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I I I I I I I I I I I I I I I I I I I U[')1952 Calculation of the Water Line Comparison Illl2k. R2CROSS uses two techniques for estimating cross-sectional area, One estimate is obtained by summing the product of "measured" water depth and cell width for all cells in the cross section (A",), This technique allows independent water surface elevations within each cell and provides the most accurate estimate of cross-sectional area at the time the field measurement was made, However, this technique cannot be used to simulate a single, flat water surface elevation at computer- modeled stream discharges. The second technique used to estimate cross-sectional area involves projecting a single water surface elevation across the stream channel. Channel bottom elevations are subtracted from this projected water surfilCe elevation to obtain a "computed" water depth at each cell vertical. Cross-sectional area is obtained by summing the product of the "computed" water depth and cell width at each cell vertical (Ae)' This technique constrains the water surface to a flat plane and is useful for simulating discharges above and below the field-measured discharge. The water line comparison table (Figure H) iteratively calculates 31 separate estimates of Ae' using projected waterlines ranging from 0,25 feet above to 0,25 feet below the mean waterline measured in the field. The single water surface elevation that results in Ae equal to A", is interpolated from the water line comparison table and is used in the staging table as the best estimate of the waterline at the field-measured discharge. Calculation of the Stl\~in~ Tahle The final product of the R2CROSS macro is the staging table (Figure I), In addition to the three critical biologic criteria (xd ' %WP, and xv), R2CROSS also calculates incremental estimates of top width (TW), maximum depth (Dmax), cross-sectional area (A), wetted perimeter (WP), hydraulic radius (R), and flow (Q) at a number of waterline elevations. The upper limit of the model occurs at bankfull discharge which is defined as the lower of the two grassline elevations measured in the field, The lower limit is either 1.75 feet below the watetl;ne calculated in the water line comparison table or stage of zero flow' (the lowest field-measured channel profile), whichever is higher in elevation. The formulae for each of the parameters estimated in the staging table are summarized in Table 4, -31-