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I I I I I I I I I I I I I I I I I I I V. FLOOD HAZARD AREA DELlNEATIor: The lOa-year flood discharges shown in the figures mentioned above are conservative in that the highest discharge rate calculated for a subreach is used for the entire subreach length, not just for the point for which it was calculated. In general, discharge values were selected such that they increased instep f ashi on as the fl ood progressed downstream. One exception to this is near the upstream end of the study area (just north of South Highlands Ranch Parkway) where a slight reduction in the peak discharge occurs. This exception is attributed to the change in input parameters for the CUHP computer program (i .e., change in % imperviousness). A. FLOOD FREQUENCY AND DISCHARGE Floods have occurred in the past and will continue to occur in the future along Marcy Gulch with magnitudes equal to and greater than those that have already occurred. As development progresses within the drainage basin the flOOding potential will increase and actual flooding will occur more frequent 1 y. The Marcy Gulch drainage basin has been analyzed to determine the discharge at various key locations along the flood pl ain for the lOO-year flood event. The IDO-year flood event is defined as that which has a one percent chance of being equalled or exceeded in anyone year. The 100-year flood is considered by the Urban Drainage and Flood Control District, the Colorado Water Conservation Board, and the Federal Insurance Administration, as the flood magnitude for which flood plains should be designated for regulatory and improvement purposes. In Colorado, the 100-year flood plain is an area of state interest as defined in H.B. 1041. Lesser floods with higher probabil ities of occurrence have, therefore, not been considered here. Flood magnitudes greater than the lOa-year flood magnitude can and will occur and plans for land improvement adjacent to the lOO-year flood plain should consider the probability of flood damage. Sample flood hydrographs at key locations are shown in Figures 5 through 7 in the Appendix. B. FLOOD PLAIN AND FLOODWAY DATA The Marcy Gulch flood plain has been divided into four reaches in order to more readily identify the various tributary branches of the waterway. Refer to Basin Boundary Map in the Appendix of this report for exact locations of each of the four reaches. The lOO-year flood plain limits and water surface profiles for each of these reaches of the Marcy Gulch flood plain are shown on the Flood Hazard Area Del i neat i on drawi ngs, Sheets 2 through 12 (see Append i x), and the computed lOa-year flood elevations for the various cross section locations are tabulated in Tables 3 and 4 as discussed earlier. A brief description of the flooding for each of these reaches follows: The peak flow profiles given in Figures 8 through 10 in the Appendix show the discharges developed in the hydrologic portion of this study for the laO-year flood event and are the discharges used in the hydraulic analYSis portion of the flood hazard area delineation. The discharge values shown are those which could occur under fully developed conditions without any mitigating measures being taken to control or retard the increased runoff. A subsequent report will address reduced discharge rates which are more representative of final design conditions and will describe the mitigating measures which will be taken to achieve a discharge rate which should not exceed pre-development conditions at the downstream edge of the New Town of Highlands Ranch development, i.e. U.S. 85. Reach 1 - The lOO-year flood pl ain and water surface profiles for Reach 1 are shown on the Flood Hazard Area Deli neat i on drawi ngs, sheets 2 through 11. The downstream terminus of Reach 1 is located at a secondary road just west of the Denver and Rio Grande Western Railroad (D&RGW). This secondary road, with an inadequate culvert, creates a backwater and ponding condition between the D&RGW and AT&SF railroad rights-of-way. In addition, water is backed up and ponded between the AT&SF railraod and U.S. Highway 85. Present construction of culvert extensions under these railroads and U.S. 85 required establishing an anticipated future thalweg between these structures. The culverts at the highway and both railroad crossings were found to be adequately sized to carry the 100-year flood. 10