tidally, and seasonally variable vertical mixing also modifies the structure. As a result of the persistent stratification and dominance of hydraulic dynamics, channel constrictions influence circulation and transport during ebb tides. These dynamics also generate a flood/ebb asymmetry in salt wedge structure and circulation that is modulated seasonally through discharge. We hypothesize that these dynamics lead to a seasonally modulated residual circulation. In 2001 the lower Duwamish Estuary was added to the national list of Superfund Sites. A Record of Decision was issued in 2014 recommending a $342 million remedial plan for the remaining 412 ac, 57% (235 ac) of which relies on the burial of contaminated sediment by cleaner sediment originating from upstream. We re-examine existing total suspended sediment load data to understand the magnitude and timing of suspended sediment delivery to the estuary. We use three existing datasets, with data ranging from 1966 to 2019, and develop new rating curves using a piecewise approach. The new rating curves have improved R2 values compared with previous attempts. A combined-fit rating curve accurately estimates 1960s and 2010s annual loads, whereas no single rating curve generated here or previously is appropriate for the entire period from 1961 to present. Using the combined-fit, we find that most of the 11.7±4.8à 104 mt/year long term mean annual load occurs from November-May during high discharge events. There is evidence that modern annual load estimates may be closer to 9.4±4.4à 104 mt/year, down from a 1960s annual load of 16.2±5.5à 104 mt/year. Estimates of fine and coarse loads derived from modern data indicate that 70% of the average annual total load is composed of fine sediment. We present new measurements of SSC throughout the estuary from 2011-2013 in the form of transects and bottom-mounted moorings, which we use to investigate sediment sources in the estuary, transport pathways, the role of local resuspension, and relevant timescales. Upstream sediment is delivered to the estuary in only a handful of storm events that last on the order of days. Gravitational settling transfers upstream sediment to the salt wedge during loading events. As the salt wedge retreats during ebb, retained material is resuspended and re-entrained into the upper layer. The upper layer exports some material seaward, but some re-settles into the salt wedge. The flooding salt wedge re-entrains and transports fluffy material landward. Our measurements show that retained material is reworked in the estuary for three to four months after a delivery event until it is depleted through seaward export and consolidation
The role of sand in Wave-Supported Gravity Flows over primarily muddy seabeds
No full textThesis (Ph.D.)--University of Washington, 2021Wave-supported gravity flows (WSGF) are one of the most crucial processes contributing to sediment transport across continental shelves with gentle bottom slopes. This dissertation studies the mechanisms of WSGF over primarily muddy shelves through laboratory experiments in an oscillatory water tunnel with a sediment bed of either 1\% or 13\% sand fraction. Physics behind this includes sand fraction effects on the dynamics of WSGF under equilibrium state, the role of sand in controlling sediment suspensions under equilibrium state, and key factors that affect the dynamics of WSGF during the bed adjustment period before the equilibrium state. First, sand fraction effects on the dynamics of wave-supported gravity flows in mud-dominant environments are investigated. Low and high energy regimes are differentiated based on a Stokes Reynolds number . In the low energy regime, the sand fraction influences flow dynamics primarily through ripple formation; no ripples form in the 1\% sand experiments, whereas ripples form in the 13\% experiments that increase turbulence and the wave boundary layer thickness, . 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 . 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 energy regime, the sand content reduces the relative influence of stratification in the boundary layer, shifting the elevation of critical stratification, , from approximately 1.3 to 2.5 in the 1\% and 13\% experiments, respectively. In both sets of experiments at the strongest wave energy, indicating a transition to strongly stratified dynamics. Second, sand particles control the sediment suspension over mud-dominant environments. High near-bed concentration and concentration gradient happen when the sand fractions suspend from the bed. The suspension of sand fraction in bed sediment mixture leads to the formation of a high suspended-sediment concentration layer. A modified sediment suspension criterion is created based on \citet{rijn1984sediment} and using the median particle size of only the sand fraction in the bed and is successful in predicting the necessary suspension that contributes to wave-supported gravity flow formation. This modified sediment suspension criterion provides a limited condition for the formation of wave-supported gravity flows. Finally, experimental results during bed adjustment periods are presented. The total bed adjustment before the equilibrium can be divided into three stages. Stage I, the initial adjustment stage, occurs in the first 30 - 40 min, where the non-uniformity of the bed introduces sediment redistribution