Chapter 9: Tides and Tidal Currents

Equilibrium theory and tidal types

The Moon's and Sun's gravitational and centrifugal effects raise two bulges, giving roughly two highs and two lows per lunar day (24h 50m). Spring tides occur near new and full moon when Sun and Moon act together; neap tides at the quarters. Actual tides depend on basin shape, giving semidiurnal (two nearly equal highs, US East Coast), diurnal (one cycle, Gulf of Mexico), and mixed (two unequal, US West Coast) types.

Working the tide tables

Table 1 gives times and heights of high and low water for reference stations; Table 2 gives time and height differences or ratios for subordinate stations; Table 3 interpolates intermediate heights. Height of tide is added to charted depth to obtain actual depth. For under-keel clearance, subtract draft and add a safety margin for squat and swell.

• Charted depth + height of tide = depth of water
• Vertical clearance under a bridge decreases as the tide rises
• Predictions are astronomical; wind and pressure can shift them by a metre or more

Rule of Twelfths and tidal currents

For a roughly sinusoidal semidiurnal tide, the range is consumed in twelfths over six hours: 1, 2, 3, 3, 2, 1. Tidal Current Tables predict slack water and maximum flood and ebb with their directions; flood sets landward or upstream, ebb seaward. In a standing-wave regime slack water occurs near high and low water; in a progressive-wave regime maximum current occurs near high and low water — a distinction with real consequences in narrow channels.