Tides and Currents

[Gcosloy]

Last June we moved to Maine from Mass. Consulting the tide tables I was able to plan well enough for trips from Kittery around Gerrish and Cutts. Anytime I planned a trip anywhere near Portsmouth Harbor however I was usually surprised by the current strength occurring during what I presumed was slack water time. The rule of thumb being that slack coincided with high and low tides. I've now realized from some experience that in and around Portsmouth Harbor this won't work. I recently did a reading of tide tables and tide and current predictions for the area near Seavey Island. For April 9th i.e. today LT is 9:53AM yet current does not become slack until 12:45PM at 2:45PM the current is at flood. Similar discovery for HT: 4:10PM yet slack doesn't occur until 6:50PM with current at max ebb by 9PM.

This almost seems like Progressive Wave current as opposed to the Standing Wave current I had previously grown accustomed to. see the link for the relationships: http://tidesandcurrents.noaa.gov/faq4.html#15

Or does the Great Bay emptying and filling up in Dover have more or less to do with this phenomenon?

[jason]

Last June we moved to Maine from Mass. Consulting the tide tables I was able to plan well enough for trips from Kittery around Gerrish and Cutts. Anytime I planned a trip anywhere near Portsmouth Harbor however I was usually surprised by the current strength occurring during what I presumed was slack water time. The rule of thumb being that slack coincided with high and low tides. I've now realized from some experience that in and around Portsmouth Harbor this won't work. I recently did a reading of tide tables and tide and current predictions for the area near Seavey Island. For April 9th i.e. today LT is 9:53AM yet current does not become slack until 12:45PM at 2:45PM the current is at flood. Similar discovery for HT: 4:10PM yet slack doesn't occur until 6:50PM with current at max ebb by 9PM.

This almost seems like Progressive Wave current as opposed to the Standing Wave current I had previously grown accustomed to. see the link for the relationships: http://tidesandcurre...ov/faq4.html#15

Or does the Great Bay emptying and filling up in Dover have more or less to do with this phenomenon?

You should start looking at current tables and attend Scott's class:

Currents:

http://tidesandcurrents.noaa.gov/currents11/tab2ac1.html#7

And the class:

http://www.nspn.org/forum/index.php?showtopic=7793

[EEL]

I was usually surprised by the current strength occurring during what I presumed was slack water time. The rule of thumb being that slack coincided with high and low tides. I've now realized from some experience that in and around Portsmouth Harbor this won't work.

To say tidal currents around Portsmouth Harbor are strong, complex and variable is an understatement. "Slack" can be a constantly changing 50M patch of confused water with a strong ebb current at one end and a strong flood current on the other. I second Jason's suggestions. It is no place to go without reviewing the tide current predictions for several locations if you do not have good local knowledge.

Ed Lawson

Who goes there with trepidation

[JohnHuth]

Any time there's a narrow opening into a large embayment, there's usually a lag time between high and low tides and the time of slack water, that's because it takes time to fill up the basin and a 'full basin' won't correspond to the time of high tide at the opening. The timing of the lag is related to the width of the constriction and the size of the basin being filled.

In this case, Great Bay is the culprit.

[Gcosloy]

This quote is taken from the NOAA FAQ button on tides & currents:

"The relationship between the times of high/low tide and the times of slack water or maximum current is not a simple one. There are three "base case" conditions. The first is a "standing wave" type of current. In a standing wave the times of slack water will be nearly the same time as the high and low tides, with the maximum flood and ebb current occurring mid way between the high and low tides. The second is a "progressive wave" current. In a progressive wave, the maximum flood and ebb will occur around the times of the high and low tides, with the slack water occurring between the times of high and low tide. The third case is a "hydraulic current". In a hydraulic current, the current is created by the difference in height of the tides at two locations joined by a waterway. The current will be at its maximum flood or ebb when the difference in the two heights are the greatest. The slack water will occur when the height of the tide at the two locations in nearly the same."

So John, would you say that the differential between the heights of the tides in the Great Bay and the Piscataqua represents what the NOAA labels an example of "hydraulic current"? The lag between high and low tides and slack water down stream in the harbor is so large as to imply that progressive wave current is happening. Perhaps the explanation involves both?

Jason and Ed, my post should have demonstrated to you that I am consulting tide and current charts and your recommendation to do so is unnecessary. It is not a sign of weakness or fault to reveal "surprise" encountering anything unanticipated in the ocean. That's why learning happens on a "curve". Would that all adventures in life be pre-planned and totally anticipated-but then they wouldn't be called "adventures". I've been in the mix of the Piscataqua harbor maybe a dozen times and never have I encountered current that was not manageable if not predictable. Staying away from the major bridges and the current never runs faster in the harbor than what a competent paddler can overcome. Check out the tide and currents tables. There's even low spots and rocks which makes it even more interesting and yes Ed I too approach this area with caution and concern each and every time I go out.

