#66089
04/23/2002 1:38 PM
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Joined: Jan 2001
Posts: 1,156
old hand
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old hand
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Posts: 1,156 |
Actually, Faldage, my book lists something like 12 different components of tidal periods. (There are irregularities in planetary orbits and also the effects of the other planets which are small but affect things in the long run.) And I understand that the way they do tide charts these days isn't from modelling, but from harmonic analysis. Rather than trying to explain why the components observed dominate in one particular place, they just observe it and extend the observations into predictions.
Back to the Fraser River example...my supervisor says someone tried to model that particular location but he doesn't think they got as far inland as we were...sometimes there's a lack of data...so I guess to some extent there's not much use in modelling when you can just use harmonic analysis and build the tide table quite easily. The guys who drive the tugboats probably don't care about the why of the tide so much as the when and where of it!
I had some nifty plots of not-very-sinusoidal looking tidal patterns in my book but I'm at home sick again (I made it into school this morning and gave up after a couple of hours) and can't scan them for you.
Edit: I've given it some thought and I do agree with Faldage: the tide you observe from day-to-day seems sinusoidal - but the overall picture, observed over many days, or a month, is not just a plain old sine curve, or even a superposition of just two frequency components, but much more complicated than that.
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#66090
04/23/2002 2:59 PM
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Joined: Nov 2000
Posts: 866
old hand
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old hand
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Posts: 866 |
> a fjord is a sunken river, into which the sea has pushed..
So is a ria.
Close to home for you Hev - the Hawkesbury/Nepean system ocupies a ria - a drowned river valley.
stales
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#66091
04/24/2002 6:19 AM
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Joined: Apr 2000
Posts: 3,065
Carpal Tunnel
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Carpal Tunnel
Joined: Apr 2000
Posts: 3,065 |
In reply to:
Tornadoes and other such things (like ocean circulation) rotate one way in the NH (= oceanographer-ese for Northenr Hemisphere) and t'other way in the SH. Easy weather application: Wind blows counterclockwsie around a low pressure system in the NH
Umm, what happens for us more-or-less equatorial types?
Bingley
Bingley
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#66092
04/24/2002 2:06 PM
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Joined: Jan 2001
Posts: 1,156
old hand
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old hand
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Posts: 1,156 |
what happens for us more-or-less equatorial types?
Well, the strength of the Coriolis effect depends on latitude, starting with zero Coriolis effect at the equator and reaching its maximum at the poles. Probably, in terms of large features like atmospheric pressure systems, this means the circular shape isn't very circular when the feature is near the equator. (This is party the reason for the doldrums near the equator, I believe.) As for tornadoes - I really don't know. I don't think they're able to form as easily at the equator since they are offspring of a pressure system. And they also don't travel very far (in global terms). I will see if I can find more info and PM you with it.
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#66093
04/25/2002 8:55 AM
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Joined: Apr 2000
Posts: 3,065
Carpal Tunnel
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Carpal Tunnel
Joined: Apr 2000
Posts: 3,065 |
I have noticed that Java doesn't seem to go in for wind much. A good stiff breeze before a major rainstorm but that's about it. Having said that, we did get quite a gale a couple of weeks ago with branches coming off trees and so on, but it's the first one I remember in coming up to 15 years here.
Bingley
Bingley
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#66094
04/25/2002 2:15 PM
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Joined: Aug 2001
Posts: 2,605
Carpal Tunnel
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Carpal Tunnel
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Posts: 2,605 |
Today's Chicago Tribune newspaper, on how are city fathers inflated the amount of parkland they supposedly will create under a controversial project:
When City of Chicago officials heralded the 19 acres of parkland to be creating ..., they were craftily including landscaped medians, sloped berms next to a garage and any patch of grass they could find. "A berm can have plants on it, and isn't that part of a park?" one of the project's architects reasoned.
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#66095
04/25/2002 2:22 PM
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Joined: Nov 2000
Posts: 3,439
Carpal Tunnel
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Carpal Tunnel
Joined: Nov 2000
Posts: 3,439 |
"A berm can have plants on it, and isn't that part of a park?" one of the project's architects reasoned
This is the same mind-set that decided catsup is a vegetable when served at a school lunchroom!
Sheeeeesh!
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#66096
04/25/2002 3:23 PM
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Joined: Mar 2000
Posts: 6,511
Carpal Tunnel
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Carpal Tunnel
Joined: Mar 2000
Posts: 6,511 |
... are what I get during allergy season.
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#66097
04/25/2002 3:42 PM
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Joined: Dec 2000
Posts: 144
member
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Posts: 144 |
Yeahbut(r) that still doesn't explain the Poole Harbour phenomenen.
