Tidal conundrum

[Gryphon2]

Sat on my boat watching the tide at Southwold I noticed what commonly happens here. The tide is still flooding at about a kt but the height has already dropped by 30 mm.
No one has ever given me a satisfactory explanation of what is going on.
Any offers?

 

[MontyMariner]

Maybe a confusion between HW / LW and Slack Water, for any given place they are not necessarily the same.

 

[vyv_cox]

As I understand Southwold there is a large lake inland of the channel. If the incoming tide suddenly floods a large area of dry lake it could result in a lowering of depth in the channel. The ebb in the channel is far stronger than the flood as the lake empties

 

[Gryphon2]

That makes sense if there were a large dry area.....but there isn't. The inland "lake" are just old drained areas where the weather walls have long gone and the tide just fluids them progressively.

 

[Gryphon2]

Ignore weather!

 

[srm]

30mm=3cm, just over an inch in old money. Are you sitting next to a tide gauge to measure so accurately?

 

[Gryphon2]

No sitting next to the boardwalk that comes to our mooring.It was flooded but then was not whilst the flood still sped in.

 

[Gryphon2]

These boards were just under water. Tide continued to flood whilst they emerged this much.

 

[Roberto]

del

 

[Metalicmike]

You haven't told us the state of the tide. Upper surface water running opposite to the tide is normally due to wind effect and checking windy the wind is blowing onshore in your direction.

 

[srm]

May not be relevant here but when teaching tides I would get students to look at the tidal stream atlas for HW Dover. Local HW but tidal stream still flowing due to height differences either side.
I have also observed very long period low amplitude waves that gave the appearance of the tide going in and out on a flat beach and on tide gauge records. Also saw these on one of the wave monitoring stations I ran. All locations open to the north Atlantic though.

 

[AntarcticPilot]

Tidal current and tidal height are not necessarily related. This is especially true where two larger bodies of water are linked by a narrow strait, such as in places like the Gulf of Corryvreckan, Gray Dogs and the Sound of Sleat on the West coast of Scotland, where two independent tidal basins are connected by narrow channels - it is the difference in height of tide that drives the strong currents, so the height of tide is pretty much disconnected from the tidal current. In rivers, the river flow may counteract the tidal flow, resulting in times when the height of the water is not related to the ebb and flow; I suspect this is the case where you are, And others have pointed out that surface, wind-driven flow can be contrary to deeper tidal flow.

 

[Greg2]

There is a similar phenomenon on the River Bure at Gt Yarmouth where the flood and ebb are happening simultaneously on and beneath the surface due, I guess, to the amount of water inland i.e. what came into the Broads with the flood isn’t able to get out before the tide turns.

As already mentioned, the amount of water between Southwold and Blythburgh is probably a factor.

 

[Sandy]

Sat on my boat watching the tide at Southwold I noticed what commonly happens here. The tide is still flooding at about a kt but the height has already dropped by 30 mm.
No one has ever given me a satisfactory explanation of what is going on.
Any offers?

It is when it stops moving we have a problem and there will be nothing we can do about it, so why worry?

 

[Metalicmike]

Maybe its a pre tsunami effect

 

[Gryphon2]

Lots of good thoughts but I am not totally convinced by any of them. My observations were all around local hw with the tidal drop occuring over about half an hour.The winds were light and across the river and cant have been having much effect. The amount of fresh water coming down the Blyth must be minimal, and being lighter I thought it would have the opposite effect of the fresh water draining over the top of the still ongoing salt. I can't believe the area above Southwold is large enough to have its own tidal effect as the west coast of Scotland can.
Interesting . Keep thinking!

 

[Greg2]

Lots of good thoughts but I am not totally convinced by any of them. My observations were all around local hw with the tidal drop occuring over about half an hour.The winds were light and across the river and cant have been having much effect. The amount of fresh water coming down the Blyth must be minimal, and being lighter I thought it would have the opposite effect of the fresh water draining over the top of the still ongoing salt. I can't believe the area above Southwold is large enough to have its own tidal effect as the west coast of Scotland can.
Interesting . Keep thinking!

You will be aware of the speed / strength of the ebb in Southwold, which is due to the large amount of salt water that has flooded the Blythe, particularly the large tidal area at Blythburgh. A lot of water!
.