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Mixing of an interflow into the ambient water of Lake Iseo
Abstract
River water flowing as an interflow was investigated using field data, collected in Lake Iseo (Italy), and theory. A theory for the lateral falling mechanism of plunging was developed for inflows when the initial densimetric Froude number (Fr-0) is slightly larger than unity. The ratio of the river width to the offshore extent of the plunge region was equal to Fr-0. The mixing ratio in the plunge region was 0.06. Theoretical results were quantitatively consistent with the length scale and mixing ratio of the observed plunge region. The progression of the inflow was interpreted as: initially a laterally falling plunge region with little mixing; followed by a steep underflow region with substantial mixing; and finally an intrusion. The intrusion was at first controlled dynamically by an inertia-buoyancy force balance. Further from the liftoff point, turbulent mixing effects dominated over those due to inertia. Ultimately, the intrusion diffused into the adjacent layers in such a way that the interflow fluid was effectively indistinguishable from the lake water.
... Examples of (a) a laterally confined inflow ( Hogg et al. (2013) and Hogg (2014) theorized that for Fr d numbers slightly larger than unity, buoyancy forces dominate lateral entrainment for laterally unconfined, hyperpycnal inflows entering a lake. They note that the gravitational forcing causes lateral slumping accompanied by baroclinic vorticity generation (see Figure 3b for a sketch of their conceptual flat-bed model). ...
... Hauenstein and Dracos (1984) alluded to this effect in a definition sketch of the flow structure of a laterally unconfined plunging flow, without describing it in detail. Hogg et al. (2013) and Hogg (2014) visually identified a plunging area at the inflow of the Canale Italsider into Lake Iseo. For this, they relied on a buildup of flotsam that indicated the presence of a convergence zone, which they called the plunging curve, at about 30-50 m from the river mouth. ...
... Both studies performed simulations of saline inflows, with Tseng and Chou (2018) also simulating sediment-laden inflows. Their model results showed the formation of a triangle-shaped surface pattern and the lateral slumping of the inflow plume under baroclinic forces, reinforcing the hypothesis of Hogg et al. (2013) and Hogg (2014). Shi et al. (2022) confirmed these processes in laboratory measurements. ...
The present knowledge of plunging hyperpycnal river plumes is mainly based on two‐dimensional (confined) laboratory experiments. Several hypotheses on three‐dimensional (unconfined) flow processes have been made, but not tested in situ. In this field study, the dominant three‐dimensional hydro‐sedimentary processes related to unconfined plunging were elucidated by synchronizing autonomous time‐lapse camera images with boat‐towed acoustic Doppler current profiler measurements. It was found that the flow field complies with two‐dimensional conceptualizations along the plume centerline. Perpendicular to the centerline, the plume slumped laterally due to its density excess and simultaneously converged laterally due to its vertical divergence. This combination led to a narrowing of the plume near the surface, resulting in a sediment‐rich triangle‐shaped pattern on the surface near the inflow, and a rather stable width near the bed. The formation of secondary flow cells transporting riverine water and suspended particulate matter (SPM) away from the plume near the bed, up toward the surface and back toward the plume near the surface, was revealed. An increase of the average SPM concentration and the SPM flux in the main flow direction indicates net sediment erosion under the investigated conditions of high discharge and sediment load. This suggests transient storage of sediment during conditions of lower discharge and sediment load, and high morphological activity in the plunging area. These findings allowed extending a classical conceptual plunging model to laterally unconfined sediment‐laden plunging inflows for conditions of well‐mixed ambient water in the plunging region and inflow densimetric Froude numbers exceeding 1, common in nature.
... A sketch of the unconfined plunging process is shown in Figure 2b. In unconfined plunging, Hogg et al. (2013) defined the plunge location as the place on the surface where the ambient waters from both sides of the progressively plunging plume meet, that is, the vertex of the surface triangle. Hereinafter, this location is referred to as the vertex plunge point in the unconfined case to distinguish it from the plunge location in confined plunging. ...
... Hereinafter, this location is referred to as the vertex plunge point in the unconfined case to distinguish it from the plunge location in confined plunging. Hogg et al. (2013) presented a theoretical approach to predict the shape of the triangle and the vertex plunge point as a function of the inflow parameters for the case of a horizontal bottom. ...
... with maximum (positive) and minimum (negative) values located near the edge of the surface triangle. This inward transversal velocity results from the transverse lock-exchange-type flow generated by the lateral pressure gradient between the hyperpycnal plume and ambient water as discussed in Hogg et al. (2013) and Tseng and Chou (2018). The superposition of the longitudinal and inward transversal velocity components results in a velocity direction that follows the edge of the surface velocity triangle at the interface (Figure 5a). ...
Hyperpycnal (negatively buoyant) river inflow into lakes and oceans often develops three‐dimensional (3D) plunging flow patterns when laterally unconfined. To determine the 3D flow pattern characteristics, laboratory experiments of laterally unconfined plunging on a sloping bed were carried out using salinity to control the density difference. The experiments were complemented by numerical modeling based on a high‐resolution computational fluid dynamics model. As is the case for confined plunging plumes, it was found that in unconfined plunging, the inflow densimetric Froude number Frd−0 at the river mouth and the bed slope of the receiving water body β are the dominant control parameters. However, the results documented that the hydrodynamics of laterally unconfined plunging are fundamentally different: The hyperpycnal plume in unconfined configurations forms a triangle on the surface in the plunge zone due to its convergence near the surface and lateral spreading near the bottom. The triangular pattern extends further into the receiving water when Frd−0 increases or the bottom slope decreases. The unconfined entrainment coefficient, which quantifies the amount of ambient water entrained into the plunging plume, also increases with increasing Frd−0. In general, entrainment is much higher in unconfined than in confined plunging. The plunging densimetric Froude number Frd−p takes a constant value of ∼0.5 in confined plunging, whereas it increases with increasing Frd−0 and can be ≫1 in unconfined plunging. Complex patterns of secondary currents occur in the plunging plume. A low‐velocity zone whose size increases with Frd−0 is observed near the centerline above the bed.
... When the underflow reaches the depth where inflow and lake densities are equal, it will detach from the lakebed and intrude into the ambient water as an interflow (Stevens et al., 1995;Ahlfeld et al., 2003). This conceptual model for dense riverine inflows is supported by various modeling (Hebbert et al., 1979;Akiyama and Stefan, 1984;Parker et al., 1986) and observational studies (Giovanoli and Lambert, 1985;Fleenor, 2001;Hogg et al., 2013). At the top and bottom of the interflow, modeling shows that pronounced gradients of velocity and density form, and the resulting turbulence will mix and entrain ambient lake waters (Martin et al., 1998). ...
... The entrainment rate (γ ), or mixing ratio, is given by Lee and Yu (1997), Lamb et al. (2010), and Hogg et al. (2013): ...
... The γ entrainment values of ∼2 observed at transect T1 at 400 m from the mouth (Figure 10) suggest that entrainment is probably important in the plunging and underflow areas before reaching T1. In unconfined flow, the river inflow can develop laterally after leaving the inflow channel and as a result, the plunging of a negatively-buoyant inflow can be three-dimensional (3D) (e.g., Khan et al., 2005;Hogg et al., 2013). Figure 1C is a representative example of unconfined Rhône River plunging. ...
River inflows have a major influence on lake water quality due to their input of sediments, nutrients and contaminants. After leaving the river channel, river waters form a plume and interact with ambient lake waters. Strong hydrodynamic changes take place in the nearfield. To determine the nearfield hydro-sedimentary dynamics of the negatively-buoyant Rhône River plume in thermally stratified Lake Geneva, field campaigns were carried out at high and intermediate river discharge. High-resolution full-depth Acoustic Doppler Current Profiler (ADCP) transects were taken at 400, 800, 1200 and 1500 m from the river mouth, combined with profiles of temperature, turbidity and particle size distribution. These measurements provided, for the first time in a lake, detailed velocity fields across the full-plume cross-section in each transect. Furthermore, the unique combination of measurement techniques allowed the quantification of entrainment, Suspended Particulate Matter (SPM) concentrations and fluxes. They revealed that the negatively buoyant river inflow intruded into the metalimnion as a laterally unconfined interflow and continued flowing straight out in the streamwise direction, since currents in the lake were weak. At the same time, it mainly spread laterally due to entrainment of ambient water. The size of the interflow core and its velocity progressively decreased with distance from the mouth, as did SPM concentrations and volumes of particles (by a factor of 2-3 within 1500 m), due to sediment settling and entrainment. The interflow momentum flux remained constant along the pathway, but interflow cross sections and discharge increased 2-3 times (within 1500 m). The entrainment coefficient was > 2 at 400 m and further increased nonlinearly along the pathway. These values are much higher than those reported for laterally confined laboratory studies. The particle size distribution in the interflow was dominated by fine particles (< 32 μm), which were transported up to 1500 m from the mouth and most likely beyond, whereas larger particles (> 62 μm) almost completely settled out before reaching that distance. The above processes occurred independent of river discharge; their intensity, however, changed with discharge.
... Si el río ingresa superficialmente, los nutrientes estarán directamente disponibles en la zona fótica (ZF) del embalse para apoyar la actividad biológica (Vidal et al., 2012), mientras que si un río ingresa de forma intrusiva, los nutrientes que él transporta estarán disponibles en la ZF en función de la profundidad de intrusión y de la magnitud de los procesos de mezcla superficiales y procesos advectivos (Rueda et al., 2007), dado que si la intrusión es cercana a la capa superficial mezclada (CSM), la mezcla vertical podría llevar en días o meses después de la intrusión elementos como nutrientes a la CSM (Cortés et al., 2014), mientras que en intrusiones profundas o en corrientes de fondo, solo procesos de mezcla convectivos profundos podrían lograrlo (Rueda et al., 2007). Hogg et al. (2013) expone que una corriente intrusiva posee tres etapas consecutivas con distintas dinámicas, siendo estas: 1) la etapa de sumergencia, donde la localización del punto de sumergencia responde a un balance entre la fuerza dinámica (momentum de la corriente), la fuerza estática (gradiente de presión en la interfaz) y las fuerzas resistentes (cortantes en el lecho, interfaz y superficie) (Ford y Johnson, 1983), 2) la etapa de flujo sumergido, y 3) la etapa intrusiva (en dos zonas diferenciadas por el balance existente entre la inercia y la flotabilidad, y entre la difusión y la flotabilidad, donde no necesariamente se presentan ambas en una corriente intrusiva) (Figura 1). Figura 1.-Esquema conceptual de las diferentes etapas de una corriente de densidad intrusiva En la etapa de sumergencia se presenta la denominada "dilución inicial" (Γ i ), producto de la mezcla generada por grandes remolinos formados por los flujos reversos debidos a la sumergencia del agua que ingresa (Figura 1). ...
... La dilución Γ tanto inicial (Γ i ) como total (Γ i + Γ S ) se estimó según lo expuesto en Cortés et al.(2014), ecuación [1], empleando los registros de temperatura del agua tomados con el perfilador, donde T río es la temperatura del agua en el río (aguas arriba del punto de sumergencia), T sup.embalse la temperatura del agua en la superficie del embalse (en la CSM), y T pluma es la temperatura del agua en la pluma, esta última para la dilución inicial se midió justo aguas abajo del punto de sumergencia identificado en campo, igualmente se realizaron cálculos empleando los perfiles de concentración de clorofila "a" total según lo expuesto en Hogg et al (2013) utilizando la misma relación. Adicionalmente se tomó la definición de concentración relativa ⁄ presentada en Fischer et al.(1979), que permite estimar en una mezcla las p partes de agua de afluente (río) y (1-p) partes del agua ambiente que la componen. ...
