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Sediment transport Part 1: initial motion

Sediment transport Part 1: initial motion. GEOL/CE/EEB 8601 Intro to Stream Restoration. Why does it matter?. A common requirement in channel design is that the bed be stable under some specified discharge, i.e. the sediment will not move

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Sediment transport Part 1: initial motion

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  1. Sediment transportPart 1: initial motion GEOL/CE/EEB 8601 Intro to Stream Restoration

  2. Why does it matter? • A common requirement in channel design is that the bed be stable under some specified discharge, i.e. the sediment will not move • Total transport of bed-material sediment plays a major though incompletely understood role in setting channel width

  3. Why does it matter? • Changes in the transport capacity of the reach may cause erosion or deposition • In-stream organisms are often sensitive to bed texture, especially fines content of gravel bed streams

  4. Steps in analyzing sediment mobility • Determination of bed sediment characteristics: grain size distribution and texture • Will it move? Apply the Shields criterion (Shields diagram) • Estimate bed-material transport rate if desired – note that existing formulas are highly imprecise/inaccurate • Consider the watershed, boundary conditions and natural history:

  5. Watershed and history • What is being supplied from upstream? Does it/will it/could it include material not represented in the bed (e.g. fines from upland land management)? • Is there morphologic evidence (e.g. air photos) for changes in stream type related to sediment supply (e.g. braided vs meandering)? • What is the long-term trend (depositing, degrading, bypass)? Why? • Are there downstream changes (e.g. reduction in base level) that could lead to aggradation or degradation?

  6. Step 1. Sediment characterization • Gravel beds: usually bimodal • Gravel mode: Wolman count+gravelometer, image-based measurement • Distinguish surface vs subsurface

  7. Step 1. Sediment characterization • Gravel beds: usually bimodal • Greater intrinsic mobility of sand often leads to higher gravel fraction in surface layer: “armor” or “pavement” • You can measure GSD of either depending on your purpose. Usually do surface Frey & Church Science 2009 Note higher sand content – subsurface GSD is usually closer to the GSD of material in transport

  8. Step 1. Sediment characterization • Sand beds: usually unimodal • sieve • automated size counter

  9. Either way you end up with something like this: Unimodal sand

  10. or this: Bimodal gravel-sand

  11. Summary: grain-size distributions • Logarithmic size scales: ln2 [], -ln2 [], or log10 • Standard form: percentages in size range; cumulative • Common percentiles: 90, 84, 65, 50, 16 • Unimodel or bimodal (e.g. gravel-sand) • No standard form at present for single modes (e.g. log-normal)

  12. Summary: size and mineralogy • Gravel, cobble, etc: > 2 mm; all common rock lithologies • Sand: 62 m – 2 mm; quartz, feldspar, other • Silt: 4 m – 62 m; quartz, feldspar, other • Clay: < 4 m; clay minerals • Cohesive effects important for D <~ 10 m and/or clay minerals and/or biological effects

  13. Settling velocity, ws • Two regimes, distinguished by Reynolds number: Stokes (laminar, R<~1) vs impact (turbulent, R>~100) • General formula, Ferguson & Church 2004 C1 = 18 C2 = 0.4 1 R = s/f – 1  = kinematic viscosity

  14. Settling velocity • Rule of thumb, qtz density in water: for D < 100m, ws in diam/s  D in m for 100 < D < 1000, ws in diam/s  100 diam/s D > 1000, ws increases as D1/2

  15. R < 1 C2 1 C2 0.4 R > 104

  16. 2. Will it move? Shields initial motion From Buffington (1999) Shields stress:

  17. 2. Will it move? Shields initial motion

  18. 2. Will it move? Shields initial motion

  19. Initial motion: standard conditions Motion No motion stolen from Peter Wilcock, JHU

  20. What not to use Less objectionable if this is interpreted as initial motion, but still better to use shear stress Hjulstrom diagram

  21. What to do about size mixtures? When grain sizes are clearly segregated into patches like this, you have to apply Shields to each patch separately.

  22. Within a mixture, all sizes tend to move together up to very large clasts mixture effects diminish for extremely large grain sizes Di / D50 Parker; Wilcock; Proffitt & Sutherland

  23. Modifying Shields for slope effects Streamwise slope Lateral slope

  24. Transport ofBiota Hondzo & Wang 02

  25. Initial motion -- summary • Brownlie formula for Shields curve: • Correction for streamwise slope: • Correction for side slope: • Correction for mixtures: NB Parker et al. (2003) have suggested reducing this by a factor of 2

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