How to create new 1D river cross-sections from a map
    • 05 Aug 2022
    • 10 Minutes to read
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    How to create new 1D river cross-sections from a map

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    Article summary

    1. Load ground data

      You will need to load a suitable ground grid into the viewport, in a compatible grid format: binary (flt), ASCII (asc) or ESRI. This grid will provide the vertical elevation data for the new cross-section. It also provides geo-referencing data to set the viewport's boundaries. On the Home tab, click Add GIS Data and browse to your grid.

    2. Define a river centre line.

      The river centre line will define the direction of the river, and the left and right banks of the cross-sections. If the river consists of many tributaries and branches, the centre line will consist of multiple polylines.

      There are three options for defining a river centre line:

      • If a centre line shapefile already exists, you can load it into the viewport using the 'Add GIS Data' in the 'GIS Data' section of the 'Home' tab.

      • If a relevant GXY file exists, then this can be loaded into Flood Modeller as a point shapefile. These points can then be joined up to define the centre line.

        A river centre line can be drawn as a standard format polyline shapefile, using the Flood Modeller shapefile editing tools. You can access these using the 'New' button from the 'GIS Data' section in the 'Home' tab, and selecting 'Shapefile' from the drop-down menu. Select 'River centre line' from the window that opens. Alternatively, you can select Additional Model Build Tools > 1D Cross Sections > Generate River Centre Line in the modelling toolbox. Then select the cross-section Shapefile, and specify the river centre line name and location to save it.

        1DRiverimagesshapefileselector.jpg

      • This process will work with any polyline shapefile, but the specific centre line option creates an attribute river-label which you can use to name the new cross-sections automatically. There are numerous options available in Flood Modeller for drawing a standard shapefile polyline, eg from the 'Home' tab; GIS Data > New > Shapefile.

    3. Set river centre line direction.

      • Select the appropriate layer in the 'Layers' panel.

      • Select the 'Start Edit' option in the right-click drop-down menu on the layer.

        1DRiverimageslayeredit.jpg

      • Use the 'Select' button from the 'Selection' section of the 'Layer Editor' tab to select the centre line.

      • Once selected, the line will appear as shown in the diagram below. The start and finish of a polyline in Flood Modeller are denoted by a red and green point respectively.

        1DRiverimagesstartfinish.png

      • The direction of the river can be changed by using the 'Reverse' button in the 'Edit Features' section of the 'Layer Editor' tab.

        1DRiverimagesreverse.jpg

      • These stages can be repeated for any feature which requires reversing.

      • Once you have finished editing, select 'Layer Editor' tab and 'Save' to save changes. Exit the 'Layer Editor' tab using 'Stop Edit' button.

      • If a shapefile is loaded that can provide guidance for the centre line path eg gxy points shapefile, then you can use the snapping control when drawing the new centre line. Select the snapping control from the 'Properties' section of the 'Layer Editor' tab. A new panel is displayed below the viewport. Select to snap the new centre line polyline to any points of the gxy file by selecting the settings shown below.

        1DRiverimagessnapping.jpg

      • Select the 'Line' button from the 'Draw' section of the 'Layer Editor' tab. The mouse cursor will change to a pencil when over the viewport. Use the left-click button to draw new points on screen. The cursor should snap to the gxy file points when positioned close by, enabling you to join points form a new river centre line.

      • Right-click to see a menu with options to 'Undo Last Point Added' or to 'Finish Shape'. A double left-click completes the line. Note, you need to perform a save operation (Layer Editor/Save) to save new edits.

      • You can add more lines by repeating the three previous steps eg for multiple tributaries.

      • When the centre line is complete, access the attribute table by right-clicking on the shape file in the 'Layers' panel and select 'Show Attributes' from the drop-down menu. A new window is displayed containing the shapefile attribute table.

        1DRiverimagesattributetable.jpg

      • Double-click on the river label field entries to allow you to enter the appropriate names. Note, Flood Modeller will add enumeration to these names to distinguish each cross-section generated along each line. Consequently, the maximum string length is seven characters and additional characters will be ignored.

      • When all shapefile editing is complete, select 'Stop Edit' from the 'Layer Editor' tab and save the file when prompted to record all changes. Note, you can use a save function to save changes during the process.

    4. Create cross-sections.

      You can add cross-sections to the centre line in two ways:

      1. Use the cross-section generator in the modelling toolbox (covered below).

      2. Draw polylines on screen to represent cross-sections.

    5. Access cross-section generation tool.

      You can use the cross-section generator to create cross-sections along the river centre line. Access by clicking the 'Cross Section' button in the 'Map Tools' tab of the main toolbar (it is also available from the Toolbox under Additional Modelling Tools >1D Cross Sections > Cross Section Generator).

    6. Define new cross-section properties.

      You can use the tools in this window to perform the following steps:

      • Select the relevant river centre line shapefile.

      • Select a grid to calculate the elevations of all the points on the cross-section automatically (Z-values) (optional).

      • If a cross-section is located where no elevation data are available, the cross-section Z values will be assigned the missing data flag -9999.

      • Select a GXY shapefile to obtain node names for the points on the centre line (optional).

        NB: if the required shapefile or grid is loaded in the 'Layers' panel, it can be selected from the drop-down list. If not, the folder icon can be used to browse for the relevant file.

      • Define the node names for the new cross-sections. You can: type in a name that Flood Modeller will use, combined with an incremental counter (eg IM_001, IM_002); or select a field from the centre line *.dbf file or GXY shapefile that contains node names.

      • Identify global parameters (applies to all cross-sections) - Manning’s roughness coefficient and the relative path length. Default values are set automatically.

