PERMFILE127751 - Laserfiche WebLink

Home Browse Search

' 2.04.7(2) Essentially, "probability of occurrence" is implied. Thus, during the term of an average mortgage (30 years), the chance of a 100-year flood occurring is approximately 1 in 4; during an average lifetime (70 years), the chance is approximately ' 1 in 2. In a 100-year period, the chance of a 100-year flood is approximately 3 in 5. ' To determine flows of the 100-year and other frequency floods for this report, a flow-frequency analysis was made for five index points on the P]orth Fork Gunnison River. The annual peak flows for rain and snowmelt were analyzed. independently ' and frequency curves were developed for each. All-event peak flow-frequency curves were develope3 by combining the ' rainfall and snowmelt flow-frequency curves for each index point. The upstream index point was the U.S. Geological Survey gaging station near Somerset. Those down-stream along the study ' reach were at points that reflect significant inflow. Criteria outlined in the U.S. ;dater Resources Council Bulletin 17a were used in the analysis. ' In general, the flow-frequency analysis involved developing peak flow frequency curves reflecting natural (unregulated) ' flow prior to construction of Paonia Reservoir, and adjusting these curves to account for the regulatory effect of the reservoir. ' Specifically, the unregulated peak flow frequency curve for the Somerset index point was established from analysis of unregulated flow at the gage: ' 1. for the period of record, 1934-1960, and 2. for the years 1897-1933, as estimated from correlations with the A~orth Fork Gunnison River near Paonia gage (1922-1932), the Gunnison River ' below Gunnison Tunnel gage (1906-1960), and the Gunnison River near Grand Junction gage (1897-1960). ' Unregulated peak flow frequency curves for the four index points downstream from Somerset were developed from generalized fre<~uency REl'ISED t4ay 15, 1981 47-2c