[JohnHuth]

Yes, that matches quite well with the definition of an hydraulic current. It's the difference between the height of water at the constriction and in the embayment that drives the current flow.

There's perhaps one more rare case that is not mentioned in those NOAA definitions, that's the case of a tidal bore, which I'd even hesitate to call a 'current'. When tidal differences become very large in a steadily narrowing estuary (really a transition from an embayment into a river), it can create a solid wall of water. This happens in parts of the Bay of Fundy, the Severn River in England and few other locations.

[Paul Sylvester]

Figure about 2-1/2 hours lag after high tide in Portsmouth for the max ebb.

There are many small play spots on the ebb, more so with a big tide. At times I have seen alot of interesting water on the river: standing waves, boils and eddies everywhere and once saw swells coming in the channel and standing up against the current. I was in a 16' outboard and had my hands full getting out of it.

Beware when you get boat wakes hitting the eddies (or play and smile wide)

Play at the very small race at the mouth of Great Bay (ebb, last 15 mins)

Or the bridge abutments

West of the 95 bridge on the Maine side there is a rock outcropping that goes off rather neat on the ebb (across from power plant)

[scamlin]

To illustrate the comments above, in the trip pre-planning workshop yesterday, we were looking at the NOAA tables for currents and noted that for some locations, the column for slack water had currents listed--meaning there is no slack at all.

In fact, the complex mix of currents is three dimensional: not just ebbs and flows occurring simultaneously in the same channel, but also surface water going one way and deeper water going the other. I imagine it's a combination of different densities due to temperature and (in rivers) salinity, plus momentum as local circulation patterns emerge to allow flows to pass each other rather than collide. The physics of fluids and turbulence is one of the more difficult natural phenomena to model.

Scott

[PeterB]

To illustrate the comments above, in the trip pre-planning workshop yesterday, we were looking at the NOAA tables for currents and noted that for some locations, the column for slack water had currents listed--meaning there is no slack at all.

In fact, the complex mix of currents is three dimensional: not just ebbs and flows occurring simultaneously in the same channel, but also surface water going one way and deeper water going the other. I imagine it's a combination of different densities due to temperature and (in rivers) salinity, plus momentum as local circulation patterns emerge to allow flows to pass each other rather than collide. The physics of fluids and turbulence is one of the more difficult natural phenomena to model.

Scott

to add to the list of useful resources in planning for slack before ebb, slack before flood, max currents etc. If you have an I phone or an Ipod touch:

"Aye Tides" is a good app; (costs $9.99) and it lists many current stations. The ones in bold have a full menu of information: slack before ebb, slack before flood, max current, etc. Fortunately, an area like Portsmouth has many stations to draw from.

[Gcosloy]

I don't think this horse is dead yet! Examining the current predictions for the entrance to Portsmouth Harbor there is approximately a delay of slack water 1-1/2 hours after high tide and 2 hours after low tide. During these lags current exists.Slack by definition means no current. Is the height of the water the same throughout the delay? In other words if HT is 9' of water is there still 9' of water 1-1/2 hours later? My guess is yes but I can't find any tables to confirm this.

[leong]

Last June we moved to Maine from Mass. Consulting the tide tables I was able to plan well enough for trips from Kittery around Gerrish and Cutts. Anytime I planned a trip anywhere near Portsmouth Harbor however I was usually surprised by the current strength occurring during what I presumed was slack water time. The rule of thumb being that slack coincided with high and low tides. I've now realized from some experience that in and around Portsmouth Harbor this won't work. I recently did a reading of tide tables and tide and current predictions for the area near Seavey Island. For April 9th i.e. today LT is 9:53AM yet current does not become slack until 12:45PM at 2:45PM the current is at flood. Similar discovery for HT: 4:10PM yet slack doesn't occur until 6:50PM with current at max ebb by 9PM.

This almost seems like Progressive Wave current as opposed to the Standing Wave current I had previously grown accustomed to. see the link for the relationships: http://tidesandcurrents.noaa.gov/faq4.html#15

Or does the Great Bay emptying and filling up in Dover have more or less to do with this phenomenon?

Granted NOAA tables are available to predict tides and tidal currents at various locations. This is the “what” part of the question. But what about the “why” part of the question? The “why” of the standing wave and hydraulic cases are probably obvious, since everyone knows that water runs down hill. So the standing wave tidal current is proportional to the slope of the tidal height versus time curve. The slope is zero at both high and low tide (thus no standing wave current) and the magnitude of the slope is maximum somewhere between high and low tide (thus maximum standing wave current). But what is the simple physics that explains the progressive wave tidal current? Why is the progressive wave tidal current near a maximum near high and low tide? And why is the progressive wave tidal current near a minimum where the slope of the tide height versus time curve is near a maximum? Somebody please splain this to me. (Dr. Huth?)

[jdkilroy]

A place like Portsmouth will never fall into one of the neat categories that allows easy description or prediction. There are simply too many variables.

