Slack tide and Slack Current

[Jim Cate]

Here in eastern Oz, Alan Lucas's excellent cruising guides have guidelines for most of the river barred entrances that we must use. This includes time delays for slack on flood and ebb (often quite different) and times of high water further inland on the rivers. Very useful info that isn't available from any "official" sources that I'm aware of.

What becomes obvious when making the trips inland, say up the Clarence to Grafton, some 45 miles inland, is that it is easy to go upstream staying with the flood, favorable current and deeper water. Not such a good outlook for the return trip!

Jim

[mvweebles]

Inertia is a pretty easy to understand answer. The waters that feed San Francisco Bay are tidal to Sacramento and beyond. Sacramento is 100 nms Inland from the GG Bridge. High tide at GG Bridge today was 0130 at 5.5 ft. At Sacramento, high tide was 0900 at 3.4 ft. Once the Sacrament River and American Fork River have a tidal push in addition to their natural flow, it evacuated a lot of water. Tidal flush (ebb) at GG Bridge will continue even after the incoming (flood) current starts. It's can be really turbulent and creates what is called a Tidal Bore, a wave that advances with current reversing behind the wave. Areas with a large tidal range and a lot of area in the drainage basin will see bigger differentials.

Bottom line is high tide and slack current (or their inverse) are not synonymous. You need tide tables to correlate. Crossing bars/inlets, important to know when slack water is, not high tide

[chrisr]

lake macquarie (NSW) where we spend a fair bit of time, is a very big lake with a very small entrance. the amount of water flowing into or out of the lake simply cannot fit through the channel until the tide height at one end (or the other) reaches a certain pressure. this means that there is a significant overrun of tidal flow

in the case here, current flow continues to run for about 3hrs after the posted time of high or low water. as current through the narrow bridge into/out from the lake can run at 4 knots or more, it's pretty important to get it right !

cheers,

[ggray]

Regarding tides and currents on a river, tide can be described as a mound of water (very low and broad, of course) moving up the river. This results in progressively later highs, lows, and slacks at points going up the river.

OK, for example it is high tide at a particular point. The tide is still rising at all points further upstream. That water causing water levels to be rising at points further upstream is coming from downstream of that point and making its way upstream. That is the current you still see there at that point at high tide.

Of course, when you approach the upper reaches of a tidal river, the current is also affected by water flow from above the fall line.

But current sure gets complicated when there are islands diverting the tidal flows on a large body of water.

[wefleenor]

I was astounded at the breadth of this discussion. I have a PHD in hydrodynamics and spent most of my career researching hydrodynamics and water quality of estuaries. If you want a reasonably concise book on the subject I would suggest Waves, Tides, and Shallow-water Processes which you can generally pick up for 6-15 dollars. I always recommended that students read the book before taking my graduate course in Estuarine Hydrodynamics and Water Quality. If you are particularly interested in the San Francisco estuary I suggest you watch some videos I put together simply to assist people in understanding the complexity of the system
https://californiawaterblog.com/?s=unraveling+the+knot

Bill Fleenor
Double-Wide

[Adelie]

Originally Posted by evm1024

OH, let me throw mu 2 cents in.



In a land locked basin (ignoring inertial effects and having no "side chambers") with only 1 way in and the same 1 way out slack and high tide will happen at the same time. This is the image that most people visualize.



In other places there are more than one way in and one way out we end up with differential flows.



High tide is reached on inflow when the rate of input of water is equal the to the rate of outflow. For that circular basin written about above the condition is outflow = 0 and in flow = 0 and height is max.



Add in a side basin or a second (or third) inlet/outlet and when the inflow is greater than the outflow the current runs and the height increases. As the inflow slows (with increasing basin height) the height gain slows also. At some point the inflow rate and the outflow rate are equal and then the height does not change (max height). But as noted there is an outflow and an equal inflow. Current with no change in height.



Anyway that is the gist of what is happening. It is quite complex and has many modes. Add in a river (additional inflows) and you get even more modes.



I spent some time when I was supporting a NSF regional science center that modeled coastal ocean and riverine water flows using the ELCIRC and SELFE model setting working on this.



A single model run can be 30 GB per hour depending on the resolution. Here is a starting point if interested: Virtual Columbia River | Center for Coastal Margin Observation & Prediction



Let's see if this GIF works:

Nope, number of “chambers” is not it.

The closer you are to the “dead” end of an inlet the closer the slacks will be. At the dead end they will be simultaneous. The closer to the mouth of the inlet the bigger the disparity.

More fingers, constrictions, general width, atmospheric effects, .... all make this more complex. But the basic issue is inertia.

[Cheechako]

Originally Posted by dougweibel

A64,

Lots of answers here and most are not wrong as there are many things that affect tidal current flow. The guys in the PNW have a very complicated current picture up there as the current is winding its way between many obstructions, and often coming from or going to multiple directions in the bigger picture.

But for the simple case I think you are asking about, where the current flow from the ocean into/out of a sound through an inlet that restricts the flow, the answers in posts #7 and #9 are pretty good. It is all about restriction of the tidal flow coming in from or going out to the ocean. The situation can be hard to see but you can simplify the situation in your mind by imagining a big sound of water connected to the ocean by a single long channel. The channel restricts the flow of water so that the level at the two ends of the channel can be quite different. When the tide is at peak height in the ocean the water level at the other end of the channel will be significantly lower and current will be flowing quickly in. An hour after peak tide (for example) the ocean level will have gone down, but will still be above the level of water in the sound so water continues to flow in. Another hour later the ocean level is down so that the water level on both ends of the channel is equal and the tidal current is slack. Etc. How late after high tide the current goes slack is a function of how restrictive the channel is and how big the sound is. In the extreme slack current occurs at mid tide.

