In this paper, we will describe consistent numerical methods for power-law viscoplastic free-surface flows. From the rheological viewpoint, associated models are of Herschel-Bulkley type, which is a generalization of the Bingham model. On the one hand, Bingham model is the simplest model when it comes to describe viscoplasticity. On the other hand, power-law model is a natural extension of a rate-of-shear dependant viscosity, as opposed to the canical case of the (often) constant viscosity used in the Navier-Stokes equations. After describing a shallow-water asymptotics of a 3D Navier-Stokes-Herschel-Bulkley model with free surface, we will end up with a model which has various mathematical difficulties. We will show how to handle optimization problems arising from the variational inequalities associated to the model, as well as their coupling with finite-volume discretization. Several numerical tests will be shown, including a comparison with an analytic solution, to confirm the well balanced property and the ability to cope with the various rheological regimes associated with the Herschel-Bulkley constitutive law. See : http://www.iccfd.org/iccfd7/assets/pdf/papers/ICCFD7-3302_paper.pdf