Which theory/model of tides accounts for finite ocean depth, basin resonances, and the interference of continents on tide waves?

• A. Equilibrium Theory of Tides
• B. Dynamic Theory of Tides
• C. Ebb Current
• D. Flood Current

Answer: (B)

Tides behave as real waves whose speed and pattern depend on the depth of the ocean and the shape of the ocean basins. The dynamic theory treats tides as propagating gravity waves, so it accounts for how water moves and interacts as it travels, slows, and reflects in different environments.

Finite depth matters because wave speed in shallow water is set by depth (roughly c ≈ sqrt(g h) for long waves). As depth changes along coastlines and across shelves, tides slow down, change wavelength, and alter arrival times and heights. This is essential for understanding why tides aren’t uniform across the globe.

Basin resonances occur when the period of the tidal forcing matches a basin’s natural sloshing frequency. In such cases, energy becomes concentrated and the tides can become unusually large in enclosed or semi-enclosed seas and bays. That amplification and the specific timing depend on the basin’s size, depth, and shape.

Continents interfere with tidal waves by blocking, reflecting, and diffracting them, creating complex patterns of interference and standing waves that change tide heights and timing along coastlines.

Equilibrium theory, by contrast, treats tides as simply bulges in a static ocean with no wave propagation or coastline effects, which can’t explain depth-related changes, resonances, or continental interference. Ebb and flood currents describe the two main tidal current directions, not the theory that explains how tides behave in real, physically structured oceans.