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I I I I I I I I I I I I I I I I I I I C. METHODOLOGY The drainage basin is contained wholly within Douglas County and is crossed by rights-of-way owned by the State Highway Department, AT&SF Railroad, D&RGW Rai 1 road. Easements for the City of Aurora and the Denver Water Board also cross this drainage basin. Marcy Gulch basin is contained wholly within Urban Drainage and Flood Control District jurisdiction. This report addresses only that portion of the drainage basin flood limits which lie upstream of the D&RGW Railroad. The basic mapping was made at 1 inch = 40 feet scale with one-foot con- tours, and subsequently reduced to 1 inch = 200 feet scale for use in this report. Land use data were obtained from the approved Development Plan for the New Town of Highlands Ranch. The Marcy Gulch drainage basin was divided into 12 drainage sub-basins (see Basin Boundary Map), and peak discharges at the downstream end of each of these sub-basins were deter- mined using the Colorado Urban Hydrograph Procedure (CUHP). Water surface profiles and flood plain limits, as well as floodway limits due to en- croachments, were determi ned us i ng the HEC-2 Water Surf ace Profi 1 es com- puter program. The reader shou 1 d refer to the fo 11 owi ng sect ions for greater detail on the hydrologic and hydraul ic analyses used and to the tables, figures and drawings included in the Appendix of this report for the results of these analyses. B. MARCY GULCH Marcy Gulch basin consists of four major waterways (reaches) which are all tributary to the basin's outfall point. The extent and relationship of these reaches can best be seen on the previously referenced Basin Boundary Map in the Appendix. The channels delineated on this map vary considerably along their lengths in terms of channel cross section, side slopes, stream bed (thalweg) slopes, roughness and channel material. Detailed mapping as described elsewhere in this report has been prepared for the hydraulic analyses of these major reaches. II. DRAINAGE BASIN CHARACTERISTICS A. GENERAL The New Town of Highlands Ranch Development Plan delineates that major reaches and significant surrounding area will remain in non urban use; therefore, for hydrologic analysis, the hydraulic lengths and slopes of the major basin channels are assumed not to change between existing and future development conditions. It should be noted that the presence of a number of ex i st i ng check dams with in the Marcy Gu lch drainage bas i n have been consi dered in determi ni ng the maximum water surface profi 1 e and IOO-year flooding limits since this would represent the worst possible flooding cond it i on. The Marcy Gulch drainage basin consists of approximately 4.25 square miles of undeveloped area in the northwest corner of Dougl as County, Colorado. Currently, the gulch carries runoff following precipitation, but it is normally dry. Further study is needed to determine if development will cause more frequent runoff due to urbanization characteristics. The basin originates at Section 23, Township 6 South, Range 68 West (see Vicinity Map, Basin Boundary Map and Sheet Index, sheet 1 of 12 in Appendix), and extends approximately 6.5 miles in a northwesterly direction to its eventual outfall point at the South Platte River. The basin is presently an open undeveloped area, but will be developed over approxi- mately a twenty to twenty-five year period. Development in this basin will consist of single and multifamily residences, commercial, light industrial, school sites and nonurban areas and is expected to increase runoff rates over existing condition flows. Existing culvert-type road crossings at U.S. Highway 85, and the AT&SF and D&RGW Railroads, as well as the Highl ine Canal crossing have been included in the hydraulic modeling of the Marcy Gulch flood plain. Proposed future culvert/embankment type road crossings have been located and have also been analyzed for their potential backwater effects on the flood plain. The topography of the Marcy Gulch basin ranges from rolling rangeland in the majority of the basin to steeply-eroded, northwest-trending ridges in