NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Fw: A little off topic _ Ebb and Flow
From: George Huxtable
Date: 2006 Jul 3, 15:03 -0500
Red wrote-
" Yes, there is a relationship for every specific point on the earth,
but no, there is no general relationship you can apply universally.
To oversimplify: Once the "basin" has filled with water (high tide has
come all the way in) there will be no current as the water stands
still for a short time. Then, it will run out of the basin (bay,
inlet, whatever) and there will be a current. The speed of that
current and the timing of it will depend on the local underwater
geography, which largely controls how fast that water can escape
wherever it is."
True(ish), but as Red says, an oversimplification. Yes, with a basin
or harbour being filled by a narrow channel from a tidal sea, the
current through that channel will stop when the level in that harbour
is at its highest, which is at high water (in the harbour). But not
necessarily when the sea outside is at its highest, because by that
time, the harbour may not yet have filled, and may never reach the
level of the sea outside.
Guy states that he's an engineer, and presuming he has done some
electrical engineering, there's a simple analogy which he might find
helpful. Think of a capacitor, representing the volume of the harbour,
connected across the terminals of a sine-wave generator, but with a
resistor in series; the resistor representing the restriction to the
flow presented by the entrance channel. If that resistance is
negligibly small, then the voltage swing (= level change) across the
capacitor is identical with that across the generator, and there is
negligible phase shift between the sea and the harbour. And then,
there's a 90-degree phase shift between the current through the
resistor and the voltage across the capacitor; that is zero current at
maximum voltage. But if the resistance to flow is increased, the
voltage across the capacitor lags, and the amplitude decreases. It's
possible to take the analogy further; if the energy involved in the
momentum of that water flow in the channel becomes important, then
that corresponds to adding an inductive component to the resistance,
and resonant buildup of amplitude can occur. Taking it further still,
the propagation velocity of such waves in the channel can be taken
into account, and the resonant effects of standing waves can be
allowed for. Unfortunately, life is rarely simple enough to allow for
real values of those components to be estimated and put into an
equation, but I have found, from my own familiarity with electrical
stuff, that the analogy can provide a useful mental picture.
Red adds ...
"the time difference will vary with the height of the tides, the
prevailing winds and fetch (pushing more water) and gravitational
tidal pulls from the moon, etc."
Well, maybe a bit, but not much, I would say (other than in
exceptional conditions).
"So if you need a universal answer, you get a list of tide/current
differences for tide stations, or software that has the lists built
in, or you stand at the location and write it down as observed. I
suppose you could generalize and say the maximum current MIGHT be
halfway between the tides, but I've never heard of any firm rule to
calculate it, versus measuring it and listing it."
In the circumstance of a small harbour, filled through a channel that
presents little obstacle to its filling, so that the tidal range
within is equal to the range outside, one can say with some confidence
that maximum current through that channel will be halfway between the
tides; but not otherwise.
None of this deals with the question of the time-relation between
current flow along a coast, and height of tide at a point on that
coast. In general, there will be a reasonably constant time-shift
between them, but not something you can predict from first-principles.
Lu Abel's contribution is very much to-the-point.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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From: George Huxtable
Date: 2006 Jul 3, 15:03 -0500
Red wrote-
" Yes, there is a relationship for every specific point on the earth,
but no, there is no general relationship you can apply universally.
To oversimplify: Once the "basin" has filled with water (high tide has
come all the way in) there will be no current as the water stands
still for a short time. Then, it will run out of the basin (bay,
inlet, whatever) and there will be a current. The speed of that
current and the timing of it will depend on the local underwater
geography, which largely controls how fast that water can escape
wherever it is."
True(ish), but as Red says, an oversimplification. Yes, with a basin
or harbour being filled by a narrow channel from a tidal sea, the
current through that channel will stop when the level in that harbour
is at its highest, which is at high water (in the harbour). But not
necessarily when the sea outside is at its highest, because by that
time, the harbour may not yet have filled, and may never reach the
level of the sea outside.
Guy states that he's an engineer, and presuming he has done some
electrical engineering, there's a simple analogy which he might find
helpful. Think of a capacitor, representing the volume of the harbour,
connected across the terminals of a sine-wave generator, but with a
resistor in series; the resistor representing the restriction to the
flow presented by the entrance channel. If that resistance is
negligibly small, then the voltage swing (= level change) across the
capacitor is identical with that across the generator, and there is
negligible phase shift between the sea and the harbour. And then,
there's a 90-degree phase shift between the current through the
resistor and the voltage across the capacitor; that is zero current at
maximum voltage. But if the resistance to flow is increased, the
voltage across the capacitor lags, and the amplitude decreases. It's
possible to take the analogy further; if the energy involved in the
momentum of that water flow in the channel becomes important, then
that corresponds to adding an inductive component to the resistance,
and resonant buildup of amplitude can occur. Taking it further still,
the propagation velocity of such waves in the channel can be taken
into account, and the resonant effects of standing waves can be
allowed for. Unfortunately, life is rarely simple enough to allow for
real values of those components to be estimated and put into an
equation, but I have found, from my own familiarity with electrical
stuff, that the analogy can provide a useful mental picture.
Red adds ...
"the time difference will vary with the height of the tides, the
prevailing winds and fetch (pushing more water) and gravitational
tidal pulls from the moon, etc."
Well, maybe a bit, but not much, I would say (other than in
exceptional conditions).
"So if you need a universal answer, you get a list of tide/current
differences for tide stations, or software that has the lists built
in, or you stand at the location and write it down as observed. I
suppose you could generalize and say the maximum current MIGHT be
halfway between the tides, but I've never heard of any firm rule to
calculate it, versus measuring it and listing it."
In the circumstance of a small harbour, filled through a channel that
presents little obstacle to its filling, so that the tidal range
within is equal to the range outside, one can say with some confidence
that maximum current through that channel will be halfway between the
tides; but not otherwise.
None of this deals with the question of the time-relation between
current flow along a coast, and height of tide at a point on that
coast. In general, there will be a reasonably constant time-shift
between them, but not something you can predict from first-principles.
Lu Abel's contribution is very much to-the-point.
George.
contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
