Tide behaviour

[scubajunky17]

Can someone please clarify for me at what point the tide is at its strongest, how does it generally behave after slack water.

 

[johndiver999]

Tide current is slack at high and low tide and speed probably increases as you move away from those two points - is probably a reasonable conceptual model for most locations.

 

[Russjstewart]

Courtesy of Tide Patterns and Currents | manoa.hawaii.edu/ExploringOurFluidEarth.
“The greatest tidal currents occur midway between high and low tide. A slack tide is when there is no current. In most areas of the world, slack tides occur near high and low tide when the flow of water is changing direction.”

Tides can be diurnal, generally one high and one low per day or semi-diurnal, two high and two low per day. The times between tides is not aneven6 or 12 hours which is why the time of high and low tides change each day.

There are exceptions of course, in Melbourne,Aus Port Phillip heads slack is midway between high and low. This is because you have a large body of semi enclosed water linked to the ocean by a narrow opening.

 

[Russjstewart]

On semi- diurnal tides the rules of 12 ths can be used.
hour 1 = 1/12 of total movement
hour 2 = 2/12 of total movement
hour 3 = 3/12 of total movement
hour 4 = 3/12 of total movement
hour 5 = 2/12 of total movement
hour 6 = 1/12 of total movement

The strength of the tidal current is dependant on much difference between high and low, the greater the difference, the stronger the flow. Sock water is a bit mis leading, it is really the smallest tidal flow, it never really drops to zero. The duration of slack also varies

 

[scubajunky17]

Thanks for that, so in general after high or low slack the power builds until mid tide when it starts to slowly taper off until the high or low is reached then becoming slack?
Thanks, seems logical

 

[Angelo Farina]

Thanks for that, so in general after high or low slack the power builds until mid tide when it starts to slowly taper off until the high or low is reached then becoming slack?
Thanks, seems logical

Yes. Water speed is a sinusoidal wave, whilst water level is a cosinusoidal one.
So water speed is null when water level is max or min.
This is called a 90° phase shift...

 

[Russjstewart]

Thanks for that, so in general after high or low slack the power builds until mid tide when it starts to slowly taper off until the high or low is reached then becoming slack?
Thanks, seems logical

As general rule correct, but there are exceptions, as I mentioned, Port Philip heads for example where slack occurs about 3 hours after high or low water,
if in doubt check with a knowledgeable local

Slack water - Wikipedia

en.wikipedia.org

 

[scubajunky17]

Same in the Menai Straights in Anglesey.

 

[Wibble]

Can someone please clarify for me at what point the tide is at its strongest, how does it generally behave after slack water.

It varies MASSIVELY with the geography of the coastline and seabed.

Many locations are simple sine waves with slack at the peaks and troughs.

However, most locations aren't that simple with varying waveforms from triangular, sawtooth, all the way to double-dips that look like molars!

Tidal height at Swanage, England​

This is caused where two bodies of water meet and they push against each other. This is seen in the Solent (SE England) and at Dover and surrounding coastlines where the tide comes down the North Sea and up the channel. The Slack moves around as the currents converge.

There's tidal atlases available; need to look in sailing almanacs for these... Example of SE England where the slack moves all over the place throughout the tide. Times are for HW (high water) Dover: