Computational area

A Flood Modeller 2D simulation requires a computational area; a‘grid’ within the confines of your ground data grid.

The computational area defines the extent of your model and the resolution within that extent (i.e. a grid cell size). When a DTM is specified, Flood Modeller will automatically read the maximum extents and cell size of this file to define the computational area. However in many cases you will not be interested in modelling the complete DTM , you may want to define your own computational area that is a subset of the DTM extent. This is a rectangular area and can be defined in two ways: 1) by specifying as a new polygon shapefile ; or 2) by manually editing the attributes of the calculated computational area. After a rectangle is drawn in the map view, Flood Modeller will populate the associated attribute file (dbf ) with the parameters required (i.e. grid cell size, no. of rows and columns, rotation angle, etc).

This grid of calculation cells that Flood Modeller 2D will operate with is defined by a series of parameters:

• “angle” – rotation angle of grid, default = 0
• “cellsize” – calculation cell size, default is the same cell size as your specified DTM dataset.
• “ncols” – number of columns in computational mesh, i.e. rectangle width divided by cell size. An imported shapefile rectangle may be reduced in width slightly to the nearest whole number of columns that can be fitted within it.
• “nrows” - number of rows in computational mesh, i.e. rectangle height divided by cell size. An imported shapefile rectangle may be reduced in height slightly to the nearest whole number of rows that can be fitted within it.
• “xll” – x coordinate of lower left corner of rectangle or DTM (fixes the computational grid to a GIS location)
• “yll” – y coordinate of lower left corner of rectangle or DTM (fixes the computational grid to a GIS location)

By default the computation area is initially defined automatically using the full extent and cell size of the specified ground elevation grid. However, these values can then be edited in the 2D user interface.

The computation area can alternatively be defined by a single (rectangular) polygon shapefile (ESRI format), although it must have the above data defined as shapefile attributes. You can create this file, with all required attributes automatically assigned, using Flood Modeller (other compatible GIS systems can also be used although the required attributes must then be defined manually).

By default the area is aligned with your x-y coordinate system, however it can also be provided with an orientation angle enabling the computation grid to be aligned with the predominant flow direction (this is not essential, but may produce better results).