How to define an embedded structure for a 2D model

Updated on 21 Jun 2023
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A modelled 2D domain may contain key flow paths that are not picked up by the topography data you have available. These may be channels, open or closed, conduits transporting flows through embankments or even short pathways such as archways under a bridge. Flood Modeller provides a few options to incorporate these into a 2D model:

Modify the baseline topography (ASCII ground grid, e.g. DTM) by overlaying shapefiles detailing localised elevation changes. See How to create a topographic feature for further details.
Fully integrate the 2D model with a 1D network, using conduit units or structures (e.g. weirs, bridges or orifices) coupled to “dummy” boundaries to transfer flows to and from the 2D domain. This option has the advantage of modelling flows through an embedded channel. See Linking River Networks and 2D Components for further details.
Embed a 1D structure within the 2D model. The 2D solver offers an option to define certain 1D components using a shapefile and associated structure file (similar to a 1D event file, but with an extension “.str”). This option allows one or multiple culverts, weirs or orifices to be added to a 2D domain. The advantage of this method is the flow through the embedded structure is calculated by new components of the 2D solver. These allow the 1D component to run dry, thus can deliver a more stable solution. In addition, the interface includes simple to use tools for defining these embedded 1D components, i.e. you just draw a line to represent a channel or structure and the tool prompts for appropriate 1D properties to be defined.

This guide focuses on the final option above - embedding a 1D structure directly into the 2D domain.

Embedding 1D structures into a 2D model involves two aspects; initially defining the 1D elements, and then adding these elements to the 2D simulation.

Defining the 1D Elements

Once you have defined your 2D model extent, i.e. one or more active area polygons, you can then proceed to define any structures that lie within these areas. The following 8 steps explain how to do this.

On the '2D Build' tab of main toolbar, select the '1D Structure' tool. You will be prompted to specify a filename for the new shapefile that will be created.
The 'Layer Editor' tab will be activated. This will be configured specifically for creating new polyline shapefiles that will represent your structure locations. For each structure, you will need to first draw a polyline to represent the structure.
Representing a culvert
For culverts, the line drawn should represent the path taken by the culvert. The 2D solver will calculate the direction from the line; the direction of flow at the inlet will be represented by the first two points in your polyline and the direction at the outlet comes from the last two points. The length of the culvert will be set to the length of the line on the map, however this can be overridden by a 'length' attribute in the shapefile (see later).
Representing an orifice
For orifices, the line drawn defines the 2D grid cells that the calculated orifice flows will pass between. Thus, an orifice structure can provide a simple alternative method for representing a culvert. Lines representing an orifice must always be straight lines consisting of just two points.
Representing a weir
For weirs, the line drawn represents the direction of the weir crest. The 2D solver will therefore assume flows are perpendicular to the line. The structure length is not taken from the line length – this property will be entered by the user and the structure will assume to extend from the mid-point of the line. Lines representing a weir must always be straight lines consisting of just two points.
Note:
It is possible to define multiple structures, i.e. multiple polyline features, within a single structure shapefile. However, it is recommended to limit each shapefile to one structure, i.e. one polyline per file, as this will create a more stable solution. A 2D model can incorporate multiple embedded structure shapefiles, so you can still have multiple 1D elements defined for a single 2D domain.
After drawing a structure line, you can proceed to define your associated structure properties. Click the 'Edit 1D Structures' icon on the 'Layer Editor' tab to open a new window (this may take a few seconds to appear).
A new window is displayed listing the polyline features (lines) in your shapefile. This allows you to select one feature and associate structure data. Click the button indicated to select a structure type.

