Stefan Szylkarski

You need to ensure that the cross sectional area of the culvert is less than the cross sectional area of both the upstream and downstream cross sections for each of the levels in the processed data table.  The structure/culvert flow equation requires that a flow contraction must occur at all level to ensure the correct calculation of the head losses. If the area of the culvert is even slightly greater than the cross section areas at a specific level then you have an expansion and not a contraction and MIKE11 will prevent you pre-calculating the processed data.  If you make your cross sections slightly wider or lower the invert then you will overcome the problem.