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River tides, why the flood is faster than the ebb
On the coast the time between high tide and the next low tide is about the same as the time between low tide and the next high tide (approx 6 1/4 hours).
But in a tidal river it's different: the tide comes in faster than it goes out (fast flood, slow ebb), and the further upstream you are the bigger the difference. So high tide at Hammersmith Bridge is about 40 mins later than at London Bridge, but low tide at Hammersmith is about 2 hours later than LBridge. For example here are the tide times for today (from easytide.ukho.gov.uk):
High at L Bridge 03.10
High at H'smith 03.49 + 39 mins
High at R'mond 04.14 + 1 1/4 hrs
Low at L Bridge 09.52
Low at H'smith 11.48 + 2hrs
Low at R'mond 13.00 + 3 hrs
Can anyone explain why this is?
But in a tidal river it's different: the tide comes in faster than it goes out (fast flood, slow ebb), and the further upstream you are the bigger the difference. So high tide at Hammersmith Bridge is about 40 mins later than at London Bridge, but low tide at Hammersmith is about 2 hours later than LBridge. For example here are the tide times for today (from easytide.ukho.gov.uk):
High at L Bridge 03.10
High at H'smith 03.49 + 39 mins
High at R'mond 04.14 + 1 1/4 hrs
Low at L Bridge 09.52
Low at H'smith 11.48 + 2hrs
Low at R'mond 13.00 + 3 hrs
Can anyone explain why this is?
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I presume the explanation lies in the flow of the fresh river water..
The incoming tide acts as a dam so as well as the incoming tide itself, there is a build-up of water that has flowed down the river. That adds to the rising tide and makes the water rise even faster making the high tide advance quicker.
When the tide eventually goes out, there is all that extra water that has been dammed up by the tide that then has to flow out whilst still being added to by more fresh water arriving down the river. This larger volume therefore takes longer to drain away making the low tide delayed.
The incoming tide acts as a dam so as well as the incoming tide itself, there is a build-up of water that has flowed down the river. That adds to the rising tide and makes the water rise even faster making the high tide advance quicker.
When the tide eventually goes out, there is all that extra water that has been dammed up by the tide that then has to flow out whilst still being added to by more fresh water arriving down the river. This larger volume therefore takes longer to drain away making the low tide delayed.
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