Mean Lower Low Water Springs (MLLWS) refers to what measurement?

Mean Lower Low Water Springs (MLLWS) is specifically defined as the average height of the lower of the two lower waters that occur during spring tides over a designated period. This measurement is crucial in nautical navigation and hydrology, as it provides a consistent reference point for charting and understanding tidal movements in coastal areas.

The definition underscores the significance of the "lower low water" phenomenon, which is the lowest tide reached during a given tidal cycle, and explains its focus on spring tides, which are characterized by higher tidal ranges due to the alignment of the sun and moon. MLLWS is often used to determine safe underwater clearances for vessels and to assess coastal resilience against climate change.

Contextually, the other options relate to different tide measurements but do not align with the specific definition of MLLWS. The average height of a spring high tide measures the top half of the tide cycle rather than the lower extremes, while the average height of both lower tides does not distinguish between the specific low water standard derived from spring tides. Therefore, the correct choice clearly articulates the unique focus of MLLWS on the lower water during spring conditions, which is vital for accurate navigation and water level assessments.