Drag down is when drag forces, often waves or strong currents, act on a mooring and result in an instrument being dragged down and away from its original position. Since the instrument is moved downwards in the water column, drag down can be discovered by a pressure spike. It is also common to see higher roll and pitch values at the time of displacement. Figure 1 shows an example of how drag down can look in the data for an instrument mounted on a subsurface buoy. You can see the accordance between pressure and tilt spikes and the apparent change in speed. Drag down is often noticed by abnormal high readings of speed.
The principle of current measurements based on the Doppler effect is to measure relative movement between an instrument and the water. When an instrument is in a fixed position, the measured current velocities represent the water's motion exclusively. There is no way to distinguish between the movement of the instrument and the water when the instrument itself is moving. The consequence of drag down is hence invalid current data that affects the entire profile.
Drag down typically occur when nearby currents get so strong that the mooring is unable to withstand the forces. An effective way to reduce the risk of drag down is to minimize the amount of volume that the current can take hold of. This can be to use a thinner rope or a shorter rope if possible.
For some, the main purpose of measurements with ADCP is to determine the maximum speed in an area. This can unfortunately be difficult to achieve if the deployment setup is susceptible to the drag down effect.
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