Friction decoupling and loss of rotational invariance in 2D flooding models

Friction decoupling, i.e. the computation of friction vector components making separate use corresponding velocity components, is common in staggered grid models SWE simplifications (Zero-Inertia and Local Inertia Approximation), due to programming simplicity consequent calculations speed-up. In present paper, effect decoupling has been studied from theoretical numerical point view. First, it found that causes reduction computed force rotation vector. Second, demonstrated decoupled-friction lack rotational invariance, model results depend on alignment reference framework. These have confirmed by employing experiments flat smoothly variable bed. Since speed-up obtained with modest, coupled-friction models, which are more general, should be preferred every case.