Module 11 - Tides & Tidal Streams
Tidal Anomalies
The simple six-hour rise-and-fall model does not describe every port. Basin and coastal response can produce double high or low waters, long stands, asymmetric rise and fall, bores and stream behaviour that does not reverse at local HW or LW.
An anomaly changes the passage decision. A stand can extend one part of an entry window, while a rapid fall after it can close depth unexpectedly. A stream turn can precede or follow the local height event. The source note and local curve therefore outrank a generic rule of twelfths.
Read the almanac or sailing-direction note, the named reference port and the port-specific curve. Build a latest safe time and an earlier abort gate. Do not infer a precise cause or timing from a regional label alone.
- Local tides can have double waters, stands, asymmetry or bores
- Stream turns need not coincide with local HW or LW
- Use the port-specific note and curve
- Translate the anomaly into an explicit timing and abort decision
| Observed or published feature | Planning risk | Required response |
|---|---|---|
| Prolonged stand | Window looks wider than the later fall allows | Use the full local curve |
| Double high water | Wrong peak selected | Identify both events and required one |
| Rapid fall | Depth margin closes quickly | Set an earlier leave/abort time |
| Stream offset | Wrong atlas hour or gate | Use named reference and local note |
Optional quick check
Section 7 of 13
A port has a prolonged stand followed by a rapid fall. What should control the departure window?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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