... En lo que corresponde a la dilución inicial (Tabla 1), las estimaciones realizadas con las expresiones propuestas en Fleenor (2001) exponen errores entre 0.37% y 14.21% frente a los valores estimados con mediciones de temperatura, porcentajes adecuados considerando la incertidumbre asociada a la ubicación (en campo) del perfil aguas abajo del punto de sumergencia, de tal manera que no se mida en demasía al interior de la etapa de sumergencia de la corriente. El ajuste apoya lo expuesto por Hogg et al. (2013), frente que a que la propuesta de Fleenor (2001) es adecuada si F 0 <1, condición que se cumplió en los casos valorados. Los cálculos muestran que la dilución inicial producto de la sumergencia del río varía en un rango amplio, ya que en términos de los valores de concentración relativa (fracción de volumen de afluente) se obtuvieron valores entre 0.75 a 0.92 partes de agua del río frente a 0.25 a 0.08 partes del agua del embalse. ...
RESUMEN: Se investigó la forma como el río Chico ingresa al embalse Riogrande II, y su posible influencia en la formación de florecimientos de algas localizados hacia la entrada del embalse, empleando de forma conjunta información de campo, ecuaciones empíricas, y el modelo hidrodinámico tridimensional ELCOM. Se encontró que la profundidad de intrusión del río Chico posibilita que los procesos de mezcla superficial integren agua rica en nutrientes de la corriente intrusiva a la zona fótica, lo anterior en conjunto con diluciones importantes en el punto de sumergencia, favorece los florecimientos casi constantes y localizados hacia la entrada del río Chico. ABSTRACT: We investigated how Chico River inflow to Riogrande II reservoir, and its possible influence on the algal blooms located in the plunge point zone using field information, empirical equations, and the three-dimensional hydrodynamic model ELCOM. We were found that intrusion depth enables the surface mixed layer entrain a fraction of the interflow, generating nutrient-rich river water supply the photic zone, this in conjunction with major dilutions in the plunge point, promot the occurrence of located algal blooms near to plunge point zone. PALABRAS CLAVES: corrientes intrusivas, dilución, florecimiento algal
... In normal flow conditions (ca 50 m 3 .s -1 ; Groppelli et al., 2011;Hogg et al., 2013), the water underflow coming from the Oglio inlet only influences the first 100 m of depth of the lake basin, while the horizontal length of overflow intrusion is limited to 3 km (Hogg et al., 2013). Thus, most of sediment input from the Oglio river during normal flow conditions is accumulated close to the delta or on the north-western shores of Lake Iseo. ...
... In normal flow conditions (ca 50 m 3 .s -1 ; Groppelli et al., 2011;Hogg et al., 2013), the water underflow coming from the Oglio inlet only influences the first 100 m of depth of the lake basin, while the horizontal length of overflow intrusion is limited to 3 km (Hogg et al., 2013). Thus, most of sediment input from the Oglio river during normal flow conditions is accumulated close to the delta or on the north-western shores of Lake Iseo. ...
The sedimentary processes in the deep basin of large peri-Alpine lakes have not been studied much on long timescales due to high coring complexity of such lake systems. In 2018, a 15.5 m long sediment section was retrieved from the deep basin of Lake Iseo (Italy) at 251 m water depth. A seismic survey associated to a multi-proxy approach with sedimentological and geochemical analyses, reveals that event deposits correspond to 61.4% of the total sedimentation during the last 2000 years. The great heterogeneity of textures, colours and grain-size distribution between the different types of event layers can be explained by the high number of potential sources of sediment in this large lake system. By combining a proxy of sediment sources with proxies of transport processes, flood events were distinguished from destabilisations of the slopes and the main delta. The three thickest mass wasting deposits correspond to major regional earthquakes events of 1222 CE, 1117 CE and around 700 CE. From a thorough comparison with regional climatic fluctuations and human activities in the watershed, it appears that periods of high sediment remobilization can be linked to a preceding increase in erosion in the watershed mainly under human forcing. Hence, even in large catchments, human activities play a key role on erosion processes and on sediment availability, disrupting the recording of the extreme events in lacustrine archive.
... Considering constant null salinity is common for the modeling of large perialpine lakes (e.g. Hogg et al., 2013;Laborde et al., 2010). Null turbidity was also attributed to the inflows, as field measurements are not available. ...
... Indeed, such distances ensure the complete mix of interflows. Hogg et al. (2013) reported horizontal length scales for the mixing of the two main tributaries of Lake Iseo ≈ 4 km. This enforces the validity of the adopted 1D approach. ...
Atmospheric exchanges largely dominate the heat budget of deep lakes in temperate regions. Heat import and export by through-flows is of much lower entity and has been neglected or simplified in many numerical thermodynamic models of lakes. This is often due either to the unavailability of data for inflows and outflows, or to the difficulties in forecasting the evolution of their discharge and temperature in climate change studies. While disregarding through-flows may seem correct, riverine intrusions can bring warmer water than the deep hypolimnetic one to the lower metalimnion and upper hypolimnion, where sunlight does not penetrate and mixing is poor. For holomictic lakes with significant inflow contributions, this can affect the thermal structure at intermediate depths, hampering any numerical model which neglects through-flows. This study focuses on a relevant basin under such aspect, Lake Maggiore (Northern Italy/Southern Switzerland), which drains the rainiest watershed of the Southern Alps. First, we quantify to what extent a one-dimensional fixed-level model ignoring through-flows is able to predict the observed evolution of the thermal structure of the lake and the improvements resulting from reproducing the main inflows and outflows. Then, we directly discuss the influence of through-flows on the thermodynamic structure of Lake Maggiore. The General Lake Model (GLM) was here adopted, reproducing the 1998–2014 period, spanning years with different meteorological and hydrological features. Results show that a calibrated enclosed-lake model can give satisfactory results only if it employs an unrealistically low light extinction coefficient to allow heating of the deep metalimnion and hypolimnion, whose real warming strongly depends on interflows.
... This target seems an ambitious task in a strongly urbanized watershed like that of Lake Iseo. Although already suggested by Bonomi and Gerletti (1967), who underlined the study of the circulation mechanics of this lake as a priority, only recently a set of experimental and modelling activities have been accomplished to specifically investigate its internal hydrodynamics (Pilotti et al., 2013), with a focus on the internal wave structure (Valerio et al., 2012;Vilhena et al., 2013) and on plunging at the entrance of the Oglio River (Hogg et al., 2013). Beside the internal movements triggered by the wind, inflows-induced currents are also central to the understanding of oxygenation, deep-water renewal and nutrient transport in deep lakes. ...
... These channels clearly show that occasionally plunging flow characterizes the tributary water. Although a recent study (Hogg et al., 2013) theoretically investigated this mechanism of plunging, to the knowledge of the authors, no previous study has ever investigated the horizontal path of the inflow intrusion. ...
The influence of Coriolis force on the currents of large lakes is well acknowledged; very few contributions, however, investigate this aspect in medium-size lakes where its relevance could be questionable. In order to study the area of influence of the two major tributary rivers in Lake Iseo, a rotating vertically distorted physical model of the northern part of this lake was prepared and used, respecting both Froude and Rossby similarity. The model has a horizontal length scale factor of 8000 and a vertical scale factor of 500 and was used both in homogeneous and in thermally stratified conditions. We explored the pattern of water circulation in front of the entrance mouth for different hydrologic scenarios at the beginning of spring and in summer. We neglected the influence of winds. The primary purposes of the model were twofold: i) to increase our level of knowledge of the hydrodynamics of Lake Iseo by verifying the occurrence of dynamical effects related to the Earth’s rotation on the plume of the two tributaries that enter the northern part of the lake and ii) to identify the areas of the lake that can be directly influenced by the tributaries’ waters, in order to provide guidance on water quality monitoring in zones of relevant environmental and touristic value. The results of the physical model confirm the relevant role played by the Coriolis force in the northern part of the lake. Under ordinary flow conditions, the model shows a systematic deflection of the inflowing waters towards the western shore of the lake. The inflow triggers a clockwise gyre within the Lovere bay, to the West of the inflow, and a slow counter-clockwise gyre, to the East of the inflow, that returns water towards the river mouth along the eastern shore. For discharges with higher return period, when only the contribution by Oglio River is relevant, the effect of the Earth’s rotation weakens in the entrance zone and the plume has a more rectilinear pattern, whilst in the far field the current driven by the inflows keeps moving along the western shore. On the basis of these results one could expect that the north-western part of the lake between Castro and Lovere, although not aligned with the tributaries’ axes, is more sensitive to accumulation effects related to river-borne pollution. The results obtained with the physical model are critically compared with data obtained from different sources: the trajectory of a lagrangian drogue; a map of reflectivity data from the lake floor; a map of water turbidity at the intrusion depth. The findings are also confirmed by the results of a 3D numerical model of the lake.
... Another thermistor chain (TC), consisting of a series of 18 fast-response thermistors vertically spaced at 1 to 6 m intervals from 1 to 33 m depth, was installed in the southern basin and provided water column temperature data during the experiment period. River loggers, labeled RL1-3, were installed at the two main inflows (Oglio River and Canale Italsider) and at the lake outlet (Oglio River), providing measurements of water temperature, conductivity (except for RL2), and surface level (Hogg et al. 2013). Additionally, four on-lake wind stations (WS1-4) were installed at different locations. ...
... Inflows from Oglio River and Canale Italsider and the Oglio River outflow were considered in the simulation; flow rate and water temperature data were obtained from the river loggers RL1-3 installed for the experiment. The Oglio River inflow rates were estimated by applying the weir rating curve to the measured water level, and the Canale Italsider inflow rates were taken from the hydropower plant that controls the flow to the Canale (Hogg et al. 2013). The Oglio River outflow rates were obtained from Consorzio dell'Oglio. ...
The energy transfer from basin-scale internal waves to internal nonlinear waves was investigated in a large, deep subalpine lake through a combination of field data and three-dimensional hydrostatic and nonhydrostatic modeling. The response of the internal wave field induced by two storm events, with distinct characteristics, showed that, for the whole lake, around 15% of the total potential energy contained in the basin-scale internal waves was transferred to nonlinear internal waves in response to moderate forcing, the large transfer being the direct result of the small surface layer thickness compared with the depth of the lake. Locally, the energy transfer to nonlinear waves was up to 30% for the more severe forcing. To model such energy transfers, a nonhydrostatic three-dimensional hydrodynamic model was required; this implies that the inclusion of nonhydrostatic effects is essential for accurate modeling of ecological processes in deep large lakes, which is a challenge considering currently available computational resources.
... The lake underwent a relatively rapid eutrophication in the 1970s, mainly due to large nutrient loading from the inflows (Garibaldi et al. 2003). Since 1992, many studies have been conducted in Lake Iseo (see Salmaso et al. 2012 and references therein, Valerio et al. 2012, de Vilhena et al. 2013, Hogg et al. 2013; water samples taken continuously over the last decades at the deepest point of the basin (Site 3 in Fig. 1) form the basis for a description of the trophic evolution of the lake and phytoplanktonic community (Garibaldi et al. 2003, Salmaso et al. 2012). ...
... The survey period was selected because previous studies have indicated that density and diversity of phytoplankton is highest at this time of year (Garibaldi et al. 2003, Salmaso et al. 2012. Two replicates of each sample were collected, and the sampling depths were varied slightly to correspond to the depths of peak chlorophyll a (Chl-a) concentrations as recorded with a Multi-Scale Profiler (MSP; Hogg et al. 2013). ...
The spatial distribution of phytoplankton in the surface waters of a typical, deep subalpine lake was investigated by analysing water samples collected at 3 depth ranges (0-1, 9-13, and 15-19 m) at 7 sites (21 total samples) using a generalized linear model. The biovolume of different phytoplanktonic taxa significantly differed between depth ranges. Different algal taxa showed maximum population density at different depths as a result of different optima of light, physical conditions, and nutrients. The layer at 9-13 m depth showed the greatest abundance of phytoplankton, mainly represented by Bacillariophyceae and Cyanobacteria. Conversely, Chlorophyceae, which require few nutrients and are not inhibited by high light energy, were most abundant at the surface. The vertical heterogeneity in phytoplankton distribution closely paralleled that of nutrients, which in turn depend on the stable thermal stratification of the lake water column. Biovolume of phytoplankton taxa and chemical parameters did not differ significantly between sampling sites some kilometers apart due to large horizontal water movements in the lake that create relatively uniform conditions throughout the basin.
