Final answer:
Wave celerity decreases as waves shoal due to the energy compression in the shallower water that creates higher waves and wave refraction, which causes waves to bend and slow down as they move into shallower areas.
Step-by-step explanation:
As waves shoal and enter increasingly more shallow water, wave celerity, or their speed of propagation, decreases. This occurs because as waves reach shore, where the water depth decreases, the energy of the wave is compressed into a smaller volume.
This effect is known as shoaling, and it results in waves with higher amplitude but lower speed. Additionally, waves experience refraction as they travel from deeper to shallower water, bending in a path closer to perpendicular to the water's surface, which further decreases their speed and also shortens their wavelength.