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Draw and Assign Roadway Crossings Command

HEC-RAS allows the user to model roadway crossings, consisting of single or multiple bridge openings and culverts. In GeoHECRAS, the user can draw roadway crossings interactively on the map view or assign the already imported GIS shapefiles as a roadway crossing.

Creating a Roadway Crossing

The Draw Roadway Crossings or Assign Roadway Crossings commands can be used to create a roadway crossing. Both commands work similarly.
Draw and Assign Roadway Crossings Command Image 1

Draw Roadway Crossings

The Draw Roadway Crossings command allows the user to interactively draw the centerline of the roadway crossing on the Map View.

Follow the steps below to use the Draw Roadway Crossings command:

  1. From the Input ribbon menu, click the Roadway Crossings dropdown menu, and then select the Draw Roadway Crossings command.
    Draw Roadway Crossings Command Image 2
  2. The Draw Roadway Crossings dialog box will be displayed.
    Draw Roadway Crossings Dialog Box Image 3

The below sections describe how to draw a roadway crossing on the Map View and interact with the above dialog box.

Drawing Roadway Crossing Polyline

The Draw Roadway Crossing Polyline section is used to draw the roadway crossings on the Map View using polylines.

To draw a roadway crossing polyline, follow the steps below:

  1. Click on the [Draw] button adjacent to the Structure centerline field. Note that the user can use the Create curvilinear polyline checkbox option to make a smooth outline for the drawn polyline.
    Draw Roadway Crossings Polylines Image 4
  2. The Draw Roadway Crossings dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user on what to do next.
  3. Draw the roadway crossing polyline on the Map View. While drawing elements, the user can use the [Ctrl] key to switch between the curvilinear and the linear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.
  4. When finished, press the [Enter] key or right-click and select Done from the displayed context menu.
  5. The Draw Roadway Crossings dialog box will be redisplayed, and the status of the Structure centerline read-only field will be changed from Not Drawn to Drawn.
    Draw Roadway Crossings Polylines Image 5

Roadway Crossing General Specifications

This section is used to provide general specifications for the roadway crossing.
Roadway Crossing General Specifications Image 6

Note that the software automatically picks up the value for the Roadway crossing river station. This river station ID identifies the upstream end of the roadway crossing. The user can use the Description box to describe the location of the roadway crossing in more detail.

General Options

The General Options tabbed panel is used to specify the structural dimensions and weir overflow specifications of the roadway crossing.
General Options Dialog Box Image 7

Roadway Structure Dimensional Specifications

This section is used to define the width, distance, and slope of the road crossing.

Roadway Width (Parallel to Flow)

The Roadway width (parallel to flow) entry field is used to enter the width of the roadway crossing along the stream. Alternatively, the user can click the [Pick] button to measure roadway width parallel to flow from the Map View.

Distance from Railing to Upstream XS

The Distance from railing to upstream XS entry field is used to enter the distance between the upstream side of the roadway and the cross section immediately upstream of the roadway. Alternatively, the user can click the [Pick] button to measure the distance from the Map View.

Upstream Embankment Side Slope (V:H)

The Upstream embankment side slope (V:H) entry field is used to enter the slope of the road embankment on the upstream side of the roadway. The slope should be entered as the horizontal to vertical distance ratio of the roadway crossing.

Downstream Embankment Side Slope (V:H)

The Downstream embankment side slope (V:H) entry field is used to enter the slope of the road embankment on the downstream side of the roadway. The slope should be entered as the horizontal to vertical distance ratio of the roadway crossing.

Roadway Weir Overflow Specifications

This section is used to define the minimum weir elevation, maximum submergence ratio, weir crest shape, and weir coefficient for an overflow weir on a roadway.
Roadway Weir Overflow Specifications Image 8

Minimum Weir Elevation (optional)

The Minimum weir elevation (optional) entry field allows the user to enter the minimum elevation for which weir flow will begin to be evaluated. If this field is left blank, the elevation that triggers weir flow is based on the lowest high chord elevation on the upstream side of the roadway. Alternatively, the user can click the [Pick] button to select elevation from the cross section on the Map View.

