Sediment transport Part 1: initial motion GEOL/CE/EEB 8601
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 bedmaterial 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
Frey & Church Science 2009 Note higher sand content subsurface GSD is usually closer to the GSD of material in transport 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, ws Two regimes, distinguished by Reynolds number: Stokes (laminar, R<~1) vs impact (turbulent,
R>~100) General formula, Ferguson & Church 2004 ws RgD 2 3 0.5 C1 (0.75C 2 RgD ) C1 = 18 C2 = 0.4 1 R = s/f 1
= kinematic viscosity 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 R<1 C2 1 C2 0.4
R > 104 ws RgD 3 0 .5 C1 (0.75C 2 RgD ) 2. Will it move? Shields initial motion From Buffington (1999) Shields stress:
2 u g ( s 1) D 0 u u D Re
2. Will it move? Shields initial motion 2. Will it move? Shields initial motion c 0.22 0.6 Re p 0.06 10
Re p ( 7.7 Re p0.6 ) (s-1)gD D Initial motion: standard conditions Motion No motion
stolen from Peter Wilcock, JHU What not to use Hjulstrom diagram Less objectionable if this is interpreted as initial motion, but still better to use shear stress 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 ci c50 x 10 1
mixture effects diminish for extremely large grain sizes 0.1 0.1 1 10 x Di / D50 Parker; Wilcock; Proffitt & Sutherland
Modifying Shields for slope effects Streamwise slope c tan cos 1 c co
streamwise slope S x tan Lateral slope 2 c co tan cos 1 2
c c 0.7 1/ 2 lateral slope S y tan Transport of Biota Hondzo & Wang 02
Initial motion -- summary Brownlie formula for Shields curve: c 0.22 Re p0.6 0.06 10 ( 7.7 Re p0.6 ) Re p RgD D
NB Parker et al. (2003) have suggested reducing this by a factor of 2 Correction for streamwise slope: c co tan cos 1
c streamwise slope S x tan c 0.7 Correction for side slope: tan 2 c co cos 1 2
c 1/ 2 lateral slope S y tan Correction for mixtures: ci c50 Di
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