How to extend river sections
  • 02 Aug 2022
  • 3 Minutes to read
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How to extend river sections

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Flood Modeller provides a few options for extending 1D river cross sections onto the floodplain:

  • Using the extend sections tool in the Layer editor tab of the main toolbar

  • Using the extend all river sections tool from the Additional Model Build Tools > 1D Cross Sections area of the Toolbox

  • Converting polyline shapefiles into new 1D river sections (as .sec files)

This page explains the first of these options. The procedure for defining extended sections is as follows:

  1. Load the 1D network to be extended into your Project panel and make this the active network.

  2. Display the cross sections on the map view by dragging the network file from the Project panel onto the map. A network file entry should be added automatically to the Layers panel and you should see the cross sections on the map. Hint: If you turn off the node icons by unchecking the box labelled “Nodes” in the main toolbar you should see the cross sections more clearly.


  3. Use the Add GIS Data button to load any additional GIS data you will use to determine how far to extend each cross section, e.g. a DTM / DEM in grid format (i.e. ASCII or binary) or a polygon shapefile. Note this will load on top of all existing layers so you will need to drag it down the order in the Layers panel to see the 1D cross sections on top of it.


  4. Put the 1D network layer into edit mode (using the Layers panel right-click menu).


    Check the Extend settings in the Layer Editor toolbar to ensure you are adding sufficient points to define the extended cross section accurately.


    These settings will define a minimum spacing between points and the number of points added with each line segment defined (recommended settings are 5m and 10points).

  5. Use the Layer Editor select function to highlight a cross section (by dragging a line across a section). Then use the appropriate extend function (left or right – NOTE: these are the left/right banks as you are looking downstream, not West and East as they appear on the map) to add line segments to your highlighted cross section (each click adds a line segment). Double click to stop adding segments. Do not extend the line beyond the area defined by your underlying ground elevation data (as you won’t be able to specify a bed elevation outside of this area).


  6. Repeat process of highlighting a section and extending it left or right (or both) until all sections adjacent to your floodplain extend across it. Each time you want to extend a new section, or in a new direction on the same section, you need to use the select tool again to re-highlight the appropriate section and then click the required extend tool to turn on the drawing function.


  7. The new points added to each section now require ground elevation data, as they will initially be defined with elevations set to the missing data flag (-9999). These values will come from a DTM – if not already loaded in your map view, load a suitable grid file (move down the layer order to enable sections to still be visible on top). Then use the Set Missing Z function in the Layer Editor to extract ground elevation data from the DTM for each cross section. We use the Set Missing Z function to ensure the in-bank survey data is preserved and only the missing data flags are replaced with underlying ground elevation data.

  8. Save your 1D network using the Save button in the Layer Editor (and not the Save Network button on the Home tab of the main toolbar). Then exit edit mode by clicking Stop Edit. The network should be refreshed in all views within the interface allowing you to inspect the revised cross sections.


  9. After extending the sections within a network you may need to run a new 1D steady simulation and use the results to refresh the initial conditions of the revised network file.

More about River Section Nodes

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