Acoustic Doppler & Velocity Measurement Flashcards

The 'moving bed' condition during ADCP measurements occurs when:

Answer: Bedload sediment transport causes the bottom to appear to move relative to the instrument

A moving bed condition exists when bedload sediment moves fast enough to bias bottom-tracking velocity estimates, causing apparent upstream vessel movement and discharge overestimation.

Which test is performed to detect a moving bed condition during ADCP field measurements?

Answer: Stationary moving bed test with vessel anchored or held fixed

In the stationary moving bed test, the vessel is held stationary while the ADCP bottom tracks; any apparent displacement indicates bedload movement.

In Doppler velocity measurement, the Nyquist velocity limit defines:

Answer: The maximum unambiguous velocity measurable without phase aliasing

The Nyquist limit is the maximum velocity the system can measure unambiguously; velocities exceeding it produce aliased (wrapped) readings that appear as opposite-sign velocities.

What does the 'error velocity' output from a four-beam ADCP indicate?

Answer: Inconsistency between redundant beam-pair velocity solutions, flagging data quality

Error velocity is computed from the difference between the two independent estimates of vertical velocity from opposite beam pairs; large values flag acoustic interference or inhomogeneous flow.

The speed of sound used by an ADCP to convert travel time to range is primarily a function of:

Answer: Water temperature, salinity, and pressure (depth)

Sound speed in water increases with temperature, salinity, and pressure; the ADCP uses a sound speed value (often from a sensor or default profile) to scale range calculations correctly.

According to USGS Techniques and Methods 3-A22, the recommended minimum number of transects for a standard ADCP discharge measurement is:

Answer: 4 transects

USGS guidance recommends a minimum of four transects (two in each direction) to obtain a reliable mean discharge estimate and assess measurement repeatability.