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. . . . . . . . . . . . . I -. . I . I ABSTRACT roadway crossings and/or an elevation to guide in the location of develop- ment upstream of the roadway crossings. The design of outlet control facil ities in the roadway embankment will be such that the maximum back- water elevation never be exceeded during the 100-year storm. This report was prepared for the Urban Drainage and Flood Control District, Douglas County, and Mission ViejO Company, and describes that portion of the Dad Clark 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 limits described in th i s report, therefore, represent the worst po ss ib 1 e 100-year flood ing conditions after the site is developed. If it is determined in a later study that more storage in addition to what is being 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 lOO-year flood plain after development occurs in Dad Clark Gulch. I. INTRODUCTION For the most part, Dad Clark Gulch has a well defined natural channel and, with a few exceptions, the 100-year flood is contained within its existing banks. Existing highway C470 (County Line Road) and the existing Highlands Ranch access road at Broadway will be flooded where they cross the Gul ch. The underside of the Highline Canal structure which crosses the Gulch would be very close to the peak water surface e 1 ev at ion and may be subject to flood damage if no mitigating measures were implemented. A. SCOPE OF WORK This study was prepared for Highlands Ranch to determine the infrastruc- ture facilities required in Dad Clark 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 Dad Clark Gulch by request of the Urban Drainage and Flood Control Di strict. These crossings include Centennial Parkway (Future C470), Canal Road, Broadway, Dad Clark Drive, North Highlands Loop (3 crossings), and West Parker Road. The predicted maximum flooding limits for conditions before and after the construct ion 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 max imum water surface e 1 ev at ions shown on the drawings are based upon anticipated roadway elevations and not upon a storage or discharge requirement. This maximum backwater 1 imit will be used as the control elevation for determining available storage at the This report describes that portion of the Dad Clark Gulch drainage basin that would be inundated by a flood with a return frequency of 100 years after the drainage basi.n is fully developed in accordance with the Develop- ment Plan for Highlands Ranch. The IOO-year flood flows considered 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 deve 1 opment of the area. In order to determ ine the fl ood ing 1 im its for such a flow, it was necessary to evaluate 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. Hy- draul ic cal cul ations were then performed to determine the water surface 1