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Draw and Assign Inline Structures Command

In GeoHECRAS, inline structures are used to model inline dams, weirs, and gated structures with radial gates (often called tainter gates), vertical lift gates (sluice gates), overflow gates (open to the air or with a closed top), gates modeled with user-defined curves, culverts, culverts with flap gates, user-defined outlet rating curves, and user-specified outlet time series. The spillway crest of the gates can be modeled as an ogee shape or broad crested weir.
Water being released from all 28 spillway gates

Inline structures are used to locate the centerline of hydraulic structures that span a water course (e.g., dam, weir, etc.). The inline structure should be created from left to right when looking downstream, crossing a river line exactly once.

HEC-RAS inline structures can be defined by either drawing or assigning the polyline using the following commands:

  • Draw Inline Structures
  • Assign Inline Structures

The following conditions should be met in order to draw or assign inline structure polylines:

  • Inline structure polylines cannot be drawn without an underlying river reach alignment, which has already been defined. At least two bounding cross sections must have already been defined for the underlying river reach.
  • Inline structure polylines must not overlay more than one river reach.
  • Inline structure polylines should not cross another cross section.
  • Inline structure polylines must be continuous (not broken into separate segments).

Drawing Inline Structures

The Draw Inline Structures command is used to manually draw polylines, one-by-one, as inline structures. Generally, these polylines are drawn along the centerline of the dam structure across the river.

Follow the steps below to draw inline structures:

  1. From the Input ribbon menu, expand the Inline Structures dropdown combo box and select the Draw Inline Structures command.
    Draw inline structure ribbon menu command
  2. The Draw Inline Structures dialog box will be displayed.
    Draw inline structure dialog box

The following sections describe how to draw inline structures and interact with the above dialog box.

Drawing Inline Structure Polyline

The Draw Inline Structure Polyline section allows the user to interactively draw individual polylines on the Map View to associate as inline structures.

The user can click the [Draw] button to interactively draw the inline structure centerline (polyline) on the Map View. Use the Create curvilinear polygon checkbox to draw the polyline using curvilinear segments.
[Draw] button

After clicking the [Draw] button, the Draw Inline Structures dialog box will temporarily disappear, and a prompt will be displayed on the status bar instructing the user what to do next. The user can interactively draw the inline structure polyline on the Map View. While drawing elements, the user can use the [Ctrl] key to switch between linear and curvilinear digitizing. Refer to this article in our knowledge base to learn more about drawing elements on the Map View.

When drawing the inline structure polyline, click on a starting station for the inline structure and click on station locations where the inline structure changes direction. Finally, click an ending station for the inline structure and then press the [Enter] key or right-click and select Done from the displayed context menu.

After the polyline is drawn, the dialog box will be redisplayed, and the status of the Structure centerline read-only field will change to Drawn. In addition, the drawn polyline will be highlighted on the Map View. The user can then proceed with entering the remaining data.

Note that the software is able to determine the direction of the inline structure (left to right, looking in a downstream direction) by referencing the flow direction of the underlying river reach.

Inline Structure General Specifications

This section is used to enter general specifications for the drawn inline structure.
Inline structure general specifications

The software automatically defines the river station for the drawn inline structure polyline in the Inline structure river station numerical field. This field represents a unique value (lookup key) for the current river reach, used to identify an inline structure among other cross sections, roadway crossings, inline structures, and lateral structures.

Alternatively, the user can use the Description field to describe the current inline structure in more detail.

General Options

The General Options tabbed panel is used to specify the structural dimensions and weir overflow specifications of the inline structure.
General options tabbed panel

Inline Structure Dimensional Specifications

This section defines the dimensions of the inline structure.
  • Structure width (parallel to flow)
    This entry field specifies the width of the inline structure (measured along the flow direction). By default, the software uses a value of 46 feet. The user can enter a different value or click the [Pick] button to interactively measure this width from the Map View.
  • Distance from US face to US XS
    This entry field specifies the distance from the upstream side of the inline structure to the cross section immediately upstream of the inline structure. By default, the software uses a value of 10 feet. The user can enter a different value or click the [Pick] button to interactively measure this distance from the Map View.
  • Upstream embankment side slope (V:H)
    This entry field specifies the embankment side slope of the inline structure (measured in the direction of flow) on the upstream face of the structure. The slope is represented by the number of horizontal units per vertical unit. For vertical side slope embankments, enter a value of 0. By default, the software uses a value of 3.0 (i.e., 1:3).
  • Downstream embankment side slope (V:H)
    This entry field specifies the embankment side slope of the inline structure (measured in the direction of flow) on the downstream face of the structure. The slope is represented by the number of horizontal units per vertical unit. For vertical side slope embankments, enter a value of 0. By default, the software uses a value of 3.0 (i.e., 1:3).

