Gyres and Seiches in a Large and Shallow Lake

Gyres and seiches are two prominent features of lakes. Gyres largely transport sediments, nutrients, and algae in the horizontal direction. Seiches, on the other hand, can contribute to the vertical mixing in lakes. Theoretical analysis, statistical methods, and numerical models are used to investigate gyres and seiches in Lake Okeechobee, the largest subtropical/tropical lake in North America. The lake has a 1,730-km2 surface water area, a typical length of more than 50 km, and a mean depth of 3.2 m. Both the Empirical Orthogonal Function (EOF) method and the numerical model results indicated that lake circulation is typically dominated by a two-gyre pattern, especially in the winter. The northwest wind or southeast wind leads to a cyclone (a counterclockwise rotation gyre) in the southwest and an anticyclone (a clockwise rotation gyre) in the northeast. Because the mean velocity field in the lake is very weak, the first two EOF modes play an important role in lake transport. The mechanism of gyre formation in the lake is clearly explained in a theoretical analysis. Power spectra analysis on measured and modeled water elevations at four stations revealed that Lake Okeechobee has a seiche signal of 5 hours or so. The seiche range is typically around 10 cm. Results from the theoretical analysis, power spectral analysis, and numerical modeling all agree with each other very well. The findings in this study should be useful to understand the lake processes, to guide field data collection programs, and to assist decision making on lake management.     

Physicochemical gradients and water fluxes between Nyanza Gulf and main Lake Victoria,East Africa: Tracing dynamics of gulf-main lake interaction

Variability in physico-chemical water column structure along a gradient between Nyanza Gulf and Lake Victoria was characterized, and the exchange between the two basins estimated. Specific conductivity (SC) measurements between March 2005 and March 2006 identified three morphologically distinct zones, the inner-gulf, the connecting Rusinga Channel and the main lake basin. The relatively shallow, wide and river-influenced inner-gulf had higher turbidity and SC compared to the channel and the main lake. The deep, narrow and more wind-exposed mid-channel area was significantly cooler than the rest of the study areas. Observational data revealed “density-driven” underflow of the cooler channel water into the main lake and compensatory surface flow into the gulf as well as seasonal patterns of gulf-main lake exchange in relation to variation in tributary input. The wind field was dominated by a diurnal lake-land breeze which resulted in wind-driven diurnal reversal of water flow in and out of the gulf, where the constriction of flow within the channel induced strong currents. The exchange between the gulf and the main lake was estimated using a box model, with SC as a conservative tracer. A net annual residual water flow of 35?m³?s^?1 occurred from the gulf into the main lake although during months with low tributary discharge the flow was towards the gulf. The daily flow in and out of the gulf resulted in relatively high exchange flux compared to residual flow across the Rusinga Channel leading to the development of the well-defined physicochemical gradient along the Rusinga Channel.     

Circulation and mixing in Sodus Bay due to water exchange with Lake Ontario

Embayments on large lakes may be affected by water exchange with the lake that, in turn, impact water quality in the embayment. In this study we examine the influence of hydrodynamic factors that may play a role in controlling water quality in Sodus Bay, the largest enclosed embayment on the U.S. shoreline of Lake Ontario. The motivation for this study was the occurrence of a blue-green algal (cyanobacterial) bloom in 2010, and the need to understand the factors that influence this and other water quality issues. A hydrodynamic model with high spatial and temporal resolution was applied and calibrated to field data collected in a detailed sampling program in 2013. The model, along with field data collected over several additional years, was then used to develop a comprehensive understanding of the hydrodynamic impacts on physical conditions in the bay. A primary result of this process is a determination of the importance of flow exchange between Lake Ontario and Sodus Bay, particularly during lake upwelling periods that cause colder water to enter the bay as an underflow. Hydrodynamic features identified to have played a potential role in the bloom formation of 2010 include a lake upwelling event, strong stratification, and relatively warm conditions throughout much of the summer. Lack of continuous monitoring in 2010 precludes a specific comparison of the model and data when the bloom formed, but the model clearly shows conditions that would have led to a bloom, assuming other preconditions were in place. The customized hydrodynamic model provides opportunities for future ecological modeling, hypothesis development, and what-if scenario testing. This study reinforces the importance of hydrodynamic interactions between lakes and embayments, and their impacts on water quality.     

