Laserfiche WebLink
<br />and District Office in Albuquerque, and the Colorado State Engineers' <br />Office); (4) adjusting short-period statistics to reflect long- <br />period values if they are closely correlated; (5) considering outliers <br />;>/Inn hi c::!t-nrit"";:Il rl~i-~ nnring t:hp- deYeJ.o-p-~t of floodflow-:trequencv <br />relationships for each gaging station; and, (6) developing regional <br />relationships of basin area to peak discharge for the 10-, 50-, 100-, <br />and 500-year floods. <br /> <br />Stream gaging record analysis was used to determine the peak dis- <br />charge-frequency relationships for the study area on the San Juan <br />River in the vicinity of pagosa Springs. Synthetic hydrograph <br />analysis (Reference 7) was used for McCabe Creek. Because these two <br />streams were studied by the U.S. Soil Conservation Service and the <br />preliminary report (Reference 8) was available to the study contrac- <br />tor, a review was conducted to check these results. The results in <br />the Flood IIazard Study for San Juan River and McCabe Creek (Refer- <br />ence 8) are reasonable and, therefore, were adopted for this Flood <br />Insurance Study. <br /> <br />Peak discharge-drainage area relationships for the san Juan River, <br />McCabe Creek, West Fork McCabe Creek, North Fork McCabe Creek, and <br />Rio Blanco are shown in Table 2. <br /> <br />3.2 Hydraulic Analyses <br /> <br />~ses et" the hydraulic characteristics et" streams in the commu- <br />nity were carried out to provide estimates of the elevations of <br />floods of the selected recurrence intervals along each stream <br />studied in the town and county. <br /> <br />Cross section data for streams in the area were obtained from aerial <br />photographs (Reference 9). The below..water sections were obtained <br />by field measurements. All bridges and culverts were measured to <br />obtain elevation data and structural geometry. <br /> <br />Locations of selected cross sections used in the hydraulic analyses <br />are shown on the Flood Profiles (Exhibit 1). For stream segments <br />for which -a flOodwayis c:oinputed (Sec:tion 4.2);--selected cross . <br />section locations are also shown on the Flood Boundary and Floodway <br />Map (Exhibit 2). <br /> <br />. <br /> <br />Roughness coefficients (Manning's "n") were established by field <br />inspection at each cross section arid checked against photographs and <br />a recognized standard text (Reference 10). Roughness values for the <br />main channels ranged from 0.04 to 0.045 on the San Juan River; from <br />0.075 to 0.1 on North Fork McCabe Creek, West Fork McCabe Creek, and <br />McCabe Creek; and, from 0.03 to 0.04 on the Rio Blanco. Roughness <br />values "for the flood plains ranged from 0.1 to 0.12 on the San Juan <br />River; from 0.085 to 0.11 on North Fork McCabe Creek, West Fork <br />McCabe Creek, and McCabe Creek; and, from 0.045 to 0.06 on the Rio <br />Blanco. <br /> <br />. <br /> <br />lO <br />