This isolation can be seen by taking the mean low water springs (MLWS) height for Conwy
from Reeds as +1.1mLAT. Then plot on Admiralty Chart 1463 the contour line for 1m around
the sands and channels in Conwy Bay and one can see just how little of the area is flooded
and thus being used at the beginning of the flood to fill the channels and Deganwy Narrows
into the harbour.
The lowest predicted LW height in 2015 derived from data in Reeds for Holyhead corrected
for Conwy & Deganwy will be about +0.2mLAT [21 st March] which would on the face of it
isolate Deganwy Narrows from the channel completely. When the tide eventually does start
to flood the channels again, the flow will be slow because of the narrow and shallow channels offering an additional resistance to flow and hence delaying the time taken to flood into
Deganwy Narrows. The effect of the bars however have an effect also on the minimum
height of tide experienced at Conwy Quay and hence down to The Perch bar and beyond to
the approach channel.
Tony Mead, having spent 20+ years observing tidal heights within Conwy Harbour on an almost daily basis has never seen a LW height of tide off Conwy Quay, where the calibrated
electronic tide gauge is situated, of less than 0.9m. This is explained as being entirely due to
the shallow (above chart datum) areas of the approach channel which prevent the tide level
inside the harbour from falling to its predicted height. In addition to this the tide level inside
the harbour on springs when predicted LW height is less than 1m, is further raised from the
bar restriction by the river flow over the bar areas. This is normally 0.5m but can fall slightly
below this at periods of low river flow.
In summary, whenever the predicted height of tide is 1m or below, the tide within the harbour does not go down to the predicted level because it is stopped from doing so by the bar
restrictions of 0.5m above chart datum plus ~0.5m of river flow over these bar areas.
Below is a plot of readings taken with Morgan le Fay’s depth sounder whilst on the mooring
on 26th September 2014, which although imprecise, demonstrates this phenomenon for a
spring tide. The mooring is on the edge of a pool and the scope of chain is long so the boat
swings across the steep pool-side and readings of depth vary from time to time; each point is
the average of three readings varying by + or – 0.3m. The actual depth recorded is that below
the transducer which is ~0.6m below the surface.
Superimposed on these readings are the predictions using the Holyhead tidal curve [Reeds
Western Almanac 2014, p119] offset for clarity so that the HW depth +7.8mLAT predicted
from Reeds using Conwy & Deganwy data [Reeds p115 loc cit], falls on the maximum depth
measured at HW. [Prediction was LW 0728A, 1.3m; HW 1243A, 7.8m] The minimum sounder
measurement on the mooring at LW was ~2.1 +/– 0.3m depending on the position of the
boat (see above).
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