Theoretical background

Introduction

Flood Modeller's 2D solver (FAST) computational engine is based on a simple set of rules to simulate spreading of flood water over a given flood plain. It estimates a region's response on receiving a certain amount of water as input, based on data provided as a detailed digital terrain map (DTM). The basic sequence of events is:

The first stage involves pre-processing the input raster grid. The pre-processor identifies every depression in the DTM that has all its neighbouring points at a higher elevation. It also finds the set of all neighbouring points such that water falling on these points will flow towards the identified depression. This set of points is termed as a depression. Hence, the entire DTM can be broken into a collection of depressions. Furthermore, the pre-processor sets up stage-area-volume relationships for each depression, defines its neighbours and finds the minimum connection level with each.

In addition to preparing input data, the pre-processor checks that the allowable number of active computation cells and number of depressions are not exceeded. If these limits are exceeded, the simulation will be aborted and a message explaining which limit is exceeded will be written to the FAST solver log file. Unless you are using a Professional edition licence with the unlimited cell 2D solver enabled, the number of computational cells and depressions in Flood Modeller are limited as follows:

• Standard – 400,000 computational cells; 4,000 depressions
• Professional – 1,000,000 computational cells; 10,000 depressions

The computational engine then introduces water into the depressions, linked to the boundary conditions specified in the model. It checks the water level in each depression. If the water level in any depression is higher than the connection level with its neighbouring depression and the water level in the neighbouring is lower than the water level in the depression being considered, then the water is distributed evenly between the depression and its neighbour such that volume is conserved and water levels are equalised.

Finally, the post-processor projects the water levels computed for each depression onto the DTM and then tries to find the path the water may have taken (based on terrain data) to generate flood maps.