# Slack Tide vs. Slack Current



## Sabreman (Sep 23, 2006)

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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## Ulladh (Jul 12, 2007)

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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## sailingdog (Mar 19, 2006)

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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## rikhall (Feb 7, 2008)

Sabreman

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 ½ feet higher than the river. At low tide the tidal waters are 14 ½ 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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## Tim R. (Mar 23, 2003)

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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## sailingdog (Mar 19, 2006)

But rik, that looks like so much fun...


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## Faster (Sep 13, 2005)

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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## Ulladh (Jul 12, 2007)

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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## RichH (Jul 10, 2000)

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'.


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## AdamLein (Nov 6, 2007)

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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## jrd22 (Nov 14, 2000)

I agree with Adam. "Slack" water/current/tide is used to identify times of little or no *current*. "High" or "Low" tide has little relevance to the time of slack water, they can be hours apart. There seems to be a lot of confusion amongst recreational boaters as to the correct meaning of the terms to describe the stages of the tides and currents.


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## hellosailor (Apr 11, 2006)

Tide refers to the vertical rise/drop of the water level.

Current refers to the horizontal motion of the water.

Since the motion of water is restricted by geographies (i.e. a narrow inlet into a large bay or sound) the water at that choke point may have already risen/sunk as far as it is going to, but the water on the other side is going to continue to rush in/out as it seeks equilibrium.

There is no real "slack tide", tides are either high or low. Only the current "slacks" when it stops rushing around trying to equalize water depths in different locations. If you hear someone refer to "slack tide" that's a tip-off that they aren't really clear on what they're saying.


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## jackdale (Dec 1, 2008)

I am with Adam, John and HS.

To get even a bit more pedantic, Canadian Tide and Current Tables use the term "turn" to identify that time at which the current changes from flood to ebb, or ebb to flood. Even at a turn there is some water moving around as a result of the inertia of the water.


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## Boasun (Feb 10, 2007)

Slack Tide is a misnomer. It is 'Stand of Tide' at either high or low tide when there no change in the height of tide, high or low, for a few moments.
Slack water is inter-changable with Minimum Current in your Current Tables.
for movement throught some areas you should arrive about a half hour before slack water. This is in accordance to the capabilities of your vessel. But in some areas...Transit only during slack water...Safer that way.
Keep both Tide & Current Tables on board your boat and learn to use both tables. The Tide table can keep you from running aground in some areas. Such as you can cross that mud bank at high tide but at low tide you will rip the keel off of your boat. 
And the Current table will give you informed data as to when you can safely transit some area where the currents can wreck your vessel.
You cannot use one table to predict the other. The Tides and Tidal Currents are NOT in sync with each other. This is caused by the shapes of the land masses surrounding the areas you are sailing in.


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## Boasun (Feb 10, 2007)

Did I say there is no such animal called Slack Tide? Well there isn't one. It is STAND of TIDE. When there is no change in the height of tide at either High or Low Tide.


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## AdamLein (Nov 6, 2007)

jackdale said:


> Even at a turn there is some water moving around as a result of ty\he inertia of the water.


Hm.... this is something I'm not so sure about. Certainly you get some water moving when it should be slack, or moving the wrong direction before or after a turn, but I think that that's more likely due to back eddies and the general non-uniformity of the current than the water's inertia.

Basically current flows from higher gravitational potential to lower gravitational potential. When the water levels even out there's no longer any gradient to push the water. This process _generally_ (all due respect to Bay of Fundy folks and people with tidal bores in their backyard) happens slow enough that inertial effects would disappear very very quickly.

Imagine a long trough of water on a see-saw. If you change the angle of the see-saw rapidly through a large angle, yeah, even after you bring it level again the water will still be moving, for a matter of seconds. If you slow it down and only move through a very small angle, then the water's flow will correspond very close to the angle of the see-saw. You will still get a wave (effectively a tidal wave) that bounces back and forth from one end of the trough to the other, but this is not current.

I think most inlets are more like the latter case, as tidal heights change over a matter of hours and the "angle" between the line from one end to the other and the surface of constant gravitational potential is quite small (a few feet over a few miles).


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## Bene505 (Jul 31, 2008)

At high tide, current could still be flowing in order to bring the "downstream" water level up to high tide, even though it's already high tide where the current is flowing. This happens at an inlet, or where tidal flow is contstained by islands or shallow water.

If I understand it correctly (mostly from electronics, looking at voltage and current in the presence of resistance and capacitance), current will lag tidal height whenever there is a large surface area of water behind a land mass that constricts the flow of water. The land mass could be shallow water or a narrow inlet or both (a shallow inlet). The smaller/shallower the inlet and the greater the surface area of the water behind it, the greater will be the lag between tidal height and tidal current at that inlet.

Regards,
Bra


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## Boasun (Feb 10, 2007)

Tides are caused by the gravitational pull of the moon & sun and by the weather. When the sun, moon (new & full) and the earth are in line you have spring tides, and when the moon is at quadrature with the earth & sun you have neap tides. The weather affects the height of tide, by heavy run offs from snow melt and rain along with storm surges or air masses of high or low pressures. 
The Currents are affected by the same forces along with temperature and salinity differences and the coriolis effect of the Earth's rotation. And along with the land masses this throws the Tidal Currents out of sync with the Tides.
Best bet is to have both Tide and Current tables on board and learn to use them. They are required by law on all commercial vessels and we do use them.


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## AdamLein (Nov 6, 2007)

Bra(d): why does it depend on the surface area of the water behind it?

I'm guessing you're drawing an analogy here between water pressure and voltage. If so, the water pressure gradient between two connected reservoirs depends only on the difference in depths between the two reservoirs, not their volumes or surface areas.


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## Faster (Sep 13, 2005)

I'll go along with that, Adam, but the duration of the 'leveling time' (ie flowing current condition) and hence the delay of actual slack current will depend on volume (and the capacity of the restriction, or narrows).


