2011-12-16_REVISION - M1977572 (71)

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5.3 Operation of Ponds for River Flow Conditions 5.3.1 River Levels Below Berm A Inlet On any given day, during a normal year, the average daily discharge in the Arkansas River can range between near 0 cfs and 600 cfs. The average daily discharge during the non - irrigation season (October to April) will typically be less than 50 cfs while flows up to 500 cfs are common during the irrigation season (May to August). The normal everyday flows in the river are estimated to be between 10 cfs and 300 cfs more than approximately 50% of the time. The regional groundwater table will be generally flowing toward the river, from the south to the north, through the unlined pits. Water surface elevations in the pits themselves will gradually rise and fall in conjunction with the water surface elevation in the river. Based on historic discharge data (from 1949 -2010) and the hydraulic modeling performed for the current study it is estimated that the normal annual fluctuation in the pond's water surface elevations will be on the order of 2 feet. The normal high water surface elevation in the pit could occur during the irrigation season and the lower water surface elevation will occur in the winter season. During normal everyday operating conditions the flows in the river should: (a) stay in the main channel; (b) not utilize the Berm A Inlet; and (c) bypass the pits. 5.3.2 River Levels Near and Above Berm A Inlet Occasionally, in a normal year, the average daily discharge in the Arkansas River will exceed 600 cfs. On an annual basis the river discharge will exceed 600 cfs approximately 4% of the time. However, during the irrigation season 600 cfs maybe exceed as much as 10% of the time. At approximately 865 cfs the Berm A Inlet will begin to operate. In any given normal year, it is estimated that the Berm A Inlet will operate around 2% of the time or for approximately 7 total days. The Berm A Inlet could operate for a portion of one day or for multiple days several times in any given year depending on precipitation events and reservoir releases upstream. When the Berm A Inlet operates, it will fill Pond #1 with water in 1 to 2 hours and then overtop the ground between Ponds #1 & #3 (the former location of Pit #2) in a shallow /sheet flow fashion. It is estimated that Ponds #1, #3, and #4 will be filled in approximately 4 to 7 hours after water starts flowing through the Pond #1 Inlet. If a flood event has a long enough duration, water will overtop the east end of the Pond #4 and depending on the river's stage will either be captured by the irrigation ditch along the east property line or if the ditch is full move out on to the overbank floodplain downstream. The east and west sides of each pond were design to be wide and very flat thereby allowing shallow sheet flow to fill the downstream pond and minimizing the potential for significant erosion of the berm between the ponds. After the passing of a flood event through one or more of the ponds; the inadvertently captured volume of water will be returned to the river, as the river's stage decreases, thru an outlet pipe in each pond. Infrequent Operations Infrequent flood events on the Arkansas River can range from 10- percent annual chance of occurrence (10 -year) flood of 11,200 cfs to the 1- percent annual chance of occurrence (100 -year) flood of 40,000 cfs COVALC001_Phase 11 Report_082211.docx 5.10 ANdERSON CONSULTING ENCjiNEERS, INC.