Well, it would tell you whether the sediment originated locally or in the
upper basin; that is, give you an idea whether it was lacustrine or
flood-transported. On the other hand, if you have various samples from the
upper basin, you could perhaps tell the origin of 'most' of the floodwater,
which in turn might tell you something about the hydraulic regimen under
which the flood took place. For instance, suppose a relict paleobar
containing way more than its share of grains localized to the Dolores side
of the San Juan; it's odds-on the flood occurred when the Colorado was
still traversing Unaweep Canyon, before Westwater was excavated.
>
>Another question that has tickled my imagination for some time now --
>750000 cfs (to pick a number) sounds impressive, but can we relate it
>to something more concrete -- for example, would the water reach the
>footbridge at Phantom?
I believe the water was within 6 - 8 feet of the footbridge when the river
was at ca 220000 in 1927. Somebody with a USGS water supply paper could
check the station heading for Colorado River in Grand Canyon and compare
the highest discharge and stage to the height of the bridge.
My intuition tells me that the velocity of the
>water, past some point, is purely determined by the overall slope of
>the bed, which means that at any point you should be able to
>calculate (to a rough approximation) the water level.
Well, the highest velocity measured in an open channel was 22 fps at Great
Falls during one of those floods of the Potomac when the whole of the
metagraywackes that make up that area were under (!). Harding mentioned to
me that when Mulholland's dam in the foothills of the San Gabriels failed
that severed powerlines showed a velocity of ca 27 fps. When the Johnstown
dam failed, a steam train engine raced down to town with the whistle
blowing all the way to give a warning. It handily beat the flood. (As I
recall, the engineer ended up in the trainyard, which was lost: an act of
selfless heroism.) It appears that in open channel flow there's a velocity
limit and it's under 30 fps; you absorb discharge by deepening, widening,
and abating internal friction so that the velocity of the whole mass of the
water approaches more and more nearly to the highest velocity of any of the
water. This in turn occurs because the internal turbulence is abated by
transporting sediments (ever notice how at high water on the San Juan you
float faster than the slope would suggest you should?) and because the bed
friction has less and less influence on velocity as the bed is further and
further away from the preponderance of the water.
So what we need is to feed good contours into Harding's not-yet-existent
model and it will calculate water surface height and velocity at every
point in the canyon for a given discharge. Have to ask him how it thinks
about scour, though.
====================================================================
To subscribe, send email to majordomo@songbird.com, with "subscribe
gcboaters" as the only line in the message body. To unsubscribe send
"unsubscribe gcboaters". For further information send "info
gcboaters", or see http://www.songbird.com/gcboaters
====================================================================