... This target seems an ambitious task in a strongly urbanized watershed like that of Lake Iseo. Although already suggested by Bonomi and Gerletti (1967), who underlined the study of the circulation mechanics of this lake as a priority, only recently a set of experimental and modelling activities have been accomplished to specifically investigate its internal hydrodynamics (Pilotti et al., 2013), with a focus on the internal wave structure (Valerio et al., 2012;Vilhena et al., 2013) and on plunging at the entrance of the Oglio River (Hogg et al., 2013). Beside the internal movements triggered by the wind, inflows-induced currents are also central to the understanding of oxygenation, deep-water renewal and nutrient transport in deep lakes. ...
... These channels clearly show that occasionally plunging flow characterizes the tributary water. Although a recent study (Hogg et al., 2013) theoretically investigated this mechanism of plunging, to the knowledge of the authors, no previous study has ever investigated the horizontal path of the inflow intrusion. ...
The influence of Coriolis force on the currents of large lakes is well acknowledged; very few contributions, however, investigate this aspect in medium-size lakes where its relevance could be questionable. In order to study the area of influence of the two major tributary rivers in Lake Iseo, a rotating vertically distorted physical model of the northern part of this lake was prepared and used, respecting both Froude and Rossby similarity. The model has a horizontal length scale factor of 8000 and a vertical scale factor of 500 and was used both in homogeneous and in thermally stratified conditions. We explored the pattern of water circulation in front of the entrance mouth for different hydrologic scenarios at the beginning of spring and in summer. We neglected the influence of winds. The primary purposes of the model were twofold: i) to increase our level of knowledge of the hydrodynamics of Lake Iseo by verifying the occurrence of dynamical effects related to the Earth's rotation on the plume of the two tributaries that enter the northern part of the lake and ii) to identify the areas of the lake that can be directly influenced by the tributaries' waters, in order to provide guidance on water quality monitoring in zones of relevant environmental and touristic value. The results of the physical model confirm the relevant role played by the Coriolis force in the northern part of the lake. Under ordinary flow conditions, the model shows a systematic deflection of the inflowing waters towards the western shore of the lake. The inflow triggers a clockwise gyre within the Lovere bay, to the West of the inflow, and a slow counter-clockwise gyre, to the East of the inflow, that returns water towards the river mouth along the eastern shore. For discharges with higher return period, when only the contribution by Oglio River is relevant, the effect of the Earth's rotation weakens in the entrance zone and the plume has a more rectilinear pattern, whilst in the far field the current driven by the inflows keeps moving along the western shore. On the basis of these results one could expect that the north-western part of the lake between Castro and Lovere, although not aligned with the tributaries' axes, is more sensitive to accumulation effects related to river-borne pollution. The results obtained with the physical model are critically compared with data obtained from different sources: the trajectory of a lagrangian drogue; a map of reflectivity data from the lake floor; a map of water turbidity at the intrusion depth. The findings are also confirmed by the results of a 3D numerical model of the lake.
... The water of the rivers always flows in the layer of lake water that has similar temperature and density. Incoming water with a lower density than lake water will flow on the surface of the reservoir, and water with a higher density will fall to a layer of the same density or fall to the bottom of the reservoir [3]. Depending on the season and the river water temperature relative to that of lake water, the mixing of river and lake waters may also vary, occurring either at the surface or at the bottom of the reservoir [4,5]. ...
The mixing of river and lake waters is important for the functioning of a reservoir, especially in the case of shallow polymictic reservoirs such as Lake Swarz˛edzkie. The extent of this mixing depends largely on the river flow rate. In lakes, which rivers with low flow values flow through, it should be expected that the flow currents only reach the narrow zone adjacent to the mouth of the river to the lake. The water of rivers generally has different chemical compositions and physicochemical parameters in relation to lake water. Therefore, to determine the range of the river in the lake and characterize the water mixing, measurements of temperature, electrolytic conductivity, and the concentrations of selected chemical elements were made in the estuary zone and at other points located on the lake and on the river near the tributary. In addition, the values and directions of horizontal currents were determined, and thermal photos were taken from a low-altitude ceiling.
... a Present address: Commission internationale pour la protection des eaux du Léman (CIPEL), Nyon, Switzerland detaches from the lakebed at the depth where river and lake densities are comparable, and transforms into an interflow. This conceptual framework has been applied to describe negatively buoyant river inflows in stratified lakes (Giovanoli and Lambert 1985;Churchill et al. 2003;Hogg et al. 2013;Scheu et al. 2015) where river interflows develop in a layer at the base of the lake surface mixed layer just above the abrupt change of water density in the thermocline. ...
Based on field investigations carried out in Lake Geneva during summer stratification in 2019, this study documents, for the first time, the rapid change of negatively buoyant river inflow dynamics caused by the passing of a coastal upwelling. Under calm conditions, the negatively buoyant Rhône River, entering at the eastern end of the lake, interacts with the lake density profile such that the river intrusion moves as an interflow in the thermocline layer straight out into the lake. A strong, large‐scale spatially homogeneous wind that lasts for several days causes a downward thermocline tilt at the western end of the lake, coastal downwelling on the northern shore, and coastal upwelling on the southern shore. This cold‐water upwelling progresses like a Kelvin wave around the lake after wind forcing ceases. When it arrives at the river inflow area, it homogenizes the lake water temperatures, and the river inflow transforms into an intrusion that spreads over the whole water column. Trapped by strong alongshore currents generated by the progressing coastal upwelling, the river plume is sharply deviated and flows along the shore in the nearshore zone, potentially bringing nutrients directly into the phototrophic near‐surface layer. Following the passage of the coastal upwelling, the river inflow transforms back into an interflow. This change in inflow dynamics, which is documented for five strong wind events between June and September by combining in situ measurements, remote sensing, and numerical simulation, can be expected to occur in other large lakes with comparable wind‐induced large‐scale thermocline displacement.
... Oglio river waters and thus the sediment inflow by the Oglio plume are deflected towards the Western shore of Lake Iseo due to the Coriolis influence (Pilotti et al., 2018(Pilotti et al., , 2013. In normal conditions, the water underflow coming from the Oglio inlet only influences the first 100 m of the lake basin, while the horizontal length of overflow intrusion is supposed to be limited to 3 km (Hogg et al., 2013). Thus, most of sediment input from the Oglio river during normal flow conditions is accumulated close to the delta or on the North-Western shores of Lake Iseo. ...
Few of the large Southern peri-alpine lakes have been studied with a sedimentological approach in their deep basin to understand the dynamics of their long-term sedimentation due, among other factors, to the high complexity of the coring in such deep lakes. In 2018, a 15.5 m-long sediment section was retrieved from the deep basin of Lake Iseo (Italy) at 251 m of water depth. Seismic survey associated to a multi-proxy approach with sedimentological and geochemical analyses, reveals a high number of event layers that corresponds to 61.4 % of the total sedimentation during the last 2000 years. The great heterogeneity of textures, colours, and grain-size distribution between the different types of event layers can be explained by the high number of potential sources of sediment inputs in this large lake system. By combining proxies for sediment source with transport processes, we were able to distinguish: i) flood events, and ii) destabilisations of slopes and deltas due to an increase of the sediment load and/or to seismic shaking. From a thorough comparison with both, the regional climatic fluctuations, and the human activity in the watershed, it appears that periods of high sediment remobilization can be linked to a previous increase in Critical Zone erosion in the watershed mainly under human forcing. Hence, even in large catchments, human activities play a key role on erosion processes and on sediment availability, disrupting the recording of the Critical Zone functioning in such lacustrine archive.
... . These values are similar to those reported in the literature. For example,Hogg et al. (2013) estimated γ ¼ 1.5 from field experiments in the river inflow of the temperate Lake Iseo. Rueda and MacIntyre (2010) estimated and modeled γ ¼ 2 in the arctic Toolik Lake, andCortés, Fleenor, et al. (2014) reported values for the temperate Lake Béznar of the same order ...
The effect of large‐amplitude isopycnal displacements, frequently observed in deep medium‐size arctic lakes during the ice‐free period, on the near‐ and far‐field fate of negatively buoyant river inflows is explored in this work. A three‐dimensional transport and hydrodynamic model of sub‐arctic Lake Lagarfljót was used to simulate the fate of river inflows during the summer stratification period. The intrusion dynamics are strongly affected by the amplitude and direction (downwelling/upwelling) of the isopycnal displacements induced by the wind near the river inlet. These displacements control the distance of travel of the river plume until reaching the layers with maximum density gradients (pycnocline) and, thus, the mixing ratio between the river plume and the lake water. Specifically, strong upwelling near the inlet causes the river to flow to the bottom as an underflow. Under downwelling, river plumes tend to form metalimnetic intrusions. The influence of the isopycnal displacements on the initial river fate can be parameterized using a time‐varying density Richardson number, which needs to be smoothed to account for the effects of unsteadiness. Large amplitude internal motions, of up to 70 m in Lake Lagarfljót, move deep underflows upwards to shallower basins where they could be readily incorporated into the surface mixed layer and rapidly flushed out of the lake. Metalimnetic currents associated with the V2H1 internal circulation can also accelerate riverine transport out of the lake.
... Gravity currents are nearly horizontal flows driven by lateral density gradients (Simpson 1982(Simpson , 1997. They play a key role in the transport of scalars in a range of environmental settings, including dense inflows into lakes (Fischer & Smith 1983;Hogg et al. 2013;Cortés et al. 2014a), snow avalanches (Sovilla et al. 2018), katabatic winds down mountain faces and valleys (Samothrakis & Cotel 2006b), turbidity currents and mudslides (MacIntyre et al. 1999;Marti & Imberger 2008), volcanic lava gravity currents (Huppert & Dade 1998), and effluent discharges (Rueda et al. 2007;Baines 2008;Meiburg & Kneller 2010;Hodges et al. 2011). When an inclined gravity current descends down a slope, it entrains ambient fluid and is gradually diluted as it flows (Ellison & Turner 1959;Hebbert et al. 1979;Dallimore et al. 2002;Fernandez & Imberger 2006;Cenedese & Adduce 2010). ...
Interaction between an inclined gravity current and a pycnocline in a two-layer stratification - Volume 887 - Yukinobu Tanimoto, Nicholas T. Ouellette, Jeffrey R. Koseff
... The Mann Kendall test highlighted a significant positive trend for Cl − (p < 0.001), with a slope of 0.03 mg·L −1 ·year −1 and an increase over the 25 considered years of about 33%. Data concerning chloride concentrations between 0 and 10 m of depth are reported (Figure 2b), as this layer responds to the variations of the hydrological parameters and watershed discharge more quickly than the entire water column (Hogg et al., 2013). Besides the presence of a positive long-term trend, a seasonal component could be highlighted. ...
Recent studies have highlighted an increase of chloride in many lakes worldwide, with negative effects on chemical and physical properties of inland waters and freshwater biota. In this study, we assessed the long‐term trend (1993–2017) of chloride in Lake Iseo, located in the midlatitudes, and analyzed its relationship with discharge data of the inflows and hydrological and hydroclimatic changes. Additionally, we performed a specific annual survey, collecting water samples at a biweekly frequency in tributaries, calculating chloride load, and comparing the result with the chloride increase observed in lacustrine water. The combined use of Geographic Information System (GIS) and multivariate analyses allowed identification of possible sources of chloride and their seasonal dynamic within the mixed‐land‐use watershed: impervious surface and deicing salt, population and sewage effluents, livestock, and rainwater. Their potential contribution to total mass balance was assessed, cross‐checking the results between chloride source evaluation and load estimation. Chloride load from wastewater treatment plants played an important role, albeit we highlighted that the main source was the road deicing salt, with a primarily winter contribution. The increased anthropization in the watershed was likely the main cause of the chloride enhancement. This study demonstrates that the problem of salinization can also affect lakes located in the midlatitude areas and provides a reproducible method for the identification and quantification of the different sources. Our results will help to better understand the potential sources of salinity in rivers that may reveal processes controlling the salinization of freshwater systems, with implications for future management practices.
