Tidal Curves and Mean Ranges of Tides
The tidal curve for Hamilton sound can be found on page 75 of the RYA Training Almanac (Fig 5.23). As well as having their own tide tables Standard Ports are furnished with individual Tidal Curves. These curves are fantastic tools that describe how the water floods and ebbs over time in an individual port.
There is a Mean Ranges box above the curve that gives us information about the mean or average ranges we can expect during Spring and Neap tides. We can see that the average range at Springs is 5.2m and average for Neaps is 3.2m.
From the tide table information above (Fig 5.22) we have determined for the 3rd March that the tidal rage is 6.0m and 5.3m. Remember, the Mean Spring Range is the difference between Mean High Water Springs MHWS and Mean Low Water Springs MLWS. We can see that the tidal range on the 3rd of March is nearer but higher then the Mean Spring Range listed in the information box on the tidal curves (Fig 5.23).
I have used this date as an example of an extraordinary spring tide. The range of tide on the 3rd of March is higher than the average for the whole year and probably due to the date of 3rd March being closer to the spring equinox, when the sun and moon are lined up with the earth and experience higher tidal generating gravitational forces (Fig 5.5).
We can now illustrate a tidal range that is closer to Neaps, again we will use the tide tables for Hamilton Sound on page 72 of the RYA Training Almanac (Fig 5.24) and the Hamilton Sound Tidal Curve mean ranges information on page 75 (Fig 5.23).
On the 24th March
At 0105hrs the height of the tide is 1.5m above chart datum
At 0754hrs the height of the tide is 4.6m above chart datum
At 1425hrs the height of the tide is 1.5m above chart datum
At 2036hrs the height of the tide is 4.7m above chart datum
The tidal range between the first low tide and high tide is 3.2m
The tidal range between the first high tide and second low tide is 3.1m
The tidal range between the second low tide and second high tide is 3.2
The Mean Neap Range (Fig 5.8) is the average difference between mean Mean High Water Neaps MHWN and Mean Low water Neaps MLWN. In the Mean Range information box on the Hamilton Sound Tidal Curve (Fig 5.23) we can see that the mean range is similar to the tidal range calculated from the Hamilton Sound tide tables.
It is very important when consulting the Mean Tidal Range information that you select the appropriate tidal curve. Every location has its own specific tidal range values, which correspond to the way the tide floods and ebbs. This curve describes the flow of water in and out of the harbour, and is modified by the local geomorphology and shape of the bays, estuaries and harbours. It is therefore very important that the tidal curve page for the right standard port is used. If you compare the Mean Ranges information on different Tidal Curves you will see that there ids a difference between the average range of tides between individual places and the curves have slightly different shapes.