WSP03196 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I I 2915 II-14 Conclusions 1. Moisture maximization of storms involves the estimation of the actual moisture in the air flowing into the storm and the maximum moisture that might be available under optimum conditions. The assumption is that, other precipitation-producing factors being equal, precipitation is proportional to the amount of moisture inflow. Surface dewpoints are used to estimate the atmospheric moisture. 2. Simple transposition of storm isohyetal patterns in mountainous regions is at best an oversimplified, relatively crude procedure. There are other procedures besides direct transposition of storm patterns for estimating PMP in such regions, but they all require a great deal of judgment, and the accuracy of the results is generally uncertain also. 3. There is a general agreement that the outstanding June 7 to 8, 1964 Montana storm on the eastern slopes of the Continental Divide is trans- posable to the project basin. This storm was definitely an outstanding event, During its short life of about 48 hours, the storm deposited rainfall exceeding SO percent of the mean annual precipitation over an area of almost 3000 square miles. 4. Comparison with PMP estimates for adjacent basins and nearby areas (Table 11-2) indicated that the design storm recommended by Reclamation (Table 11-3) is at the general level of PMP for the region. 5. Because of seasonal differences in storm efficiency between the Montana storm site and the project basin, the design storm proposed by Reclama- tion could be reduced by 6 percent to adjust for a possible May 23 to 24 occurrence. A reduction of 9 percent could be made for an early June occurrence, when the snowmelt potential would be most likely to be at a maximum. Hourly values for these two prospective design storms may be obtained by multiplying the amounts of Table 11-3 by 94 and 91 percent, respectively, These reduced values are given in Tables 11-4 and 11-5. DAMES e MOORE