Page 182 - Fundamentals of Ocean Renewable Energy Generating Electricity From The Sea

In Situ and Remote Methods for Resource Characterization Chapter | 7 171

FIG. 7.9 Time series of (A) backscatter and (B) current speed from a 2-week 600 kHz ADCP
deployment at the northeast approach to the Menai Strait, UK.

7.1.5 Lagrangian Drifters

Moored current meters and ADCPs measure properties of the sea in an
1
Eulerian context. However, it is often desirable to measure properties in a
2
Lagrangian framework. This could be useful, for example, when attempting to
understand how larvae propagate through a region, how a contaminant such as
an oil spill is advected by the current, or for measuring longer-timescale residual
flow. Such measurements are conducted using drogues (Fig. 7.11). For short-
term deployments, a drogue can be an inexpensive and relatively simple device
with two key elements: a surface buoy (so that the drogue can be identified
and visually tracked) and a (weighted) parachute with a suitable length of cable
appropriate to the representative depth of water that is to be tracked. In the
absence of a parachute, the movement of the drogue would simply correspond
to the wind-influenced surface currents. Using an inexpensive arrangement
and differentiating between surface markers, it is possible to release and track
(e.g. with a small boat and a hand-held GPS) many drogues simultaneously
over relatively short timescales. There is minimal financial impact if a drogue
is lost, and it is more statistically meaningful to track the dispersal of a

1. Observations of fluid motion based on a fixed point in space.
2. Observations of fluid motion where the observer follows an individual fluid parcel as it moves
through space and time.