The rising and falling of the water level in the surf zone at intervals in the vicinity of 2 to 5 minutes, especially noticeable on a flat beach. This is caused by the pattern of incoming waves being such that groups of high waves and low waves follow each other at the same intervals. This is in turn due to the interaction of wave groups with slightly different frequencies, a process that leads to a much longer envelope or beat frequency modulated the short wavelength waves.