High mobility of subaqueous debris flows and the lubricating layer by sanmelody

VIEWS: 31 PAGES: 38

									  Experiments on subaqueous mass
transport with variable sand-clay ratio

                        Fabio De Blasio
                          Trygve Ilstad
                        Anders Elverhøi
                          Dieter Issler
                         Carl B. Harbitz
             International Centre for Geohazards
              Norwegian Geotechnical Institute, Norway
           Dep. of Geosciences, University of Oslo, Norway.
                                 .

   In cooperation with the SAFL group, University of Minnesota


                                                                 1
               Basic problem!

How can we explain that 10 -    Debris
1000 km3 of sediments can       flow
  • move100 - > 200 km
  • on < 1 degree slopes
  • at high velocities
   ( -20 - > 60 km/h)




                                         2
         Inferring the dynamics of
          subaqueous debris flow
• Field observations (long runout, outrunner
  blocks, geometry of sandy bodies, velocity...)
• Experiments:
  – (Experiments +Numerical modeling) × Extrapolation   Field
  – composition change

• Physical understanding and numerical
  simulation

• Important application:
  – Emplacement of massive sand in deep water
  – Offshore geohazards
                                                                 3
           Experimental settings
           St. Anthony Falls Laboratory

                      Experimental Flume: “Fish Tank”




                              turbidity current


        debris flow


                                   6° slope


                                     10 m



Video (regular and high speed) and
pore- and total pressure measurements
                                                        4
       Runout distance in laboratory
   Same: GSD, % Water, Discharge, Volume


                  1
                0.8

                0.6
                0.8
                                                  How to explain the various
Elevation (m)




                              Subaerial
                                                  styles of run out!
                0.6
                                                  Subaerial
                0.4                                 Short and thick

                                                  Subaqueous
                0.2       Subaqueous                Thin and long

                 0
                      0      2      4     6   8
                      Horizontal Distance (m)
                                                                               5
6
 High clay content – video record



Turbidity current




Hydroplaning
debris flow




                                7
Flow behavior - High clay content ( 30 % kaolinite)




High speed video
record
(250 frames/sec)




                                                       8
Low clay content – video record

                        Turbidity
                        current




                        Dense flow
                        Deposition of
                        sand




                                        9
Debris flows- low clay content (5%)




Turbulent front         Deposition of sand




                                             10
High clay content-
- Plug flow- “Bingham”




High sand content
-Macro-viscous flow?
-Divergent flow in the
 shear layer




                         11
                         Thickness of sandy deposits – versus clay content

                                   Deposition          Dense flow         Turbidity current
Deposition height (cm)




                         3
                                                           clay
                                                   5   wt%
                         2


                                                          lay
                         1
                                                10 wt% c
                                                                        15 wt% clay
                         0

                             0            3                         6                    9
                                                 Time (s)
                                                                                              12
                                                                    Pore pressure
   Pressure interpretation                                          Total pressure

                                                         Pressure
            Flow   Grains in constant contact with bed



                   Total pressure
Pore pressure                                                                Time
                                                         Pressure

           Flow           Fluidized flow


                                                                             Time
                                                         Pressure

           Flow     Rigid block over a fluid layer

                                                                              13
                                                                             Time
Pressure measurements at the base of a debris flow as
          pressure develops during the flow
Low clay content                High clay content




      Total pressure
      Hydrostatic pressure
                                                        14
         High clay content
viscoplastic/hydroplaning/lubrication




                                        15
Material from the base of the debris flow is eroded and
incorporated into the lubricating layer.
                                                 L2
                             Ls
            L1

                                                               H2
                                  Hs

  H1



                                       Downslope gravitational forces
                                       Bottom shear stresses

                                                                    16
 Detachment/stretching dynamics
Neglected physics:
• Changing tension due to slope and velocity
  changes
• Friction, drag and inertial forces on neck
• Changes in material parameters of neck due to
  – shear thinning, accumulated strain and wetting, crack
    formation


More sophisticated treatment is possible
  Coupled nonlinear equations, use a numerical model
  Main difficulty is quantitative treatment of crack
   formation and wetting and lubricating effects
                                                       17
      Clay rich sediments
• Visco-plastic materials
• Model approach:
  – ”Classical Bingham fluid” (“BING”)
  – R-BING: Remolding of the sediment
             during the flow
  – H-BING: Hydroplaning/Lubricating


                                         18
Velocity profile of debris flows
         Bingham fluid
              •Classical Bingham fluid:
                 •Yield strength: constant during flow



              •Bingham fluid – with remolding
              (R-BING):
                 •The yield strength is allowed to vary
                 during flow




                                                   19
           Water film/lubricating layer
     shear stress reduction in a Bingham fluid
                                                                 u=1
                                                                          Lid
                                                                          (Debris flow)
                                                           =1
                                                               Water, w, w, uw
                                                         =1-

                                                                 Mud
                                                                 m, m, um



                         1                   1

Shear layer
                                            
                                       1                    1
                                  u                     
              Velocity                                      +      Shear stress
                           1                   1            
                         1-                  1-
                                            

                                                
                               (R-)/ 1                   1
                                                     R(1+)/
                                    u                     +                              20
                                                             