... Much of what is known about the transport induced by sidearm differential cooling comes from studies of river underflows (Alavian et al. 1992, Hogg et al. 2013, Cortés et al. 2014). There, the flow path is split into three dynamically different regimes, starting with a plunging stage, followed by an underflow stage, and finally an intrusion stage. ...
Lakes and other confined water bodies are not exposed to tides, and their wind forcing is usually much weaker compared to ocean basins and estuaries. Hence, convective processes are often the dominant drivers for shaping mixing and stratification structures in inland waters. Due to the diverse environments of lakes - defined by local morphological, geochemical, and meteorological conditions, among others - a fascinating variety of convective processes can develop with remarkably unique signatures. Whereas the classical cooling-induced and shear-induced convections are well-known phenomena due to their dominant roles in ocean basins, other convective processes are specific to lakes and often overlooked, for example, sidearm, under-ice, and double-diffusive convection or thermobaric instability and bioconvection. Additionally, the peculiar properties of the density function at low salinities/temperatures leave distinctive traces. In this review, we present these various processes and connect observations with theories and model results.
... The lake drains a large watershed (1807 km 2 ) which is about 30 times larger than the lake surface area) and is also one of the most industrialised districts in the Italian Alps. The complex lake hydrodynamics have been previously studied by Valerio et al. (2012), Pilotti et al. (2014aPilotti et al. ( , 2014b and Hogg et al. (2013). ...
During an initial field survey in 2012, we observed an unexpected asymmetry of dissolved oxygen distribution between the western and eastern side in northern Lake Iseo. Motivated by this apparent anomaly, we conducted a detailed field investigation, and we used a physical model of the northern part of the lake to understand the influences that might affect the distribution of material in the northern section of the lake. These investigations suggested that the Earth's rotation has significant influence on the inflow of the lake's two main tributaries. In order to further crosscheck the validity of these results, we conducted a careful analysis at a synoptic scale using images acquired during thermally unstratified periods by Landsat-8 and Sentinel-2 satellites. We retrieved and post-processed a large set of images, providing conclusive evidence of the role exerted by the Earth's rotation on pollutant transport in Lake Iseo and of the greater environmental vulnerability of the northwest shore of this lake, where important settlements are located. Our study confirms the necessity for three-dimensional hydrodynamic models including Coriolis effect in order to effectively predict local impacts of inflows on nearshore water quality of medium-sized elongated lakes of similar scale to Lake Iseo.
... Much of what is known about the transport induced by sidearm differential cooling comes from studies of river underflows (Alavian et al. 1992, Hogg et al. 2013, Cortés et al. 2014). There, the flow path is split into three dynamically different regimes, starting with a plunging stage, followed by an underflow stage, and finally an intrusion stage. ...
Aquatic physics in inland water is a crucial subject for studying aquatic ecosystems. Transport and mixing are of tremendous importance for the pace at which chemical and biological processes develop. Recent observations allow to distinguish mixing and transport processes in stratified lakes and reservoirs. The surface and bottom boundary layer are turbulent while the lake interior remains comparatively quiescent.
... The main tributary of the lake is the river Oglio, whose waters enter the northern part of the lake mainly as interflow [Hogg et al., 2013;Pilotti et al., 2014a], with a temperature and discharge regime that affects both the thermal structure of the lake [Valerio et al., 2015] and its water renewal time [Pilotti et al., 2014a[Pilotti et al., , 2014b. Considering a volume of the lake of about 7.9 3 10 9 m 3 and the average inflow of 55 m 3 /s, the theoretical renewal time of the lake is about 4.5 years. ...
The representation of spatial wind distribution is recognized as a serious difficulty when modeling the hydrodynamics of lakes surrounded by a complex topography. To address this issue, we propose to force a 3D lake model with the wind field simulated by a high-resolution atmospheric model, considering as a case study a 61 km2 pre-alpine lake surrounded by mountain ranges that reach 1800 m above the lake's surface, where a comprehensive data set was available in the stratified season. The improved distributed description of the wind stress over the lake surface led to a significant enhancement in the representation of the main basin-scale internal wave motions, and hence provided a reference solution to test the use of simplified approaches. Moreover, the analysis of the power exerted by the computed wind field enabled us to identify measuring stations that provide suitable wind data to be applied uniformly on the lake surface in long-term simulations. Accordingly, the proposed methodology can contribute to reducing the uncertainties associated with the definition of wind forcing for modeling purposes and can provide a rational criterion for installing representative measurement locations in pre-alpine lakes.
... The inflow from As Conchas reservoir is expected to have a higher nutrient concentration than the receiving water in Salas reservoir. Nutrient-rich inflows entering stratified water bodies can produce different ecological consequences depending on their intrusion depth [15,22], the most direct link being an increase in phytoplankton biomass with an increase of nutrient availability in the photic zone, hence the importance of the depth in which the inflow from As Conchas reservoir propagates into Salas reservoir. As noted above, the inflow is denser than the surface water of the reservoir in all the tested cases, therefore reducing the effective nutrient loading in terms of biological growth. ...
Pumped-storage hydroelectric power plants are generally perceived as an environmentally respectful technology. Nevertheless, the pumping of water from a lower reservoir to an upper impoundment, and the return of that water during power generation, can strongly affect the water quality of the reservoirs. In particular, plant operation can alter their thermal structure, deep water mixing, and water circulation characteristics. The objective of this study is to quantify, through the use of 3D hydrodynamic modeling, the potential impacts of a pumped-storage hydroelectric plant on the thermal stability and mixing of two reservoirs in Galicia, northwest of Spain. To this end, three-dimensional hydrodynamic simulations were conducted using the model Delft3D. Two different coupled models, one for each reservoir, were constructed and subsequently tested for several stratification scenarios, according to measured temperature profiles during the spring and summer season. Several reservoir minimum and maximum operation water levels were also considered. Model simulations demonstrated a high level of mixing in the vicinity of the intake-outlet structures, in particular during startup of the power plant, regardless of the water level in the reservoir. Beyond this area, the results showed a limited overall effect on stratification and mixing in the upper reservoir, owing to the relation between the inflow temperatures and the initial temperature profile of this reservoir. A more significant alteration of the thermal structure is expected in the lower reservoir due to its narrow shape and shallow depth at the structure location, as well as the temperature differences between receiving waters and inflow.
... The impact of typhoon-induced inflow was large although the inflow effect is usually minor in physical processes in large lakes (Michalsky and Lemmin, 1995). For understanding localized mixing for stratified water bodies, numerous spot measurements are necessary to reveal more precise characteristics of inflow-induced vertical mixing in specific areas with density plume phenomena (Laborde et al., 2010;Hogg et al., 2013). The overall suggestion is that the inflow had an important effect on vertical mixing although wind shear stress was also influential. ...
p>Studying mixing processes in a stratified lake is important for understanding the biological, chemical and physical processes occurring there. Statistical analyses were performed of data from a small, shallow, stratified lake in a subtropical alpine region (Yuan-Yang Lake in Taiwan) to determine the predominant physical factors in heavy-rainfall-induced mixing. This study focused on both vertical mixing in the entire water column and surface-layer mixing extending to the upper thermocline. The effects of meteorological driving forces, such as wind, heating/cooling and inflow on vertical mixing and surface layer mixing, were evaluated using the relationships between each driving force and the change in thermal stability between the pre-mixing and mixing periods. For surface layer mixing, a comparison between penetrative convection related to heating/cooling and wind-related friction velocity was conducted for each heavy rainfall event. A heat content parameter measuring thermal potential energy was introduced to further investigate inflow effects ( e.g. effects of changes in discharge volume and temperature) on vertical mixing during heavy rainfall events. Results show that wind input affected vertical mixing more significantly than did other meteorological forcing factors during storm-dominant events. Indeed, wind energy input in the surface layer was more pronounced than was energy of heating/cooling for surface layer mixing. Furthermore, inflow effect was shown to be crucial during large scale and extreme weather events ( i.e. lower air pressure events) in the vertical mixing process. Forcing by heating/cooling likely contributes less to mixing because it is likely less dynamic than the wind and inflow inputs with respect to internal response of the lake. In addition, a principal component analysis (PCA) modified by partial correlation was performed to verify the results quantitatively. The first and second components, which accounted for more than 90% of the total variance in the PCA, showed that the intensity of vertical mixing was affected primarily by wind-induced turbulence and inflow intrusion and was only weakly associated with the effect of net heat balance. Considering the interactions between chemical and physical processes, inflow intrusion may have an effect on dissolved oxygen concentration in the lake.</p
... hyperpycnal plume). Their dynamics are divided into three distinct stages (Alavian et al., 1992; Cortés et al., 2014; Hogg et al., 2013). First, the river dense water pushes the ambient lake water until the resulting baroclinic pressure created by the local density difference between the river and the lake water balances the force of the momentum inflow. ...
In deep stratified lakes, such as
Lake Geneva, flood-driven turbidity currents are thought to contribute to the
replenishment of deep oxygen by significant transport of river waters
saturated with oxygen into the hypolimnion. The overarching aim of this study
was to test this long-standing hypothesis directly. It combines direct
observational data collected during an extreme flooding event that occurred
in May 2015 with dark bioassays designed to evaluate the consequences of
river-borne inputs for the hypolimnetic respiration. The exceptional
precipitation events of May 2015 caused floods with an annual return time for the
Rhône River, the dominant tributary of Lake Geneva, and with 50-year
return time for the Dranse River, the second-most important tributary.
Sediment-loaded river flows generated turbidity currents plunging into the
lake hypolimnion. The observed river intrusions contributed to the
redistribution of dissolved oxygen, with no net gain, when occurring in the
lowermost hypolimnetic layer. In the uppermost hypolimnion above the last
deep-mixing event, the intrusions coincided with a net oxygen deficit.
Consistent with field observations, dark bioassays showed that 1 to 50 %
substitution of riverine organic matter to deep (< 200 m)
hypolimnetic water did not affect microbial respiration, while the addition of 1
to 10 % of riverine water to the uppermost hypolimnetic waters resulted
in a respiration over-yielding, i.e. excess respiration of both river-borne
and lacustrine organic matter. The results of our study conflict with the
hypothesis that flood-driven turbidity currents necessarily increase
hypolimnetic oxygen stocks in Lake Geneva. In contrast, results show that
flood-driven turbidity currents can be potential hot spots for priming effect
in lakes.
... hyperpycnal plume). Their dynamics are divided into three distinct stages (Alavian et al., 1992;Cortés et al., 2014;Hogg et al., 2013). First, the river dense water pushes the ambient lake water until the resulting baroclinic pressure created by the local density difference between the river and the lake water balances the force of the momentum inflow. ...
In deep stratified lakes, such as Lake Geneva, flood-driven turbidity currents are thought to contribute to the replenishment of deep oxygen by significant transport of river waters supersaturated with oxygen into the hypolimnion. The overarching aim of this study was to test directly this long-standing hypothesis. It combines direct observational data collected during an extreme flooding event that occurred in May 2015 with dark bioassays designed to evaluate the consequences of riverborne inputs on the hypolimnetic respiration. The exceptional precipitations of May 2015 caused floods with annual return time for the Rhône River, the dominant tributary of Lake Geneva, and with 50-year return time for the Dranse River, the second most important tributary. Sediment loaded river flows generated turbidity currents plunging into the lake hypolimnion. The observed river intrusions contributed to the redistribution of dissolved oxygen, with no net gain, when occurring in the lowermost hypolimnetic layer. In the uppermost hypolimnion above the last deep mixing event the intrusions coincided with a net oxygen deficit. Consistent with field observations, dark bioassays showed that 1% to 50% substitution of riverine organic matter to deep (<200 m) hypolimnetic water did not affect microbial respiration, while addition of 1 to 10% of riverine water to the uppermost hypolimnetic waters resulted in a respiration overyielding, i.e. excess respiration of both riverborne and lacustrine organic matter. The results of our study conflict the hypothesis that flood-driven turbidity currents necessarily increase hypolimnetic oxygen stocks in Lake Geneva. In contrast, they show that flood-driven turbidity currents can be potential hot-spots for priming effect in lakes.