and transient ripples start to form, and the bed elevation might increase or decrease. Stage II, the decreasing erosion stage, occurs during 30 - 90 min for 1\% sand experiments or 40 - 105 min for 13\% sand experiments, where the bed erosion rate is near a constant value. Stage III, the near-equilibrium stage, occurs in the final 15 min, where the bed erosion rate drops to near zero, upward transport flux is balanced with the settling flux, stratification is induced at the top of the wave boundary layer, and the system reaches an equilibrium state. Two different controlling mechanisms of the bed erosion and deposition are discussed: bed control, and stratification control. In our experiments, bed armoring due to the surface coarsening of the bed is observed, which decreases the bed erodibility. Transient ripples are observed in the total bed adjustment period, which elevates near-bed shear stress. The formation of transient ripples might slightly increase bed erosion. Stratification plays a limited role in the near-bed sediment system during the bed adjustment period but becomes important when the equilibrium state is reached
The dynamics of cross-shore sediment transport in the Rhine region of freshwater influence
No full textThesis (Ph.D.)--University of Washington, 2018River plumes are critical to the exchange of suspended particulate matter (SPM) between nearshore regions and shelf seas. These exchanges contribute fundamentally to the health and function of the coastal ecosystem, and have important implications for the overall sediment and biomass budgets. The physical processes that determine these exchanges depend on the dynamical characteristics of the river plume, which are largely affected by the freshwater input, water depth and the tidal flow, among many other factors. Here, we present field measurements and numerical modeling of hydrodynamics and sediment transport in order to investigate the dynamics of cross-shore exchange processes along a shallow river plume system. Measurements from moorings and bottom frames along the Rhine region of freshwater influence (ROFI) are used to evaluate the importance of baroclinic (stratified) and barotropic (unstratified) dynamics on the cross-shore transport of fine sediments, in a site located 10 km north of the river mouth. These measurements allowed for the evaluation of sediment transport dynamics over a wide range of stratification, wind, wave and tidal conditions. Both barotropic and baroclinic processes are found to be relevant for the cross-shore transport of fines at depth. The observations suggest that wind and wave-driven transport during storms tends to move fine sediment offshore, while calmer, more stratified conditions move it back onshore. Data collected during an intense 2-day storm are used to document the occurrence of a wave-supported gravity flow (WSGF) in a shallow inner shelf site along the Rhine ROFI. These observations are the first to document a WSGF on a predominantly sandy environment; previous observations had been restricted to muddy shelf deposits with very fine particle size distributions. The observed WSGF dynamics support previous studies regarding the use of a friction-buoyancy balance, and suggest that the same balance can be used on sandy seabeds. The offshore transport associated with the WSGF was much higher than other modes of transport, such as the suspended transport in the upper water column or the bedload, highlighting the importance of these events to cross-shelf transport and morphological evolution on inner-shelf regions. The occurrence of a WSGF under conditions unique from previous observations suggest that WSGF may occur in a much wider range of shelf locations than previously thought. In the Rhine, the tidal flow interacts with the cross-shore density gradients to generate tidal straining. The influence of tidal straining in the generation of a turbidity maximum zone (TMZ) along the Rhine ROFI is investigated using idealized numerical simulations. Tidal straining leads to cross-shore sediment convergence and the formation of a nearshore TMZ, that is detached from the coastline and confined to the near-bed region. Subtidal landward sediment fluxes are created by asymmetries in vertical mixing between the stratifying and de-stratifying phases of the tidal cycle. Model simulations show the development of a coastal TMZ for a wide range of horizontal density gradients, latitudes and settling velocity of bed sediments, suggesting that these phenomena are not limited to the Rhine ROFI. A parameter space for the occurrence of tidal straining and a coastal turbidity maximum is proposed in terms of a horizontal Richardson number and a Stokes number. Lastly, near-bed turbulence and bedform measurements are used to investigate the influence of bed roughness in the estimation of bed shear stress under the influence of large waves and currents. Direct measurements of the near-bed Reynolds stresses are compared to bed stress estimates obtained from a 1D bottom boundary layer model that accounts for wave-current interaction. Model-derived bed stresses compare well with measurements when field measurements of bedform dimensions are incorporated into the model calculations. The use of standard bedform predictors based on bulk wave properties results in a severe overestimation of the bed stresses at this particular site. We find that the combination of a time-dependent bedform evolution model and a 1D wave-current boundary layer model provides a simple approach that allows improved bed stress estimates in cases where bedform data is not available. Estimates of bed stresses and measured vertical turbulent sediment fluxes are then incorporated into a linear erosion formulation to obtain field-based estimates of the critical stress for erosion and resuspension constant
