Slack tide and Slack Current

[Pete7]

Originally Posted by conachair

Too complicated for me In the English channel the water comes from both directions so it can be slack as both cancel each other out but still rising.

In addition we have noted on many occasions whilst diving, slack water can vary with with weather often 20 minutes each way. Strong Westerlies will blow water up the channel which being funnel shaped increases the further East you go. The air pressure again effects the local conditions with higher or lower tides so different volume of water moving and variations to high water or slack times.

[Montanan]

https://en.wikipedia.org/wiki/Slack_water

[Macblaze]

This is a fact of life here in the PNW. If you are making a journey involving any of the numerous tidal passages you always look at the current guides — high and low tide can wait until you are actually at the anchorage

If you have the North American Navionics it is fun to play with it around Dent Rapids or Surge Narrows and compare the tides and current flows. The math is completely beyond me and my response is usually "Ooooh neat!"

[Hudson Force]

Have you ever sat in a long line of traffic at a stop light and noticed that sometimes you're moving forward when the light ahead is red and you and your neighbors are stopped when the light ahead is green. 'seems pretty normal to me!

[Macblaze]

Originally Posted by Hudson Force

Have you ever sat in a long line of traffic at a stop light and noticed that sometimes you're moving forward when the light ahead is red and you and your neighbors are stopped when the light ahead is green. 'seems pretty normal to me!

I don't buy that explanation (although I assume there is some truth in it). Slack at Arran Narrows is 7:01 and 1:16 today. High tide at Turnback Point is 7:10 am and low tide is 2:41. They are less than .5 nm away from each other.

Given the convoluted nature of this area I can see how Steamgoat's explanation is likely the cause but its way more complicated in real life.

[MicHughV]

The J'ville inlet is a classic case.

The St. Johns River is always trying to run out, even when the tide is coming in.
This is usually clearly seen as the river water will be a murky brown and the ocean water green. This separation of color is noticeable several miles from the inlet and can be particularly pronounced where the ICW crosses the St. Johns.

Low tide does not mean that the St. Johns River stops flowing. The St. Johns river is always trying to flow out. It's only as the tide significantly rises does it push the river water back.

Low tide at the Mayport jetties does not mean slack water as the St. Johns River will continue to run out for for some time.

I have fished these jetties on numerous occasion. It's always been an interesting experience to time low tide with slack water.

Any tide table will clearly show the stages and times of the tide along the length of the St. Johns River.

The J'ville Bar pilots are particularly knowledgeable about this phenomena.

I'm sure other river mouths are the same...Savannah, Brunswick, etc...

[ferrailleur]

SAINT MALO (France) on the English channel....
I spent time dinghy sailing there.... it's rough but beautiful!

BBC - Travel - Danger: Saint-Malo and the highest tides in Europe

[Boatyarddog]

Originally Posted by a64pilot

What does slack current not occur until a couple of hours after tide reversal? I observe that but it doesn’t make sense me, it would seem that current and tide should occur at the exact same times, yet current lags tide by about two hours, why?

Is it just inertia?

Lots of tidal exchange here in the PNW.
I believe your correct about Inertia.
Although, other conditions affect it, baro pressure, wind, lunar cycles, bottom topography, seasonal cycles.
Here, our exchanges can vary 20 ft. Minus 4 to plus 16.
Huge, getting on a tidal river is like a rollercoaster in those conditions.
SV Cloud Duster

[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:

[Macblaze]

Originally Posted by evm1024

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.

Best/simplest explanation I have read to help wrap your brain around it.

[Hudson Force]

Originally Posted by Macblaze

I don't buy that explanation (although I assume there is some truth in it)...............

'intended more as a simple analogy than an explanation. There are other factors in play that don't have a role in the roadway traffic analogy. The width is not fixed and the water molecules are not staying in a lane. In addition, as pointed out by McHughV there is the flow of rivers. also the effect of neighboring lagoons and inlets and the funneling effects of capes and bays. There's enough variety to require a different time table for each location and not one simple rule to apply for all.

[SV__Grace]

Living on the water is fascinating and trying to figure out the behavior of water is mind bending and multi-dimensional, being influenced by the moon cycle, land shapes, depths, etc.

I purchased the book How to Read Water to see if I can learn the secret mysteries and have only made it through a couple of chapters so far.

I don't pretend to fully understand tides and currents, but it helps me to imagine the momentum/energy of water movement. Watching waves on the beach you can clearly see the energy and speed of the water at different times, such as an incoming wave (tide) picking up speed, crashing into land, slowing down towards the end, then receding slowly and picking up speed as it goes back out.

I'm getting used to the behavior tides and currents of the PNW, so different than SF Bay where I came from.

DeepZoom has become my go-to resources for trip planning around tides and currents and I added a bookmark to my smart phone so I can check it any time. I no longer buy tide tables, pilot books or current tables when all that info is easily accessed via internet.

The hardest thing for me to figure out is the behavior of the current in a bay or cove. You have the energy of the flood tide pushing the water towards shore, then the energy of the water bouncing back from shore, and the variations of land shapes that create eddies and such. I would love to understand all this an be able to find a spot to anchor that has more stable motion, like we're able to sail or motor using eddies in different directions (e.g. local knowledge about Tacoma Narrows is that the east side always flows north and the west side always flows south).

Is it possible for a boater to fully understand all of this?

[MicHughV]

The company I used to work for had similar computer programs. We had several Ph.D's doing nothing but computer modeling of numerous inlets and water bodies. Every once in a while they would present a show 'n tell to explain what they were doing with the above graphic presentations. It was fascinating stuff and I learned a lot.
But I also fished the Mayport inlet and could visually see when slack tide occurred and when the river flow got pushed back. Wind direction often played a major part in this phenomena.

.

[AndyEss]

I like Hudson Force’s traffic analogy. This is of course for the simplest system.

Here’s another really simple analogy - blowing up a rubber balloon. The forcing function (tide) fills a reservoir (some sort of simple inlet with no inflowing river(s). You stop blowing up the balloon - and yet there us plenty of flow out of the balloon out of its neck, depending on its size and how much you managed to blow into it (tidal range).

Now complicate that with several little balloons somehow attached to the parent balloon, with different neck sizes, different body sizes - and add several small air supply lines here and there to the Rube Goldberg assembly to mimic rivers flowing into the inlet.

Add erosion/deposition to dynamically vary throat sizes, reservoir capacities, vary the amount of air flow through the additional air supply tubes - - and you have a crude approximation of how complicated a tidal system can become.

[Hudson Force]

Originally Posted by MicHughV

.............
........ Wind direction often played a major part in this phenomena.

The wind is unique among the many other factors because it doesn't fall in line with the consistent phenomena that allow for predictable tide and current tables.