The landslide velocity plays a dominant role in estimating impact force and
devastated area. Here, based on Pudasaini and Krautblatter (2022), I develop a
novel extended landslide velocity model that includes the force induced by the
hydraulic pressure gradient which was neglected by all the existing analytical
landslide velocity models. By a rigorous conversion between this force and
inertia, I develop two peer systems expecting to produce the same results.
However, this contradicts with our conventional wisdom. This raises a question
of whether we should develop some new balance equations. I compare the two
velocity models that neglects and includes the force induced by the hydraulic
pressure gradient. Analytical solutions produced by the two systems are
different. The new model is comprehensive, elegant, and yet an extraordinary
development as it reveals serendipitous circumstances resulting in a
pressure-inertia-paradox. Surprisingly, the mass first moves upstream, then it
winds back and accelerates downslope. The difference between the extended and
simple solution widens strongly as the force associated with the hydraulic
pressure gradient increases, demonstrating its importance. Viscous drag plays
an important role in controlling the landslide dynamics. However, no explicit
mechanical and analytical model exists for this. The careful sagacity of the
graceful form of new velocity equation results in a mechanically extensive,
dynamically evolving analytical model for viscous drag, the first of this kind.
A dimensionless drag number is constructed. Contrary to the prevailing
practices, I have proven that drags are essentially different for the expanding
and contracting motions, an entirely novel perception. Drag coefficients are
close to the often used empirical or numerical values. But, now, I offer an
innovative, physically-founded analytical model for drag in mass flow
simulation