Overtide

An overtide is a higher harmonic of a single astronomical constituent generated by nonlinear distortion of the tidal wave in shallow water, with a frequency that is an integer multiple of its parent. M4 (6.2103 h) is the first overtide of M2, M6 the second, and S4 of S2. Overtides arise because friction, the advective terms, and the finite ratio of tidal amplitude to depth steepen the wave as it enters estuaries and over banks, producing tidal asymmetry between a fast flood and a slow ebb or the reverse. They control sediment transport and make harmonic prediction harder near shore, where shallow-water constituents must be added to the deep-water set.