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Fluvial Geomorphologic Analysis CE154 Hydraulic Design Lecture 4

Fluvial Geomorphologic Analysis CE154 Hydraulic Design Lecture 4. Fluvial Geomorphology. Fluvial – water Ge – earth, land Morph - form Ology – knowledge Fluvial Geomorphology – the study of river forms and the processes that shape them. Why study geomorphology?. Natural rivers

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Fluvial Geomorphologic Analysis CE154 Hydraulic Design Lecture 4

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  1. Fluvial Geomorphologic Analysis CE154 Hydraulic DesignLecture 4 CE154

  2. Fluvial Geomorphology • Fluvial – water • Ge – earth, land • Morph - form • Ology – knowledge • Fluvial Geomorphology – the study of river forms and the processes that shape them CE154

  3. Why study geomorphology? • Natural rivers • Purposes- Flood protection- Navigation- Recreation- Habitat preservation • Yesterday’s practice – hardscape, straightening at our will, our way • Today’s practice – natural material, nature’s way CE154

  4. Objectives • Learn - Fundamental geomorphic concepts - how a river functions - Relevant geomorphic parameters – how to simulate nature’s way - Natural channel design CE154

  5. Important Fluvial Geomorphic Concept I 1. Flowand Sedimentcombine to shape a channel CE154

  6. Sediment Transport Mechanisms CE154

  7. Calabazas Creek at Pruneridge 1999 CE154

  8. Calabazas Creek at Pruneridge 2005 CE154

  9. Flow and Sediment shape a channel • Optimum channel cross-section and slope = most efficient in transporting flow and sediment of the watershed CE154

  10. Important Fluvial Geomorphic Concept II 2.Channel evolves toward an equilibrium condition- dynamic equilibrium process Example: b. No lasting disturbance c. Increased flow CE154

  11. Lane’s Equilibrium Scale CE154

  12. Dynamic Equilibrium • Channel geometry and slope remain relatively constant with time, given flow and sediment fluctuations • A long-term stable condition CE154

  13. Important Fluvial Geomorphic Concept III • 3. Changes take time – time scale CE154

  14. Comer Debris Dam CE154

  15. Time Scale • Geologic, Modern and Present time scales • Project focus – Present time scale – tens of years CE154

  16. Important Fluvial Geomorphic Concept IV 4. Geomorphic responses are not continuous – geomorphic threshold exists CE154

  17. Geomorphic thresholds • Shear stress threshold • Stability threshold - degradation • Planform threshold - channel avulsion – creation of a new channel CE154

  18. Summary – Geomorphic Concepts • Flow and sediment – equally important • Dynamic equilibrium • Time scale • Thresholds CE154

  19. Geomorphic Parameters • Planform- meander- channel length- valley length- sinuosity CE154

  20. Geomorphic Parameters • Braided stream- steep slope- high sediment load- erodable banks • Meandering stream • Straight stream CE154

  21. Geomorphic Parameters • Riffle • Pool • Point bar CE154

  22. Point bar CE154

  23. Geomorphic Parameters • Low flow channel • Bankfull Channel- bankfull width- bankfull depth- floodplain- terrace • Large flow channel CE154

  24. Typical bankfull channel CE154

  25. Incised channel CE154

  26. Entrenchment Ratio • An index of channel incision 2Dbf Dbf CE154

  27. Analysis of Geomorphic Properties CE154

  28. Analysis of Geomorphic Parameters • Approach to develop generalized empirical relationships (e.g., bankfull width vs. drainage area, bankfull dimension vs. flow, slope vs. drainage area) is not recommended for rivers in urbanized areas • Relationships should include Q, Qs, S, and ds plus effect of artificial hardpoints CE154

  29. Stable Channel Design Procedure Determine bankfull cross-sectional geometry Develop equilibrium longitudinal slope via. sediment transport modeling Identify hardpoints in project reach Layout cross-section and profile and, if necessary, locations of grade control structures Design surface protection, if necessary CE154

  30. 1. Design Cross-Sectional Geometry • Determine low flow channel geometryLow flow = base flow or min 7-day average flow • Determine bankfull channel geometryBankfull flow  see next slide • Design floodplain within real estate constraints CE154

  31. Bankfull Concepts • Bankfull channel is hypothesized to be the cross-section shaped by nature to most efficiently transport flow and sediment loads • Bankfull depth is determined where the width/depth ratio is a minimum, or where vegetation changes to perennial trees CE154

  32. Bankfull Channel Indicators • Change in bank slope • Tops of Point bars formed on bends • Change in vegetation - Change from shrubs/grass to woody trees • Change in particle size • Finer material on flood plain 5. Analytically – moving the most sediment – Effective Flow (see Slide #39) CE154

  33. Shear Stress = o CE154

  34. Shear stress – Initiation of motion o/(s-w)ds V*ds/ CE154

  35. Shear Stress • V* = shear velocity • Critical sediment diameter subject to motion CE154

  36. Why bankfull channel? CE154

  37. Why bankfull channel? CE154

  38. Effective Discharge • To determine the most efficient cross-section in carrying flow and sediment:- Effective flow calculation: compute sediment frequency curve using flow data from gauge station and sediment rating curve for the reach CE154

  39. Effective Discharge Effective discharge CE154 Guadalupe River near Almaden Expressway

  40. Effective Discharge – verified using HEC-RAS Cross section of Guadalupe River near Almaden Exp. CE154

  41. Results of effective discharge calculation for Guadalupe River near Almaden Expressway • Observed bankfull flow = 900 cfs • Calculated effective flow = 800 - 1000 cfs • Corresponding recurrence interval = 1.2 year CE154

  42. 1. Design Cross-Section Geometry • Calculation shows Bankfull flow = flow that carries most sediment = effective flow = channel-forming flow = dominant flow • Preserve bankfull channel characteristics through modification • Superimpose large flow area allowed by right of way CE154

  43. 2. Determine equilibrium slope • Lane’s qualitative relationship Qsd50 QS • Quantitatively, use momentum and energy balance Sediment in = sediment out CE154

  44. 2. Determine equilibrium slope • Determine sediment size distribution for project reach • Use SAM to calculate sediment transport capacity in project reach • Do the same for upstream and downstream reaches • Compare sediment capacity (±15%) and adjust slope as necessary • Extend bankfull flow to cover other return-period flows to estimate sediment yield and verify equilibrium (±30%) • Details described in Chapter 6 of Hydraulic Design Manual • Sediment transport workshop in February will cover application of SAM and HEC-6 CE154

  45. Transport Capacity Calculation Example of SAM sediment transport model results for Calabasas Creek CE154

  46. Transport Modeling + measured elevation in 2004 HEC-6 sediment transport model results for Calabasas Creek CE154

  47. 3. Identify hardpoints in project reach • Hardpoints – hard surface cover that prevents degradation and controls grade CE154

  48. 4. Layout x-section & profile • Determine need for grade control • Design grade control structures (chapter 8, hydraulic design manual) • Layout plan and profile (workshop in Apr 07) CE154

  49. 5. Design surface protection • Although equilibrium slope provides sediment balance, there is continuous sediment deposition and erosion taking place • Surface armor concept – armor will occur if Dcritical≤ D90-95 and the D90-95 materials will cover bed surface (pavement) • Surface cover should be compatible with flow regime CE154

  50. Armoring Example • Bankfull channel Velocity = 2.8 ft/secDepth = 2.5 ftSlope = 0.003Manning’s n = 0.03Hydraulic Radius = 1.875Bed Materials s = 165 lb/ft3 d50 = 6 mm d90 = 15 mm d95 = 25.4 mm • Will the bed armor? CE154

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