Warning
This theory has not been fully reviewed yet. Read its content with a critical mind.
The governing equations of the dense flow avalanche are derived from the incompressible mass and momentum balance on a Lagrange control volume (Zw2000,ZwKlSa2003
).
Where qent represents the snow entrainment rate.
We introduce the volume average of a quantity P(x, t):
and split the area integral into :
Fient represents the force required to break the entrained snow from the ground and to compress it (since the dense-flow bulk density is usually larger than the density of the entrained snow, i.e. ρent < ρ) and Fires represents the resistance force due to obstacles (for example trees). This leads to in momentum-balance1
:
Using the mass balance equation mass-balance1
, we get:
The free surface is defined by :
Fs(x, t) = z − s(x, y, t) = 0
The bottom surface is defined by :
Fb(x) = z − b(x, y) = 0
The boundary conditions at the free surface and bottom of the flow read:
σi(b) = (σklnlnk)ni represents the normal stress at the bottom and τi(b) = σijnj − σi(b) represents the shear stress at the bottom surface. f describes the chosen friction model and are described in theoryCom1DFA:Friction Model
. The normals at the free surface (ni(s)) and bottom surface (ni(b)) are:
The previous equations will be developed in the orthonormal coordinate system (B, v1, v2, v3), further referenced as Natural Coordinate System (NCS). In this NCS, v1 is aligned with the velocity vector at the bottom and v3 with the normal to the slope, i.e.:
The origin B of the NCS is attached to the slope. This choice leads to:
In this NCS and considering a prism-like Control volume, the volume content
The snow entrainment is either due to plowing at the front of the avalanche or to erosion at the bottom. The entrainment rate at the front qplo can be expressed as a function of the properties of the entrained snow (density ρent and snow thickness hent), the velocity of the avalanche at the front
The entrainment rate at the bottom qero can be expressed as a function of the bottom area Ab of the control volume, the velocity of the avalanche
This leads in the mass balance mass-balance1
to :
The force Fient required to break the entrained snow from the ground and to compress it is expressed as a function of the required breaking energy per fracture surface unit es (J.m − 2), the deformation energy per entrained mass element ed (J.kg − 1) and the entrained snow thickness (Sa2007,SaFeFr2008,FiFrGaSo2013
):
Fient = − wf (es+ qent ed)
The force Fires due to obstacles is expressed as a function of the characteristic diameter f-res
). The effective height heff is defined as
The surface integral is split in three terms, an integral over Ab the bottom x3 = b(x1, x2), As the top x3 = s(x1, x2, t) and Ah the lateral surface. Introducing the boundary conditions boundary-conditions
leads to:
Which simplifies the momentum balance momentum-balance3
to:
The momentum balance in direction x3 (normal to the slope) is used to obtain a relation for the vertical distribution of the stress tensor (Sa2007
). Due to the choice of coordinate system and because of the kinematic boundary condition at the bottom, the left side of momentum-balance5
can be expressed as a function of the velocity Zw2000
):
rearranging the terms in the momentum equation leads to:
The previous equations momentum-balance5
and sigma33
can be further simplified by introducing a scaling based on the characteristic values of the physical quantities describing the avalanche. The characteristic length L, the thickness H, the acceleration due to gravity g and the characteristic radius of curvature of the terrain R are the chosen quantities. From those values, it is possible to form two non dimensional parameters that describe the flow:
- Aspect ratio: ε = H/L
- Curvature: λ = L/R
The different properties involved are then expressed in terms of characteristic quantities L, H, g, ρ0 and R (see fig-characteristic_size
):
The normal part of the stress tensor is directly related to the hydro-static pressure:
σii = ρ0 g H σii*
The dimensionless properties are indicated by a superscripted asterisk. Introducing those properties in sigma33
, leads to :
The height, H of dense flow avalanches is assumed to be small compared to its length, L. Meaning that the equations are examined in the limit ε ≪ 1. It is then possible to neglect the last term in sigma33star
which leads to (after reinserting the dimensions):
And at the bottom of the avalanche, with x3 = 0, the normal stress can be expressed as:
Calculating the surface integral in equation momentum-balance5
requires to express the other components of the stress tensor. Here again a magnitude consideration between the shear stresses σ12 = σ21 and σ13. The shear stresses are based on a generalized Newtonian law of materials, which controls the influence of normal stress and the rate of deformation through the viscosity.
Because ∂x1 and ∂x2 are of the order of L, whereas ∂x3 is of the order of H, it follows that:
and thus σ12 = σ21 is negligible compared to σ13. σ13 is expressed using the bottom friction law boundary-conditions
.
