LOW WATER SPRINGS DEPTH CALCULATIONS
FOR DEGANWY & CONWY
The behaviour of tidal flows in sand-choked estuaries is difficult to predict accurately. At low
water (LW) when the effect of channels and sand banks is more pronounced and particularly
at low water springs when tidal heights may be near chart datum, the lowest astronomical
tide (LAT), the effects can be really quite significant.
Morgan le Fay is moored opposite Conwy Marina entrance and the raised marina tide gate is
visible above the water surface when the tidal height is less than about (~) 3.7 metre (m)
above chart datum (~+3.7mLAT) by my (jfa) calculation. This gate provides a useful tide
gauge for me and there is a feature that I recognise ~ 0.8m below the top of the gate. When
it becomes covered on a flood tide that tells me there will be enough water over The Perch
beacon bar by the time I get there, to give me ~1m clearance below the keel in the deepest
part of the passage over the bar (+0.7mLAT charted depth).
These events coincide also with the footings of the guys on The Perch beacon becoming covered, an indicator which is sometimes quoted in pilotage advice notes. The 1m clearance
may seem generous, but it gives me a bit of leeway when manoeuvring in the narrow channel either side of the bar in what can be a strong cross tide.
The next shallowest part of the channel on the way out to sea is at The Scabs bar on the corner, charted +0.5mLAT. On a flood tide therefore, safely crossing The Perch bar will assure
one of being able to cross The Scabs bar at its deepest part.
Although the indicators described above are useful, at night and for visitors to Conwy one
might reasonably want to calculate the times when there will be enough water safely to
cross the bars near LW springs. Reeds Western Almanac gives the data for Conwy and
Deganwy using Holyhead as the Standard Port for both high water (HW) and LW predictions.
In my experience these give accurate predictions on neap tides for HW and adequate for LW,
and accurate predictions for HW on springs. Around spring tides however the LW predictions
are not reliable in giving the correct time for a given depth of water on the flood outside
Conwy Marina entrance and hence over the bar.
To cut short a long story of observations and calculations, I find the need to add 40-45
minutes to the time calculated from the tidal curve for Holyhead in Reeds corrected for
Conwy & Deganwy for the tide to reach +3.1mLAT, to match the observed time this occurred.
This correction becomes nil at neap tides when there is a small tidal range and the sands
remain largely flooded (see below).
To understand why this happens two factors at least need to be taken into account.
The tidal curve given in Reeds