The Input | Cross Sections | Assign Cross Sections command enables the user to quickly assign an existing polyline as a cross section on the Map View and have the software extract the HEC-RAS cross section geometry from the underlying ground terrain.
Selecting this command will display the Assign Cross Sections dialog box.
The following sections are used for defining data in assigning polylines as cross sections.
Select Cross Section Polyline
This section is used to interactively select a polyline from the Map View for assigning as a HEC-RAS cross section. Click on the [Pick] button and the dialog box will temporarily disappear. A prompt will be displayed at the status line, informing the user what to do next. Within the Map View, click on a polyline to assign as a cross section. Note that the selected polyline must overlay a pre-existing river reach or the software will report this as an issue. See this section on defining river reaches.
After selecting a polyline, the user will be returned to the dialog box showing that a polyline was selected.
Cross Section Specifications
This section is used to define the river station of the HEC-RAS cross section, along with an optional description for the cross section. After selecting a cross section polyline, the software will automatically suggest a river station for the HEC-RAS cross section based upon other cross sections already assigned to the river reach.
Process Selected Cross Section Polyline
This section is used to define whether the selected polyline should be deleted after the cross section is constructed. By default the delete selected polyline checkbox is unchecked.
Extract Elevation Data
This optional section is used to define the elevation data source to be used for extracting the HEC-RAS cross section geometry. Depending upon the elevation data source type that is selected, different options are provided to specify additional elevation data information.
See this section for information on the types of terrain elevation data that can be used for constructing HEC-RAS cross sections.
If the checkbox for this section is unchecked, then the subsequent sections below it are unavailable (i.e., grayed out). In addition, no geometry will be created when the cross section is created. The cross section will just be a flat horizontal line at elevation 0.
Cross Section Geometry Extraction Control
This optional section is used to control the amount of HEC-RAS cross section geometry to extract for the selected polyline. This assures the user that an adequately deep cross section is created on both sides of the river reach. The software will attempt to retrieve the cross section geometry data to the depth specified within the maximum cross section width specified.
If the selected polyline does not extend outward far enough to get the cross section depth requested, the software will automatically extend the constructed cross section further outward. Similarly, if the selected polyline extends too far outward for too large of a depth, the software will automatically trim the constructed cross section.
Assign Bank Stations
This optional section is used to construct HEC-RAS channel bank locations based upon an assumed normal flow depth and a maximum channel width search distance. The software will first determine where the thalweg location is on the cross section. It will then move outward from the thalweg until the requested channel depth is reached within the maximum channel width specified.
Note that there are other methods for assigning channel bank stations, as described in this section.
Assignment of Manning’s Roughness and Flow Lengths
During the construction of the HEC-RAS cross section, the software will automatically assign a default Manning’s roughness for the left overbank, channel, and right overbank areas.
See this section on how to define the default roughness values for the software to use. In addition, the flow length to the next downstream cross section is determined. If inserting a new cross section between two adjacent cross sections, the software will automatically adjust the flow length of the next upstream cross section to account for the insertion of the new cross section.