Question
How can I convert a MIKE FLOOD model, which includes MIKE 21 'Classic'/Single Grid, to MIKE+? Can there be differences in the results?
Answer
Conversion of a MIKE FLOOD model that includes MIKE21 'Classic'/Single Grid model to MIKE+ is an automatic workflow using the 'Import' functionality in MIKE+ 'File' menu, as shown in Fig. 1.
Fig. 1 - Predefined imported procedures for MIKE supported models
However, you should be aware of the differences between MIKE 21 Classic/Single Grid and MIKE+ 2D overland (coupled, or not, to a 1D model, as part of MIKE FLOOD and MIKE+ models, respectively).
First, note that the 2D Overland model component in MIKE+ uses the MIKE 21 FM engine, which is based on the Finite Volume method, whereas MIKE21 Classic/Single Grid (.m21), our old 2D Model, is based on the Finite Difference method.
Once imported, the new model may show a number of validation errors. For instance, an error may be that the file path referenced to the boundary conditions time series is not correct, as shown in Fig. 2.
Fig. 2 - Imported Boundary conditions to MIKE+
Furthermore, you must verify that the result files used as network loads in the MIKE URBAN-MIKE FLOOD coupled setup ('Computation' dialog in Fig. 3) are properly imported into MIKE+ as boundary conditions. This is due to changes in data handling from MIKE URBAN to MIKE+.
Fig. 3 - Lumped rainfall result file used as boundary condition in MIKE URBAN
The rainfall runoff file should be imported into the boundary conditions section of MIKE+ (in contrast to MIKE URBAN the result file cannot be plugged directly in the simulation tab of MIKE+) - see Fig. 4.
Fig. 4 - Network load from rainfall runoff result file imported to MIKE+
In 'Simulation setup' under 'Simulation specifications' a new entry will be added containing the configuration (modules selection and simulation period) of the MIKE 21 model component (as part of the coupled MIKE FLOOD model). Please note the description text is auto generated by MIKE+ during the import process of the MIKE FLOOD .couple file - see Fig. 5.
Fig. 5 - Simulation launcher in MIKE+, the General tab shows the implemented MIKE 21 configuration and the simulation Description
Make sure to check that the time step defined in MIKE+ is the same as the one specified in the MIKE 21 model (please note that MIKE+ can support smaller time steps than MIKE FLOOD).
Fig. 6 - Simulation launcher in MIKE+, the HD tab shows the time step implemented in the simulation
Regarding model parallelisation, be mindful that (compared to MIKE FLOOD (MIKE 21 Classic/Single Grid)) the MIKE 21 FM numerical engine of the MIKE+ 2D overland model takes full advantage of the computer's computational power (CPU cores) and the GPU available.
In general, the results should be similar (Fig. 6), however there may be cases where the results may differ given the changes in the numerical engine, so models previously calibrated are encouraged to be revalidated after MIKE+ migration.
Fig. 6 - Result comparison of MIKE FLOOD (MIKE 21 Classic/Single Grid, left) vs MIKE+ (MIKE+ 2D overland, right)
Please note that even if the MIKE 21 Classic/Single Grid model has been decommissioned, you can still produce grid results as .dfs2 files in MIKE+.
Conclusion
The import process of MIKE FLOOD model into MIKE+ is a straightforward one by means of the predefined import schemes of MIKE+. However, validation and double checks are recommended after migrating the model.
FURTHER INFORMATION & USEFUL LINKS
Release Notes
MIKEPlus Release Notes
MIKE 21 Release Notes