HEC-RAS Cross Sections
Cross sections are developed based on the location layout of cross section lines and the properties of other layers such as the rivers, bank lines, and terrain layers. Cross sections should be laid out perpendicular to where water will flow in the channel and overbank areas. Most cross section lines are created from a minimum of four points (i.e., the end points and points at the edge of the main channel). Cross sections will also be visualized when looking in the downstream direction; therefore, they should be created from left to right when looking downstream. There are many considerations when developing cross section data in terms of orientation, location, and spacing, but keep in mind that the cross sections should represent a smooth transition in geometry (elevation and area) and properties (conveyance, surface roughness, etc.). It is always recommended to use the terrain, river centerline, bank lines, flow path lines, inundation mapping, and other data to properly place cross section lines.
Follow the steps below to view or modify the cross section data in GeoHECRAS:
- From the Input ribbon menu, expand the Cross Sections dropdown menu and select the Cross Section Data menu item.
- The Cross Section Data dialog box will be displayed.
The following sections describe how to define the cross section data and interact with the above dialog box.
Selecting Cross Section
The Select Cross Section panel allows the user to select a cross section for purposes of defining the cross section data.
This section contains the following options:
- River
This read-only dropdown combo box displays the river(s) defined in the project. When the user changes a river, the Reach dropdown combo box automatically updates to display a valid corresponding river reach. - Reach
This read-only dropdown combo box displays all the reach(s) defined in the project that correspond to the selected river. - River station
This editable dropdown combo box displays all the cross sections defined in the project that correspond to the selected reach. The up and down arrow buttons adjacent to the dropdown combo box allow the user to switch between the next downstream and upstream cross section. If there are no other cross sections defined for the selected river reach, then these buttons are disabled (grayed out). Alternatively, the user can click the […] pick button to select the cross section from the Map View. - Node name
This entry field allows the user to assign an optional text label to the current cross section. - Description
This optional entry field allows the user to include information on the current cross section. - Less/More
The [< Less] and [More >] buttons at the Select Cross Section header allow the user to hide and display the right side of the dialog box containing the cross section plot. This allows the dialog box to be smaller when the user does not want to see the cross section plot view. - New
The [New] button allows the user to draw a new cross section on the Map View. - Copy
The [Copy] button allows the user to create a copy of the current cross section on the Map View. - Delete
The [Delete] button allows the user to delete the current cross section.
Once the desired cross section is selected, the user can switch between the following panels from the Cross Section Specifications dropdown entry to view and edit the associated cross section data.
Cross Section Specifications
This section contains a dropdown combo box with the following data panel entries, which allow the user to define cross section data:
- Geometry Data
- Ineffective Flow Areas
- Levees
- Conveyance Obstructions
- Cross Section Lid
- Internal Rating Curve
- Vertical Roughness
- Geometry Adjustment
- Geometry Comparison
- Geometry Point Reduction
- Profile Results
- Hydraulic Calculator
Geometry Data
This data panel allows the user to define the geometry data of the current cross section.
The Geometry Data panel contains the following sections:
Cross Section Geometry
This section of the panel provides a table for entering and editing cross section geometry. The top right corner of the table header displays the number of points for the current cross section. This allows the user to quickly determine which cross sections need to be reduced in terms of total number of ground points.
The table contains the following columns:
- Horizontal Station
This table column defines the horizontal station for the cross section geometry data. The user can click the […] pick button adjacent to the Horizontal Station column header to select a specific horizontal station of the cross section from the Map View. - Ground Elevation
This table column defines the ground elevation for the corresponding horizontal station of the cross section geometry. - Horizontal Roughness
This table column is enabled only when the Manning’s checkbox option under the Other Cross Section Data section is unchecked. The user does not need to enter a roughness at every ground station, except the locations where the roughness changes. The acceptable Manning’s range is from 0 to 1. Clicking the […] lookup button displays the Manning’s Roughness lookup dialog box for reference values, as shown below.
Other Cross Section Data
This section of the panel defines the following additional cross section data.
- Bank Stations
This field defines the left and right bank stations. The defined bank station must match an existing ground station. The user can click the […] pick buttons under the Left and Right entry fields to select the right and left bank stations from either Map View or the cross section plot. - Manning’s
This checkbox entry defines Manning’s n roughness values for left overbank, channel, and right overbank. Clicking the […] lookup button displays a Manning’s roughness lookup table. Unchecking this checkbox entry disables the underlying fields and enables the Horizontal Roughness column under the Cross Section Geometry table. - Flow length
This entry defines the flow length for the left overbank, channel, and right overbank. Typically, these values are automatically determined using flow length polylines drawn on the Map View. However, the user can override those values by either entering a flow length or clicking the […] pick button to measure the flow distance from the Map View. - Contraction / Expansion
These entries define the contraction and expansion coefficients. Clicking the […] lookup button displays the Expansion Contraction Coefficients lookup table, as shown below. Refer to this article in our knowledge base to learn more about expansion and contraction losses in HEC-RAS.
- Skew angle
This entry defines the skew angle of the cross section. This entry is blank by default.
Ineffective Flow Areas
This data panel allows the user to define ineffective flow areas of the cross section, which represent water that is not actively being conveyed. Refer to this article in our knowledge base to learn more about ineffective flow areas.
