Deep-water waves travel to shore and become shallow water waves when the water depth is less than half of their wavelength, and the wave motion becomes constrained by the bottom, causing the orbit paths to be flattened to ellipses. The bottom exerts frictional drag on the bottom of the wave, which decreases the celerity (or the speed of the wave form), and causes refraction. Slowing the wave forces it to shorter which increases the height and steepness, and the top (crest) falls because the velocity of the top of the wave becomes greater than the velocity of the bottom of the wave where the drag occurs.