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  #1  
Old 03-01-2011
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Slack Tide vs. Slack Current

Over the past year, I've seen several references that Slack Tide and Slack Current are not the same. Can someone explain why? Is it because the current lags the tide and takes time to "catch up"? That's about the only plausible reason that I can see.

I've done some searches and many people quote that there is a difference, but I've not seen the reason why.

Sailing the Chesapeake and New Jersey shore, I've generally ignored the tide but now my interest in piqued.
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Old 03-01-2011
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As I understand;

Tide is the rise and fall of water surface level due to interaction between Earth and Moon.

Tidal current is the product of the interaction of the rise and fall of water surface level, shore line shape, underwater features and other restraints on bodies of water.
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Old 03-01-2011
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Your understanding is pretty good... just because the water has stopped rising or falling doesn't mean that it has stopped moving or vice versa.

Water takes time to reverse direction, so if it was flowing out, just because the water level isn't dropping anymore doesn't mean the water stops instantly and reverses...that takes time so the flow lags behind the changes in height.
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Old 03-01-2011
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Here, where we sail, to get to the salt water from the river system we pass through the Reversing Falls in the Saint John river. At high tide the tidal waters are actually 14 ˝ feet higher than the river. At low tide the tidal waters are 14 ˝ feet lower than the river level. So, almost 30 feet difference. We refer to the correct time to go through the falls as "Slack Water". To calculate low slack, add 3 hours and 50 minutes to low tide in Saint John harbour. Similarly, to determine high slack, add 2 hours and 25 minutes to high tide at Saint John harbour.

This is when you don't go through.



This is when you do - and you have about a 15 minute window three or four times a day.



Not sure if it helps your question.

Rik
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Slack tide means the water level has stopped rising or falling but the current could still be moving. This one is tricky becasue the fact that water is moving from one area to another implies a change in height. So for most tidal, non-river areas slack tide and slack current will be similar times.

Slack current means the current has stopped but the water could be rising or falling. This can happen when a river current is opposed to a tidal current.
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But rik, that looks like so much fun...

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Look at a tide graph... slack tide occurs at the peaks and valleys of the tide height.

When slack current occurs depends on the area you're looking at, and is when there is no horizontal movement of the water.. with bays and lagoons behind narrows this will happen when the water level on either side is the same.. and could occur quite some time after slack tide.. and may only last a moment before reversing.
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Persistent high and low pressure systems, and wind direction may cause water levels to be low or high in constrained bays and inlets extending periods when current is out of phase with predicted tide.
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Old 03-01-2011
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The classic example for the difference between the two is the filling and ebbing at inlets ... The tide has to rise BEFORE it starts to flow through an inlet, all due to the 'flow restriction' due to the geometry of the inlet. It also means that the tide on the land side of the inlet will be delayed in time due to the 'restricted flow'
.... just the opposite during the ebb, max current flow 'out' will be long delayed until max low on the 'ocean side', leaving the tide level somewhat higher on the land side of the inlet --- again another 'delay'.

In all of these situations the max current only occurs at the max. DIFFERENCE between the tidal heights, not the typical 'half way' (6/12th of the tide range interval) rise or fall of the ocean side tide; the 'ocean side' tide being the controlling factor. All this is not 'calculable' but is rather entirely based on the historical record of exactly how each inlet 'works' and retards the change due to the geometry of the 'restriction'.

Wind, etc. influenced tides across such inlets radically changes the current and the timing of the 'slack' conditions in the inlets --- no 'prediction' is possible when 'wind tides' overlay across the current in an inlet.

An example of the restriction geometry: take two styrofoam cups and at equal level connect them 'at the bottom' with a very small tube .... slow rate of transfer as one cup is filled; change the hose to a much larger diameter connection (geometry) and the 'rate of change' becomes 'faster'.

So, when navigating across especially small inlets with large bays 'behind' the inlet .... the proper way to know when to 'shoot' or for 'slack water' is to consult the historic CURRENT tables - such as published in "Eldridge" (East Coat tide and current tables). If you use 'just a tide calculation program', etc. you will invariably be entering at the WRONG time. "Hellgate" in NYC, Barnegat Inlet in NJ and "Hellgate" and Elliott Cut on the AICW in SC are prime examples of the differences between calculated tide tables and 'historic' actual performance data.
If you really want to take a look at 'confusion' look at the C&D Canal - influenced by radically out of phase tides and radically different tidal heights of the Chesapeake and Delaware Bays and the 'alteration difference' that occurs in the canal ... and then overlay the influence of wind tides and heavy rains (or 'drought') in the Delaware River basin: can be a 'crap shoot' to get it 'right' to navigate during 'slack'.

Last edited by RichH; 03-01-2011 at 10:15 AM.
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A minor pedantic point:

"Slack" or "slack water" refers to the time when there is no appreciable tidal current. Sometimes it's referred to as "slack tide", but it still refers to current. So technically there's no difference between slack tide and slack current

Near high or low water when the depth is not changing appreciably, this is technically referred to as "standing water" or "stand of tide".

For reference: NOAA Tide and Current Glossary or chapter 9 of Bowditch (article 904)
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