How to use the Link-Line Generator Tool

This section explains how to define link lines to link a Flood Modeller 2D model to a 1D network using the link line generator tool:

1. Open Flood Modeller and use the Add GIS Data button tool (Home tab of main toolbar) to load your required datasets to the map view. Your required data is selected from the following:
   • A 1D model network. The tool will read cross section data from the active 1D network and create link lines that start / end at the closest locations to the ends of your sections (or ends of active portions if partially deactivated). Therefore, specified network must be fully geo-referenced to work in the tool. If not, then first review the Flood Modeller user guide section on how to geo-reference a 1D model.
   • As an alternative to a georeferenced 1D model network, use a point shape file representing your model nodes (e.g. an imported gxy file from a 1D model). The shapefile must include 1D node labels as attributes.
   • A polyline or polygon shape file representing the boundary of your 2D model, e.g. a bank line marking the edge of your 1D model extent. The link line tool will “trace” along this when creating new link lines (with links starting/ending at the locations on the shapefile closest to each 1D node). This shapefile does not require specific attributes. Note the tool offers the alternative option of using the specified active 1D network. In this case link lines will be created as straight lines between section end points (or ends of active portions if partially deactivated).
   • Ground elevation dataset, i.e. DTM. This is an optional dataset. Link lines include attribute fields for underlying elevation data at the start and end of each link. If these fields are set to the missing data flag -9999 (the default option), then the 2D solver will extract these data at simulation runtime from the composite topography defined in each model.
     However, if you want fixed elevations assigned to your link lines then the tool allows you to populate the height attributes with levels taken either from an underlying ground grid or from the 1D cross section end points. You might do this if these data include flood defence heights that are not present in the ground grid input for your 2D model.
2. Select the link line generator tool from the 2D Build tab (main toolbar), as shown below:
   
3. The following new window will then appear:
   
   The tool requires the following information to be specified:
   • Specify source for 1D nodes - this can be a 1D network (dat) file or a point shape file created by importing a network gxy file into the map. Any compatible files loaded into your map or project view can be selected from a dropdown list in the tool. Alternatively, browse buttons are provided to select a file that is external from your project.
   • Set start and end nodes – the available nodes in the selected 1D network will be listed in two dropdown lists for you to select a start and end node. These are the nodes that define the start and end of the reach that will be linked to your 2D domain, i.e. where water can pass between the models.
     Useful tip:

     Note that the tool can append links calculated for the specified reach to an existing link line shapefile (as an alternative to creating a new file). Thus, if you need to link multiple reaches you can repeat the linking exercise multiple times and append all links to the same shapefile.

   • Specify end of cross section – If you are linking using a georeferenced network then you can link between (or close to) the extreme left or right ends of each defined cross section or the ends of the active portions if you have deactivated areas. 
   • Specify path for link lines – If your 1D network is georeferenced then you can select to create link lines as straight lines between the end points of each 1D section. You can also choose to create link lines on just the right or left banks of your 1D river or along both banks at the same time.
     Alternatively, instead of drawing straight lines between adjacent 1D cross sections, you can specify a polyline or polygon shapefile to trace along. These can be selected from the dropdown list populated with compatible files from your project or by using the browse button to select an external file. If the selected shapefile contains multiple lines or polygons (as separate features) select also the feature that the tool will trace along (a second dropdown list is automatically populated with the ID value of all features in the selected shape file).
   • Elevation data – the tool provides the option to add elevation data from a specified source to the “height1” and “height2” attribute fields of your link lines. Source options are a grid of ground levels (DTM/DEM) or elevations read from cross sections in the specified 1D network (section edges or deactivation markers). Levels taken are those underlying the locations of the start and end of each link line. By default no elevation data will be added to a link line file and the height attributes will be populated with the missing data flag, -9999.000 (see point 1 above for explanation for this).
   • Select link type – Links to 1D models can be flow, level or weir types (default is level link). This setting determines how the passage of flood water from the 1D to the 2D domain will be calculated (see 2D modelling technical reference for further details).
   • Set flow adjustment factors – Link lines include factors used to modify the flow data calculated across them. The adjustment equation is modified flow = (flow x A) + B. Default values (A = 1, B = 0) are automatically included in the interface, but these can be edited to a valid numeric value.
   • Specify output file – The name and location for the link line shape file to be generated can be set using the adjacent browse button or by typing the name and location (full path) into the space provided. This can be a new file or you can choose to append calculated link line data to an existing link line shape file, e.g. where multiple river reaches are linked to the same active area (each reach will be calculated separately, but can be combined into a more efficient single file).
     You can choose to also automatically load these data to your map view upon completion of the link line generation process. A tick box is provided entitled “Add to View”, which by default is ticked. 
4. When all the above settings are defined you can proceed with the automated link line generation (by clicking the OK button). Depending upon your input data selection the tool will create link lines in one of the following ways:
   

   	Settings	Link line definition
   1	1D network nodes – use active network Link line path – use active network	Link lines defined as straight lines, joining between end points of cross sections (either full sections or just active portions).
   2	1D network nodes – use shapefile (i.e. node points shapefile) Link line path – use shapefile (i.e. polyline or polygon)	Link lines trace along specified shapefile. Start and end points are created at closest location along shapefile to points representing each cross section (in node points shapefile).
   3	1D network nodes – use active network Link line path – use shapefile (i.e. polyline or polygon)	Link lines trace along specified shapefile. Start and end points are created at closest location along shapefile to specified end points of cross sections (either full sections or just active portions).
   4	1D network nodes – use shapefile (i.e. node points shapefile) Link line path – use active network	This is not a valid option (and cannot be selected in the tool)

   Note:

   The link line tool will create links for interpolate and replicate 1D sections. As these unit types do not include cross section extent data the link line tool will assume a section with the same extent as the upstream river section, extending perpendicular to the river centreline. In some cases, these links may need to be further edited manually to locate correctly. Thus, it is always recommended the link line tool output be reviewed before it is used in a model.

   Furthermore, note the link line tool will not draw link lines that extend across structures within the 1D network. However, if your 1D network is represented by a shapefile then this functionality depends on your shapefile including a node type attribute to enable structures to be identified.

5. When the calculation process is completed the tool will ask you if you want to append further link line analyses to your new file. If you answer no, then the window will close and return you to the map view. If you answer yes, then the tool will remain open for you to specify linking between a different range of 1D nodes. These links can then be added to a new link line shapefile or appended as new features in your existing link line shapefile.
   
   Note:

   The link line generator tool is only provided to help speed up the process of building link lines. It will not always be able to produce all required links in a single “one-click” operation. Furthermore, it will not be able to traverse structures in your 1D network and in some scenarios, it may be confused by a particular 1D setup and make a mistake. Hence it is always recommended that you check your automatically generated link lines carefully as further manual editing may be required to complete your model setup. Manual edits might be:

   • Correcting links that overlap with the adjacent link lines (can sometimes happen when linking around bends). Correction needed to avoid double counting of 1D model inputs to the same cells of a 2D model.
   • Add additional links across reaches that the automatic tool was unable to link across, i.e. fill in any gaps.
   • Positioning of link lines for interpolate and replicate 1D section nodes.