FLOOD06039 - Laserfiche WebLink

Home Browse Search

low flow water surface elevation since the surveys were conducted by aerial photogrammetric methods. The profiles on plates 14 and IS show, in addition to the water surface profile of the la-year, 50-year, lOa-year, and SOO-year floods, the water surface profile of the maximum release from Boulder Reservoir. Because Boulder Reservoir has two outlet works to release water, the channels of which join at reference point I lA, the maximum release water sur- face profile is composed of a 730 cubic feet per second (c.f.s.l reach and.a 1,100 c.f.s. reach upstream and downstream from refer- ence point I lA, respectively. This water surface profile has been shown until it Intersects the lOa-year flood profile. This occurs at reference point 34. The frequency of occurrence of the maximum release from Boulder Reservoir Is unknown. This release may occur each year or it may never occur. The hydrologic analysis to deter- mine the magnitude of the la-year, SO-year, lOa-year, and SOO-year floods assumed a zero release from Boulder Reservoir, thereby reflecting only runoff conditions downstream from the reservoir. Because of these condItions, the water surface profile for the maximum release from Boulder Reservoir is higher than the SOO-year flood upstream from reference point 12 and higher than the lOa-year flood upstream from reference point 34. We, therefore, recommend that flood plain management considerations such as zoning and flood plain construction do such in accordance with water surface eleva- tions reflecting the maximum reservoIr release or the lOa-year flood - whichever is higher. . The water surface elevations of the lOa-year and SOO-year floods were used to determine flood limits on the cross sections and topographic maps. The results are shown on the Flooded Areas, plates 3 through 13. Plates 3 and 4 also show the flood limits resulting from the maximum release from Boulder Reservoir. An index to the Flooded Areas plates is on plate 2. Representative cross 22 sections are Illustrated on plates 28 through 33. The cross sec- tions show ground elevations across the valley on both sides of the channel and the depths of overbank flooding. Reference points, coinciding with the locations of the cross sections and pertinent planimetric features are shown on the Flooded Areas and profile for correlation between the drawings. Table 3 provides data at refer- ence point locations, Including distance along the main channel centerline, elevations of the streambed, and elevations of the 10- year, 'SO-year, lOa-year, and SOO-year floods. The Flooded Areas show the area that would be flooded by the lOa-year and SOO-year floods and the maximum release from Boulder Reservoir where the maximum release Is larger than the 100- year flood. The flood limits were located at each cross section and the Intervening flood outlines were drawn based upon detailed topographic mapping, engineering judgment, and field observations. It is, however, possible that more or less flooding should be shown on the Flooded Areas. For a specific situation, where more detailed accuracy of flooded area Is required, the flood limits can be more accurately established by determining the water surface elevation from the profile or reference table and then .Iocatlng that elevation by survey on the flood plain. The detailed topographic mapping util ized to assist In delineating the outline of the lOa-year and SOO-year floods has a scale of I Inch equals 200 feet with a 2-foot contour interval. These maps were prepared in December 1976 by Parker Aerial Surveys of Denver, Colorado under contract with the Colorado Water Conserva- tion Board in cooperation with Boulder County. These maps, now used for administrative purposes, are on file and available for public inspection at the Publ ic Works Department, Boulder County Courthouse Annex, Boulder, Colorado. 23