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r . I -- I I. ,- I. Overview This hydrologic investigation was undertaken to provide a basis for floodplain delineation within the Corporate Limits of the City of Cortez, Colorado. Currently, there are five designated Special Flood Hazard Areas within the city limits, which are regulated under the National Flood Insurance Program. Although the Special Flood Hazard Areas are shown on the Flood Insurance Rate Map (FIRM), no detailed study was performed. This report will provide the hydrologic basis for a detailed study, which will provide new floodplain delineations intended to replace the currently ;regulated flood hazard areas. In addition to providing more accurate floodplain data, the flood hazard area delineations will be expanded to reflect annexations into the city limits, which have occurred since the time of the original effective date. II. Methodology '-----'\, The SCS Curve Number Runoff Method was employed to generate rainfall-runoff simulations. The simulations were carried out using PondPack 9.0 software by Haested Methods. The Curve Number Runoff Method takes into account the hydrologic soil group, the nature of development, and the state of existing vegetation to determine a runoff depth for a given amount of rainfalL The Curve Number Runoff Method uses the SCS Unit Hydrograph to generate actual runoff hydro graphs for various storm return frequencies. . III. Rainfall Rainfall totals selected for analysis include the 2-, 10-, 125-, 50-, and IOO-year '24-hour events. The total rainfall for these events was taken from the NOAA Atlas 2, Precipitation Frequency Atlas of the Western United States, (1973). The temporal distribution of the rainfall is given by the SCS Type II 24-hour rainfall distribution. Figure 1 presents the cumulative rainfall depths over the 24-hour duration. IV. Basin Delineation Five major basins have been delineated within the City of Cortez corporate limit, namely the Carpenter Wash, Denny Lake, Glade Draw, South Central, and Wal-Mart Major Basins. The major drainage basins are identified on Figure 2 (City of Cortez Hydrologic Exhibit) along with the Special Flood Hazard areas, to be delineated. Each of the Major Basins was subdivided into basins, which were delineated based on physical drainage features- and locations of structures within the Major Basins. The basins were delineated to facilitate hydraulic modeling of the drainage courses. V. Curve Number Selection To assign appropriate curve numbers to each basin, the basin delineations were overlaid onto a soils map taken from the Soil Survey of Cortez Area, Colorado, Parts of Dolores and Montezuma Counties (USDA & NRCS). The map det~ls the soil map units throughout the area of interest. Using ArcView, queries were performed to determine the 1