From: Ronald Wong (ronwong@inreach.com)
Date: Tue Feb 26 2002 - 09:43:39 PST
Message-Id: <l03102800b8a0f363272e@[209.209.18.100]> Date: Tue, 26 Feb 2002 09:43:39 -0800 From: Ronald Wong <ronwong@inreach.com> Subject: re: Question on Tidal Effects
Sue asked:
>...
>
>Second question, what is the tidal effect on a landlocked lake? On a lake
>that drains into another body of water?
Sue:
Keep in mind that "tidal" effects refer to tides and that tides are due to
the gravitational influence that the moon - and, to a smaller degree, the
sun - has on the earth, it's atmosphere, and it's oceans.
Most diagrams that are drawn to show the tidal effects of the moon on the
earth and it's oceans are exaggerated to bring out why, in general, there
are two high tides and two low tides every day. As a result, in the
diagrams, the range between high tide and low tide appears to be enormous.
In fact, the range between high tide and low tide is quite small. On
average, it is about 2 meters compared to the earth's diameter of 10^7
meters. So, for a shell of water around 10 million meters in diameter with
the earth in the center, the tidal range, on average, only amounts to 2
meters. That is to say that when the difference in distance between the
near side of a body of water and the moon is 10 million meters less than
from the far side of the same body of water and the moon - at an average
distance equal to the present earth-moon distance - you end up with two
high tides and two low tides with a tidal range of 2 meters each day.
Local conditions can alter this. At Honolulu, the range is less than 1
meter and, at the other end of the spectrum, the Bay of Fundy has a 14 m
range. But, overall, it's 2 m.
So, unless one end of a lake is 10 million meters closer to the moon than
it's other end, there won't be much in the way of tidal action.
It's this difference between one end of the body of water/earth/atmosphere
and the other relative to the moon that is responsible for tidal effects.
Since landlocked lakes are considerably smaller than 10 million meters, you
won't find any tidal effects in them. Even the Mediterranean Sea is too
small to exhibit tidal effects.
Rivers and bays that empty into an ocean will exhibit tidal effects. This
is in response to the rising/falling level of the ocean. But this effect
becomes less and less as you go upstream away from the ocean. Here in San
Francisco's bay, the daily cycle of tides is very noticeable but as you
move up through the delta towards Sacramento, where the San Joaquin and the
Sacramento rivers come together, the tidal range drops down to less than a
meter and, as you continue up either of the two rivers, it drops to zero.
This doesn't mean that water levels in a landlocked lake won't rise or fall
as if the lake had "tides". It's just that the rise and fall is not due to
tides.
Weather conditions may occur in such a way that a high pressure system
moves over one end of a lake while a significantly lower pressure system
moves over the other. As this pressure difference moves across the lake,
the water levels will rise and fall in response to the lower and higher
pressures.
Another condition that will produce tide-like effects is wind moving over a
lake. Strong winds can generate waves that move across the lake. When they
reach the end of the lake they may be reflected back on themselves. What
happens next is exactly the same as what happens when you tie one end of a
cord to a door knob and then start sending waves down the cord towards the
knob by wiggling the other end up and down. The reflected wave runs into
the incident wave and, in both cases, you get a standing wave.
In the case of the lake, the standing wave is called a seiche (pronounced
like "sash" but with a long "a" sound). The size of the standing wave tends
to be quite large with a trough at one end of the lake and a crest at the
other. Like the standing wave in the cord, the antinodal regions oscillate
between being a crest and being a trough ("high tide" and "low tide").
Depending on the size of the lake, the period of vibration can be as short
a minute or as long as an hour (i.e. considerably less than the period of
an actual tide).
>What are some good resources to read more about all of this?
Go to the library and find the section in the stacks where the books on
physical geography are kept. A good one should be able to tell you what you
want to know without overwhelming you with minutiae.
Happy hunting.
ron
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