CH Acoustic Doppler & Velocity Measurement 2 — Questions and Answers
Question 1: The index velocity method for computing streamflow relies on a relationship between:
- Stage and cross-sectional area only
- A measured velocity at a fixed point and mean channel velocity (Correct answer)
- Salinity and acoustic backscatter
- Water temperature and discharge
Correct 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.
Question 2: What is the primary source of uncertainty in ADCP discharge measurements caused by the region near the water surface?
- GPS signal multipath errors
- The unmeasured top layer due to blanking distance and draft (Correct answer)
- Electromagnetic interference from the boat engine
- Refraction of acoustic beams by temperature gradients
Correct 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.
Question 3: Which extrapolation method is recommended by USGS for estimating discharge in the unmeasured near-bed zone of an ADCP measurement?
- Linear extrapolation from the deepest bin
- Power-law or constant extrapolation based on bed roughness (Correct answer)
- Gaussian curve fitting to the full profile
- Assuming zero velocity at the bed
Correct 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.
Question 4: A specific acoustic frequency of 600 kHz for an ADCP is best suited for which measurement environment compared to a 300 kHz unit?
- Deeper oceanic water columns
- Shallower rivers and estuaries (Correct answer)
- Profiling below the thermocline
- Measuring salinity gradients
Correct 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.
Question 5: In an ADCP moving-vessel measurement, 'GPS-referenced' mode is used when bottom tracking fails because:
- GPS is always more accurate than bottom tracking
- Strong bed sediment transport or very deep water can make bottom tracking unreliable (Correct answer)
- Regulations require GPS backup at all times
- GPS eliminates the need for magnetic compass correction
Correct 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.
Question 6: What quality control check involves comparing ADCP discharge measurements from two or more repeated transects across the same cross section?
- Compass calibration check
- Moving bed test
- Transect repeatability check (Correct answer)
- Salinity calibration loop
Correct 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.
CH Acoustic Doppler & Velocity Measurement 2
The index velocity method for computing streamflow relies on a relationship between: