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NPDES water quality parameters can be met. The valve(s) will be periodically adjusted to <br />accommodate for the varying water quality coming off the ever changing vegetation covered <br />reclaimed fields. When the 10yr-24hr storm event enters the pond, the water will be allowed to rise <br />and if necessary, go through the Primary Structure but not through the Emergency Spillway. After <br />the storm event, the pond retains the water for several hours while the gate valve(s) slowly dewater <br />the pond. In the case of the 25yr-24hr event ,the inflow of storm water will be allowed to enter the <br />pond, some of it will go through the gate valve(s), the Primary Spillway and the remaining, if <br />necessary, goes through the Emergency Spillway. <br />Sediment deposits within the pond will be allowed to accumulate to a point within 0.5ft of the lowest <br />gate valves. When this occurs, then a 6" diameter pipe extension of approximate 1-2ft will be <br />attached to~the flanged valve body to raise the opening of the valve above the sediment bed. This <br />will then raise the point to which the sediment bed can accumulate too. It has been observed in <br />other ponds that sediment in the pond forms in a delta type fan starting from the bottom of the <br />pond inlet structures and goes toward the outlet. Even though SEDCAD and the USLE equation <br />calculates the amount of sediment inflow and volume of such, the true sediment dispersion within <br />the pond will not be uniform. For that reason, the elevation difference between the pond bed load <br />and the valve opening will be monitored and periodically adjusted to keep adequate freeboard. <br />Clean out of the sediment will occur when the sediment elevation reaches to within two (2) feet of <br />the Primary Spillway Inlet. The SEDCAD4 program suggest that with an Average Annual R value <br />that they supply for the Nucla area, it will take approximately 260 years for the pond to fill to an <br />elevation of 5611.5 (0.5ft below the lowest adjustable valve opening. <br />Conclusion <br />The size of Sediment Pond 011 was enlarged to accommodate water pumped from the pit and <br />irrigation runoff to be retained for a period of time to allow the NPDES water quality to stabilize <br />before being discharging. Then the 10yr-24hr and 25yr-24hr storm events were factored in. The <br />SEDCAD4 program was utilized in the design to size the pond and to determine the size, location <br />and placement of the adjustable 6 inch gate valves, Primary and Emergency Spillway Pipes. The <br />SEDCAD4 10 & 25yr-24hr models can be viewed within this document. SEDCAD4 shows that <br />neither the 10yr or 25yr event will initially reach the top of the Primary Riser. <br />Attachment 2.05.3(3)-20-4 <br />