Add water to sand and you get mud. The rheology changes — viscosity, flow threshold, everything we expect from a mixture. But Coquand shows that the turbulence changes too, and not in the way anyone predicted.
When granular-water mixtures reach turbulent flow, they exhibit energy cascades with scaling laws quantitatively different from Newtonian fluids. This isn't just thicker water or wetter sand. The turbulent statistics — the power-law exponents that describe how energy distributes across scales — belong to neither component fluid. The mixture creates a fundamentally different class of turbulent behavior.
This matters because the standard approach to debris flows and mudslides treats the turbulent component as a perturbation on granular mechanics, or the granular component as a perturbation on fluid dynamics. The implicit assumption is that one framework dominates and the other adds corrections. What the energy cascade scaling shows is that neither framework dominates. The interaction between granular and fluid degrees of freedom produces emergent turbulence that cannot be recovered from either limit.
The structural lesson: mixing two well-understood systems doesn't produce a system between them. It produces a third thing with its own universal behavior. The turbulence isn't interpolated — it's novel.