FLOOD06098 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 9 The flooded areas on the South Platte River in Adams County can be divided into two major reaches. The first reach be- gins at the downstream limits of the study, Baseline Road, and extends upstream to the Union Pacific Railroad crossing. The second major reach extends from the Union Pacific Railroad cross- ing, upstream to Franklin Street, the upstream limit of the study. Manmade obstructions to flood flows along this reach are limited in that most of the flood plain is still agricultural. Road crossings include Baseline Road, East 160th Avenue, Henderson Road, McKay Road, East 104th Avenue, and East 88th Avenue. Al- though the 100-year event will not overtop any of these struc- tures, debris will be a major problem in that bridge piers and girders will tend to collect debris leading to possible scour around the bridge piers and approach embankments. Even though the bridge structure is not overtopped, roadway embankments will be overtopped at all of these locations. Special problems were encountered upstream from East 88th Avenue in that levees were ----found to be partially effective. In order to analyze the reach from 88th Avenue to Station 810+, it was necessary to separate channel flow and the discharge of 12,300 cfs estimated to be lost over the levee along the east side of the channel. Separate water ---surface profiles were then computed for the main channel flow and the overbank flow. The overbank water surface profile is plotted on Sheet 32. Flood magnitudes greater than the 100-year magnitude can and will occur. Land improvement adjacent to the 100-year flood plain limits should also consider the chance for possible flood damage. Flooded Areas The 100-year flood plain limits and water surface profiles are shown on the attached Flood Hazard Area Delineation maps and profiles, Sheets 1 through 35. The computed 100-year flood ele- vations at each reference point are tabulated in Tables I and II. description of the extent of flooding is discussed in the fol- lowing paragraphs. Along the reach from Baseline Road to the Union Pacific Railroad, the 100-year flood plain is generally confined by natural bluffs located along the east side of Riverdale Road and along the west side of Brighton Road. The 100-year flood plain is further con- fined by manmade levees near 104th Avenue and upstream from 88th Avenue to protect gravel mining operations. The levees around these gravel pits are for the most part adequate. Levees located along the main channel below the Union Pacific Railroad bridge, however, proved to be ineffective during the 1973 flood and were so considered in the hydraulic analysis completed for this report. In the reach between the Union Pacific Railroad and Franklin Street, the flood plain is generally confined by manmade facili- ties including levees, railroad embankments and roadway embank- ments. Flood plain widths generally range from 200 feet to 1500 feet excluding those areas outside existing levees which may be subject to flooding, should the levees fail. These areas are located downstream from I-270. Other areas susceptible to shallow flooding are located west of the East 58th Avenue cross- ing where low banks allow water to back-up behind the Chicago, Rock Island and Pacific Railroad embankment. The major crossings in this reach include the union Pacific Railroad, East 74th Ave- nue, Interstate Highway 76, the Metropolitan Denver Sewage Treat- ment Plant access road, the CRI & P Railroad, York Street and Franklin Street. All these bridges will pass the 100-year flood without overtopping, assuming there is no blockage.