As I understand it, semi-diurnal means that you get the high high water, low high water, high low water, and low low water across the course of the tidal day (ie 24 hours ish). In Poole Harbour you still get the twice daily high tide eg at 7.00am and 7.00pm (approximate), but in each 12 hour period you will have 1st high, 1st low, 2nd high, 2nd low. The 2nd is high is your main high water. Basically, it comes in a bit, goes out a bit, comes in all the way and then goes out all the way, completely throwing out any rule of twelves! So, what would that be? Or is it just that all the curves and technicalities are getting me confused?
As far as I'm concerned, as long as I know what times they are, where and when the flow is strongest and which direction they're going in relative to my course so that I can make sure that I use them to maximum effect, I'm happy!
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#66098
04/25/2002 3:49 PM
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Joined: Dec 2000
Posts: 13,803
Carpal Tunnel
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Carpal Tunnel
Joined: Dec 2000
Posts: 13,803 |
the Poole Harbour phenomenon
If you dump a sinusoidal driving force into a chamber that has a natural resonance twice the frequency of the driving force it will oscillate at the higher frequency. Perhaps it is something like this that is happening. Where is this Poole Harbour, how big is it and how open?
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#66099
04/25/2002 3:57 PM
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Joined: Dec 2000
Posts: 144
member
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Posts: 144 |
Poole Harbour:
http://makeashorterlink.com/?E1EA321C
It's a large natural harbour with a very small entrance. I seem to remember back in the mists of time that it's something like the 2nd or 3rd biggest natural harbour in the world.
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#66100
04/25/2002 4:10 PM
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Joined: Dec 2000
Posts: 13,803
Carpal Tunnel
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Carpal Tunnel
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Posts: 13,803 |
Love the map site. On second thought, I'd think that this was a pretty small body of water to have a natural frequency of twice a day. Bean? You're the most likely to have anything intelligent to say on this. Any ideas?
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#66101
04/25/2002 4:40 PM
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Joined: Mar 2002
Posts: 1,692
Pooh-Bah
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Pooh-Bah
Joined: Mar 2002
Posts: 1,692 |
Only just looked at this thread, golly gosh it is interesting! I am not a sailor or an expert, but I know that the double tide effect operates in the Solent and in Southampton Water. If you zoom out on the map of Poole Harbour you will see where these features are. I have understood that this occurs because as the water rises for high tide in the English Channel it does so first at the western end of the Channel and then gets progressively later as you move eastward up the Channel, so you get a high tide pushing into the Solent from the west as the tidal effect passes the western arm and then a second body of water from the east as it passes the eastern arm. If this is truly the cause of the double tide, however, I am surprised that it impacts on Poole harbour, and there may some other reason. When I was a very junior Civil Engineer, more years ago than...., I worked on a project to extend the capability of Poole sewage works. Up to that point they were discharging macerated sewage out to sea. If there were double tides.... well it doesn't bear thinking about really does it...  dxb
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#66102
04/25/2002 4:53 PM
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Joined: Dec 2000
Posts: 13,803
Carpal Tunnel
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Carpal Tunnel
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it does so first at the western end of the Channel and then gets progressively later as you move eastward
Now I'm confused. I know, I know. I'm often confused.
Shouldn't the tide move from east to west?
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#66103
04/25/2002 5:09 PM
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Joined: Mar 2002
Posts: 1,692
Pooh-Bah
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Pooh-Bah
Joined: Mar 2002
Posts: 1,692 |
Dear Faldage,
Well....yeah, I guess you're right, I didn't make a considered choice of direction there, I subconsiously assumed that because the Channel, the North Sea and the Baltic are just a little adjunct off the Atlantic, they would be fed from the European Atlantic coast tide. I am happy to accept that the effect works east to west, but the double tide is still the result - if the reason I was given is correct. I will try and check on which way the tide moves in the Channel - as said I am not an expert, or even knowledgable, on the theory!
dxb
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#66104
04/25/2002 5:29 PM
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Joined: Dec 2000
Posts: 13,803
Carpal Tunnel
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Carpal Tunnel
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Posts: 13,803 |
they would be fed from the European Atlantic coast tide
That's an interesting point. I hadn't thought of that. We already know that local effects can have huge overriding consequences. All this stuff is very impotant to me in my long term goal of unsuccessfully understanding the universe.
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#66105
04/25/2002 5:33 PM
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Joined: Aug 2001
Posts: 2,605
Carpal Tunnel
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Carpal Tunnel
Joined: Aug 2001
Posts: 2,605 |
my long term goal of unsuccessfully understanding the universe
A most optimitic goal. It's difficult enough to meet the goal of unsuccessfully understanding the universe.
