Wind effect on diurnal thermally driven flow in vegetated nearshore of a lake

Abstract

In this study, a highly idealized model is developed to discuss the interplay of diurnal heating/cooling induced buoyancy and wind stress on thermally driven flow over a vegetated slope. Since the model is linear, the horizontal velocity can be broken into buoyancy-driven and surface wind-driven parts. Due to the presence of rooted vegetation, the circulation strength even under the surface wind condition is still significantly reduced, and the transient (adjustment) stage for the initial conditions is shorter than that without vegetation because of the reduced inertia. The flow in shallows is dominated by a viscosity/buoyancy balance as the case without wind, while the effect of wind stress is limited to the upper layer in deep water. In the lower layer of deep regions, vegetative drag is prevailing except the near bottom regions where viscosity dominates. Under the unidirectional wind condition, a critical dimensionless shear stress $\Gamma _{cri}$ to stop the induced flow can be found and is a function of the horizontal location $x$. For the periodic wind condition, if the two forcing mechanisms work in concert ($\theta =0$), the circulation magnitude can be increased. For the case where buoyancy and wind shear stress act against each other ($\theta =\frac{1}{2}$), the circulation strength is reduced and its structure becomes more complex. However, the flow magnitudes near the bottom for $\theta =0$ and $\theta =\frac{1}{2}$ are comparable because surface wind almost has no influence.