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I I I I I I I I I I I I I I I I I I I Flooded Areas The 100-year floodplain limits and water surface profile for the Little Dry Creek main stem and tributaries are shown on the Flood Hazard Area Delineation Maps. The sheets are ordered to follow the main stem and Tributaries "B" and "C" beginning at the upstream end. The sheets are ordered beginning at the downstream end of the Shaw Heights Tributary. The upstream region of the main channel of Little Dry Creek, in Jefferson County, has a very narrow floodplain because the topography is dominated by the surrounding hills and ridges. The channel is predominately in a natural state except for a few small ponds and road crossings which have been constructed acrO$ it. It is at these crossings where the floodplain widens and becomes hazardous, see Sheets 1 through 5. A flow separation structure has been constructed at the Croke Canal to prevent storm flows from flowing beyond Little Dry Creekin the canal. Between Kipling Street and Hoyt Way in Arvada, Sheets 6 and 7, .the channel is being improved and a double 6' x 14' concrete box culvert is being constructed at Kipling Street as part of a residential development. Once construction of the culvert and channel sections is complete the 100-year flood will be well contained within the channel at this location. Culverts at Hoyt Way and Club Crest Drive have sufficient capacity to carry the 100-year flood without overtopping the banks or the road. Between Club Crest Drive and 80th in Arvada, Sheets 7 and 8, Pomona Lakes 2 and 3 have been relocated and now act as a side channel detention dam. This facility effectively reduces the peak flow from flood events exceeding 600 cubic feet per second, about the l5-year event, by storing exce$ runoff and releasing it at a slower, controlled rate back into Little Dry Creek. This detention facility significantly reduces the flood hazards in the reach below the dam to Sheridan Boulevard in Arvada. -26- The flood hazards shown on the Flood Hazard Area Maps, Sheets 9, 10, 11 and 12, are due mainly to undersized culverts. The worst places are at 80th Avenue, where only a 24" diameter corrugated metal pipe presently exists, and at Wadsworth, Webster Way, Pierce, and Marshall Streets. Culverts at the latter locations are sized for only a 5-year event. The channel section between Wadsworth and Sheridan has been improved as a green belt and includes an 8 foot wide asphalt trail adjacent to the creek. The channel itself is adequate to convey the 100-year flood except at the culverts. The flood flows in Little Dry Creek are increased at the upstream side of Sheridan by flows entering from Lake Arbor, Sheet 12, and on the downstream side of Sheridan Boulevard by flows entering from the Shaw Heights Tributary, Sheet 13. It is at Sheridan where the channel section suddenly becomes grossly inadequate, able to convey only minor flows. Between Sheridan and 72nd Avenue, the channel passes through medium and high density residential areas. Because no right-of-way has been deeded to the City of Westminster, the channel encroachment has been severe and maintenance non-existent. The 100-year floodplain in this area becomes much wider and includes many single and multi-family structures, Sheets 13 and 14. Between 72nd Avenue and Federal Boulevard in Westminster and Adams County, Sheets 15 and 16, the channel meanders through commercial and industrial properties. The floodplain is wide and velocities are generally lower than 7 feet per second. Culverts beneath Irving Street, West 69th Avenue, Lowell Boulevard and Osceola Streets carry only minor flows and are subject to washing out during higher flows. At Federal Boulevard, a 10' x 10' concrete box culvert has been placed beneath the overpass embankment. The floodplain is wide and deep upstream of the Federal overpass because the embankment effectively dams the channel. Excess floodwaters are forced beneath the overpass at the railroad tracks. The embankment at Federal Boulevard causes depths of up to 20 feet at some points and the resulting floodplain includes numerous residential structures.