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# Draw and Assign 2D Flow Areas Command

2D flow areas are regions of a model in which the flow through that region will be computed with the HEC-RAS two-dimensional flow computation algorithms. 2D flow areas are defined by laying out a polygon representing the outer boundary of the 2D flow area and then specifying the computational mesh. Refer to this article in our knowledge base to learn more about 2D flow areas.

The following image shows a 2D flow area polygon boundary for an area that is protected by a levee.

HEC-RAS 2D flow areas can be defined by either drawing or assigning the polygon boundary using the following commands:

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

## Drawing HEC-RAS 2D Flow Areas

HEC-RAS 2D flow areas can be defined by drawing the polygon boundary of the 2D flow area on the Map View using the Draw 2D Flow Areas command.

Follow the steps below to draw a 2D flow area:

1. From the Input ribbon menu, click the 2D Flow Areas dropdown menu and then select the Draw 2D Flow Areas command.
2. The Draw 2D Flow Areas dialog box will be displayed.
3. Click on the [Draw] button adjacent to the 2D flow area polygon field. Note that the user can use the Create curvilinear polygon checkbox option to draw the polygon using curvilinear segments.
4. The Draw 2D Flow Areas dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user to draw polygons 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.
5. When finished, press the [Enter] key or right-click and select Done from the displayed context menu.
6. The Draw 2D Flow Areas dialog box will be redisplayed and the status of the 2D flow area polygon read-only field will be changed from Not Drawn to Drawn.
7. Provide the name for the 2D flow area in the 2D flow area ID entry field.
8. From the 2D meshing scheme dropdown combo box, select the mesh type. GeoHECRAS can create the following 2D mesh types:
• Uniform Mesh
• Hexagonal Mesh
9. The Element spacing entry allows the user to define the cell spacing used in the 2D flow area generation routine. By default, the software uses a value of 30. However, the user can enter a different value or click the […] button to measure the cell spacing from the Map View.
10. The Fix adjacent 2D flow area edges checkbox is used to add internal break lines between adjacent 2D model domains and causes adjacent 2D elements from the 2D model domains to align with each other.
11. Click on the [Apply] button and the 2D flow area will be created.

## Assigning HEC-RAS 2D Flow Areas

HEC-RAS 2D flow areas can be defined by assigning an already drawn polyline or polygon as the boundary of the 2D flow area using the Assign 2D Flow Areas command.

Follow the steps below to assign an already drawn polyline or polygon as a 2D flow area:

1. From the Input ribbon menu, click the 2D Flow Areas dropdown menu and then select the Assign 2D Flow Areas command.
2. The Assign 2D Flow Areas dialog box will be displayed.
3. Click the [Pick] button adjacent to the 2D flow area polyline/polygon field.
4. The Assign 2D Flow Areas dialog box will temporarily disappear, and a prompt will be displayed on the status bar directing the user what to do next.
5. From the Map View, click on a polyline or polygon representing the 2D flow area boundary.
6. When finished, press the [Enter] key or right-click and select Done from the displayed context menu. The Assign 2D Flow Areas dialog box will be redisplayed and the status of 2D flow area polyline/polygon read-only field will be changed from Not Selected to Selected.
Note that the 2D Flow Area General Specifications section of the Assign 2D Flow Areas command is similar to that of the Draw 2D Flow Areas command. Hence, refer to the above section of this article to learn about the options available in this section.
7. Click on the [Apply] button, and the software will treat the polyline or polygon as a 2D flow area boundary.