Initial Conditions

In 1D river modelling, initial values of flow and stage are required at every node before commencing an unsteady or steady timestepping 1D simulation. Failure to supply adequate values of these may cause the model to fail or lead to initial instabilities in the simulation.

Flood Modeller provides options to Define new Initial Conditions for a 1D river network.

Further technical details about initial conditions

Additionally, it may be required to specify Unit Mode and Unit State for some structures, for example sluices. The latter (for example when setting the initial gate opening for a Vertical Sluice) is particularly important when starting a model with the gate operating in LOGICAL mode. Initial values of other 1D model variables, for example velocity, Froude Number and thalweg (lowest bed elevation) may be supplied, although these have no impact on the model solution.

Initial conditions are specified in the INITIAL CONDITIONS block. In the Flood Modeller Interface, this lies in the lower portion of the Network Panel, separated from the Network Chart (above) by a splitter bar in the data file, this must be the last data block after all the hydraulic units.

Initial conditions for unsteady runs can either be entered directly by the user, or obtained from a steady run. If entered directly the flows and stages for all nodes must be entered (for Muskingum and Muskingum-Cunge units only flow is required). Normally, however, a steady run is used to provide the initial conditions for an unsteady run (this may not be possible for Muskingum/Muskingum-Cunge nodes).

The Direct Method for computation of steady flows does not require initial conditions for single channels and only requires a flow estimate at confluences in looped models. Node labels are required to be present in the initial conditions block for all nodes, even if values of all the variables are not necessary.

Following a steady simulation the new values of flows and stages are written to the .zzs file. These steady results can then be used as the initial conditions for an unsteady run if desired. By far the easiest way of creating initial conditions for a new model is to run a Direct Method steady state run and to use the steady results. This can be done either by importing these into the model directly, or by ensuring the "Initial Conditions from Initial Conditions File" option is selected from the Files Tab of the 1D Simulation Interface.