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I I I I I I I I I I 4 I I I I I I I I I overland or in the streets through the mobile home park. Granby Drainageway crosses under Sable Boulevard about a block north of Colfax and then continues southeast across Colfax Avenue. At Colfax Avenue approximately 440 cfs of flow is intercepted by an 84-inch storm sewer. The 84-inch storm sewer follows Colfax Avenue west to Toll Gate Creek. The drainageway upstream of Colfax Avenue consists of an open channel through an open field to a storm sewer leading up Granby Street. At this point the storm sewer continues east of Chambers Road 1n the Hinkley High School parking lot. High flows 1n Granby Drainageway will overtop Chambers Road and flow down Granby Street. Upstream of Hinkley High School, the initial surface runoff 1n the Granby Drainageway crosses under the Highline Canal in a 36-inch storm sewer, while larger flows will pond and ultimately overtop the Canal. Between the Highline Canal and 6th Avenue, Granby Drainageway is confined within a City-owned drainageway easement. Upstream of 6th Avenue, the City recently built a 100 acre-foot detention basin. The basin is drained by two 24-inch storm sewer p1pes. The hydrologic effect of the detention basin was considered in this study. Above the detention basin, an open channel continues upstream under Buckley Road, and drains the central portion of Buckley Air National Guard Base. Development in the Granby Basin includes commercial and industrial buildings along Colfax Avenue, Buckley Road, and 6th Avenue; and residential and school buildings throughout the remainder of the basin. Buckley Air National Guard is located in the headwaters of the basin east of Buckley Road. In addition to the detention basin on Granby Drainageway and the Buckley Road storm sewer (Sable Drainageway) other drainage improvements have been constructed since the 1977 FHAD study and the Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS). The improvements include the installation of the 84-inch diameter storm sewer along Colfax Avenue and the Toll Gate Creek channel improvement project. The Colfax Avenue storm sewer diverts flows in Granby Drainageway to Toll Gate Creek. The Toll Gate Creek channel improvement resulted in lower 100- year flood elevations along the creek and reduces the Toll Gate Creek floodplain limits east of 1-225 at Colfax Avenue. Flood History Little information 1S available on past flooding along Granby and Sable Drainageways. Discharge measurements, high water marks and other flood data have not been officially recorded. Most published accounts of past flooding in the area are related to Toll Gate Creek, Sand Creek, and the South Platte River. When there was flooding on these other streams, however, verbal accounts indicate that Granby and Sable Drainageways were also carrying unusually high flood flows. The only known written account of flooding on Granby Drainageway was reported 1n the Aurora Advocate Sentinel on May 9, 1973. This flooding was caused by heavy rains leading to the failure of the Highline canal bank upstream from Eighth Avenue and Hannibal Drive. Several basements and streets were flooded as a result. Drainage problems have occurred at several locations 1n the Granby and Sable Drainageways. Improvements to the drainage system have addressed specific drainage concerns. For example, the Colfax Avenue storm sewer was constructed to alleviate frequent street flooding. Flood Related Studies There are several previous studies related to del ineating and mitigating flood hazards associated with Granby and Sable Drainageways. These studies have been performed by various organizations as summarized below: