Wind sheltering of a lake by a tree canopy or bluff topography

A model is developed to quantify the wind sheltering of a lake by a tree canopy or a bluff. The experiment‐based model predicts the wind‐sheltering coefficient a priori, without calibration, and is useful for one‐dimensional (1‐D) lake hydrodynamic and water quality modeling. The model is derived from velocity measurements in a boundary layer wind tunnel, by investigating mean velocity profiles and surface shear stress development downwind of two canopies and a bluff. The wind tunnel experiments are validated with field measurements over an ice‐covered lake. Both wind tunnel and field experiments show that reduced surface shear stress extends approximately 50 canopy heights downwind from the transition. The reduction in total shear force on the water surface is parameterized by a wind‐sheltering coefficient that is related to the reduction of wind‐affected lake area. While all measurements are made on solid surfaces, the wind‐sheltering coefficient is shown to be applicable to the lake surface. Although several canopy characteristics, such as its height, aerodynamic roughness, and its porosity affect the transition of velocity profiles and surface shear stress onto a lake, a relationship based on canopy height alone provides a sufficiently realistic estimate of the wind‐sheltering coefficient. The results compare well with wind‐sheltering coefficients estimated by calibration of lake water temperature profile simulations for eight lakes.