Mean Low Water Springs (MLWS)
The average level of the lowest low tides that occur during spring tide conditions, calculated over a standard period of time.
Mean Low Water Springs (MLWS) is a tidal datum used in marine navigation, hydrography, and coastal engineering. It represents the average height of the lowest tides that occur during spring tides, which are the tides with the greatest range. Spring tides happen approximately twice a month, during the full moon and new moon phases, when the gravitational forces of the sun and moon align to exert a stronger pull on the Earth’s oceans. This alignment results in higher high tides and lower low tides than usual.
MLWS is determined by observing tidal patterns over an extended period, typically 19 years, to account for variations caused by astronomical and meteorological factors. The resulting average provides a reliable reference point for charting water depths, designing coastal infrastructure, and assessing navigational safety. For example, MLWS is often used as a baseline for measuring the depth of water on nautical charts, ensuring that mariners can anticipate the shallowest conditions they might encounter.
Understanding MLWS is particularly important in areas with significant tidal ranges, such as estuaries or coastal regions with strong tidal currents. It helps mariners plan safe passages and avoid grounding during low water periods. Additionally, MLWS is a key consideration in environmental and legal contexts, such as defining the boundary between land and sea for property rights or conservation zones.
