2D Ineffective Flow Areas

Ineffective flow areas can be assigned to the 2D mesh to account for regions where the water is not actively being conveyed. The water will pond in such areas, and its velocity in the downstream direction will be close to zero. This water is included in the storage calculations, but it is not included as part of the active flow area. When using ineffective flow areas, no wetted perimeter friction is included at the boundary between the ineffective flow area and active flow area.

Ineffective Flow Areas

2D ineffective flow areas are assigned using polygons. The software will automatically refine the 2D mesh to accommodate the ineffective flow area shape.

HEC-RAS 2D ineffective flow areas

2D ineffective flow areas can be defined using the following commands:

  • Assign 2D Ineffective Flow Areas
  • Draw 2D Ineffective Flow Areas

Assign 2D Ineffective Flow Areas Command

If polygons have already been drawn on the Map View or a polygon shapefile layer loaded to represent ineffective flow areas, then the Assign 2D Ineffective Flow Areas command can be used to assign 2D ineffective flow areas. Follow these steps:

  1. From the Input ribbon menu, click the 2D Flow Areas menu item, and then select the Assign 2D Ineffective Flow Areas command.Assign 2D ineffective flow areas command
  1. The Assign 2D Ineffective Flow Areas dialog box will be displayed.
    Assign 2D Ineffective Flow Areas dialog box

There are two options for assigning ineffective flow areas to a 2D mesh:

  • Select Multiple Polygons
  • Select Polygon Shapefile

The following sections describe how to use these assignment options.

Select Multiple Polygons Option

This option allows multiple polygons to be interactively selected from the Map View and then assigned as ineffective flow areas. Follow these steps:

  1. Choose the Select Multiple Polygons radio button option. Then, click the [Pick] button.
    Assign 2D Ineffective Flow Areas - [Pick]
  1. The Assign 2D Ineffective Flow Areas dialog box will temporarily disappear and a prompt will be displayed on the status line informing what to do next.
  2. Select the polygons on the Map View. Then, right-click and choose Done from the displayed context menu.
  3. The Assign 2D Ineffective Flow Areas dialog box will redisplay.
  4. Click the [Apply] button. The selected polygons will be assigned as 2D ineffective flow areas.
    Assign 2D Ineffective Flow Areas - Select Polygons

Select Polygon Shapefile Option

This option allows a polygon shapefile layer to be used in assigning 2D ineffective flow areas. Follow these steps:

  1. Choose the Select Polygon Shapefile radio button option.
    Assign 2D Ineffective Flow Areas - Select Polygon Shapefile Layer
  1. The software will display all loaded polygon shapefile layers in the Shapefile layer dropdown combo box. Select the corresponding shapefile layer.
  2. Click the [Apply] button. The polygons on the shapefile layer will be assigned as 2D ineffective flow areas.
    Assign 2D Ineffective Flow Areas - Apply Polygon Shape FIle Layer as Ineffective Flow Areas

Draw 2D Ineffective Flow Areas Command

The user can interactively draw polygons on the Map View for defining the ineffective flow areas. Follow these steps:

  1. From the Input ribbon menu, click the 2D Flow Areas menu item, and then select the Draw 2D Ineffective Flow Areas command.
    Draw 2D Ineffective Flow Areas command
  1. The Draw 2D Ineffective Flow Areas dialog box will be displayed.
    Draw 2D Ineffective Flow Areas dialog box
  1. Click the [Draw] button. The Draw 2D Ineffective Flow Areas dialog box will temporarily disappear and a prompt will be displayed on the status line informing what to do next.
  2. Draw the polygon on the Map View.
  3. After the polygon has been drawn, the Draw 2D Ineffective Flow Areas dialog box will redisplay.
  4. Click the [Apply] button.
    Draw 2D Ineffective Flow Areas - [Apply]
  1. Repeat the process for all other polygon areas to be defined as 2D ineffective flow areas.

Incorporating Ineffective Flow Areas into the 2D Mesh

After the ineffective flow area polygons have been defined, the software will stamp them into the 2D mesh flow area and refine the mesh to account for each polygon’s shape. Follow these steps:

  1. Double-click on the 2D mesh on the Map View to display the 2D Flow Area Data dialog box.
  2. Click the [Update] button and the software will incorporate the 2D ineffective flow areas into the 2D mesh.
    Update 2D Mesh

About the Author Chris Maeder

Related Articles