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## Boasun (Feb 10, 2007)

NOTE: you cannot apply Electrical/Electronic Science to the Earth Science of Tide & Currents. So Please Don't Try, because it will only frustrate you totally.
But then I have found that some people will try to make Orange Juice with a bussel of Apples... It just don't work..


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## Bene505 (Jul 31, 2008)

If a bay with a 4 foot tidal difference is 1000 feet deep or 20 feet deep, it doesn't matter in terms of the volume of water needed to raise the water level in that bay. In the case of the 1000 foot deep bay, the first 996 feet of depth will still be there regardless. So what matters is the surface area. (Or at least the average surface area, if the surface area varies a lot between high and low tide.)

Regards,
Brad


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## Bene505 (Jul 31, 2008)

Boasun said:


> NOTE: you cannot apply Electrical/Electronic Science to the Earth Science of Tide & Currents. So Please Don't Try, because it will only frustrate you totally.
> But then I have found that some people will try to make Orange Juice with a bussel of Apples... It just don't work..


It's all physics, right?

I really like your explanation because it filled in what my description lacked. I meant only to describe the situation for bays/lakes with narrowish inlets. That explains what can happen. Your description filled in a lot of what happens in the open ocean and more generalized cases. Hopefully both posts helped the OP.

Regards,
Brad


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## jackdale (Dec 1, 2008)

Time to buy the Canadian Tide Manual.


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## AdamLein (Nov 6, 2007)

Faster said:


> I'll go along with that, Adam, but the duration of the 'leveling time' (ie flowing current condition) and hence the delay of actual slack current will depend on volume (and the capacity of the restriction, or narrows).


Volume to be transferred is not really related much to the volume of the source reservoir. If water is flowing into the Strait of Juan de Fuca, it doesn't matter that it's coming from the Pacific Ocean and not the Atlantic. That's my only point there. Sure, there will be a limit if you are looking at tides between very small reservoirs that simply don't have the capacity to fill one another up, but such reservoirs really don't see tides much anyway (though actually I don't really know what the Great Lakes are like... if they have tides, that would shed some light on the subject).



> If a bay with a 4 foot tidal difference is 1000 feet deep or 20 feet deep, it doesn't matter in terms of the volume of water needed to raise the water level in that bay. In the case of the 1000 foot deep bay, the first 996 feet of depth will still be there regardless. So what matters is the surface area. (Or at least the average surface area, if the surface area varies a lot between high and low tide.)


True... but I'm not talking about depth of water anywhere. I agree that the surface area at the head of an inlet or in a bay (or, really, anywhere) is a factor in determining how much water flows into that inlet to raise the water level. Where I disagree is that it depends on the surface area or volume _outside_ the inlet; so long as the surface area outside is much much larger than the area of the inlet or bay, the interior area is all that should matter.

I guess I misunderstood you when you said "large surface area behind land mass", as I was picturing a flooding tide from an ocean into a bay or inlet.


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## rikhall (Feb 7, 2008)

sailingdog said:


> But rik, that looks like so much fun...


I went with my sister-in-law who was visiting. Linda had just had back surgery and so the pounding we took would not have been good for her. She stayed on the shore and took pictures. We really were going up and down 10 foot waves.

But, it was an absolute blast!!!!! I highly reccomend the experience. But, you will get soaked to the skin!

More photos at Reversing Falls - Rik

Rik


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## Faster (Sep 13, 2005)

Adam

But the extreme tidal currents found on our narrows and passages, while primarily dependant on differential water level across the narrows, the length of time it takes to equalize will affect the duration of the current flow.

If Indian arm was only 1/2 mile long and 25 feet deep long instead of 20 odd/1000 feet, your 2nd narrows would behave very differently.


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## Sabreman (Sep 23, 2006)

I really love stirring up a hornet's nest in the morning and then checking in on all the debris in the evening (actually taking the time to do some work in between). 

Good answers. I'm glad that I raised the question because the answer is not at all straightforward, judging by the responses. So I wasn't the only only one wondering. I *will* be reading Bowditch tonight.

From the answers, it appears that the responses can be summarized:
1. Tide and Current will vary not necessarily because of the earth/moon interaction, but due to bottom/shore configuration.
2. If tide and current are ever to be the same, it would only be at the point of measurement. That is, the place for which the tide chart was published (e.g., Annapolis Harbor).
3. Even at the measurement point, it is possible for tide and current to be out of sync, but not by much (If it's high tide, how could water still be flooding? If so, then it's not high tide because it will get higher).
4. We now know why Rik has gray hair and on his third boat.


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## Faster (Sep 13, 2005)

Sabreman said:


> ......3. Even at the measurement point, it is possible for tide and current to be out of sync, but not by much (If it's high tide, how could water still be flooding? If so, then it's not high tide because it will get higher).
> 4. We now know why Rik has gray hair and on his third boat.


We have two tide stations about 5 or 6 miles apart. One (Pt Atkinson) is on a relatively open stretch of water, on the edge of Georgia Strait. The other (Vancouver harbour) is behind a rather tight narrows, subject to currents up to 9 knots at spring tides.

The strait itself can have currents in the order of 1 or maybe 2 knots, with slack occurring very near standing tide times give or take the intertia of the mass of the water in the strait.

However as the tide rises on the strait side of the narrows, it cannot get through narrows as fast as it rises outside, so the water level in the harbour lags behind. This differential generates current as the higher water outside tries to fill the harbour basin. Lets say the level lags by 2 feet on a large tide. When the tide outside peaks, the water is flowing into the harbour where it's two feet lower.