Evaporation from Lake Superior: 2: Spatial distribution and variability

Evaporation is a critical component of the water balance of each of the Laurentian Great Lakes, and it is expected that because of their shear size, evaporation cannot be spatially or temporally uniform. Despite this, examples of spatially distributed estimates of evaporation in the scientific literature are rare for most of the lakes and non-existent for Lake Superior. Direct measurements of evaporation taken at an offshore site on Lake Superior from June 2008 to October 2010 were used with concurrent satellite and climate model data to extrapolate evaporation measurements across the entire lake. Evaporation rates, large scale forcing mechanisms, and spatial patterns and variability are presented. Spatial patterns of evaporation tend to follow synoptic-scale air masses traveling over the lake. While most evaporation occurs during relatively short term events, these episodes tend not to occur in isolated locations on the lake, but are spatially widespread. The exceptions to this rule are during periods of a stable atmosphere and low evaporation, and when a transient ice cover can limit evaporation from some areas.     

基于GPS的洪泽湖水流流场质点示踪及数值模拟

针对不同水文气象条件下湖泊真实流场描述的难题,通过在洪泽湖主要入口分别投放GPS示踪器,实现了对湖泊表层水流轨迹、方向的实时监测,形成洪泽湖初步流场图。野外示踪实验结果表明:流经湖泊中心区域的水流运动轨迹均为自西向东的曲线;入湖流量较小时水流难以进入湖泊中心流场,入湖流量较大时流速空间分布不均,流场较复杂;小流量入湖条件下水流运动受到湖泊表面风场的影响,流量较大时水流流动的主要影响因素为湖泊自身的流场。同时,构建了不同入湖流量条件下的洪泽湖二维水动力数值模型,水动力模型模拟的流场分布基本符合水流质点GPS示踪实验的结果,两者拟合较好。本次研究揭示了洪泽湖的真实流场,基于GPS的示踪方法可用于分析河流湖泊的水动力空间变化,为泥沙、污染物质的输移和扩散提供了重要参考依据。     

Impacts of wind waves on sediment transport in a large,shallow lake

Lake Okeechobee is a large, shallow subtropical lake, located in south Florida. Over the last several decades, Lake Okeechobee has experienced accelerated eutrophication due to excessive phosphorus loads from agricultural run‐off. Recycling of phosphorus from bottom sediments through resuspension is critical to addressing eutrophication of the lake and for water quality management. The present study investigates the impacts of wind waves on sediment transport in Lake Okeechobee, using measured data and the Lake Okeechobee Environmental Model (LOEM). The LOEM was fully calibrated and verified with more than 10 years of measured data in previous studies. Analysis of the measured data indicates significant wave height (SWH) and suspended sediment concentration are closely correlated to the wind speed in the lake. The nonlinear interaction of high‐frequency wind waves with relatively low‐frequency currents in the boundary layer plays a key role in sediment deposition/resuspension. Without considering the effects of wind waves, the bottom shear stress can be greatly underestimated. The spatial variations of key variables for sediment modelling, including SWH, water depth, orbital velocity, current velocity, bottom shear stress and sediment concentration, are discussed. In general, the near‐bottom wave velocity (and the associated bottom shear stress) is greater than or the same order of magnitude as the near‐bed current velocity (and the associated bottom shear stress) in this shallow water system. Although the sediment zones of Lake Okeechobee were described in previous studies, few published papers discussed its formation mechanisms. The findings of the present study include that the multiyear averaged bottom shear stress with wind‐wave effect plays a key role in forming the spatial patterns of the sediment zones. The study results are currently being used in lake management and in developing strategies for reducing phosphorus in the lake.     

