How to tell where the tide will take the current?

[brambor]

When I look at marine maps I am unsure whether it is possible to read which way the current will flow during outgoing or ingoing tide. For example (discounting wind direction) a circumnavigation of an unspecified island as observed on a map ... how will I be able to tell which way to go...clockwise or counterclockwise?

[jtmllr]

That's a good question I'd like the answer to so I tried looking it up on the Internet as well as some books. I didn't find an answer. I don't see it explicitly listed on the charts but I suppose you could make an educated guess based on bottom topography since water will want to flow in the direction of least resistance. If the island is quite on its own I would think the tide would go around it fairly symmetrically and the only situations where it would favor a "side" would be when close to other islands or a mainland. Such as with Cape Ann or MDI and you are talking about the narrows/river sections exhibiting the phenomenon and you need to consider timing for that particular piece of the trip. Perhaps the side with the most open water or least restrictions would likely be the "from" direction? In those particular examples I think I've read mention in books describing paddles for the area, so it would be a local knowledge thing. It might also be in the Coast Pilot if there's a reason to mention it.

In looking this up I noticed a new NOAA resource compared to the last time I went for charts online. Previously I had to find a 3rd party viewer, and the free viewers were few (esp on Linux and OS X) and not particularly feature rich making printing a chore. Now they are in PDF as Booklets http://ocsdata.ncd.noaa.gov/BookletChart/A...okletCharts.htm which should make printing to letter & legal as easy as selecting pages to print. This is the reason I replied as I don't have an answer to the actual question!

[scamlin]

When I look at marine maps I am unsure whether it is possible to read which way the current will flow during outgoing or ingoing tide. For example (discounting wind direction) a circumnavigation of an unspecified island as observed on a map ... how will I be able to tell which way to go...clockwise or counterclockwise?

In my experience, you can make an educated guess, but you can't tell for certain until you get local information. Why? Because the direction, timing and speed of currents depend not only on the shape of the channel at your site--in your case around the island--but also the waters before (downstream) and after (upstream so to speak). In addition the shape and depth of the bottom of the channel and basins before and after the island also influence currents. These affect not only how much water goes in and out, how far and how fast, but also when. You also have to take into account the possibility of a resonance effect, where the "wave" of tide water interacts with the size and shape of tidal basin, sort of like a wave sloshing back and forth in a bathtub. To make it even more interesting, the currents depend also on changes in tide height during the month and over the course of the year, as well as air pressure and any on-shore or off-shore winds. Basically, it can get pretty complicated...but mostly it's not.

Your educated guess depends on a few rules of thumb, most based on estimates of how much water is moving where, which you can make based in a close inspection of your chart:

How big is the channel? How wide and how deep is it? This will give you an idea of how much water is likely to move through it. If there are significant differences in volume between either side of the island, more water is likely to move through the larger volume channel, other things being equal.

What is the shape of the channel? Of the bottom? Water in a wide channel that comes to a narrow section will speed up (the Venturi effect), but the constriction can also back up the flow. Shallow shoals or sand bars in an otherwise deep channel can have the same effect (as well as interesting bumps in the surface). For some channels, the slope of the bottom is a factor as the water runs downhill much like a river. The Western Rivers section of the Maine Coast between Popham Beach and Pemmaquid Point has many examples of these effects.

What's beyond the channel in question? Big or small? Long and narrow or wide and short? How deep? The bigger and deeper the basin above (upstream) of the channel, the faster the current in that direction, the later it starts, and the longer it will continue. Why? Because there is more water going into and out of the basin. For example, the current in Plum Island Sound changes direction about an hour after high tide because the Parker River basin is still filling up long after the water height starts to go down. Same on the ebb except in reverse. In some locations, the current direction can change three or four hours AFTER the tide changes. (If you're talking outside the Gulf of Maine, it can get really interesting with dirunal or semi-diurnal tides.)

What other channels or inlets are nearby? Moving water in channels or inlets nearby can either suck water out of your channel or push water volumes into it, through differences in tide heights (water runs down hill) or pressure (much like a point in a current pulls water out of an eddy).

How can you know for sure?

1. Through predictions based on historical readings at established current stations (available on NOAA website or mapping software). Unfortunately, current stations are relatively few in number and may not be available for your island or even your stretch of the coast. If there is station near your paddling target, you're in luck.

