Swish Floats: An Inexpensive Neutrally Buoyant Float to Monitor Dispersion in Coastal Seas

Neutrally buoyant floats have been widely used to measure flows in the ocean, but deploying them in large numbers can be costly and impractical. This is particularly true near coastlines due to the elevated risk of instrument grounding or vessel collisions, resulting in a lack of subsurface Lagrangian measurements in coastal regions. Here, we describe an inexpensive neutrally buoyant satellite-tracked float (named Swallow-ish, or Swish floats) that has been designed and tested as a costeffective strategy to measure subsurface dispersion in coastal areas on time scales up to a month. These autonomous instruments are inexpensive, constructed at a material cost of CAD $300 per unit; lightweight, with a mass of 5.4 kg; isopycnal; and constructed from commercially available components, using recently available global navigation satellite system technology to provide the user with a point-to-point measure of subsurface transport. We describe the float design, ballasting techniques, and the governing equations that determine their behavior. Further, through 29 deployments in two coastal seas, we calculate an uncertainty budget and determine a ballasting error of 61.6 g, corresponding to a local depth targeting error of 16-30 m, analyze the float resurfacing data to calculate subsurface dispersion coefficients, and examine the float depth records to quantify the local internal wave field. Finally, we evaluate surface dispersion using the postresurfacing trajectories. Our findings indicate that Swish floats offer a cost-effective alternative for Lagrangian measurements of subsurface flows in coastal regions.

Automated summary

This article describes an inexpensive neutrally buoyant satellite-tracked float (Swish float) for measuring subsurface dispersion in coastal areas. These floats are cost-effective, lightweight, and constructed from commercially available components, using global navigation satellite system technology. Through field deployments and data analysis, the study demonstrates that Swish floats provide a cost-effective alternative for measuring subsurface flows in coastal regions.