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<br />Because Manning's coefficient of roughness 'n' depends on such <br />factors as type and amount of vegetation, channel configuration and <br />stage; several options are available to vary 'n'. When three 'n' values <br />are sufficient to describe the channel and overbank roughness, the first <br />three fields of the NC card ('n' value change) are used. Any of the 'n' <br />values may be permanently changed at any cross section by using another <br />NC card. Often three values are not enough to adequately describe the <br />lateral roughness variation in the cross section in which case the NH <br />card ('n' value - horizontal) is used. The number of 'n' values used <br />to describe the roughness is entered as variable NUMNH in the first <br />field and the 'n' values and corresponding cross section stations are <br />entered in subseouent fiel ds. These 'n' val ues will be used for all <br />subsequent cross 'sections unless changed by another NH card. Normally <br />the NH card 'n' values should be redefined for each cross section with <br />new geometry. <br /> <br />Data indicating the variation of Manning's 'n' with river stage <br />may be used in the program. Manning's 'n' and the corresponding stage <br />elevation (beginning with the lowest elevation) are entered on the NV <br />card ('n' value - vertical), beginning in the second and third fields, <br />respectively. Variable NUMNV in field one is the number of 'n' values <br />input on the NV cards. This 'n' value option applies only to the channel <br />area. <br /> <br />If for subsequent jobs of the same run it is desired to modify the <br />'n' values specified on the NC, NH, and NV cards by a factor, variable <br />FN on the J2 card may be used. The desired factor is entered as variable <br />FN for each job. If the value of FN is negative, the factor is multiplied <br />by the channel 'n' values on the NC card but the overbank 'n' values are <br />not changed. <br /> <br />Reference 1 shows Manning's 'n' values for average channels. A more <br />extensive compilation of 'n' values for streams and flood plains is <br />listed in reference 2. References 3 and 4 use pictures of selected streams <br />as a guide to 'n' value determination. <br /> <br />Contraction or expansion of flow due to changes in the channel cross <br />section is a common cause of energy losses within a reach. Whenever this <br />occurs, the loss may be computed by specifying the contraction and expan- <br />sion coefficients as variables CCHV and CEHV, respectively, on the NC card. <br />The coefficients are multiplied by the absolute difference in velocity <br />heads between the cross sections to give the energy loss caused by the <br />transition. Where the change in river cross section is small, coefficients <br />CCHV and CEHV are typically on the order of 0.1 and 0.3, respectively. <br />When the change in effective cross section area is abrupt such as at bridges, <br />CCHV and CEHV may be as high as 0.6 and 1.0, respectively. These values <br />may be changed at any cross section by inserting a new NC card. These new <br />values will be used until changed again by another NC card. For additional <br />information concerning transition losses and for information on bridge <br />loss coefficients see Appendix IV. <br /> <br />13 <br />