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    Re: New Moon, Perigee, and Solstice
    From: Trevor Kenchington
    Date: 2003 Dec 26, 11:15 +0000

    Thank you for your explanations. I am still trying to get my mind fully
    around them but they are enlightening.
    I'll only pick up on a few of your points:
    > In all these divisions, the general form of the differential equation
    > would be the same. The number of terms and the numerical coefficients
    > would be different. In each of them the forcing function is a boundary
    > to a larger world. It is not something special, and cannot really be
    > called a "cause" except in a local sense.
    In the case of any one local area, I can see that. However, the tides of
    the World Ocean are all interconnected and, seen as a whole, their
    forcing function is gravitational, not water movements in some adjacent
    basin. If non-zero "ages" of tides are widespread (albeit not
    universal), then I would have expected some "cause" rooted in the energy
    from the lunar and solar gravitational attraction. That seems, to me, to
    be fundamentally different from the tides of the Minas Basin being
    driven by the tides immediately outside Cape Split. Mathematically, it
    may not be different but physically it seems so -- at least to me.
    > A practical interface for this question would be tide tables for a
    > mid-Pacific Island, such as Canton and Enderbury. That should give a
    > good handle on the phase of the tidal bulge as it would be in a
    > uniformly water-covered planet. I don't have such a tide table.
    Neither do I but I don't think it would show what you expect. The tides
    of the mid-Pacific are dominated by amphidromic systems just as much as
    those of the North Sea are. They do not resemble a "tidal bulge" on a
    planet that lacked land masses. (I'm not sure that a planet without land
    would have recognizable bulges anyway, unless the ocean was also
    extremely deep and covering a very small solid core.)
    >>On the effects of a tidal barrage at the head of Fundy: I'm not sure
    >>that anyone really knows what it would do to amplitudes elsewhere.
    > News accounts of the proposal stated that it was known (by whom??) that
    > the mill would tune toward resonance. If you are engaged in active
    > Fundy research you can probaly find the relevant papers faster than I
    > could, if you are so inclined.
    Thanks. But I'm not that interested in 30-year old tidal modelling. It
    was Frank who claimed that it was quite precise. I only doubt that
    because similar, contemporary estimates for the Severn were not precise
    at all (though I think they all agreed that that system would also be
    moved closer to resonance). If somehow the Fundy case allowed for much
    more precise predications, maybe Frank can point to the evidence.
    {Now the British are building (or just contemplating?) an underwater
    tidal mill to extract energy from the Bristol Channel tides without
    constructing a barrage. Presumably, that will fractionally weaken the
    tides, through the energy removed.}
    >>say that the marine geologists here have recently found active erosion
    >>of the seabed in the Bay. One possible explanation is that the system is
    >>gradually getting nearer to perfect resonance with the semi-diurnal
    >>lunar tide (the likely cause being isostatic rebound, which is lifting
    >>the New Brunswick side of the bay, while dipping the Atlantic coast
    >>of Nova Scotia into the sea -- though the blockage of various tidal
    >>rivers by road causeways might be contributing). The other possible
    >>explanation is that the scallop draggers and groundfish trawlers are
    >>breaking stony lag deposits on the bottom and exposing the underlying
    >>glacial deposits to erosion by tides no stronger than they have been in
    >>past centuries. I guess both could be simultaneously true.
    > That is startling news to me. By "active erosion" do you mean
    > noticeable depth changes year to year?
    I have never asked the sedimentologists to show me just what they have
    seen. However, I think it came from multibeam acoustic surveys, which
    give a high-precision 3-D view of the seafloor. I think that imagery
    revealed erosional features, just as an aerial photograph of the land
    can show water-eroded gullies and the like.
    >>This week, I am trying to make sense of a data set which may provide a
    >>bit more information on the effects of the fishing gear (though on the
    >>seabed biota rather than the seabed itself). That won't answer any of
    >>the above questions but may warn off anyone who wants to ask about gear
    >>impacts -- I am liable to drown any inquiry in a torrent of unwanted,
    >>and off-topic, information!
    > I would be happy to receive your torrent off list, but only for
    > curiosity, not scholarly interest.
    After spending much of Christmas Eve and even a portion of yesterday on
    it, I really don't want to type any more on that topic! Inquiries I will
    answer, of course, but I'm not volunteering more labour. (I do have a
    1,000-item bibliography on the topic on my web site, if anyone _really_
    wants some holiday reading.)
    Happy New Year to you and all members of Nav-L !!
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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