How to create a flood map from 1D model results
    • 20 Oct 2022
    • 7 Minutes to read
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    How to create a flood map from 1D model results

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    Article Summary

    A 1D model will yield water level data at each node location within the model network. To process these data to obtain a flood extent map you first need to calculate a water surface by interpolating between the nodal water levels. You then subtract the underlying ground levels to determine flood depths (the ground is considered wet where depth > 0). In Flood Modeller the procedure to generate a flood map is detailed below.

    Load ground data

    A ground grid, e.g. digital elevation model (DEM) from LiDAR data, is required to be loaded into the map view to calculate flood extent maps from your 1D modelled water levels. This can be any compatible grid format, i.e. binary (flt), ASCII (asc) or ESRI.

    The grid provides the ground elevation data to intersect with the water level data to calculate flow depths and hence create a flood map (i.e. areas where depth > 0).

    Load TIN

    A triangulated irregular network (TIN) is a GIS layer containing an array of triangular structures joined together to represent a surface (file extension expected for a TIN is ".htn") . Flood Modeller enables you to associate data from 1D network nodes to the points within a TIN file. It will then use these data to interpolate values anywhere across the surface of the TIN (between the known data points).

    TIN files can be loaded into the viewport using the Add Layer button in the main toolbar (Home tab or Map Tools tab). If you use an external TIN file then it should contain a reference to a node from your 1D model network at each point. Flood Modeller needs this information to be able to assign water level data from your 1D model results to the points of your TIN. The required attribute field name for node labels within a TIN is "Node Label A". You also have the option of specifying a second node label under the field name "Node Label B". If this is defined for any point in the TIN then Flood Modeller will calculate the average water level of the two defined model nodes at this point.

    Flood Modeller can generate a new TIN from the sections of a georeferenced 1D network. A number of options are available for generating TINs in Flood Modeller:

    • Use river cross sections only – the points of each river section in a georeferenced 1D network can be plotted on your map view (drag the 1D dat file from the Project window onto the map to “draw” cross sections). Along each section the water level will be constant, i.e. the value for that node within the 1D model results. Flood Modeller provides a tool to join these lines to create a TIN (highlight the cross section layer in the Layers panel and select Toolbox > Triangulate selected file). As the water level value along each line is constant it is not necessary to use every surveyed point of each section (which would lead to an unnecessarily complex TIN containing many triangles). Instead Flood Modeller will use just enough points to define the shape of the section.
      It should be noted that this method will only join sections where the upstream section has a non-zero distance to next value. Where structures (and other unit types) interrupt a series of sections there will be a gap in the TIN. Gaps can be filled in by adding extra triangles to the TIN. This can be done using the layer editor tools to manually draw triangles (use snapping to join to existing triangles in TIN).
    • Create pseudo sections at all 1D nodes – a water level is calculated for every node in a 1D model. Thus, for 1D networks with many different node types you will not be fully utilising the available model results data if your TIN only contains river cross sections. Flood Modeller provides a tool to generate a section at every node location irrespective of node type (Map Tools tab > cross section generator). These sections are generated as a polyline shapefile and each feature (section) is given the corresponding 1D network node label as an attribute. Use the Triangulate selected file function (as above) to create a TIN file from this shapefile (for shapefiles this function will call the Flood Modeller TIN Creator tool, which is designed to triangulate more complex input data).
    • Combine sections with reservoir polygons – a 1D network may include reservoir nodes. These nodes represent an area that can store water, e.g. a floodplain. You can define polygon shapefiles to represent the areas covered by each reservoir type node. The Flood Modeller TIN Creator tool (Toolbox > Flood Mapping > 1D Models > Triangulate selected file) can combine a number of shapefiles (polygons representing reservoirs and polylines representing sections) into a single TIN (note all features must have a fully populated node label attribute).

    Further information on TIN file generation is provided in elsewhere in this user guide (see the page on TIN Creator tool).

    Add 1D model binary results

    To assign Flood Modeller model binary results to the loaded TIN:

    1. Highlight the TIN layer in the Layers window
    2. Right-click on the highlighted layer and select Load Results from the displayed menu.
    3. A standard Windows browser is displayed. You can browse to the required results file. This can be the binary output file from your 1D simulation (extension zzn or zzc) or it can be a text file (csv format) containing only outputs from selected single or multiple timesteps. The latter is generated using the Flood Modeller TabularCSV tool (which post-processes the binary 1D model outputs).
      If you select a csv file then all data from this will be automatically associated to your TIN.
      If you select a binary results file, you need to make some further selections as follows:
      1. The Node Selector tab will be displayed initially:
        This displays the model nodes with results that will be associated to the TIN file. If any TIN node references are not present in the specified results file, then you can manually cross reference alternative model nodes to the TIN (e.g. a node from a similar location).
        If there is a Water Quality results file (.zzc file) available in the same folder as the 1D model results file (.zzn file), then you can load the water quality results into Flood Modeller. In this case the drop-down box above the standard list of 1D results parameters will be enabled and populated with the available water quality output parameters. If a parameter is selected from this list, then it will be loaded to the TIN instead of any of the hydraulic parameters (only one results parameter at a time, either water quality or hydraulic, can be loaded to a TIN).
      2. Click on the Time Steps Selector tab. The following is displayed:
        The default setting is to only load the maximum water levels for each node from the results file.
        It is possible to select multiple individual time steps from the time series of results, which can then be converted to an animated sequence using the Flood Modeller 1D Flood Calculator tool. Also different model variables can be loaded, e.g. flows or velocities. Details of these procedures are given elsewhere in the user guide.
      3. Click on the Upload button to add the results data to the TIN.
    4. The time series data in the selected results file are then associated with the TIN. When the TIN file is highlighted in the Layers window all associated time steps are listed in the time step tab on the right of the interface. Click on a time step to display the associated water surface data in the map. As the map is 2D (rather than 3D) there is no interaction between the water levels within the TIN surface and any loaded ground level grids. If the TIN layer is higher in the hierarchy of loaded GIS layers then the entire surface will be displayed in the map, overlaid on the grid layers. An example of this is shown in the figure below (in this example the TIN layer has also been set in the layer properties window to display partially transparent):

    This is not a representation of the extent of flooding as there may areas within the surface that are not flooded, i.e. the underlying ground level is higher than the interpolated water level. The surface will be filled in with colours relating to water level values as defined in the associated colour ramp, which is located in the layer properties window (accessed by double clicking on the TIN layer in the Layers window)

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