      • Decide where you require the centre line cross-sections. The options are:

        • At each point defined along the centre line - option to be used when centre line has been defined from existing model data.

        • A fixed number of sections along centre line - Flood Modeller calculates a fixed separation distance between sections.

        • A fixed separation distance between sections - Flood Modeller calculates the number of sections required.

          1DRiverimagescrosssectiongenerator.jpg

          NB: for the final option, the user can choose to add extra cross-sections at end, ensuring there is a cross-section at each end of the centre line.

      • Specify the length of the cross-sections. The cross-sections are drawn away from the centre line. The user must specify the distance to the right and left bank, and the number of points along these lengths. By default, the left and right bank are set as the same length.

      • Specify the output sec filename and location for saving the new cross-section.

    7. Generate new cross-sections

      Select OK to create new cross sections. They will be automatically loaded into the view port, and the .sec file added to the 'Layers' panel.

    8. Assign Z values to points on cross-sections (optional)

      There are three options for assigning new z values to the extended cross-section:

      1. Assign z values from a grid to the whole cross-section.

      2. Assign z values from a grid to points with no values eg if you have extended surveyed cross-sections, you can choose to protect the original survey data and only update levels in the extended segments.

      3. Manually edit of cross-section attributes.

    Assign Z values from grid

    Both options will prompt you to select a relevant grid data source.

    1. Highlight the cross-section .sec file in the 'Layers' panel.

    2. Select 'Start Edit' from the drop-down menu when right-clicking on the file in the 'Layers' panel.

    3. The Layer Editor will be displayed.

    4. To assign the z values from a grid to the whole cross-section, click the 'Toggle Protection' button and unlock the padlock icon. To assign z values from a grid to the extended cross-section, keep the 'Toggle Protection' button with the padlock icon locked.

    5. There are now two options:

      • Highlight an individual cross-section and set Z to the value for one section on the 'Layer Editor' toolbar

      • Select 'All Z' to assign the Z values for every section.

    6. Once you have made the changes, you should save the sec file by selecting 'Save' from the 'Layer Editor' tab.

    Adding Z values manually

    If the elevations (Z values) for the points have not been assigned using a grid, you can input them manually using the file 'Layer Editor' tab.

    1DRiverimagesnodeattributes.jpg

    1. Select the newly created cross-section layer (.sec file) in the 'Layers' panel, right-click and select 'Start Edit' from the drop-down menu.. This will start the Layer Editor, which is signified by a red star in the 'Layers' panel and a new tab entitled 'Layer Editor'.

    2. Select 'Attribute Window' button from the 'Layer Editor' tab. This window displays each cross-section with its ID, label, type and chainage. You can select a feature by clicking it. This will highlight the relevant cross-section in the viewport.

    3. In order to edit the elevations for the points on a cross-section, double-click the relevant row in the attribute table.

    4. This will open a new window:

      1DRiverimagescrosssection.jpg

    The 'Elevation' tab shows a graph of the cross-section selected previously. You have two options for plots: map distance, representing the separation of points calculated on the map view; and model distance which plots points using distances defined by the sec file attributes (field ‘Model x (m)’).

    • You can use the cursor on the graph to zoom in on specific areas. To zoom out, use the scroll button on the mouse.

    • The buttons on the top-right of the window can be used to cycle through each cross-section. These have been highlighted.

    On selecting the 'Node Attributes' tab, the following window is displayed:

    1DRiverimagesnodeattributes2.jpg

    • You can edit data in any of the fields. First, select the 'Toggle Data Protection' button. This action will allow you to edit any data. Otherwise you will only be able to edit points added to the original cross-section with no elevation data.

    • Once the button is selected, the fields will turn white. You can edit the 'y' values. Here the -9999 value represents missing data.

    • When you have finished entering the appropriate elevation details for the cross-section, select 'Apply' and switch the 'Toggle Data Protection' to the locked position to protect the changes.

    • The changes will then be confirmed, and you can view the new cross-section under the 'Elevation' tab. You will still need to save changes to the layer ('Layer Editor' tab).

    1. Associate a river centre line to cross-sections.

      Depending on how the cross-section has been created using the tool or manually drawn - it may be necessary to assign a river centre line (shapefile). This is used when saving the cross-sections to a .sec file to define distances between cross-sections, the order of cross-sections and which sides are the left and right banks. Flood Modeller can save cross-section data directly to the .sec file without specifying a centre line file, so this action is optional (if no centreline is specified, Flood Modeller will use the currently specified values and order of sections).

      To associate a river centre line:

      1. Highlight the cross-section layer in the 'Layers' panel.

      2. Right-click the cross-section (*.sec) file from the 'Layers' panel, and select 'Associate River Centre Line'.

      3. A new window will appear, displaying the available polyline shapefiles loaded in the 'Layers' panel.

      4. Select the appropriate file from the list and click 'OK'.

    2. Save to Flood Modeller model file format.

      Flood Modeller will always save cross-section data to a .sec file, even if it has been imported from a .dat file. The reason is that Flood Modeller allows .sec files to be associated with .dat files to provide updates to existing sections. The .dat file can hold the original survey data and the revised cross sections in Flood Modeller can be stored in separate .sec files.

      1. With the cross-sections loaded in the 'Layers' panel in edit mode, 'Save' under the 'Layer Editor' tab.

      2. You must then give a location for saving and a file name.

      Useful tip: Isolated cross-sections - not joined to the specified river centre line - can sometimes occur if original sections have zero chainage. These will not be written to an output sec file unless first extend the river centre line to pass through them (using shapefile editing tools).

    More about River Section Nodes


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