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Draw and Assign Cross Sections Command (HEC-HMS)

Cross sections are located at intervals along a stream to characterize the flow-carrying capability of the stream and the adjacent floodplain. They should extend across the entire floodplain and should be perpendicular to the anticipated flow lines. Sometimes it is necessary to layout cross sections in a curved or a dog-leg alignment to meet this requirement. They should not intersect each other and should remain perpendicular to the main channel.

In GeoHECHMS, cross sections can be defined by either drawing or assigning polylines on the Map View using the following commands:

  • Draw Cross Sections
  • Assign Cross Sections

Drawing/Assigning Cross Sections

The Draw/Assign Cross Sections command allows the user to manually draw/assign polylines on the Map View as cross sections one after another until completed as well as to extract the cross section geometry from the underlying ground terrain.

Follow the steps below to use the Draw/Assign Cross Sections command:

  1. From the Input ribbon menu, click the Cross Sections dropdown menu and then choose the Draw/Assign Cross Sections command.
    Draw/Assign Cross Sections Input ribbon menu commands
  2. The following dialog boxes will be displayed.
    • Draw Cross Sections:

      Draw Cross Sections dialog box
    • Assign Cross Sections:
      Assign Cross Sections dialog box

Note that the user can press the shortcut key (Ctrl+D) to automatically start the Draw Cross Sections command.

The following sections describe how to use the Draw and Assign Cross Sections commands and interact with the above dialog boxes.

Drawing Cross Sections

Drawing Cross Sections Polylines

This section is used to interactively draw a polyline on the Map View to assign it as a cross section. It is highly recommended that the rest of the dialog box input parameters be defined before drawing the cross sections on the Map View. These parameters define information, such as how the cross section IDs should be numbered and where the cross section geometry will be extracted from.

To draw cross section polylines, follow the steps below:

  1. From the Draw Cross Section Polylines section, click the [Draw] button, and the Draw Cross Sections dialog box will temporarily disappear. Use the Draw curvilinear polyline checkbox option to draw the polyline using curvilinear segments.
    [Draw] button
  2. Draw cross section polylines across the river from one side to the other on the Map View. While drawing elements, the user can use the [Ctrl] key to switch between curvilinear and linear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.
    Note that the drawn polyline must cross an existing river reach. Otherwise, the software will report this as an issue.
  3. After completing the cross section drawing, the Draw Cross Sections dialog box will be redisplayed, and the Cross section polylines read-only field will be changed from Not Drawn to Drawn.
    Cross section polylines read-only field

Assigning Cross Sections

Selecting Cross Section Polylines

The Select Cross Section Polylines section can be used to manually assign polylines on the Map View as a cross section.

To assign polylines as a cross section, follow the steps below:

  1. From the Select Cross Section Polylines section, click the [Pick] button and the dialog box will temporarily disappear.
    [Pick] button
  2. The status bar (shown under the Map View) will prompt you to select a polyline from the Map View to assign it as a cross section. Click on the previously drawn polyline on the Map View to select it.
  3. Following the selection of a cross section polyline, the Assign Cross Sections dialog box will be redisplayed, and the Cross sections polylines read-only field will be changed from Not Selected to Selected.
    Cross sections polylines read-only fieldNote that if a polyline has been preselected before running this command, the Cross section polylines read-only field will be shown selected on opening the Assign Cross Sections command.

Cross Section Specifications

This section is common to both the Draw Cross Sections and Assign Cross Sections dialog box and is used to specify the cross section ID for each drawn/assigned cross section. The user can assign these IDs either manually or automatically using some predefined formats.

Follow the steps below to assign cross section IDs:

  1. If a cross section was drawn/assigned while the Cross section ID radio button option was selected, the user can manually enter the cross section ID in the corresponding field as shown below.
    Cross section ID radio button option
  2. Alternatively, the user can enable the Auto-name cross section ID radio button option to automatically name every newly drawn/assigned cross section as per the user’s predefined naming formats as shown below.
    Auto-name cross section ID radio button optionThe different cross section naming formats present in the Auto-name cross section ID option are as follows:

    • Use corresponding reach numerical ID portion: This option extracts the numerical ID from the corresponding reach ID and applies it to the cross section ID. Selecting this option will disable (i.e., grayed out) the following options:
      1. Cross section ID digits
      2. Next available cross section ID
      3. Cross section ID increment
    • Cross section ID prefix: This option allows a prefix to be added to the start of the cross section ID.
    • Cross section ID digits: This option permits the specification of a set number of digits to use for the cross section ID. For example, using 3 digits causes the cross section ID to be of the format 001, 002, etc.
    • Next available cross section ID: This entry defines the next element ID number to be used.
    • Cross section ID increment: This entry defines the increment to use when numbering elements. The default value is 1.
    • Cross section ID suffix: This option allows a suffix to be added to the end of the cross section ID.
    • Cross section ID preview: This entry provides a preview of the cross section naming specifications defined above.
  3. After providing the cross section ID, press the [Enter] key or click the [Apply] button.
    [Apply] button

Note that if the auto-name option is enabled and the user returns to the dialog box, the [Apply] button is disabled since the just drawn/assigned cross sections have already been named and created.

Other Specifications

This section is common to both the Draw Cross Sections and Assign Cross Sections dialog box and is used to define the geometry extraction specifications for each drawn/assigned cross section.
Other Specifications - Extraction Data panel

Extraction Data

This tabbed panel is used to define the data extraction specifications for the selected cross sections.

Extract Elevation Data

This optional section is used to define the elevation data source(s) to be used for extracting the cross section geometry. The user can use the Primary and Secondary Elevation Data panels to define the primary and secondary (if available in the project) elevation data sources for extracting the cross section geometry. Depending upon the elevation data source type that is selected, different options are provided to specify additional elevation data information.

Refer to this article in our knowledge base for information on the types of terrain elevation data that can be used for constructing cross sections.

When a secondary elevation data source is available, the software will form a concave hull around the primary elevation data source to identify its bounds. For locations where elevation data from the primary data source are unavailable, the software will use the elevation data from the secondary data source.

Note that the user cannot utilize the same data source to define both primary and secondary elevation data.

The user can click the [Swap Sources] button to swap the selected elevation source from primary elevation data to secondary elevation data and vice versa.

If this section checkbox is unchecked, then the subsequent section below it (Cross Section Geometry Extraction Control section) will be unavailable (i.e., grayed out).

Cross Section Geometry Extraction Control

This optional section is used to control the amount of cross section geometry to extract for the drawn cross section polylines. This assures that an adequately deep enough 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 specified maximum cross section width.

If the drawn polyline does not extend outward far enough to get the cross section depth specified, the software will automatically extend the constructed cross section further outward. Similarly, if the drawn polyline extends too far outward for the depth specified, the software will automatically trim the constructed cross section.

Assigning Manning’s Roughness and Flow Lengths

During the extraction of the cross section geometry, the software will automatically assign a default Manning’s roughness for the left overbank, channel, and right overbank areas. However, the user can adjust these Manning’s roughness values in the Roughness tabbed panel.
Other Specifications - Roughness panel

Note that while defining Manning’s n values, the user can click the […] button to display the Manning’s Roughness dialog box. This dialog box provides a reference to Manning’s roughness coefficients for some commonly used surface materials.
Manning’s Roughness dialog box

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