Going with the Flow II: Use of Drifters to Address Uncertainties in the Spatial Variation of Tidal Flows

Abstract

Drifters are one of the oldest, simplest, and most reliable methods for measuring ocean currents, and also provide a simple and low risk platform from which to gather acoustic information along flow streamlines (drift tracks). In the context of tidal energy developments, drifters provide flow speed information needed for evaluating resource, engineering designing, and supporting marine operations.

With previous funding support from OERA the project team has developed:

  • Instrumentation, methodology, and software for using several low-profile Global Positioning System (GPS) based drifters concurrently to map surface flows (SF drifters) at several tidal energy sites in Southwest Nova Scotia, with recent demonstration and application in the Minas Passage.
  • A prototype Acoustic Doppler Current Profiler (ADCP) drifter, including demonstration in the Minas Passage.
  • Drones and Drifters methodology and software for calculating flow speeds and trajectories of biodegradable drifters using aerial images collected by Unmanned Aerial Vehicles (UAVs a.k.a.drones).

The project plan for Going With the Flow 2 focuses on:

  • Comprehensive characterization of Grand Passage (and to a lesser extent Petit Passage), as high quality demonstrations of flow field mapping that can now be applied to any site. The primary focus on one site allows for more advanced development and validation of the methods for data collection and analysis.
  • ADCP drifter testing in Grand Passage and Petit Passage, with a focus on characterizing turbulence from a mobile platform.
  • Analysis of our existing SF and ADCP drifter data sets (including Minas Passage)
  • Data visualizations for information sharing
  1. The ADCP drifter to evaluate the along track:
    1. vertical structure of the velocity
    2. vertical structure of turbulent dissipation rate
    3. vorticity (at the surface).
  2. Several SF drifters to evaluate spatial variation in flow speeds throughout the flood-ebb tidal cycle.

The project outcome is site characterization information useful for advancing research and micrositing tidal turbines for testing, demonstrations, and/or commercial developments. The information has also be utilized by a) Dr. Alex Hay to advance research on turbulence in high-energy tidal environments, and b) Dr. Richard Karsten for calibration and validation of the FVCOM numerical model (FVCOM).

The surface flow measurements have been combined with existing data and updated images of surface flow-fields are available on-line with public access. A webinar providing an overview of flow speed measurements by drfiting platforms, and how they are used for site assessment and marine operations planning is avaiable at https://vimeo.com/285279452.

Example videos of flow field progression through tide time are avaiable at:

For turbulence analysis, on the whole, the results are very encouraging, and clearly demonstrate that the ADCP drifter is a valuable tool for investigating large-scale turbulent structures in high-flow tidal channels. Especially noteworthy, one might even say revelatory, is the evidence for water-depth scale flow features with 1 m/s vertical velocities persisting along-track for time scales of minutes.

Going with the Flow II: Use of Drifters to Address Uncertainties in the Spatial Variation of Tidal Flows is located in Canada.