In some locations it is this simple but often, as in the Bahamas, you have multiple cuts connecting interior sounds with the ocean and then you can get all sorts of other things going on due to momentum, wind, bottom contours and all sorts of things.

Yeah , some pretty complicated systems at work for sure. It's interesting to think of water bodies having "bulges", but then on a larger scale, that's all tides are. On the inside passage in the PNW, I have sat aboard at anchor and watched two levels of water of 2- 3 feet height difference intersecting. it created a standing rolling wave at the intersection line across the channel for several minutes.

[MicHughV]

...some years ago I was involved in constructing a water test tank to conduct various and sundry hydro tests.

At one end of the very long tank were two turbine pumps, fitted with flow straighteners to pump a uniform x-section of water into the pumps that pumped the water to a stilling well on the far side which in turn fed the main long tank to provide smooth water flow. The speed of flow could be moderate by the pump speed as could various devices installed in the tank.

This same tank was painted with a propriety paint system which produced an almost frictional surface, but even as slight as this was, the frictional losses could be measured.

Into this tank, alongside the tank, on the bottom of the tank, etc, were fitted various and sundry devices installed there by various universities to conduct various tests.

Alongside the tank were glass windows so one could peer into the tank to view what was going on.

A lot of tests were run to validate computer models. Velocity devices were inserted into the tank which could measure incremental minute differences in water velocity. Other devices could measure the elevation drop of water from end of the tank to the other to in minute fractions.

I had an opportunity to witness some of these studies.

I learned a lot about water flow that I never knew about or cared to know about and came away with the knowledge that there is no one size fits all.

I was informed that every water body on this planet is subject to a variety of different forces and conditions that will differ from one location to another. Besides water depth, salinity, bottom contours, bottom matrix, moon and sun phases, rotation of the earth, one must also consider a gazillion other factors......

The oceans are a constantly moving mass of water with umpteen twist and turns, temperature differences, etc. that affect practically anything and everything, including weather.

It truly is the realm of the Ph.D's.

As a sailor I'm contend to study the tide and current tables and add my best guess. I can only see and feel the upper layer of any water body, but there is far more going on below me that you and I am aware of.

[tombeau]

I read an article a while back that proposed an answer for the phenomenon you observe. It framed the tide surge as a wave moving over the earth as the moon pulled the water as the earth rotated. As a wave, it was moon generated instead of wind generated. With this in mind, think of a little ball in the water. As the wave comes through, it rises and wants to move with the wave. as the wave crest passes, it starts to fall but the current is still moving with the wave. Then the wave/tide changes direction, and the ball continues falling buts its motion is in the opposite direction. When it gets to its lowest point, the current is still moving out but is slowing as the next wave comes. Looking at this ball laterally, it would describe a circle. high tide at 12 oclock. Low tide 6 oclock. slack water at 3 and 9. I think all the discussion about rivers and inlets explains the fact that the time difference between high/low and slack is not exactly between the two, but lags some. If you stand off a beach when waves are present, you will experience this phenomena. The wave will push you toward shore after the crest has passed, and still be dragging you away after the low point is reached. Hope this is helpful, and it may not be accurate at all.

[LooseGoose]

Think 3D and volume and aperture. Constraints like Current Cut may mean that all the water in the constrained area has not had time to go through the cut yet, although the gravitational tide effect has passed. Currents from rivers can also add pressure and volume to an area like Albermarle Sound. Great that you see somethingnthat looks weird....important to spend a bit of time figuring out. Good luck
Ok

[ggray]

Originally Posted by tombeau

I read an article a while back that proposed an answer for the phenomenon you observe. It framed the tide surge as a wave moving over the earth as the moon pulled the water as the earth rotated.

<snip>

Following that concept, someone wrote that were it not for continents blocking the way, especially the "wall" of N America, C America, S America, that wave circling the globe would build on itself and become enormous.

Interesting to think about.

[a64pilot]

Originally Posted by ggray

<snip>

Following that concept, someone wrote that were it not for continents blocking the way, especially the "wall" of N America, C America, S America, that wave circling the globe would build on itself and become enormous.

Interesting to think about.

Never been there, but I think maybe you have described the Southern Ocean.

[a64pilot]

Originally Posted by wefleenor

I was astounded at the breadth of this discussion. I have a PHD in hydrodynamics and spent most of my career researching hydrodynamics and water quality of estuaries. If you want a reasonably concise book on the subject I would suggest Waves, Tides, and Shallow-water Processes which you can generally pick up for 6-15 dollars. I always recommended that students read the book before taking my graduate course in Estuarine Hydrodynamics and Water Quality. If you are particularly interested in the San Francisco estuary I suggest you watch some videos I put together simply to assist people in understanding the complexity of the system
https://californiawaterblog.com/?s=unraveling+the+knot

Bill Fleenor
Double-Wide

That is what I love about this forum, just about any topic, there will be an honest to God, card carrying expert show up, and usually nicely explain why we are all wrong

[jhulmer]

Originally Posted by a64pilot

Now that’s an interesting statement, I’m sure It’s correct, but what effects it? Maybe the size of the tide? Bigger swings more lag?

My understanding is water seeking its on level. The tides put it into motion and after it slackens the water has to find an equilibrium and thus current. Always dynamic.


edit: oops! guess I should have read to the end of the thread. LOL. My over simplification.
cheers,
Jim

[Rasselas36]

What you are questioning occurs daily on the Saint Johns Rivers, Florida. Normally, there is a hour+ lag in the time of slack current versus the high/low tide.