Long waves, also called infragravity waves, have a wave period of 30 seconds to several minutes. Astronomical tides represent one important example of long waves.
Long waves can occur in shallow water, such as the Chesapeake Bay. But long waves are not limited to what is normally considered shallow water. In fact, most tides are long waves over the entire ocean because their wavelengths are on the order of 1,000 km and depths can reach for many kilometers. Similarly, seismic-forced phenomena such as tsunamis propagate across the Pacific Ocean in depths of up to 20 km but have wavelengths on the order of hundreds of kilometers.
One important difference between long waves and short waves is their effect on the ocean floor. The velocity of long waves allows them to sometimes generate bottom currents. The fact that long waves affect the bottom is important in that bottom sediments can be eroded and transported by tidal and other long-wave currents. For example, tidal flood and ebb currents contribute to the transport of sediments to form ebb and flood shoals.