Even though the water level outside is now falling, it's still higher than the harbour until that two foot differential has been crossed. During that time the water in the narrows is still 'flooding' even though the tide is actually ebbing. When the water levels even out, the current goes slack, but it's relatively momentary because now the tide continues to fall and the same restriction of the narrows delays the 'draining' of the harbour. Now the narrows is 'ebbing' (finally!) This delay is typically between 1.5 and 2 hours in this case.

At the other end, when the tide reaches low ebb, the level in the harbour is still higher than the strait, so until the tide rises to equal the lagging level drop in the harbour it continues to 'ebb' even though the tide is on the flood again. When the levels are equal (again briefly) the current is slack until it starts to flood, again some time after 'low tide'.

This same delay generally means lower highs and higher lows inside the harbour, and a time differential as well.

Rik's got gray hair and on boat three??? We're on boat 5 and the hair's grey and thinning!!


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## AdamLein (Nov 6, 2007)

Faster said:


> But the extreme tidal currents found on our narrows and passages, while primarily dependant on differential water level across the narrows, the length of time it takes to equalize will affect the duration of the current flow.


You're absolutely correct... I had misread Bene's post.


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## rikhall (Feb 7, 2008)

jackdale said:


> Time to buy the Canadian Tide Manual.


Take a look at:

Tides, Currents and Water Levels - Canada

Rik


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## jackdale (Dec 1, 2008)

rikhall said:


> Take a look at:
> 
> Tides, Currents and Water Levels - Canada
> 
> Rik


Hi Rik

I sail on the left coast. The Tidal Manual explains the theory



> The Canadian Tidal Manual is designed to provide the theoretical background and the technical instruction necessary for the effective performance of the tasks involved in gathering and using tide, current, and water level information on hydrographic field surveys. (Warren D. Forrester, Ph. D; Canadian Hydrographic Service, 1983, 148 pages)


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## jrd22 (Nov 14, 2000)

Sabreman said:


> 3. Even at the measurement point, it is possible for tide and current to be out of sync, but not by much (If it's high tide, how could water still be flooding? If so, then it's not high tide because it will get higher).
> )


As Faster has explained high tide and high water slack (turn) can be hours apart. Generally the larger the tidal change the larger the difference in the two times; a twelve foot tidal change will usually have a longer time until slack than say a four foot change. The pass that we live on has as much as a two or three hour difference between high or low water and slack sometimes. A body in motion tends to stay in motion (It's magic, don't try to figure it out)


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## tempest (Feb 12, 2007)

I'm not sure if I can add too much to this discussion, that hasn't been covered.

The purpose of knowing the current flows is to be able to make a favorable passage. So you are moving too. In a place like the east river of NY where the current runs 5 kn. it's important to get the timing correct. So in addition to the flow of current ( direction and speed) we also look at the time of the Turn.

If I hit one spot on the river at a certain time when the current tables tell me is slack water ( before it turns to go back ) and I am traveling at 5 kn. it's conceivable that as I travel past that point I may still encounter an opposing current further ahead. 

There are some points ( choke points) where is desirable to hit a point at slack, for others you might try to time your arrival at or after the current turns in your favor.


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## dacap06 (Feb 2, 2008)

sailingdog said:


> But rik, that looks like so much fun...


I wonder what that would be like on an old inner tube?  Of course that would be on a flood tide only, for obvious reasons!


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## Sabreman (Sep 23, 2006)

I have to ponder this a bit and read Bowditch.

It's still not making sense to me. I understand the discussion regarding choke points, but what I can't get past is that if it's "high" tide at Point A, then it should be as high as it's going to get. That implies no current because if there is, then it would get higher (or lower). The two discussions seem disconnected to me. As an engineer, I'm looking for the connection. Just need to think a bit....


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## jackdale (Dec 1, 2008)

Tempest said:


> There are some points ( choke points) where is desirable to hit a point at slack, for others you might try to time your arrival at or after the current turns in your favor.


This is one that *most* want to hit the turn (slack).






Yes, this is a pass on the West Coast. It goes up to Sechelt Inlet.


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## Faster (Sep 13, 2005)

Hey Jack.... we could easily have been there the day that was taken!

It's a great walk/hike into there and well worth it for the hours of entertainment.


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## jrd22 (Nov 14, 2000)

Sabre- I agree, it's difficult to comprehend. I believe that it just has to do with the momentum of a large body of water. Once it get's moving it takes awhile to stop, even after the height of the tide has stopped moving up or down. The volume and the energy is staggering when you stop and think about it. It's moving horizontally until it runs out of energy (slack) and starts to go the other way. This may just be hogwash since I don't have a PHD (or any other degree for that matter) in fluid dynamics, but it's the only thing that makes any sense to me.


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## Sabreman (Sep 23, 2006)

> it runs out of energy (slack) and starts to go the other way


That makes sense which is why I buy that slack current and tide could be off a little, but not hours.



> Once it get's moving it takes awhile to stop, even after the height of the tide has stopped moving up or down


I buy that too.


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## CaptainForce (Jan 1, 2006)

There's another dynamic too. If you're considering the current at a river or estuary, then there is the net flow out toward the ocean which will cause the ebb to run longer than the flood. Another simple analogy that helps demonstrate the delay between tides and current would be to consider your wait in a long line of cars at a traffic signal. If you are far from the light you willl likely be stopped while the light is green and moving while the light is red! Take care and joy, Aythya crew


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## Faster (Sep 13, 2005)

Sabreman said:


> That makes sense which is why I buy that slack current and tide could be off a little, but not hours.


In an open body of water there will be little difference in current slack and tide stand, other than that from the inertia of the mass. But in those conditions the current (even at mid-tide) is generally light (1-2 knots) and while obviously potentially significant to someone only able to do 5 or 6 knots, it's not impossible to deal with.

Where currents build in narrows to difficult, impossible, or even dangerous levels is where the consequences of this confusion become of concern.

