Acoustic Doppler & Velocity Measurement Flashcards

The index velocity method for computing streamflow relies on a relationship between:

Answer: A measured velocity at a fixed point and mean channel velocity

The index velocity method uses a rating curve that relates a continuously monitored point velocity (index velocity) to mean cross-sectional velocity, which is then multiplied by area to compute discharge.

What is the primary source of uncertainty in ADCP discharge measurements caused by the region near the water surface?

Answer: The unmeasured top layer due to blanking distance and draft

The zone from the water surface to the first valid bin (blanking distance plus transducer draft) contains unmeasured flow that must be extrapolated, introducing uncertainty.

Which extrapolation method is recommended by USGS for estimating discharge in the unmeasured near-bed zone of an ADCP measurement?

Answer: Power-law or constant extrapolation based on bed roughness

USGS guidance recommends power-law (1/6-power) or constant extrapolation for the near-bed unmeasured zone, selected based on observed velocity profile shape and bed conditions.

A specific acoustic frequency of 600 kHz for an ADCP is best suited for which measurement environment compared to a 300 kHz unit?

Answer: Shallower rivers and estuaries

Higher acoustic frequencies (600 kHz) provide finer range resolution and work well in shallow environments but attenuate faster, limiting maximum profiling depth compared to lower frequencies.

In an ADCP moving-vessel measurement, 'GPS-referenced' mode is used when bottom tracking fails because:

Answer: Strong bed sediment transport or very deep water can make bottom tracking unreliable

Heavy bedload transport or excessive depth can cause bottom tracking to lose the return signal, so GPS provides an independent vessel velocity reference in these conditions.

What quality control check involves comparing ADCP discharge measurements from two or more repeated transects across the same cross section?

Answer: Transect repeatability check

Repeatability checks require measuring at least four transects and comparing results; the mean should be within ±5% of individual transect values per USGS guidelines.