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SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

PART 2 – SITES EARTH SPILLWAY EVALUATION B. Earth Spillway Integrity Analysis i. Three phase model of spillway performance ii. Phase 1 and phase 2 inputs and equations. SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008.

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SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

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  1. PART 2 – SITES EARTH SPILLWAY EVALUATION B. Earth Spillway Integrity Analysisi. Three phase model of spillway performance ii. Phase 1 and phase 2 inputs and equations SPECIALTY WORKSHOP: SITES TRAINING AND INTRODUCTION TO WINDAM ASDSO Dam Safety 2008

  2. Auxiliary Spillway Integrity Analysis • Applied to all reaches, including natural hillslope areas • Based on integral of the FBH • Determines whether spillway breach is expected • Uses a three phase failure model iteratively applied • Surface Erosion (vegetal cover failure) • Headcut Development • Headcut Advance • Generally a vegetal cover with minor discontinuities is assumed for design • Requires geologic information for all materials that could potentially be exposed to hydraulic attack during failure • CONCEPT IS THAT DAMAGE IS ACCEPTABLE SO LONG AS BREACH DOES NOT OCCUR

  3. UNIFORM SPILLWAY

  4. UNIFORM COVER

  5. COVER FAILURE INITIATION

  6. EXPANSION OF ERODING AREA

  7. COVER LOCALLY REMOVED Phase 1 is complete when the cover is locally removed such that the erodible boundary is no longer protected.

  8. CONCENTRATED FLOW EROSION

  9. CONCENTRATED FLOW EROSION Phase 2 is complete when an overfall is formed such that the erosion is the result of plunging action of the flow.

  10. HEADCUT Phase 3 includes simultaneous deepening and upstream advance of the headcut.

  11. EXPANDING HEADCUT

  12. ADVANCING HEADCUT

  13. BREACH

  14. SPILLWAY EROSION PHASES 1. SURFACE EROSION (Cover Destruction) 2. CONCENTRATED FLOW EROSION 3. HEADCUT ADVANCE

  15. kd 1 Erosion Rate, er 0 0 Effective Stress, te tc

  16. SURFACE DETACHMENT . r = kd (e - c)a . r = the rate of detachment kd = coefficient of detachment e = erosionally effective stress c = critical tractive stress a = exponent (~ 1)

  17. APPLICATION OF DETACHMENT RATE EQUATION WITH VEGETATION e=  ds(1-Cf) (ns/n)2 . r = kd (e - c) tc would be expected to be small for topsoil material supporting vegetation

  18. edt, (lb/ft2)-h Plasticity Index, Iw

  19. MINOR DISCONTINUITY Minor discontinuities cause erosion to begin where there is no vegetal cover and/or the surface is disturbed.

  20. MAJOR DISCONTINUITY Major discontinuities concentrate the flow where there is no vegetal cover and/or the surface is disturbed.

  21. EFFECTIVE STRESS • Maintenance code effect • e= dS(1-Cf) (ns/n)2 If MC=2, Cf0 If MC=3, Cf0 and n ns

  22. SHALLOW-ROOTED COVER

  23. SOD STRIPPING

  24. Potential Rooting Depth, ft Peak Stress, lb/ft2

  25. Parameters Impacting Phase 1 Erosion

  26. SPILLWAY EROSION PHASES 1. SURFACE EROSION (Cover Destruction) 2. CONCENTRATED FLOW EROSION (Headcut Formation) 3. HEADCUT ADVANCE

  27. PHASE 2 CONCENTRATED FLOW EROSION

  28. PHASE 2 Concentrated Flow Surface Detachment . r = kd (e - c) kd = coefficient of detachment (material property) c = critical tractive stress (material property) te = g(d+Dd)S Stress equation applies when eroded depth (Dd) is less than approach critical flow depth (phase 2).

  29. CRITICAL STRESS for Detachment Critical stress based on loose particle condition for all materials.

  30. DETACHMENT RATE COEFFICIENT The detachment rate coefficient may be externally determined and entered directly in SITES or estimated by SITES from soil properties.

  31. Parameters Impacting Phase 2 Erosion

  32. SPILLWAY EROSION PHASES 1. SURFACE EROSION (Cover Destruction) 2. CONCENTRATED FLOW EROSION 3. HEADCUT ADVANCE

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