Performance characterization and relative dispersion of drogued drifters deployed in the fjords of Nootka Sound

Abstract

[author abstract] Current velocities and dispersion were analyzed in Muchalat inlet using drifters as a Lagrangian sensor. The recorded velocity measurements were compared to two moored ADCP’s that were placed in the channel of Muchalat near the base of its two sills. The classic Davis style drifter design was modified in this study and used for subsurface measurements at a depth of 30 m. Since this was the first known study to use Davis style drifters for subsurface measurements, the main objective was to characterize the performance of the drogued drifters using the moored ADCP to act as a ground truth. The surface component of the drifter incorporated the use of a handheld GPS unit to log its time and position while deployed. Four experiments were designed to evaluate the performance of the drogues with deployments on both flood and ebb tides and adjustment of the initial spacing of the drogues from 10, 25 and 50 m separation to resolve the impact of tidal forcing and across channel variability on the circulation in the channel. The commonality between all experiments was the velocity of the drogue being greater than the speed recorded by the ADCP from 0.01 m/s to 0.1 m/s. This suggests that the current was not the only mechanism forcing the drogue and other variables such as wind, stratification, GPS fix and drifter design were affecting the recorded data values. This implies that the surface area of the Davis drifter may not be large enough to accurately record subsurface measurements.