Diffusional mass transfer coefficient at the water-sediment interface for wind-induced flow in very shallow lagoons

Abstract

Very shallow lagoons that are a few centimeters deep are common in the arid Andes of Northern Chile, Argentina, Bolivia and Peru . The dynamics of these lagoons are dominated by the water-sediment interface (WSI) and strong afternoon winds. Although many studies have examined the diffusional mass transfer coefficients (k(t)) of open channel flows, estimates for wind-induced flows are still unknown. The aim of this article is to propose and validate an analytical expression for computing k(t) at the WSI for wind-induced flow. The laboratory measurements were conducted in a wind tunnel with a water tank of variable depth located at its downwind end. Natural muddy sediments were placed in the middle of the tank so that the dissolved oxygen (DO) was consumed in the sediments. The diffusional mass transfer coefficient that characterizes the DO uptake in the sediment was obtained from DO micro-profiles measured with an OX-25 Unisense microelectrode. Water velocity profiles were measured with a 2D side-view Sontek acoustic doppler velocimetry (ADV), and the wind shear velocity was computed based on wind velocity profiles that were measured with an Extech hot-wire anemometer. A total of 16 experiments were conducted with different water depths and wind shear stresses. The constants required by the model were determined from these experiments, and the analytical expression was successfully validated by the laboratory observations. The analytical expression obtained for computing k(t) was also validated with field observations that were conducted in October, 2012, in Salar del Huasco, Northern Chile (20.274 degrees S, 68.883 degrees W, 3800 m above sea level). The comparison between the observed and predicted values of k(t) provides a determination coefficient of r(2) = 0.48 and a p value < 0.01. The results show that the value of k(t) for wind-induced flow is proportional to the wind shear velocity and the inverse of the Reynolds number of the wind-induced current.