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===Solution Scheme and Parallelisation===
TUFLOW HPC is an explicit finite volume solution.
[[File: HPC_Cell_Design.PNG |360px]]<br>
The ZC point:
* Defines the volume of active water (cell volume is based on a flat square cell that wets and dries at a height of ZC plus the Cell Wet/Dry Depth);
* Controls when a cell becomes wet and dry (note that cell sides can also wet and dry); and
* Determines the bed slope when testing for the upstream controlled flow regime.
The ZU and ZV points:
* Control how water is conveyed from one cell to another;
* Represent where the momentum equation terms are centred and where upstream controlled flow regimes are applied;
* Deactivate if the cell has dried (based on the ZC point) and cannot flow; and
* Wet and dry independently of the cell wetting or drying (see Cell Wet/Dry Depth). This allows for the modelling of “thin” obstructions such as fences and thin embankments relative to the cell size (eg. a concrete levee).
ZH points:
* Play no role hydraulically. this poi8nt location is used for output processing;
* The only elevations written to the SMS .2dm mesh file (by default, binary output is interpolated/extrapolated to the cell corners).
within the above sub-grid framework, using TUFLOW HPC time derivatives of cell averaged water depth, u-velocity and v-velocity are computed on a cell-by-cell basis and the model evolved using an explicit ODE solver. Calculation of the cell based derivatives are highly independent of each other making it possible to run this solution scheme across multiple processors or GPU cards. Parallelisation is done by breaking up the model into vertical ribbons. Each ribbon of the model is run on a different processor (or GPU card) with boundary information shared between processors at each timestep.<br>
[[File: Mesh_Ribbon_Splitting.png |360px]]<br>
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