新疆博斯腾湖二维水动力数学模型研究

风场是影响湖泊水动力的重要因素,它可以改变水体运动速度和方向,影响各种物质在湖泊内的输移扩散。通过分析博斯腾湖大湖区风场,准确估算出风力、风向变化情况,构建了博斯腾湖平面二维水动力模型。应用计算模型对博斯腾湖水动力进行模拟预测,分析不同风向、风速对湖泊流场结构的影响,为进一步研究博斯腾湖水动力和污染物输移扩散提供理论依据。     

洪泽湖换水能力的时空分布特征EI北大核心CSCD

为研究洪泽湖换水能力的时空分布特征,基于MIKE21建立洪泽湖二维水动力模型,模拟分析了洪泽湖不同湖区的换水周期;考虑洪泽湖的流场呈现风生流和吞吐流双重特点,分析了不同季节和不同风场条件下换水周期的空间变化特征。结果表明:洪泽湖换水周期空间异质性明显,从南到北梯度递增,冬季和春季全湖平均换水周期较长,分别为75 d和60 d,秋季和夏季的全湖平均换水周期较短,分别为49 d和31 d;实际风场对洪泽湖的换水能力起到正向作用,东南风对洪泽湖的换水能力起正向作用,东北风对洪泽湖的换水能力起到抑制作用。     

大型浅水湖泊纳污能力核算的风场设计条件分析

分析了东南风、西北风及东北风3种不同主导风场下太湖的风生湖流形态特征、环流结构及湖流流速大小变化,研究了不同风生湖流特征条件下环湖入湖污染物的输移扩散规律和湖区水质空间分布特征差异,以及在不同主导风场和常年主导风向下风速变化对太湖各主要湖湾纳污能力的影响。同时,参照水情设计条件要求,选择相对不利的主导风向和保证率为90％时的风速,对太湖进行了纳污能力计算。计算结果表明：在平均风速1.0m/s的西北风和枯水年型设计来水条件下,太湖的CODMn、TP、TN指标纳污能力分别为29732t/年、393t/年和7550t/年,较常年主导风向及多年平均风速条件下的纳污能力分别减小13.5％、 11.3％、23.6％。     

Finite Element Circulation Model for Lake St. Clair

In the numerical modeling of a toxic chemical in a lake, it is necessary to solve the hydrodynamics of the transporting flow as well as the transformation processes of the pollutant in the lake. It is important to calibrate and verify the model before predictive applications. A simple numerical hydrodynamic model which includes wind stress, bottom friction, Coriolis force, inflow, outflow, and the bottom topography of the lake has been developed and verified with field data for Lake St. Clair. The overall root mean square differences between predicted and measured current magnitudes and directions were 1.30 cm s^-1 and 22.5°, respectively, while the correlation coefficients were 0.99 and 0.95, respectively. The hydrodynamic model was tested for stability, convergence, and sensitivity to parameters such as wind shear, wind direction, and vertical eddy viscosity effects. This model is used to generate the typical lake circulation patterns for different steady state wind and ice conditions that are required for the long-term pollutant simulation models such as the EPA (TOXIWASP) model. The depth average velocities were also used in a finite element pollutant transport model. An upwind finite element formulation was used to obtain a stable solution for the convective transport phenomena. The predicted pollutant (chloride ion) concentration pattern was compared with observed field data and fairly good agreement was obtained.     

Thermal habitat characteristics for warmwater fishes in coastal embayments of Lake Ontario

Along 40 km of the Lake Ontario shoreline near Toronto, Ontario, small coastal embayments (4.38-848 × 10³ m²) have been constructed or modified by lake-infilling to restore warmwater fish habitat. We describe how the thermal regimes of these embayments differ from those of small inland lakes, how embayment bathymetry alters the degree of exchange with Lake Ontario, and predict how embayment thermal regimes affect warmwater fish growth. The accumulated growing-degree days, calculated from a 14 °C baseline (∑ GDD₁₄), of seven South-Central Ontario inland lakes and an embayment disconnected from Lake Ontario ranged from 844 to 1157 GDD. Compared to the inland lakes and disconnected embayment, the coastal embayments connected to Lake Ontario achieved fewer ∑ GDD₁₄ and had a greater range, 390-1047, reflecting differences in their degree of exchange with and the cooling effect of Lake Ontario. The thermal regime of coastal embayments differs most from inland lakes during May-late-July. During early summer, mean embayment depth explains over 50% of the variability in ∑ GDD₁₄, with deeper embayments cooling more from lake exchange than shallow embayments. After mid-summer, the cooling embayments are warmed by exchanges with Lake Ontario. This late-summer warming is insufficient to compensate for the cooling effect of the Lake earlier in the summer. Bioenergetic simulations of bluegill, Lepomis macrochirus, growth indicate that most of these embayments are too cool for adequate summer growth of YOY sunfish. Nevertheless, small coastal embayments that are shallow with suitable controls on exchange with the Lake can provide acceptable warmwater fish habitat.     