... In Lake Iseo, the horizontal length scale of the inlet water intrusions (Oglio River and Italsider Canal) is important from a biological perspective because it is the length scale over which the inflow's potential energy transports nutrients and allochthonous DOC. This length scale is of the order of 4 km as supported by both, conductivity measurements and calculations based on the intrusion dynamics (Hogg et al., 2013). ...
... Density-driven intrusions with their associated staircase-like density profiles have been observed in lakes (see e.g. Caldwell et al. 1978;Hogg et al. 2013) and in the ocean (see Armi 1978). ...
Localized regions of turbulence, or turbulent clouds, in a stratified fluid are the subject of this study, which focuses on the edge dynamics occurring between the turbulence and the surrounding quiescent region. Through laboratory experiments and numerical simulations of stratified turbulent clouds, we confirm that the edge dynamics can be subdivided into materially driven intrusions and horizontally travelling internal wave-packets. Three-dimensional visualizations show that the internal gravity wave-packets are in fact large-scale pancake structures that grow out of the turbulent cloud into the adjacent quiescent region. The wave-packets were tracked in time, and it is found that their speed obeys the group speed relation for linear internal gravity waves. The energetics of the propagating waves, which include waveforms that are inclined with respect to the horizontal, are also considered and it is found that, after a period of two eddy turnover times, the internal gravity waves carry up to 16 % of the cloud kinetic energy into the initially quiescent region. Turbulent events in nature are often in the form of decaying turbulent clouds, and it is therefore suggested that internal gravity waves radiated from an initial cloud could play a significant role in the reorganization of energy and momentum in the atmosphere and oceans.
... Although it was originally developed for use in Australian reservoirs, DYRESM has been used on a wide range of lake systems (e.g. Balistrieri et al., 2006;Trolle et al., 2011;Hogg et al., 2013), and its applicability to boreal lake systems has recently been established (Tanentzap et al., 2007(Tanentzap et al., , 2008. For example, Tanentzap et al. (2007) calibrated the DYRESM model for Clearwater Lake in the Sudbury (Ontario, Canada) region and then employed the model to hindcast wholelake water temperature over a 28-year period, from 197328-year period, from to 200128-year period, from (Tanentzap et al., 2008. ...
Recent climate change represents one of the most serious anthropogenic threats to lake ecosystems in Canada. As meteorological and hydrological conditions are altered by climate change, so too are physical, chemical and biological properties of lakes. The ability to quantify the impact of climate change on the physical properties of lakes represents an integral step in estimating future chemical and biological change. To that end, we have used the dynamic reservoir simulation model, a one-dimensional vertical heat transfer and mixing model, to hindcast and compare lake temperature-depth profiles against 30 years of long-term monitoring data in Harp Lake, Ontario. These temperature profiles were used to calculate annual (June–September) thermal stability values from 1979 to 2009. Comparisons between measured and modelled lake water temperature and thermal stability over three decades showed strong correlation (r2 > 0.9). However, despite significant increases in modelled thermal stability over the 30 year record, we found no significant change in the timing of the onset, breakdown or the duration of thermal stratification. Our data suggest that increased air temperature and decreased wind are the primary drivers of enhanced stability in Harp Lake since 1979. The high-predictive ability of the Harp Lake dynamic reservoir simulation model suggests that its use as a tool in future lake management projects is appropriate. Copyright © 2013 John Wiley & Sons, Ltd.
The interaction of a uniform cooling rate at the lake surface with sloping bathymetry efficiently drives cross-shore water exchanges between the shallow littoral and deep interior regions. The faster cooling rate of the shallows results in the formation of density-driven currents, known as thermal siphons, that flow downslope until they intrude horizontally at the base of the surface mixed layer. Existing parameterizations of the resulting buoyancy-driven cross-shore transport assume calm wind conditions, which are rarely observed in lakes and thereby restrict their applicability. Here we examine how moderate winds (≲ 5 m s-1) affect this convective cross-shore transport. We derive simple analytical solutions that we further test against realistic three-dimensional numerical hydrodynamic simulations of an enclosed stratified basin subject to uniform and steady surface cooling rate and cross-shore winds. We show cross-shore winds modify the convective circulation, stopping or even reversing it in the upwind littoral region and enhancing the cross-shore exchange in the downwind region. The analytical parameterization satisfactorily predicted the magnitude of the simulated offshore unit-width discharges in the upwind and downwind littoral regions. Our scaling expands the previous formulation to a regime where both wind and buoyancy forces drive cross-shore discharges of similar magnitude. This range is defined by the non-dimensional Monin-Obukhov length scale, χ_MO : 0.1 ≲ χ_MO ≲0.5. The information needed to evaluate the scaling formula can be readily obtained from a traditional set of in-situ observations.
This study presents the findings from several field campaigns carried out in Lake Idro (Northern Italy), a deep (124 m) meromictic-subalpine lake, whose water column is subdivided in a mixolimnion (~0–40 m) and a monimolimnion (~40–124 m). Hydrochemical data highlight two main peculiarities characterizing the Lake Idro meromixis: a) absence of a clear chemocline between the two main layers, b) presence of a high manganese/iron ratio (up to 20 mol/mol). The high manganese content contributed to the formation of a stable manganese dominated deep turbid stratum (40–65 m) enveloping the redoxcline (~45–55 m) in the upper monimolimnion. The presence of this turbid stratum in Lake Idro is described for the first time in this study. The paper examines the distribution of dissolved and particulate forms of transition metals (Mn and Fe), alkaline earth metals (Ca and Mg), and other macro-constituents or nutrients (S, P, NO3-N, NH4-N), and discusses their behavior over the redoxcline, where the main transition processes occur. Field measurements and theoretical considerations suggest that the deep turbid stratum is formed by a complex mixture of manganese and iron compounds with a prevalence of Mn(II)/Mn(III) in different forms including dissolved, colloidal, and fine particles, that give to the turbid stratum a white-pink opalescent coloration. The bacteria populations show a clear stratification with the upper aerobic layer dominated by the heterotrophic Flavobacterium sp., the turbid stratum hosting a specific microbiological pool, dominated by Caldimonas sp., and the deeper anaerobic layer dominated by the sulfur-oxidizing and denitrifier Sulfuricurvum sp. The occurrence in August 2010 of an anomalous lake surface coloration lasting about four weeks and developing from milky white-green to red-brown suggests that the upper zone of the turbid stratum could be eroded during intense weather-hydrological conditions with the final red-brown coloration resulting from the oxidation of Mn(II)/Mn(III) to Mn(IV) compounds.
The formation of chemical and biological heterogeneity in lakes can be favored by physical and morphometric constrains. This study describes the results of four whole-lake field campaigns carried out in Lake Como (north Italy) during thermal stratification. The aim was to analyze the distribution of chemical-biological variables in a multi-basin lake as a result of internal and external physical drivers and constrains. Lake Como has a y-like shape encompassing three main sub-basins: northern, south-eastern, and south-western. Field data underlined: the presence of chemical-biological gradients between the south-western basin and the rest of the lake and the propagation of a fresher water-plume (formed by the two main northern inflows) into the northern basin and then into the south-western closed arm. The use of a three-dimensional hydrodynamic model showed a periodic movement of this plume that tends to enter the south-western basin first and then to return toward north, moving forward and back through the junction of the three arms. The entrance into the eastern basin, instead, occurs only secondarily. Wavelet transform analysis revealed common periodicity between the plume movement and the action of different external and internal lake-stressors, including: the discharge of the main inflow (period centered at 1, 3.3, and 6.5 day), the wind intensity (0.5 and 1 day) and the two main basin-scale internal wave motions: (3.3 day and 7.1 day). The periodic movement of the fresher water-plume enhances the water exchange and reduces the chemical-biological gradients between the western closed basin and the main lake, playing a crucial role in distributing both inorganic and organic materials at the lake basin-scale.
The flow duration curve (FDC) is effectively the cumulative distribution function of streamflow. For a long time, hydrologists have sought deeper understanding of the process controls on the shape of FDC, which has been a challenge due to contrasting processes controlling the fast flow and slow flow components of streamflow, and their interactions. In this paper, we outline a new framework for exploring the process controls of the FDC, which involves studying fast and slow flow components of FDC separately, and combining them statistically, explicitly accounting for their dependence. We illustrate the potential of the framework by constructing empirical fast flow duration (FFDC) and slow flow duration (SFDC) curves from the flow components obtained by traditional baseflow separation. Streamflow time series data from 245 MOPEX catchments across the continental United States are used. The dependence of FFDC and SFDC components is captured by the Gumbel copula, the strength of which varies regionally. In catchments where FFDC and SFDC are independent, FDC can be approximated by a simple convolution of FFDC and SFDC. The proposed conceptual framework opens the way for future modeling studies to explore process controls of fast and slow components of streamflow separately, their dependence, and their relative contributions to the shape of the FDC.
A common assumption in models of water flow from soil to root is that the soil can be described in terms of its representative or effective behavior. Microscale heterogeneity and structure are thereby replaced by effective descriptions, and their role in flow processes at the root‐soil interface is neglected. Here the aim was to explore whether a detailed characterization of the microscale heterogeneity at the scale of a single root impacts the relation between flow rate and pressure gradient. Numerical simulations of water flow toward a root surface were carried out in a two‐dimensional domain with a randomized configuration of spatially variable unsaturated hydraulic conductivities and varying boundary conditions, that is, increasing and decreasing root water uptake rates. By employing Matheron's method, the soil hydraulic properties were varied, while the effective hydraulic conductivity (corresponding to the geometric mean) remained unchanged. Results show that domains with a uniform conductivity could not capture important features of water flow and pressure distribution in spatially variable domains. Specifically, increasing heterogeneity at the root‐soil interface allowed to sustain higher root water uptake rates but caused a slower recovery in xylem suction after transpiration ceased. The significance of this is that, under critical conditions, when pressure gradients and flow rates are high, microscale heterogeneity may become an important determinant and should not be neglected in adequate descriptions of water flow from soil to root in dry soil.
Bayesian inverse modeling is important for a better understanding of hydrological processes. However, this approach can be computationally demanding as it usually requires a large number of model evaluations. To address this issue, one can take advantage of surrogate modeling techniques. Nevertheless, when approximation error of the surrogate model is neglected, the inversion result will be biased. In this paper, we develop a surrogate‐based Bayesian inversion framework that explicitly quantifies and gradually reduces the approximation error of the surrogate. Specifically, two strategies are proposed to quantify the surrogate error. The first strategy works by quantifying the surrogate prediction uncertainty with a Bayesian method, while the second strategy uses another surrogate to simulate and correct the approximation error of the primary surrogate. By adaptively refining the surrogate over the posterior distribution, we can gradually reduce the surrogate approximation error to a small level. Demonstrated with three case studies involving high‐dimensionality, multi‐modality and a real‐world application, it is found that both strategies can reduce the bias introduced by surrogate approximation error, while the second strategy that integrates two methods (i.e., polynomial chaos expansion and Gaussian process in this work) that complement each other shows the best performance.
