Received: from hpoclrf.sj.hp.com by opus.hpl.hp.com with SMTP (1.37.109.8/15.5+ECS 3.3+HPL1.1) id AA09277; Wed, 14 Dec 1994 07:42:05 -0800 Return-Path: <kirk@hpmsd3.sj.hp.com-DeleteThis> Received: by hpmsd3.sj.hp.com (1.37.109.8/15.5+IOS 3.22) id AA01745; Wed, 14 Dec 1994 07:43:08 -0800 Date: Wed, 14 Dec 1994 07:43:08 -0800 From: Kirk Lindstrom <kirk@hpmsd3.sj.hp.com-DeleteThis> Message-Id: <9412141543.AA01745@hpmsd3.sj.hp.com-DeleteThis> To: poulton@zonker.hpl.hp.com-DeleteThis, wind_talk@opus.hpl.hp.com-DeleteThis Subject: Re: Re: Tide Data
> > Just curious whether anyone actually uses tide data for anything?
Yes. For reasons stated by others.
>
> > On a related note, does anyone know why high/low tide times differ
> > (sometimes substantially) from slack times? I noticed this in my tide
> > book and it didn't make much sense to me. Could it be that there's a lag
> > between when slack occurs and the tide actually hits its highest (or lowest)
> > point (e.g. during a full moon)?
>
> EE's will have no problem with this - the Bay is just a distributed RC
> transmission line. In simpler terms, it takes a while for the tidal
> change at the ocean to propagate up the Bay - this is why there are
> delays from the Gate to other sites (like around 5 hours to Rio). The
> tranmission is not perfect, however, and the result is that
> when the tide is high at the Gate, current is still running into the
> Gate to fill other parts of the Bay. Slack water doesn't happen
> until around 2 hours later when the Gate tide level is coming back down.
>
I think an even better explaination is that it is as Ken says "a
distributed transmission line", but they are modeled by L's and C's.
Without having L's, there would be less of a high tide at Alviso rather that
the 3.3ft above the high tide at the Gate. A "perfect" transmission
line is lossless and thus has no "R" componments in its model.
A very simple model could look like this:
Tide Golden San Mateo
Height Gate Bridge
-----------()()()()------------()()()()------ Alviso
| L | L |
Vin === ===
| | C | C
--------------------------------------------
I think of big, deep areas as large Capacitors and tight restricted
straights like the gate and San Mateo Bridge as Inductors. Without the
inductors, the tide height (Voltage) at the different points such as Alviso
would follow the height at the Gate. If the straights were modeled as
R's, then there would be loss and the heights inside the Bay would
always be less extreme (than the max high and low) at the Gate.
I believe we'd model bottom shape as "Rs" and these would be variable
with tide current and be in series with the L's (more current, more
loss).
>
> Ken Poulton
"Corvettes are cool! Heh, heh." - Beavis and Butthead
Kirk out
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