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I I I I I I I I I I I I I I I I I I I VI. FLOOD HAZARDS The 100-year flood event is considered by the State and the UDFCD as the flood magnitude for which floodplains should be designated for regulatory and drainageway improvement purposes. The 100-year floodplain limits, and the 10- and 100-year water surface profiles are shown on the Flood Hazard Area Delineation maps, contained in the rear of this report. For Granby Drainageway, the following areas have been identified as major 100-year flood hazard areas. The residential area located on the east side of 1-225 and between Sand Creek and Sable Drainageway (Sheet 1) is subject to 100-year flooding due to overflows from the Sable and Granby Drainageways. Some of the residential buildings would be inundated and basement flooding is likely to occur. The area between Colfax Avenue and Billings Street 1S subject to flooding during a 50- or a 100-year event due to the flow split which occurs at Colfax Avenue. The existing 84-inch RCP can divert some portion of the flow to Toll Gate Creek via Colfax Avenue. The majority of the 100-year flow will flow along and/or across Sable Boulevard in a northwest direction. This area is flat with minimum ground variations on the overbanks, which tends to spread the f low in a much wider area (Refer to Sheets 2 through 5). A sheet flow area is also identified for the area on the north side of Colfax Avenue and the east side of 1-225, due to the split flow along Colfax Avenue (Refer to Sheets 3 through 5). A flood hazard area adjacent to Granby Drainageway, between 12th Avenue and 8th Avenue also exists. Two single-family homes are within the 100-year floodplain, as shown in Sheet 7. Other flood hazard areas include the Highline Canal Crossing and Laredo Street Crossing due to insufficient capacity to pass the 100-year flow. For the Sable Drainageway, the major flood hazard areas include: (1) 1-225 crossing, (2) Sable Boulevard crossing, (3) 18th Avenue crossing, (4) col fax Avenue crossing, and (5) local street crossings. The flood hazard areas are shown in Sheets 10 through 13. The 1-225 cross1ng consists of two 6-foot by 10-foot concrete box cuI vert (CBC). Due to the low, north 17 channel bank profile at the culvert, an overflow can occur during a 50-year or greater flood. This overflow drains to Sand Creek and creates a flood hazard for the adjacent residential area (Refer to Sheet 1). At Sable Boulevard, the drainageway is conveyed under the road by twin 60-inch reinforced concrete pipes (RCP's), which are inadequate to carry the flow without overtopping dur ing a 1 DO-year event. The I DO-year flow w.d 1 overtop the north bank and flow across the low point of Sable Boulevard and then gradually rejo1n Sable Drainageway after flowing across 23rd Avenue. Backwater at the upstream side of Sable Boulevard will inundate the open space to the northeast side of Sable Boulevard and 23rd Avenue (Refer to Sheets 10 and 11) . The 18th Avenue cuI vert and underground storm sewer can convey no more than the 50-year discharge. Overbank flow at this location will occur during a 100-year flood. The overbank flows will drain to Chambers Road and inundate two single-family homes ---- located on the west side of Chambers Road (Refer to Sheet 12). The area between Chambers Road and 17th Avenue is subject to ]00- year shallow flooding due to insufficient culvert capacity at Helena Street and 17th Avenue. At this location, SlX commercial/industrial buildings are affected (Refer to Sheet 12). Other identified flood hazards exist adjacent to the Elkhart Street crossing (Refer to Sheet 11), Altura Street cross1ng (Refer to Sheet 12), 22nd Avenue crossing (Refer to Sheet 11), and Montview Boulevard crossing (Refer to Sheet 11), all due to culvert overtopping during a 100-year flood. Finally, the area on the southwest side of Colfax Avenue and Laredo Street 1S impacted by flood flows. The 100-year event will flow across Laredo street and inundate the area south of Colfax Avenue, and then create sheet flow to the area north of Colfax Avenue and east of Jasper Street, as shown 1n Sheet 13. For 100-year flood discharge, depth, and velocity of selected channels and floodplain cross sections, refer to the Appendix.