Levees
This data panel allows the user to define levees to constrain the flow to the main channel by defining a left and/or right levee station and corresponding elevation at a cross section. Refer to this article in our knowledge base to learn more about levees.
Conveyance Obstructions
This data panel allows the user to define areas at a cross section that are permanently blocked from conveying flow. Refer to this article in our knowledge base to learn more about conveyance obstructions.
Cross Section Lid
This data panel allows the user to define long pipes or tunnels over a flow direction. Refer to this article in our knowledge base to learn more about cross section lids.
Internal Rating Curve
This data panel is used to define a rating curve (Stage vs Flow curve) at the current cross section. By defining a rating curve at a cross section, the software will use the rating curve to lookup the corresponding water surface elevation rather than computing the water surface elevation.
Vertical Roughness
This data panel allows the user to define the vertical variation of Manning’s roughness for cross sections. This panel allows the user to enter Manning’s roughness values that vary both horizontally as well as vertically. After enabling the Vertical Roughness section, the user can define the data between Roughness versus Flow or Roughness versus WSEL. In this way, the user can vary the roughness value either by elevation or discharge rate. During the computations, HEC-RAS will interpolate Manning’s roughness whenever the computed water surface elevation is between the user-defined elevations. If the computed water surface elevation is below the lowest elevation defined, then the roughness values from that lowest elevation will be used. Similarly, if the computed water surface elevation is above the highest elevation defined, the roughness values from that highest elevation will be used.
Geometry Adjustment
This data panel is used to adjust geometry for the current cross section or other cross sections.
Adjust Cross Section Geometry
This section allows the user to define the type of adjustment to be applied to the cross section geometry.
This section contains the following options:
- No change
This option is selected by default and causes no changes to the cross section geometry. - Adjust elevations
This option allows the user to adjust the cross section geometry elevations by the defined amount. - Adjust stations
This option allows the user to adjust the cross section geometry stations by the defined amount. - Shift stationing
This option allows the user to shift the cross section geometry stationing using an existing reference point. The user selects the corresponding reference point from the dropdown combo box and then assigns the station to be assigned to the reference point. The following reference points are provided in the dropdown combo box entry:- Leftmost Station
- Left Bank
- Thalweg (lowest elevation between channel banks)
- Centered Between Banks
- River Reach Intersection
- Right Bank
- Rightmost Station
- Scale stationing
This option allows the user to scale the stationing by the defined amount for the following regions:- Entire Cross Section
- Left Over Bank
- Channel
- Right Overbank
- Reverse ground station order (flip end for end)
This option causes the cross section stationing geometry to be reversed (flipped end for end). This option should be used when the cross section was entered with the survey data in a reverse fashion relative to the other cross sections.
Adjustment Extents
This section is used to select which cross sections and river reaches are to be changed by the specified adjustment. If bridge structures or inline structures are defined for selected reach(s), then their corresponding cross section geometry is also adjusted.
Clicking the [Apply] button applies the defined cross section adjustment method.
Geometry Comparison
This data panel allows the user to select a comparison cross section to display in the cross section plot, align the comparison cross section with the current cross section, and replace the current cross section with the selected portion of the comparison cross section. Refer to this article in our knowledge base to learn more about geometry comparisons.
Geometry Point Reduction
This data panel allows the user to automatically filter out unnecessary station-elevation points for Cross Section Geometry, Roadway Geometry, Inline Structure Crest Geometry, and Lateral Structure Crest Geometry. Refer to this article in our knowledge base to learn more about geometry point reduction.
Profile Results
This data panel allows the user to select the analysis results on the cross section plot. The table displayed under the Profile Results section lists all of the analyzed water surface profiles, allowing the user to specify for which profiles the results should be displayed.
This data panel contains the following sections:
Profile Results
This section contains the following options:
- Profile
This read-only table column lists the profile name. - Water Surface
This table column lists a checkbox option that controls the display of computed water surface on the cross section geometry. - Energy Grade
This table column lists a checkbox option that controls the display of computed energy grade line on the cross section geometry. - Critical Depth
This table column lists a checkbox option that controls the display of computed critical depth on the cross section geometry. - Filled-in water surface
This checkbox option causes the selected water surface to be filled-in on the cross section geometry. If more than one profile has been selected which has the water surface checkbox option selected, then the lowest water surface elevation value is used.
Other Options
This section allows the user to enable additional curves for the plot terrain surface, fixed sediment, and pilot channel in the cross section plot.
Hydraulic Calculator
This data panel allows the user to choose an unknown variable and the software will then automatically compute the solution after the user enters the known parameters. For example, for a given water surface elevation (WSEL) and energy (or bed) slope, the software will compute the discharge rate. It accounts for Manning’s roughness subareas, but does not account for ineffective flow areas, conveyance obstructions, levees, or lids.
Selecting Normal Depth from the Calculate property dropdown combo box enables the Discharge and Gradient entry fields. The user can enter these entry fields and click the [Compute] button to compute all the associated hydraulic parameters under the Hydraulic Calculator Data section.
Similarly, selecting Normal Discharge from the Calculate property dropdown combo box enables the Water surface elevation, Flow depth, and Gradient entry fields. The user can enter these entry fields and click the [Compute] button to compute all the associated hydraulic parameters.
