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4. Interpolate Cross Sections Command

# Interpolate Cross Sections Command

The Interpolate Cross Sections command can be used to automatically add interpolated cross sections to a river reach. The interpolated cross sections can be added for an entire river reach or just part of a reach.

While it is always better to define “real” cross sections by extracting the cross sections from an underlying terrain surface, there are times when this is not feasible. For example, the existing cross sections might have been created by compositing the overbank geometry with the channel geometry using different data sources. The overbank geometry might have been extracted from a terrain surface, whereas the channel geometry may have been constructed from field survey data. In these situations, the Interpolate Cross Sections command can be used to fill-in cross sections between the existing cross sections. If desired, after the interpolated cross sections have been constructed, the user can then re-extract only the overbank geometry from the terrain surface using the Extract Cross Section Geometry command to create more accurate interpolated cross sections. Refer to this article in our knowledge base to learn more about the Extract Cross Section Geometry command.

Interpolated cross sections are useful when there is too large of a change in the velocity head between existing cross sections and the software cannot accurately calculate the energy gradient. An adequate depiction of the energy gradient change is important for accurately modeling friction losses as well as contraction and expansion losses. HEC-RAS will report the following warning messages for these situations as well as other situations where additional cross sections may be required:

• The energy equation could not be balanced within the specified number of iterations. The program used critical depth for the water surface and continued on with the calculations.
• The energy equation could not be balanced within the specified number of iterations. The program selected the water surface that had the least amount of error between computed and assumed values.
• During the standard step calculations, the final energy answer that was computed is less than the downstream energy. This is not physically possible. Please check your data.
• Critical depth could not be determined within the specified number of iterations. The program used the iteration with the lowest energy.
• The energy loss was greater than 1.0 ft (0.3 m) between the current and previous cross section. This may indicate the need for additional cross sections.
• The maximum number of iterations were exceeded in calculating the wide river ice jam thickness between this cross section and the adjacent upstream cross section.
• The energy computed by the inline structure equations at the upstream cross section is lower than the energy at the downstream cross section. The energy at the upstream cross section has been set to the energy at the downstream cross section.
• The split flow optimization for the lateral structure failed to converge within the maximum number of iterations. The results from the final iteration were used.

When adding interpolated cross sections, the software will automatically add and space the cross sections along the river reach as per the user-defined specifications, positioning the cross sections perpendicular to the channel centerline.

1. From the Input ribbon menu, click the Cross Sections menu item, and then select the Interpolate Cross Sections command.
2. The Interpolate Cross Sections dialog box will be displayed.

The following sections describe the Interpolate Cross Sections command and how to interact with the above dialog box.

## Selecting River Reach

The Select River Reach section describes how to select the river reach in order to perform automatic cross section interpolation. Follow the steps below:

1. Click on the River name dropdown combo box and then select the river identified for cross section interpolation.
2. If there are multiple reaches for the selected river, then click on the Reach name dropdown combo box and then select the reach identified for cross section interpolation.

Alternatively, click the [Pick] button. The Interpolate Cross Sections dialog box will temporarily disappear. The software will then prompt the user to select the river reach from the Map View. After selecting a river reach, the Interpolate Cross Sections dialog box will redisplay with the river reach shown selected.

## Selecting Cross Section Range

The Select Cross Section Range section is used to define the cross section range along the selected river reach, in which the cross section interpolation will be applied. Follow the steps below:

1. Click on the Upstream cross section dropdown combo box and then select the cross section that will be used as the upstream most river station for the interpolation. By default, the program will select the cross section at the upstream end of the river reach.
2. Click on the Downstream cross section dropdown combo box and then select the cross section that will be used as the downstream most river station for the interpolation. By default, the program will select the cross section at the downstream end of the river reach.

Alternatively, click the [Pick] buttons adjacent to the Upstream cross section and Downstream cross section dropdown combo boxes and select the corresponding cross sections from the Map View. After clicking on a [Pick] button, the Interpolate Cross Sections dialog box will temporarily disappear. The software will then prompt the user to select the associated cross section from the Map View. After selecting a cross section, the Interpolate Cross Sections dialog box will redisplay with the cross section shown selected.

## Cross Section Specifications

This section is used to define the maximum allowable distance between cross sections. If the channel distance between any two existing cross sections is greater than the maximum allowable distance defined by the user, then the software will interpolate cross sections between these two cross sections. The software will interpolate as many cross sections as necessary to get the distance between the cross sections below the specified maximum allowable distance.

Enter the distance for the Maximum cross section spacing entry field. Or, click the […] button. The Interpolate Cross Sections dialog box will temporarily disappear, and a prompt will be displayed on the status line informing the user what to do next. From the Map View, measure the maximum cross section spacing distance to be used, and then right-click and choose Done from the displayed context menu. The Interpolate Cross Sections dialog box will reappear with the measured distance displayed.

The [Delete Interpolated Cross Sections] button can be used to delete previously created interpolated cross sections for the selected river reach. Note that this button is disabled if there are no interpolated cross sections defined for the selected river reach. Alternatively, the Delete Cross Sections command can be used to interactively select cross sections to delete.

After the cross section interpolation options have been defined, click the [OK] button and the software will interpolate the cross sections.