High mobility of subaqueous debris flows

1. High mobility of subaqueous debris flows 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. Flow behavior -reduced clay content • Massive deep sea sand- the problematic child • Deep water sands (> 1 -30 m tick) units devoid of primary sedimentary structures • Often associated with water escape features • Poor to moderate sorting • Two processes for transport and emplacement: • debris flows ? • turbidity currents ? • “low-density” • “high-density”

4. Overview Map, Norwegian margin geometry

5. Ormen Lange Field development in the middle of a gigantic slide area Location of the Ormen Lange gas field in the middle of the Storegga slide area. Possible pipeline routes across the continental slope are marked red. Subsea production Ormen Lange gas field Seismic profile 250 m Slope stability ? 3-D Bathymetric map from Norsk Hydro Seismic profile across the Storegga slide area shows slide escarpment and debris to be considered during development of the Ormen Lange gas field. In addition to verification of slope stability, historic slide debris makes the seabed topography a challenge for future installation of subsea structures in the reservoir area. Sub-zero water temperature at the seabed is another challenge for subsea production Slide 2 Area for exploration 5 km Slide 3 Slope stability ? 1150 m Seismic profile from Norsk Hydro

6. 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

7. Sub aqueous slides: • thinner and longer • runout than their • subaerial • counterparts! • Despite: • reduced gravitational • force • increased viscous drag

9. Flow behavior II A – Front 25 wt% clay B – Behind the head C – Front 32.5 wt% clay D – Front 20 wt% clay E – Behind the head F – Main body G – Front 5 wt% clay H – Main body (layered)

10. Flow behaviorExperimental studiesVideo records High clay content ( 30 % kaolinite) Front Subaerial Fixedposition High speed (250 frames/sec) High yield strength

11. Frontal behavior – “out runner blocks”Video records Video from above “ detachment” Sub aqueous record (oblique) Sub aqueous record (front)

12. Flow behaviourVideo records 15% kaolinite 5 % kaolinite 33% kaolinite Front Increasing clay content Fixed camera High speed Increasing sand content High speed

13. Frontal behavior – “out runner blocks” Depositional pattern

14. Frontal behavior – “out runner blocks” Depositional pattern Finneidfjord

15. Particle tracking – high speed video cameras- velocity profiles

16. Velocity profiles • High clay content- • - Plug flow ? • High sand content • Macro-viscous flow? • Divergent flow in the • shear layer

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

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

20. Pressure measurements at the base of a sand rich mass flow as pressure develops during the flow

21. Cross section of subsequent debris flows • Distance from outlet (in m) 1.8 5.0 7.7 • Subaerial Subaqueous

22. Thickness of sandy deposits – versus clay content

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

24. 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

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

26. Flow behavior:Debris flow at intermediate mass fraction of clay

27. 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

28. 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

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

30. 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

31. Use of statistics from ancient slides • Plot n. 1.: Log(V/W) ------ Log(Runout) • Plot n. 2.; Log(V/W) ------ Log(H/R) (Runout ratio) Energy (or force) arguments suggest that for a granular material: → H/R= tan (internal friction angle) Initial deposit Drop height (H) Final deposit Log (H/R) H slide Log R Runout (R)

33. Comparison between Storegga slides and selected cases

34. Dependence of the run­out ratio H/R on the release volume.

35. Model calibration and sensitivity analyses • Various debris flow models is tested using data from two medium-sized slides

36. Models of a Bingham (viscoplastic) fluid • ”Classical” BING • Variant 1: B-BING: Bing with blocks • Variant 2: C-BING: Yield stress increasing with depth • Variant 3: R-BING: Remolding of the sediment during the flow • Variant 4: H-BING: Hydroplaning

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

38. 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

39. 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+)/

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

41. Velocity of Storegga phase 1 debris flow

42. 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

43. 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?

44. 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 • Stretching and thinning of material • 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.