2. Other boating publications. For instance, the old Eldridge tide book has wonderful charts showing current direction (and speeds) for Boston Harbor, Vinyard Sound, etc. Other piloting books such as those for big boats may have such charts as well as textual descriptions. NOAA publishes a Coastal Pilot that has similar information; you can download it for free from the NOAA website. These kinds of publications go beyond what you can learn from the current stations.

3. Local knowledge from other paddlers, boaters, fishermen, marinas, etc. Many are happy to help, but you may not get precise enough information. There is a good chance someone in the club has paddled where you want to go, so you can always post on the message board.

4. Paddle it yourself. Plan your trip to leave a margin for times, speeds and direction of the current to differ from your educated guess. And a backup plan if the margins are not enough. Once you find out what happened, you are then a source of local knowledge, especially if you record not only the currents (direction, speeds and times) but also what the tide heights were that day.

Have fun.

Scott

[EEL]

When I look at marine maps I am unsure whether it is possible to read which way the current will flow during outgoing or ingoing tide.

Just to augment the excellent advice given, a very rough guide for the coast of Maine is that tidal currents flow north and east on flood and south and west on ebb. Of course as Scott shows, there are many exceptions to this, especially where rivers are involved or odd bays. Away from areas often traveled by commercial vessels such as harbors the NOAA tidal current information tends to be sketchy and currents can vary significantly both in velocity and direction over a short difference. For example, check the stations in the Hussey Sound area in Portland.

Here is the NOAA site that provides current predictions which I have found helpful.

http://co-ops.nos.noaa.gov/curr_pred.html

Ed Lawson

[JohnHuth]

Nantucket Sound, Newport RI, and LI sound are fairly tricky as regards tides. The general flow begins around Newport and moves both east and west.

Here's a nice animation of the currents in Nantucket sound based on a computer simulation:

http://fvcom.smast.umassd.edu/research_pro...vies/tide1.html

you can see how complicated the flow is, depending on where you're located.

If you look at tide tables, you'll find that in Boston Harbor, high tide corresponds fairly well to the lunar transit, but at Newport, it's off by 90 degrees of phase.

Flooding of estuaries can be reasonably predictable, but you need to know the tidal lag in a particular location (time before/after the lunar transit).

[NPSheehan]

Here is some interesting information on the subject from Maine Coast Sea Kayaking

http://www.mainecoastguide.com/kayak/kayak...escurrents.html

A well planned trip if you are trying to avoid strong currents would be to abide by the 50/90 Rule stated in the above article which read..

"The “50-90 rule†estimates current speeds between slack water and maximum flow. Divide the time between slack water and maximum ebb or flood into thirds. In the first third, the current will increase to 50% of its maximum. In the second third it will reach 90%. And in the last third, it only increases the last 10%."

The NOAA National Current Observation Program (NCOP) has real time current info for some locations but unfortunately does not plot the Maine Coast locations yet. Pretty cool stuff though. Ed's link in a previous reply gives the predicted currents in many Maine locations and that's probably as close as your gonna get right now.

http://co-ops.nos.noaa.gov/station_retriev...state=&id1=

Neil

[Paul Sylvester]

"The “50-90 rule†estimates current speeds between slack water and maximum flow. Divide the time between slack water and maximum ebb or flood into thirds. In the first third, the current will increase to 50% of its maximum. In the second third it will reach 90%. And in the last third, it only increases the last 10%."

It is better stated that "at the end of the first hour the current will reach 50% of its max"

and "at the end of the second..........key word is ---end --of the hour or third.

[Bmach]

You might want to check out a book called Eldridge Tide and Pilot Book. It shows the currant for the various stages of the tide. It covers the Northeast well and a lot of the East coast.

[Tom Tieman]

In some places, the wind can also change the direction of the current.