Remote Sensing of Water Surface Variability Near River Mouths
No full textThesis (Ph.D.)--University of Washington, 2020Rivers discharge approximately 1,000,000 cubic meters per second of freshwater to the oceans yet less than 60% of this discharge is measured at the river mouth. The standard method of measuring river discharge is to use in-situ current meters that are expensive to deploy and not easily used at remote locations. Remote sensing of river discharge could address the problem of global coverage and is becoming an active area of research due to the recent and upcoming improvements of the resolution of satellite altimeters, including the Surface Water and Ocean Topography (SWOT) altimeter. Remote measurements of river discharge have been made from altimeter type water surface elevation data before but only far upstream where the flow is uniform. In order to accurately quantify the amount of freshwater being discharged into the ocean the discharge measurement must be made at the river mouth where the flow is not uniform and the relationship between water surface elevation and discharge is not well understood. This dissertation uses airborne Lidar data, numerical model output, and new analytical expressions to characterize the relationships between river discharge, water surface elevation, tides, and bathymetry at and near the river mouth. It is found that the slope of the water surface elevation at the mouth of the Columbia River changes sign over the tidal period and that wave amplification increases with river flow velocity. Numerical model output of an idealized river discharge describes the shape of the water surface elevation near the river mouth under both low and high discharge conditions. During high discharge conditions an offshore ridge develops on the water surface whose location and height are related to river discharge. Equations are derived to calculate the location and height of the ridge and relate these signatures to the river discharge. These relationships are then used to predict whether the upcoming SWOT altimeter will be able to detect the ridge for flow from the Mississippi, Connecticut, and Columbia Rivers. This research provides an understanding of how water surface elevation changes at the river mouth are related to tides, waves, and discharge and quantifies which of these signals will be observable by SWOT. Finally, remotely sensed infrared videos of the surface are used to infer the bottom drag coefficient in a river. The surface turbulent kinetic energy is extracted from the videos and an equation is derived to calculate the river bottom drag coefficient from the turbulent kinetic energy. The equation is tested with field data from six locations in the Snohomish River and the drag coefficients calculated compare well with drag coefficients calculated from in situ velocity data. In the future, the turbulent kinetic energy equation for the drag coefficient and remote sensing techniques presented here could be applied to data from unmanned aerial vehicles and used in the surf zone
Feedbacks between river morphodynamics and overbank flooding
No full textThesis (Ph.D.)--University of Washington, 2023River morphodynamics (the way rivers adjust their shape and form) can affect overbank flooding if changes to cross-sectional area or roughness reduce the flow conveyance capacity of the channel. Extreme floods can also cause drastic adjustments to river morphology on relatively short timescales. The potential simultaneity of these processes raises the question: how do flood dynamics and river morphology co-evolve? Additionally, are river morphodynamics a substantial contributor to flood hazards? Existing research on channel adjustment and recovery during a flood hydrograph has been limited by a paucity of field data recording river bathymetry during peakflows. Additionally, the computational expense of morphodynamic modeling has challenged the application of models to investigate feedbacks between morphodynamic river adjustments and flood hazard. This thesis seeks to fill the gaps in scientific knowledge about feedbacks between morphodynamics in flooding by (I) examining the importance of river morphodynamics in modifying flood hazards; (II) explaining causes of spatial variation in morphodynamic response to floods; and (III) investigating interactions between flood hydraulics and river morphodynamics during a flood event in a location with persistent overtopping during peak flows. Part I investigates long-term changes to river channel conveyance in northwest Washington State, U.S., and compares the relative importance of these to shifts in moderate flood streamflow using 50 long term river gaging datasets. River conveyance is unsteady in most rivers in western Washington, but its importance for modifying flood risk depends on the style of conveyance response. Conveyance responses can be linear, oscillating, punctuated by sudden sediment-supply