Experiments on intrusions generated by boundary mixing in a lake
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SUMMARY A technique is given for obtaining exact analytical solutions for wind-driven currents in a shallow lake or sea with a large class of bottom topographies which may or may not contain an island. Welander's second-order partial differential equation for the lake's surface displacement is used for determining the lake circulation. The technique, together with some results based on conformal mapping, is used to reduce the problem of finding solutions to Welander's equation for a closed body of water to the problem of solving an ordinary differential equation. The method is applied to an elliptically shaped lake and a circular lake containing an eccentrically located circular island.
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Stratification (seeds)
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The physical processes responsible for the exchange of water and particles
between lake boundaries (littoral) and interior (pelagic) are investigated using field data and
the three-dimensional hydrodynamic Estuary and Lake Computer Model (ELCOM) in Lake
Kinneret, Israel. The field data revealed large-scale metalimnion oscillations with amplitudes
up to 10 m in response to westerly diurnal winds and the existence of a well-defined
suspended particle intrusion into the metalimnion of the lake. The changes in the thermal
structure explained the observed vertical and horizontal movements of the suspended particle
intrusion. The horizontal advective transport via the metalimnion, associated with the
velocities induced by the basin-scale mode-two Poincare wave, controlled the exchange
between lake boundaries and interior on daily timescales. Detailed comparison of simulation
results with field data revealed that the model captured the lake hydrodynamics for time
scales from hours to days. The numerical simulation showed bottom shear velocities capable
of resuspension and so a source of the observed suspended particle intrusion. Together, field
data and numerical modelling clarified the flux path of water and particles between littoral
and pelagic waters in the lake and demonstrated the importance of a proper characterization
of the processes involved required for modelling and management of water quality resources.
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Dynamics
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Circulation (fluid dynamics)
Inflow
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Turbulent benthic boundary layers generated by breaking of internal waves along the sloping bottom of a stratified lake play a significant role in vertical mixing. Benthic boundary mixing in density stratified lakes was investigated by conducting laboratory measurements. The turbulence characteristics of a benthic boundary layer were investigated by using MSCTI (Micro Scale Conductivity and Temperature Instrument) in conjunction with LDV (laser-Doppler-velocimetry). The measurements indicate that the dominant flux path, the path along which particles travel, is along the benthic boundary layer and then horizontally into the main water body via intrusions. The experimental measurements conducted in the Hydromechanics Laboratory at Purdue University suggest that the: (a) breaking of internal wave on sloping boundaries generates a turbulent benthic boundary layer; (b) the thickness of the boundary layer can be scaled as H ∼ (λωξL/(N cosθ))(1/3), where λ, ω, ξ, are internal wave length, frequency, and amplitude respectively, L is the length of the sloping boundary, N is the buoyancy frequency, and θ is the slope of the boundary; and (c) the turbulence intensity in the benthic boundary layer exhibits an oscillatory behavior.
Stratified flows
Stratification (seeds)
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Abstract New mixing algorithms to model the vertical mixing processes in stratified lakes have been developed for the Dynamic Reservoir Simulations Model, DYRESM, and have been validated using five lakes of different size, shape and wind forcing characteristics. An analysis of temperature profiles from Lake Kinneret, Canning Reservoir and Mundaring Reservoir, were used to develop a strong inverse relationship between the Lake number and lake‐wide average vertical eddy diffusion coefficient. Analysis of microstructure data collected in Lake Kinneret using the portable flux profiler suggests that the development of a turbulent benthic boundary layer (BBL) accounts for a large proportion of the lake‐wide average vertical flux. A pseudo two‐dimensional model with explicit BBL and internal fluxes was developed based on the Lake Kinneret field observations and similar investigations in the literature. A sensitivity analysis revealed that improvements in the ability of DYRESM to model the diverse range of lakes considered without user‐calibration was attributable to a wind‐sheltering algorithm and a process‐based description of BBL and internal fluxes.
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