8.5 Production Runs

After calibration and verification are completed, the modeler can finalize the existing condition profiles for the stream. This effort should focus on developing the hypothetical frequency profiles and stage-frequency relationships for all streams under study. Different study objectives require preparation of different profiles. Four or more profiles are often run simultaneously, since HEC-RAS has the capability to run as many as 500 profiles in a single run. If a flood insurance study is being performed, the production runs will typically feature the 10-, 50-, 100-, and 500-year profiles. A flood-reduction study focuses on these and other events, including the 2-, 5-, 25-, and 200-year floods. These profiles can be linked with economic data (river stage versus damage values) to develop average annual damage values for evaluating the effectiveness of flood reduction options.

Other very rare events, such as the Probable Maximum Flood (the greatest flood reasonably possible for an area) could also be included, particularly when a dam and spillway are being designed or analyzed. Navigation studies may evaluate in-bank flows and concentrate on the lowest flows at which navigation is still possible. For whatever purpose the hydraulic model is to be used, production runs will be necessary for the various discharges selected.

Even though the debugging and calibration process is complete, the development of water surface profiles for a wide range of flow events usually results in some additional modifications of the hydraulic input during the production runs. The events used during the calibration process are seldom as large as the hypothetical events to be studied, like the 100- and 500-year floods. Although the model agrees well for the calibration events, a few problems normally occur during the development of existing condition water surface profiles for large events.

Large Changes of Key Parameters

Graphical and tabular output for all sections in the reach should be examined for sharp changes in important parameters. A modeler would expect a plot of all flood profiles to show reasonably parallel shapes. If one or more show sharp increases at certain locations, these cross sections should be reviewed further. Notes and warnings for higher flows not studied during the calibration process should also be reviewed. If there is a large elevation difference between the lowest and highest profiles being studied, consideration should be given to varying Manning's n in the vertical direction. For example, if the 5-year flood results in 1-2 ft (0.3-0.6 m) of water in the floodplain, but the 100-year event results in 20 ft (6.1 m) of water in the overbank areas, the overbank n for the rarer floods is probably smaller than that of the more frequent events. Adjusting Manning's n in the vertical direction can simulate this change. HEC-RAS allows the modeler to modify n for different ranges of elevation.

Constraint Elevations and Ineffective Flow Areas

Bridges and culverts passing large flows should be reviewed again to ensure proper modeling. Even though the constraint elevations and ineffective flow area locations may have performed adequately during calibration, the production runs may have much larger flow values. Consequently, these higher flows may not be properly constrained upstream and/or downstream of the bridge or culvert and some modification of the constraint elevations may be necessary.

A final review of the production runs by another experienced hydraulic engineer is recommended but not always possible. There is also automated model checking software available, such as CHECK-RAS and CHECK-2 available from FEMA's website. These programs can be used to supplement manual review and can also be applied during the initial model development and calibration process. If a modeling project is going to be submitted to FEMA for their review and approval, they will most likely use these programs to review the hydraulic work. It is worth the time and effort to use these programs before submitting work to FEMA, potentially avoiding future problems. CHECK-RAS (discussed further in Chapter 9) may be applied for any steady flow hydraulic study, not just FEMA work. The program will check roughness and expansion/contraction coefficients, cross-section input and output data, floodways, structures (such as bridges and culverts), and multiple profiles.