I am currently using Delft3D to solve problems in my research project. It is about calculation of river channel erosion and sedimentation. When I mix two sediment fractions on the bed, the diameter of which, for example, are 1.25 mm and 30 mm, and using Van Rijn's formula (1993), sediment transport is wrongly calculated. I think the problem may be related to the implementation of the software, and explain it in detail in the attachment. Could you give me some suggestions to solve this problem?  I would be very grateful indeed for  any help you could give me.