Mean Lower Low Water (MLLW) for a tide station is calculated by averaging the lowest tide recorded each day over a specific 19-year period. This period is called the National Tidal Datum Epoch.
Understanding Mean Lower Low Water (MLLW)
MLLW is a crucial reference point for nautical charts and coastal management. It represents the average height of the lowest low tides experienced at a location. Because tides vary due to astronomical and meteorological influences, a long-term average is required for accuracy.
Calculation Process
The determination of MLLW involves these key steps:
- Data Collection: Tide gauges at the station continuously record water levels over a 19-year period (National Tidal Datum Epoch). This period is used because it accounts for the 18.6-year lunar nodal cycle, which significantly affects tidal ranges.
- Identification of Daily Lowest Low Tides: For each day of the 19-year period, the lowest tide is identified and recorded.
- Averaging: All the recorded lowest low tides from the 19-year period are then averaged together. The resulting value is the MLLW.
Example:
Imagine a tide station has recorded the following lowest low tides (simplified example):
| Year | Lowest Low Tide (feet) |
|---|---|
| 1 | -2.5 |
| 2 | -2.0 |
| 3 | -3.0 |
| ... | ... |
| 19 | -2.2 |
To calculate the MLLW, you would sum all 19 values and divide by 19.
Importance of the 19-Year Epoch
Using a 19-year epoch ensures that the MLLW value is representative of long-term tidal patterns and minimizes the influence of short-term variations.
Practical Applications
- Nautical Charting: MLLW is often used as the chart datum, which is the reference level for depths shown on nautical charts. This ensures that mariners have sufficient clearance when navigating.
- Coastal Zone Management: MLLW is also used in coastal zone management for determining property boundaries and regulating coastal development.
