Mass Balance Files and Calculations

Flood Modeller 2D solver models can be set up to produce mass balance data with each simulation. It is highly recommended that you review these mass balance files to help validate the results produced by your model. Any problems present within your model set up will often lead to a mass balance error, i.e. water will either be lost or created by the model.

An Flood Modeller 1D - 2D linked model simulation can be set up to produce two mass balance files; one for the 2D element of the model and one for the 1D element.

Mass balance files are created as comma separated (CSV format) text files. Filenames and locations are specified by the user, either in the 2D Simulation window or by directly editing the 2D solver control XML file. This format of file can be opened in a normal text editor, Excel or other spreadsheet software.

The 2D Simulation window provides the facility for specifying mass balance files for a simulation. The mass balance file for the 2D element of your model is defined in the Domain > Model Outputs tab of the window. This contains a set of sub-tabs, one of which is entitled Mass Balance Output:

The ‘Generate mass balance output file’ box must contain a tick to activate this functionality. You then need to specify a filename for your output file. If you define this without a full path then 2D solver will create it in the same folder as your other model outputs. A browse button is provided should you wish to save this file to a different location.

You are also required to define the frequency that mass balance data will be calculated and written to your output file. This can be the same as or greater than the model time step. The shorter the selected time step the more detailed a representation of mass balance you will produce for your simulation, however this is offset by the increased size of the file produced.

For 1D 2D linked simulations an additional mass balance output can be produced with data calculated from the 1D element of the model. The specification for this file is defined in the General tab of the Simulation window. As with the 2D mass balance file you must specify a filename (with or without a full path) and an output frequency (in seconds).

The table below shows an example mass balance file:

The columns of this file are defined as follows:

• The first column T displays time in seconds (from the start of the simulation). The frequency of output in this column (and the simulation) is defined by the user in the 2D solver control XML file (See the Description of the Format of the 2D Solver Control XML File ).

• The second column Percentage Vol Err indicates the mass balance error in terms of a percentage of the total volume in the model domain.

• The third column Wet Cells indicates the number of wet cells for this time step.

• The fourth column Q->1D indicates the inflow into the linked 1D model (e.g. water spills from the floodplain into the river). The above example is a 2D only model, so Q->1D is a zero value throughout the model run.

• The fifth column Q BC->2D shows the 2D model boundary inflows onto the 2D domain (the values normally are the linear interpolation of the inflow Hydrographs).

• The sixth column Q BC<-2D indicates the boundary outflows (i.e. flow out of the 2D domains at downstream boundaries).

• The seventh column 2D DV/Dt shows the rate of change of volume in the model over each time step.

• The eighth column 2D V records the total water volume accumulated in the 2D domain.

• The ninth and tenth columns, Trapped (+) and Trapped (-) show the volume of water held in cells with depth below the dry threshold (see section ”Wetting and drying ” on the page The ADI solver of the section How Flood Modeller 2D Solvers Work for more information on this).

• The last column Q e is the mass balance error, defined as:

The mass balance error indicates the amount of water created (positive value) or lost (negative value) by the model per second.

It is important that you check the mass balance file carefully to ensure your model is behaving correctly. The total water volume, boundary inflow, and mass balance error are good indications of whether a model is valid.