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#66106
04/26/2002 4:38 PM
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Joined: Jan 2001
Posts: 1,156
old hand
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old hand
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Posts: 1,156 |
OK, sorry to take a while to reply. Some thoughts on tides and resonance: Yes, harbours that are enclosed can have a resonant frequency, and I'm sure that can modify the tidal dynamics. There's a formula to work that out, what I need is the length and width of the harbour. It might be tricky because it's a really funny shape which doesn't even approximate a rectangle. Give me some time on that one. (I don't feel like solving the differential equations again when I know the info is in a book on my desk at the university!) Regarding direction of the tide: Tides are actually very long-wavelength waves known as Kelvin waves. They travel with the coast on their right in the NH (which means on the left in the SH). Looks like this one should be travelling East to West. The best I can do on short notice for St. John's Harbour (which is practically rectangular and I just marked an assignment where the students figured out the period of one of the modes of vibration), much more so than Poole Harbour) is http://makeashorterlink.com/?H2F7213C, if anyone's interested. Here's a much better map, courtesy of the Port Authority: http://www.sjpa.com/images/Facilities/HarMap1.jpg
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#66107
04/26/2002 7:27 PM
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Joined: Oct 2000
Posts: 5,400
Carpal Tunnel
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Carpal Tunnel
Joined: Oct 2000
Posts: 5,400 |
Speaking of harbors.. here is a boat trip round NY upper harbor.. the slide show is about 350 slides, it covers about 10 hours.. of a commercial tourist attraction ride around Manhattan island. the url is to a home page, with a live web cam, or click on link to take the virtual tour.http://www.earthcam.com/usa/newyork/circleline/
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#66108
04/26/2002 10:56 PM
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Joined: Sep 2001
Posts: 6,296
Carpal Tunnel
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Carpal Tunnel
Joined: Sep 2001
Posts: 6,296 |
Well, this thread is ready to go on out to sea, and I've found a lovely ocean word I don't believe we have here:
spendrift
It's defined in Random House Word Menu:
spray swept by wind from waves during storm at sea...
and then an equally lovely term is given as a synonym:
spoondrift
...I can go to sleep on those two words, stormy though they may be. What do you call a word that sounds as though its one thing (e.g., spendrift and spoondrift sound laidback and easy), but is actually quite something else? Spoondrift just doesn't sound stormy at all to my ear.
Beach regards,
WordWave
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#66109
04/29/2002 3:25 PM
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Joined: Mar 2002
Posts: 1,692
Pooh-Bah
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Pooh-Bah
Joined: Mar 2002
Posts: 1,692 |
Dear faldage, High Tides along the south coast of the UK:This is all strictly non-technical and whether it will assist you in universal understanding I hesitate to predict! I don't think it helped me much - knowledge is one thing, understanding another. What I am told and what I read do not seem to agree. An amateur yachtsman has assured me, I think mistakenly, that the high tide along the south coast moves from west to east. The Atlantic high tide he tells me swells back around both sides of Ireland and around the north of Scotland, moving up the channel and down the North Sea (would this form a sort of mælstrom where they meet, I wonder?). When I look at tide tables for south coast ports, however, my interpretation is that the tide moves east to west along the Channel coast. I think I prefer to trust the tide tables and to avoid cruising on my friend’s yacht! It seems from the tables that there is a high tide approximately every 12 hours 20 minutes. The double tide effect at Poole Harbour causes a secondary peak that occurs about 4 hours after the main high tide. I don't know why it occurs, perhaps Bean can explain; it is generally lower than the main peak, but there appears to be variation in this, I guess depending on where you are in the spring/neap tide cycle. A similar effect is seen at Southampton due I believe to the Isle of Wight and the two arms of the Solent, but there the two peaks are mostly about the same height with a drop of only a few inches between them, so this has the effect of lengthening the duration of high water – obviously very desirable for a port. Again this effect seems more or less marked depending, I guess, on the spring/neap cycle. Where I have given conclusions rather than facts above they are based on my deductions arrived at from looking at the tide tables so could well be completely erroneous! Two interesting links: http://www.ukho.gov.uk/easytide.htmlhttp://www.alia.ie/sailing/tides.htmldxb
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#66110
04/29/2002 4:32 PM
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Joined: Jan 2001
Posts: 1,156
old hand
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This has been very interesting indeed! From a glance at the dimensions of Poole Harbour, I would at first guess that the situation is much, much more complicated than first appears. I Googled "double tides" and got this great link for a place in Scotland, which at least states that it's due to the what we call the bottom topography (which is what I suspected all along): http://www.rampantscotland.com/know/blknow31.htmI think to get a good, definitive answer to your question may take some hunting in research papers. Perhaps someone's done a model of the water level and currents in Poole Harbour for a thesis or something. Indeed, from the bit I've read in my book (an introductory oceanography book with merely a single chapter on tides) it would be quite complicated! And from looking at the map, with two sets of narrows and Brownsea island in the middle - I'd say it must be quite complicated - no simple rectangular box for this model! Edit: Oh! Oh! I've found something great, and not technical! Try http://www.weston.org.uk/tides.htm. (I am reading it right now, haven't finished it yet. Just wanted to post in case you were still on Board!)
