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High mobility of subaqueous debris flows and the lubricating layer model

High mobility of subaqueous debris flows and the lubricating layer model. Anders Elverhøi Fabio De Blasio Trygve Ilstad Dieter Issler Carl B. Harbitz International Centre for Geohazards Norwegian Geotechnical Institute, Norway Dep. of Geosciences, University of Oslo, Norway.

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High mobility of subaqueous debris flows and the lubricating layer model

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  1. High mobility of subaqueous debris flows and the lubricating layer model Anders Elverhøi Fabio De Blasio Trygve Ilstad Dieter Issler Carl B. Harbitz International Centre for Geohazards Norwegian Geotechnical Institute, Norway Dep. of Geosciences, University of Oslo, Norway. .

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

  3. Experimental settingsSt. 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. Flow behavior Clay rich debris flows Hydroplaning front “Auto-acephalation” 32.5 wt% clay cited from G. Parker

  5. Pressure measurements at the base of a clay rich debris flow as pressure develops during the flow

  6. Flow behavior:Debris flow at high mass fraction of clay

  7. 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

  8. Grossly simplified detachment/stretching dynamics • Tensile force in the neck • Viscoplastic stretching of the neck is volume-conserving • The growth rate of the length is the product of the stretching rate with the neck length • Solution of the simplified stretching equations: • The neck stretches and thins at a rate that increases with time, until the height becomes zero after a finite time • detachment occurs

  9. 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 effects

  10. Grossly simplified detachment dynamics: Tension in the “neck”: Viscoplastic stretching of the neck is volume-conserving:Solution of the simplified stretching equations:

  11. Simulation of the giant Storegga slide400-500 km runout • Clay-rich sediments • Visco-plastic materials: • Model approach: • “Classical” BING • BING: Remolding of the sediment during flow • H-BING: Hydroplaning

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

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

  14. Simulation: final deposit of the large-scale Storegga

  15. Conclusions • Experiments • water enhances the mobility of debris flows via the formation of a lubricating layer/stretching • The giant Storegga slide • BING • reproduced with extremely low yield stresses, 200-300 Pa • R-BING • starting from yield stresses between 6 and 10kPa, residual stress of 200 Pa • Hydroplaning • extreme runout distances, even with stiff sediments • independence of sediment rheology

  16. Future directions (II) • Modification of the existing models • Incorporation of water in the slurry • Detachment mechanism of a hydroplaning head • Parameterizations of the rheological properties as a function of water content

  17. Subaqueous conditions -increased mobility Basic concept – based on experimental studies: • Hydroplaning • Lubricating • Stretching (not yet implemented)

  18. Comparison between Storegga slides and selected cases

  19. Future direction (I) • Modification of the existing models • Incorporation of water in the slurry • Detachment mechanism of a hydroplaning head • Parameterizations of the rheological properties as a function of water content (and stretching?) • Important question: How is the basal “water” layer distributed?

  20. D2 D1

  21. Velocity profile of debris flows Bingham fluid – with remolding The yield stress is allowed to vary according to: Plug layer initial yield stress residual yield stress total shear deformation dimensionless coefficient quantifying the remolding efficiency Shear layer Yield strength at start: high; 10–20 kPa Yield strength at stop: low; < 1 kPa

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