Observations and modeling of the surface seiches of Lake Tahoe, USA

A rich array of spatially complex surface seiche modes exists in lakes. While the amplitude of these oscillations is often small, knowledge of their spatio-temporal characteristics is valuable for understanding when they might be of localized hydrodynamic importance. The expression and impact of these basin-scale barotropic oscillations in Lake Tahoe are evaluated using a finite-element numerical model and a distributed network of ten high-frequency nearshore monitoring stations. Model-predicted nodal distributions and periodicities are confirmed using the presence/absence of spectral power in measured pressure signals, and using coherence/phasing analysis of pressure signals from stations on common and opposing antinodes. Surface seiches in Lake Tahoe have complex nodal distributions despite the relative simplicity of the basin morphometry. Seiche amplitudes are magnified on shallow shelves, where they occasionally exceed 5 cm; elsewhere, amplitudes rarely exceed 1 cm. There is generally little coherence between surface seiching and littoral water quality. However, pressure–temperature coherence at shelf sites suggests potential seiche-driven pumping. Main-basin seiche signals are present in attached marinas, wetlands, and bays, implying reversing flows between the lake and these water bodies. On the shallow sill connecting Emerald Bay to Lake Tahoe, the fundamental main-basin seiche combines with a zeroth-mode harbor seiche to dominate the cross-sill flow signal, and to drive associated temperature fluctuations. Results highlight the importance of a thorough descriptive understanding of the resonant barotropic oscillations in any lake basin in a variety of research and management contexts, even when the magnitude of these oscillations tends to be small

Use of tracers to quantify subsurface flow through a mining pit

Three independent tracer experiments were conducted to quantify the throughflow of water from Herman Pit, an abandoned mercury (Hg) mine pit adjacent to Clear Lake, California, USA. The tracers used were Rhodamine-WT, sulfur hexafluoride, and a mixture of sulfur hexafluoride and neon-22. The tracers were injected into Herman Pit, a generally wellmixed water body of ̃81 000 m2, and the concentrations were monitored in the mine pit, observation wells, and the lake for 2-3 months following each injection. The results for all three experiments showed that the tracer arrived at certain observation wells within days of injection. Comparing all the well data showed a highly heterogeneous response, with a small number of wells showing this near-instantaneous response and others taking months before the tracer was detectable. Tracer was also found in the lake on four occasions over a onemonth period, too few to infer any pattern but sufficient to confirm the connection of the two water bodies. Using a simple mass balance model it was possible to determine the effective loss rate through advection for each of the tracers and with this to estimate the through-flow rate. The through-flow rate for all three experiments was ̃630 L/s, at least 1-2 orders of magnitude larger than previous estimates, all of which had been based on geochemical inferences or other indirect measures of the pit through-flow. © 2008 by the Ecological Society of America

Snowmelt Timing as a Determinant of Lake Inflow Mixing

Snowmelt is a significant source of carbon, nutrient, and sediment loads to many mountain lakes. The mixing conditions of snowmelt inflows, which are heavily dependent on the interplay between snowmelt and lake thermal regime, dictate the fate of these loads within lakes and their ultimate impact on lake ecosystems. We use five decades of data from Lake Tahoe, a 600 year residence-time lake where snowmelt has little influence on lake temperature, to characterize the snowmelt mixing response to a range of climate conditions. Using stream discharge and lake profile data (1968–2017), we find that the proportion of annual snowmelt entering the lake prior to the onset of stratification increases as annual snowpack decreases, ranging from about 50% in heavy-snow years to close to 90% in warm, dry years. Accordingly, in 8 recent years (2010–2017) where hourly inflow buoyancy and discharge could be quantified, we find that decreased snowpack similarly increases the proportion of annual snowmelt entering the lake at weak to positive buoyancy. These responses are due to the stronger effect of winter precipitation conditions on streamflow timing and temperature than on lake stratification, and point toward increased nearshore and near-surface mixing of inflows in low-snowpack years. The response of inflow mixing conditions to snowpack is apparent when isolating temperature effects on snowpack. Snowpack levels are decreasing due to warming temperatures during winter precipitation. Thus, our findings suggest that climate change may lead to increased deposition of inflow loads in the ecologically dynamic littoral zone of high-residence time, snowmelt-fed lakes

