Received: from hplms26.hpl.hp.com by opus.hpl.hp.com with SMTP (1.37.109.8/15.5+ECS 3.3+HPL1.1) id AA14961; Wed, 14 Dec 1994 09:24:43 -0800 Return-Path: <Geoffrey.Boehm@wj.com-DeleteThis> Received: from gatekeeper.wj.com by hplms26.hpl.hp.com with SMTP (1.36.108.4/15.5+ECS 3.3+HPL1.1S) id AA07321; Wed, 14 Dec 1994 09:25:06 -0800 Received: from internal.wj.com (internal.wj.com [144.172.5.12]) by gatekeeper.wj.com (8.6.9/8.6.9) with SMTP id JAA07219 for <wind_talk@opus.hpl.hp.com-DeleteThis>; Wed, 14 Dec 1994 09:19:33 -0800 Received: from ccsmtp.wj.com by internal.wj.com with SMTP id AA10866 (5.67b/IDA-1.5 for <wind_talk@opus.hpl.hp.com-DeleteThis>); Wed, 14 Dec 1994 09:22:06 -0800 Received: from ccMail by ccsmtp.wj.com (IMA Internet Exchange v1.03) id eef2a0d0; Wed, 14 Dec 94 09:23:25 -0800 Date: Wed, 14 Dec 1994 09:13:48 -0800 Message-Id: <eef2a0d0@ccsmtp.wj.com-DeleteThis> From: Geoffrey.Boehm@wj.com-DeleteThis Subject: Re[3]: Tide Data To: wind_talk@opus.hpl.hp.com-DeleteThis Content-Type: text/plain; name=Text_Item Content-Transfer-Encoding: 7bit Content-Description: cc:Mail note part
I had never realized there was such a large interval between slack and
high/low tides. Can we assume, however, since tidal motions are close
to harmonic, that even if there is an hour betwen slack and high tide,
that the current at high tide is still very small? After all, nobody
cares about small currents - what we all really want to know is when
the current becomes significant.
What I would really like to know is a RULE OF THUMB which would allow
me to figure out the percentage of full current as a function of time
(relative to high/low tides).
To be more precise:
First, let's simplify and just assume an outgoing tide:
Let TH = Time at High Tide
Let TL = Time at Low Tide
Let TI = Tidal Interval = TL-TH
Let T = Current Time
Let C(T) = Current at time T (in the ebb direction, to be precise)
Let TR = Relative time = (T - TH)/TI
= Fraction of the interval between high and low tides that
has elapsed so far
Let CR(T)= Relative current = C(T)/(Maximum current)
So, we want to be able to calculate CR(TR), ie,
the percentage of maximum current as a function of the percentage
of the tidal interval that has elapsed so far.
If there were no corrections (no land), this would simply be:
CR(TR) = sin(TR x pi)
So, you oceanographers and mathematicians out there, what are the
corrections to be added to the above?
==============================================
And now, on a different aspect of this topic, I wonder how much effect
the background river current has at Sherman Island? Even if we ignore
all the resistive/capacitance corrections being discussed so far, it
is evident that at high tide there is an ebb current which is equal to
this background river current, so slack current will only occur when
there is actually a substantial flood due to the tidal component,
which will then cancel the river current component.
So, what is this background river current, and does it vary by season?
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