At the start of a 2D simulation, the solver will generate a composite ground grid from the layers you have specified in the topography table. The data in this table is read from top to bottom and therefore data from layers in the lower rows will overwrite data from layers in higher rows.
If you want to check the composite grid your simulation is using, ensure the Write check grids checkbox is ticked (on the Domains> Outputs tab of the 2D interface). The 2D solver will then write out the composite grid it uses to the check grid entitled output.chk.zcen.asc (furthermore, the check grid output.chk.zmod.asc shows only the cells that have been modified from your main DTM later, i.e. the top row in the topography table).
::: The tool now accepts the following formats for topography data: * ASCII Grid (ASC) * GeoTIFF (TIF) * Binary Grid (FLT) * ESRI Shapefile (SHP) The computational area and time step details need to be entered next. When a grid data file has been selected for the topography (above), the parameters will be automatically filled based on this data, but these can be edited. Note that for the ADI 2D solver, it is recommended that the **Time Step** (in seconds) should be between ¼ and ½ of the **Grid Size** (in metres), i.e. for a 5-metre grid, the time step should be between 5/4 and 5/2 seconds. A **Reset** button is given on this panel to reset the parameters based on the underlying grid data. Alternatively, new parameters can be loaded into the tool from a shapefile: click the **Load** button to browse to the relevant file.  An active area must be added: this can be selected from the **Layers** panel of Flood Modeller and dragged into the **Active Area** field, or the browse button can be used to search for and select the file. A deactivation area can also be added; this allows you to enter a second shapefile (residing inside the active area) where computations will not be carried out, i.e. to create a “hole” in the active area. Drag a shapefile loaded in the **Layers** panel of Flood Modeller into the **Deactivation** field, or use the browse button to locate the required file. Note that polylineZ shapefiles can also be used. Details of the roughness can be entered into the tool in the roughness data section of the tab. To specify uniform roughness over the entire active area, a **Default Roughness Value** can be set. This will default at 0.1 but this value is editable. The default **Roughness Law** is “Manning”. Click on the drop-down box provided to adjust this to “Chezy” if required.  Roughness grid files can also be added to describe spatially varying roughness. Click **Add** to add a new roughness file and the following window will appear:  Choose a file type from the drop-down box provided and enter the filename and path directly or click to browse to the relevant file. Click **OK** to add the file to the table. Alternatively, click to highlight a file loaded in the **Layers** panel of Flood Modeller and drag it into the table. This will bring up the above window but automatically filled with the filename and file type. Note there are two options for shapefiles, MasterMap shapefiles and Roughness polygons, so the file type should be double-checked if dragging a file in from the **Layers** panel. If a MasterMap shapefile is added, a Lookup Table (.fric file) must also be provided. Type the location directly into the appropriate field or browse to the file in the usual way. Multiple roughness grid files can be added to the table. To **Edit** or **Remove** a file, click on the appropriate button to the right-hand side of the table. The directional arrows to the left of the table allow you to move a highlighted file up or down the list. As with the topography layers, roughness grids will be read from top to bottom in the table, and therefore data from layers in lower rows will overwrite data from layers in higher rows.