Great bay does act as a large reservoir to create a hydraulic dimension but this is not a simple reservoir; Great Bay is constantly being filled by several streams/rivers.

The constant flow from the Piscatiqua River and Great Bay which empties into it, act to oppose/enhance any tidal movement. These effects can vary significantly depending upon recent weather phenomenon (think, a few days of heavy rains) or more long term phenomenon such as a very dry summer.

Add to that the unpredictable variables of tidal movement ....

All this creates a place where the tide/current prediction tables can only hope to provide a guide to expected behaviour. I don't think I've ever seen the tables and reality coincide there.

Its a great back yard though...

Jon

[spider]

Though I seldom paddle Portsmouth area. I seem to remember that both Piscatiqua and Merrimack Rivers were on a list of the 10 strongest current rivers in the country..but I often wonder how true that is.

I avoid all the bridges from route 4 on down. I do however paddle from Adams Point and ride the tides into Great Bay or Exeter...We met the Exeter Harbor Master the other day..I didn't even know we had one there.

Thought I would mention that along with the various terminology for the currents and what not.

There is at times an audible sound as one current slices over the top of a deeper current.

I have only heard it on a few occasions but I bet those that know the water better could explain it better than I can...but it is kind of cool when you hear it.

[JohnHuth]

I was going to draw a diagram of the hydraulic current condition, but then realized that it appears in the literature, so I scanned one of the pages and attach it. It shows how you can get different slack tides when you have a large embayment next to a constricted opening.

For what some others said - yes, the mean height of the water in Great Bay is going to have an effect on the timing. At spring runoff, I would reckon that slack might be delayed further after ebb and pushed earlier after flood.

On the difference between progressive and standing waves. A progressive wave is a traveling wave, where the crest of the high tide moves up an estuary or narrowing embayment and the maximum current coincides with the high tide.

A standing wave happens when water moves in, but reflects off an obstacle, so that there's effectively a rise and fall of the water level, but the "wave" of the tide doesn't really transfer any energy. Another example of a standing wave is called a 'seiche' - which is a standing wave that can occur in a trapped body of water. You often see these in harbors and they have a period of several minutes, which is dictated by the size of the harbor.

Here's an animation of a seiche

http://earthguide.ucsd.edu/earthguide/diagrams/waves/swf/wave_seiche.html

Generally, when you have standing waves, there's a phenomenon called 'resonance', where an embayment has a natural frequency and responds to the 'pushing' of the water from the tides associated with the moon. This is called a 'forcing function'. When the natural frequency of the system coincides with the frequency of the moon passing overhead, it creates the strongest tides. This is why the tides are large in the Bay of Fundy and the Gulf of Maine - they have natural frequencies around 1/13 hours^(-1), while places like Nantucket have natural frequencies that are higher than the moon - something like 1/8 hours^(-1).

The resonance response of embayments is classic for a standing wave, where you expect the highest current to be midway between high and low tide.

On long, narrow estuaries, you tend to get more of a case where there's a progressive wave and the highest current is at high or low tide.

The case of Great Bay and the constriction is such that there's no direct forcing because the constriction is too small, hence no standing wave, but because it's not really long narrow estuary, it's not really a progressive wave either, so the category of an hydraulic wave seems to fit best.

Tides can get rather complicated if the bodies of water are linked together. The flow into Long Island Sound and Nantucket Sound are linked and have a common source around Newport, RI. This is how we get oddities like the flood moving east past Monomoy Island, when you might naively think it would flood west.

[djlewis]

Tides can get rather complicated if the bodies of water are linked together. The flow into Long Island Sound and Nantucket Sound are linked and have a common source around Newport, RI. This is how we get oddities like the flood moving east past Monomoy Island, when you might naively think it would flood west.

My favorite graphic of that one...

http://fvcom.smast.umassd.edu/research_projects/GB/tidal_simulation.html

Click the 2d tidal simulation of here...

http://fvcom.smast.umassd.edu/research_projects/GB/Tidal_simulation/m2_curr_el.gif

Or read the whole thread here...

http://www.nspn.org/forum/index.php?showtopic=5172&st=0&p=27557&hl=eddy&fromsearch=1entry27557

--David

[leong]

On the difference between progressive and standing waves. A progressive wave is a traveling wave, where the crest of the high tide moves up an estuary or narrowing embayment and the maximum current coincides with the high tide.

Is it correct to assume that the progressive wave type of tidal current moves at about the group velocity of the progressive wave, which is probably much smaller than the phase velocity

[JohnHuth]

The distinction between phase velocity and group velocity only has meaning if you have a 'wave packet' - that is to say, the wave is the result of the addition of different wavelengths.

I'd think that a progressive wave would be dominated by the wavelength of the tide, but depending on the width of the opening, I suppose shorter wavelengths could be created. In any case, the real power delivered comes from the group velocity, not phase velocity.