The most important secondary flow in rivers is the so-called helical flow, with its name derived from Helios (the Sun in Greek). The name helical is used because the flow arises as the water in the lower portions of the water column flowing towards the local center of curvature, and away from the local center of curvature along the water surface. This has only a minor impact on the hydrodynamics, usually only pronounced on a laboratory scale, but it has profound impacts on the sediment transport and morphology because the helical flow influences the otherwise zero transverse sediment component. MIKE 21C applies standard theory for the helical flow, which can be found in e.g. Rozowsky (1957). Standard helical flow theory provides a secondary flow velocity profile that is fully characterised by friction and the deviation angle between the main flow direction and the direction of the shear stress at the river bed.