This study investigates the accuracy and reproducibility of air-water interfacial areas measured with high-resolution synchrotron x-ray microtomography (XMT). Columns packed with one of two relatively coarse-grained monodisperse granular media, glass beads or a well-sorted quartz sand, were imaged over several years, encompassing changes in acquisition equipment, improved image quality, and enhancements to image acquisition and processing software. For the glass beads, the specific solid surface area (SSSA-XMT) of 31.6 ±1 cm-1 determined from direct analysis of the segmented solid-phase image data is statistically identical to the independently calculated geometric specific solid surface area (GSSA, 32 ±1 cm-1) and to the measured SSSA (28 ±3 cm-1) obtained with the N2BET method (NBET). The maximum specific air-water interfacial area (Amax) is 27.4 (±2) cm-1, which compares very well to the SSSA-XMT, GSSA, and SSSA-NBET values. For the sand, the SSSA-XMT (111 ±2 cm-1) and GSSA (113 ±1 cm-1) are similar. The mean Amax is 96 ±5 cm-1, which compares well to both the SSSA and the GSSA values. The XMT-SSSA values deviated from the GSSA values by 7-16% for the first four experiments, but were essentially identical for the later experiments. This indicates that enhancements in image acquisition and processing improved data accuracy. The Amax values ranged from 74 cm-1 to 101 cm-1, with a coefficient of variation (COV) of 9%. The maximum capillary interfacial area ranged from 12 cm-1 to 19 cm-1, for a COV of 10%. The COVs for both decreased to 5-6% for the latter five experiments. These results demonstrate that XMT imaging provides accurate and reproducible measurements of total and capillary interfacial areas.
Aproximación con datos de campo y expresiones teóricas al comportamiento de un río ingresando a un embalse encañonado, y como este afecta algunas variables de calidad del agua
The depths of entry of municipal wastewater into receiving lakes importantly affects associated impacts on water quality. The plunging behavior of two negatively buoyant inflows that carry municipal waste, an urban tributary and an effluent discharge, in Onondaga Lake, NY, is characterized and quantified based on an integrated program of monitoring, density calculations, and modeling. In-lake signatures of plunging from the two inflows are differentiated according to constituents in which each is enriched. Under common contemporary conditions, the summer averages of the fraction of the urban stream and effluent discharge inflows plunging to stratified depths is predicted, with a calibrated hydrodynamic model, to be approximately 0.7 and 0.35, respectively. Recent short-term increases in salinity levels from construction site dewatering caused greater plunging of the effluent discharge and interfered with normal complete fall turnover in the lake. Water Environ. Res., 88 (2016).
A combination of field observations and 3D hydrodynamic simulations were used to identify the phytoplankton species and to estimate the various time scales of the dominant physical and biological processes in Lake Iseo, a deep subalpine lake located in northern Italy, during a stratified period (July 2010). By ordering the rate processes time scales we derive a phytoplankton patch categorization and growth interpretation that provides a general framework for the spatial distribution of phytoplankton concentration in Lake Iseo and illuminates the characteristics of their ecological niches. The results show that the diurnal surface layer was well mixed, received strong diurnal radiation, had low phosphorus concentrations and the phytoplankton biomass was sustained by the green alga Sphaerocystis schroeterii. The vertical mixing time scales were much shorter than horizontal mixing time scales causing a depth-uniform chlorophyll a concentration. The horizontal patch scale was determined by horizontal dispersion balancing the phytoplankton growth time scale, dictating the success of the observed green algae. The strongly stratified nutrient-rich metalimnion had mild light conditions and Diatoma elongatum and Planktothrix rubescens made up the largest proportions of the total phytoplankton biomass at the intermediate and deeper metalimnetic layers. The vertical transport time scales were much shorter than horizontal transport and vertical dispersion leading to growth niche for the observed phytoplankton. The study showed that time-scale hierarchy mandates the essential phytoplankton attributes or traits for success in a particular section of the water column and/or water body. This article is protected by copyright. All rights reserved.
Appendices to this thesis can be found at https://www.repository.cam.ac.uk/handle/1810/245309
In many important natural and industrial systems, gravity currents of dense fluid feed basins. Examples include lakes fed by dense rivers and auditoria supplied with cooled air by ventilation systems. As we will show, the entrainment into such buoyancy driven currents can be influenced by viscous forces. Little work, however, has examined this viscous influence and how entrainment varies with the Reynolds number,
Re. Using the idea of an entrainment coefficient, E, we derive a mathematical expression for the rise of the front at the top of the dense fluid ponding in a basin, where the horizontal cross-sectional area of the basin varies linearly with depth. We compare this expression to experiments on gravity currents with source Reynolds numbers,
Res
, covering the broad range 100 < Res
< 1500. The form of the observed frontal rises was well approximated by our theory. By fitting the observed frontal rises to the theoretical form with E as the free parameter, we find a linear trend for E(Res
) over the range 350 < Res
< 1100, which is in the transition to turbulent flow. In the experiments, the entrainment coefficient, E, varied from 4 × 10−5 to 7 × 10−2. These observations show that viscous damping can be a dominant influence on gravity current entrainment in the laboratory and in geophysical flows in this transitional regime.
Water level changes in a lake are balanced by inflows and outflows. They also fluctuate because of wind-generated waves, seiches, precipitation, groundwater inflows, evaporation, and outflows, each of which has a different spatiotemporal scale. Surface waves can lead to short-term changes in water level, whereas heavy precipitation results in larger and more significant changes in water level. To date, the response of water levels in Lake Biwa to heavy precipitation has not been examined in detail. When heavy precipitation falls directly over a lake, water levels respond quickly. In contrast, precipitation falling over a catchment is conveyed to a lake through rivers or as groundwater with a time delay. In this study we examined how lake water levels fluctuate on different time scales, and the relationships between lake water level fluctuations and the physical phenomena that control the fluctuations. By separating Lake Biwa water level data into different time scales, we estimated the ratio of heavy precipitation on catchment areas to lake inflows. We measured water levels and precipitation in the South Basin of Lake Biwa from 2010 until 2012, from which we calculated the influence of riverine inflows on lake water levels during heavy precipitation events. Although the average contribution ratio (â) is thought to be additionally influenced by precipitation characteristics and the land surface conditions before precipitation, we demonstrated that the ratio increased with increased cumulative precipitation. We also found that the time taken for the water level to reach a steady state increased with cumulative increases in precipitation. We found that there was a significant relationship between the â value and cumulative precipitation: (P): â = 100 (1 - e -P/100) (R2 = 0.95, n = 42, p < 0.001). From this study, we have been able to demonstrate that multiple time scales can be identified from analysis of water level data from large lakes such as Lake Biwa, thereby allowing us to identify different influences on lake water levels.
This paper presents a synthesis of the most important studies performed on the deep insubrian lakes (Maggiore, Lugano, Como, Iseo and Garda). Samples taken continuously over the last decades form the basis for a description of the trophic evolution of the lakes, inferred from concentrations of algal nutrients and dissolved oxygen. The results show an improvement in the condition of Lake Maggiore, currently approaching oligotrophy, and in that of lakes Como and Lugano, which are still nevertheless in an unacceptable eutrophic state. Lake Garda has been affected by an increase in mean P concentration since the 1970s, but it has stabilised in recent years. Also in Lake Iseo P concentration has not changed in the last twenty years, but the lake still remains in an eutrophic condition. It is noteworthy that the consequences of eutrophication are accentuated by the incomplete circulation of most of these lakes. This situation has particularly affected lake Iseo, causing permanent anoxic conditions in the water below a depth of 150 m.
Results of a field campaign and numerical simulations are used to show how physical mechanisms impose length scales and timescales that determine the dominant biogeochemical process. As an example, the dynamics of the Snake River inflows into Brownlee Reservoir is investigated to explain the onset and maintenance of an oxygen-depleted region (the oxygen block) in the surface layer of the upstream part of the reservoir. The oxygen block was located in a region of the reservoir in which the surface layer was warmer as a result of smaller wind stresses and reduced evaporation rates. Numerical simulations reproduced the hydrodynamic field observations resulting from inflow, outflow, wind stress, and atmospheric heat fluxes. When the wind stress opposed the inflow, the surface layer was arrested, forming a zone of convergence, stagnating the fluid and allowing the biological oxygen demand in the water to deplete the dissolved oxygen (DO) in the surface water; direct measurements showed that vertical mixing was small and contributed only marginally to the oxygen depletion. Net DO production in the water column was consistent with the observed variation with the buoyant inflow pattern, that is, a sink during overflows and overcast days and a source during interflows and intense sunlight. These observations provided further evidence that the water in this region was biologically isolated as confirmed by a scaling analysis. Modern numerical hydrodynamic simulations have reached a level of accuracy where they may be used to identify and quantify ecological niches.
Field data, theoretical analyses, and numerical simulations were used to investigate the effects of lake basin bathymetry and spatial and temporal structures of the wind field on the basin-scale internal wave spectrum in Lake Iseo. This deep Italian basin is characterized by the presence of a large island, imparting attributes of an annulus to the lake. During the summer of 2010, the internal wave activity was dominated by a basin-scale internal wave of vertical and horizontal modes 1 (V1H1), superimposed on which were occasional higher vertical modes (V2H1) and higher horizontal modes (V1H5) trapped by the main island. The occurrence of these motions was interpreted as forcing by the wind components with similar horizontal structures and with energies at frequencies near the natural oscillations of the excited modes. The modifications of the wind field by the topography, in particular, controlled the excitation of an anticyclonic wave trapped around the island, whose features were investigated on the basis of an extension of the circular model to the case of an annular basin of constant depth.
Field measurements were used to validate predictions for the initial dilution of negatively buoyant, cold‐water inflows to Lake Taupo, as part of a study to quantify mixing processes associated with the two largest inflows to the lake. The predictions were made using a formulation originally derived for positively buoyant, warm‐water inflows to cooling ponds. The formulation predicts the total dilution of an inflow during its inertia‐dominated phase between its entrance to the lake and the point where buoyancy forces are great enough to cause the inflow to plunge and form a submerged density current. In one of the measured inflows, the inflowing jet was free to entrain from both sides; in the other, entrainment was restricted on one side by attachment of the inflowing jet to the shoreline of a bay just upstream of the plunge point. In the former example, the unmodified coefficients from the cooling pond formulation provided an excellent prediction of initial dilution. In the latter example, entrainment was reduced and different coefficients were derived. In both examples the inflows remained attached to the lake bed throughout their course until their liftoff at depths of 45–55 m to form interflows. The difference between coefficients for the two inflows indicates that the coefficient values should be considered site‐specific. The formulation is not valid for inflows that separate from the bottom of the inflow channel before plunging. The entrance mixing formulation was incorporated in a more general model of lake stratification, DYRESM, which already includes a well‐documented routine for routing underflows down submerged channels on the bed of a lake. Application of the model to the inflows measured in Lake Taupo gave good results for two model outputs that were not involved in the calibration of the entrance mixing formulation, but that are affected by the result of the initial dilution calculation—the temperatures in the river plume after it has plunged, and the insertion depth.
A method to match the response of the SBE-3 temperature sensor and the SBE-4 conductivity cell is described. The technique uses a recursive filter in the time domain, which allows direct calculation of salinity and density, and thus offers a significant computational advantage over other methods. The response of any sensor may be matched or sharpened using this method provided that the sensor can be modeled appropriately. Using this method the useful bandwidth of the SBE-3 temperature sensor may be improved by a factor of between 3 and 7, depending on the permissible signal-to-noise ratio degradation. It is also possible to match the SBE-3 and SBE-4 responses closely and thus remove spikes in the profiles of calculated salinity and density.
The Wigner-Vie distribution, a new tool in the time-frequency analysis of signals, is applied to temperature gradient microstructure records. In particular, the Wigner-Vie distribution is used to compute the local in- stantaneous and maximum frequencies of the signal as a function of depth, and these Frequencies are then related to the dissipation of turbulent kinetic energy. The method is applied to two temperature 8radient mi- crostructure records from the Wellington Resewoir. It is shown that a high resolution estimate of the dissipation is obtained that provides in&&t into the patchiness, the wavenumber comer& and the Reynolds-Froude number variability of the integral scales of motion in a strongly stratied water column.