Use the radio button to select the structure required.
Adding a culvert
Culvert – select a culvert shape type, default is rectangular, but any of the shapes offered for 1D river networks can be selected in the dropdown list. In addition, there is the option to include the effects of a single bend in the culvert, i.e. additional energy losses will be calculated (tick box to include a bend).
Adding an orifice
Orifice – there are no additional options to select with this structure type.
Adding a weir
Weir – select a weir type, default is general, but any of the types offered for 1D river networks can be selected in the dropdown list.
After selecting the required structure type click 'OK' to return to the main embedded structure editor window.
Note:
If you access the structure type pop-up window for an already fully defined 1D structure, the window settings still always default to show a rectangular culvert selection (with no bend). Thus, press the 'Cancel' button to close the pop-up window without applying any changes.
After specifying a structure type for a polyline, the 1D structure editor table will show some fields automatically populated. Some of these fields can be edited directly within the table.
Items you can edit in the table:
Parameters automatically filled are 'Node1', 'Node2', 'Structure Type' and 'Sub Type'. The node labels are always fixed for embedded structures and so are locked for editing. 'Structure Type' and 'Sub Type' can only be changed via the 'Structure Type' pop-up window. However, the following fields in the table can be edited directly within the table:
- Name - you can specify a unique name for each structure by clicking on the 'Name' field and typing as required (this is an optional setting).
- Length – the length value can override particular properties for each structure type:
  - For culverts it represents reach length. The default value is zero, which instructs the solver to use the length of the drawn line. A non-zero (positive) value overrides the length of the drawn line. Note: The line on map defines the 2D cells that interact with the start and end of a culvert.
  - For weirs it represents the breadth of weir. If set to a positive, non-zero value then this will override the breadth entered in the unit property window. Note: The direction of the line drawn on the map for a weir defines the orientation of the weir crest, i.e. flow will be considered perpendicular to the drawn line.
  - For orifices the entered length represents the orifice width. It is used to calculate bore area, overriding the bore area entered in the unit property window. Note: The start and end of the line on the map defines the 2D cells that calculated orifice flows are passed between.
- Include bend – setting applies only to culverts. Tick box to include additional energy loss due to single bend in culvert reach (this can also be set in the structure type pop-up window)
- Invert drop – setting applies only to culverts. This is the change in invert level between upstream and downstream conduit sections. Although the cross-section shape is always assumed constant in the conduit, you should specify a change in elevation to create a bed slope (to ensure water can flow through the culvert).
  Note: If you try to exit this tool without setting an Invert drop a warning will be displayed informing you that invert change is set to zero (you can still proceed to close and save if zero is the desired value).
- Distribution factor – setting determines how many 2D grid cells to distribute the outflow from the 1D structure across. This factor can only be 1, 2 or 3. The default factor is 1, which puts the entire outflow into the single 2D cell underlying the end of the structure line. In most cases this setting will be sufficient to achieve a stable model. However, if a model is showing an instability in the vicinity of an embedded structure, then distributing structure outflows over an increased area might help. If the factor is set to 2 then the outflow will be distributed across the same 2D cell and the immediately adjacent cells on either side (i.e. 3 cells in total). If the factor is set to 3 then the outflow will be distributed between all cells surrounding the cell underlying the end of the structure line (i.e. 9 cells in total).
  Note: This factor applies to culverts and orifices, but not weirs. Also, outflows will only be distributed across cells that lie within the 2D active area.
You now need to define the structure properties in the same way as they are defined for a 1D river network. You can access the related 1D node property window by clicking on the appropriate 1D node icon displayed in the 'Edit' field for the polyline. This will display the appropriate 1D node properties window ready for data entry. Further details on each set of properties can be seen in the Culverts, Orifice and Weirs sections of this guide.
Note: Some of the properties available for these structures in a 1D river network are not required for embedded structures and so are disabled here (e.g. slots for conduit sections to maintain a minimum flow – as embedded structures can handle running dry).
In addition, you can instruct the 2D solver to set culvert invert level to the elevation of the underlying ground grid cell. This is achieved by setting the invert elevation in the culvert unit property window to “-9999” (i.e. the missing data flag).
This functionality is not available for symmetrical and asymmetrical conduit types as these don’t have a single invert elevation property. The functionality is also not available for weir and orifice structure types.
After defining your embedded 1D components to the required specification, close the component wizard and click the 'Save' button in the 'Layer Editor' tab. This will save both the polyline shapefile that locates your component and the associated data. The latter is saved to a 1D structures file, which is a text file with a “.str” extension. This file will always take the same base filename as your polyline shapefile (the 2D solver expects both shapefile and str file to always use a common name).
Once all edits are saved, click the 'Stop Edit' button in the 'Layer Editor' tab (or right-click on the file in the 'Layers' Panel and select the 'Stop Edit' menu option).

Note: The Edit 1D Structure tool can be used to review the properties defined for existing embedded structure shapefiles. As you need to put your shapefile in edit mode to access this functionality, it is also possible to edit your structure further in the tool. Remember that if you close the tool by clicking the OK button any edits will be saved automatically in the .str file.

What if I already have a shapefile to represent my structure?

If you already have a polyline shapefile drawn and want to use this to represent an embedded structure in a 2D model, then there are two options:

Start a new 1D structure polyline (see point 1 in the previous sequence). Then, in the Layer Editor ribbon, activate the snapping tool and choose to snap to any point in your existing polyline.
Edit the attributes of the existing shapefile, adding a field called “Type” that should be a text format.

