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Dolores River Wa':er-surface elevations were t:hen computed using the latest version of the Corps HEC-2 step-backwater computer program (Version 4.6.2, dated May 1991). Ute basic input data were that used in the FEMA and Flood Hazard Information Report studies and obtained through FEMA headquarters in Washington, D.C., and the Corps Sacramento District. These data contained cross sections upstream and downstream of the abovE!-mentioned newly surveyed cross s'~ctions (as well as basic information such as reach lengths and other coincide with those of the 1989 FEMA study and the 197il Flood Hazard Information Report study. The cross- section locations are shown on Attachments 2 and 3. The hydraulic analysis consisted of th,; following: - Develop the computer model. - Update and run the model using current information. - 'lary "n" values in the model to determine sensitivity. - Revise the model to reflect potential project conditions. Th,~ original FEMA(Flood Hazard Information study data were used to create an executable computE!r file. All the data were entered exactly as originally writtE!ll, including the old cross section,;, flows, etc. The file was then run and debugged until it compared closely with the original output for the 100-year water-surface profile. The few remaining differences were due to changes made to the HEC-2 program it:self (the original FEMA model being run with an August 1977 version of the program). 'I'he HEC-2 file (input and output), identified a!l "OLDDOL.OUT," is included in Appendix B. Th,~ computer model was then r,;vised to incorporate current informa':ion, including the new survey data referenced earlier and adjustm,mts to channel flows based on the levees which have been constru,~ted. A few old cross sections (Nos. 1, 5, 6, and 8) at the dO~lstream end are no longer applicable because of the USBR project and were dropped from the lnodel. Several other cross section:; were revised to reflect thE! fill placed by the tTSBR below 4':h Street. Also, it appears that with the higher banks and lev'~es, most of the overbank flow intI) Dolores would be just upstream of the bend in the river, south of the high school (at cross s'~ction 19). Once the flow is out of bank at this location, it is hydraulically cutoff from the channel flow until it reaches the reservoir (unless the levees were to fail or be overtopped) . Th,~ resulting new water-surface profile is shown (along with the old FEMA(Flood Hazard Information Report profile) on Attachm,mt 4. The HEC-2 file is included as "NEWDOL4. Ou'l'" in Appendi:( B. The old and new profilEls compare very closely, suggesting that the original modeling accounted quite well for the eff'~cts of the McPhee Reservoir and levees. Therefore, there 5