events or influenced by flow regulation. The relative behavior and importance of conveyance and streamflow variability depends on flow regulation; moderate flood streamflows have increased in unregulated rivers, but this trend is suppressed and/or reversed in regulated rivers. Flow regulation does not necessarily reduce total flood hazard, because downstream channel conveyance losses can exceed reductions in flood flows. Part I concludes that channel conveyance unsteadiness is an important modifier of total flood hazard variability and incorporating this factor may improve flood risk predictions. Part II explains the spatial variation of river bed response during floods and recovery during low flows in relation to floodplain geometry and topography using 20 years of bi-weekly bathymetry data from the Waal River, an engineered channel in the Netherlands. Bed response and recovery to peak flows are a result of floodplain geometry and topography that forces flood flows to enter or exit the main-channel including: 1) streamwise changes in floodplain width, and 2) topographic routing of floodplain flow that causes flow to plunge into and out of the main-channel during floods. The latter factor is primarily important for spatial variation in main-channel bed response and recovery to floods. Where floodplain flows enter the main-channel, erosion (deposition) occurs during floods (low flows) and where floodplain flows exit the main-channel, deposition (erosion) occurs during floods (low flows). Part II concludes that bed elevation within and between flood events may be understood by floodplain topography and geometry that results in flow exchange between the main channel and floodplain. Part III applies a hydro-morphodynamic model to analyze hydraulic and morphodynamic feedbacks during the November 2021 flood in the Nooksack River, Washington State–a system with persistent overbank flooding during peak flows at an historic avulsion node. Cross-sectionally averaged patterns of modeled bed elevation change suggest that meter-scale adjustments in bed elevation occurred during the flood. Bed elevation changes in the main-channel are commonly located where flow spatially accelerates or decelerates during the flood; in particular, flow deceleration is observed at locations of overbank flooding. At the historic overflow location, bed deposition co-occurs with abrupt flow deceleration related to overbank flooding. Part III demonstrates that the hydrodynamics of overbank flooding can cause channel adjustments that affect channel conveyance capacity for future floods. These results augment our understanding of the importance of river morphodynamics in modifying overbank flooding and the feedbacks between overbank flooding and river bed response. The thesis concludes with a suite of possible questions and future research directions relating to: 1) the preferential style of morphodynamic adjustment to peak flows, 2) river bed response to climate variability, 3) feedbacks between floods, sediment supply, and channel response, and 4) compound channel structure as it relates to morphodynamic response to floods. Overall, this work furthers our understanding of fluvial response to floods with implications for improving flood safety of river-bounding communities
Wave-supported gravity currents in continental shelves
Get PDFThesis (Ph.D.)--University of Washington, 2015-12Wave-supported gravity currents (WSGC) are one of the most important processes causing cross shelf sediment transport. This dissertation studies the physics behind WSGC in continental shelves using experimental observations. An analytical model for predicting sediment transport due to these events on the shelf is proposed and validated. First, the presence of the sediment bed and its effects on the flow structure is investigated in detail. The presence of sediment on the bed significantly alters the structure of the wave boundary layer relative to that observed in the absence of sed- iment, increasing the turbulent kinetic energy (TKE) by more than a factor of three at low wave orbital velocities and suppressing it at the highest velocities. In the low velocity regime, the flow is significantly influenced by the formation of ripples, which enhances the TKE and Reynolds stress and increases the wave boundary layer thick- ness. In the high velocity regime, the ripples are significantly smaller, the near-bed suspended sediment concentrations are significantly higher and density stratification due to suspended sediment concentration 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 transition between regimes 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 stratification near the bed. Second, experimental results for bulk, gradient and flux Richardson numbers are shown and proper scaling is proposed. It is shown that a bulk Richardson number using the maximum buoyancy frequency has a critical value of 1/4 , which is the value theoretically predicted. However, a bulk Richardson number using the average buoy- ancy frequency and a prescribed sediment concentration profile has a critical value of 0.03. This value is nearly one order of magnitude smaller than the assumed critical number of 1/4 from steady tidally driven currents. Third, WSGCs are modeled using an analytical solution to the Navier-Stokes equations, and validated using experimental results. The velocity field