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Ii I II II I III I III I I Ii I' Ii Ii I] 11 Ii IJ II II II The flood hydrographs for the subbasins were routed and combined at vari- ous flow concentration points. Routing was performed by the Muskingum method. The Muskingum parameters were estimated based on mean flow velocities of 5 to 10 ft/sec. These velocities were estimated using assumed channel sections and the Manning equation. In general, channels in the upper watershed had veloci- ties of about 5 ft/sec, and main channels in the lower end had velocities of about 10 ft/sec. The sensitivity of the Muskingum parameters was evaluated. MODEL RESULTS The results of the HEC-l model for various sets of input are shown in Table 4. The peak discharge, time to peak, and runoff volume for two concen- tration points are shown; Boulder Creek near the Orodell gaging station and Boulder Creek at the canyon mouth. HEC-l model number BC2B is judged to be the best model of the watershed based on a comparison of the 100-year flood discharge from the model to other accepted results (discussed below). FLOOD FREQUENCY ANALYSIS The results of the model were compared against flood frequency estimates based on stream gage records. Two gaging stations in the watershed are avail- able for flood frequency analysis; Boulder creek near Boulder and Boulder Creek near Orodell. The data are shown in Appendix B. Graphical flood frequency analyses were performed using normal, log-normal, extreme value, and log-extreme value probability papers. The Cunnane plotting position was used for all distributions. The Orodell record is more reliable because of the longer length of record. The data fits the log-normal distribution better than the other distributions, and the graphical analyses are shown in Appendix B. The flood frequency estimates at Orodell are: 100-year = 1,690 cfs 50-year = 1,490 cfs 10-year = 1,050 cfs 4