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I I I I I I I I I I I I I I I I I I I East 85th Avenue Crossing The East 85th Avenue crossing presently consists of Q 36-inch CMP culvert. The existing discharge capacity is negl igible; therefore, this culvert should be re- placed with a culvert with ample capacity to poss the IOO-year flood flow. Tributary l Detention - Plate 111-8 Tributary l at Interstate 25 was recommended in the Phase A report as a site for detention. This detention pond will significantly reduce the amount of flow carried in the Niver Creek channel during flood conditions. A low dike and modification of the existing culvert under 1-25 to reduce its discharge capacity will be required to form the detention pond. The detention facilities were studies in order to reach an optimum solution between cost (both structure and land acquisition), physical limitations and net effect of detention on Niver Creek flows. It was determined that the facilities should be designed such that the detention pond level would reach elevation 5233 under 100-year flood inflow conditions. The planned detention does not meet any of the State Engineer's criteria to classify it as a dam; therefore, no consideration of PMP flow was made. The water level behind 1-25 will reach elevation 5230 under present chan- nel conditions, without detention, during a IOO-year storm. This represents an area of about 0.75 acre. With detention, the 100-year flood plain would be about 1.5 acres; therefore, 0.75 acre of additional land will have to be purchased to insure no future development will occur within the JOO-year flood pool. Niver Creek Detention - Plate 111-9 The Niver Creek main channel at East 88th Avenue was also identified as a detention site in the Phose A report. The optimization study for this facility showed that it should be designed for a maximum water surface level of elevation 111-9