Extract Cross Section Geometry command

The Extract Cross Section Geometry command can extract the cross section geometry from an underlying elevation grid where there is valid elevation data. The software will automatically recognize the limits of the elevation data and extract the cross section geometry where this data exists. It will not try to extract the cross section geometry data where there is no valid elevation data.

For example, the underlying elevation grid might be of the river channel only. Previously, extracting the cross section geometry for this area required that the user define the channel bank stations at the limits of the elevation data and then extract the cross section geometry only for the cross section channel. If the user tried to extract the cross section geometry for the overbank areas, the overbank geometry would be flat.

Follow these steps to use the Extract Cross Section Geometry command:

  1. From the Input ribbon menu, select the Cross Sections menu item and then choose the Extract Cross Section Geometry command.
    Extract hec-ras cross section geometry ribbon menu
  2. The Extract Cross Section Geometry dialog box will be displayed.
    Extract Cross Section Geometry dialog box

The following sections describe the Extract Cross Section Geometry command and how to interact with the above dialog box.

Extract Geometry for Selected Cross Sections

This section is used to manually select the cross sections for extracting geometry. If a cross section is already selected on the Map View prior to running this command, the same cross section will be shown selected within the table.

Alternatively, click the [Pick] button to interactively select the cross sections from the Map View. Clicking the [Select All] button causes all the cross sections to be selected. Clicking the [Clear All] button causes all the cross sections to be deselected. In addition, the user can filter the specific river reaches to be shown in the table listing.

Extract Data

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

Extract Data

Extract Elevation Data

This section is used to define the elevation data source to be used 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.

See this section for information on the types of terrain elevation data that can be used for constructing cross sections.

Cross Section Geometry Extraction Control

This optional section is used to control the amount of the cross section geometry to extract for the selected polylines. This assures the user that adequately deep cross sections are 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 polylines do not extend far enough outward for the cross section depth specified, the software will automatically extend the cross sections. Similarly, if the selected polylines extend too far outward for the cross section depth specified, the software will automatically trim the cross sections.

Cross Section Extraction Options

This section is used to control what portions of the cross section(s) should be extracted. For example, the user may have survey data for the cross section channel and a LIDAR digital terrain surface for the overbank areas. In this example, the user would only want to extract the cross section geometry for the overbank areas since the surveyed channel geometry is already accurate.

The dropdown combo box entry has the following options available:

  • Entire Cross Section (default)
  • Both Overbanks
  • Channel Only
  • Left Overbank Only
  • Right Overbank Only

In addition, the cross section flow lengths are automatically updated.

Other Data

This tabbed panel is used to define channel bank locations based upon the options selected for the selected cross sections.

Other Data

Assignment of 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. In addition, the software will compute the cross section flow lengths based upon the length of the river reach polyline from the current cross section to the next downstream cross section.

Extracting the Cross Section Geometry

When all the options have been defined, click the [OK] button and the software will extract the cross section geometry from the elevation terrain.

About the Author Chris Maeder

Chris Maeder

Chris is an experienced civil engineering and software technology leader, with over 30 years industry experience. With proven expertise in global software development, he has built engineering teams that adapt quickly, focus on what’s important and, most importantly, deliver. He is a licensed professional civil engineer with extensive experience in water resource engineering. He has performed and supervised engineering projects in urban stormwater drainage, transportation and roadway drainage, storm sewer design, detention pond design, stormwater quality, green infrastructure, watershed management planning, wastewater sewers, water distribution networks, pump stations, FEMA flood studies, bridge and culvert design, bridge scour and armoring, dam failure analysis, seepage and groundwater modeling, and environmental permits.

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