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I I I I I I I I I I I I I I I I I I I ABSTRACT If it is determined in a later study that more storage in addition to what is beinq provided by the roadway embankments is necessary, upstream storage facilities will need to be studied. This, however, is beyond the scope of this report, since this report is designed to study the maximum limits to be encountered in the 100-year fl ood pI ai n after development occurs in Marcy Gulch. This report was prepared for the Urban Drainaqe and Flood Control District, Colorado Water Conservation Board, Douglas County, and Mission Viejo Company, and describes that portion of the Marcy Gulch drainage basin that would be inundated by a flood with a return frequency of 100 years. Fully developed site conditions were assumed, but without any mitigating features which would maintain flood discharges at pre-development levels. The flooding I imits described in this report, therefore, represent the worst possible 100-year flooding conditions after the site is developed. I. INTRODUCTION A. SCOPE OF WORK For the most part, Marcy Gulch has a well defined natural channel and, with a few exceptions, the 100-year flood is contained within its existing banks. Existing U.S. Highway 85 and both the AT&SF and D&RGW railroads will not be overtopped where they cross the Gulch. The underside of the Highline Canal structure which crosses the Gulch will be inundated ap- proximately 1.0 feet and may be subject to flood damage if no mitigating measures were imp I emented. Mit i gat i ng measures wi 11 be addressed in the Master Plan of Drainage yet to be developed. This study was prepared for the New Town of Highlands Ranch to determine the infrastructure facilities required in Marcy Gulch to support initial development, and to develop drainage and erosion control concepts for the proposed development. Maximum 100-year flood backwater conditions were also evaluated at the sites of proposed road crossings along the various reaches of Marcy Gulch. These crossings include the access road to Marcy Gulch Treatment Plant, Rangeline Road, North Highlands Ranch Parkway, South Highlands Ranch Parkway, and McLellan Drive. The predicted maximum flooding limits for conditions before and after the construction of these future road crossings are shown on the Flood Hazard Area Del ineation drawings included in the Appendix of this report. These road crossings mayor may not be used to retard flow and reduce peak discharges; therefore, the maximum water surface elevations shown on the drawings are based upon anticipated roadway elevations and not upon a storage or discharge requirement. This maximum backwater limit will be used as the control elevation for determining available storage at the roadway crossings and/or an elevation to guide in the location of development upstream of the roadway crossings. The design of outlet control facilities in the roadway embankment will be such that the max imum backwater e I ev at i on never be exceeded duri ng the IOO-year storm. This report describes that portion of the Marcy Gulch Drainage Basin that would be inundated by a flood with a return frequency of 100 years after the drainage basin is fully developed in accordance with the Development Plan for the New Town of Highlands Ranch. The IOO-year flood flows con- sidered in this report are to be taken as maximum values and do not reflect expected peak flow reductions that would result from the construction of retarding basins and other mitigating measures which will be employed during and after development of the area. In order to determine the flooding I imits for such a flow, it was necessary to eval uate the site hydrologic conditions and, through the various methods described later in this report, peak flood flow rates were calculated at key locations in the drainage basin. Hydraulic calculations were then performed to determine the water surface profile and flooding limits corresponding to these flow rates. Also, floodway limits were evaluated and backwater flood plain limits and profiles were determined for tentative culvert/embankment type of road cross i ngs. B. COORDINATION Appropriate methodologies for performing the necessary analyses were established through consultations with the Urban Drainage and Flood Control District.