8.1 Input Data Checking

All input data comprising a hydraulic model of a stream reach must be correct and reasonable. The modeler and at least one other professional engineer should closely review the information to ensure that it is appropriate and defensible. In addition, HEC-RAS automatically performs many internal checks that are used during data input and hydraulic computations.

Checks Performed by the Modeler

The modeler should check all data as it is input into HEC-RAS for correctness and reasonableness. The cross-section plot routines can be used to quickly determine whether some of the elevation or stationing data are obviously bad. Bridge and culvert data can be quickly checked by plotting the completed cross sections. The modeler can use these plots to readily spot incorrect constraint elevations or the absence of ineffective flow areas, as was presented in Chapters 6 and 7. The previously selected Manning's n values (presented in Chapter 5) should be reviewed to confirm the validity of the values selected. In addition, the peak discharges that were developed to verify that the techniques used are commensurate with the accuracy requirements and physical features of the study watershed should be reviewed. The more care the modeler takes in preparing and entering the basic data, the fewer problems will occur during the initial operation and calibration of the model.

Checks Performed by HEC-RAS

HEC-RAS performs several checks of model input before performing computations, unless the modeler chooses otherwise. The default is for HEC-RAS to perform the checks.

The checks performed by HEC-RAS can be grouped into two general types. The first type is performed as the modeler is entering the data. Every time data is inserted into a field, the program performs various checks, including:

Alphanumeric checks - Most fields allow either alpha or numeric values, not both. The program checks the characters to ensure that the proper format is used.
Checks for stationing increase - Each station value inserted to define an individual cross section must equal or exceed the previous value.
Check of channel bank stations - The program reviews the specified channel bank stations and makes sure that the named stations are in the cross-section data.
Check of minimum and maximum ranges for variables - For example, if a value greater than 1 is mistakenly entered for an expansion or contraction coefficient, the program immediately indicates that 1 is the maximum allowable value.
Bridge deck geometry check - The program ensures that the deck/roadway data intersect with the ground data.
Deletion checks - If the modeler tries to delete data, the program issues a warning to see whether the data really are to be deleted.
File save checks - The program notifies the modeler to save the existing file before opening another or before closing the program.

The second type of check performed by HEC-RAS occurs when the modeler begins the computation process. The program checks data completeness and consistency to determine whether all required data are present. Incomplete data errors include such items as a missing pier coefficient for bridges with piers, a missing n value at one or more locations, and an expansion or contraction coefficient that was not defined.

The program then checks to determine whether the data are appropriate for the type of computations being performed. These checks include comparing the number of profiles to be computed with the flow data supplied and determining if the appropriate boundary conditions have been specified. HEC-RAS does not perform computations until the modeler has supplied all required data.