Field data, collected in a shallow underflow into a medium-sized reservoir, show that the dimensionless entrainment, E, is non-negligible at high Richardson numbers. For such underflow, the field data demonstrate that E is mainly dependent on the stress generated at the bottom, directly determined by the relative bed roughness and the properties of the current. The flow is shown to quickly reach a condition of normal flow where Ri is constant; the value of Ri being a strong function of the bottom slope. An entrainment law is presented, in terms of both Richardson number and bottom drag coefficient, written as E = E(Ri,CD), that shows good agreement with the values of E measured in the field.
The time development of two-dimensional fluid motion induced by a line sink in a rectangular, density stratified reservoir with a free surface is given. It is shown that the initiation of such a sink gives birth to a spectrum of internal expanding shear fronts with a progressively decreasing vertical wavelength. These fronts move out from the sink and travel towards the far wall, where they are reflected. This process ceases once the front with a vertical wavelength equal to the steady withdrawal-layer thickness has reached the end wall. The fronts so introduced continue to move back and forth, expanding to standing waves if the viscosity of the fluid is small enough. The evolution and nature of the withdrawal layer are shown to depend critically on the relative magnitude of the convective inertia and viscous forces, the number of reflexions from the rear wall and the Prandtl number.
Temperature, oxygen concentration, conductivity and ionic composition of the water of Lake Iseo (z max 251 m) between 1978 and 2003 were used to analyze the lake's evolution towards meromixis, through stability calculations following Walker (1974). Total, thermal and chemical stability is assessed, both as an integrated quantity on the whole water column and in its vertical distribution. The results show an increase over time of total stability in winter, caused by the increase of solutes present in the deep layers, the result of the isolation of these layers due to a lack of complete vertical mixing, and in summer by an increase in the thickness of the surface water layer involved in the annual heat exchange. It emerges clearly that the anoxia of the deep layers, which first occurred in 1994, is related to the increase in the solutes present in the deep hypolimnion, and that the solutes, via chemical stability, are related to the depth of the vertical winter mixing. In addition, the high density present in the deep layers due to the solutes also prevents them from being oxygenated by hydro-meteorological mechanisms resulting from the inflow of river water to the deep layers, mechanisms which are common in deep lakes like Iseo. The vertical distribution of the stability values makes it possible to identify the water layers that present the greatest resistance to mixing, and to evaluate how their volume has continually expanded over the years, so that now more than 50% of the lake volume is affected. As a result the deep layers are increasingly stable and isolated. The vertical distribution of the chemical stability reveals that at the end of winter Lake Iseo still has below 50 m a level with a chemical stability of more than 0.02 J m -2 , which means that a full circulation cannot occur.
The identification of the factors and mechanisms determining a particular lake's typology is crucial for the correct interpretation of trophic evolution. Nutrient concentrations are not the only properties which determine the trophic characteristics of lakes; others, such as morphometry, hydrology and climatic conditions, also have a major influence on the development of algal biomass and spe-cies composition. Large, deep lakes belong to a well defined typology. Their trophic status depends not only on algal nutrient loads, but also on the extent of the spring vertical mixing and renewal time. These factors are closely interrelated, for the actual renewal time approaches the theoretical renewal time only during the complete overturn. This paper compares the influence of different cir-culation patterns on the trophic status of two deep subalpine lakes with contrasting mixing characteristics. Lake Garda is the largest lake in Italy (S=368 km 2 , V=49 km 3 , Z max =350 m). It is characterised by irregular circulation events (oligomixis); during the 1990s, complete homogenisation of the water column occurred in 1991 and 1999-2000. The years between these episodes showed an in-crease of hypolimnetic temperatures and a progressive vertical stratification of nutrients. Full overturn episodes were characterised by a sudden decrease of temperature and complete homogenisation of the chemical and physical variables along the water column, with a corresponding increase of nutrients and algal biomasses at the surface. In contrast, the last mixing involving the deepest waters in Lake Iseo (S= 60,9 km 2 , V=7,6 km 3 , Z max =251 m) occurred at the beginning of the 1980s, after which the reduced water renewal and the high trophic level resulted in a marked decrease in oxygen concentrations and in the establishment of conditions of anoxia during the 1990s. At present the lake is meromictic. In 1999 and 2000, when a complete overturn was observed in lakes Maggiore and Como as well as in Lake Garda, Lake Iseo experienced a spring mixing which was still limited, but able to determine an increase in the algal biomass.
The Regione Lombardia geological mapping program (CARG), in collaboration with the Istituto Nazionale di Oceanografia e Geofisica Sperimentale of Trieste (OGS), realized a geophysical study of Iseo Lake (Sebino).
A multibeam survey and high-resolution single-channel seismic lines allowed the recognition of five physiographic units with different morphologies and evolution: the Oglio prodelta in the northern part of the lake; a central basin with a regular flat floor between –240 and –256 m in depth; the Monte Isola submerged escarpment, bounding the western shores of Monte Isola and emerging at the small islands of San Paolo and Loreto; the Sale Marasino plateau, with a maximum depth of –100 m, between Monte Isola and the eastern shores of the lake; and the southern Sarnico Basin.
In the single-channel seismic profiles crossing the central basin, the uppermost part of the sedimentary infill of Iseo Lake can be subdivided into three sequences, interpreted from top to base as recent lacustrine deposits, glacial deposits recording the last glacial maximum expansion, and lacustrine deposits.
The morphology of the lake was to a large degree shaped prior to the last glacial maximum (LGM) expansion, as erosional action exerted by the LGM glacier was much reduced. The Oglio prodelta, the fan deltas skirting the lake shores and the landslides along the submerged slopes of the lake occurred subsequent to the retreat of the LGM glacier.
Field data gathered from a Western Australian reservoir are used to obtain an estimate for the entrainment coefficient of the underflowing river. By the use of a steady normal flow theory, the entrainment coefficient is related to the flow and river channel characteristics. By combination with a general turbulent entrainment law, it is shown that the entrainment coefficient and dilution may be directly related to the physical characteristics of the river channel, and thus may be estimated without recourse to a field experiment. In order to determine the initial underflow depth and the location of the plunge line, a simple theory using the concept of a normal flow depth is used, the results of which compare favourably with observation. (A)
An algorithm is developed to compute the conductivity of lake and dilute ocean water from measured chemical composition at arbitrary temperature and pressure. The complex mixed electrolyte is considered as a sum of binary electrolytes rather than a sum of ions. Effects of ion association are included, and it is found that pairing effects are important in natural freshwaters. Bounds on the accuracy of the algorithm for specific classes of binary electrolytes are assessed and it is estimated that the algorithm has an overall accuracy of better than 2% for salinities less than about 4 g L-1. Comparison with seawater conductivities is much better than 1%, but predicted conductivities of some published analyses of river waters are about 3% too high. Some of this difference may be due to a lack of data on ion pairing effects between bivalent metals and bicarbonate, but also may result from uncertainties in the measured chemical composition and measured conductivity. An iterative procedure incorporating this algorithm is used to compute reference conductivity at 25°C and salinity from in situ measurements of conductivity in waters where only relative amounts of ions are known. It is found that the conversion to reference conductivity is reasonably independent (to within about 1%) of the ionic composition for most world river waters, but is somewhat different than that for KCI solutions. However, derived salinities are quite sensitive to the composition, and the ratio of ionic salinity to reference conductivity varies between 0.6 and 0.9 mg L-1 (μS cm-1)-1. © 2008, by the American Society of Limnolngy and Oceanography, Inc.
Negatively buoyant (plunging) flow entering a horizontal and diverging channel has been studied in the laboratory. Such flow is found at the heads of reservoirs and near effluent sites into lakes. Six flow regimes could be distinguished in the experiments. The flows are similar to nonbuoyant, twodimensional diffuser flow, but negative buoyancy adds several variations. The dependence of the flow regimes on the inflow channel aspect ratio, inflow densimetric Froude number, diffuser half-angle, and channel-to-diffuser transition are presented. The experimental results can be used to predict if and where flow separation will occur and what flow regimes will be present.
Outbreaks of waterborne disease have been associated with rainfall when pathogens are washed from the watershed and transported into reservoirs via riverine inflows. To help quantify the concentration and viability of Cryptosporidium contamination in lakes arid reservoirs after such occurrences, the authors have developed a simple model that can be accessed online. In addition to the basic dynamics of inflows, the model includes fate transformations associated with temperature, ultraviolet light exposure, and sedimentation. The model's ability to predict inflow characteristics was tested during several inflows at three reservoirs in Australia. The data presented demonstrate the model's effectiveness as well as provide insight into the dynamics of inflow behavior in reservoirs and the processes affecting source water supply. Source water management is an increasingly important area of the water industry. Water utility operations and eventually public health can benefit from the effective use of a timing tool that can also help reduce treatment costs by selecting the best water for use in the plant.
In this paper the viscous, slightly stratified flow towards a sink is investigated. The fluid is assumed incompressible, linearly stratified in density, and the flow steady. The theoretical portion of this paper includes both the two-dimensional and the axisymmetric cases, whereas the experimental portion includes only the two-dimensional case. The solutions obtained indicate that there exists a with-drawal layer symmetrically situated about the horizontal plane of the sink. The flow occurs in this layer while outside this layer there is essentially no motion. This withdrawal layer grows in thickness with the distance x (or r) from the sink at a rate proportional to $x^{\frac{1}{3}}$ (or $r^{\frac{1}{3}}$). The velocity distributions u(y) (or u(z)) are similar from one station x (or r) to another.
The interaction of multiple inflows entering a reservoir (Thomson Reservoir, Australia)is investigated with a field experiment andthree-dimensional numerical model simulations.The focus of the study is the mixing and transport patterns within the reservoir of the inflowing water.Data from the field experiment showed the existence of multiple inflows intruding horizontally into the reservoir immediately below the thermocline and selectively propagating into the reservoir at a depth determined by the individual density of each inflow. The results serve to show that inflows slot into a stratified reservoir in an orderly fashion, their depth dependent on the separation of inflow densities and reservoir bed, with minimal mixing between intrusions. Inflows thus do not take their nutrient load into the reservoir as a whole, but rather slot their load into an intrusion layer at an appropriate depth. Nutrients brought in via inflows become accessible only when vertical mixing, either directly at the base of the surface layer, or indirectly via the benthic boundary layer flux, transports the intrusion waters into the surface layer.
The three-dimensional hydrodynamic Estuary, Lake and Coastal Ocean Model (ELCOM) was coupled to the ecological Computational Aquatic Ecosystem Dynamics Model (CAEDYM) and to an underflow model to simulate the fate of the constituents from three flood underflow events in Lake Burragorang, Australia, in order to verify the changes in the hydrodynamical behavior that could lead to an algal bloom when the lake water level is low as a consequence of climate change. Simulated patterns of temperature, dissolved oxygen, and turbidity compared well with field data. The ELCOM-CAEDYM simulations demonstrated that the vertical excursion induced by an intrusion depended on the volume of the lake before the arrival of the inflow, on the volume of water inserted by the inflow, and on the proximity between the top insertion of the inflow and the surface layer of the lake. When the water level was low and the inflow volume was high, the underflow constituents mixed into the surface layer and triggered a major algal bloom.
Under certain tidal conditions, a saline underflow originating in the Pacific Ocean moves into Lake Ogawara, Japan. The underflow consists of a uniform saline bottom layer that is slightly warmer than the ambient and an interfacial shear layer in which the velocity and density are decreased. Within the experimental area the underflow is confined to a channel approximately 1 km wide and is essentially two-dimensional. The underflow had a bulk Richardson number, defined in terms of the mean properties, between 1 and 2. The rate of entrainment into the bottom layer was calculated using two distinct methods. The first method used the change in the maximum salinity of the underflow measured at two stations along the path of the underflow to infer the amount of ambient water entrained. The second method made direct measurements of vertical mass fluxes with a profiler. The agreement between the two methods was excellent. The measured entrainment coefficients were consistent with the derived entrainment law. The turbulent structure of the flow was mapped for a 3 h quasisteady period of the flow. Turbulence is predominantly generated on the bottom boundary and is transported vertically to the density interface, where it leads to mixing.
