Navigation and Chart work - Tidal anomalies
Tide curves based on low water
The curves we use to calculate tidal heights are centered on HW at most ports. the reason for this is that normally the time of HW is fairly clearly identified, by examining the curve you can see that it rises to a peak then quick drops off again.
In some areas high water is not so clearly defined, one of these is around the Isle of Wight on the south coast of the UK. Because of the way that the tide floods in to the Solent from the west as the tide sweeps up the English Channel, then some of it floods back in to the eastern end of the Solent as the tide ebbs in the English Channel there are double high tides on some tides.
This phenomena is most obvious in Southampton Water on spring tides, the water level rises to HW then falls as much as half a metre, then rises again, the whole process taking several hours. On a neap tide the period of HW last several hours with little change in sea level.
Because the time of high water is not clear, the tidal curves are drawn centered on LW. There may even be curves to use for each secondary port as well, as the pattern varies so much from place to place.
In the Training Almanac an example of one of these curves is on page 43 for Dunbarton. Notice how clear the time of low water is, the curve is much flatter near the HW ends, indicating that the time of HW is less clear cut.
It is used in exactly the same manner as the ones centered on HW but our focus is now the time of LW.
What is the height of the tide at 1837 UT at Dunbarton (page 40) on 28th January?
What is the height of the tide at 0828 Summer Time at Suzy Bay Marina (page 44) on 3rd June?
As far as I know the Solent area is the only place in the World that has tidal curves like these.