Here in BC many newbies consider it an accomplishment to decipher the tide tables.. and everyone tells them to go through the pass at "low slack" or "high slack" - and so they attempt transit at the turn of the tide. In this area that can really catch one unawares and we have many many passages where the actual slack current is indeed an hour or more off of the high or low stand.

It all stems from the difference in water levels on either side of the choke point as I tried (perhaps poorly) to explain before. Another issue that nails beginners is making assumptions as to which direction a given pass 'floods' vs 'ebbs' - expecting favourable current when encountering the opposite.

Another danger is underestimating the hazards of even favourable current. We have one pass in particular that has little in the way of shoals or rocks, but it develops boat-swallowing whirlpools that also can eject logs and deadheads at torpedo trajectories without warning.

The lesson in all of this is to learn to use the appropriate charts and publications - and transit only when safe and reasonable to do so.

The good Captains reference to the effects of rivers is also noted... into all of this we do have several significant rivers that add their contributions to the mix... Combine a heavy runoff from spring melts and rainstorms with a contrary gale, LOOK OUT!!!!


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## Faster (Sep 13, 2005)

Skookumchuck narrows, (as per the video Jack posted earlier) is one of our most extreme.. running up to 16 knots at times. The kayakers can play in it until it gets beyond 12 knots or so, when the white water overfalls actually disappear and they can no long stay 'on the wave'.

These two shots are at around the 12 knots range.. I'm standing on the rocks at about 40-50 feet down stream from the crest . Hopefully the pics show it, but the level of the water is at or above eye level, so in that short distance there's a drop of nearly 6 feet at mid flood.

Note.. the 'planing' RIB has a ground speed of 0 knots in the shot.


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## dillybar (Nov 10, 2009)

And just when you think you have it figured out we have Johnston Strait where one side will miss the flood entirely or the top will ebb while below it floods, or you have areas that can go for days without an appreciable flood. 
As I recall the difference in salinity during times of heavy freshets is a contributing factor. 
The current tables actually show cross sections of the strait and where you can expect what!


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## jrd22 (Nov 14, 2000)

Here are the current and tide tables for the pass where we live. Notice the time difference between high and low tide and slack water. The bottom row of each shows the time of low slack and low tide on Mar. 3. Low slack is at 1123 while low tide is at 0936, and this is on only a 6' tidal change. On spring tides we often get 4-5 knot currents through Peavine Pass. I have also seen where slack *precedes* the high or low tide (still trying to figure that one out).
Currents:
2011-03-01 9:52 PM PST 0.00 knots Slack, Flood Begins
2011-03-02 1:06 AM PST 2.54 knots Max Flood
2011-03-02 4:36 AM PST -0.00 knots Slack, Ebb Begins
2011-03-02 6:51 AM PST Sunrise
2011-03-02 7:33 AM PST -1.86 knots Max Ebb
2011-03-02 10:48 AM PST 0.00 knots Slack, Flood Begins
2011-03-02 12:33 PM PST 0.72 knots Max Flood
2011-03-02 2:51 PM PST -0.00 knots Slack, Ebb Begins
2011-03-02 5:56 PM PST Sunset
2011-03-02 6:50 PM PST -2.67 knots Max Ebb
2011-03-02 10:33 PM PST 0.00 knots Slack, Flood Begins
2011-03-03 1:32 AM PST 2.56 knots Max Flood
2011-03-03 5:03 AM PST -0.00 knots Slack, Ebb Begins
2011-03-03 6:49 AM PST Sunrise
2011-03-03 7:57 AM PST -2.18 knots Max Ebb
*2011-03-03 11:23 AM PST 0.00 knots Slack, Flood Begins*

Tides:
2011-03-02 4:23 AM PST 8.42 feet High Tide
2011-03-02 6:51 AM PST Sunrise
2011-03-02 9:25 AM PST 4.77 feet Low Tide
2011-03-02 2:57 PM PST 7.04 feet High Tide
2011-03-02 5:56 PM PST Sunset
2011-03-02 9:02 PM PST 0.85 feet Low Tide
2011-03-03 4:47 AM PST 8.27 feet High Tide
2011-03-03 6:48 AM PST Sunrise
2011-03-03 9:53 AM PST 4.13 feet Low Tide
2011-03-03 3:47 PM PST 7.00 feet High Tide
2011-03-03 5:57 PM PST Sunset
*2011-03-03 9:36 PM PST 1.12 feet Low Tide*


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## jackdale (Dec 1, 2008)

John 

Thanks for that. I was going to do something similar for Porlier; but you made the point.

Jack


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## Omatako (Sep 14, 2003)

Faster said:


> 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.


What you're saying is not always evident in a tide chart though and one can get mislead quite easily. In the harbour where we moor, the tide charts are the same timing as Auckland Harbour but with the bay being a large expanse of water, the water is still streaming out long after the tide chart reports low slack.

As it happens the bay is benign and the currents are not a problem but in a place like were Rik is it could be quite testing.


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## Faster (Sep 13, 2005)

Omatako said:


> What you're saying is not always evident in a tide chart though and one can get mislead quite easily. In the harbour where we moor, the tide charts are the same timing as Auckland Harbour but with the bay being a large expanse of water, the water is still streaming out long after the tide chart reports low slack.
> 
> As it happens the bay is benign and the currents are not a problem but in a place like were Rik is it could be quite testing.


If I understand what you're saying Andre, we don't disagree at all. This is generally the point of this thread - the question being why "high" and "low" tide times don't coincide with slack water times - esp when discussing narrows and bay entrances....


