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e -. <' . , e I " , e i. Other factors. Ongoing or near-future, changes in the watershed should be considered in developing water surface elevations. Consideration of urbanization effects on future discharges has long been a requirement of Corps analysis. Other localized effects should also be considered. Local channel modifications and bridge replacements that are ongoing or scheduled 10 be com- pleted prior 10 implementation of a Corps project should be incorporated inlo the hydraulic study. Bridge obstruc- tions, particularly culverts under a high fill, can cause significant upstream poDding and induce damage to nearby structures. If the local community has no plans (or funds) to rectify a severe local flooding problem such as this, the Corps study team should include this obstruc- tion in the future condition, without project, analysis. On several occasions, however, in the time between the Corps' feasibility report and the fmal design document, such obstroctions have been replaced, greatly decreasing project benefits and affecting the authorized plan. Sensi- tivity tests on economic effects 10 the Corps' recom- mended plan of potentia! modifications 10 culverts or bridges are encouraged. The project manager should maintain continuous contact with the local community and highway department to obtain information on poten- tial bridge replacements that may affect the project. 3-5. Calibration of Hydraulic Analysis Models The reliability of the results of a hydraulic model study depends on the skills and experience of the hydraulic engineer performing the study, applicability of the model 10 the physical situation, and the quality of the data used to both model the study reach and calibrate the model. The overall calibration process incorporates three distinct steps: obtaining the necessary data and translating it inlo input for a numerical model, calibrating the model, and verifying the model. Additional guidance on calibration is given in Chapters 4 through 7 and Appendix D. a. Purpose of calibration. The objective of the calibration process is to match the output of the model with observed data (usually water surface elevations). This process is performed by adjusting one or more parameters, such as Manning's n, until a satisfactory match of model results with known data is achieved. When a set of known conditions has been approximately matched by the model, one can apply the model to unknown conditions (the I-percent chance flood, the Standard Project Flood, etc.) with more confidence that the model output is reasonably representative of the physical processes associated with that event. However, to be confident, the observed data for calibration should EM 1110-2-1416 15 Oct 93 be obtained from an event that is near the scale of the events 10 be modeled. b. Observed data. This includes data rec.orded at gages along with that obtained from field observations by Corps personnel, and from interviews with local resi- dents. Recorded discharges, stages, and velocities are valuable for calibration purposes; however, it is rare that sufficient gage data are available for comprehensive calibration. The preponderance of calibration data usually comes from local observations during and after an event. The hydraulic engineer should plan for several days of field work 10 obtain highwater marks from local residents' observations or following an event that occurs during the study. The best data often come from people who have lived near the stream for many years. They can supply information concerning flood elevations, erosion or deposition tendencies, local channel modifica- tions (when and where), tendencies for debris 10 obstruct bridge openings, how often the stream gets out of banks, and possible flow transfers between watersheds during floods. As much information as possible should be obtained from local residents for use in the calibration process. While all information is useful, the hydraulic engineer should recall that the further back in time, often the hazier the memory of the individual is for exact flood heights. The exact water level of the flood may not be accurately recalled. The engineer should not expect that model results will match every highwater mark exactly. c. Calibration process. The calibration process normally focuses on matching stage and discharge data at gaging sites with highwater marks used 10 calibrate the model at ungaged sites. This section addresses only the stage or highwater mark calibration. (1) The first step in the process does not begin until the study reach data have been assembled and entered into an input file, several discharges have been simulated, and the data me corrected as necessary. Effective flow area transitions between adjacent cross sections should be reasonable; profiles through bridges should be closely inspected 10 ensure that faulty modeling procedures are not leading 10 incorrect head losses and computed water surface profiles; and all warnings or messages from a numerical model should be reviewed and corrected if necessary. The hydraulic engineer should ensure that the model is performing reasonably well before "fine tuning" is initiated to match model results to field data. (2) For subcritical flow, one-dimensional steady flow water surface profile computations begin 3-11