should be the better predictor of wetted perimeter based on the 

 greater accuracy of the predictions of water surface elevation. 

 However, the wetted perimeters generated by the IFG-4 model 

 are only an approximation and may be subject to some error. . 

 This error was assumed to be negligible for the relatively 

 large waterways the model was applied. 



Wetted perimeter curves generated by the WSP and IFG-4 

 models for the composite of cross-sections in four of the five 

 river reaches are compared in Figures 35 through 38. The inflec- 

 tion points on the curves for the two models generally occurred 

 at approximately the same flows (Table 31) . The obvious excep- 

 tion was the Beaverhead reach in which the inflection point 

 occurred at 100 cfs for the WSP model and 225 cfs for the IFG-4, 

 model. Neither model provided discernible inflection points 

 for the Gallatin reach. . : 



Table 31. Comparison of the flows at which inflection points 



occurred on the wetted perimeter-discharge relation- 

 ships generated by the WSP and IFG-4 hydraulic 

 simulation models for a composite of cross-sections ',. 

 in reaches of the Madison, Beaverhead, Gallatin and u 

 Big Hole rivers. 



Reach 

 Madison (#1) 

 Beaverhead (#2) 

 Gallatin(#2) 

 Big Hole(#l) 



Inflection Point Flows (cfs) 



WSP Model 

 900, 1,400 

 100 



400, 700 



IFG-4 Model 

 900 i 

 225 



450, 700 



On a statistical basis, the wetted perimeter curves gen- 

 erated by the two models were significantly different (P <..05) 

 from one another in the Madison #1, Beaverhead and Gallatin 

 reaches. The most obvious discrepancy occurred in the Beaver- 

 head reach (Figure 36) . Some of this difference between models 

 may be due to the extrapolation of the IFG-4 data beyond the 

 lowest calibration flow. The IFG-4 wetted perimeter curve may 

 be subject to some error due to the problems previously dis- 

 cussed. In a previous study, a wetted perimeter curve gener- 

 ated by the V7SP model for a composite of 20 cross-sections in 



86 