Inline Structure Weir Specifications

This section defines the parameters defining spillway flow through the inline structure.
  • Spillway weir crest shape
    This dropdown combo box defines spillway shape at the inline structure. The following options are provided:

    1. Broad Crested (default) – This option should be used with typical spillways.
    2. Ogee – This option should be used with ogee spillways.
      If the user selects the Ogee shaped weir, two additional parameters: Spillway approach height and Design energy head are displayed in the Inline Structure Weir Specifications section.
      Spillway approach height and design energy head fields
  • Spillway approach height
    This entry field is used to specify the height that is equal to the elevation of the spillway crest minus the mean elevation of the ground just upstream of the spillway.
  • Design energy head
    This entry field is used to specify the height that is equal to the energy grade line elevation (at the design discharge) minus the elevation of the spillway crest.
  • Weir coefficient (Cd)
    This entry field specifies the coefficient of discharge for use in the weir flow equation for spillway flow. By default, the software uses a value of 2.60. The user can click the […] browse button to display a weir coefficient reference dialog box.
    Weir coefficients dialog boxIf an ogee shaped spillway is defined, then after the Spillway Approach Height and Design Energy Head have been defined, clicking on the [Compute] button causes the software to calculate the weir coefficient at the design discharge (corresponding to the design head).

Other Specifications

This section contains an optional field: Pilot (low base) flow. This field is used only for unsteady flow computations. This entry defines the minimum flow that will be released from the structure. The HEC-RAS unsteady flow computations require no cross sections to go dry during the simulation. Therefore, if water is not flowing over the spillway at an inline structure, the user can use this field to ensure that there is always some minor flow going through the structure.

Terrain Options

This tabbed panel is used to define the specifications for extracting the weir crest geometry of the drawn inline structure.
Terrain options tabbed panel

Extract Weir Crest Geometry

This optional section is used to define the elevation data source(s) to be used for extracting the weir crest 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 weir crest geometry. Depending on the selected elevation data source type, 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.

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 is unavailable, the software will use the elevation data from the secondary data source.

Note that the user cannot utilize the same data source for defining the 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 the section checkbox is unchecked, then the subsequent panels below it will be unavailable (i.e., grayed out). In addition, no geometry will be created when the inline structure is created. The inline structure will just be a flat horizontal line at elevation 0.

When the data has been defined in the Draw Inline Structure dialog box, click the [Apply] button. The software will create an inline structure where the user has drawn the centerline.

Assigning Inline Structures

The Assign Inline Structures command is used to manually associate, one by one, previously drawn polylines as an inline flow control structure (i.e., dam). Generally, these polylines are drawn along the centerline of the structure, across the river.

Follow the steps below to assign a polyline as an inline structure centerline:

  1. From the Input ribbon menu, expand the Inline Structures dropdown combo box and select the Assign Inline Structures command.
    Assign inline structure ribbon menu command
  2. The Assign Inline Structures dialog box will be displayed.
    Assign inline structure dialog box

The below section describes how to associate individual polylines with specific inline structures and interact with the above dialog box.

Selecting Inline Structure Polyline

This section allows the user to interactively select individual polylines on the Map View to be associated as inline structures.

Follow the steps below to select the inline structure polyline from the Map View:

  1. Click the [Pick] button adjacent to the Structure centerline field.
  2. The Assign Inline Structures dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user what to do next.
  3. Select the inline structure polyline from the Map View.
  4. After selecting the polyline, the dialog box will be redisplayed, and the status of the Structure centerline read-only field will be changed from Not Selected to Selected.
    Structure centerline read-only field
  5. The remaining sections of the Assign Inline Structures command are similar to that of the Draw Inline Structures command. Refer to the above sections to learn how to define the data for these sections.

Note that if the user has first selected a polyline from the Map View before running this command, then the Structure centerline read-only field will show Selected. Otherwise, the field will show Not Selected, and the user can interactively select the inline structure polyline from the Map View.

When the data has been defined, click the [Apply] button. The software will create the inline structure and associate the selected polyline as the structure’s centerline.

About the Author Chris Maeder

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