R2001-120 AUTHORIZING AGREEMENT - Laserfiche WebLink

Home Browse Search

Basin 6 Volumes and Sedimentation Rates Calculations of the lake capacities were made using methods described in the A�ational 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. The reference elevation used for the lake was the top of the spillway, 614.5 feet NGVD. Table 1 presents the volume calculation results for each survey. Sedimentation has reduced the basin capacity from 2,797 acre-feet (ac-ft) in 1922 to 1,451 ac-ft in 2000. The 2000 basin capacity was 48.1 percent of the 1922 potential basin capacity. For water supply purposes, these volumes convert to capacities of 911 million gallons in 1922 and 473 million gallons in 2000. Sedimentation rates for Lake Decatur's Basin 6 were analyzed in terms of deposition rates in the reservoir. Table 1 presents the sedimentation rates for Basin 6 for all available sedimentation periods. These rates indicate a decline in annual sediment deposition rates from 35.4 ac-ft for the period 1922-1936 to 8.3 ac-ft annually from 1983-2000. The long-term average annual deposition rate for 1922-2000 was 17.3 ac-ft. Table 2 presents capacity loss rates for Basin 6. The capacity loss rates compare the rate of accumulation of sediment in the lake to the original reservoir capacity of the Iake. For Lake Decatur Basin 6, the rate of capacity loss has been reduced from 13 percent per year for the period 1922-1936 to 0.3 percent per year for the most recent period (1983-2000). Factors Affecting Lake Sedimentation Rates Sedimentation rates in a lake can vary over time due to changes in either watershed or in-lake conditions. Changes in watershed conditions, such as altered precipitation patterns, land-use patterns, and streamflow vaziability, also affect the sediment delivery rates to the lake. In-lake conditions that a�so affect sedimentation rates involve the variation of trap efficiency (due to reduced storage capacity) and sediment consolidation. Representative streamflow values for the Sangamon River at Monticello from October 1921- September 1999 are shown (figure 3). The most important plots for analysis of lake sedimentation are the maximum flows and the average flows. High sediment transport rates are closely related to peak water discharge periods (Demissie et al., 1983; Bhowmik et al., 1993). The plots in figure 3 do not indicate any distinct differences in high and average flow characteristics for an individual sedimentation period. Of interest for water supply yield purposes aze the generally lower minimum flows that occurred during the 1922-1936 and 1946-1956 periods. Low flow rates at the Monticello station for the early 1930s and the mid-1950s reflect the severe drought conditions that were prevalent for most Illinois streams during those time periods. 6