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<br />known profile with the least number of cross sections.) Equation (3) <br />is the friction loss formulation presently used in the U.S.G.S. Step- <br />Backwater programs E43l amd J635. Equation (4) has been shown by Reed <br />and Wolfkill to be the most suitable for M2 profiles. <br /> <br />Another feature of this option is the capability of the program to <br />select the most appropriate of the preceding four equations on a reach by <br />reach basis depending on flow conditions (e.g., Ml, Sl, etc.) within the <br />reach. It is anticipated that this capability may be incorporated into <br />the program as a standard feature at sometime in the future. At present, <br />however, the criteria shown in Table 2 below, do not select the .best <br />equation for friction loss analysis in reaches with significant lateral <br />expansion, such as the reach below a contracted bridge opening. <br /> <br />TABLE 2 <br />CRITERIA UTILIZED TO SELECT FRICTION EQUATION <br /> <br /> Is friction slope at current <br /> cross section greater than <br /> friction slope at preceding <br />Profile Type cross section? Equation Used <br />Subcritical (Ml, Sl) Yes (2) <br />Subcritical (M2) No (4) <br />Supercritical (S2) Yes (2) <br />Supercritical (M3, S3) No (3) <br /> <br />The friction loss equation is controlled by variable IHLEQ on the <br />J6 card as follows: <br /> <br />Value of IHLEQ (J6.1) <br /> <br />o <br />1 <br /> <br />2 <br />3 <br />4 <br /> <br />Friction Loss Equation Used <br /> <br />(1) Average Conveyance <br /> <br />Program selects equation based on <br />flow conditions (see Table 2). <br /> <br />(2) Average Friction Slope <br /> <br />(3) Geometric Mean Friction Slope <br /> <br />(4) Harmonic Mean Friction Slope <br /> <br />When using this option, it is appropriate to also use a J3 card to request <br />printout of the variable IHLEQ to identify the equation used for each <br />reach. <br /> <br />8. CHANNEL IMPROVEMENT <br /> <br />Cross section data may be modified automatically by the CHIMP option <br />of the program to analyze improvements made to natural stream sections. <br />The CHIMP option simulates channel improvement by trapezoidal excavation. <br />This option is requested by the CI card which specifies the location of <br />the centerline (CLSTA), the elevation of the improved invert (CELCH), a <br />new 'n' value (CNCH), the left side slope (XLSS), the right side slope (RSS) <br /> <br />20 <br />