  Computation of gas dispersal in urban places or hilly grounds requires a
large amount of computer time when addressed with conventional multidimensional
models. Those are usually based on two-phase flow or Navier-Stokes equations.
Different classes of simplified models exist. Among them, two-layer shallow
water models are interesting to address large-scale dispersion. Indeed,
compared to conventional multidimensional approaches, 2D simulations are
carried out to mimic 3D effects. The computational gain in CPU time is
consequently expected to be tremendous. However, such models involve at least
three fundamental difficulties. The first one is related to the lack of
hyperbolicity of most existing formulations, yielding serious consequences
regarding wave propagation. The second is related to the non-conservative terms
in the momentum equations. Those terms account for interactions between fluid
layers. Recently, these two difficulties have been addressed in Chiapolino and
Saurel (2018) and an unconditional hyperbolic model has been proposed along
with a Harten-Lax-van Leer (HLL) type Riemann solver dealing with the
non-conservative terms. In the same reference, numerical experiments showed
robustness and accuracy of the formulation. In the present paper, a third
difficulty is addressed. It consists of the determination of appropriate drag
effect formulation. Such effects also account for interactions between fluid
layers and become of particular importance when dealing with heavy-gas
dispersion. With this aim, the model is compared to laboratory experiments in
the context of heavy gas dispersal in quiescent air. It is shown that the model
accurately reproduces experimental results thanks to an appropriate drag force
correlation. This function expresses drag effects between the heavy and light
gas layers. It is determined thanks to various experimental configurations of
dam-break test problems.
