How to create a 2D active area

Required type of file:Polygon shapefile

Overview

The computational area is defined as a rectangle, however the actual area of interest for your model may not cover all of this defined area. Therefore 2D modelling in Flood Modeller allows an active area to be specified within the computational area. During a simulation, Flood Modeller will only consider cells that lie within the active area. Those outside the area are ignored thus streamlining the model and reducing simulation run times.

Flood Modeller calculates the output variables that you specify (such as Depth, Elevation, Flow, Velocity) for the area bounded by the active area. If the active area shapefile is not specified than Flood Modeller will calculate the variables for the whole 2D computational area, which may lead to significantly longer simulation runtimes.

Active areas are defined by an ESRI polygon shapefile. Flood Modeller only requires the geographic information contained within the shapefile, therefore it does not matter what attributes are assigned to the shape. There can be several active areas defined within the computational area; however (for a single 2D domain) they must all be part of the same active area file, i.e. each active area polygon is a separate feature in the same active area shapefile. Note that if you define a multi-domain model then each domain will have its own computational area, which means each domain can have its own active area, e.g. left bank and right bank floodplains.

Method

The process for creating an active area shapefile in Flood Modeller is the same as creating a polygon shapefile. Unlike the computational area shapefile, link line shapefile or topographic feature shapefile (explained later), which are all custom shape files, the active area shapefile is a standard shapefile that does not require any additional parameters to be set up. Thus, you could utilise a polygon shapefile defined in separate GIS software. Alternatively, Flood Modeller includes functionality for drawing a new active area. This includes an option to “trace” along an existing polyline (loaded in your map view), e.g. a link line shapefile, to incorporate the path of the polyline into your new active area.

The method for defining an active area in Flood Modeller is as follows:

1. Once your DEM and computational area have been created or loaded into Flood Modeller, you can specify an active area shapefile using the “Active Area” the 2D Build tab and clicking on “Active Area” (in the 2D Model section of the tab). You will be prompted to specify a filename and location for saving your shapefile. At this stage, the new file will not yet be created, but the filename will appear in the Layers Panel.

   

   Alternatively, you can create a new Active Area shapefile from the Home tab by clicking on New > Shapefile and selecting “2D Active area (custom polygon)”.

2. Flood Modeller will automatically activate editing mode and the Layer Editor toolbar tab will be selected.

   

   A new active area shapefile is created by clicking the Polygon button and then “drawing” the required shape in the map view. Clicking the left mouse button on the map creates the first point of your polygon. Each subsequent left click adds a new point at the current cursor location and a line joining the new point to the previous point. Clicking the right mouse button opens a menu with the option to remove the last point added.

   To finish drawing your polygon, double-click the mouse or select the “Finish shape” option from the right-click (on map) menu. The polygon will be closed with the last point being joined to the first. Another left click of the mouse in the map view will start drawing a new polygon. This will then be a new feature within the same shapefile, rather than a new shapefile.

3. Drawing active area to edges of 1D channel (for a 1D-2D linked model):

   If you are linking your 2D model to a 1D network then you may want the 2D active area to extend right to the bank edges of the 1D river sections. You can ensure your active area is located correctly by adding the 1D network to the map and then drawing the active area to touch the edge of each 1D cross section:

   • Flood Modeller drawing tools provide the facility to draw to the precise edge of a 1D section. If your 1D network contains sections that extend beyond the bank edges onto part of the floodplain you need to decide whether these out of bank areas should remain in the 1D model or become part of the 2D model. If you decide on the latter then you need to ensure only the in-bank sections are displayed in the map. There are a few options to achieve this:

     • You can manually edit all 1D sections, curtailing them to the bank edges – this can be time consuming in larger models and you will ‘lose’ the out of bank areas from your network.
     • You can mark the out of bank areas in each cross section as inactive by adding deactivation markers. The 1D engine will then ignore these inactive areas at simulation runtime and only model the in-bank areas. These markers are set in the 1D section properties window (accessed by double-clicking on a 1D section in the network table or the related icon on the map).
     • If your 1D network already has markers that signify bank edges then you can go to the Toolbox > Additional Model Build Tools > 1D Cross Sections to access the Change River Section Markers tool and change all bank edge markers in a network to deactivation markers in a single operation.
     • After setting up the required extent of your 1D sections for use in a 1D-2D linked model you need to display the sections on the map as polylines. Add your 1D sections to the map view by dragging the network from the Project panel onto the map. Note your cross sections must be fully georeferenced to be drawn on the map (the Toolbox has tools to georeference cross sections).

     Flood Modeller will check the network and if sections include deactivation markers you will be prompted to decide whether to show all section data or just the active portion on the map. Choose the latter option and only the active portion of each section will be drawn on the map.

   • In the Layer Editor toolbar for your active area select the snapping tool as shown below:

     

   • When the snapping control window is displayed, irrespective of whether the full 1D cross sections are displayed on the map or just the active portions, you should set snapping to one or both end points of the 1D sections layer, as shown above. Close the snapping control window (red “x” in upper right corner) to apply your settings.

   • You can now proceed to draw your active area polygon as described in point 2 above. As you add each polygon point the mouse cursor should jump to the section end points of the specified 1D network as you hover close to each section. Thus the active area can be constructed joining up to the 1D model it will link to.

   Note : In some cases you may decide to draw your 1D-2D link lines before defining your active area (link lines would be drawn manually using snapping to link precisely between end points of 1D sections). In this case you would have the option to snap your active area to your link line shapefile and then draw the active area by tracing along your link lines.

4. Tracing active area along existing polyline:

   If you have a (high) bank line or 1D-2D link lines already defined, then you can quickly trace your active area along these lines in a single operation (rather than separate clicks to extend your area over every point along the line). The tracing procedure is as follows:

   • Ensure the polyline shapefile you wish to trace along is loaded in your map view.
   • Start drawing a new active area, as described in point 1 above.
   • In the Layer Editor tab of the main Toolbar, select the snapping control. In the pop-up window that is then displayed, select to snap to the polyline shapefile you will be tracing along – tick the box next to the appropriate file and select to snap to any point on the line (the default option).

   It is recommended that when tracing you should only have snapping defined for the polyline selected to trace along and no additional files.

   

   • Close the pop-up window to apply the snapping setting.
   • Click on the Draw Polygon button in the Layer Editor to start drawing a new active area. Move the mouse cursor (which should change to a pen icon) close to the point where you wish to start tracing on your polyline. The snapped point should be shown with a dotted line rectangle surrounding it. Click the left mouse button to start drawing from this point.
   • With the Shift key depressed, move the cursor to where you want to trace to on the polyline. When the correct point is displayed with the “snapped to” rectangle around it click the left mouse button (with Shift key still depressed). Active area points should be added at all points along the traced polyline (between your specified start and end points).
   • To add further, non-traced points to your active area continue to left-click on the map, but without depressing the Shift key.
   • Double-click to stop adding points and to close the active area polygon.

   Note: Any errors in the ordering of features in a polyline (or errors in the order of the points that make up each feature) will be highlighted if it is used for tracing an active area. The tracing function will follow whatever order is present in the polyline file between the user selected start and end points.

5. When all features of your active area polygon are defined click the Save button in the Layer Editor toolbar and then the Stop Edit button to exit edit mode (you can just click the Stop Edit and you will be prompted to save recent changes).