[NBLUG/talk] OT: physic puzzle reviewed.

sms at sonic.net sms at sonic.net
Fri Jan 9 16:27:00 PST 2004


Thanks to all who've commented!

Sadly, all the arguments to date have been "merely" common-sense; I won't
win this argument (nor be defeated!) without hard numbers.  I'll "come
clean" and admit that I'm in the "bridge goes down" camp (I try to hide
my side of any argument, when I pitch it to a 3rd party); BUT, I'm not at
9 9's of certainty  ;-)   Frankly, I think any of us lay scientists who
DO claim incredibly-high confidence are speaking either from ignorance
(there are a LOT of factors involved, and I know *I* am not competent to
evaluate them!), or from modesty (i.e. they aren't ignorant of the forces,
but are "modestly" claiming less awareness than possessed).

I'm really hoping for someone with the civil engineering / hydrodynamics
skills to make a educated guess, instead of my own ignorant guess.  From
my (admittedly ignorant) perspective, I don't see any replies from anyone
whose got the specialized training.  :(

Specifically replying to folks:  Gandalf -- yes, I'm taking an out-of-
county class.  Others -- Sorry if I misattribute.


Cal -
Yes, water has inertia; BUT, enough mass can resist the inertia and will
redirect _ANY_ amount of water.  I *THINK* the wave will utterly overwhelm
the bridge.  But, I don't know it for absolutely certain; I'm not sure to
what extent my "gut feel" for this problem is led astray by the scales
involved.

Your wall, for example:
   rigid (likely old) concrete,
   a solid surface designed to catch/stop everything
   not designed to accept any flexion.

The GG Bridge:
   Steel & cement, well-maintained
   with NARROW components (i.e. mass of the wavedoesn't have to divert very
     far to "flow around").
   it's designed to flex/stretch

It's not entirely clear what degree of lesson can be learned from your wall.



Troy -
Never surfed, but I've played in the surf; yes, I've planted face on the
bottom because a wave nailed me.

But...
I weigh ALMOST the same as water; when the water rises, so do I.  Then,
I go where the water sends me.  I *DO* know from experience that if I'm
in SHALLOW water and can brace myself, I can hold firm in one place
against a MUCH larger wave.

MY question is, what would happen if I borrowed concrete overshoes from
the mafia, and a steel exoskeleton from the military/university robotics
teams... (yah I know:  I'd drown; but would I drown _in_one_place_?)



Eric -
> Things required to analyze this:
> 1) force (weight) rating of the bridge for straight downward force
> 2) same rating for lateral forces
> 3) some kind of measurement of how high the wave is
> 4) some kind of measurement of how much water is there (gallons, feet
thick,
> etc.)

Also:

We need coefficients of friction (if the bridge survives the initial
impact):  how much stress will the water exert as it flows by, & for how
long?

I'm sure we'd need to know how much torque the GG can take, too:  the water
is gonna exert different forces on the bridge at different places along the
length, different front/back, different top/bottom.  That bridge is gonna
TWIST...

There's probably other factors we've both missed.

> there's no "just water and will flow freely".  Really.  Try diving in a
> pool wrong.

Yah.  The question is, will the _bridge_ dive in _right_?  Or at least,
well _enough_.  OK, an Ohio-class boomer it isn't, but it *MAY* be strong
enough & shed ENOUGH water...


> I suspect that as much water as depicted in that poster, any water that
> missed the bridge would sweep away enough earth from under the bay

Actually, I tend to doubt that.  The gate has been opened for c. 10K years,
with continuous net flow out (from rivers), & intermittent bidirectional
scrubbing (from the tides).  Ain't much stuff LEFT to get swept away; AFAIK,
the brige sits on bedrock (I *THINK* it's actually embedded, concrete poured
into holes blasted into the bedrock).

> that not only would the bridge be smashed, but all its parts washed away.

Agreed; if the wave took down the bridge, we'd find parts of the bridge all
through the East Bay, the Sacto Valley, and likely on to the Sierra
foothills.

> (the infamously damaging "tidal wave" tends to be about an inch or so
> high; water has a lot of mass)

Out in  the open ocean:  yes tidal waves are very small; most open-ocean
craft won't even notice a tsunami pass them.  But these waves DO rise (a
LOT) as they come to shore.  Most seriously-damaging tidal waves are feet
high by the time they come onshore.



Meg-
> I also don't have enough physic skills to show exactly what would
> happen, and agree that there is not enough information given, as Eric
> pointed out,

ALL the info available is what's in the picture (in the end, this is a
beer-N-pretzels question, not "hard" physics); sadly, I haven't found the
pic online or I'd offer a link.  It's obviously been diddled (darkroom or
photoshop).  Shows the GG bridge from inside the gate, Marin side.  We
see a beautiful "pipeline" breaker, the kind a skilled surfer likes to
crouch inside and race through... but the scale is wrong:
 - The "froth" atop the wave hides the exact top, but it appears that the
   wave is continuous to about the same height as the towers of the GG.
 - The front face of the wave is reaching into the bay; the bottom edge
   extends forward between the towers, & the hollow "pipeline" actually
   wraps around the road-bed where traffic drives.

> Also consider that a wave of that size would most likely originate
> outside the Bay, and most likely wider than the entrance to the bay.
> When it comes to the coast, the much of the water is going to take the
> path of least resistance, ie into the bay, instead of hitting land.
> This means that the wave will be hitting the bridge with much more force
> than it originally had outside, due to this tunneling effect it will
> experience.

Yes; as per my reply to Eric, above, the wave will follow the lay of the
land.  Presuming it's a tsunami, with a 'quake epicenter out near the
Farallons, it'll hit everything from Pt. Reyes in Marin, to Moss Beach in
San Mateo, within a few seconds.  There will be a fair bit of "funnel"
effect, but less than in many places because the 'Gate is relatively
recent & is more sheer (less sloping/gentle) than many "bay" entrances;
so, the water won't have as much TIME to funnel together, and much of it
will head straight inland (I gotta grin imagining the "WTFO??!?" of folks
on the bay side of the peninsula, a a huge wall of saltwater comes down
from the hills above 'em!).

Thanks to everyone who has been playing the game!

If I come up with a more-satisfying solution or relevant info, I'll post
it...



- Steve S.







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