Water Quality Modeling of Caged Aquaculture Impacts in Lake Wolsey,North Channel of Lake Huron

Nutrient release from fish farming could have an impact on water quality in restricted areas of the Great Lakes. Unprecedented hypolimnetic anoxia, discovered in a small bay of the LaCloche Channel in the vicinity of a caged aquaculture operation (Ontario Ministry of Environment, September 1997), stimulated an environmental review by provincial regulators of the increasing number of caged aquaculture operations in the North Channel area of Lake Huron. They found that water exchange rates between the restricted zones around the cages and surrounding waters are a key component in determining assimilative capacity and evaluating potential impacts on water quality.A caged aquaculture operation exists in Lake Wolsey (area 2,315 ha), a restricted embayment along the northwestern shoreline of Manitoulin Island and which ultimately drains into the North Channel of Lake Huron. The morphometry of Lake Wolsey lends itself to the formulation of a simple box model for the change in total phosphorus concentration due to loading from the caged operation and a specified exchange across the entrance of the embayment.Using water level readings at the entrance of the bay, the geometry of the entrance, and a one-dimensional exchange flow model, the exchange was found to be in the order of 14.3 m³/s on average. This relatively vigorous exchange is due to the presence of large wind and lunar tides in the North Channel. Such an exchange flow would result in a residence time of 215 days for the 25 m deep basin. Based on an estimated open water season (May to November) loading of 1,352 kg phosphorus for the fish culture operation, the resulting elevation of the total phosphorus concentration would be 5.1 μg/L above the measured background level of 8 μg/L, a total close to that observed, 12 μg/L. Similarly, in the ice-covered season with lower feeding rates the model predicted 7.9 μg/L comparing favorably to the observed spring turnover value of 8.0 μg/L. In Lake Wolsey, the long-term effects of caged aquaculture at its current level of production are probably minimal but in the short-term could result in algal blooms and increased oxygen demand.     

3D-modelling of Lake Kivu: Horizontal and vertical flow and temperature structure under spatially variable atmospheric forcing

With the increasing extraction of methane from Lake Kivu, there is a growing need to evaluate the effect of such operations on the lake’s permanent density stratification. This requires understanding of the spatial structure and variability of flow velocities and constituents in Lake Kivu. In this study, we develop a 3D hydrodynamic model of Lake Kivu, set-up within DELFT3D at a 750 m grid spacing and forced by COSMO-CLM atmosphere model results at a 2.8 km grid spacing. Validation shows that the model correctly reproduces the generation and breakdown of the temperature stratification in the upper mixed layer and predicts flow velocity magnitudes and directions similar to measurements both at the surface and at greater depth. Analysis of currents reveals a surface current pattern with two clockwise circulations, one around the whole lake and a smaller one in the northern part, with velocities around 0.1 m/s. This pattern is consistently present over an (ensemble-)averaged day, both in the wet and in the dry season, while day-by-day variations are large. Time-averaged deep currents are found to be a few mm/s at maximum. However, the variations can be substantial, with standard deviations up to 2 cm/s for the currents at 220 m depth, attributed to internal seiches. The temperature stratification, present during the entire wet season, is found to first break down in the dry season in the southern part of the lake. This is explained by the spatial differences in the wind stress and the evaporation heat fluxes during the dry season.     

Spatio-temporal trends in the food habits of age-0 lake whitefish

We compared diets of juvenile lake whitefish among six sites in Lake Michigan and one in Lake Superior during 2005 and 2006 to assess spatial and temporal patterns in food habits and evaluate if ontogenetic diet shifts occur that may influence growth and survival. A total of 262 and 496 juveniles were captured in 2005 and 2006, the majority of which were captured during June and July. Sites in southern Lake Michigan tended to have larger juveniles, and the smallest juveniles were observed at Naubinway, northern Lake Michigan, and Whitefish Point, Lake Superior. The mean number of prey items per stomach differed among sampling sites and years. Copepods were the most prevalent prey item, and were present in greater than 70% of juvenile stomachs from most sites. However, the percent by number of copepods decreased during July as chironomids and other benthic macroinvertebrates increased in number. There was a significant positive relationship between percent of benthic prey items and mean length of juvenile lake whitefish. A substantial increase in the percent of benthic prey consumed after 40 mm (total length) was observed and likely resulted from juvenile lake whitefish crossing a size threshold for benthic feeding relating to morphological changes (i.e., transition of mouth opening from terminal to sub-terminal) in addition to a potential increase in the availability of emergent macroinvertebrates. Timing of the transition to benthic feeding is likely regulated by the number of prey per juvenile and the overlap with peak emergence of important benthic aquatic invertebrates such as chironomids. A better understanding of these factors will increase our understanding of juvenile lake whitefish growth and survival, which are necessary for improving year-class strength predictions.     