is solved for zero slope condition and a down-slope velocity due to gravity is added for slope conditions. The final velocity profiles agrees very well with experimental down-slope velocities. The solution of the model is provided for laminar and turbulent conditions, and for exponential and a constant suspended sediment concentration profiles in the mud layer. Additionally, the criteria for laminar or turbulent regimes are proposed based on the Reynolds number of surface waves. Finally, experimental results for suspended sediment concentration profiles, ve- locity, turbulence and wave boundary layer are compared to field observation and numerical simulations of WSGCs. The predicted down-slope velocity of a layer sus- pended and moved by surfaces waves is 1-2 orders of magnitude smaller than that observed in the field correlated with similar wave conditions
Long-term sediment yields from glacierized basins on Mount Baker, 1947-2015
Get PDFThesis (Master's)--University of Washington, 2022Where glacierized watersheds are adjacent to population centers, sediment-laden rivers may catastrophically flood and the frequency of such flooding has been predicted to increase along with increased sediment supply. Despite these concerns, upland sources of sediment and sediment connectivity between proglacial uplands and downstream river plains are poorly understood. We utilize automated Structure from Motion software to process historical aerial imagery and, in tandem with publicly available LIDAR data, produce a DEM time series with coverage that spans from 1947 to 2015 in glacierized watersheds on Mount Baker, a stratovolcano in Washington State. We measure basin-wide sediment yields in 10 distinct basins and quantify the relative sediment contributions of different upland erosion processes. Sediment yields are correlated with lithology, Little Ice Age moraine slope, upslope contributing area, and glacier retreat. We found that upslope contributing area and moraine slope are the strongest predictors of basin-wide sediment yield, and stream channel slope is a poor predictor of sediment yield, indicating that debris flows originating from glacier moraines are a primary erosion mechanism in proglacial basins. Our estimates of proglacial sediment yields account for 10 to 56% of estimated total sediment load in the Nooksack river, which drains multiple basins on Mount Baker, and sediment input from proglacial basins exceed contributions from erosion of glacial and volcanic terraces. On intra-decadal timescales, the irregularity of hillslope erosion events disconnect proglacial sediments from the downstream system. On decadal timescales, infrequent debris flows bypass proglacial stream channels and carry sediment downstream of proglacial limits. Our findings indicate that pro-glacial sediments may contribute a significant portion of riverine sediments where upland topography and lithology are favorable and when the climate is relatively wet
Wind and wave-influenced mixing and dynamics in the Fraser River plume
No full textThesis (Master's)--University of Washington, 2018-08Rivers play a large role in the transport of matter between ecosystems by connecting interior regions with lakes and oceans. River outflows, or plumes, constitute a significant delivery mechanism of pollutants, sediment, and nutrients to the coastal environment. Such plumes are common along the worlds coastlines, but are poorly understood and difficult to sample due to their complexity. The effects of external forcing (such as winds and waves) on river plumes is particularly understudied, but has been shown to play a role in the behavior of the plume. The Fraser River (the site of proposed and existing oil infrastructure) is a key system to understand, as its mouth is near both a major population center (Vancouver, BC) and unique coastal ecosystems. This study presents observations of plume mixing and stream-normal momentum balances in the Fraser river plume using Lagrangian surface drifters. The Strait of Georgia commonly experiences calm to moderate winds from the Southeast and strong storm winds from the Northwest in the winter, and there are differences in plume behavior between these conditions. Under SE winds, the plume thins, spreads, and turns to the right (North) upon exiting the river mouth, mixing intensely for a short period of time. This process is dominated by a balance between stream-normal pressure gradient, Coriolis, and rotational acceleration, as previous studies have shown. Under NW winds, the plume stays thicker, narrower, and flows directly across the Strait while forming a lateral front on its northern side, mixing at a slower rate for longer. Different momentum terms dominate under this wind condition: the stream- normal balance pits Coriolis and rotational acceleration pressure gradient against interfacial shear stress, wind stress, wave radiation stress gradient, and ambient current body force. The two configurations of the plume show that variable winds can have a substantial impact on the shape of the plume, which can lead to changes in the sea surface anomaly associated with the dome of freshwater exiting the river mouth. Additionally, while spreading causes the plume to mix intensely under SE winds, the net mixing under NW winds is larger due to the longer extent of the plume. This indicates that large spreading is not always necessary for substantial plume mixing. Understanding the conditions that change plume mixing and behavior is a critical step in describing the pathways of pollutants, sediment, and nutrients as they enter the coastal ecosystem through a river plume
