As a user, I need to burn the 1D river into ground model, so the flooding extent inside the 1D river channel is shown accurately
HEC-RAS has a tool in RAS Mapper doing just that.
Martin Spiers commented
It doesn't seem possible to extract the flood extents within a 1D river reach from ICM: neither an export of the 2D mesh results (understandably) or of flood contours (less understandably) includes all the sections of the 1D channel which are wet.
HECRAS and FloodModeller are capable of combining 1D and 2D results to create an output showing the extent of flooding