Can I squeeze over this tidal sill on neaps?
James Stevens considers a problem sent in by a Yachting Monthly reader who asks if you should squeeze over a tidal sill on neaps?
Tom and Jane are on holiday in North Brittany with their two children. They are on their 11m yacht with a 2m draught.
The weather has been hot, with a large area of high pressure 1036mb and little gradient wind but pleasant breezes.
They are planning to visit Binic, an attractive port with berths for visiting yachts in a wet basin in the centre of the town.
The entrance to Binic dries 5.5m so entry is only possible at high water. The marina has a hydraulic flap gate which opens when the height of tide reaches 8.5m.
The French tidal information refers to coefficients which indicate range of the tide. For Binic the coefficient required for entry has to be over 40. This means for most tides, access is possible a few hours either side of high water.
However, during some neap tides the height of high water is below 8.5m and the coefficient below 40, so the flap stays shut and the marina becomes entirely inaccessible.
Jane has been studying the tidal heights for Binic. The tides are on neaps but she calculates that if they arrive today they should be able to enter the marina at the top of the tide which is predicted to be 8.6m and the coefficient just over 40.
The family are sailing along the coast aiming to reach Binic at high water. Is Jane’s plan is going to be successful?
Can I squeeze over this tidal sill on neaps?
A Probably not. The problem on this day is the exceptional high pressure. Tidal height predictions in tide tables are based on a standard barometric pressure of 1013 millibars (or 1013 hectopascals). A change of 1mb can cause a difference in tidal height of approximately 1cm. High pressure depresses the water level so in these conditions the height of high water will be less than predicted.
It is unrealistic to try and predict tides to the nearest centimetre but Jane should be aware that the height could be 20cm lower than shown in the tables, and therefore the flap may not open making the marina inaccessible. Even if she can enter the marina, she needs to consider whether she will be able to exit in the next few days.
It would be sensible to call the marina and ask their advice before setting out.
They should be aware of the problem and will have planned accordingly.
Atmospheric pressure can also cause higher tidal heights than predicted. A very deep low pressure system can increase tidal heights and if combined with a storm surge can cause serious flooding and damage ashore.
In these conditions, predicting tidal heights accurately is almost impossible but the forecast will give enough warning to trigger the implementation of flood defences such as the Thames barrier. Yacht skippers sailing in extremes of high and low pressure therefore need to give themselves a safety margin when calculating tidal heights, particularly in high pressure when crossing sand bars and shoal water.
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The post Can I squeeze over this tidal sill on neaps? appeared first on Yachting Monthly.