FLOOD10395 (2) - Laserfiche WebLink

Home Browse Search

e e e'l The concurrent peak flow on Lost Canyon Creek, about 820 cfs, is more frequent than a 5% chance event and not in conflict with the rainflood frequency curve on Plate 22. The HMS model with the 1 % West Fork centering produces reasonable results that are in agreement with the flow frequency curves. The flow frequency analysis validates the rainfall runoff model. 9. 1 % Peak Flow Comparison Table 18 presents a comparison between the 1 % peak flow frequency values developed for the Rico and Dolores gage locations for this flood insurance study, for the 1976 flood hazard information study, and flow frequency analyses done for other federal agencies (Reference 4-m and 4-0). The 1 % peak flow value of 14,500 cfs used in the 1976 hydrology report for Dolores River below Lost Canyon Creek was computed by combining the rainflood and snowmelt frequency curves statistically, using the Addition Rule for independent events: Pa = Pr + Ps - (Pr * Ps) where Pa = annual exceedence probability of peak flow for combined populations, Pr = annual exceedence probability of peak flow in rainflood season, Ps = annual exceedence probability of peak flow in snowmelt season, and the rainflood and snowmelt peaks are independent of each other, that is, neither event influences the other. However, combining flow frequency, statistics is not appropriate for the 2004 study, because the peak rainflood curve incorporates rain-on-snow events that have occurred during the snowmelt season of April through July. Most of the peak flows during snowmelt season occur in May. Several times in the record, a rainfall event toward the end of snowmelt season has caused a high peak flow, with snowmelt comprising a small part of it, as in 1927, 35