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BASIC FLUVIAL SEDIMENT CONCEPTS

Understand the principles of sediment collection, transport mechanics, and physical properties of fluvial sediment. Explore the various origins of sediment and the impact of human activities. Learn terminology and techniques for sediment data collection.

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BASIC FLUVIAL SEDIMENT CONCEPTS

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  1. BASIC FLUVIAL SEDIMENT CONCEPTS Sediment Data-Collection Techniques USGS National Training Center Course SW1091 Toutle, Castle Rock, and Vancouver, WA March 24, 2014 John R. Gray (jrgray@usgs.gov) National Sediment Specialist (retired) USGS Office of Surface Water

  2. OVERVIEW • Ya dip your sampler, ya get a sample. Who cares about the details? • Well…if you don’t understand mechanics of sediment transport, it’s nigh impossible to develop a credible network-design monitoring program or collect reliable sediment data over a range of flows. • So Listen Up!

  3. WHAT IS SEDIMENT? • Sediment consists of particles derived from rocks or biological materials. • When transported by, suspended in, or deposited by flowing water, referred to as fluvial sediment. • Be careful using the term, “Sedimentation” which can be ambiguous

  4. SEDIMENT ORIGINS:The Geologic Cycle • Weathering • Mechanical (freeze/thaw, abrasion, other) • Chemical (carbonic acid, biotic interactions, other) • Erosion • raindrop impact • sheet, gully, bank/channel erosion • mass wasting, glaciation, volcanic eruption, eolian • Transport (a number of mechanisms) • Deposition • streambeds, floodplains • Digenesis • compaction, cementation, mineral replacement

  5. FLUVIAL-SEDIMENT ORIGINS Natural: • Channels (bed-material load) • Uplands (washload and bed-material load) Some Alterations by Human Activities: • Agriculture seasonally changes ground cover, exposes soil. • Tree harvests indirectly increase erosion rates (PNW). • Construction (all types). • Riparian vegetation removal. • Channel straightening (Kankakee River example). • Dams (about 90,000 dams in the COE National Inventory of Dams). • Dredging. • Jetties (Outer Banks, NC). • Stream Corridor Restoration

  6. PHYSICAL CHARACTERISTICS OF SEDIMENT • Size • lengths of long, intermediate, short axes. • “fall diameter” calculated as that equivalent to a quartz sphere that falls at same rate in still water • Wentworth size classes (USGS TWRI 5C1; Folk, p. 23). • we often classify as 3 natural populations: pebbles & larger, sand/silt, clay. • coarsest material normally related to high slope (ex., mountains); finest correlated to low slopes (ex., coastal plains).

  7. 2 mm Sands 0.062 mm Silts 0.002 mm Clays

  8. PHYSICAL CHARACTERISTICS OF SEDIMENT • Shape. • prismatic or angular ("new"). • elliptical or spheroidal (after abrasion). • flat or bladed (nature of particle -- e.g., mica, halite). • Mineral. • quartz, felspars, gold, etc.

  9. Cape Cod Quart Sand Plain Light Vertical scale 0.38 mm

  10. Cape Cod Quart Sand Cross-Polarization Vertical scale 0.38 mm

  11. Cape Cod Quart Sand Epiflouresence Vertical scale 0.38 mm

  12. PHYSICAL CHARACTERISTICS OF SEDIMENT • Density (mass per unit volume). • dry wood <1 • water 1.0 (pure, 4° C) • coal 0.9-1.4 • quartz & feldspar ~2.65 (prevalent minerals in nature) • iron 7.9 • silver 10.5 • lead 11.4 • mercury 13.5 • gold 19.3 • ??? 22.6

  13. STOKES LAWGeorge Gabriel Stokes, derived 1851 • For our purposes: If particles fall in a viscous fluid by their own weight due to gravity, a terminal velocity – also known as the settling velocity – is reached when this frictional force combined with the buoyant force exactly balance the gravitational force. where: • Vs = particle settling velocity (m/s) (vertically downward if ρp > ρf, upwards if ρp < ρf ), • g = gravitation acceleration (m/s2), • ρp = mass density of the particles (kg/m3), • ρf = mass density of the fluid (kg/m3), • μ = fluid's dynamic viscosity (kg/(s·m)), and • R = radius of the spherical object (in m). George

  14. PHYSICAL CHARACTERISTICS OF SEDIMENT -- Density August 2006, photos from the yacht 'Maiken' in the South Pacific

  15. FLUVIAL SEDIMENT TERMINOLOGY From ASCE Manual 110, figure 5-1, p. 308

  16. FLUVIAL SEDIMENT TERMINOLOGY • SEDIMENT CONCENTRATION: (actually suspended-sediment concentration) ratio of mass of dry sediment in a water-sediment mixture to the mass of the entire mixture. Expressed as parts-per-million, which can be converted to mass-per-volume units, such as milligrams per liter.

  17. FLUVIAL SEDIMENT TERMINOLOGY • SEDIMENT DISCHARGE: The mass, volume, or weight of sediment passing a stream cross-section in a unit time. It can be divided into categories: • Defined operationally, and as mode of transport: Suspended- sediment load (discharge), and Bedload (discharge). • Defined by origin: Washload, and Bed-material load. Sediment discharge is not the same as sediment load. The latter by itself requires additional information to be meaningful.

  18. FLUVIAL SEDIMENT TERMINOLOGY • SUSPENDED-SEDIMENT LOAD: That part of the sediment load which is in suspension. • WASH LOAD: Finer material (<<<0.062 mm) that tends to flow into and out of a reach sans bed interactions • BEDLOAD: Material moving on or near the stream bed by rolling, sliding, and skipping. In strict sense, material finer that about 0.2 mm in diameter is rarely a part of bedload (but can be part of near-bed suspended load).