Simulation: final deposit of the large-scale Storegga


                            Initial deposits
                            Present deposits
                             = 10 kPa
                             = 10 kPa with remoulding to 0,5 kPa
                             = 10 kPa with remoulding to 0,1 kPa
                             = 5 kPa with hydroplaning




                                                                    21
   What happens during flow at
        low clay content?
• 1) disintegration of         y  k exp   C  
                                                    
  the mass: the yield
  stress drops
  dramatically            dependent on
                                                           Reference
                          the clay       solid fraction in solid
                          content        the slurry        fraction
• 2) settling and sand
  stratification within
  few seconds


                                                                 22
        Low clay content
Turbulence, disintegration, layering




                                       23
 Existing models adapted to low
clay debris flows: e.g.: NIS model

• Mud with plug and shear layers
   –   plasticity, viscosity, and visco-elasticity
          • dry friction (no cohesion in code)
          • dynamic shear (thinning)
          • dispersive pressure

                                                          r
                                           dv ( y ) 
              x  pe  pu   ( 1  2 ) x 
                                           dy 
                                                    
                                                  r
                                   dvx ( y ) 
              y  pe  pu   2 
                                   dy       
                                             
                                                      r
                                       dv ( y ) 
              xy  c  pe tan    m x 
                                       dy 
                                                
                                                              24
      Iverson- Dellinger model
• Depth integrated, three-dimensional model
• Accounts for the exchange of fluid between
  different parts of the slurry due to diffusion and
  advection.
                p '    p '    p '     2p '
                     u      v      
                t      x      z       y2
• Limitations for our purpose: water content of the
  slurry must not change, no cohesion, no
  turbulence

                                                       25
    In short: high clay debris flows
•   Viscoplastic behaviour
•   Vertically quasi-homogeneous
•   Hydroplaning/lubrication
•   Dynamical forces important
•   The material remains compact
•   Front detachment/outrunner block
•   Modeling: rheological flow,
    – Modified “BING”
• THEY ARE VERY MOBILE BECAUSE OF
  LUBRICATION

                                       26
     In short: low clay debris flows
•   Granular + turbulent behaviour
•   Settling and vertical layering (“Brazil Nut Effect” )
•   Lubrication only at the very beginning
•   The material breaks up catastrophically
•   Blocks do not form
•   Modeling: Fluid dynamics + granular
•   THEY ARE VERY MOBILE BECAUSE OF
    DRAMATIC DROP IN YIELD STRESS AND
    FLUIDISATION IN THE SAND LAYER
                                                        27
                  Conclusions
• Slurries with a high clay content:
   – transported over long distances preserving the initial
     composition
• Slurries with low clay content:
   – sandy materials may drop out during flow, alternatively
     being transformed into turbidity currents
• Flow behavior:
   – Strongly influenced by the amount of clay versus sand
     in the initial slurry


                                                              28
29
        Iverson-Dellinger model
• the Coulomb frictional force (diminished of the water
  pressure at the base of the debris flow),
• the fluid viscous shear stress,
• the earth-pressure force (namely, the lateral forces
  generated in the debris flow due to differences in the
  lateral pressure),
• the earth-pressure contribution of the bed pressure,
• a diffusive term of water escaping from the bottom,
• an earth-pressure term along the lateral (z) direction,
• the diffusive term of water along the lateral direction, and
  finally
• the pressure at the base of the debris flow.

                                                            30
                     Conclusions
• At high clay content:
   – a thin water layer intrudes underneath the front part = lubrication!
   – progressive detachment of the head
   – the thin water underneath the head is a supply for water at the
     base of the flow
   – a shear wetted basal layer with decreased yield strength is
     formed

• At low clay content:
   – water entrainment at the head of the mass flow
   – low slurry yield stress = particles settlement and continuous
     deposition
   – a wedge thickening depositional layer is developed some
     distance behind the head
   – viscous effects in the diluted flow, Coulomb frictional behavior
     within the dense flow. High pore pressures → near liquefaction.
                                                                       31
      Dispersive pressure

• When solid particles are present
• Particles forced apart
• Ability to move large particles
   – proportional to square of the particle size
     for given shear rate (Bagnold, 1954)
   – larger particles forced towards area of
     least shear                               32
33
   Velocity profile of debris flows
            Bingham fluid



                                                     u 
                                            y   
                                                     y 
                                                     
                                       shear         dynamic
                                       stress        viscosity
                                                yield            shear
Yield strength: constant during flow            strength         rate

                                                                         34
   Water film shear stress reduction in a
               Bingham fluid u=1
                                                                          Lid
                                                                          (Debris flow)
                                                           =1
                                                               Water, w, w, uw
                                                         =1-

                                                                 Mud
                                                                 m, m, um



                         1                   1

Shear layer
                                            
                                       1                    1
                                  u                     
              Velocity                                      +      Shear stress
                           1                   1            
                         1-                  1-
                                            

                                                
                               (R-)/ 1                   1
                                                     R(1+)/
                                    u                     +                              35
                                                             
36
Debris flows- high clay content
             A: 32.5 wt% clay, hydroplaning front
                Dilute turbidity current

             B: 25 wt% clay hydroplaning front
             D: Behind the head, increasing
                 concentration in overlying turbidity current




                                                         37
Debris flows- low clay content (5%)




Turbulent front         Deposition of sand




                                             38

								
To top