The difference between mean high water and mean low water is that mean high water is the average height of high tides, while mean low water is the average height of low tides.
Understanding Mean High Water and Mean Low Water
The terms "mean high water" (MHW) and "mean low water" (MLW) are crucial in coastal areas, especially concerning navigation, property boundaries, and ecological studies. These terms are not based on the absolute highest or lowest tides but rather on the *average* of these extremes over a specific period, typically 19 years. This averaging process helps to account for the variations caused by lunar cycles, weather patterns, and other environmental factors. Here is a more detailed breakdown:
Mean High Water (MHW)
As explained by the reference, mean high water represents the average height of all high tides observed over a long period. It's a crucial reference point for understanding the extent of tidal inundation in coastal zones. Specifically:
- MHW is not a single, absolute water level, but rather an average calculated from numerous tidal cycles.
- It is used to define certain property lines and coastal construction guidelines.
- MHW provides a reliable marker to predict and assess risks associated with flooding during high tide events.
Mean Low Water (MLW)
The provided reference also describes MLW as the average water level at low tide. Like MHW, MLW isn't the single lowest low tide; instead, it's the average across multiple low tide events over a specified period. This is important for several reasons:
- MLW helps to define the lower extent of many intertidal habitats.
- It serves as a critical reference for nautical charts to indicate the depth of navigable waters.
- MLW plays an important role in determining the jurisdictional boundaries of coastal lands.
Key Differences in Tabular Form
| Feature | Mean High Water (MHW) | Mean Low Water (MLW) |
|---|---|---|
| Definition | Average height of all high tides | Average height of all low tides |
| Significance | Marks the upper limit of tidal influence | Marks the lower limit of tidal influence |
| Application | Property lines, coastal construction, flooding | Navigation, intertidal habitat, land boundaries |
Practical Implications
Understanding the difference between MHW and MLW has many practical implications:
- Property Rights: Property lines along the coast often relate to the MHW line, which impacts construction permits, land use, and ownership.
- Navigation: MLW is the datum used for charted depths. This is important for safe navigation.
- Ecology: Coastal ecosystems depend on the tidal fluctuation defined by MHW and MLW for their biodiversity and productivity.
- Flood Management: Knowledge of MHW is essential for managing flood risks and coastal erosion.
In summary, MHW and MLW represent distinct average water levels that are fundamental for various aspects of coastal management and research. The key takeaway is that they represent the average high and low tides over a period of time and are not based on the absolute highest or lowest tide levels.
