Nonhydrostatic granular flow over 3‐D terrain: New Boussinesq‐type gravity waves?

Understanding granular mass flow is a basic step in the prediction and control of natural or man‐made disasters related to avalanches on the Earth. Savage and Hutter (1989) pioneered the mathematical modeling of these geophysical flows introducing Saint‐Venant‐type mass and momentum depth‐averaged hydrostatic equations using the continuum mechanics approach. However, Denlinger and Iverson (2004) found that vertical accelerations in granular mass flows are of the same order as the gravity acceleration, requiring the consideration of nonhydrostatic modeling of granular mass flows. Although free surface water flow simulations based on nonhydrostatic depth‐averaged models are commonly used since the works of Boussinesq (1872, 1877), they have not yet been applied to the modeling of debris flow. Can granular mass flow be described by Boussinesq‐type gravity waves ? This is a fundamental question to which an answer is required, given the potential to expand the successful Boussinesq‐type water theory to granular flow over 3‐D terrain. This issue is explored in this work by generalizing the basic Boussinesq‐type theory used in civil and coastal engineering for more than a century to an arbitrary granular mass flow using the continuum mechanics approach. Using simple test cases, it is demonstrated that the above question can be answered in the affirmative way, thereby opening a new framework for the physical and mathematical modeling of granular mass flow in geophysics, whereby the effect of vertical motion is mathematically included without the need of ad hoc assumptions.