  Friction decoupling, i.e. the computation of friction vector components
making separate use of the corresponding velocity components, is common in
staggered grid models of the SWE simplifications (Zero-Inertia and Local
Inertia Approximation), due to the programming simplicity and to the consequent
calculations speed-up. In the present paper, the effect of friction decoupling
has been studied from the theoretical and numerical point of view. First, it
has been found that friction vector decoupling causes the reduction of the
computed friction force and the rotation of the friction force vector. Second,
it has been demonstrated that decoupled-friction models lack of rotational
invariance, i.e. model results depend on the alignment of the reference
framework. These theoretical results have been confirmed by means of numerical
experiments. On this basis, it is evident that the decoupling of the friction
vector causes a major loss of credibility of the corresponding mathematical and
numerical models. Despite the modest speed-up of decoupled-friction
computations, classic coupled-friction models should be preferred in every
case.
