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Draw and Assign SA/2D Connections Command

In GeoHECRAS, storage areas or 2D flow area connections are used to link two storage areas together with a hydraulic structure or two 2D flow areas, or a storage area to a 2D flow area. The user can draw storage area or 2D flow area connections interactively on the map view or assign the already imported GIS shapefiles as a storage area or 2D flow area connection. The software allows the user to place a hydraulic structure in the middle of a 2D flow area to control how flow travels from one series of cells to another series of cells. Storage area or 2D flow area connections can consist of a weir and gated spillway, culverts, bridge, just a weir, or linear routing option.

Creating SA/2D Connections

To establish a hydraulic connection between two storage areas, 2D flow areas, or inside a 2D flow area, the Draw SA/2D Connections or Assign SA/2D Connections command can be used. Both commands work similarly.
SA/2D connection dropdown menu

Drawing SA/2D Connections

The Draw SA/2D Connections command allows the user to interactively draw the centerline of the storage area or 2D flow area connections on the Map View.

Follow the steps below to use the Draw SA/2D Connections command:

  1. From the Input ribbon menu, click the SA/2D Connections dropdown menu, and then select the Draw SA/2D Connections command.
    Draw SA/2D connection command
  2. The Draw SA/2D Connections dialog box will be displayed.
    Draw SA/2D connection dialog box

The following sections describe how to draw SA/2D connections and interact with the above dialog box.

SA/2D Connection General Specification

This section is used to specify the storage area or 2D flow area connection ID. The SA/2D connection ID field is used to enter the storage area or 2D flow area connection ID to identify each connection location uniquely. The user can use the Description box to describe the location of the storage area or 2D flow area connection in more detail.

Drawing SA/2D Connection Polyline

The Draw SA/2D Connection Polyline section is used to draw the storage area or 2D flow area connection centerline on the Map View using polylines.

To draw a SA/2D connection 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] button
  2. The Draw SA/2D Connections dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user what to do next. The user can then draw polylines 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.
  3. When finished, press the [Enter] key or right-click and select Done from the displayed context menu. The software will then return the user to the dialog box, and the status of the Structure centerline read-only field will be changed from Not Drawn to Drawn.
    Structure centerline read-only field drawn
  4. The Cell spacing along polyline entry represents cell spacing along the storage area or 2D flow area connection polyline. If this entry is left blank, then the cell spacing used in the vicinity of the storage area or 2D flow area connection will be used. By default, the software uses a value of 20 ft. The user can enter a different value or click the […] button to measure the cell spacing from the Map View.
  5. The Relaxed cell spacing in 2D flow area entry represents the cell spacing further away from the storage area or 2D flow area connection polyline. This value should not be the same as the Cell spacing along polyline value, or many cell errors may be introduced along the polyline. Click on the […] button to measure the cell spacing from the Map View.

Extract Weir Crest Geometry

This section allows the user to select the source elevation data for extracting the weir crest geometry. By default, the Extract Weir Crest Geometry section is enabled. If this section is disabled, the software does not extract the weir geometry.
Extract Weir Crest Geometry section

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 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 for constructing 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 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.

Overflow Weir Specifications

The Overflow Weir Specifications section is used to define specifications for the overflow weir.
Overflow Weir Specifications section

Structure Width (Parallel to Flow)

The Structure width (parallel to flow) entry field defines the width of the top of the structure. By default, the software uses a value of 10 ft. Alternatively, the user can click the [Pick] button to measure the structure width from the Map View.

Overflow Weir Crest Shape

The Overflow weir crest shape dropdown combo box entry allows the user to specify weir types. Two options are 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 Overflow Weir Specifications section.
Overflow weir crest shape - Ogee

Spillway Approach Height

The Spillway approach height entry field is used to define 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 is used to define 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 are used depending on units such as US Units or Metric (SI) 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 dialog box

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

Compute Flow

The Compute flow dropdown combo box entry allows the user to define the computational method, which can be one of the following:

  • At Each Time Step
  • As Curves (faster)

When the data has been defined in the Draw SA/2D Connections dialog box, click the [Apply] button. The software will then create a connection on the location where the user drew the centerline.

Assigning SA/2D Connections

The Assign SA/2D Connections command allows the user to manually assign polylines as storage areas or 2D flow area connections. To use this command, a polyline that can be selected must already exist on the Map View to represent the storage area or flow area connection.

Follow the steps below to use the Assign SA/2D Connections command:

  1. From the Input ribbon menu, click the SA/2D Connections dropdown menu, and then select the Assign SA/2D Connections command.
    Assign SA/2D connection command
  2. The Assign SA/2D Connections dialog box will be displayed.
    Assign SA/2D Connections dialog box
  3. From the Select SA/2D Connection Polyline section, click the [Pick] button.
    [Pick] button
  4. The Assign SA/2D Connections dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user what to do next. The user can then select polylines from the Map View.
  5. When finished, press the [Enter] key or right-click and select Done from the displayed context menu.
  6. The software will then return the user to the Assign SA/2D Connections dialog box and the Structure centerline read-only field will be changed from Not Selected to Selected.
    Structure centerline read-only field selected
  7. The software will now treat the polyline as a storage area or 2D flow area centerline connection. Note that the remaining sections of the Assign SA/2D Connections command are similar to that of the Draw SA/2D Connections command. So, the user can refer to these sections explained above for further information.
  8. When the data has been defined in the Assign SA/2D Connections dialog box, click the [Apply] button. The software will then assign the polyline as a storage area or 2D flow area connection.

About the Author Chris Maeder

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