How do you determine the values of μ and ξ?

Question:

I read the manual of debrisflow, but I am still confused about how to to determine the values of μ and ξ.

Answer: 

The major problem is that debris flow parameters vary strongly related to the water content. They therefore vary much more than rockfall parameters. As a first approximation we take (μ=0.2, ξ=200m/s2) which represents the friction of a standard, granular debris flow (say water content of about 30 percent). If you have flows with more water I would suggest using lower mu values (μ between 0.05 and 0.2). A μ of 0.05 is for highly saturated flows (water content > 50 percent). Some possible xi values are provided in the figure below: