Assigning Survey Data to HEC‑RAS Cross Sections

LiDAR has become an established method for collecting very dense and accurate elevation data across landscapes, shallow-water areas, and project sites. Many LiDAR systems operate in the near-infrared region of the electromagnetic spectrum and cannot penetrate water, as shown below. As such, the terrain model is flat in these areas.

Cannot Penetrate Water-Assign Survey Data to HEC-RAS Cross Sections

However, some LiDAR sensors operate in the green region of the electromagnetic spectrum and can penetrate water and detect the channel bottom features. These bathymetric LiDAR systems can be used in areas with relatively clear water to measure channel bed elevations. However, many times it is necessary to combine survey of airborne LiDAR (green) and sonar (red), as shown below.

Lidar & Sonar Mapping - How to Assign Survey Data to HEC-RAS Cross Sections?

In order to reduce bathymetric survey costs, it is only necessary to field survey the river to extract channel cross sections. These channel cross sections can then be merged with overbank cross sections created from a LiDAR derived surface terrain model.

LiDAR River Channel Survey

A common problem with surveyed channel cross section data is that the surveyed point data does not follow precisely along a straight line across the river, but instead waivers in the X-Y plane. To solve this problem, the Input | Cross Sections | Conflate Point Data command can be used to collapse the surveyed channel cross section data to a selected cross section alignment.

Conflate Point Data dialog box

The following survey cross section point data can be used with the Conflate Point Data command:

  • AutoCAD Drawing
  • GIS Point Shapefile
  • XYZ Point Elevation Data

The Conflate Point Data command allows the user to define the buffer distance for the selected cross sections in which the surveyed cross section data should be snapped to the cross sections. The assignment of the surveyed cross section data can be assigned based upon the following methods.

Conflate Point Data Description

The Conflate Point Data command allows the user to control the extent of the surveyed cross section data to be assigned as geometry data to the selected cross sections:

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

The Conflate Point Data command also provides the option to replace the existing cross section geometry data with the surveyed cross section data.

Below is an example of the Conflate Point Data command using channel survey data for an existing cross section that was originally created using LIDAR data, replacing the channel geometry of the cross section.

Sample Conflate Command

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