Snowmelt Timing as a Determinant of Lake Inflow Mixing

Snowmelt is a significant source of carbon, nutrient, and sediment loads to many mountain lakes. The mixing conditions of snowmelt inflows, which are heavily dependent on the interplay between snowmelt and lake thermal regime, dictate the fate of these loads within lakes and their ultimate impact on lake ecosystems. We use five decades of data from Lake Tahoe, a 600 year residence-time lake where snowmelt has little influence on lake temperature, to characterize the snowmelt mixing response to a range of climate conditions. Using stream discharge and lake profile data (1968–2017), we find that the proportion of annual snowmelt entering the lake prior to the onset of stratification increases as annual snowpack decreases, ranging from about 50% in heavy-snow years to close to 90% in warm, dry years. Accordingly, in 8 recent years (2010–2017) where hourly inflow buoyancy and discharge could be quantified, we find that decreased snowpack similarly increases the proportion of annual snowmelt entering the lake at weak to positive buoyancy. These responses are due to the stronger effect of winter precipitation conditions on streamflow timing and temperature than on lake stratification, and point toward increased nearshore and near-surface mixing of inflows in low-snowpack years. The response of inflow mixing conditions to snowpack is apparent when isolating temperature effects on snowpack. Snowpack levels are decreasing due to warming temperatures during winter precipitation. Thus, our findings suggest that climate change may lead to increased deposition of inflow loads in the ecologically dynamic littoral zone of high-residence time, snowmelt-fed lakes

An SF6 tracer study of the flow dynamics in the stockton deep water ship channel: Implications for dissolved oxygen dynamics

A sulfur hexafluoride (SF6) tracer release experiment was conducted in the Stockton Deep Water Ship Channel (DWSC) to quantify mixing and transport rates. SF6 was injected in the San Joaquin River upstream of the DWSC and mapped for 8 days. From the temporal change in SF6 distributions, the longitudinal dispersion coefficient (K x ) was determined to be 32.7 ± 3.6 m2 s-1 and the net velocity was 1.75 ± 0.03 km day-1. Based on the decrease in SF6 inventory during the experiment, the pulsed residence time for waters in the DWSC was estimated at ∼17 days. Within the DWSC from Stockton downstream to Turner Cut, dissolved oxygen concentrations maintained a steady state value of 4 mg l-1. These values are below water quality objectives for the time of year. The low flow rates observed in the DWSC and the inability of oxygen-rich waters from downstream to mix into the DWSC upstream of Turner Cut contribute to the low dissolved oxygen concentration. © 2008 Coastal and Estuarine Research Federation

Shadowgraph observations of the transition of the thermal boundary layer in a side-heated cavity

The transition of the thermal boundary layer from start-up to a quasi-steady state in a side-heated cavity is observed using a shadowgraph technique. Three stages of the transition, including an initial growth stage, an entrainment development stage and a quasi-steady stage, are demonstrated. A significant feature of the transition revealed from the present flow visualisation is the formation of a double-layer structure along the sidewall at the entrainment development stage. It is believed that the reverse flow in the double-layer structure is the likely cause responsible for the unstable travelling waves at the quasi-steady state

Numerical models for management of anabaena circinalis

The control of nuisance species of cyanobacteria in reservoirs is a critical issue for the internationalwater industry, as these organisms can produce toxins and compounds that taint potablewater with unpleasant tastes and odours. To assist with effective management of cyanobacterial growth, numerical models that are either site specific or universally applicable can be employed. An artificially destratified reservoir was modelled with the coupled hydrodynamicecological numerical model DYRESM-CAEDYM. The validation site was Myponga Reservoir, South Australia, a highly managed drinking water supply reservoir. Chemical dosing (CuSO4) and artificial mixing via an aerator and two raft-mounted mechanical surface mixers (hereafter referred to as surface mixers) are used at Myponga to manage the growth of the scum-forming cyanobacterium Anabaena circinalis. The dominant phytoplankton community was adequately modelled, and combinations of the various management options were simulated whereupon informed operational strategies could be implemented. Without any intervention, permanent stratification would occur and the growth of Anabaena circinalis would peak above 3 μg L−1, producing geosmin that would exceed the taste and odour threshold (10 ng L−1); with the individual use of the aerator or surface mixers, growth of Anabaena circinalis was significantly reduced to below 1 μg L−1.David M. Lewis, Justin D. Brookes and Martin F. Lamber