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## Sabreman (Sep 23, 2006)

I promised that I would read Bowditch and I have. Everyone posting is correct, but getting back to the original question - *Why?* - Bowdtch has a rather concise answer that is in sync with most of what has been posted anecdotally. As always, Bowditch has the last word:



> 918. Time Of Tidal Current And Time Of Tide
> At many places where current and tide are both semid- iurnal, there is a definite relationship between times of current and times of high and low water in the locality. Cur- rent atlases and notes on nautical charts often make use of this relationship by presenting for particular locations, the direction and speed of the current at each succeeding hour after high and low water, at a place for which tide predictions are available.
> 
> Where there is considerable diurnal inequality in tide or current, or where the type of current differs from the type of tide, the relationship is not constant, and it may be hazardous to try to predict the times of current from times of tide.Since the relationship between times of tidal current and tide is not everywhere the same, and may be variable at the same place, one should exercise extreme caution in us- ing general rules.
> ...


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## Bene505 (Jul 31, 2008)

jrd22 said:


> ... I have also seen where slack *precedes* the high or low tide (still trying to figure that one out)....


Picture a lake-behind-narrow-inlet that is located off another lake-behind-narrow-inlet. The first lake may have a 4 hour delay between high tide and slack current. The second lake (off of the first) may have another 4 hour delay off the first lake. So the second lake would be 8 hours behind (4 hours ahead) of the ocean's high tide. [I know this doesn't really qualify, but it's an interesting thought experiment.]

OR

Picture a big lake with a really narrow inlet. It may take 8 hours for water to flow into the lake. That's now preceding the next tide.

Note that in both cases above, the height of the tide will be significantly diminished. Without doing the math, I'd guess that every 3 hour delay results in 1/2 the tidal height. (It feels a lot like polarization of radio waves, and a lot like the bending of swells around a point.)

Regards,
Brad


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## captbillc (Jul 31, 2008)

AdamLein--- the great lakes do not have tides. they have seiches caused by wind and difference in barometric pressure from one end of the lake to the other, which can change water level 6ft or more. if you go to seiches you will find some interesting information about them.


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## jackdale (Dec 1, 2008)

From the Canadian Hydrographic Service Tidal Manual



> TIDES AND TIDAL STREAMS
> In a tide wave the horizontal motion, i.e. the particle velocity, is called the tidal stream. The vertical tide is said to rise and fall, and the tidal stream is said to flood and ebb. If the tide is progressive, the flood direction is that of the wave propagation: if the tide is a standing wave, the flood direction is inland or toward the coast, i.e. «upstream.» The flow is the net horizontal motion of the water at a given time from whatever causes. The single word «current» is frequently used synonymously with «flow», but the term residual current is used for the portion of the flow not accounted for by the tidal streams. A tidal stream is rectilinear if it flows back and forth in a straight line, and is rotary if its velocity vector traces out an ellipse. Except in restricted coastal passages, most tidal streams are rotary, although the shape of the ellipse and the direction of rotation may vary. The ellipse traced out by a tidal stream vector is called the tidal ellipse. Slack water refers to zero flow in a tidal regime. The stand of the tide is the interval around high or low water in which there is little change of water level: this need not coincide with slack water.
> 
> Since the observed tide consists not of a single wave, but of the superposition of many tide waves of different frequency and amplitude, it will never fit exactly any of our simple descriptions. Because of this, we cannot expect the heights of successive High Waters (HWs) or of successive Low Waters (LWs) to be identical, even when they occur in the same day. Thus, the two HWs and two LWs occurring in the same day are designated as higher and lower high water (HHW and LHW), and higher and lower low water (HLW and LLW). It is likewise only the tidal stream associated with a single frequency tide wave that traces a perfect tidal ellipse. The composite tidal stream each day traces a path more closely resembling a double spiral, with no two days patterns identical. Also, no tide is ever a purely progressive or a purely standing wave, so that slack water should not be expected to occur at the same interval before HW or LW at all locations.


CHS-Phenomena


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## AdamLein (Nov 6, 2007)

> In a tide wave the horizontal motion, i.e. the particle velocity, is called the tidal stream. The vertical tide is said to rise and fall, and the tidal stream is said to flood and ebb. If the tide is progressive, the flood direction is that of the wave propagation: if the tide is a standing wave, the flood direction is inland or toward the coast, i.e. «upstream.»


Do you know exactly what "wave" they're referring to here?


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## hellosailor (Apr 11, 2006)

Sabreman-
" it should be as high as it's going to get. That implies no current because if there is, then it would get higher"
You're missing the whole crux of the matter. "Current" has nothing to do with "higher" or "lower" !
Current only refers to horizontal movement of the water, NOT A CHANGE IN HEIGHT.
Once you realize that the vertical water level has nothing to do with the horizontal movements of the water, it should be a "D'OH!" moment.
What happens to the water in a narrow creek, when you throw a beaverdam up across the creek? The water level GOES UP. Once it reaches the top and starts to spill over, or through, the beaver dam, the horizontal flow of the creek continues as it did before. But the HEIGHT has GONE UP at the locaiton of the dam, and for some distance upstream of it.
Height=tidal changes. Flow=current changes. Yes, they do "pull" on each other somewhat and there is some interaction, but for our purposes, ignore that, they are two separate movements of the water.
Now, if you really want to be confused, look up the movement of water in ocean waves. The waves don't go up and down or "that-a-way", the movement of the water in each wave is actually circular! Up, and over, and down, and back. Over and over again, and the energy just travels in the wave pattern.


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## Bene505 (Jul 31, 2008)

Sabreman said:


> ...If it's high tide, how could water still be flooding? If so, then it's not high tide because it will get higher)...


Going with the large lake or bay that is behind a narrow inlet (and simplifying a bit by leaving out any oscillations of height within the bay itself)....

If you measure tidal height on the bay side of the inlet, then the tidal height won't be at maximum until the incoming current stops.

But, if you measure just outside of the inlet, the tide may start to fall before the bay's tidal height has "caught-up". The outside tidal height will actually be dropping and current will still be flowing into the lake/bay. That's an example of how you get more incoming current without more tidal height.

-

The tidal range (high minus low) inside the inlet is smaller that the tidal range outside the inlet. This is obvious because the lake never gets all the water needed to equal the outside tidal height before the outside tide starts falling.