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#66111
04/29/2002 5:08 PM
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Joined: Jan 2001
Posts: 1,156
old hand
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old hand
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OK, dxb, here you go. Online, I found a reference to a book:
Author = Officer, Charles B.
Title = Physical Oceanography of Estuaries (and Associated Coastal Waters)
Publisher = John Wiley & Sons
Year = 1976
Pages = 465
ISBN = 0-471-65278-4
Apparently, it has a chapter which has a section dealing with tides at Solent, etc. So there you go! In a free moment maybe you or I will pick it up from a university library and have a gander at it!
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#66112
04/29/2002 9:34 PM
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Joined: Jun 2001
Posts: 2,636
Carpal Tunnel
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Carpal Tunnel
Joined: Jun 2001
Posts: 2,636 |
Hi y'all. I have moved these posts to the Spendrift thread. I'd hate to see such interesting material lost in cyberspace.
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#66113
04/30/2002 4:48 AM
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Joined: Apr 2000
Posts: 3,065
Carpal Tunnel
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Carpal Tunnel
Joined: Apr 2000
Posts: 3,065 |
crossthreading.
Bean: tides at Solent.
I think UK usage would be tides in the Solent.
Bingley
Bingley
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#66114
04/30/2002 10:46 AM
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Joined: Mar 2002
Posts: 1,692
Pooh-Bah
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Pooh-Bah
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Posts: 1,692 |
Bean, thanks for the links and the book ref. In my bookshop browsing I shall keep an eye open for it. Meanwhile there are all the links that have come out of this discussion to explore.
dxb
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#66115
04/30/2002 2:43 PM
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Joined: Jan 2001
Posts: 13,858
Carpal Tunnel
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Carpal Tunnel
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Posts: 13,858 |
I had an unpleasant experience with a "sinusoidal" tide. My brother forgot to mention he had used up all the shear pins that protected outboard motor when propeller got tangled in seaweed. My father and I were half a mile down the channel from the mooring place when some weed caused pin to shear, and we found there was no spare pin. I elected to swim back up the channel to the skiff at our mooring place. When I started, the tide was high, but halfway to my goal, it started to turn, and picked up speed quite rapidly, until I was going back down the channel. I had to swim to channel bank, and walk through horrible ooze filled with sharp broken sheel that cut my bare feet. If I had plotted my progress during the swim it would have made a sort of sinusoidal curve, I now recognize.
One of the most spectacular tides in the world is the one in the Bay of Fundy, which in spring can reach forty feet. Alas the few times I was able to visit the Tidal Bore it was comparatively tame. There is a rather long list of terms used to describe tides, too long, and too easy to forget to be worth posting.
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#66116
05/01/2002 10:52 AM
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Joined: Jan 2001
Posts: 1,156
old hand
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Yes, I know this thread is getting long, but I have one last thing for dxb on this subject. I was in the library today anyway and went to look at that book. It is basically a review of the scientific work in estuaries up to that date (already more than 25 years ago). I read the bit on the Solent, etc., and found it wasn't very helpful. I asked my supervisor what he thought and we looked at a map and worked it out. The Kelvin waves in the NH, as I said, travel with the coast at their right. If the English Channel is about 100 m deep, the the velocity of the Kelvin waves is about 100 km/h. If you look at a map and picture the tide propagating, first along the north coast of France from west to east. Then since the Channel near Dover is so narrow, the Kelvin wave "sees it" as closed altogether, and instead continues along the south coast of England, in an east-west direction. The distance between the closest point near the Isle of Wight and Cherbourg, France, if you measure along the coastline, is such that it takes about three hours for the tide to work its way around there (i. e. ~300 km). So if, say, it is high tide at Poole, the tide is on its way out at Cherbourg, and though they are some distance apart it is not so far that the tide going out at Cherbourg is "unnoticed" by the water at Poole. There are also probably strong topographical effects as well, and what you get is the tide "meeting itself" in the region between the Solent and Poole Harbour. Whew! What an odyssey. If you have any more questions, do PM me (and don't forget to turn your PMs on in that case). (I think others may not be as interested in this discussion as we are!) 
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#66117
05/01/2002 4:10 PM
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Joined: Mar 2002
Posts: 1,692
Pooh-Bah
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Pooh-Bah
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Posts: 1,692 |
Thanks Bean, I shall sit down quietly with a map and a glass (scotch!) and think on that. As you say, any more by PM.
dxb
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