ECO-HYDRAULICS TECHNIQUES FOR CONTROLLING EUTROPHICATION OF SMALL SCENERY LAKES-A CASE STUDY OF LUDAO LAKE IN SHANGHAI

The Ludao Lake with an area of 0.86 km2 and 50% water surface ratio, was taken as an example to study the eco-hydraulics techniques for preventing lake eutrophication. Besides external water inflow and outflow, the term related to internal local flow circulation was added in the continuity equation of two-dimensional horizontal hydrodynamic model, and further the hydrodynamic model was calibrated by the scenario of no water exchange. The velocity of 0.2 m/s was suggested to be the critical velocity of controlling algal bloom. To achieve the critical velocity in the whole lake, three factors were analyzed, which are wind, artificial external inflow augmentation and internal local flow disturbance by pump circulation. It is found that the role of wind can be disregarded. For the eco-hydraulics technique of external lake water inflow augmentation, the water flowing route should be firstly optimized, further, the lake inflow has a critical value under specified water level due to the narrow inlets, so the whole lake is difficult to reach the critical velocity to prevent algal bloom, and a combination of external inflow augmentation and internal local flowing disturbance should be considered. Simulation results show that the combination of external water inflow augmentation and internal local flow disturbance requires less eco-flow to achieve the global critical velocity than the sole internal local flow disturbance, for the Ludao Lake, the former requires total eco-flow of 25 m3/s, which reduces by 50% than the latter requiring total eco-flow of 52 m3/s.     

High-resolution hydrodynamic modelling to study year-round circulations and inter-basin exchanges in Lake Winnipeg

A new high-resolution (500 × 500 m), three-dimensional hydrodynamic model was applied to Lake Winnipeg to study summer and winter water circulation, temperature, and ice-cover during 2016–17. The model was run with a combination of buoy-based observations and the outputs from the Global Environmental Multiscale model forcing. Four primary riverine inflows and two outflows were considered in the model. The bathymetry from a previous study by the authors was revised using a 2018 survey covering the South Basin and the Narrows. Comparisons of this new model with the previous model setup (2 km resolution) show noticeable improvements in all simulated parameters. In the Narrows, where seiche-driven flows have predominant oscillation periods of ~27 h and ~17 h, the RMSE of simulated currents is 0.1 m s^?1, half of that of the previous simulations. The new model was able to reasonably simulate the spatial development of ice-cover over the lake. The ice-free period circulation results show that there are two clockwise and counterclockwise gyres in the North Basin, and a weak seasonal clockwise gyre in the South Basin. Monthly circulation patterns differ from those during short wind events due to spatiotemporal variability of wind patterns. The materials from the Red and the Winnipeg Rivers need ~50 days to reach the Narrows before transported from the South Basin to the North Basin. The daily inter-basin exchange flow oscillations during the ice-free period can range up to ~3.5 × 10⁴ m³ s^?1, while under-ice daily exchanges are always northwards with values depending on inflowing riverine discharge up to ~0.5 × 10⁴ m³ s^?1.     

Response of Hydrodynamics and Water-quality Conditions to Climate Change in a Shallow Lake

Lake water resources operation and water quality management come up with higher challenges due to climate change. The frequency and intensity of extreme hydrological events are increasing under global warming, which may directly lead to more uncertainty and complexity for hydrodynamic and water-quality conditions in large shallow lake. However, studies about effects of climate change on lake hydrodynamic and water-quality conditions are not enough. Thus, a coupled model is es-tablished to investigate the potential responses of lake water level, flow field and pollutant migra-tion to the changing climatic factors. The results imply that water flow capacity and self-purification in the Hongze Lake can be improved by west, northwest, north, south and southeast winds indi-cating wind filed change has a great effect on the hydrodynamic and water-quality conditions in large shallow lake. It is further observed that both hydrodynamics and water quality are more sensitive to rainfall change than to temperature change; compared to the effect from temperature and rainfall, the effect from wind field appear to be more pronounced. Moreover, the results verify the feasibility of coupling basin hydrological model with lake hydrodynamic and water quality model. To the best of knowledge, the coupled model should not be used until independent calibra-tions and verifications for hydrodynamics and water quality modeling, the hydrological model and the coupled model.

