Post-processing

At the post-processing stage, the water levels computed for each depression are projected onto the DTM. Then the post-processor tries to find the path the water may have taken (based on terrain data) to generate flood maps.

This section of the manual describes these processes in more detail.

The standard algorithm in Flood Modeller 2D solver (FAST) generates a table of water levels for each depression. Volumes are also calculated and summed as a check on model mass balance. The water level output is combined with a grid, where each cell contains the ID of the depression to which it belongs. This generates a grid of water levels (for some cells, this will be below ground level in depressions that are not full). The DTM is subtracted from the water levels to give depths, and negative values are removed. The depth and water level grids are output as ASCII grid raster files. This post-processing stage is implemented as a separate segment.

The algorithm focuses on obtaining the final flood extent from any source of flooding involved (pluvial or fluvial). Although this is acceptable in many cases, it is a severe limitation in others – especially where the flood water travels a significant distance (such as in cases involving inflow from dam or embankment breaching or other point sources).

To overcome this constraint, an additional post-processing functionality called 'probable flowpath mark-up' can be used in combination with standard post-processing.  As the name suggests, this tries to find the path water may have taken (based on terrain data) to reach the current flood extent. The following section describes the algorithm, followed by case studies used to test the method.

Probable flow path mark-up method overview

The flow path identification is a version of a rolling ball algorithm and finds the likely flow path between two isolated flooded areas. This flow path connects up the flood extent but is essentially the width of a single cell. To have a more realistic approximation, this width is padded by the 'padding width' defined by the user. The algorithm tries to expand the path to match the padding width, while taking into account the terrain layout (for example, it would not extend into a 'valley' side if the ground rises steeply and it is unlikely to become wet from  flowing water).

Flow path examples