Product(s):	OpenFlows FLOOD
Version(s):	10.xx.xx.xx

Problem

Some parameters necessary to build a 2D flood model are spatially variable and can greatly influence water flow. The main one is of course terrain elevation which in OpenFlows FLOOD is provided as a Grid Data file (See more information on How to create a digital terrain model). A Grid Data file contains information attached to a computational grid, where a value is set for each grid cell.

Other parameters such as Manning's n (used to compute friction depending on the type of terrain surface) or SCS Curve Number (used to compute infiltration rates depending on the type of surface) are also very important in calculating flows and can also be setup in the model using Grid Data files which can easily be created within OpenFlows FLOOD. These two parameters are normally based on land use/land cover data which is mainly available in a polygon shapefile. Each polygon has an attribute describing the type of land use/land cover, e.g. forest, buildings, roads, agriculture, pasture, etc.). 

This article describes the workflow to create a Grid Data layer from a polygon shapefile, where each computational grid cell will be set with a Manning's n or SCS Curve Number. 

Solution

To create a Grid Data file from a shapefile, you'll first need to load your Grid Data digital terrain model and a shapefile with land use/land cover data. 

When loading the shapefile you can select the attribute to be color coded in the MapWindow. In the image below an example of the dialog where this selection is made is shown. The shapefile has several attributes (Oid, fid, country, etc...), and in this example the "class_2018" will be selected as it contains a text description of the type of land use. 

Once loaded you can access the shapefile properties dialog by right-click in the layer list on the left and the following dialog will appear: 

In "Style Type" choose "Color Range" and automatically a list of the attributes for "class_2018" will populate the list on the right, with a single color for each type of land use. You can alter the color codes by selecting a different colormap or manually changing the color by double-clicking on the color rectangle. 

Below is shown an example of the land use/land cover shapefile loaded in OpenFlows FLOOD: 

After loading your Grid Data terrain model file and land use shapefile, open OpenFlows FLOOD toolbox and in the Grid Data tools select "Create Grid Data from Shape".

The following dialog will appear on the right. 

In Step 1, select the terrain model Grid Data file, the land use shapefile and the shapefile feature (attribute) that describes the type of land use (in this case "class_2018"). Then press the button to analyze the contents of the shapefile. 

This will populate the Step 2 list as shown below.

Then, in the value column on the right, you must enter a value of Manning's or SCS Curve Number (or other parameters) corresponding to the type of land use. See below an example (the values introduced on the right side of the list are for demonstration purposes only). Make sure to set a default value in case some parts of your computational grid are not covered by the land use data. 

Once the list is filled, you can save the list in a XML file. This will enable you to load this relationship table without having to manually introduce the values again. You can load the XML file using the button above the save button. 

Finally, in Step 3, choose the file path for the Grid Data that will be created, containing the Manning's n or Curve Number values for each grid cell and press the button to process. 

When the tool finishes press OK in the following dialog:

and the Grid Data file will be automatically be loaded on the Map Window as shown below: 

You can then reference the Grid Data file in the model calculation options. For Manning's n set the option in the RunOff module:

And SCS Curve Number set the option in the Basin module: