A novel drifter designed for use with a mounted Acoustic Doppler Current Profiler in shallow environments

Abstract

Lagrangian drifters have been used for many years to measure surface currents over a wide range of spatial and temporal scales. The cost and design of these drifters varies depending on the application. Building on deployments of drifters in the deep ocean (e.g., Davis 1985), and taking advantage of cheap and accurate modern GPS systems, researchers have recently developed low-cost drifters for use in nearshore, estuarine (Johnson et al. 2003;Austin and Atkinson 2004;Thomson 2012) and surfzone (Schmidt et al. 2003;MacMahan et al. 2009) environments. However, the drifter designs above typically have drafts of around 0.5 to 1 m, rendering them unsuitable for use in very shallow flows.Modern compact Acoustic Doppler Current Profilers (ADCPs) are capable of measuring velocities over range bins of a few centimeters. The small size, high resolution, and short blanking distance of these instruments have raised the possibility of using drifter-mounted ADCPs to resolve near-surface flows, even in very shallow environments.We present a novel design for surface drifters intended for use with a Nortek Aquadopp ADCP. The design yielded highresolution vertical profiles of velocity, as the drifter tracked the propagation of baroclinic surface fronts over very shallow tidal flats (water depths of 0.2-1 m). The highest bin of the velocity profiles was around just 0.18 m below the water surface, a notable improvement over use of majority of the above designs, which would result in a smallest measurement depth of around 0.62 m below the surface (based on a representative 0.5 m drifter draft plus 0.07 m ADCP draft plus 0.05 m blanking distance). The Thomson (2012) drifter used an uplooking ADCP, which allowed measurements to be made close to the water surface; however, the drifter had a 1.25 m draft preventing its deployment in very shallow water depths. For water depths of between 0.15 m and 0.18 m, the ADCP-mounted drifter described here was still freely advected, but in this case, ADCP measurements were not obtained. The drifters were robust and of very simple construction, requiring only use of hand tools and using off-the-shelf components. Although the cost of the ADCP was substantial (exceeding US$10,000), the cost for each drifter body was minimal (less than US$100, similar to costs of the other drifters above). Drifter positions were tracked by an inexpensive off-the-shelf GPS logger (<US$100). Materials and proceduresDrifters (Fig. 1) were constructed from sections of PVC pipe (outside diameter 60mm chosen to ensure a low-draft) and connectors. Pipe, connectors and PVC cement are easily obtained and assembly was straightforward. Crossing pipes provided attachment points for the ADCP, which was secured using hose clamps. The width of the drifter (0.72 m) was chosen to ensure the drifter body extended beyond the ends of the ADCP, providing protection in the event of collision with a solid object. To minimize windage, drifters were filled with granular ballast (feedcorn) until nearly neutrally buoyant, with only the top ~ 10 ...

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