Flood Modeller is used to model open channel and overbank flows in any network of channels. Any sensible looped or branched network can be modelled using Flood Modeller, which incorporates a wide range of hydraulic units including a variety of conduit types, hydraulic structures, and so on.

Flood Modeller computes flow depths and discharges using a method based on the equations for shallow water waves in open channels - the Saint-Venant equations.

Flood Modeller can be used to solve systems under both steady and unsteady flow conditions. Steady solutions are discussed further in the Steady Flows topic. For unsteady solutions Flood Modeller uses the governing hydraulic equations for each unit. These equations are inevitably a combination of empirical and theoretical equations many of which are non-linear. The non-linear equations are first linearised and the solution to the linear version of the problem is then found via matrix inversion. An iterative procedure is used to account for the non-linearities.

The Preissmann four-point implicit finite difference scheme is employed for the channel equations and the matrix is inverted using a powerful sparse matrix solver.