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Sepetember 13, 2019 <br /> Page 2 of CIVIL RES"WURCES <br /> ➢ A side overflow spillway was placed on the north side of Reservoir 4 adjacent to the river. This spillway will allow water <br /> to flow into and out of the reservoir as the invert elevation will be set one-foot lower than the existing grade at that point <br /> of the reservoir which is a minimum of one foot below the base flood elevation. <br /> The reservoir is 299 acre-feet in volume and the spillway has 413 cubic feet per second (cfs) capacity at 1 foot of <br /> overtopping depth. Each spillway was designed to be able to fill the reservoir so the banks are less susceptible to <br /> major washouts. A one-inch per hour rise in flood water would fill the reservoir in approximately 3.5 hours, where-as <br /> a six inches per hour rise would fill the reservoir in two and a half hours. This is adequate to protect the non-reinforced <br /> banks from severe erosion as the reservoir would be full by the time the banks would overtop and potentially erode. <br /> Reservolr5: 100 percent of Reservoir 5 is within the 100-year floodplain. The following information was considered in designing <br /> the spillways and erosion protection: <br /> A side overflow spillway was placed on the west side of Reservoir 5 adjacent to the river. This spillway will allow water <br /> to flow into the reservoir. An alternate spillway will also be placed at the southeast end of the reservoir to allow for <br /> water to enter and exit the spillway as this reservoir is long in length from west to east. Water will also enter this <br /> spillway which will allow for a quicker fill time and greater equalization of flood waters through the reservoir. Both <br /> Spillways will be set one-foot lower than the existing grade at that point of the reservoir. <br /> ➢ The reservoir is 680 acre-feet in volume and the spillway has 600 cubic feet per second (cfs) capacity at 1 foot of <br /> overtopping depth. Each spillway was designed to be able to fill the reservoir so the banks are less susceptible to <br /> major washouts. A one-inch per hour rise in flood water would fill the reservoir in approximately 4.5 hours, where-as <br /> a six inches per hour rise would fill the reservoir in under two hours. This is adequate to protect the non-reinforced <br /> banks from severe erosion as the reservoir would be full by the time the banks would overtop and potentially erode. <br /> Conclusion <br /> The analysis was completed based on general hydraulic characteristics of the Big Thompson as a flooding source. The pattern <br /> of storms is largely unpredictable and therefore the recommended improvements cannot guarantee that the facilities will not be <br /> damaged during large-scale storm events. <br /> JABestway-2131bernhardtORMS Permit updatelAdequacylfloodcontrol-9-11-19.docx <br />