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US 40 traverses the study area in a general north-south direction. Hydraulic structures under the highway have sufficient capacity so that floodflows are generally unaffected. However, north of Winter Park, insufficient capacity of the culvert under US 40 restricts flow from Leland Creek, on the west side ofthe highway, from entering the Fraser River. The Denver and Rio Grande Western Railroad also traverses the valley in a north-south direction. It is located at the western edge of the valley and crosses all the tributaries I included in this study except Jim Creek. Some minor ponding at these crossings is expected, although most of the culverts do not flow full. 3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods, standard hydrologic and hydraulic study methods were used to determine the flood-hazard data required for this study. Flood events of magnitudes that are expected to be equaled or exceeded once on average during any 10-, 50-, 100-, or 500-year period (recurrence interval) have been selected as having special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, and 500-year floods, have a 10-, 2-, 1-, and 0.2-percent chance, respectively, of being equaled or exceeded during any year. Although the recurrence interval represents the long-term, average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. The risk of experiencing a rare flood increases when periods greater than 1 year are considered. For example, the risk of having a flood that equals or exceeds the 100-year flood (I-percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10); for any 90-year period, the risk increases to approximately 60 percent (6 in 10). The analyses reported herein reflect flooding potentials based on conditions existing in the community at the time of completion ofthis study. Maps and flood elevations will be amended periodically to reflect future changes. 3.1 Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for each flooding source studied in detail affecting the communities and for the approximate studies of Elk Creek and S1. Louis Creek. The Worrall Engineering report developed regression equations for this area; the 100-year flow equation is Q 1 00 = 70.13 x A 0.686. Discharge values prepared by Worrall Engineering and presented in their 1983 report (Reference 1) were used in this FIS. Several key assumptions made in that analysis were reviewed during the hydrology analysis of this study. A detailed summary of the hydrologic analysis is contained in a separate Hydrology Report to FEMA (Reference 2). The Worrall report used data from eight USGS gages (Table 1) to develop flood discharges versus drainage-area curves for several recurrence intervals. Seven ofthe gages are located in the Fraser River basin, and one is in the nearby Williams Fork basin. All the gage records are affected by diversions. Worrall's hydrologic analysis added the recorded diversion flows, obtained from records of the Denver Water Board, to the USGS gage records for each peak flow date to estimate the total flow produced. This assumes that the diversion structures are non-operational during peak runoff events. This is a conservative assumption. These combined annual flow peaks were analyzed according to the log-Pearson Type III procedures described by the Water Resources Council in Bulletin 17 A (Reference 3). Floodflows for the 2-year, 10-year, 50-year, 100-year, and 500-year recurrence intervals were calculated for all the gages. These values were then used to develop the final regression equations. 6