Mean High Water Springs (MHWS)

The average height of the high waters that occur during spring tides, calculated over a specific period of time.

Mean High Water Springs (MHWS) is a tidal datum used in marine navigation, coastal engineering, and shoreline management. It represents the average level reached by high tides during spring tide conditions, which occur approximately twice a month when the Sun, Moon, and Earth align, resulting in the greatest tidal range. This alignment amplifies the gravitational pull on Earth’s oceans, causing higher-than-average high tides and lower-than-average low tides. MHWS is calculated over a long-term period, typically spanning 19 years, to account for variations caused by factors such as the lunar nodal cycle and atmospheric conditions.

In practical terms, MHWS is used as a reference point for defining the upper boundary of the intertidal zone, where marine and terrestrial environments meet. It is also a key factor in determining legal boundaries for coastal property ownership and maritime jurisdiction. For mariners, understanding MHWS is essential for safe navigation, as it helps predict the extent of tidal coverage over underwater hazards, such as sandbanks or reefs, during spring tides. Additionally, MHWS is significant in environmental planning, as it can influence the design of coastal defences, harbours, and other marine infrastructure to account for potential flooding or erosion risks. By providing a standardised measure of tidal extremes, MHWS plays a vital role in ensuring safety and sustainability in marine and coastal activities.