A coupled three-dimensional hydrodynamics and two-dimensional underflow model is adapted to provide simulation of plunging inflows in reservoirs. The new approach accounts for the effect of the barotropic term prior to the plunge point of the inflow. Simulations of plunging flows in constant width and constant slope channels are conducted and the resulting plunge depths are in agreement with prior empirical models. Simulation of a previously measured underflow in Wellington Reservoir (Australia) demonstrates the model application to a plunging inflow in a natural water body and good agreement between field and model results.
A negatively buoyant inflow to a lake or reservoir sinks to the bottom and forms an underflow. In this process, the "plunging flow" entrains ambient water and becomes diluted. The dilution alters the buoyancy and hence the flow behavior of the underflow. Since literature on entrainment by underflows does not include the "plunging process" itself, great uncertainty exists when the initial conditions of underflows have to be estimated. Therefore, the density driven plunging phenomenon is experimentally investigated in a horizontal diverging channel. Entrainment rates are determined from velocity and temperature measurements and related to channel diffuser angle, inflow densimetric Froude number, and inflow channel aspect ratio. The experiments are conducted at inflow channel Reynolds numbers larger than 10,000. Experimental diffuser angles are varied from 3° to 180°, inflow densimetric Froude numbers from 1.8 to 21 and channel aspect ratios from 0.5 to 1.8. The information is of use as input to one-dimensional stratified lake or reservoir water quality models.
If they are negatively buoyant (denser than ambient water), inflows to lakes and reservoirs sink (plunge) below the water surface after entering. The location of the plunge line or maximum penetration into the lake or reservoir is related to the ratio of inertial to buoyant forces, and lake or reservoir geometry. Experiments have been conducted in diverging horizontal laboratory channels, and the plunging phenomenon has been reproduced. For flows separating from the channel wall the relationship xp/B0 = 0.52 F40 has been found to describe the maximum horizontal penetration xp. The basic form and exponent of this relation was developed from jet flow theory, and the coefficient was experimentally determined. F0 is the inflow densimetric Froude number and B0 is the inflow channel width. Experimental data for xp/B0 are also reported for attached flows. In this case the distance to plunging can be expressed as xp/B0 = (0.74 F0 − 0.5)(tan δ)−1.
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The current state of knowledge of density currents caused by negatively (dense) or positively (light) buoyant flows entering stratified or unstratified lakes and reservoirs is reviewed and evaluated. Emphasis is given to heavier than ambient flows that plunge below the surface and move along the lower boundary of the receiving water in their initial stage of propagation. The behavior of entering flow, plunging flow, density current, separating flow, and interflow is briefly described. Methods commonly used in the analysis of various flow regimes (zones) are presented. Selected references and examples of the application of density currents are provided to assist the practicing hydraulic and environmental engineering community. Finally, gaps in the current knowledge and research needs are identified for the hydraulic engineering research community.
Measurements of velocity and density profiles were made to describe the transverse structure of the flow in Aysen Fjord, Southern Chile (45.2°S and 73.3°W). Current profiles were made with a 307.2kHz acoustic doppler current profiler (ADCP) during 20 repetitions of a cross-fjord transect during one semidiurnal tidal cycle. The transect had a ∼320° orientation, 3km length, and its bathymetry consisted of two channels, one on the southern side (230m depth) and the other on the northern side (180m depth), separated by a bank ca. 65m depth, which was located ∼1km from the northern coast. Density measurements to a maximum depth of 50m were made at the extremes of each transect repetition and over the bank. Also, a total of nine CTD stations that covered the surroundings of the bank was sampled 2 days prior to the ADCP sampling. During the sampling period the mean flow showed a three layer structure that was consistent with up-fjord wind-induced exchange: a thin (
Stable stratification seems to suppress the turbulence enough so that interesting events are somewhat spread out in space and time. Most particularly, there seem to be unique opportunities to identify and trace to its source the particular energy supply tapped by the small-scale motions that do the mixing in various circumstances. This provides the organizational theme for this review, and mixing motions in three categories are discussed: those deriving their kinetic energy from the gravitational potential energy of a water column that is in some sense top-heavy; those that covert to smaller scales the kinetic energy associated with large-scale shear flows or internal waves; those that are driven by some fairly obvious external source of small-scale agitation. Here we think particularly of the wind mixing of surface waters.
A reservoir is assumed to be filled with water which has a linear variation of density with depth. The geometry of the boundaries is simplified to a parallel walled duct with the line sink at the centre of the fluid. The primary focus is on partitioning the flow into distinct flow regimes and predicting the withdrawal-layer thickness as a function of the distance from the sink; the predictions are verified experimentally.
For fluids with a Schmidt number of order unity, the withdrawal layer is shown to be composed of distinct regions in each of which a definite force balance prevails. The outer flow, where inertia forces are neglected, changes from a parallel uniform flow upstream to a symmetric self-similar withdrawal layer near the sink. For distances from the sink smaller than a critical distance, dependent on the flow parameters, inertia forces become of equal importance to buoyancy and viscous forces. The equations valid in this inner region are derived. Using the inner limit of the outer flow as the upstream boundary condition, these inner equations are solved approximately for the withdrawal-layer thickness by an integral method. The inner and outer variations of δ, the withdrawal-layer thickness, are combined to yield a composite solution and it is seen that the inclusion of inertia forces yields layers thicker than those obtained from a strict buoyancy-viscous force balance. In terms of the inner variables the only parameter remaining is the Schmidt number.
Laboratory experiments were carried out to verify the theoretical conclusions. The observed withdrawal-layer thicknesses were shown to be closely predicted by the integral solution. Furthermore, the data could be represented in terms of the inner variables by a single curve dependent only on the Schmidt number.
Tidal outflow from Leschenault Estuary in winter forms an unsteady surface buoyant jet in Koombana Bay on each outgoing tide. As the water moves away from the exit channel and spreads radially over the bay, it assumes a plumelike nature. The mean features of this buoyant outflow, such as the plume shape, frontal celerity, internal circulation, and the bulk entrainment, were measured in an intensive field program. These features are shown to compare favorably with results from previous laboratory investigations and predictions from a simple inertia-buoyancy force balance which includes entrainment at the front. The stability of the plume frontal region was examined, and it is shown that subfronts were present, but their behavior cannot be explained by reference to previous work.
An algorithm is presented for obtaining the rate of turbulent kinetic energy dissipation by fitting the theoretical Batchelor spectrum to the temperature gradient spectrum at high wavenumbers. The algorithm is relatively robust in selecting the turbulent Batchelor component from temperature gradient spectra, which have finestructure, internal wave, and noise contributions. The theoretical curve is fitted using an error function that takes into account many of the characteristics of the Batchelor spectrum. Overall, the use of the algorithm to determine dissipation of the turbulent kinetic energy is considerably more time efficient than manual methods. Some limits on the accuracy of the method are also discussed.
Scaling arguments are presented for the magnitudes of Fr "SUB T" and Re "SUB T" . It is shown that these may vary widely and depend, in the first instance, on the physics of the underlying processes energizing the turbulence. by implication, from Part I, this means that the ratio of the buoyancy flux b to the net rate of input of mechanical energy m varies between 0 and 0.2 for events where b0. For events which are energized by a negative buoyancy flux, scaling arguments are used to recast the depth dependence, derived in Part I, to a dependence on the Reynolds number Re "SUB T" . The magnitude of the pair (Fr "SUB T", Re "SUB T" ) are then derived directly from temperature microstructure measurements taken in lakes and spanning eight different phenomena: neutral surface layers penetrative convection, shear layers, diurnal thermoclines, thermals, intrusions, hypolimnetic mixing and boundary mixing. The field data also show large variations in the values of Fr "SUB T" and Re "SUB T" . Only in the thermocline region of the lake where all mixing events are governed by an inertia-buoyancy balance is Fr "SUB T" constant with a value of between 1 and 3. (A)
We describe the flow paths of negatively buoyant river inflows in a small lake with multiple basins separated by sills (Toolik Lake, Alaska) using field data and three-dimensional simulations. Comparisons of field observations, analytical computations, and simulations show that in small basins in which the timescale for filling (tau(f)) is less than the timescale of an event (tau(0)), overflow is the dominant mechanism of interbasin exchange. Exchange is mediated by internal wave displacements if upwelling reaches the height of the sills, z(s). This criterion is met if the Lake number, the inverse of which is proportional to the degree of metalimnetic tilting, is less than z(i)/(z(i) - z(s)), where z(i) is the vertical displacement of the intrusion. Entrainment followed by horizontal dispersion is the dominant mechanism of interbasin transport and can account for similar to 65% of the exchange between large subbasins. Entrainment is enhanced when submerged sills force river water to flow close to the surface layer. Depths of intrusions depend upon discharge. They occurred in the lower metalimnion for the coldest events analyzed but were near the top of the metalimnion during low-discharge events, such as during brief cold fronts. Persistence of intrusions depends upon the time interval between cold fronts; they ranged from a few days to three weeks. Timescales of horizontal mixing vary with meteorological forcing; they ranged from on the order of 1 d for wind events with speeds of up to 6-8 m s(-1) to on the order of 10 d for light winds.
The principles of physical limnology in the Southern Hemisphere are no different to those in other parts of the world and the same mechanisms may be expected to be active in lakes of similar size and shape. However, in actuality, it is observed (Imberger, 1982; Ward, 1982; Allanson & Jackson, 1983; Powell et al., 1984) that the temperature gradients and their seasonal occurrence are more predominant in the Southern Hemisphere. The temperature range is fixed by the winter cold inflow and the summer heating and the temperature range from the bottom of a lake to the lake surface in the Southern Hemisphere is commonly up to 10 C° or even 15 C° during the summer peak period. Combined with the overall elevated mean temperature, this leads to an extremely strong buoyancy stabilization of the water column and it is this which characterizes lakes in the Southern Hemisphere.
The dynamics of negatively buoyant river plumes in a small multi-basin kettle lake with steep bathymetry (Toolik Lake, AK) are simulated using a Cartesian hydrodynamic model based on the solution of the three-dimensional shallow water equations. To validate the model, model predictions are compared with results from previous analytical and laboratory studies and with field observations. The grid resolution adopted for the Toolik Lake model is 0.5 m (= Δz) in the vertical and 20 m (= Δx) in the horizontal, so that the ratio of the bottom slope S0 to Δz/Δx is lower than 4 in 99% of the computational domain. With that resolution, the model represents correctly the rate of mixing between lake and river water and the speed of propagation of downslope gravity currents. The model provides accurate predictions of the temperature structure (RMSE = 0.25 °C) and of eddy diffusivities at the depths of the intrusions of incoming water. Measurements and modelling show similar fractions and depth distribution of river water on a cross-basin transect, which suggests that the mixing dynamics of the plume as it transits between basins are well resolved. Thus, the stage is set to quantify the ecological consequences of storm events in small lakes with several interconnected basins using coupled biological measurements and 3D modelling.
The ecological consequences of river inflows, usually carrying large amounts of nutrients, will be different depending on whether it penetrates to the hypolimnion, inserts in the metalimnion or remains on the surface. For plunging rivers, the intrusion depth is controlled by the river-reservoir density difference prior to the plunge point together with mixing processes between ambient and inflowing water, which occur both in the region of the plunge and after the flow has assumed the form of a density current. In contrast with the processes of entrainment into density currents, which have been extensively studied, entrainment and mixing within the plunge zone has received less attention in the physical limnology literature. The existing literature fails to identify adequate parameterizations of this phenomenon, while exhibiting a large variability in the initial mixing rate, measured both in the laboratory and in the field. This work examines the consequences (in terms of uncertainty) that our lack of knowledge of parameters describing river-reservoir mixing in 1-D transport models has on the estimates of river nutrient loads into the euphotic zone of a reservoir, where nutrients are readily available for phytoplankton growth.