Hi NIels.thule,
you can model open channels within MIKE URBAN by using the link type "Natural Channel".
Just start defining the stream as nodes and links. You need at least two nodes at the beginning and at the end of the channel and probably more nodes at connection points to the pipe network.
In a second step you define the cross sections in the CRS menu as X-Z-open or X-Z-R-M open (for details see CollectionSystem Users Manual page 64/65). The third step is to create one topography for each link. A topography consists of a number of cross sections, each with a chainage along the link. The best solution is to have at least two cross sections for each topography, one at Chainage 0 and one at the chainage that corresponds to the link length. In between two chainages the cross section will be interpolated.
Add the final topography to your links and connect the stream to the pipe network.
Best Regards
Ralf