Harbour dredging survey with a two-hour tide-gauge outage
A maintenance dredging survey is required in a 1.2 km harbour basin where charted depths are 8 to 14 m. You run a multibeam echosounder with RTK tide and a shore tide gauge. The primary gauge fails for 95 minutes near the middle of the survey. A backup gauge is 3.1 km away and shows a similar tidal curve, but the tidal range is 3.2 m and the phase lag between gauges is about 18 minutes. The survey is to be delivered to S-44 Order 1a and used to update the harbour chart and dredge design. Your preliminary vertical uncertainty budget gives 0.18 m from the echosounder and attitude, 0.12 m from sound velocity, and 0.10 m from the tide transfer during the outage. The dredging contractor wants the whole area accepted so the dredge can mobilise. The chart datum is based on a 19-year tide record at the primary gauge, which is now unavailable.
1. Evaluate whether the tide-outage section can be accepted within the survey order. Show how you would combine the uncertainty components and what threshold you would apply.
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I would first combine the independent uncertainty components by root sum square: sqrt(0.18 squared + 0.12 squared + 0.10 squared) = about 0.235 m. That is only the preliminary budget. The 0.10 m tide-transfer term looks optimistic because an 18-minute phase lag on a 3.2 m tidal range can produce a significant height difference near mid-tide. At a rough tidal rate of 0.5 m per hour, 18 minutes is about 0.15 m, before allowing for gauge datum differences and interpolation error. A more realistic tide term might be 0.15 to 0.20 m, giving a combined uncertainty near 0.27 to 0.30 m. I would compare this with the total vertical uncertainty allowed by S-44 Order 1a for the depth range, using the order formula rather than a guessed constant. If the combined uncertainty is within the order limit and the crossover checks show no bias, the section may be accepted with a documented caveat. If it exceeds the limit, or if the backup gauge cannot be tied to the primary gauge datum, the outage section should be re-surveyed. Commercial pressure to mobilise the dredge is not a reason to accept unverified bathymetry.
2. How would you transfer chart datum and reduce the outage section? Include what evidence you would retain.
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Chart datum is a long-term datum based on the primary gauge record; I would not redefine it from a short backup record. I would reduce the outage section using the backup gauge after correcting for phase and datum, and I would compare the result with RTK tide and a tidal model where available. The comparison should be made at overlapping soundings before and after the outage and at nearby check lines. If the backup gauge is not levelled to the same datum, I would establish the offset by simultaneous observation or by a known benchmark connection. I would retain the raw primary and backup tide data, gauge benchmark levels, time stamps, the phase-lag analysis, RTK tide records, crossover differences and the full uncertainty budget. The final chart and dredge surface should identify any area where the vertical uncertainty is higher than the surrounding survey. The evidence must allow an independent reviewer to see how chart datum was maintained and why the outage section was accepted or rejected.
3. What would you tell the contractor about mobilisation and why?
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I would tell the contractor that mobilisation can proceed only for areas that meet the survey order and have a verified vertical reduction. The outage section is not automatically accepted just because the rest of the basin is complete. If the uncertainty check passes, I would release the area with a note on the higher uncertainty and the tide-transfer method; if it fails, the dredge should not rely on those depths until the gap is re-surveyed. The reason is that dredge design and charting both depend on depth below chart datum, and a systematic tide error can make a shallow area look deeper or deeper areas look shallower. I would give the contractor a clear plan: which lines are accepted, which are provisional, and when the re-survey can be done. This protects both the dredging operation and the chart update from a false sense of completeness.