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go <br /> <br />DRAFT <br /> <br />The total inflow was calculated as the sum of the suspended load and the bed load. <br /> <br />The total annual outflow was subtracted from the total annual inflow to get the annual <br /> <br />loading. <br /> <br />The fraction of bed load depends on the characteristics of the stream bed material, texture of <br />suspended material, and the suspended sediment concentration. The bed load correction factors <br />given in Table A-3 (Design of Small Dams, Bureau of the Reclamation, Department of the <br />Interior 1987) were used to estimate the percent bed load in terms of suspended load. For <br />concentrations less than 1,000 mg/1, the table gives a bed load correction factor of 25 to <br />150 percent of suspended load for sandy stream bed material. <br /> <br />The trap efficiency of the lake was also calculated assuming different bed load concentrations <br />(Appendix D). The trap efficiency of the lake is the percent of the total sediment inflow that is <br />deposited in the lake <br /> <br />The trap efficiencies calculated were compared with Brune's trap efficiency curve (Brune, "Trap <br />Efficiency of Reservoirs" June 1953) (Appendix D). Brune's curve gives trap efficiencies of <br />normal ponded reservoirs as a function of detention time (lake volume divided by the annual <br />inflow). Lake Orono has a volume of about 1,249 acre-ft. The annual inflow rate into the lake <br />based on 1996 data is 222, 410 acre ft/yr (USGS data for Elk River near Big Lake). This gives a <br />detention time of 0.0056 years. The corresponding trap efficiency per Brune's curve is in the <br />range of 15-40 percent. This is comparable to the calculated values with bed loads of 50-100 <br />percent of suspended load. Total sediment deposition was calculated to be between 880 and <br />2,500 tons per year assuming bedload contribution in the above range. Based.on bedload at 75 <br />percent of the total suspended load, net deposition is approximately 1,700 tons, or approximately <br />1,700 cubic yards, per year in Lake Orono. Spread evenly over the lake's 254 acres, it represents <br />a deposition rate of 0.004 feet/year. This is about one fourth of the deposition rate previously <br />calculated by comparing the 1970 and 1996 DNR lake surveys. The sediment deposition is, <br /> <br />N:\0598\01 ~ELKRIVER. RPT-rlb 1 3 <br /> <br /> <br />