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I I I I I I I I I I I I I I I I I I I Immediately downstream from Federal, the creek crosses beneath the Colorado and Southern railroad in a 48" diameter concrete pipe, Sheet 16. The floodplain extends on both sides of the railroad tracks while floodwaters gradually spill over the tracks to the main channel section on the north side. The profile on Sheets 17 and 18 is of the water conveyed by the main channel only. Water surface elevations for the overland areas are identified by the wavey water surface contour lines on the plan view. Between the confluence and 64th Avenue, Sheet 18, the channel is severely constricted by 72-inch and 84-inch diameter culverts which limit the capacity of the channel to about 300 cubic feet per second. Upstream from 64th, the channel is narrow and shallow, and the 100-year floodplain becomes quite wide. As floodwaters leave the Little Dry Creek channel, they flow across agricultural, residential and industrial properties before entering Clear Creek. The flood damage potential in this reach is extreme as the channel section is insufficient for even a two-year event. Shaw Heights Tributary Tributary "A", or the Shaw Heights Branch, of Little Dry Creek joins the main channel at the northwest corner of 76th and Sheridan in Westminster. The channel is little more than a roadside ditch as it parallels 76th Avenue and the 100-year peak flow is 1880 cubic feet per second. The floodplain, shown on Sheets 13, 21 and 23, includes a residential area between 75th and 76th. Floodwaters are carried by the streets in this area with depths sufficient to cause backwater between and behind the structures. Upstream from 76th, the channel is well defined and contains the 100-year flood flow to 80th Avenue, Sheets 21 and 22. Two 3.5' x 6' semi-circular culverts have been constructed beneath 80th and these are connected to a 7.7' X 6' box culvert constructed beneath the parking lot of a small shopping center. The combined capacity of these culverts is about 220 cubic feet per second, insufficient for even a two-year storm. The channel is concrete lined upstream from 80th and multi-family residential structures have been constructed in the floodplain. - 27- At the Colorado and Southern Railroad crossing, Sheet 25, two 48" diameter corrugated metal pipe culverts have been placed in the fill to pass the storm flows. For floods with peaks exceeding about 220 cfs, floodwaters will follow the railroad tracks in a shallow swale and gradually spill over the tracks. The floodplain area outlined on Sheet 24 includes an area of sheet flow extending from the railroad tracks through a multi-family residential development. The historic channel upstream from the railroad tracks has been completely obliterated by residential development. A 36-inch diameter concrete pipe extending from the tracks to U.S. 36 has been installed to carry low flows. Excess storm runoff originating upstream is carried primarily by streets throughout the remainder of the study reach, Sheets 27 and 28. The capacity of the detention pond just east of U.S. 36 is insufficient to contain the 100-year storm runoff generated by the contributing residential area. Because the pipe beneath U.S. 36 has negligible capacity, the floodwaters will pond behind the highway embankment, submerge the spillway and flow across U.S. 36. The potential flood hazard in this situation is severe. Tributary "B" The upper sections of Tributary "B" were still primarily undeveloped at the time this study was undertaken. However, residential development is taking place rapidly in this area, which will increase the runoff significantly. From Church Lake, flow in the tributary follows the south side of 84th Avenue. At the two foot diameter pipe at Yukon Court, Sheets 29 and 30, flood flows of any magnitude are forced over the street after ponding behind 84th Avenue. A 9.0' by 4.2' concrete box culvert has been constructed beneath Wadsworth Boulevard, but because of restricted channel capacity downstream, the tail water controls the capacity of the culvert and flood waters from events larger than the five year will overtop Wadsworth Boulevard. Downstream from Wadsworth, the 100-year floodplain widens out to approximately four hundred feet. A golf course has been developed between Webster Way and Otis Drive. Low flows are carried beneath the