semidiurnaal

Semidiurnaal is a term used in oceanography and Dutch tidal literature to describe a semidiurnal tidal regime. It refers to tides in which two high tides and two low tides occur roughly every tidal day, about 24 hours and 50 minutes, with the two high tides and the two low tides of similar height. The semidiurnal pattern arises from the gravitational forces exerted by the Moon and the Sun, with the lunar semidiurnal constituent M2 and the solar semidiurnal constituent S2 typically dominating the cycle. The interval between successive high tides is about 12 hours and 25 minutes on average, leading to roughly two tidal cycles per day.

Geographically, semidiurnal tides are common along many ocean coasts, particularly on open coasts where the shoreline geometry and water depth support two nearly equal high tides per day. Local bathymetry and coastline shape can modify the pattern, producing variations such as mixed tides (two highs with different heights) or, less commonly, diurnal tides (one high and one low per day). Understanding the semidiurnal regime is important for navigation, coastal engineering, sediment transport, and tidal energy assessment.

Etymology-wise, semidiurnaal is the Dutch adjective for semidiurnal, derived from Latin semi- 'half' and diurnus 'daily'. The term is used in Dutch-language coastal and oceanographic literature to classify tidal behavior similar to the English semidiurnal tide.