🚀 go-pugleaf

I made a posting concerning reservoir sedimentation in which I stated what
I thought was a reasonably straight forward opinion, capable of being
clearly understood by all.  Apparently, my optimism was misplaced.

Before getting into the specifics, let me deal with three preliminary
matters.  First, Dr. Menten made a post which was a good-faith effort to
obtain more information.  Unfortunately, I misread his motives and
responded in an inappropriate manner.  For that I apologize.

Secondly, a word for those who have sent some rather juvenile insults my
way, including insults involving the misspelling of my name.  I am afraid
they will have to look elsewhere for their jollies.  I am a little too
long in the tooth to start entering into exchanges of sophomoric insults.

Thirdly. Mr. Penny, in his posting on this matter, represented that:
>...silt introduced in the 1986 high water event shrank Folsom reservoir's
capacity from >1.01 million acre-feet to 9.77 million acre-feet...  
This is not a correct statement,  There is, of course, no evidence that
Mr. Penny set out to deliberately post incorrect information.  However,
there is also no evidence that Mr. Penny made any effort to contact the
appropriate responsible public agencies to ascertain the actual facts.  I
find this unfortunate.

Now to the matter of reservoir sedimentation.  My words were "Siltation of large
reservoirs in the Sierra Nevada is not a significant problem."  Ms.
Schuefele turns that into the generalized statement  "..siltation is not a
problem."  So let's look at the words I actually used.

First, the words "Sierra Nevada."  This thread is an outgrowth of an
original posting dealing with floods in the Central Valley of California,
and it is pretty clear that has been the central theme of the subsequent
postings.  Flooding in the Central Valley is obviously the result of
runoff from streams tributary to the Central Valley, primarily the Sierra
Nevada and more specifically the west slope thereof.  To be complete, we
should also add some areas in the southern Cascades.  There also is some
drainage to the Central Valley from the easterly slope of the Coast Ranges
and from the Tehachapis which may cause local flooding.  Unless they have
been rerouted since I last looked at a map, the Carmel, Eel and Mad Rivers
do not drain to the Central Valley.

Second, the word "large" appears in my statement.  There are eight
reservoirs in the Sierra (and Cascade) foothills with capacities ranging
from approximately 1 to 4-1/2 million acre-feet.  The less than 150,000
acre-foot Terminus Reservoir on the Kaweah River is not a "large"
reservoir.

Thirdly, I made reference to "significant problem."  A significant problem
is one of sufficient magnitude to throw into question the achievement of
the benefits projected over the reservoir life.  So let's make some
ball-park estimates of the amount of siltation which might be expected in
the large reservoirs located in the Sierra foothills (plus Shasta
reservoir).  And let's look at the 50- to 100-year evaluation period
normally used for evaluation of benefits, costs and revenues of water
resource projects.

Ms. Schuefele cites the case of Terminus Dam on the Kaweah River and makes
reference to the World Wide Web home page of the American Whitewater
Affiliation.  That reference indicates siltation of 7,000 acre feet.  This
has presumably taken place since the reservoir was constructed in 1962,
and amounts to a little under five percent of the reservoir capacity.   
This would project as a reduction of some 15 percent over 100 years. 
Again, Terminus does not impound a large reservoir.

The available surveys of the reservoir area behind Folsom Dam indicate a
33,000 acre-foot reduction in capacity over the entire period between 1956
and 1992 (not just the year 1986).  This would extrapolate to a unit rate
of silt production per square mile about 1/3 higher than that for
Terminus.  Indicated 100-year siltation would be on the order of 9 percent
of Folsom reservoir capacity.  There has undoubtedly been additional
sedimentation since 1992, particularly at the beginning of this year. 
However, the siltation subsequent to 1992 has not been, and cannot be,
quantified at this time.

What must to be recognized is that a significant portion of the siltation
that has taken place in Folsom Reservoir (including any new siltation this
year) was contibuted by the 1986 washout of a coffer dam which had been
built in connection with the contruction of Auburn Reservoir.  Hence, the
siltation actually experienced at Folsom Reservoir is not representative
of what might be expected on other foothill reservoirs.

Figures for actual historic reservoir sedimentation undoubtedly exist for
other large Central Valley reservoirs (approximately a million acre feet
or larger).  I do not happen to have those figures at hand.  However, if
the rates of silt production per square mile above Terminus are
extrapolated to the other large reservoirs,  it would suggest that at the
end of 100 years after construction, reservoirs would have been reduced in
capacity by an amount between about one and six percent after 100 years
(depending on the particular dam site and the ratio between reservoir
storage and tributary watershed area).  I don't find that sort of
reduction significant,  particularly when it is recognized that:

     **Capacity diminution approaching the full amount does not occur
until late in the evaluation period, when (due to the time value of money)
the effect on the present worth of benefits is small.
     **Much of this siltation may well take place in the dead storage
below the elevation from which water can be drawn from the reservoir.  In
some instances, dead storage can be large--over 100,000 acre-feet in the
case of Shasta Reservoir and over 800,000 acre-feet in the case of
Oroville Reservoir.
     **Under proper evaluation techniques, the projected reduction of
reservoir capacity would have already been allowed for in the analysis of
project benefits and accomplishments.

Let me deal with one other thing.  Ms. Schuefele made reference to:
>...continuing siltation problems with some of the dams on the North Fork
Feather.

This refers to Rock Creek and Cresta Dams, two power diversion dams built
in 1949 and 1950, with an aggregate capacity of some 8,500 acre-feet and a
tributary watershed area of some 1900 square miles.  They have accumulated
about 4200 acre-feet of silt.  Of course tiny reservoirs with large
tributary drainage areas are going to have a silt problem.  That's a
no-brainer.  But that has absolutely nothing to do with what we are
talking about--large reservoirs.  I sure this will really bring out the
hate mail, but I have to say that this is why well- meaning amateurs
should not muck around in areas they don't understand without getting some
guidance on how to evaluate the data they manage to ferret out.

I see no reason to retreat from my statement concerning reservoir
sedimentation.  More refined estimates could change the numbers somewhat. 
But they are not going to change them sufficiently to invalidate the
conclusion that siltation of large reservoirs in the Sierra Nevada is not
a significant problem.  Representations that large reservoirs tributary to
the Central Valley will completely fill up in a matter of a few years or
decades are flat-out incorrect.

-- 
Dave Powell
dopowell@alumni.caltech.edu