Wind and buoyancy driven horizontal exchange in shallow embayments of a tropical reservoir: Lake Argyle, Western Australia

Abstract

Many factors including depth, vegetation density, wind, and gyres may act to influence the littoral exchange in a water body but very few studies have investigated the interaction between more than two of these factors at any time. To investigate these controls on horizontal exchange in a large tropical reservoir, we conducted a 9‐d intensive field experiment in Lake Argyle, Western Australia. The experiment began with a 7‐d cooling period that generated water in the shallows of the reservoir embayments that was persistently cooler than the interior. This led to an underflow of dense water that moved from the lake boundary toward the center of the reservoir. A three‐dimensional hydrodynamic model (ELCOM) was able to adequately reproduce this thermal structure and was used to demonstrate its sensitivity to wind‐sheltering effects and submerged macrophyte presence. Further analysis of the predictions indicated that when the ratio of the shear and buoyancy force, averaged over 6 h (B6h) and 6‐h averaged wind speed in the direction of the embayment (U6h) was (1) greater than 0.5 ms−1 and 4.5 ms−1 respectively, the exchange was dominated by a topographic gyre formation, (2) when 0.1 < B6h < 0.5 and 2.4 ms−1 < U6h < 4.5 ms−1, the resulting circulation was a combination of differential cooling flows and a topographic gyre circulation, and (3) when U6h fell below 2.4 ms−1, purely buoyancy driven flow occurred but only if the buoyancy forces across the embayments were an order of magnitude greater than wind‐induced velocity shear.

Temperature and Circulation Dynamics in a Small and Shallow Lake: Effects of Weak Stratification and Littoral Submerged Macrophytes

“…The velocity profile has large gradients near the water surface and the lake bed, but is constant at the mid-layer, which is totally different from the two-layer velocity profile under the baroclinic scenario. In other words, a smooth and log layer velocity profile under the barotropic scenario is replaced by the two-layer daytime stratification and reduce the mixed layer depth by increased solar radiation attenuation and reduced wind mixing [32,33,72]. In this paper, it is found that the three-dimensional temperature structures in a small and shallow lake can be affected by velocity retardation and vertical mixing reduction due to littoral macrophytes, which have not yet been reported in the literature before.…”

“…daytime stratification and reduce the mixed layer depth by increased solar radiation attenuation and reduced wind mixing [32,33,72]. In this paper, it is found that the three-dimensional temperature structures in a small and shallow lake can be affected by velocity retardation and vertical mixing reduction due to littoral macrophytes, which have not yet been reported in the literature before.…”

“…The hydrodynamic model is driven by meteorological data including solar radiation, air temperature, wind speed and direction, as shown in Figure 2. The wind stress at the water surface is calculated over each cell via an aerodynamic drag relationship with a wind speed-dependent drag coefficient and based on an interpolated wind field at 10 m height [53,72]. The dense forest around the shoreline can produce a sheltered zone over the lake surface, which can result in a relatively inhomogeneous wind field [20].…”

“…Based upon Secchi depth measurements and sensitivity analysis, a shortwave solar radiation attenuation coefficient of 1.0 m −1 is appropriate for the period of simulations [74]. The bottom friction coefficient is selected as 0.002 [20,72]. A time step of 60 s is chosen, yielding a ratio of simulation time to real time of about 1:15.…”

Laboratory Studies on Nearshore Density-Driven Exchange Flow over a Partly Vegetated Slope

“…In aquatic environments, convective exchange flows play an important role in the transport of nutrients, pollutants and chemical substances between the littoral and pelagic regions of lakes or reservoirs under weak wind conditions and in the absence of other sources of momentum (e.g., river flows) [1][2][3]. The convective exchange flows are mainly driven by a density difference in the horizontal direction, which can be caused by nearshore topographic changes [4], vegetation shading [5], or turbid patches in the water [6].…”