What will really rock your world is the thought that the tidal range varies along the length of the inlet. If it's a couple of long jetties forming the inlet, the tidal range at a point that's part way into the inlet will be roughly the average of the (larger) outside tidal range and the (smaller) inside tidal range.

Put another way... at low tide, the black lines on the rocks would not be level, but sloping up as you head to open ocean. The shorter the inlet, the more severe the angle. You can see that in the reversing falls picture, which has a very short inlet and thus a severe difference in tidal range over a few feet. Look at the angle of the black line on the rocks.









We just get so busy looking at the current there, that we forget to see the different tidal range. (The above reached by thought experiment, then checked in the picture.)

What's rocking my world is the thought that along the length of the inlet, the interval between high tide and slack current changes. I have a feeling it varies along with the change in the tidal height, that there is an equation that relates the two. (The height of a point along that black line can determine the delay between high tide and slack current at that spot.)

Regards,
Brad


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## jackdale (Dec 1, 2008)

AdamLein said:


> Do you know exactly what "wave" they're referring to here?


In reading this the tide is viewed as a wave.



> Generality
> Tides in the open ocean are usually of much smaller amplitude than those along the coast. This is partly due to amplification by reflection and resonance. It is, however, more generally the result of shoaling: *as the wave propagates into shallower water, its wave speed decreases and the energy contained between crests is compressed both into a smaller depth and a shorter wavelength. The tide height and the tidal stream strength must increase accordingly.* If, in addition, the tide propagates into an inlet whose width diminishes toward the head, the wave energy is further compressed laterally. This effect, called funneling, also causes the tide height to increase.


CHS-Phenomena


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## TakeFive (Oct 22, 2009)

jrd22 said:


> Sabre- I agree, it's difficult to comprehend. I believe that it just has to do with the momentum of a large body of water. Once it get's moving it takes awhile to stop, even after the height of the tide has stopped moving up or down. The volume and the energy is staggering when you stop and think about it. It's moving horizontally until it runs out of energy (slack) and starts to go the other way. This may just be hogwash since I don't have a PHD (or any other degree for that matter) in fluid dynamics, but it's the only thing that makes any sense to me.


I do have a PhD in fluid mechanics, and I too have a difficult time getting my hands around it. You are very correct, it has a lot to do with momentum and inertia, which are very significant in these flows. For the moment, forget about large bodies feeding into restricted inlets, and just imagine a straight river with uniform width. (Not unlike parts of the Delaware where I sail.) Imagine a low-amplitude "wave" moving up the river until it runs out of gas (or hits rapids). As the crest of this wave moves up the river ("flood"), a stationary observer will observe high tide when the crest passes him, but after high tide the crest continues to move upriver (where the high tide occurs later), and the current directly behind the crest continues in the "flood" direction for awhile afterwards, then eventually reverses to an ebb.

An extreme example would be standing in the ocean at the beach. You are standing out far enough that the swells are not breaking until after they are past you. As the swell passes you, you are pushed toward shore, and after it passes you are still pushed toward shore momentarily even though the water level is dropping. Then as the next swell approaches you are pulled back away from shore, and even as the level rises the "undertoe" is pulling you away from the shore. So you have two examples here where the current is actually the reverse of what you would expect from the way the water is rising or falling.


AdamLein said:


> ...Certainly you get some water moving when it should be slack, or moving the wrong direction before or after a turn, but I think that that's more likely due to back eddies and the general non-uniformity of the current than the water's inertia.


Technically this is not correct. Eddies cannot occur without inertia. Inertialess flows are perfectly laminar and reversible. Eddies violate the principle of reversibility. Look up the classic works (and videos) of G.I. Taylor for proof and interesting demonstrations of these effects.


Bene505 said:


> If a bay with a 4 foot tidal difference is 1000 feet deep or 20 feet deep, it doesn't matter in terms of the volume of water needed to raise the water level in that bay. In the case of the 1000 foot deep bay, the first 996 feet of depth will still be there regardless...


It's too late at night for me to do any calculations to back this up, but it is very possible that the depth is VERY important in the example that you cite. The balance of inertial forces (which cause eddies and push "waves" up rivers) and viscous forces (which damp out the eddies and cause waves to dissipate) is strongly affected by the length scale, which would most likely be the half-width or depth of a body of water (whichever is less). Water traveling in a 1000 foot deep body of water would likely have much higher inertia than in a 20 food deep body of water. And obviously the cresting action would be strongly affected by this depth as well.

As a short-time sailor and long-time fluid mechanics expert, I am still getting my hands around the applications of Reynolds Number, Froude Number, and other dimensionless paramters to sailing. It's a whole new set of boundary conditions that are somewhat new to me, but is strikingly similar to the things I do in my professional life.


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## Bene505 (Jul 31, 2008)

RhythmDoctor said:


> ...It's too late at night for me to do any calculations to back this up, but it is very possible that the depth is VERY important in the example that you cite. The balance of inertial forces (which cause eddies and push "waves" up rivers) and viscous forces (which damp out the eddies and cause waves to dissipate) is strongly affected by the length scale, which would most likely be the half-width or depth of a body of water (whichever is less). Water traveling in a 1000 foot deep body of water would likely have much higher inertia than in a 20 food deep body of water. And obviously the cresting action would be strongly affected by this depth as well...


In the lake-behind-inlet example, the inertia of the big wave of tide is very diminished by the narrow inlet. The surface arae of the lake becomes paramount. (The example is useful because it provide a simple example of current lagging tidal height.)

That said, I don't have my head fully around it either. What a facinating thread from a simple question!

Regards,
Brad


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## Sabreman (Sep 23, 2006)

I get it now. See my post quoting Bowditch. Reading the entire chapter provided me with insight into what is actually happening. As I indicated in the OP, I was after the "why" of it. I too, have lots of anecdotal evidence but without causal factors. 

