Question
In the previous version of MIKE URBAN we could modify the threshold for allowed differences between nodes ground level and surface model ground level (MIKE 21) by means of the parameter MM21_ALLOWEDGROUNDLEVELDIFFERENCE in the DHIAPP.INI file. Where can I change this setting in MIKE+?
Answer
The current version of the coupling engine does not set a limit for the difference between the ground levels between 2D surface Model (FM) and the MIKE 1D engine values if the flexible mesh ground level is above the urban node bottom level for manholes or similar.
If the difference between the ground levels is higher than 1 cm (about 0.39 in), a warning message is sent to the output report files. Therefore, no longer a need to set the value of the parameter “MM21_ALLOWEDGROUNDLEVELDIFFERENCE” as was the case when coupling with MOUSE.
Summary of background
The former coupling of MIKE URBAN and MIKE 21 did not allow larger differences, the user could modify the threshold. It was, however, a sign of instability and potential problems with the model.
Fig. 1 - Map of ground level differences
MIKE URBAN ran the classic rectangular grid engine (MIKE 21) for the overland flow, this engine was not as versatile and powerful as the flexible mesh engine (MIKE 21 FM) - see Fig. 2.
Fig. 2 - Classic rectangular grid
The flexible mesh engine (MIKE 21 FM) will simply overrule the ground level of the nodes running a validation check for the invert level of the node to be lower than the ground in the Digital Elevation Model (mesh). If an invert level is above the DEM, the engine will catch the error and notify the user.
Conclusion
The flexible mesh engine rules the 2D Overland Simulation setting up the mesh level as the surface elevation thus the parameter control for the engine is outdated.
FURTHER INFORMATION & USEFUL LINKS
Manuals and User Guides
MIKE+. User Guide. 2D Overland
MIKEPlus Release Notes 2022
Training options
MIKE+ CS - Getting started with urban drainage modelling
MIKE+ 2D Overland Flow - Getting started with urban flood modelling