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Using ArcView, KPD staff measured the area of descending lake bottom contours in two-foot
intervals (Figure 29). To calculate volume, each contour area was then multiplied by the depth
associated with that contour. Slope / depth variation within each depth contour was corrected
using the following method. All of the depth readings within a contour data set were tabulated,
and the mean calculated. The mean of a contour data set was then used as the depth in
calculating the volume of the contour. The volumes of all the contours were then summed to
arrive at total lake volume. Using this method, the total volume of Patriot's Park Lake was
determined to be 224.45 acre-feet (73,148,255 gallons).
Prior to the IEPA Clean Lakes Study, a study conducted by the Illinois Natural History Survey
estimated the volume of the lake to be 185.7 acre-feet. Comparing this to the current volume
estimated in this study would indicate a net gain of 38.75 acre-feet. The difference in volume
may be accounted for by extensive spillway reconstruction that was performed in 1993-1994.
Before reconstruction of the spillway, water flowing out of the lake was actually exiting the lake
at a level below that of the original spillway. Over the years water had entered cracks in the
concrete floor of the spillway and subsequent freeze/thaw cycles had caused the floor to heave
until most of the water was exiting the lake underneath the spillway floor (Jerry Sauerwein,
personal communication, 2003). Reconstruction of the spillway involved removing the old
concrete floor, setting a sub-base and pouring a new floor. Additionally, a 9-inch concrete lip
was added to the front of the spillway effectively raising the normal pool elevation. The Illinois
Natural History Survey (INHS) bathymetric mapping was completed prior to the rebuilding of
the spillway. It is unknown what the normal pool elevation would have been at the time of the
INHS study but is likely that it was at least 9 inches and possibly as much as 18 inches lower
than the current elevation. This spillway elevation difference would then account for the
difference in volume between the two studies.
Because of the lack of baseline data prior to 1987, no estimations can be made of lake volume
lost due to sedimentation since lake construction. However, as previously mentioned in this
report, sedimentation has been shown to be a significant factor affecting lake health. NRCS
investigations in 1996 revealed that within the upper sediment basin of the lake, the average
depth of silt deposits was 61 inches. Given the 63-year time span between the construction of
the lake and the NRCS study, the rate of volume loss in the sediment basin would be nearly an
inch per year. Additionally, NRCS staff observed a reduction in sediment basin water surface
area from an estimated 3.6 acres at its construction to approximately two acres at the time of the
study. Recent global positioning system (GPS) data taken by KPD staff confirms the present
surface area at normal pool. Within the remaining normally inundated area, depths have been
reduced to an average of 1.15 feet. This loss of volume has a significant impact on the
effectiveness of the sediment pond.
Hydraulically, it is likely that sedimentation rates have declined as retention time and storage
capacity within the sediment basin has decreased. This has resulted in the sediment basin
becoming a net exporter of sediments to the main body of the lake, a situation which cannot be
allowed to continue.
