FLOOD01813 - Laserfiche WebLink

Home Browse Search

the re~rence 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 which equals or exceeds the 100-year flood (1 percent chance of annual exceedance) in any 50-year period is approxi- mately 40 percent (4 in 10), and, for any 90-year period, the risk in- creases to approximately 60 percent (6 in 10). The analyses reported here reflect flooding potentials based on conditions existing in the county at the time of completion of this study. Maps and flood eleva- tions 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 county. peak discharges for floods of the 10-, 50-, 100-, and SOO-year recu=ence intervals for the Uncompahgre River were computed in April 1979 by the U.S. Army Corps of Engineers (Reference 2). In the U.S. Army Corps of Engineers study, frequency curves for rainfall and snowmelt events on the Uncompahgre River at the Delta and Colona stream gages were developed. The Delta gage is located approximately 30 miles downstream of Montrose, and the Colona gage measures flow on the Uncompahgre River approximately 10 miles up- stream of Montrose. These frequency curves were computed by log- Pearson Type III analyses of stream gage data for 58 years of record at the Delta gage and 57 years of record at the Colona gage (Refer- ence 7). The independent frequency curves for rainfall and snowmelt events were combined statistically to get "all-event" frequency curves at the gage locations. Because the two all-event frequency curves were nearly identical at the two gage locations, the peak discharges for the 10-, 50-, 100-, and SOO-year floods were computed to be constant between the gages. This indicates that attenuation and stream diversions counterbalance tributary inflow between the stations. For the San Miguel River, peak discharges for the 10-, 50-, 100-, and SOO-year floods were determined using a regional floodflow frequency analysis (Reference Bl. For the regional study, USGS stream gage records for 12 gaging stations in hydrologically similar watersheds were analyzed. The station data were separated into rainfall and snowmelt peaks in a previous study (Reference 4). Log-Pearson Type III frequency analyses were conducted on the rain- fall and snowmelt data (Reference 7). The results of the snowmelt- and rainfall-frequency analyses were combined statistically to give a maximum flow for both events. Regional curves of drainage area versus discharge were computed 17