If the description of the bathymetry is the most important task in the modelling process then the description of the water levels and flow at the open boundaries (in short called the “boundary conditions”) is the second most important task. The better the boundary conditions the better the results and the fewer the instability problems.
The MIKE 21 Flow Model solves the partial differential equations that govern nearly-horizontal flow and, like all other differential equations, these need boundary conditions. As the unknown variables are surface elevation and flux densities in the x-direction and y-direction you must, in principle, specify two of these three variables in all grid points along the open boundary at each time step. However, in most applications you only know the surface elevation and possibly the general flow direction or you know the total flow through your boundary and its general direction. The input to the hydrodynamic module of the MIKE 21 Flow Model has therefore been structured accordingly.
You can choose between the following two combinations of boundary input:
· Specify water levels and the direction of the flow.
The water levels can be constant or varying along the boundary line. The variation in time can be either constant, sinusoidal or vary as specified in a type 0 or a type 1 data file. Finally a transfer file can be used for water level, which is obtained by use of the M21 tool Transfer Boundary.
The directions can either be specified as default, meaning perpendicular to the boundary, or read from a type 1 data file. The data file must contain a direction for each individual grid point along the boundary.
· Specify a flux boundary, as either discharge (constant, sinusoidal, type 0), flow flux (type 1, transfer) or Rating curve (type 0), through the boundary and the flow direction.
The discharge can be constant in time, have a sinusoidal variation or vary as specified in a type 0 data file. The distribution of the total flow in the individual grid points along the boundary is calculated by MIKE 21 relative to the depth.
Type 1 and transfer Boundary conditions are also possible, but in this case the values in the cells of the input files represent the flow flux at each cell, instead of the total discharge.
A Rating curve is a dfs0 file with axis type ‘Relative item axis’. The data represent the variation of a discharge (as item) as a function of the water level which is specified in the axis.
The direction should be given in the same way as for level boundaries.