To edit shapefile attributes in Flood Modeller, after loading the file into the Layers Panel (and map view) right-click on it and select Start Edit from the displayed menu. This will activate the Layer Editor ribbon in the main Toolbar. Then click on the “Attribute Window” button. The attributes table will be displayed in a pop-up window, which includes a toolbar.

already2

Click the attribute editor button (as shown above). The editor appears in a new window that lists the current attribute fields and provides controls for adding new fields:

already3

The above method should then be repeated to add another three text fields entitled “node1” and “node2”. These will hold the 1D node labels used in your embedded structures in the associated 1D structure file (as setup using the 1D structure tool). Culverts require both an inlet node label (node1) and outlet node label (node2) to be referenced. Weir and orifice type structures only require one label to be specified (node1, with node2 left blank). These data are used by the 2D solver to match the correct polyline with the relevant data in the associated 1D structure file.

Finally repeat the process again to add a numeric field that is type; double and entitled “length”. Values entered in this field represent culvert reach length, weir breadth or orifice diameter. The default entry should be 0.00, which tells the 2D solver to use the equivalent length parameter specified in the associated structure file.

The new fields will now be displayed when you return to the attribute table. While in edit mode, you can type directly into the table. The text entered in the Type field should only be one of the following; “culvert”, “weir” or “orifice”. The “str file” field can be left blank as Flood Modeller will populate this when you setup your embedded structure in the 1D structure tool.

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After entering the required data, you can close the attribute window and click Save and then Stop Edit in the Layer Editor toolbar ribbon to apply your changes.

Note: Editing of the attribute table is only possible when a shapefile is in edit mode. If you access the attribute table just by right-clicking on the file in the Layers Panel, then the table is read-only (the editing options are disabled).

Adding Embedded Elements to 2D Simulations

This section explains how these embedded 1D structure data are incorporated in your 2D model (it assumes you have already defined your 1D structure data files as shapefiles).

Load a new or existing 2D model into the Simulations section of the Project Panel. This can be done from the Simulation tab of the main Toolbar (as shown below). Alternatively, right-click on the Simulation header in the Project Panel and select Add Item or New 2D Simulation from the displayed menu.
Double-click on the 2D model file to open the 2D model interface (in a new window). Note if you are creating a new 2D simulation you will be automatically taken to this interface.
The 2D model interface contains multiple tabs and sub-tabs. Embedded 1D structures are set on the 1D Structures sub-tab under the Domain tab. This contains a table which is used to list the 1D structure shapefiles associated to the 2D model. Note that the structures data is defined at the 2D domain level, so the content of the table will change depending on which domain is highlighted in the list of domains (upper table in the interface).
Select the Domains tab and then the 1D Structures sub-tab.
If your 1D structures shapefile is loaded in the Layers Panel (so it appears in the map view), then click on it in the Layers Panel and drag it (keeping the left mouse button depressed) onto the Embedded 1D Structure Files table. It should be added as a new row in the table and the 1D structure file settings pop-up window will appear.
Alternatively, use the Add button to browse to a 1D structures shapefile (i.e. if it is not currently displayed in the map view).
The 1D structure settings pop-up window is displayed. This presents metadata associated to the selected structure file. These are the data within the shapefile attribute table. The settings window also enables you to select which variables to save from the structure during your simulation. The following variables are initially selected by default:
- Discharge (i.e. flow)
- Water elevation (i.e. stage)
- Water depth
- Flow velocity
By default, the 1D structure will utilise the same time settings as the associated 2D model, i.e. save interval, simulation start time and end time. However, the settings window provides the option to change these for each embedded structure, i.e. use a different save interval for data calculated for the 1D structure and/or only record outputs from the 1D structure for a subset of the full simulation period.
After defining all settings click OK to return to the main 2D interface. The 1D structure shapefile should now appear in the table with any custom settings you’ve applied, as shown below:
Repeat steps 5 to 7 to add further embedded structures data to your 2D model. There is no limit to the number of shapefiles you can add to this table.
If you need to review or further edit settings for an embedded structure you can access the setting pop-up either by double-clicking on a row in the table or highlighting the row and clicking the Edit/View button below the table.
Furthermore, if you decide that a structure is no longer required in your model then highlight the row in the table and click the Remove button.
Finally click the Save button (located bottom right of the 2D model interface) to apply all your edits. If you close the 2D model interface without saving then all unsaved changes from that session will be lost.