A new Lagrangian-Acoustic Drogue (LAD) for monitoring flow dynamics in an estuary: A quantification of its water-tracking ability

We describe a novel in situ measuring drogue device developed for the investigation of near-surface sediment and flow dynamics in an estuary. The Lagrangian Acoustic Drogue (LAD) comprises a unique configuration of instruments including a downward-facing 600 kHz Broadband Acoustic Doppler Current Profiler mounted on a Lagrangian float; a Differential Global Positioning System; and a (Conductivity, Temperature, Depth sensor. Open ocean studies have revealed the necessity of understanding the Lagrangian nature of the drifter or drogue. As the ocean and estuarine systems are two vastly distinct environments, a separate study was conducted using the LAD in an estuarine environment. The main objective of this study was to directly quantify the LAD's ability to track and monitor a Lagrangian parcel of water over a slack-water period. It was found that the LAD confidently tracked a parcel of water to a depth of 4.5 m. It was also found that the water tracking ability (slippage) was directly related to the magnitude of tidal velocity. This study also revealed characteristics about the physical state of the estuary itself, and gave positive outlook for the future of drogue-based studies in estuaries