R2009-06 APPROVING AND PLACING ON FILE - LAKE DECATUR MINING PLAN

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DEPARTMENT OF WATER MANAGEMENT ' Memorandum No. 2009-01 January 7, 2009 TO: Honorable Mayor Carrigan and City Cou�cil FROM: Ryan P. McCrady, City Manager John A. Smith, Assistant City Manager for Public Services 0�'�/�" Keith D. Alexander, Director of Water Management �'. /� �,f'� SUBJECT: Lake Decatur Mining Plan SUMMARY RECOMMENDATION: Council adopt the attached Resolution approving the attached Lake Decatur Mining Plan. BACKGROUND: In May 2008 Council authorized H. L. Chastain & Associates to complete a Mining Plan. A summary of the tasks completed is as follows: l. Provide a h�graphic survev of the Lake to determine the current amount of sediment and water stora�e in the Lake. A survey had not been done for the entire Lake since 1983. The Plan determined that 2,961 acre feet of sediment have been deposited in the Lake since 1983 and that 2,136 acre feet (1,500 acre feet from Basin 5 and 636 acre feet from Basin 6) have been removed by dredging to date. The Lake's water storage capacity is currently 21,918 acre feet. 2. Design the most effective way to complete dred ing Basin 6 to maximize water flow to ADM's water treatment plant intakes. These designs are detailed on the Plan's aerial photography maps for Basin 6. 1,894 acre feet of sediment will be removed from this basin when dredging is completed. A majority of the area dredged will have a 6.4 to 7.4 foot water depth when the lake is full. 3. Desi�n the most effective way to dredge (mine the Big and Sand Creek areas of Basin l. These designs are detailed on the Plan's aerial photography maps for Basin 1. 1,945 acre feet of sediment will be removed from this basin. The Big Creek area will have a 9.4 foot water depth and the Sand Creek area will have an 8.4 foot water depth when the lake is full. 4. Desi�n sediment traps to be dred�ed in Basin 6 and Basin 1. These designs are identified on the Plan's aerial photography maps. The traps will significantly reduce the future amount of sediment deposited in the main areas of Basins 6 and l. It is anticipated that the Basin 6 trap will need to be dredged approximately every 5 years, the Basin 1 Big Creek trap every 7 years and the Basin 1 Sand Creek trap every 5 years. This schedule is highly dependent on the number and magnitude of high water flow events into the lake that will occur in the future.