Following suggestions from the literature that, in some cases, mud flow and debris flow may be treated in computations as viscous laminar flow, this paper contains test results compared with results obtained with a one-dimensional numerical model of a high-viscosity fluid wave on an inclined bottom. A gate that initially retained the fluid at the upstream end of a channel was quickly removed. The set up of the numerical model is discussed in detail. A finite difference scheme is applied. Viscous bottom friction is counted for. To overcome the discontinuity at the start of the break, the classical analytical solution of the zero-viscosity dam break wave on a horizontal bottom is introduced.