R2001-120 AUTHORIZING AGREEMENT - Laserfiche WebLink

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The trap efficiency (percentage portion of sediment captured by the reservoir) of the lake was determined using a predictive equation developed by Dendy (1974) based on the relationship between the annual capacity to inflow ratio and sediment-hoiding capacity. The trap efficiency of Basin 6 was 31.5 percent in 1922, which means that only 31.5 percent of all sediment entering the lake was trapped in the lake basin. In general, as sediment accumulation reduces the volume of the lake basin, the holding time for water entering the lake is reduced so tha.t there is less time for sediment to drop out of suspension, and the trap efficiency is reduced. The reduced capacity of Basin 6 has resulted in a trap efficiency of 15.7 percent in 2000. Gradual consolidation of lake sediments affects the calculated sedimentation rate of the lake by reducing the volume of accumulated sediments. Sediments accumulate on the bottom of the lake �� in a very loose, fluid mass. These sediments are subject to compaction as they are covered by I, continued sedimentation or are exposed by occasional lake drawdown. This process reduces the ', volume of the sediments while increasing their weight per unit volume. Thus, the tonnage of the 'i sediments accumulated during a period of time will not change, but the volume of the sediments may be reduced over time by up to 50 percent. This is also consistent with a reduced volumetric sedimentation rate over time. Consolidation of sediments has been quite pronounced in Basin 6 of Lake Decatur. This process of consolidation most likely has been enhanced by the winter drawdown that has been a part of the operating plan for the lake in recent years. Overall, sedimentation rates for Basin 6 of Lake Decatur were high for the initial period (1922- 1936) with a sedimentation rate of 35.4 acre-feet per year. Sedimentation rates for the basin have gradually reduced with time and for the period (1983-2000) were 8.3 acre-feet per year. Sediment Distribution The distribution of sediment in the lake is shown (table 3). This table lists the average sediment thickness and ma.ss distribution for the basin and for each lake calculation segment as shown in figure 2. Sediment thickness ranges from 1.8 to 4.1 feet. The most significant accumulation by either measure, depth or mass, is in segment 37. Density analyses of the sediment samples (appendix II) indicate that the unit weight of sediment in the northern (upstream) portions of the lake is greater than the unit weight of sediment in the southern end of the lake. In general, coarser sediments are expected to be deposited in the upstream portion of a lake where the entrainment velocity of the stream is reduced to the much slower velocities of a lake environment. These coarser sediments tend to be denser when settled and aze subject to drying and higher compaction rates as a result of more frequent drawdown exposure in the shallow water enviro�unent. As the remaining sediment load of the stream is transported through the lake, increasingly finer particle sizes and decreasing unit weight are observed. Sediment Particle Size Distribution A total of 26 lakebed sediment samples were collected for particle size distribution analysis. Figure 4 and appendix III present the laboratory analyses for these samples. Figure 4A shows particle size distribution plots for sampies collected from lines R53-R54, R55-R56, R75-R76, 9