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e . < . . , e . ~ . . e river problems are friction dominated, however, and the model results may not be very sensitive to the value selected for the turbulent exchange coefficients. A gen- eral approach is 10 first calibrate the roughness coeffi- cients (Manning's n values) to reproduce the energy loss or water surface gradient through the study reach and then adjust the turbulent exchange coefficients 10 match the observed or expected velocity distribution. The exchange coefficients should be set 10 the high end of the expected range first, then lowered until the desired veloc- ity pattern is reproduced by the model. In general, the higher the coefficients, the more uniform the velocity distribution; the lower the coefficients, the more readily does flow separation and eddy formation take place. Two-dimensional models (as with one-dimensional mod- els) should be calibrated 10 steady flow conditions first, if possible, before attempting calibration to an unsteady flow event (Cunge et al. 1980). d. Field data. In addition to thoroughly inspecting the study area, the analyst should be familiar with the manner in which field observations are made, that is, the type of instruments used and the conditions under which the data were obtained. Data reduction techniques may also affect the accuracy and variability of the observa- tions. The analyst should not consider field data 10 be perfectly accurate nor necessarily representative of field conditions over the complete range of circumstances 10 be studied. Internal consistency of field data should be checked if at all possible. For example, when using velocity observations for calibration of a two-dimensional model in steady flow conditions, one should calculate the discharge from the velocity and depth measurements and compare it to the discharge obtained from a nearby gage at the same time as the velocity measurements were made. EM 1110-2-1416 15 Oct 93 4-10. Example Applications Most applications of two-dimensional horiwntal models to date have been in estuarial environments; some of these applications are presented in "Two-Dimensional Flow Modeling" (U.S. Army Corps of Engineers 1982b), McAnally et al. (l984a, 1984b), and MacArthur et al. (1987). A recent study that evaluated the effects of deepening a ship channel on velocity patterns and shoal- ing is discussed by Lin and Martin (1989). Computation of velocity distributions in a river downstream from a hydropower project is presented in Gee and Wilcox (1985). Impacts of highway bridge crossings on water surface elevations are discussed in Lee (1980), Tseng (1975), and Heltzel (1988). Effects of dikes on the flow distribution in a river was investigated using TABS-2 by Thomas and Heath (1983). Use of two-dimensional modeling 10 analyze effects on river stage of a major channel encroachment is presented in Stewart et al. (1985). In this study use of a one-dimensional model did not produce credible results because values of the expan- sion-contraction coefficients governed the outcome and, as this was a design study, there were no field data for their calibration. Results were much less sensitive 10 the values of the turbulent exchange coefficients because the major flow patterns and separation areas were calculated directly by the two-dimensional model. It is the effects (energy losses) of these separation areas that the expan- sion-contraction coefficients attempt to describe. Use of RMA-2 to model flood movement in a large river chan- nel-floodplain system is presented in Gee et al. (1990). This paper also describes the computational resources required to perform such a study. Use of a two-dimen- sional model to analyze distribution of flow in the St. Lawrence River is documented by Heath (1989). 4-7