waves at shallow water develop when the ocean surface waves travel into the coastal area where the wavelength is much larger than the water depth. the normal circular motion of the water particles there is disrupted by the ocean bottom. as the water becomes shallower, the swell on the water surface becomes higher and steeper. after the wave breaks, the water flows violently, turbulently. this is where erosion of the ocean bottom and shore line intensifies dramatically. the most devastating effect may happen in the case of tsunami.