Direct Method

The main solution method is called the Direct Method. It overcomes some of the difficulties associated with the Pseudo-Timestepping Method. It is faster and more accurate and requires very little initial data. For steady state conditions the Saint-Venant equations can be reduced and written as ordinary differential equations; these are solved for individual reaches. This is used with an automated distance step size control option which tells you where extra interpolated sections should be inserted into the model. The method can be thus seen as having an adaptive grid.

The remaining problem is to solve the network so that Kirchoff's law is satisfied and equal water elevations are obtained at junctions and reservoirs. This is done by an iterative scheme to solve the correction of the flow splits at channel confluences and bifurcations. Convergence is achieved when the maximum correction to a flow split is less than 0.1% and the maximum elevation difference is less than 1mm at a junction.

The method is quicker than the pseudo-timestepping method and is able to pinpoint data problems and sensitive areas in a particular model. The automatic identification of local problem areas is of great advantage.

The method deals accurately and consistently with the problem of cross section spacing. During the computation, the method checks whether the solution is "grid dependent", and if necessary will add extra interpolated nodes implicitly. The user is informed where this has been done so that extra surveyed sections can be added to the model if available, or extra nodes interpolated between the existing sections. If a large number of interpolated sections have been added between two cross-sections, this indicates either that the channel properties of the two sections are significantly different (alerting the modeller to potential data discrepancies) or there is large surface curvature in the channel (for example as the Froude number approaches unity).