Sediment transport Part 1: initial motion GEOL/CE/EEB 8601 Intro to Stream Restoration
Why does it matter? 1.A common requirement in channel design is that the bed be stable under some specified discharge, i.e. the sediment will not move 2.Total transport of bed- material sediment plays a major though incompletely understood role in setting channel width
Why does it matter? 3.Changes in the transport capacity of the reach may cause erosion or deposition 4.In-stream organisms are often sensitive to bed texture, especially fines content of gravel bed streams
Steps in analyzing sediment mobility 1.Determination of bed sediment characteristics: grain size distribution and texture 2.Will it move? Apply the Shields criterion (Shields diagram) 3.Estimate bed-material transport rate if desired – note that existing formulas are highly imprecise/inaccurate 4.Consider the watershed, boundary conditions and natural history:
Watershed and history a)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)? b)Is there morphologic evidence (e.g. air photos) for changes in stream type related to sediment supply (e.g. braided vs meandering)? c)What is the long-term trend (depositing, degrading, bypass)? Why? d)Are there downstream changes (e.g. reduction in base level) that could lead to aggradation or degradation?
Step 1. Sediment characterization Gravel beds: usually bimodal –Gravel mode: Wolman count+gravelometer, image-based measurement –Distinguish surface vs subsurface
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 Note higher sand content – subsurface GSD is usually closer to the GSD of material in transport Frey & Church Science 2009
Step 1. Sediment characterization Sand beds: usually unimodal –sieve –automated size counter
Either way you end up with something like this: Unimodal sand
or this: Bimodal gravel-sand
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)
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
Settling velocity, w s Two regimes, distinguished by Reynolds number: Stokes (laminar, R ~100) General formula, Ferguson & Church 2004 C 1 = 18 C 2 = 0.4 1 R = s / f – 1 = kinematic viscosity
Settling velocity Rule of thumb, qtz density in water: for D < 100 m, w s in diam/s D in m for 100 < D < 1000, w s in diam/s 100 diam/s D > 1000, w s increases as D 1/2
R < 1 R > 10 4 C 2 1 C 2 0.4
2. Will it move? Shields initial motion From Buffington (1999) Shields stress:
2. Will it move? Shields initial motion
Initial motion: standard conditions stolen from Peter Wilcock, JHU Motion No motion
What not to use Less objectionable if this is interpreted as initial motion, but still better to use shear stress Hjulstrom diagram
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.
Within a mixture, all sizes tend to move together up to very large clasts D i / D 50 Parker; Wilcock; Proffitt & Sutherland mixture effects diminish for extremely large grain sizes
Modifying Shields for slope effects Streamwise slope Lateral slope
Transport of Biota Hondzo & Wang 02
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