FLOOD10525 - Laserfiche WebLink

Home Browse Search

and District Office in Albuquerque, and the Colorado State Engineers' Office); (4) adjusting short-period statistics to reflect long- period values if they are closely correlated; (5) considering outliers and historical data during the development of floodf1ow-frequency relatio~ships for each gaging station; and, (6) developing regional relationships of basin area to peak discharge for the 10-, 50-, 10Q-, and 500-year floods. Stream gaging record analysis was used to determine the peak dis- charge-frequency relationships for the study area on the San Juan River in the vicinity of.Pagosa Springs. Synthetic hydrograph analysis (Reference 7) was used for McCabe Creek. Because these two streams were studied by the U.S. Soil Conservation Service and the preliminary report (Reference 8) was available to the study contrac- tor, a review was conducted to check these results. The results in the Flood Hazard Study for SaIl Juan River and McCabe Creek (Refer- ence 8) are reasonable and, therefore, were adopted for this Flood Insurance Study. Peak discharge-drainage area relationships for the San Juan River, McCabe Creek, West Fork McCabe Creek, North Fork McCabe Creek, and Rio Blanco are shown in Table 2. 3.2 Hydraulic Analyses Analyses of the hydraulic characteristics of streams in the commu- nity were carried out to provide estimates of the elevations of floods of the selected recurrence intervals along each stream studied in the town and county. Cross section data for streams in the area were obtained from aerial photographs (Reference 9). The below~water sections were obtained by field measurements. All bridges and culverts were measured to obtain elevation data and structural geometry. Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit 1). For stream segments for which a floodway is computed (Section 4.2), selected cross section locations are also shown on the Flood Boundary and Floodway Map (Exhibit 2). '). Roughness coefficients (Manning's IIn") were established by field inspection at each cross section and checked against photographs and a recognized standard text (Reference 10). Roughness values for the main channels ranged from 0.04 to 0.045 on the San Juan River; from 0.075 to 0.1 on North Fork McCabe Creek, West Fork McCabe Creek, and McCabe Creek; and, from 0.03 to 0.04 on the Rio Blanco. Roughness values-for the flood plains ranged from O.l to O.12 on the San Juan River; from 0.085 to 0.11 on North Fork McCabe Creek, West Fork McCabe Creek, and McCabe Creek; and, from 0.045 to 0.06 on the Rio Blanco. 10