Currents, Waves, and Other Physical Measurements
Domain focus
This domain covers currents, waves, and other physical measurements: instrument principles, coordinate systems, and derived parameters.
Core rules
- ADCPs measure velocity profiles using Doppler shifts from scatterers. Key settings include frequency, bin size, ensemble interval, blanking distance, and profiling range.
- Side-lobe interference contaminates the near-surface and near-bottom bins; exclude invalid bins and document the processing.
- Transform beam coordinates to instrument coordinates, then to earth coordinates using compass heading and magnetic declination; tilt affects the result.
- Current direction convention matters: toward versus from, and true versus magnetic north.
- Wave parameters: significant wave height (average of the highest one-third of waves), peak period, and direction. ADCP wave bursts and directional buoys use different processing assumptions.
- Other measurements include CTD, salinity, temperature, turbidity, and dissolved oxygen; each has calibration and fouling issues.
Worked example
An ADCP in 20 m of water shows suspiciously fast near-surface currents. The analyst checks side-lobe contamination, excludes the top bins, applies magnetic declination, and compares with a nearby current meter. The corrected profile is plausible; the original was an artifact, not a real jet.
Traps
Reporting the top bin as surface current; forgetting declination; mixing knots and m/s; treating significant wave height as maximum wave height; ignoring tilt and blanking distance.