FLOOD01776

• County: Pueblo
• Community: Pueblo
• Stream Name: Pueblo Reservoir
• Title: Pueblo Reservoir 1993 Sedimentation Survey
• Date: 5/1/1994
• Prepared For: State of Colorado
• Prepared By: BOR
• Floodplain - Doc Type: Educational/Technical/Reference Information

computes an area at 0.01- to 1.0-foot area increments by linear interpolation between the
given contour intervals. The program begins by testing the initial capacity equation over
successive intervals to ensure that the equation fits within an allowable error limit, which
was set at 0.000001 for Pueblo Reservoir. This capacity equation is then used over the full
range of intervals fitting within this allowable error limit. For the first interval at which the
initial allowable error limit is exceeded, a new capacity equation (integrated from the basic
area curve over that interval) begins testing the fit until it also exceeds the error limit. Thus,
the capacity curve is defined by a series of curves, each fitting a certain region of data. Final
area equations are derived by differentiating the capacity equations, which are of second
order polynomial fonn:

y = a + a~ + asX2

where:

y = capacity,
x = elevation above a reference base,
a = intercept, and
a2 and as = coefficients

Results of the 1993 Pueblo Reservoir area and capacity computations are listed in tables 1
and 2 and plotted on figure 6. A separate set of 1993 area and capacity tables will be
published for the 0.01-, 0.1-, and I-foot elevation increments (Reclamation, 1993). The 1993
area and capacity computations results are listed in columns (4) and (5) of table 2. Column
(2) in the table gives the original measured contour areas used in the original area and
capacity computation, and column (3) gives the original capacity computed by ACAP. Both
the original and 1993 area and capacity curves are plotted on figure 6 for a visual comparison
of changes. The 1993 survey determined that the reservoir has a storage capacity of 349,940
acre-feet and a surface area of 5,671 acres at spillway crest elevation 4898.7.

SEDIMENT ANALYSES

Sediments have accumulated in Pueblo Reservoir to a total volume of 8,181 acre-feet below
elevation 4898.7, spillway crest, since storage began in January 1974. This volume
represents a 2.28-percent loss in total capacity and an average annual accumulation rate of
423.9 acre-feet for the 19.3-year period of operation. The net sediment accumulation rate
from the contributing basin was 0.102 acre-feet per square mile per year for the same period.
The measured annual inflow rate is about 41 percent of the original annual estimate of 1,031
acre-feet.

Before projecting future sediment accumulation, the original estimate needs to be reanalyzed
using the John Martin and Pueblo Reservoir sedimentation survey infonnation. John Martin
Reservoir is located downstream from Pueblo Reservoir and had a net contributing drainage
area of 18,102 square miles prior to Pueblo Reservoir closure in 1974. Ten sediment surveys
were conducted on John Martin Reservoir from April 1942 through March 1972. The
calculated average annual sediment accumulation between the surveys varied from -0.0398
to 0.443 acre-feet per square mile. For the 29.9-year period, the annual sediment
accumulation rate was 0.148 acre-feet per square mile for the contributing drainage area.
A sedimentation survey conducted in June 1980 calculated an annual sediment accumulation
rate, from March 1972, of 0.077 acre-feet per square mile for the reduced net contributing
drainage area. The John Martin Reservoir values illustrate the extreme variation of the
calculated annual sediment accumulation values between surveys.

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