Start with Substantial, "Overbank," n-values

It can be difficult to strictly separate boundary losses from internal losses in a mud and debris flow, but most of these events travel overland or include non-alluvial portions of the stream valley.  It is common to use n-values in the 0.15 -to - 0.25 range for steep models that are not stable. 

Adaptive Time Step Can Generate Instabilities at Low Velocities

Adaptive time step controls can help optimize time steps limiting the time step to an appropriate range for the velocities and cell sizes (limiting by the Courant Condition), speeding the model up with larger time steps at low velocities and increasing temporal resolution at high velocities.

However, velocities associated with mud and debris flows can get very low, as the shear stresses approach the yield stress, which can invoke the maximum time step in this tool, and lead to instability.  To avoid these issues, set the Computational Interval to the maximum acceptable time step, then set the Maximum Number of Doubling Base Time Steps to 0.

Try Constant Time Step if Model is Unstable

Because of mud and debris flows often pass through a wider range of velocities (including 0 or near 0) than clear water flows, the adaptive time step control can develop instabilities.  If your model is unstable, particularly at the beginning or end, consider using a constant time step.