How to create a 2D model topographic feature

Required type of file: Point/Polyline/Polygon shapefile (depending on the type of shape of the features you want to create) with at least these fields placed in the dbf file : height (or elevation) for point and polygon shapefiles and height1 (or elevation1) and height2 (or elevation2) for polyline shapefiles. These fields have to be the first fields in the dbf file, i.e. for topographic feature point and polygon shapefiles the first field must be height (or elevation) and for the topographic feature polyline shapefile the first field must be height1 (or elevation1) and the second field must be height2 (or elevation2).

In addition, a topographic feature polyline can have an attribute “thick”. This is an integer attribute that can take value 1, 2 or 3 (with thick = 1 the default setting). This field is optional, if it is not included then Flood Modeller will adopt the default value of 1. The “thick” value determines how the 2D solver will interpret the topographic adjustment defined by a polyline. Each option affects topography as follows:

• Thick=1: “normal”/default behaviour - elevation adjustments only applied to grid cells directly underlying the polyline. Note that this may result in modified cells only being “joined diagonally”, i.e. not hydraulically connected directly (by sharing a cell edge)
• Thick=2: Elevations of additional grid cells adjusted to “fill in” corners, i.e. corrects for cells only being “joined diagonally” so that line of adjusted elevations always at least one cell thick.
• Thick=3: Elevation adjustments applied to all eight neighbours of grid cells directly underlying polyline. Thus, a thicker line of adjusted elevations created. Note: It is not recommended to use this setting with the “Add” function (check carefully the effect of adding height to a wide range of cells – is this the desired effect for your model?)
• Thick = any value other than 1, 2 or 3: The default value (=1) is assumed by the solver

Overview

For 2D model simulations you can specify some topographic features of certain elevations. Topographical modifications can either be set explicitly via the Topography section in the 2D Simulation window, or implicitly as part of a link-line or boundary condition shape file (if either has the height attribute set and/or is a polylineZ type).

One of the cases when you could use a topographic feature is when modelling a defence. The elevations of these features will be taken in account in addition to the ground elevation data that you need by using a ground elevation grid during the 2D model simulation. A topographic feature shapefile is a shapefile that represents defence systems, points of different elevation not captured by the underlying DTM for example. This file can be a point, a polyline or a polygon shapefile depending on a shape of the topographic feature. This file is also called a topographic feature file. Flood Modeller provides the functionality to create these files.

A Topographic Feature point shapefile should have one dbf field called height or elevation; a Topographic Feature polygon should have one field called height or elevation; a Topographic Feature polyline shapefile should have two fields: height1 (or elevation1 ) and height2 (or elevation2 ). In the case of a polyline the elevation values are extrapolated between the points of the feature at the both ends of the polyline (height1 value and height2 value). These fields have to be the first fields in the dbf file, i.e. for topographic feature point and polygon shapefiles the first field must be height and for the topographic feature polyline shapefile the first field must be height1 (or elevation1 ) and the second field must be height2 (or elevation2).

PolylineZ/PolygonZ

If your topographic feature shapefile data is a polylineZ (or polygonZ) format it will be treated as follows.

All z data = 0

Z values of 0 (zero) will not be used – PolylineZ will be treated as standard polyline, using Z values from height attribute, and warning is written to 2D diagnostics to inform this assumption has been made.

Else (has at least one nonzero Z)

Process polylineZ data…

Attributes contain method/isis2d=”add” but not height

Assume the polylineZ data is being added to existing topography (e.g. of the original DEM

Attributes contain method/isis2d=”add” and also height

Assume the attribute height data is being added to polylineZ elevations

Attributes contain height, but not method/isis2d=”add”

Assume the attribute height data is being applied (overlaying polylineZ elevations; the latter is important in the case height=-9999, but polylineZ is nonzero – then the latter Z wins [over the original “DEM”])

None of the above (no height or add)

Apply polylineZ data, overlaying existing topography (e.g. of the original DEM)

Link lines

For topographic modifications set implicitly, i.e. via link-lines and boundary condition polylines, the hierarchy is the same as for topographical modifications above, with the exception that the ‘add’ option does not apply to link line or boundary line topographical modifications. That is to say, either the ‘height’ attribute, or the Z value from a polylineZ may be used to implicitly modify the topography, but any ‘add’ attribute set is ignored.

