Margaret S. Petersen
A:
No.
I knew a little bit about rivers, but I was no sedimentation expert. I guess I knew more about
the interaction between sediment, the channel, and the banks, but not so much about the
actual mechanics of sediment transport.
Well, there are very few people who are in--I mean sedimentation transport is not an
everyday skill in the area of civil engineering.
No, and there are more people dabbling in it today than there used to be.
Q ..
Today?
A:
One of the things we looked at at Dardanelle, which is somewhat higher than the other
navigation dams, was releasing what we call "clear water." Clear water releases from a
reservoir carry some sediment, but most of the sediment load entering a large reservoir is
deposited in the reservoir and the water released downstream is relatively clear. We knew
that the clear water in the channel below Dardanelle Dam would be picking up material from
the bed immediately below the dam initially and then farther and farther downstream as time
progressed, as well as eroding the banks, so that the turbines were not set at the present river
bed. We worked with an estimated future degraded bed. The sediment on the Arkansas isn't
uniform. It consists of a range of sizes of material. So as the bed degrades, the larger,
heavier bed material doesn't move, and
the bed becomes armored. The problem was
to estimate where the bed would be, at what elevation, when the normal release velocities
would be insufficient to move the armor material.
This is a problem that has become more recognized since our work on the Arkansas in the
'60's. On the Missouri River the same thing was done, but the Missouri below Gavins Point
Dam armored at a much higher elevation than they expected. Consequently, power
generation was less than they expected. This was due to a local gravel deposit that had been
missed when they sampled the bed material. They excavated that layer of material and then
they got additional degradation down to the level they had designed for.
Q:
A lot of this is basically guess work, then?
-47-
Previous Page
