Tidal forces act on our oceans because of the difference between gravitational strength on the side of the earth facing the moon, and the side facing away from the moon. The sun has a similar effect, but despite it’s much greater gravitational pull, because it is so much further away the difference from one side of the earth to the other is less than half the lunar effect. When the sun and moon line up they work together to produce spring tides, when not aligned we have neap tides. In the North Atlantic the tidal wave produced by these forces sloshes around in an anticlockwise direction a bit like the smoothie mixer in the Alnwick playhouse, but taking 12 hours (you may become thirsty waiting for a smoothie at that rate). A similar effect occurs in the North Sea with a wave ‘sloshing’ down the coast from North to South. When it is high tide in Wick it is low tide in Harwich. The tidal wave runs down in a southerly direction passing Alnmouth on the way.
In October I conducted a survey of tides in the Alnmouth estuary with the help of my wife and John Taylor. It was a spring tide and the results will be different at Neaps. We started at 10 o’clock with a high tide expected at around 430pm (based on published Coquet Island tidal predictions). A Leica laser level was set up on the seawall by the ferry man’s hut and used as datum. The level of water inside the estuary was then measured every half hour or more often. We also conducted some depth surveys as we went along.
North Shields is one of a network of real time tidal monitoring stations around the UK . The tide there is only a few minutes later than that at Coquet and the heights recorded will be almost identical. Tidal predictions based on movements of the celestial bodies are pretty accurate but do not take account of air pressure and wind direction which can cause significant change from those predictions. Real time tides can be seen on the website https://ntslf.org/data/realtime?port=North%20Shields&span=1 These were used for comparison to the observed tides in the estuary .
From 1030 (low tide) the level remained almost static until 1pm, ie 2 ½ hours after low water and 3 ½ hours before predicted high tide . It then rose rapidly as shown on the graph. The results are presented graphically along with the results for that day from North Shields adjusted so that they are aligned for time of high tide at the two locations. Low water is given as ‘0hrs’ and High water then at ‘6 hours’. The tidal heights are relative to the lowest astronomical tide (LAT). You can see that for the first 2 and a bit hours the levels inside the estuary did not change significantly. Once the tide was at 2.7 metres above LAT the levels rose sharply in a curve very close to that for North Shields, though very slightly delayed or slowed. After HW at 6 hours (430pm on this particular day) the tide ebbed rapidly though again showing a delay, slightly longer on the ebb, compared to North Shields. We continued to monitor the tide until about 1030pm which was nearly the next low tide. The level inside the estuary had reached a plateau beginning at roughly 4 hours after HW.
Although we only monitored the tide for one tidal cycle I have in the second graph manipulated the data to look like two tidal cycles. Here you will see the two HW peaks separated by a 4 hr plateau. The tide is very similar to North Shields, but with the bottom half of the curve removed. This all makes good sense if you consider the effect of the bar and the shallow exit from the estuary. At times of high rainfall it is likely that the tidal levels will remain rather higher during the plateau phase ( there had been little or no rain in the week before this survey) due to flow down the river. At neaps we can estimate that the period of rise and fall within the estuary will be slightly longer and the plateau shorter by perhaps 15-30 minutes at either end. Ie 3 hours plateau rather than 4, but of course the HW not so high.
Figure 1 Observed tidal levels in Alnmouth estuary compared with observed levels for that day in North Shields.
Figure 2 Data manipulated to show what two cycles would look like. Note the plateau stage followed by a very rapid rise as water flows over the bar, and a slightly delayed fall on the ebb.