FLOOD01577 - Laserfiche WebLink

Home Browse Search

special significance for floodplain management and for flood insurance rates. These events, commonly termed the 10-, 50-, 100-, 500-year floods, have a 10, 2, I, 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 one year are considered. For example, the risk of having a flood which equals or exceeds the 100-year flood (1 percent chance of annual exceedence) in any 50-year period is approximately 40 percent (4 in 10); and, 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. Most of the annual floodflows on Henson Creek and the Lake Fork of the Gunnison River within the study area result from rapid springtime melting of the snowpack in the high mountain basins, which comprise the majority of the drainage area for both streams studied by detailed methods. According to available streamflow records, peak flows occur primarily from May through early July. Snowmelt may occasionally be augmented by rain. Streamflows from snowmelt runoff are characterized by sustained periods of high flow and marked daily fluctuation. Periods of thunderstorm activity normally occur during the summer months after spring snowmelt has ended. Cloudbursts are normally localized and only occasionally produce high flows in the streams. A review of available runoff records indicate only one summertime peak flow (outside the normal runoff period). A hydrologic analysis was carried out to establish peak discharge- frequency relationships for floods of 10-, 50-, 100-, and 500-year recurrence intervals for the Lake Fork of the Gunnison River and Henson Creek. Streamflow records are available from two gages, one on Henson Creek (1918-1937) and one on Lake Fork (1918-1937). The station on Lake Fork is located ! mile upstream of the Henson Creek con- fluence, and the station on Henson Creek is It miles above its mouth. Peak flows calculated from the study area were compared to thirteen other gaging and study sites in nearby similar basins. These fifteen sites were chosen for comparison because of the high correlation between basin characteristics such as slope, precipitation, elevation, size, and vegetation. Table 1 lists 5