5.3 Tidal Diamonds, Stream Atlases & Tidal Gates

5.3 Tidal Diamonds, Stream Atlases & Tidal Gates

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Tidal Streams vs. Tidal Heights

High or low water does not by itself identify slack water. Ideal wave models illustrate why:

• Progressive tidal wave: Maximum current occurs near high and low water, with slack near mid-tide.
• Standing tidal wave: Slack occurs near high and low water, with maximum current near mid-tide.
• Actual coastal waters: Reflections, friction and differences in level between connected basins produce more complex timing. Use the local tidal-stream table, atlas or predictions; do not assign a real channel one model and infer its safe transit time from height alone.

Streams are classified as rectilinear (oscillating back and forth along a 180° axis in channels) or rotary (turning continuously through 360° across 12 hours with no true slack water in open bays).

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Reading Chart Tidal Diamonds

Admiralty charts depict tidal stream data via lettered purple diamonds (e.g., [A], [B]). The corresponding data is tabulated in the chart margin.

Tidal Diamond Table Architecture

• Reference Standard Port: Explicitly names the reference port for all hourly rows (e.g., 'Tidal Streams referred to HW at DOVER').
• Hourly Rows: 13 rows from 6 hours before High Water (-6 h) through High Water (HW) to 6 hours after High Water (+6 h).
• Direction (°T): The true direction toward which the stream sets (water movement direction).
• Rates (Knots): Velocity in knots at Springs (mean spring rate) and Neaps (mean neap rate).

Sample Chart Tidal Diamond Table: Diamond [B]

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Interpolating Tidal Stream Rates

Use the day’s range at the reference port with the published mean neap and spring ranges. These are averages, not absolute limits; follow the publication’s guidance outside them. The navigator interpolates the stream rate using the Range Factor:

$\text{Range Factor} = \frac{\text{Today's Range} - \text{Neap Range}}{\text{Spring Range} - \text{Neap Range}}$ $\text{Predicted Rate} = \text{Neap Rate} + \left[ \text{Range Factor} \times (\text{Spring Rate} - \text{Neap Rate}) \right]$

Worked Interpolation Scenario

• Exercise assumptions: Reference-port Mean Spring Range = 6.0m, Mean Neap Range = 3.2m; these are supplied values, not current Dover predictions.
• Today's Predicted Range = 5.3m.
• At 2 hours before HW (-2 h), Diamond [B] states: Dir 098°T, Spring 3.2 kn, Neap 1.6 kn.

Calculation

1. Range Factor: $\frac{5.3 - 3.2}{6.0 - 3.2} = \frac{2.1}{2.8} = \mathbf{0.75}$ (75% from Neaps to Springs).
2. Rate difference: $3.2 - 1.6 = 1.6\text{ knots}$.
3. Interpolated Rate: $1.6\text{ kn} + (0.75 \times 1.6\text{ kn}) = 1.6 + 1.2 = \mathbf{2.8\text{ knots}}$.
4. Resulting Vector: Stream sets 098°T at 2.8 knots.

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Admiralty Tidal Stream Atlases (NP Series)

Admiralty Tidal Stream Atlases (e.g., NP209, NP218) provide regional geographic overviews of tidal flow across 13 synchronized hourly charts:

• Flow Arrows: Indicate flow paths; length and thickness show relative velocity.
• Comma-Separated Figures (Tenths of a Knot): Each arrow displays two numbers without decimals:
  • First number: Rate at Mean Neaps in tenths of a knot.
  • Second number: Rate at Mean Springs in tenths of a knot.
  • Example: 14,28 denotes 1.4 knots at Neaps and 2.8 knots at Springs.
  • Example: 04,11 denotes 0.4 knots at Neaps and 1.1 knots at Springs.
• Weak or Reversing Streams: Read the publication’s legend and hourly data. Do not infer slack from a universal symbol or from HW/LW height alone.

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Observing the Stream at Marks

Check the predicted set against the water actually flowing past a fixed beacon or moored buoy, observed from a safe distance. The disturbed water/wake extends downstream. A boat moored at one end usually hangs downstream from its mooring, but wind can change its alignment; drifting objects are also affected by wind.

Record the time, position and observed direction. This checks the local surface flow, not a precise offshore rate: eddies and river flow may differ from the atlas prediction. Resolve an unexpected direction before relying on the planned stream allowance or tidal gate.

Source: Port of London Authority Tideway Code, “Ebb tide, flood tide and the tidal stream”. These observation principles do not import the Thames’s local navigation rules into a coastal passage.

Strategic Passage Planning Around 'Tidal Gates'

A Tidal Gate is a passage, strait or headland where tidal conditions make the timing of transit critical. Examples include Portland Bill, The Needles, Hurst Narrows and the Alderney Race.

Because cruising speeds of displacement yachts (5 to 8 knots) are comparable to stream rates, an adverse stream can leave little or no headway and may aggravate the sea state.

The Backwards Planning Principle

Chained Tidal Gates

When a passage crosses multiple tidal gates in sequence (e.g., The Needles, St Albans Head, and Portland Bill), skippers must verify whether ground speed allows carrying the fair tide through consecutive gates. If the distance exceeds 20 to 30 miles, a displacement yacht may catch fair tide at the first gate only to face foul tide at the second. The passage plan must include designated intermediate anchorages (e.g., Swanage or Weymouth) to await the next favorable cycle.