FLOOD04413 - Laserfiche WebLink

Home Browse Search

Data Sources - 37 TABLE 2.2 Maximum Peak Discharges on the American River (Unregulated Conditions at Fair Oaks) Year 1862 Discharge 265,000 Source Bossen, 1941 1963 240,000 1987 Folsom Control Manual 1964 260,000 1987 Folsom Water Control Manual 1997 295,00oa a Estimated at Folsom Dam. February 16, 1999. Roos, 1999a Source: Maury Roos, memorandum to Kenneth Potter dated Probable Maximum Flood In October 1996, the U.S. Bureau of Reclamation, in consultation with the USACE Sacramento District, used HMR 58 to estimate a mean probable maximum storm amount for the American River basin of 29.62 inches (Pick, 1996; NWS, in press). Using loss rates based on saturated soil for unfrozen ground and snow cover for frozen ground, the USBR calculated one- and three-day probable maximum flood discharges of 575,000 and 401,000 cfs, respectively, for regulated conditions upstream. Due to the combined volume of upstream storage and likely extent of occupied storage at flood time, the equivalent unregulated volumes were expected to exceed regulated values by only a few percentage points. In 1997, following the January 1 flood, USACE Sacramento District re-estimated the probable maximum flood for the basin by applying loss rates equivalent to those observed for this large event (0.7 inches loss of 11.8 inches total) to the probable maximum storm derived in 1996. The resulting three-day runoff was 29.07 inches and the maximum three-day average flow was 485,000 cfs. PMF estimates for the American River provide some information about the upper tail of the flood distribution. In theory the PMF is the maximum flood that can be expected at a site, the PMF concept is largely empirical, and hence a PMF estimate should be thought of as a very large flood discharge that is highly unlikely to be exceeded. While the committee is unable to specify the distribution of the likely values of the exceedance probability of a PMF for the American River, empirical data suggest that the exceedance probability should probably be smaller than 1 x 10-4 and almost surely smaller than I x 10-3. In the case of the American River, the committee decided to use the two PMF estimates as likely upper bounds on the flood quantile associated with a probability of 1 x 10-3. SUMMARY A variety of data are available for use .in flood frequency analysis on the American River. Based on the committee's consideration of these data, it has concluded the following: