ADCP Settlement and Heading Error at a Bridge Pier
A PORTS current station uses a bottom-mounted ADCP on a bridge pier in a tidal channel. After a spring flood, a scour hole forms near the mount, and the ADCP's pressure sensor indicates the instrument has settled 0.4 m. The station still transmits valid-looking data. A 30-day comparison with a temporary adjacent ADCP shows a persistent 15–25 cm/s cross-channel bias in the first three bins, decreasing with range, while deeper bins agree within 5 cm/s. The compass was calibrated one year ago; magnetic declination at the site is 13°E, and the pier contains steel pilings. The local pilots report that the displayed current direction is rotated about 10–15° from what they observe. The station's data are used for tug and barge transits. What are the likely causes, and what should be done before the data are used again?
1. What are the likely causes of the bin bias and the direction error?
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The first-bin bias is consistent with side-lobe contamination and changed bin geometry after the instrument settled. When the ADCP drops 0.4 m, the blanking distance and cell positions relative to the boundary change, and the near-boundary bins are more affected by sidelobe reflections and by the scour hole's altered flow. The decrease of bias with range supports a near-instrument geometry or boundary effect rather than a uniform velocity error. The 10–15° direction error is more likely a heading problem. A compass calibrated a year ago can drift, and steel pilings can distort the local magnetic field. The 13°E declination must be applied with the correct sign and reference; if true north and magnetic north are confused, an offset of roughly the declination magnitude can appear. The pilots' independent observation is useful evidence. Both causes should be treated as separate problems: bin geometry and heading.
2. What field and processing checks should be performed before the station is returned to service?
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Recover or inspect the mount, measure the actual depth and tilt, and update the bin mapping and blanking distance. If the instrument cannot be reset immediately, reprocess with the measured depth and exclude the affected near-boundary bins rather than publishing them. Perform an in-situ compass calibration or a comparison against an external heading reference, and check the declination sign and units in the configuration. Compare the ADCP direction with the temporary adjacent ADCP and with the pilots' observations over several tidal cycles. Check for sediment or biofouling on the beams and transducers. Review the pressure record to confirm the settlement time and flag data from that point. Do not apply an arbitrary 10–15° rotation to make the display match the pilots; correct the heading source and document the change.
3. How should the data be flagged and communicated while corrections are underway?
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Flag the first three bins as invalid or suspect from the time of settlement, and flag direction as suspect from the time the heading error is identified. If deeper bins are still reliable, publish only those bins with a clear note that near-surface or near-bottom coverage is limited. Update the station metadata with the instrument depth, tilt, bin configuration, heading source, and declination. Notify tug and barge operators that the current direction and near-boundary currents are under review and that they should not use the affected products for close-quarters maneuvering. Keep the raw data and document the reprocessing method. The key judgement is that valid-looking numbers are not enough: a 15–25 cm/s bias and a 10–15° rotation can materially affect transit decisions, so the station should publish a restricted, clearly flagged product until the mount, bins, and heading are corrected.