FLOOD10696 - Laserfiche WebLink

Home Browse Search

3.0 ENGINEERING METHODS For the flooding sources studied by detailed methods in the community, standard hydrologic and hydraulic study methods were used to determine the flood hazard data required for this study. Flood events of a magnitude that are expected to be equaled or exceeded once on the average during any 10-, 50-, 100-, and SOD-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 O.2-percent annual 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 I-percent annual chance flood 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 of this 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 by detailed methods affecting the community. The discharges on the Big Thompson River, the North Fork Big Thompson River, and their major tributaries were developed based upon a combination of gage records and regression equations contained in the Colorado Water Conservation Board's Technical Manual No.1, prepared by the U.S. Geological Survey (Reference 10). The locations of the stream gages analyzed and their respective years of record are as follows: Drake gage located on the Big Thompson River (47 years), Big Thompson River below Lake Estes (17 years), Big Thompson River above Lake Estes (27 years), and North Fork Big Thompson River at Drake (30 years). The gage records were analyzed using the log-Pearson Type ITI distribution as recommended in the U.S. Water Resources Council Bulletin 17 (Reference 11), and the discharges were adjusted as recommended in Technical Manual No. 1 (Reference 10). The Storm Water Management Model (SWMM) (Reference 12) was used to determine discharges on drainage basins that are representative of the smaller Big Thompson River and North Fork Big Thompson River tributaries. These discharges were plotted on semi- log paper to develop discharge-drainage area curves for the region. These curves were entered with the drainage areas of the smaller tributaries, and the appropriate discharges were tabulated. 19