FLOOD07458 - Laserfiche WebLink

Home Browse Search

HYDROLOGIC AND HYDRAULIC DETERMINATIO~S TABLE 2 FLOOD DISCHARGES Hydrologic Analysis An hydrologic analysis was performed to co~pute the peak discharge-frequency relationships for floods of 10-, 50-, 100- and SOD-year recurrence intervals for the Mancos River and Chicken Creek. The flood peak estimates for the Mdncos River were based on a flow frequency analysis prepared by th eU.S.Army Corps of Engineers. For the Mancos River, the f100d peak estimates were determined from a regional analysis of peak flow data from stream gaging stations in the area. The 10-, 50-, 100- and 500-year flood peaks for the ~~ncos River were computed and are listed in Table 1. Chicken Creek at Mancos, Colorado (Drainage Area: 22.3 square miles) Flood Frequency Flood Discharge 10-year 490 of; 50-year 800 of; IOO-year 950 of; 500-year 1,370 of; TABLE 1 FLOOD DlSCHARGES The 100-year flood has a I-percent chance of being equalled or exceeded in any given year and a frequency of occurrence of about once in 100-years on the long-term average. 5imilarly, the 10-year, 50-year and SOD-year floods have aID-percent, 2-percent and 0.2 percent chance of being equalled or exceeded in any given year, and an average frequency of occurrence of about once in the number of years indicated. Ilrdncos River at Mancos, Colorado (DrdinageArea:33squaremiles) Flood Frequency Flood Discharge 10.year 1,250 cfs 50-year 2,230 of; :OG-y"ar "7On .<. ~,' ~~ ,- SGG-year 4,420 of; A detailed description and summary of the hydrologic analysis and co~utations are provided in the Technical Addendum on file with the Colorado ~ater Conservati~~ ~0o~d. Tne flood flows for Cnicken Creek were computed by the ,"udy consultant, usingtheregionalizedbasinpardlnetersdeterm;nedbytheU.S.Corpsof Engineers for the Mancos River. Comparative hydrologic calculations 'o'/ere also made for the Chicken Creek basin using the Manual for Estimating Flood Characteristics of Natura.l FlowStredlTIsinColoradO,aswell as unit area discharges. J:l1.draulicAnalysiS Analyses of the hydraulic characteristics of the Mancos River and Chicken Creek were completed to provide estimates of the water surface elevations for the selected recurrence intervals. The water surface elevations were computed using the Corps of Engineers' HEC-2, Water Surfa.ce Profiles step-backwater computer program. Cross sections for the backwater analysis of both streams were taken from the topographic maps prepared for the study. Digitized cross sections were obtained for the study reach at the ti~e the topographic ~~ps wereculnpiled. The locations of the cross sectiorls utilized in the study are -15- -16-