Before creating a defence shapefile you should have a ground elevation grid loaded or some other files that will provide some geographic boundary dimensions for your new shapefile and will help you locate it correctly.

The following steps show you how to create a custom shapefile of type '2D Model Topographic Feature'.

Method

1. Load boundary data

   A GIS layer representing the 2D model extent is initially required to define the extents of the map view. The standard Add GIS Data function is used to add these data to the Layers panel. Select Add GIS Data from the Home Tab

2. Start drawing a new topographic feature shapefile

   To start drawing a new shapefile use one of the following options:

   • Select one of the topographic feature buttons from the 2D Build tab of the main toolbar (point, polyline or polygon).

   • Alternatively, you can create a new shapefile from the Home tab by clicking on New > Shapefile and selecting one of the 2D Topographic Feature options (from the 2D Model Build section), as shown below:

     

     Select the required file type (point, polyline or polygon) and specify a filename and location for the new topographic feature shapefile. Click the OK button to continue.

     After a filename is specified the new shapefile is added to the Flood Modeller Layers panel and automatically put into edit mode.

3. Draw a Topographic Feature

   Start drawing the new feature. Click on the Line/Point/Polygon button (depending on file type selected previously) from the displayed Layer Editor toolbar. Move the cursor over the map and use the left click mouse button to add new points to the current feature. Double-click the left mouse button to stop drawing a line or polygon, a single left click will then start drawing a new feature (i.e. a new feature in the same shapefile).

   The completed features will then show in a different colour to a current feature being drawn. The changes made to create the shapefile can be saved by selecting the save option in the Layer Editor tab.

4. Define shapefile height attributes

   The shapefile generated by Flood Modeller will be automatically defined with an attribute field entitled 'Height' or ‘height1’ and height2’ (in addition extra text fields are added for descriptions of features to be included). These fields must be populated manually.While in edit mode for your topographic feature shapefile, access the attribute window by clicking the button provided in the Layer Editor toolbar (another option is to right-click on the shapefile layer in the Layers panel and select Show Attributes from the displayed menu). This will display a table listing the features in your shapefile and their associated attributes. Select an attribute to edit by clicking on the appropriate cell in the table. Then type in your new value, i.e. height value for the feature.

   In addition to specifying revised elevation values, if you type ‘add’ in the "method" column the 2D numerical engine will then add the elevation specified in the height1 and height2 columns on top of the existing ground elevation. You can also specify a negative value in the height column. In this case Flood Modeller will subtract this value from the existing ground elevation. Such an operation may be found useful when there is a need to show the road network which is lower than the neighbouring terrain.

   If you leave the "method" column blank, the 2D numerical engine will replace the original value of the ground grid with the value specified in the height column(s) in the shapefile attribute table.

   For reference, “method” – this is an optional field that allows you to add or subtract the specified levels in the height fields to the underlying ground elevation. To activate this function enter “add” here. Then, for example, if you entered a height value of “-0.5” then underlying ground elevations will be lowered by 0.5m.

   If this field is missing or its value left blank (or set to “replace”) then the 2D solver will just replace underlying ground grid values with those specified in the topographic feature shape file.

   In some legacy models this field may not be present and there may be an attribute field entitled “isis2d” present instead. In this case the “isis2d” field will perform the same role as the “method” field and if left blank for a feature then the default option to replace underlying ground grid values will be invoked.

   Note:

   the optional field “thick” (as described above) is not included in the default list of attributes created by the Add Topographic Feature tool. Thus, you will need to add this field and populate it manually. This is achieved by:

   • While in shapefile edit mode, access the attributes table and click the edit attribute fields button

     

   • A new pop-up window is displayed listing existing attributes and enabling you to define the new “thick” attribute (as an “integer” type).

     

     

     Once thick attribute has been created, close the field editor pop-up window. You can now type values for the “thick” field in the table (while in edit mode). Note that the default value added by the field editor (for integer attributes) is 0, i.e. an invalid value for “thick”.

     

5. Save shapefile

   Once completed (or at any time) your new shapefile can be saved by accessing the Save option in the Layer Editor Toolbar. Alternatively if you right click on the file name in the Table of Contents you can select the Stop Edit option in the same section of this tab. This will give you an option to save your file as you exit the edit mode.