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Table 11 Observed vs. Reproduced Flood Peak Flows (cfs) September 1970 Raiinflood Event upper mainstem Dolores River below Rico and Dolores River at Dolores Gage Peak (cfs) . Maximum 72-hour Location Observed Reproduced Percent Observed Reproduced Percent Difference Difference Rico gage 1,930 1,929 0.05% 670* 559 16.6% Dolores gage 5,190 5,143 1.0% 1,677* 1,802 8.3% * Maximum 72-hour is midnight to midnight data e 7. Modeling the 1 % Flood Event The 1 % chance exceedence flood event at the Dolores River at Dolores gage is the result of a general rainstorm on the watershed. This subject is discussed further in Section 8, Flow Frequency Analysis. Section 7 includes a discussion ofthe development of the 1 % chance general rain and the HMS basin parameters used to develop the 1 % chance flood hydro graph at the Dolores gage. 7.1 Previous Storm Modeling. A 2-day Standard Project Storm (SPS) was developed in e 1976 for the HEC-l rainfall-runoff computer model. The hourly distribution was based on the distribution patterns at the Pleasant View, Silverton, and Durango recording precipitation gages for the September 1970 storm (Appendix A Table 5 and Reference 4-c.) A 1 % chance rainstorm was not developed for the 1976 study. 7.2 1 % Chance Precipitation Dat~~. 1 % point precipitation depths for the upper Dolores River watershed were obtained from the NOAA Atlas 2 web page for the 1-,2-,3-,6-, 12- and 24-hour durations. See Plate 11 for the 24-hour precipitation amounts on the basin. These precipitation amounts were developed in 1973 and have not been revised yet for the state of Colorado. The 24-hour precipitation amount is the maximum duration available from NOAA Atlas 2. A 2-day general storm was needed for the 1 % rainflood; a relationship between the 1 % 24- and 48-hour durations for the upper Dolores watershed was developed based on a similar - 24