In addition, a relation linking the horizontal normal stresses, σii, i = (1, 2), to the vertical pressure distribution given by sigmab
is introduced. In complete analogy to the arguments used by Savage and Hutter (SaHu1989
) the horizontal normal stresses are given as:
σii = K(i) σ33
Where K(i) are the earth pressure coefficients (cf. ZwKlSa2003,Sa2004
):
With the above specifications, the integral of the stresses over the flow height is simplified in equation momentum-balance5
to:
and the momentum balance can be written:
with
The mass balance mass-balance2
remains unchanged:
The unknown sigmab
, momentum-balance6
and mass-balance3
. In equation momentum-balance6
the bottom shear stress τ(b) remains unknown, and and a constitutive equation has to be introduced in order to completely solve the equations.
The problem can be solved by introducing a constitutive equation which describes the basal shear stress tensor τ(b) as a function of the flow state of the avalanche.
With
Several friction models already implemented in the simulation tool are described here.
The Mohr-Coulomb friction model describes the friction interaction between twos solids. The bottom shear stress simply reads:
τ(b) = tan δ σ(b)
tan δ = μ is the friction coefficient (and δ the friction angle). The bottom shear stress linearly increases with the normal stress component σ(b) (Zw2000,BaSaGr1999,WaHuPu2004,Sa2007
).
With this friction model, an avalanche starts to flow if the slope inclination is steeper than the friction angle δ. In the case of an infinite slope of constant inclination, the avalanche velocity would increase indefinitely. This is unrealistic to model snow avalanches because it leads to over prediction of the flow velocity. The Mohr-Coulomb friction model is on the other hand well suited to model granular flow. Because of its relative simplicity, this friction model is also very convenient to derive analytic solutions and validate the numerical implementation.
The Chezy friction model describes viscous friction interaction. The bottom shear stress then reads:
τ(b) = cdyn ρ0 ū2
cdyn is the viscous friction coefficient. The bottom shear stress is a quadratic function of the velocity. (Zw2000,BaSaGr1999,WaHuPu2004,Sa2007
).
This model enables to reach more realistic velocities for avalanche simulations. The draw back is that the avalanche doesn't stop flowing before the slope inclination approaches zero. This implies that the avalanche flows to the lowest local point.
Anton Voellmy was a Swiss engineer interested in avalanche dynamics Vo1955
. He first had the idea to combine both the Mohr-Coulomb and the Chezy model by summing them up in order to take advantage of both. This leads to the following friction law:
τ(b) = tan δ σ(b) + cdyn ρ0 ū2
This model is described as Voellmy-Fluid Sa2004,Sa2007
, and the turbulent friction term ξ is used instead of cdyn.
SamosAT friction model is a modification of some more classical models such as Voellmy model Voellmy friction model
. The basal shear stress tensor τ(b) is expressed as (Sa2007
):
With
The minimum shear stress τ0 defines a lower limit below which no flow takes place with the condition SaGr2009
).
Note
This is an experimental option to account for wet snow conditions, still under development and not yet tested. Also the parameters are not yet calibrated.
In addition, com1DFA provides an optional friction model implementation to account for wet snow conditions. This approach is based on the Voellmy friction model but with an enthalpy dependent friction parameter.
τ(b) = μ σ(b) + cdyn ρ0 ū2
where,
μ = μ0 exp ( − enthalpy/enthRef)
The total specific enthalpy of the particles is initialized based on their initial temperature, specific heat capacity, altitude and their velocity (which is zero for the initial time step). Throughout the computation, the particles specific enthalpy is then computed following:
enthalpy = totalEnthalpy − g z − 0.5 ū2
The dam is described by a crown line, that is to say a series of x, y, z points describing the crown of the dam (the dam wall is located on the left side of the line), by the slope of the dam wall (slope measured from the horizontal, β) and a restitution coefficient (describing if we consider more elastic or inelastic collisions between the particles and the dam wall, varying between 0 and 1).
The geometrical description of the dam is given on the figure fig-DamToolSide
. The dam crown line (xcrown) is projected onto the topography, which provides us with the dam center line (xcenter). We compute the tangent vector to the center line (tf). From this tangent vector and the dam slope, it is possible to compute the wall tangent vector (tw). Knowing the wall tangent vector and height, it is possible to determine normal vector to the wall (nw) and the foot line which is the intersection between the dam wall and the topography (xfoot).
When the dam fills up (flow thickness increases), the foot line is modified (
In the initialization of the simulation, the dam tangent vector to the center line (tf), foot line (xfoot) and normal vector to the wall (nw) are computed. The grid cells crossed by the dam as well as their neighbor cells are memorized (tagged as dam cells).