Thank you for all the photos and explanations. Some were even correct! :laugher


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## TakeFive (Oct 22, 2009)

One other point to remember. Since high tide travels through rivers and bays as a "crest" driven by both gravity and inertia, the only way for a crest to form is for water to flow into the crest from both sides. This also is a large part of the reason why the slackwater lags the high tide.


RichH said:


> 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...


I have a screen shot of OpenCPN's current prediction for the C&D Canal that is a great illustration of my above point:








This totally freaked me out when I saw it - so much so that I started a thread on it here thinking it was a bug in OpenCPN. But in the context of our discussions it makes a little more sense.


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## AdamLein (Nov 6, 2007)

RhythmDoctor said:


> Technically this is not correct. Eddies cannot occur without inertia.


I didn't mean to say that they could. I was trying to make two statements:

1) That the gross inertia of a large volume of water is not a significant factor in observations of unintuitive currents, like current flooding towards a point at high tide. I suppose I could be wrong about this, but at one point you mentioned the motion of the crest of the wave moving past observers at differing times, and my main belief has been that that is primarily what causes the difference in times between standing of tide and slack water.

2) That you often see current flowing in two directions at once in a given channel, because of eddies. Really I shouldn't have brought this up at all as I don't really want to complicate the matter more than it already is. Certainly I will defer to your expertise on the mechanics of eddies.

My own expertise in this area is far less... I did my bachelor's in applied math, but haven't touched it since then. I am fairly comfortable considering systems at equilibrium, or trying to reach equilibrium, and while I realize that a high-dimensional system like the ones we're describing may never approach equilibrium, I have been sticking to that analysis until it has been shown clearly inadequate.

To me the main thing consider is that the surface of the earth's water wants to be at a constant gravitational potential, and so flow is induced from areas of high potential to low potential. So long as there is a reservoir farther inland whose surface is at a lower potential, you'd expect current to be pushed in that direction.

I'm not sure where inertia comes into play here and how exactly to deal with it. If I consider a volume element of water moving at a greater speed than the volume element immediately downstream, the faster element either has to change its speed or flow around the slower one, or more likely, some combination of the two. In a restricted channel, this means that water has to flow over the water downstream, which means it's now at a higher potential.

I guess I'm just trying to picture a flood of water through a channel from a large ocean to a reservoir with a level surface at high tide. Where's that water going to go? It can't raise the level of the reservoir anymore. More precisely, where's the _energy_ of that water going to go? The only outlet I can see is gravitational surface waves. The only flawed assumption that I can see is that the reservoir is completely level and uniformly at high tide. I will remove it by dividing the reservoir into two pieces, one further inland of the other, and assuming it is level and is uniformly at high tide.

In short, I feel like inland water at high tide should be pretty good at "halting" the flow---or at least converting its energy into something else---of a flood current at the same or lower gravitational potential. The inertia of such a current shouldn't be an excuse for the water to continue flowing in the same direction.


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## TakeFive (Oct 22, 2009)

Adam - Your points are all valid. I played with a few numbers this afternoon, and you are actually correct that inertia is relatively small relative to gravitational forces driving the tides in most cases. I am used to dealing with process equipment, where the two dominant forces are viscosity and inertia and gravitational effects are very small. I am not as used to dealing with geological length scales, and on that length scale viscous effects are much smaller than inertia (as I stated before), but gravitational effects are often larger than both of them. So the dominant dimensionless parameter is the Froude number, which is the same number used to determine "hull speed." The applicable length scale is not the LWL, but the depth or width of the body of water (whichever is less). If the water is very shallow (less than a typical boat length), then inertial effects might be important, as demonstrated by the eddies that you often see in fast flowing, shallow rivers. But for deep water moving at less than a typical "hull speed," the inertia does become negligible relative to gravitational effects.

However, eddies are not the only way to get current flowing in opposite directions in a channel. Like I showed in the screenshot I showed above, the presence of a crest can account for opposing flows on either side without the presence of any eddies.

Now my brain hurts!  :laugher


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## Sabreman (Sep 23, 2006)

Wow All this brain power in action.... we have Chemical Engineering PhD's, Applied Mathematicians.. amazing. I've learned much more than I expected so it's been a good couple of days.

Good on ya, SailNet.

So the next time someone raises the topic, I'll say with authority that the difference between tidal current and tidal height is due to very large sturgeon on the seabed moving their tails in a mating frenzy. :laugher


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## Ulladh (Jul 12, 2007)

Question for the fluid dynamic folks;

What is the effect of salt water - fresh water stratification in estuary and tidal river systems?


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## TakeFive (Oct 22, 2009)

Ulladh said:


> Question for the fluid dynamic folks;
> 
> What is the effect of salt water - fresh water stratification in estuary and tidal river systems?


This site explains it better than I can.


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## SlowButSteady (Feb 17, 2010)

RhythmDoctor said:


> One other point to remember. Since high tide travels through rivers and bays as a "crest" driven by both gravity and inertia, the only way for a crest to form is for water to flow into the crest from both sides.


Not quite. A "crest", or tidal bore, is also indicative of a "shallow water wave", one where the water depth is less than 1/2 the wavelength of the wave in question (technically, waves in water <1/2 wavelength deep and >1/20 wavelength deep are "transitional" waves, while waves in water <1/20 wavelength deep are "true" shallow water waves). The crest seen when the current and the tide are opposing one another is closely related to this phenomenon.



RhythmDoctor said:


> This also is a large part of the reason why the slackwater lags the high tide.


Slack water lags behind high, or low, simply because the embayment in question has too small an opening to fill, or drain, efficiently. Look at the difference between San Francisco Bay and Monterey Bay. In open bays (e.g., Monterey Bay) the turn of the tide and slack water (for what little current there is) are identical. In semi-enclosed bays (e.g., SF Bay) the two can differ by several hours. Water depth also play a role here, as shallow bays (rivers, et cetera) slow the tide wave as it progresses upstream.


