When waves approach shallower water near shore, their lower reaches begin to drag across the ocean floor, and the friction slows them down. The wave energy below the surface of the ocean is pushed upward, causing the waves to increase in wave height. The longer the swell period, the more energy that is under the water. This means that long-period waves will grow much more than short-period waves. A 3-foot wave with a 10-second swell period may only grow to be a 4-foot breaking wave, while a 3-foot wave with a 20-second swell period can grow to be a 15-foot breaking wave (more than five times its deep-water height depending on the ocean floor bathymetry). As the waves pass into shallower water, they become steeper and unstable as more and more energy is pushed upward, finally to a point where the waves break in water depth at about 1. 3 times the wave height.