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Experimental study of failures in weakly cohesive clinoform foresets

Experimental study of failures in weakly cohesive clinoform foresets. Antoinette Abeyta Department of Geology and Geophysics University of Minnesota. How are sedimentation rates partitioned into clinoform growth?. Classical view – clinoform built by a succession of failure deposits

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Experimental study of failures in weakly cohesive clinoform foresets

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  1. Experimental study of failures in weakly cohesive clinoformforesets Antoinette Abeyta Department of Geology and Geophysics University of Minnesota

  2. How are sedimentation rates partitioned into clinoform growth? • Classical view – clinoform built by a succession of failure deposits • Sediment gravity flows • Self-similar, regular Progradation • Would higher feed rates yield larger or more frequent failure events? • How does cohesion change this system?

  3. What are failure events? • Major sediment transport mechanism • Dense fluid flows • Down slope gradient • Can be viewed as submarine avalanches Photo courtesy of UC Santa Cruz

  4. Significance • Proposed mechanism for clinoformprogradation • Major sediment transport mechanism in the deep ocean • Potential natural hazard • Generate deadly tsunamis • Destruction of coastal and marine infrastructure • Economically viable deposits Photo courtesy of Canadian Natural Resources

  5. Previous studies and limitations • Experimental methods • Slurry injected into standing body of water • Flows artificially generated • Properties of flow predetermined Photo courtesy of David Mohrig

  6. Proposed methods • Allow flows to self-organize • Observe the initiation mechanisms of mass failure • Understand controls on flow size and rheological properties • Determine what influences the frequency of events • Develop better hazard assessments and prediction patterns

  7. Experimental method • Feed sediment and water into a delta • Use mixture of a light and cohesive sediment • Test range of sediment and water discharges

  8. Flows and deposits generated • Typically debris flows • Laminar and viscous flow • Move as a plug, down slope • Coarsening upward deposits • Dewatering structures present

  9. Flows generated

  10. Pre-failure morphological changes • Development of convex structure at knick point

  11. Pre-failure morphological changes • Exaggeration of convex structure at knick point, swelling

  12. Pre-failure morphological changes • Knick point cannot sustain the weight and fails • Overlying material no longer supported, domino effect

  13. Pre-failure morphological changes

  14. Post-failure morphological changes • Steep concave morphology left in slope • Sediment is deposited down slope at a low angle

  15. Post-failure morphological changes • Steep concave morphology left in slope • Small grain flows start to deposit • Grain flows stack

  16. Post-failure morphological changes • Grain flow stacking allows for the development of convex morphology

  17. Role of discharge rates • What influence does sediment and water discharge influence the occurrence of mass failures? • Tested range of water and sediment discharges • Sediment discharge from 0.6 – 1.3 g/s • Water discharge from 7.1 to 36.7 cm3/s

  18. Role of progradation rates • Analyze the image before and after failure • Map non-mobilized sediment to get size of failure • Failure size normalized to water depth at the clinoform toe

  19. Failure size

  20. Failure size

  21. Failure size

  22. Failure frequency

  23. Failure frequency

  24. Summary of discharge rate experiments • Failure size is constant for all discharge rates • Cannot predict frequency of events based on discharge rates • Conflict with classical view of clinoformprogradation

  25. Classical view of clinoformprogradation • Clinoform built by a succession of failure deposits • Self-similar, regular • Slope constant – angle of repose Progradation • If failure rates and frequencies do not correspond to discharge, creates issue of mass balance

  26. Current system of clinoformprogradation • Clinoform are not built by a succession of failure deposits • Not self-similar, irregular • Slope varies with time • Clinoforms are not built on a succession of failure deposits

  27. Classical measure of progradation rate • Movement of the shoreline with time • Classical view of progradation under constant forcings should yield a straight line Position Shoreline Time

  28. Shoreline position with time • Irregular, variable • Sensitive to mass failures

  29. Shoreline position with time

  30. New proposed method for measuring progradation • Center of mass • Insensitive to internal forcings • Can be implemented in the field

  31. Conclusions • Series of morphological changes that lead to failure • Failure size and frequency independent of progradation rates • Scale invariant process • Clinoforms do not prograde by a series of failure deposits • Shoreline position is not a reliable measurement of progradation

  32. Future work • Conduct experiments at a larger scale in the SAFL Fish tank • More depth may allow for more complex flows to organize • Measure pore fluid pressure changes on the failure front • Determine the relationship to mass failure

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