1 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

2 Debris flow How can we explain that km 3 of sediments can move100 - > 200 km on < 1 degree slopes at high velocities ( > 60 km/h) Basic problem!

3 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

4 Experimental settings St. Anthony Falls Laboratory 10 m turbidity current debris flow 6° slope Experimental Flume: “Fish Tank” Video (regular and high speed) and pore- and total pressure measurements

5 How to explain the various styles of run out! Subaerial Short and thick Subaqueous Thin and long

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7 High clay content – video record Turbidity current Hydroplaning debris flow

8 High speed video record (250 frames/sec) Flow behavior - High clay content (  30 % kaolinite)

9 Low clay content – video record Turbidity current Dense flow Deposition of sand

10 Debris flows- low clay content (5%) Turbulent front Deposition of sand

11 High clay content- - Plug flow- “Bingham” High sand content -Macro-viscous flow? -Divergent flow in the shear layer

12 Thickness of sandy deposits – versus clay content

13 Pressure interpretation Flow Pressure Time Pressure Time Pore pressure Total pressure Grains in constant contact with bed Rigid block over a fluid layer Total pressure Flow Pore pressure Pressure Time Fluidized flow

14 Pressure measurements at the base of a debris flow as pressure develops during the flow Low clay contentHigh clay content Total pressure Hydrostatic pressure

15 High clay content viscoplastic/hydroplaning/lubrication

16 Material from the base of the debris flow is eroded and incorporated into the lubricating layer. L1L1 L2L2 LsLs H1H1 H2H2 HsHs Downslope gravitational forces Bottom shear stresses

17 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 FMore sophisticated treatment is possible FCoupled nonlinear equations, use a numerical model FMain difficulty is quantitative treatment of crack formation and wetting and lubricating effects Detachment/stretching dynamics

18 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

19 Velocity profile of debris flows Bingham fluid Plug layer Shear layer Classical Bingham fluid: Yield strength: constant during flow Bingham fluid – with remolding (R-BING): The yield strength is allowed to vary during flow Plug layer

20 Water film/lubricating layer shear stress reduction in a Bingham fluid Water,  w,  w, u w Mud  m,  m, u m Lid (Debris flow)  =1  =1-  u=1 Shear layer Plug layer 1 + R  (1+ )/  1  1 +   u (R  -  )/    1 1 u 1  1-   VelocityShear stress

21 Simulation: final deposit of the large-scale Storegga

22 What happens during flow at low clay content? 1) disintegration of the mass: the yield stress drops dramatically 2) settling and sand stratification within few seconds solid fraction in the slurry dependent on the clay content Reference solid fraction

23 Low clay content Turbulence, disintegration, layering

24 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

25 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. Limitations for our purpose: water content of the slurry must not change, no cohesion, no turbulence

26 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

27 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

28 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

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30 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.

31 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.

32 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 (up and front) Further research required

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34 Velocity profile of debris flows Bingham fluid shear stress yield strength dynamic viscosity shear rate Plug layer Shear layer Yield strength: constant during flow

35 Water film shear stress reduction in a Bingham fluid Water,  w,  w, u w Mud  m,  m, u m Lid (Debris flow)  =1  =1-  u=1 Shear layer Plug layer 1 + R  (1+ )/  1  1 +   u (R  -  )/    1 1 u 1  1-   VelocityShear stress

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37 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

38 Debris flows- low clay content (5%) Turbulent front Deposition of sand