     

A Semi-Lagrangian Study of Circulation and Transport in Lake Ontario

A particle tracking model (PTM) is linked with a hydrodynamic model to evaluate mean seasonal circulation patterns in Lake Ontario, and also to provide a basis for predicting movement of algal blooms. The PTM is based on a random walk algorithm that combines a deterministic advective component with a stochastic component associated with the turbulent diffusivity field to calculate trajectories of neutrally buoyant particles, where both the advective and diffusive velocities are obtained from the hydrodynamic model. Mean circulation is calculated using 30-year average meteorological forcing data collected from five stations around the lake. Seasonal variations in lake circulation are demonstrated, and a clockwise flow in the eastern basin during summer and early fall is identified, contrary to some previous observations that suggest counterclockwise flow. The impacts of Niagara and St. Lawrence river flows on general lake circulation are found to be small, except within approximately 10 km of the river mouth. Development and application of the PTM demonstrate its potential to provide calculations of (Lagrangian) movements as determined from the hydrodynamic output, and to serve as a first step toward development of an algal transport model. Particle tracking helps to visualize flow patterns and provides a means of evaluating the probability a bloom will reach a specified area, given an initial position and the predicted velocity and diffusivity fields. This capability, when set up for real-time applications, can provide an important tool to support management decisions that may be needed when a bloom is observed, for example in predicting potential impacts of the bloom on a beach or a water intake.     

Foredune dynamics at a Lake Michigan site during rising and high lake levels

On Great Lakes dunes, the link between foredune dynamics and coastal processes is seen in dune responses to changing lake levels. This paper investigates foredune dynamics during a recent period of rising and high lake levels. The study location was an active foredune in P.J. Hoffmaster State Park on the east coast of Lake Michigan, where field data were collected from 2000 through the final destruction of the foredune by wave removal in November 2019. Foredune dynamics were studied with erosion pins, direct observations, photographs, mapping, and on-site wind measurements. Regional climate and lake-level data were obtained from established data collection programs. The response of the foredune to rising lake levels was compared to several models of foredune behavior. During the study, the Lake Michigan-Huron level rose 1.89 m from January 2013 to July 2020. After an early transitional period, foredune activity was characterized by scarp retreat (4–19 m per year) and dune narrowing from 2014 to 2019. When the foredune completely disappeared in November 2019, erosion/scarping began on the next landward dune. The foredune activity fits Olson’s (1958) model for foredune growth and erosion through lake-level cycles. The foredune migration predicted by the revised Davidson-Arnott (2021) model of foredune response to relative water level rise did not occur, most likely because the rate of lake-level rise was too high. The six years of foredune narrowing before wave erosion started affecting the next landward dune represent a time-lag in Lake Michigan dune history models of increased dune activity during high lake-level stands.     

Relationships between wind-driven and hydraulic flow in Lake St. Clair and the St. Clair River Delta

A three-dimensional hydrodynamic forecasting model of the Great Lakes Huron-Erie Corridor is used to investigate mixing and the relationship between hydraulic and wind-induced currents in a shallow lake system in which lake inflows come through several channels of a river delta. The hydrodynamics in Lake St. Clair and the channels of the St. Clair River Delta are evaluated for (1) a one-year simulation from 1985 including water age calculation, (2) 8 different wind direction scenarios, and (3) a storm event. Observations and model simulations show distinct regions in the lake in which currents are forced by either hydraulic flow from the river system or from wind stress over the lake. However, during severe storm events, these regions are found to shift or even disappear due to changes in the delta channel inputs into the lake. These changes underscore the need for realistic, unsteady river flow boundary conditions at interfaces between a shallow lake and river delta. Steady inflow conditions will not allow for potential shifting of these current zones, and will also fail to resolve flow retardation or reversals during storm events.