Mean high water springs
Mean high water springs (MHWS) is the highest level that spring tides reach on the average over a period of time (often 19 years). The height of mean high water springs is the average throughout the year (when the average maximum declination of the moon is 23.5°) of two successive high waters during those periods of 24 hours when the range of the tide is at its greatest.
This level is generally close to being the "high water mark" where debris accumulates on the shore annually.
- Definitions of various tide related terms on the Proudman Oceanographic Laboratory site.