FLOOD00734 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 24 VII. DESCRIPTION AND DELINEATION OF THE lOa-YEAR FLOOD HAZARD FHAD Sheets 1 through 40 illustrate the thalweg (valley bottom) profile of the channel, the water surface elevations of the la-year and lOa-year floods, and the areas which would be inundated by the lOa-year flood. FHAD Sheets 41-50 show only the plan of the lOa-year flood plain for the study area east of Riverdale Road to the South Platte River. The remainder of the text in this Section presents des- criptions of flooding characteristics to supplement the FHAD Sheets. The descriptions are keyed to the sheets as well as to geographical locations. Figures 13, 14 and 15 show photographs of the channels at selected points. The lOa-Year Flood Presented at the end of this Section are the Flood Hazard Area Delineation (FHAD) Sheets illustrating the estimated extent of flooding which would occur on Brantner Gulch, its tributaries and the Regional Park Tributaries in the event of a lOa-year flood. No records are available to indicate whether or not floods of the magnitude described herein have occurred in these drainage basins. However, statistically a rainfall equal to the lOa-year storm could occur at some time. The hydrologic and hydraulic analyses reveal that such a storm will generate the flood depicted in this report. Table 3 presents a tabulation of data relating to antici- pated flooding in a lOa-year storm. Table 3 also presents data describing a "floodway" along each channel. The floodway is that portion of the channel which carries the majority of the flood flow. It is defined as the remaining channel, after a hypothetical encroachment of land is allowed on each side of the channel, which will not cause more than a nominal rise in the water surface or energy grade over the natural lOa-year flood. The allowed nominal rise in energy grade used in these computations was approximately one-half foot. Where possible, the hypothetical encroachments are allowed only so far as they remove an equal conveyance of floodwater from each side of the channel. Brantner Gulch from the Union Pacific Railroad to Colorado Boulevard (Sheets 1-3) This reach of Brantner Gulch consists of two detention dams, Eastlake Reservoir No. 2 and Eastlake Reservoir No.3, and appurtenant discharge channels at the western region of the reach. Downstream, flow travels in a small open roadside earth channel paralleling the north side of East 124th Avenue from Steele Street east to Colorado Boulevard. Presently, rainfall runoff from the watershed west of the Union Pacific Railroad flows under the railroad bridge, without constriction, into Eastlake Reservoir No.2. This dam will contain the lOa-year storm from its upstream watershed with only a minor discharge flow from the principal spillway.