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VII. HYDRAULIC ANALYSIS TIMPAS CREEK, OTERO COUNTY, COLORADO YEAR DATE PEAK DISCHARGE (cfs) 1903 June 8 11,200 1927 July 7,700 1942 Aug. 14 4,300 1945 Aug. 15 4,580 1950 July 24 9,510 1953 July 12 10,000 1956 July 23 15,500 1958 July 5 23,000 Water surface profiles have been computed for the study reach for the 10., 50., 100-, and SOD-year frequency floods. The flows arc listed in Table-3 presented on pages 13 through 17. The hydraulic characteristics of the study reach were documented based on a field investigation. Culvert and bridge geometry, channel and overbank roughness coefficients, flow obstructions and the overall characteristics of the area were assessed during this investigation. The Manning's roughness cocfficeints used in the hydraulic analysis were documented in a separate report prepared as part of the technical addendum. Photographs were included in the report to support the recommended values. Recommended roughness coefficients range from 0.030 to 0.035 for the channel and from 0.040 to 0.090 for the overbank areas. (*Information for these Tables taken from Reference 9) The water surface elevations were coroputed using the U.S. Army Corps of Engineers HEC-2 St"p a..ckw..t..r Computf'T ProgrAm (Reference 11). The locations of the field surveyed cross sections, digith;ed cross sections from the rectified photographic maps of the Arkansas River, and the digitized cross sections from the topographic maps of Timpas Creek are shown by stream stationing on the flooded area maps (Plates 3 through 11) and the flood profile~ (Plates 12 through 22). The str"arn stationing, 100-YI!ar flood elevations as "'-ell as the 10 SO 100, and 500.year flood discharges ar~ also listed in Tabie 3' Pages 13 through 17. ' V. FLOOD CHARACTERISTICS Flooding on the Arkansas River results from the annual snowmelt in its headwater regions, which is augmented by pe:riods of very heavy rain. Historical data indicates that peak discharges on the Arkansas River and Tilllpas Creek within the study area are closely related to heavy thunderstorm rainfall originating downstream of Pueblo. ThiB type of cloudburst flow in the river is characterized by high peak flows of short duration. Flooding due to snowmelt is usually abated as the river reaches the plains, and has had little impact upon the study area. Pueblo Reservoir controls approximately 40% of the study watershed, consisting of the upper reaches and headwaters of the Arkansas Rivl!r. This major flood control structure hall a large impact on flood flows in the vicinity of Pueblo, but will have only limited effect on flood flows in the atudy area. The water surface elevations presented computations which assume no reduction capabilities due to debris. W"ter increase if significAllt debris blockage in Tablc-3 are based on in the bridge "ooveyanc_", surface elevations may should occur. VI. HYDROLOGIC ANALYSIS The 10., 50- 100., and SOO-year flood events "'ere used as the flood frequenciea for this floodplain analysis. Thus the data developed in this report will be compatible not only for regulation purposes, and H.B. 1041 designation but also for Federal Insurance Administration flood insurance rate studies. These various flood events have an IivI!r"S" occurrence of once in th.. number of years IlS indicated. For example, the 100-year flood occurs, on the average, once in a 100 year period, and has a one percent chance of being equaled or exceeded in any given year. The 10, SO, 100, 500-year discharges for the Arkansas River and Timpas Creek are shown in Table 3. The hydrological analysis for the Arkansaa River was performed by the U.S. Army Corps of Engineers in 1978. Their ..nlllYlli8 was based on statistical analyses of streamflow records covering up to an 80wyear period at selected gaging stations operated by the U.S. Geological Survey (Reference 10). The flood flows for Timpas Creek ",ere provided by the Colorado Water Conservation Board, and are based on sn anslysia of stream flow records using Colorado Water Conservation Board Technical Manual No.1. Detailed description of the hydrologic analyses are docum",nted in the Technical Addendum. Thc particular L:ses for thc various flood events in addition to those stated above lire as follows: -12. -11-