Maximum Submergence Ratio

The Maximum submergence ratio entry field defines the maximum allowable submergence ratio that can occur during weir flow calculations over the roadway. By default, the software uses a value of 0.98 (98 percent submerged).

Overflow Weir Crest Shape

The Overflow weir crest shape dropdown combo box entry allows the user to specify weir types. There are two options available: Broad Crested and Ogee. The user can choose the weir type that best matches the problem.

If the user selects the Ogee shaped weir, two additional parameters: Spillway approach height and Design energy head are displayed in the Roadway Weir Overflow Specifications section.
Roadway Weir Overflow Specifications Image 9

Spillway Approach Height

The Spillway approach height entry field defines the height which is equal to the elevation of the spillway crest minus the mean elevation of the ground just upstream of the spillway.

Design Energy Head

The Design energy head entry field defines the height which is equal to the energy grade line elevation minus the elevation of the spillway crest.

Weir Coefficient (Cd)

The Weir Coefficient (Cd) entry field allows the user to enter a weir coefficient that will be used in the weir computations. Note that different coefficients may be selected, depending on use of Metric (SI) or US Units. By default, the software uses a value of 2.6. Clicking on the […] button will display a lookup dialog box containing weir coefficients.
Weir Coefficients Image 10

Note that if the Ogee shaped weir is selected, the user can click [Compute] and compute the weir coefficient based on the approach and design energy head.

Terrain Options

The Terrain Options tabbed panel is used to define the source elevation data that can be used to extract the roadway geometry defined from the digital terrain layers.
Terrain Options Dialog Box Image 11

Extract Roadway Geometry

This optional section is used to select the source elevation data necessary to extract roadway geometry. By default, the Extract Roadway Geometry section is enabled. The user can uncheck the Extract Roadway Geometry checkbox to disable this section. If this section is disabled, the sections that appear below it will be unavailable (i.e., grayed out), and the software will not extract the roadway 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 roadway 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 to construct 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 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.

Select Elevation Data Extraction Source

This section is used to define the location of the extracted terrain source on the Map View.

The Extract elevation data from roadway centerline option is used to extract the roadway geometry from the centerline of the roadway. By default, this option is selected.

The Extract elevation data from roadway edges option is used to extract the roadway geometry from the edges (left and right) of the roadway.

When the data has been defined in the Draw Roadway Crossings dialog box, click the [Apply] button. The software will create a roadway crossing in accord with the user’s centerline drawing.

Assign Roadway Crossings

This command allows the user to manually assign an already existing roadway centerline or feature line as a roadway crossing.

Follow the steps below to use the Assign Roadway Crossings command:

  1. From the Input ribbon menu, click the Roadway Crossings dropdown menu, and then select the Assign Roadway Crossings command.
    Assign Roadway Crossings Command Image 12
  2. The Assign Roadway Crossings dialog box will be displayed.
    Assign Roadway Crossings Dialog Box Image 13

The below section describes how to assign a roadway crossing on the Map View and interact with the above dialog box.

Selecting Roadway Crossing Polyline

The Select Roadway Crossing Polyline section allows the user to select the existing polyline representing the roadway centerline from the Map View.

Follow the steps below to select the roadway centerline from the Map View:

  1. Click the [Pick] button adjacent to the Structure centerline.
  2. The Assign Roadway Crossings dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user on what to do next.
  3. Select the polyline that represents the roadway centerline from the Map View.
  4. After selecting the polyline, the Assign Roadway Crossings dialog box will be redisplayed, and the status of the Structure centerline read-only field will be changed from Not Selected to Selected.
    Select Roadway Crossings Polylines Image 14

Note that the remaining sections of the Assign Roadway Crossings command are similar to that of the Draw Roadway Crossings command. To learn more about them, see the above sections of this article.

When the data has been defined in the Assign Roadway Crossings dialog box, click the [Apply] button. The software will then assign the selected polyline as a roadway crossing.

About the Author Chris Maeder

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