The direction of the ebb and flood varies in passages tending east and west. In Fox Island Thorofare, the flood comes in from both directions, meeting at Iron Point, and it also ebbs in both directions. In Eggemoggin Reach, the current floods northwest and ebbs southeast (see Penobscot Bay). In Deer Island Thorofare and Merchant Row, the current floods east and ebbs west, but strong winds can reverse the flow (see Mount Desert Island region). In Casco Passage and Moosabec Reach, the current floods east and ebbs west. At Bass Harbor Bar, the current floods west.

http://www.mainecoastguide.com/intro/tides.html

[JohnHuth]

In some places, the wind can also change the direction of the current.

http://www.mainecoastguide.com/intro/tides.html

A general rule of thumb is that a wind blowing across a long fetch for 10 hours will cause the surface waters to flow at 2% of the wind velocity. In the northern hemisphere, the current created tends to be deflected to the right of the wind direction. For the savants, this is called "Ekman transport"

[EEL]

A general rule of thumb is that a wind blowing across a long fetch for 10 hours will cause the surface waters to flow at 2% of the wind velocity.

What constitutes "surface waters"? Would this mean that while the wind might make it appear the tidal current is reversed, it is only the water close to the surface which is flowing in reverse while at greater depths the tidal current continues to flow normally? Something akin to the different flows in a tidal river at different depths.

Ed Lawson

[JohnHuth]

What constitutes "surface waters"? Would this mean that while the wind might make it appear the tidal current is reversed, it is only the water close to the surface which is flowing in reverse while at greater depths the tidal current continues to flow normally? Something akin to the different flows in a tidal river at different depths.

Ed Lawson

Well, since you asked, the Ekman layer is about 10-20 meters deep. It is doubtful that the wind would be strong enough to reverse a tidal current. For a 40 knot wind blowing over a long fetch, this would generate a current of about 0.8 knots, not enough to reverse a tidal current.

Providing the water was deep enough, it wouldn't reverse a deep tidal current at all, but there might be a gradient. I could see that it could produce some interesting wave patterns.

[EEL]

Well, since you asked, the Ekman layer is about 10-20 meters deep. It is doubtful that the wind would be strong enough to reverse a tidal current.

According to Coastal Pilot 1 the tide current in the Deer Isle Thorofare is wind dependent as in a strong easterly means the flood and ebb run westward while with a strong westerly the both run eastward. Of course what constitutes a strong wind is undefined. The current in the Thorofare N of Russ is varies from .3 to .6Kts and further out near Grog it is even less so its not like the wind needs to alter big velocities on the surface. There is no indication the wind has any effect upon the tidal current in Merchant Row father out, but no current stations in the Row either.

Its all a wonderment. Using lobster buoys to estimate current is a frequent suggestion and a good one, but of course that means you are in it and relegated to coping with whatever you are floating in and not much help planning to avoid the slog. Other than for next time of course which is why logs are nice things to keep.

Ed Lawson

[JohnHuth]

Local ocean topography makes a huge difference!

[EEL]

Local ocean topography makes a huge difference!

For kayakers all weather is local. Or at least I think someone once said something like that during a talk on weather.

Ed Lawson

[rcohn]

Well, since you asked, the Ekman layer is about 10-20 meters deep. It is doubtful that the wind would be strong enough to reverse a tidal current. For a 40 knot wind blowing over a long fetch, this would generate a current of about 0.8 knots, not enough to reverse a tidal current.

I'd need to pull out my copy of Knauss to be sure, but as I recall, the flow is in a sort of spiral with the current direction changing with depth. Currents are all in the horizontal plane with no water transport in the vertical direction.

Ralph

[JohnHuth]

Ekman transport is mainly a phenomenon that's created by large scale wind patterns, not local patterns. It's a combination of the Coriolis Effect, wind shear and gravity, which take place over thousands of miles to produce large scale currents like the Gulf Stream

The Ekman spiral, where horizontal currents in layers of water move in different directions (always to the right in the northern hemisphere and always to the left in the southern) create gyres of current flow.

But, these are large scale phenomena, there's probably a modest effect of some small current being generated by the wind, but the rule of thumb that wind of a given speed will produce a current that is 2% of that speed. That's over a minimum 10 hour fetch.

I should add that near the edge of a continent, one will get vertical transport of water due to this effect - it produced upwelling and downward currents, depending on the hemisphere, and whether the current is moving northward or southward along a coastline.

For grins - attached is a draft of a chapter I'm writing for my students on ocean currents. The last bit contains quotations on using waves as a sign of currents in blue water conditions.

Ocean_Currents.doc