174
12 Consequences
Thus the evaluation of a dam breach outflow requires attentive analyses using
complex rheological relationships, such as Bingham fluids. The parameters of the
Bingham fluid equation are not easy to determine.
For each failure mechanism above, a set of dam breach parameters can be combined with the three alternative flow characteristics to produce a representative set
of dam breach scenario combinations for assessment in a dam break study. The scenarios matrix then incorporates the range of likely outflow volumes from “smaller”
to “larger” breach events. Although this may not be considered an exhaustive set of
the possible outcomes, it may be assumed to provide a basis for screening the range
of possible outflow extents.
The storage volume for the dam during normal operations includes tailings, supernatant pond, and freeboard below the design elevation crest. The storage volume
curve is generally available from the design engineers. The storage curve provides
the potential total volume for the dam breach scenarios. This is then modified to
reflect the estimated outflow from the scenarios where the outflow volume is only a
fraction of the total potential volume.
12.2.1 Tailings Rheology
The tailings rheology can of course play an important role in the fluid dynamics
and spatial extent of the dam breach flood. The tailings rheology in the TSF can
vary temporarily and spatially (horizontally and vertically) depending on numerous factors such as solid concentration, mineralogical composition, tailings delivery
system, local hydrology and hydro-geology, and operating protocol. When the concentration of solids increases, the tailings rheology starts deviating from Newtonian
fluid behaviour to become a visco-plastic fluid. Furthermore, tailings can liquefy
under a critical shear stress value (the yield stress or yield strength). Bingham fluids
are a subset of visco-plastic fluids that are well-suited for describing “muddy flows”
where there is a high concentration of fine-grain sediment without the presence of
much sand or coarser particles.
We have seen cases where efforts were made to divide tailings volumes into two
or more classes as a function of volumetric concentrations (for example, normal and
high or somewhat consolidated). Volumetric concentration C v is the percent volume
of solids to the total mixed volume of solids and water (%v/v).
Examples of the histograms for this type of data are shown in Figs. 12.1 and 12.2.
In this example the clay mineral content for the tailings was assumed to be minimal
based on extant description of the tailings. The particle density for the tailings solids
in the TSF was 2.8 t/m
3 , based on reported particle densities. These graphs can be
used to determine the rheological parameters for the Bingham fluid.
The primary Bingham fluid parameters are:
• Yield strength (τ y ) in Pa;
• Dynamic viscosity (μ m ) in Pa s;
12 Consequences
Thus the evaluation of a dam breach outflow requires attentive analyses using
complex rheological relationships, such as Bingham fluids. The parameters of the
Bingham fluid equation are not easy to determine.
For each failure mechanism above, a set of dam breach parameters can be combined with the three alternative flow characteristics to produce a representative set
of dam breach scenario combinations for assessment in a dam break study. The scenarios matrix then incorporates the range of likely outflow volumes from “smaller”
to “larger” breach events. Although this may not be considered an exhaustive set of
the possible outcomes, it may be assumed to provide a basis for screening the range
of possible outflow extents.
The storage volume for the dam during normal operations includes tailings, supernatant pond, and freeboard below the design elevation crest. The storage volume
curve is generally available from the design engineers. The storage curve provides
the potential total volume for the dam breach scenarios. This is then modified to
reflect the estimated outflow from the scenarios where the outflow volume is only a
fraction of the total potential volume.
12.2.1 Tailings Rheology
The tailings rheology can of course play an important role in the fluid dynamics
and spatial extent of the dam breach flood. The tailings rheology in the TSF can
vary temporarily and spatially (horizontally and vertically) depending on numerous factors such as solid concentration, mineralogical composition, tailings delivery
system, local hydrology and hydro-geology, and operating protocol. When the concentration of solids increases, the tailings rheology starts deviating from Newtonian
fluid behaviour to become a visco-plastic fluid. Furthermore, tailings can liquefy
under a critical shear stress value (the yield stress or yield strength). Bingham fluids
are a subset of visco-plastic fluids that are well-suited for describing “muddy flows”
where there is a high concentration of fine-grain sediment without the presence of
much sand or coarser particles.
We have seen cases where efforts were made to divide tailings volumes into two
or more classes as a function of volumetric concentrations (for example, normal and
high or somewhat consolidated). Volumetric concentration C v is the percent volume
of solids to the total mixed volume of solids and water (%v/v).
Examples of the histograms for this type of data are shown in Figs. 12.1 and 12.2.
In this example the clay mineral content for the tailings was assumed to be minimal
based on extant description of the tailings. The particle density for the tailings solids
in the TSF was 2.8 t/m
3 , based on reported particle densities. These graphs can be
used to determine the rheological parameters for the Bingham fluid.
The primary Bingham fluid parameters are:
• Yield strength (τ y ) in Pa;
• Dynamic viscosity (μ m ) in Pa s;