Lake Basin Volumes
Depth to refusal sounding data from the initial surveys were used to calculate the original
storage capacities in 1908 (Lake Paradise) and in 1958 (Lake Mattoon) at current spillway levels
and also the 1931 lake surface area for Lake Paradise. Water depth soundings from the initial
surveys, using the sounding pole, and the 2001 depth soundings from the depth sounder were
used to calculate capacities for the date of construction and at the time of each survey. The
difference between these storage capacities is the lake volume that has been lost to sedimentation
since reservoir construction or between surveys.
Lake capacities were calculated using a method described in the National Engineering
Handbook of the U.S. Soil Conservation Service (USDA-SCS, 1968). This method can
determine the original and present volume of each segment by using the surface area of the lake
segments, the cross-sectional area and widths of their bounding segments, and a shape factor.
These volumes are then summed to determine the total lake volume. Reference elevations used
were the top of the spillway for each lake, 684.5 ft-NGVD for Lake Paradise and 632.3 ft-NGVD
for Lake Mattoon. These spillway elevations differ from values presented in the 1982 report
(684.1 ft-NGVD for Lake Paradise and 632.0 ft-NGVD for Lake Mattoon) due to discrepancies
discovered during the 2001 survey.
Volumes determined by the sedimentation survey were the 1979 water volume contained
in the reservoir and the 1979 volume of sediment contained in the reservoir. The sum of these
values is the potential water volume of the reservoir if the 1931 dam and spillway had been
constructed in 1908.
However, when the dam and spillway were constructed in 1931, a portion of this volume
already had been filled by sediment from the original (1908) dam and spillway. To develop a
1931-1979 sedimentation rate for Paradise Lake, it was necessary to estimate volume losses due
to sedimentation from 1908 to 1931. This was accomplished by prorating sediment
accumulations in segments 3-9 between 1908-1931 and 1931-1979.
The 1931 water volume in each segment was determined by using the formula:
Water volume (1931) = potential water volume (1908) – sediment volume (1908-1931)
By this method, the lake volume at the 1931 spillway elevation was reduced 6.7 percent due to
sedimentation during the period 1908-1931. This approximate adjustment cannot be used to
determine a 1908-1931 sedimentation rate.
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