The effects of mixing on arsenic mobilization and transport in a shallow lake
No full textThesis (Ph.D.)--University of Washington, 2023Arsenic, a heavy metal, is a neurotoxin and carcinogen that has been released into the environment at accelerated rates due to human activities. Arsenic has accumulated in the sediments of many lakes around the world and continues to have negative impacts on lake ecosystems and lake users. The negative effects of arsenic are magnified in shallow lakes, where spatial proximity between high arsenic concentrations and biota allow for the uptake of elevated levels of the arsenic into the aquatic food web. In this dissertation, I examine the basis of this phenomenon by examining how physical factors affect biogeochemical conditions and processes in a shallow, urban lake. In Chapter 2, I investigate lake mixing and arsenic distribution in Lake Killarney on a seasonal timescale during 2018 and 2019. We found that stratification and mixing exhibited seasonal patterns and greatly affected the fate of arsenic within the lake. Arsenic built up in lake bottom waters during the early and mid-summer when the water column remained stratified for week to multiweek periods. Warmer sediment temperatures during early summer, compared to the springtime, were shown to expediate the production of reducing conditions in the bottom water, a prerequisite for the buildup and maintenance of mobile arsenic in this region. Stratification was weaker and mixing occurred more frequently in the late summer. Frequent lake mixing during the late summer led to a more homogenous distribution of arsenic throughout the lake water column, with elevated surface water concentrations and diminished bottom water concentrations compared to early summer conditions. Thermal convection, rather than wind, was the main mixing force during the summertime in Lake Killarney and was responsible for the vertical transport of arsenic through the lake water column. This work provides a mechanistic understanding for why contaminated, polymictic lakes have high ecosystem and human health risks and underscores the need to prioritize these systems in management efforts.
In Chapter 3, we present observations of repeated diel cycles in bottom water arsenic concentrations. Arsenic concentrations were highest in the early morning to midday and lowest in the evening. Bottom water manganese and iron concentrations exhibited diel signals that coincided with those in arsenic, demonstrating that the fluxes of these three metals were interconnected and likely caused by redox processes at the lakebed. Redox conditions were thus determined to control near-bed availability of arsenic. However, timescale analysis illustrated that redox processes occurring at the sediment water interface could not solely have led to the observed diel cycles in bottom water metals. Significantly, convective mixing was shown to occur nightly and create spikes in vertical turbulence that overlapped with periods when arsenic concentrations were increasing. Estimates of turbulent diffusivities were used to calculate an approximate transport time of arsenic from the lakebed to the bottom water and yielded results that could reasonably explain the observed diel arsenic cycles. Although short term oscillations in arsenic concentrations have been documented in rivers, there is a paucity of studies describing and investigating this phenomenon in lentic systems. Consequently, this study provides an important example of diel cycling in lakes and presents an explanation of the mechanisms behind these cycles. Our findings illustrate the need for lake sampling to include methods that integrate metal concentrations over multi-day periods in order to attain accurate representations of contaminant levels in lake water and the potential effects on biota and lake users.
Finally, in Chapter 4, I expand on the findings in Chapter 2 by investigating the interannual variability in arsenic concentrations in the lake water and biota between the summers of 2018 and 2019. Mixing frequency varied between the two study years and was found to be a key control on aqueous and biological arsenic concentrations. Specifically, less frequent mixing caused a two-fold increase in arsenic concentrations in lake bottom water, phytoplankton, and zooplankton. Summertime meteorology did not differ significantly between the two years; although meteorology did have a prominent effect on surface thermal structure in both summers, surface stratification was transient and had minimal implications for the release of arsenic into lake bottom waters. Instead, bottom thermal structure, which was set by sediment temperature and the resulting sediment – water temperature difference, was the most important factor in lake mixing frequency and, thus, on the fate of arsenic diffusing upwards from sediment porewaters. Our findings demonstrate that lake mixing behavior and the fate of contaminants can change significantly from year-to-year, highlighting the necessity of understanding the ranges of these processes for effective lake management
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