Re: [Paddlewise] Tides & Currents

---Dave Kruger <dkruger_at_pacifier.com> wrote:
> > ... That is, maximum forward speed (maximum flood)
> > would coincide with high water (as the wave crest passes), max ebb
> > would occur at low water (wave trough), and slacks would occur
halfway
> > between. This model conforms to the two observations you made
(below):
> > slacks are (ideally) 3 hours out of phase with high/low water, and
> > currents are the derivative of the heights (since sine and cosine
are
> > derivatives of each other and 90 degrees out of phase).
> 
> Richard, there is an inconsistency in the above.  I
> think if  "currents are the derivative of the heights"  then max
current
> should occur mid-tide, not at max-tide (and min-tide), which is what
> "maximum forward speed (maximum flood) would coincide with high water"
> suggests.  I believe you meant to say both tides and currents were
> *in-phase* for this model.  Correct?

I stand corrected before God and Paddlewise. Forgive the hurried-over
math. Max speed at max elevation would indeed be in-phase. (Corrected
by a chemist--oh, the shame of it!)
 
> What we have here corresponds more closely to the "standing wave"
model
> because in model 2), max-tide (and min-tide)
> both correspond roughly to *slack* (attributing the hour or so
offset to
> the effects of superimposed downriver current).  This is the model
which
> has tide height and current velocity 90 degrees out of phase, I
believe.

Right again. I would have to analyze the "offset" to confirm whether
it results from river current. At the risk of getting this mixed up
again in translating my island mentality to a river mentality, the
bias toward ebb generated by the river flow should, over the long
term, make the flood phase of the cycle shorter than the average 6 hrs
25 minutes and the ebb phase longer than that.

As I visualize it, analyzing the relationship of currents to height
would be affected by your position along the river, since there would
be a salt water "front" that would propagate upstream during the flood
and downstream during the ebb. (The leading edge of the front would be
along the bottom and the surface would lag, but it is the surface we
are interested in as paddlers, yes?) Upstream of the front, the
current should always be ebbing even as the water level is rising (at
decreasing speed closer to the front as the front forges upstream
during flood). Downstream of the front, the current & height should be
correlated more as we have discussed. Thus the tidal pattern at a
given site would change as the front swept back and forth past it. So
it would seem to be a fairly complex situation. Perhaps the situation
is simplest in Astoria, close to the mouth.

Maybe some more insights will come to me in the hot tub tonight ;-).
And maybe you have some ground-truth observations to share. I don't
know offhand the river distances over which the tidal front ranges in
the Columbia, but if I recall correctly, tidal effects are felt as far
upstream as Portland.

> The piece of the dialog which really illuminated the difference
between
> what we have here and what happens in the San Juans was this:
> 
> > Currents in that area are considered to be partly hydraulic, that
is,
> > driven by differences in sea level from one side of the channel to
the
> > other. These differences result, for example, when incoming tides
> > "pile up" water on the upstream sides of the islands faster than the
> > water can drain downstream through the channel. Then, when the water
> > level starts falling after high tide, water level is still higher on
> > the upstream side than on the downstream side. Thus the flood
> > continues (even after high water has passed) until the level is
equal
> > at the two ends of the channel, resulting in a slack. Then the
situation
> > [reverses].
> 
> I only see this in minor side-channels of the Columbia which are
separated
> from the main River by a very narrow opening.  The main stem
Columbia down
> here never functions like this -- and now (thanks to you) I understand
> why:  the mouth of the River is too large relative to the vertical
> excursion in tide (and consequent horizontal exchanges of water mass).

Excuse me, I need to get back to work. Isn't this more fun than
metallurgy?

Richard in Seattle


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