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## TakeFive (Oct 22, 2009)

SlowButSteady said:


> ...Slack water lags behind high, or low, simply because the embayment in question has too small an opening to fill, or drain, efficiently. Look at the difference between San Francisco Bay and Monterey Bay. In open bays (e.g., Monterey Bay) the turn of the tide and slack water (for what little current there is) are identical. In semi-enclosed bays (e.g., SF Bay) the two can differ by several hours. Water depth also play a role here, as shallow bays (rivers, et cetera) slow the tide wave as it progresses upstream.


But in straight bodies of water like rivers of relatively uniform width (with no constrictions forming bays), slackwater still lags behind the high and low tides. Another explanation must exist for those cases.

And my use of the term "crest" was not meant to meet any rigorous technical definition of the word. I chose the word to help visualize the point of maximum amplitude of the stream in question, without specific reference to any relationship between its wavelength, amplitude, depth of water, or any other length scale.


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## SlowButSteady (Feb 17, 2010)

RhythmDoctor said:


> But in straight bodies of water like rivers of relatively uniform width (with no constrictions forming bays), slackwater still lags behind the high and low tides. Another explanation must exist for those cases.


Reread the last sentence of my previous post. The depth of the water acts to slow both the flow and the tide wave. Wave celerity (i.e., the velocity of the wave propagation) is proportional to wavelength only for deep water waves, those wave traveling in water greater than 1/2 wavelength deep. Tide waves are never deep water waves, as their "natural" wavelength would be about half the Earth's circumference (or about 20,000 km). As they enter bays, deltas, rivers, et cetera, they become very shallow water waves, and are slowed dramatically. Also, the flow of the water out of a river (et cetera) is constrained by the depth of that water. When shallow water flows fast enough, it always forms standing waves. This indicates that the flow is being constrained by the depth of the water. So, the river can't flow fast enough to keep up with the falling tide, AND the tide is slowed as it races upstream (often "piling up", or forming a tidal bore).

Actually, there HAS to be something of a lag. Tidal flow can only happen if the water at the mouth of the river is lower than slightly up river (no difference in height, no flow). So, the tide drops, THEN the flow starts. Since the tide doesn't stand still for very long, even as it turns, there will always be tidal flow lagging behind the actual turn of the tide - often for quite some time.


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## AdamLein (Nov 6, 2007)

Sabreman said:


> I'll say with authority that the difference between tidal current and tidal height is due to very large sturgeon on the seabed moving their tails in a mating frenzy.






SlowButSteady said:


> Not quite. A "crest", or tidal bore, is also indicative of a "shallow water wave",


This is why I asked above, "what's the wave we're talking about here?"

Your statement above indicates you're thinking in terms of gravitational waves that we're so familiar with as seas and swells. I think the Canadian Tide Whatever Manual, when it talked about a wave, is not referring to a wave in this sense.

Obviously there is horizontal propagation of water, and it may be that it is reasonable to describe this propagation as a wave where the phase velocity is identical to the particle velocity (I had not considered this notion before reading this thread). Gravitational surface waves are affected as they are by depth of water, and exhibit the relationship you describe between wavelength and wave depth, because of the nature of gravitational wave propagation via particles moving in closed loops. If we're talking about purely horizontal motion of particles, there's no reason to think that the same shallow water effects would be relevant.

In the end I still don't really know what is meant by "wave" here. I know the idea of a moving "crest" has been put forward, but I have never seen such a crest, despite the large tidal exchanges and tidal currents in the waters where I sail. I've always assumed the water surface is basically planar when you ignore disturbances due to wind, boats, seagulls, etc.

On the other hand, the CHS book also refers to the shoaling effect acting on the wave, so.... maybe I'm completely wrong? All I've realized is that the CHS explanation doesn't actually help my understanding at all. Yay.


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## AdamLein (Nov 6, 2007)

SlowButSteady said:


> Actually, there HAS to be something of a lag. Tidal flow can only happen if the water at the mouth of the river is lower than slightly up river (no difference in height, no flow). So, the tide drops, THEN the flow starts. Since the tide doesn't stand still for very long, even as it turns, there will always be tidal flow lagging behind the actual turn of the tide - often for quite some time.


Err, I disagree here. Sadly because I've been saying all along that tidal flow would only happen if there's a difference in water levels (i.e. inertia is not important).

However, the levels at two points can be there same and there can still be flow between them. As long as some _distant_ level is different, there will be flow. Like in a siphon.

Come to think of it, the siphon effect pretty much covers why the tidal current may continue after standing water at a given point.

Moreover, there doesn't need to be a delay. It's not like the water moves straight up or straight down at one point, and then a few seconds later, flow begins away from or toward that point. There's a relationship between level difference and flow, for sure, but it acts continuously and instantaneously. You will see a lag in principle, since the information about a given change in level travels at finite speed, but it's still much faster than the time scale on which the tide changes.


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## SlowButSteady (Feb 17, 2010)

Adam,

Yes. The inertia of the system does have an effect. This can be most dramatically seen in situations where an embayment has a natural frequency close to the frequency of the tidal cycle (e.g., the Bay of Fundy, or South SF bay to a lesser extent). If the two are in resonance (or close to it) the sloshing back and forth of the water is accentuated, creating extreme tidal heights in the upper reaches of the embayment. However, think about what happens if the tidal cycle is MUCH longer than the natural frequency of the embayment -- the embayment "drains" completely before refilling, and fills completely before draining. This is what happens in embayments with wide mouths (e.g., Monterey Bay). The apparent difference between slack water and the turn of the tide is only significant when the embayment can't drain, or fill, before the tide reverses itself.


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