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<br />16 <br /> <br /> <br />15 <br /> <br />21 22 <br /> <br />12 <br /> <br />7 <br /> <br />8 <br /> <br />1 <br /> <br />2 <br /> <br />3 <br /> <br />NUMBERS REFER TO CROSS SECTION <br />LOCATIONS. <br /> <br />Figure 5. Second Order Stream System <br /> <br />The arrangement of cross section data (Xl and GR cards) for the stream <br />system in Figure 5 for a tributary analysis in a single execution of the <br />program is as fo 11 ows : 1, 2, 3, 4, 11, 12, 1 3, 14, 15, 16, -4, 5, 6, 7, <br />8, -14, 21, and 22. <br /> <br />Tributary stream profiles should not be calculated simultaneously <br />with encroachment methods 3 - 6. <br /> <br />11. SOLVING FOR MANNING'S 'n' <br /> <br />The program can be utilized in two ways to solve for Manning's 'n'. <br />HEC-2 can compute 'n' values automatically from high water data if the <br />discharge, relative ratios of the 'n' valwes for the channel and over- <br />banks and the water surface elevation at each cross section are known. <br />The "best estimate" of 'n' for the first cross section must be entered <br />on the NC card since it is not possible to compute an 'n' value for this <br />cross section. The relative ratio of 'n' between channel and overbank <br />is set by the first cross section and will be used for all subsequent <br />cross sections unless another NC card is used to change this rato. High <br />water marks are used for the computed water surface elevation by setting <br />variable NINV on the Jl card equal to one and entering the known water <br />surface elevation as variable WSELK on the X2 card for each cross section. <br />The average friction slope equation (see J6 card description) is utilized <br />by the program to solve for 'n' values. If one of the other friction <br />equations is to be utilized for profile analysis then the program-deter- <br />mined 'n' values should be varified using the appropriate friction equa- <br />tion. Because of the sensitivity of calculated results to slight errors <br />in observed high water marks, a weighted 'n' (WTN) value is also calculated <br />at each cross section. WTN is the length weighted channel 'n' calculated <br />from the first cross section to the current cross section. When an adverse <br />slope is encountered, computations restart using 'n' values from the <br />previous section, but W1N computations continue. <br /> <br />23 <br />