Comparison of Observed Height of Tide with that Predicted
26th September 2014 on Mooring
The predicted values have been offset so that they coincide at HW
with the observed depth under the sounder
Depth Under Sounder /m
12
10
8
6
4
Observed Depth
Reeds Prediction + Offset
2
0
7
8
9
10
11
12
13
Time /A (BST)
For all the approximation in this method, the effect is illustrated quite well: there is some
delay in the onset of the main flood, starting 20-30 minutes before the time The Perch
footings become covered (0912A(BST)) and the sands also, witnessed by the appearance of
scum and foam on the flood. The tide then runs in at a faster rate than predicted by Reeds
data, the two rates becoming similar as HW is approached.
My estimated time of departure from the Reeds tidal curve data for that day was, for
+3.1mLAT, ~0830A whereas the observed time The Perch footings covered was 0912A, once
again a +40-45 minute offset.
Next plot the neap tides MLWN contour which is +2.6mLAT and one can see how much of the
sands remain flooded at LW and consequently how the flood tide will have an immediate
effect on the water flow in the channels, which is probably why the predicted times for
heights become more accurate when using the Conwy & Deganwy-corrected Holyhead tidal
curve. Different tidal curves for springs and neaps are not uncommon, for example Greenock,
Milford Haven and Bristol Standard Ports [Reeds pp 84; 141; 151 loc cit] and it would seem
Conwy is another.
Entering the channel into Conwy from The Fairway buoy on a flood tide, one is much more
reliant on calculations using almanac derived data, particularly if a stranger to the port. We
are fortunate that buoys C1 and C2 are both moored on the -2mLAT contour and so give an
approximate tide height measurement one can make from a sounding on crossing the transit
between the buoys. I generally wait until the sounding reads more than 5m which indicates
5.6m depth or +3.6mLAT height of tide. That gives me a reasonable clearance over The Scabs
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