Q ..
You've mentioned backwater studies several times, would you want to explain what they
are.
Well, a backwater study is trying to find out what the water surface profile looks like for
A
various discharges. What you do is you start with an estimated elevation at a lower cross
section. Take cross sections out of the stream so that you know what the shape of the
channel and the
area look like. Then you start with the water surface down at
the lower reach, and you step- by-step you go from one cross section to the next.
As you move upstream the first couple of cross sections probably won't be too accurate
unless your initial water surface was real accurate. But as you move upstream you'd
become more and more accurate. It dampens out the poorer accuracy as you start off and
so that you end up with a good water surface profile upstream. For example, if you want
it in the flood insurance program, which I'm working with now, you need to know what
area is covered by the
flood.
First of all, you have to know what the water surface profile is for the
flood.
Then you take that elevation horizontally from the stream until you reach the ground at
that same elevation, and that's the limit of the flooding. Then you'd draw the outline of
all that area between the cross sections. Then whatever is in that area is considered the
floodplain.
The flood insurance people require
involved to force the residents to prohibit
building in the floodplain and other agencies and other programs force people to buy flood
insurance. Mortgage people say if you're going to get a mortgage from us for you to live
in the floodplain, you're going to buy flood insurance. So that's part of the program. But
that is a primary use for backwater studies.
Then another thing you use those for is to get what I called rating curves before. You
draw a curve that shows elevation on one limb and discharge on the other. You'd get a
curve so that you can use that curve to estimate other elevations for other discharges than
the known ones. You need that downstream from dams to compute your hydraulics to
design your outlet works and to decide on how much hydropower you can get out of the
dam.
Hydropower is based on the volume of water as well as the head you have on it--the head
downstream and upstream. They are the primary two things that decide on how much
power you get out of dams. You need that backwater relationship so that you establish the
tailwater elevation and the headwater elevation, that difference in elevation at the same
gives you the total head that you use in your computations. But, anyway, those are
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