8.3 Adjusting HEC-RAS Input
The modeler makes many adjustments to the data to improve the hydraulic modeling of the study reach. Unlike the older HEC-2 program, HEC-RAS makes data adjustments easy. Cross-section points may be added or deleted at any section, graphs and tables may be used to inspect the data and modify it directly, and templates are available to modify section identification, reverse section stationing, or to add cross sections. The following sections identify several areas where HEC-RAS simplifies the editing process.
Changing Station ID
Identifying the river station ID by its distance in either ft (m) or mi (km) from a specific reference point (usually the mouth or confluence of the river) is good practice. Some engineers, however, may use an arbitrary numbering system (1, 2, 3 or a, b, c), possibly based on the cross-section number. This method may have been employed on older data sets that are imported from HEC-2. Cross sections with arbitrary numbering schemes cannot be easily located on a map by other users, thus modifying the stations to more meaningful ID values is worthwhile.
Cross Section Points Filter
The Tools tab on the Geometric Data Window containing the reach schematic includes many useful tools for data editing including the Graphical Edit tool. For example, modelers can develop geometry data by using GIS techniques with cross-section data developed from "cutting" the elevations from a GIS elevation reference map. When these techniques are used, elevation-station points can be included at close intervals across the section. For instance, cross sections cut from elevation reference maps may contain over 500 points but is desired that these be filtered down to 200 values. Profiles with extremely detailed geometry definition typically show little change compared to the same profiles using less detailed geometry data. As described in Chapter 5, 15 to 30 points describing the section geometry are usually adequate for hydraulic computations.
Point filters are available within HEC-RAS to reduce the number of ground points to a more manageable level without sacrificing computational accuracy. For example, the Cross Section Points Filter tool enables the modeler to discard extraneous or near duplicate points within defined tolerances. A single cross section up to the entire reach may be run through the filter. The filter includes both horizontal and vertical tolerances for multiple points sited close together and for collinear points that fall nearly on a straight line along a portion of the section. The modeler can also limit the filter to only sections containing more than 500 points.
Reverse Stationing
The modeler may have accidentally coded one cross section from right to left instead of from left to right (looking downstream), as first discussed in Chapter 5, or the modeler may need to use an old model where all the sections were mistakenly coded as right to left. Either direction will give acceptable answers if all sections are coded in one direction or the other, but left to right, looking downstream, is the accepted convention. HEC-RAS provides a tool for reversing cross-section stationing for a single section or all sections in the model. The modeler should closely review the reversed cross sections, especially at bridges, culverts and other obstructions. If the stationing for the bridge or culvert embankment is different than that of the bounding cross sections, a cross-section reversal could result in the bridge's or culvert's opening no longer coinciding with the channels in the bounding cross sections.
Cross-Section Interpolation
Often, the best way to minimize errors and warnings in a model is to add additional cross sections to better meet the gradually varied flow criteria. The HEC-RAS cross-section interpolation feature lets the modeler add interpolated cross sections between two existing cross sections or to a whole reach. Results of the interpolation should be carefully reviewed to ensure that the new sections reasonably represent the actual field geometry between known surveyed sections.
Interpolation is based on the five master chords connecting the two sections (first and last geometry points in each section, bankline stations, and lowest channel elevation). Figure 8.8 shows four interpolated cross sections created between two surveyed cross sections (sections 1 and 2 at a bridge) using only the five standard master chords. However, one problem in this example is the ineffective flow area at the upstream cross section at the bridge. These interpolated sections would have to incorporate the ineffective area option to correctly transition from the full-width section at RS 5.29 (Section 1 for a bridge) to the section just downstream of the bridge at RS 5.39 (Section 2).
Figure 8.8 Interpolated cross sections.
If points besides the five master chords appear in both sections and definitely should be connected, HEC-RAS has tools that add additional minor chord(s) to properly connect the points between the two cross sections.
The modeler must determine an appropriate interval for interpolated cross sections for each instance of interpolation. It should be emphasized that interpolating more cross sections does not improve the computed profile if the initial cross-section definitions are poor. Interpolated cross sections every 100-200 ft (30-60 m) are often appropriate for steeper streams, with increased spacing as slopes become milder. The modeler should also not interpolate sections at very close intervals (say every 10-50 ft, 3-15 m), because excessive cross sections make it difficult for easy viewing and analysis. Interpolating hundreds or thousands of cross sections is also not desirable from a technical review standpoint. Technical reviewers have been known to make negative comments when excessive cross sections were interpolated for a floodplain study.
