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reaches (Reference 7). Additional cross sections used for the Fraser River were taken from field surveys (Reference 8). For Cooper Creek, Mary Jane Creek, Leland Creek, and Little Vasquez Creek, digitized sections from aerial photographs flown in June 198 I were used where photography was available. Where photography was not available, field surveyed cross sections were utilized (Reference 8). All bridges, dams, and culverts were field surveyed to obtain elevation data and structural geometry . Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibit I). For stream segments for which a floodway was computed (Section 4.2), selected cross-section locations are also shown on the FIRM. Roughness factors (Manning's "n" values) used in the hydraulic computations were chosen by engineering judgment and based on field observations of the streams and floodplain areas. They were also checked using a regression developed by the CWCB (Reference 10). The regression was developed for mountain streams under flood conditions. Values from the regression are for a bankfull situation and, therefore, include the effect of the dense bank vegetation commonly found in this area. Values of roughness factors computed using the regression ranged from 0.060 to 0.075. These are higher than normally associated with a channel roughness factor of this type of stream, but are explained by the fact that the regression includes the full effect of bank vegetation, whereas the roughness factor is generally selected only for the characteristics ofthe bed of the main channel. The impact of the vegetation is increased because of the relatively narrow, 20- to 40-foot stream channels in the study area. The regression was used as a guideline; standard procedures (Reference 1 I) were used for the final selection. Photographic documentation of channels and overbanks was used extensively in this process. Roughness values for the main channel of the Fraser River and its tributaries ranged from 0.035 to 0.050, while floodplain roughness values ranged from 0.080 to O. I 10 for all floods. All the streams are supercritical. Therefore, critical depth was used as the starting WSEL. The only exception is the upper reach of the Fraser River, which is controlled at the downstream section by a box culvert. Starting WSELs at the upstream end of the culvert were calculated using a rating curve developed from standard nomographs from the Federal Highway Administration (FHW A). Floodflows along Leland Creek escape the channel on the left overbank and cause shallow flooding along the west embankment of US 40. This shallow flooding, with an average depth of less than 1.0 foot, was determined from normal-depth computations. Discharges downstream of US 40 on Leland Creek are limited to 8 cfs for the 10-, 50-, 100-, and 500-year events. Since flooding is contained within the channel downstream of US 40, no profiles are presented for that portion of Leland Creek. Manning's equation methodology was used to estimate depths for the approximate study of Elk Creek (Reference 12). Three cross sections were spaced an average of600 feet apart. No cross sections were evaluated within areas upstream of stream crossing structures. The approximate study of the upstream portions of Elk Creek, St. Louis Creek, and the Fraser River was based on an assumed depth. Appropriate ranges of estimated flood depths were determined for each stream reach by examining the detailed hydraulic analyses on the Fraser River and Leland Creek and the Manning's equation calculations performed on Elk Creek. A safety factor also supplemented the values. The ranges were conservative in the interest of public safety. Backwater effects of stream crossing structures, which further 10