Re[3]: Tide Data

From: Geoffrey.Boehm@wj.com-DeleteThis
Date: Wed Dec 14 1994 - 09:13:48 PST 

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Date: Wed, 14 Dec 1994 09:13:48 -0800
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From: Geoffrey.Boehm@wj.com-DeleteThis
Subject: Re[3]: Tide Data
To: wind_talk@opus.hpl.hp.com-DeleteThis
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     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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