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- . . Detailed flood information before 1900 is limited. Information is based on newspaper accounts and interviews with longtime residents of the area. Large floods occurred at Florence in 1909,1921,1923,1932,1941,1948, 1949, and 1965. These floods caused major damage, disrupting highway and railroad traffic and communication services, drowned livestock, and destroyed agricultural lands, roads, bridges, and buildings. Several persons lost their lives, and others were injured. The flood on June 1921 is the most severe peak flood recorded in the area. The u.S. Geological Survey (USGS) estimated peak discharges at 2,760 cubic feet per second (cfs) for Oak Creek and 3,720 cfs for Coal Creek. The flood on July 1923 was conservatively estimated to be 11,250 cfs for Oak Creek. The Weiland Engineering Company did the estimate following the flood. The flooding problems in the city of Florence are aggravated by restriction of drainage channels due to vegetation growth and accumulated debris, plus siltation of major culverts under the Denver and Rio Grande Western Railroad (D&RGW Railroad) and street drainage problems. Floodwaters in Oak Creek are forced from the channels by inadequate bridge openings and channel capacities, and the water fans out over much of the city. MAPPING AND ROUTING CROSS-SECTIONAL DATA u.S. Geological Survey (USGS) 7.5-minute-quadrangle maps served as the basis for drainage area delineation, measurement of basin parameters, and evaluation of channel flow lengths for routing reaches used in the HEC-l applications. Cross sections used to route flow in the HEC-l watershed computer model are a composite of data from the USGS mapping and field measurements. HYDROLOGIC ANALYSIS Hydrologic Watershed Model The Albuquerque District used the HEC-l Flood Hydrograph Package computer program, developed by the U.S. Army Corps of Engineers Hydrologic Engineering Center, to model the Coal Creek and Oak Creek watersheds to determine the 50.0%-, 20.0%-, 10.0%-,4.0%-,2.0%-, 1.0%-, and 0.2%-chance floods at selected locations within the watersheds for the future development of flood hazard areas. The following sections discuss unit hydro graph methodology, channel and detention pond routing techniques, precipitation amounts, infiltration, and the rainfall distribution applied to the Coal and Oak Creek watersheds. Unit Hydrograph Methodology The Albuquerque District developed unit hydrograph parameters based on the Natural Resources Conservation Service (NRCS) Dimensionless Unit Hydrograph method. Exhibit 1 shows the subareas in the Coal and Oak Creek watersheds. The basin lags and steam slope measurements were extracted from the USGS 7.5-minute quadrangle maps. The Albuquerque District computed the time of concentration based on the Kirpich formula used in the development of the NRCS triangular unit hydro graph: 5