Tides: Generation, Patterns, Prediction, and Coastal Effects

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What is a tide?

A tide is a periodic rise and fall of sea level, generated mainly by the differential gravitational influences of the Moon and Sun and modified by Earth’s motion and local conditions.

What does a tide gauge measure?

A tide gauge at a fixed station records water level over time. Its readings can be compared with a reference datum, and short-period wave variations are generally excluded.

What are flooding and ebbing?

Flooding is the rising-water stage after low tide, when more of the intertidal zone is covered. Ebbing is the falling-water stage after high tide, when that zone is exposed.

What is slack water?

Slack water, or slack tide, is the period when a tidal current ceases or reverses direction. Its timing is often near high or low water but can differ by location.

Name three factors that modify observed tides.

Examples include ocean-basin geometry, coastline boundaries, bathymetry, the Coriolis effect, frictional dissipation, and tidal resonance.

Quiz questions

1. What primarily generates astronomical tides on Earth?Answer: Differential gravitational effects of the Moon and Sun, with lunar forcing usually stronger.
2. Which assumption is part of equilibrium tide theory?Answer: A uniform ocean responds immediately, without land barriers.
3. What does harmonic analysis do to support tide prediction?Answer: Separates long water-level records into repeating components with local amplitudes and phases.
4. Why can actual sea level differ from an astronomical tide prediction?Answer: Wind and air pressure can change water levels beyond the predicted astronomical tide.
5. How does tidal-turbine power output vary approximately with current speed?Answer: It varies approximately with the cube of current speed.