1. 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. 2 Debris flow How can we explain that 10 - 1000 km 3 of sediments can move100 - > 200 km on < 1 degree slopes at high velocities ( -20 - > 60 km/h) Basic problem!

3. 3 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

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5. 5 Flow behavior Clay rich debris flows Hydroplaning front “Auto-acephalation” 32.5 wt% claycited from G. Parker

6. 6 Pressure measurements at the base of a clay rich debris flow as pressure develops during the flow

7. 7 Flow behavior: Debris flow at high mass fraction of clay

8. 8 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

9. 9 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

10. 10 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 effects Detachment/stretching dynamics

11. 11 Tension in the “neck”: Viscoplastic stretching of the neck is volume-conserving: Solution of the simplified stretching equations: Grossly simplified detachment dynamics:

12. 12 Simulation of the giant Storegga slide 400-500 km runout Clay-rich sediments –Visco-plastic materials: Model approach: –“Classical” BING –BING: Remolding of the sediment during flow –H-BING: Hydroplaning

13. 13 Velocity profile of debris flows Bingham fluid shear stress yield strength dynamic viscosity shear rate Plug layer Shear layer Yield strength: constant during flow

14. 14 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 +  1 1 1-  u (R  -  )/    1 1 u 1  1-   VelocityShear stress

15. 15 Simulation: final deposit of the large-scale Storegga

16. 16 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

17. 17 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

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19. 19 Subaqueous conditions - increased mobility Basic concept – based on experimental studies: –Hydroplaning –Lubricating –Stretching ( not yet implemented )

20. 20 Comparison between Storegga slides and selected cases

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22. 22 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?

23. 23 D1 D2

24. 24 Velocity profile of debris flows Bingham fluid – with remolding Plug layer Shear layer Yield strength at start: high; 10–20 kPa Yield strength at stop: low; < 1 kPa The yield stress is allowed to vary according to: initial yield stress residual yield stress total shear deformation dimensionless coefficient quantifying the remolding efficiency