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

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 (B 6h) and 6-h averaged wind speed in the direction of the embayment (U 6h) 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 < B 6h < 0.5 and 2.4 ms−1 < U 6h < 4.5 ms−1, the resulting circulation was a combination of differential cooling flows and a topographic gyre circulation, and (3) when U 6h 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.