Which statement describes wave shoaling?

• A. Slack water
• B. Rip current recognition
• C. Tidal bore
• D. Waves increase height in shallow water

Answer: (D)

Wave shoaling is what happens when waves move into shallower water: their height grows as the depth decreases. This occurs because the wave speed slows down in shallower water, and the wavelength shortens. To keep the same energy moving in the direction of the shore, the wave height increases, and the waves often become steeper and can break.

In more detail, in shallow water the speed is roughly proportional to the square root of the depth. As depth drops, speed drops, and the wavelength shortens to match the slower pace. Energy carried by the wave (which is tied to the wave height squared) has to move forward, so the height must rise to conserve energy flux along the coast. This is why waves appear taller and more powerful as they approach the shore.

Other terms describe different shore phenomena: slack water is the moment of calm around low tide, rip currents are strong alongshore flows, and tidal bores are large incoming waves that travel upriver in some estuaries. They are not about the height change caused by approaching shore, which is the essence of shoaling.

So the statement that waves increase height in shallow water best captures what shoaling describes.