  19. FLUVIAL SEDIMENT TERMINOLOGY • BED MATERIAL: Sediment composing the streambed. • BED-MATERIAL LOAD: That material in transport that is characteristic of the bed material; excludes wash load. • TOTAL LOAD: Equal to suspended-sediment discharge plus bedload discharge (with caveat); includes wash load. In general, the bulk of sediments transported in the world's streams occurs in the suspended phase; however, this is not a true for all streams.

  20. FLUVIAL SEDIMENT TERMINOLOGY REPRESENTATIVE SAMPLE: • A characteristic of a sample that is proportional to its occurrence in the local environment at the time of collection. • A representative suspended-sediment sample will have a sediment concentration & size distribution “equal” (similar) to that averaged over the stream cross-section when the sample was collected.

  21. SOME FACTORS IN SEDIMENT TRANSPORT • Drainage area, mean slope, sinuosity, braided • Supply or transport limitations • Geology, climate, land use • Stage & discharge range, stream power • Sediment sources & flow history • Bank and bed characteristics • moveable or cohesive bed (ripples, dunes, plane bed, antidunes) • stable versus sloughing banks • riparian and floodplain vegetation • flow resistance

  22. General Relation between Annualized Precipitation and Sediment Yield (Waite Osterkamp, USGS, modified from Langbein and Schumm)

  23. Relation of Specific Energy To Flow Depth Subcritical Region Depth Supercritical Region Specific Energy, H

  24. SECONDARY MOTION http://nptel.ac.in/courses/IIT-MADRAS/Hydraulics/pdfs/Unit5/5_1.pdf

  25. SECONDARY MOTION

  26. RELATIONS AMONG SUSPENDED-SEDIMENT CONCENTRATIONAND: • Discharge: Although suspended-sediment concentration generally increases with water discharge, the relation is not simple. • Sediment concentration peak might match, precede, or follow water-discharge peak. • Physical basis for bedload peak rate lagging hydrograph. • System might be supply-limited, or in disequilibrium. • Seasonal effects. • Ice cover. • other.

  27. Hysteresis

  28. Less Hysteresis

  29. RELATIONS AMONG SUSPENDED-SEDIMENT CONCENTRATIONAND: • Depth: Turbulence maintains sediments in suspension. Fine sediments tend to be uniformly distributed in the water column; the concentration of coarser sediments may increase with depth (Stoke's Law).

  30. Silt-Clay Distribution in X-Section BC=~1.1 BC=~1.1 Mean Values BC=~1 BC=1.03 BC is “Box Coefficient” X-Section SSC/Point SSC From Culbertson et al., 1964

  31. Clay Distribution in X-Section BC=~4 BC=~5 Mean Values BC=~1.7 BC=~1.5 BC is “Box Coefficient” X-Section SSC/Point SSC From Culbertson et al., 1964

  32. Sampled & Unsampled Zoneswith an Isokinetic Sampler

  33. Effect of Relative Sampling Rate on Errors in Sediment Concentrations + Bias % Error In Sed. Conc. Sub-efficient Super-efficient 0 - Bias 1 Relative Sampling Rate: Intake Velocity Divided by Stream Velocity

  34. RELATIONS AMONG SUSPENDED-SEDIMENT CONCENTRATIONAND: • Width: Sediments may be non-uniformly distributed in the cross-section; this may be due to incomplete mixing (inadequate time to mix, weak turbulence, density gradient, weak secondary motion, etc.); or to difference in source and flow characteristics in the cross-section. • Season, flow history, man's works, etc.

  35. CHARACTERISTICS OF SEDIMENTS IN TRANSPORT • Suspended Sediment: Kept in suspension by turbulence. • Temporal variability small on small time scales (seconds to minutes), increases with time scale. • Spatial variability usually slight-to-moderate, but can be large. • Bedload: • Temporal variability large on scale above minutes. • Spatial variability large.

  36. METHODS OF QUANTIFYING FLUVIAL SEDIMENT DISCHARGE • Indirect: Equations based on incipient motion orenergy for suspended-sediment discharge, bedload, total load. • Many total load equations; Modified Einstein Procedure (MODEIN, Colby, Hembree, Schroeder) widely used for sand-bed channels (http://water.usgs.gov/software/modein.html) • SEDDISCH Program (Stevens and Yang) provides 13 sub-programs to calculate sediment transport in various modes and for various stream types (http://water.usgs.gov/software/seddisch.html)

  37. METHODS OF QUANTIFYING FLUVIAL SEDIMENT DISCHARGE • MODEIN and SEDDISCH require: • discharge measurements. • depth-integrated suspended-sediment samples analyzed for concentration and size distribution. • Bed-material size distribution. • water temperature. • Many other suspended-sediment, bedload, total load equations. • Rating curves (quasi-direct). (Modified Einstein: http://water.usgs.gov/software/modein.html ) (Sed Discharge EQ: http://water.usgs.gov/software/seddisch.html )

  38. METHODS OF QUANTIFYING FLUVIAL SEDIMENT DISCHARGE • Direct, total: • Total-flow collection (buckets, barges, other means). • Bedload, sediment trap (see Emmett, East Fork, WY).

  39. METHODS OF QUANTIFYING FLUVIAL SEDIMENT DISCHARGE • Direct, sampling: • Suspended sediment: Collect representative suspended-sediment sample, measure discharge, calculate instantaneous suspended-sediment discharge. • Bedload: Collect sufficient bedload samples using a sampler such as the FISP-approved BL-84 sampler over the stream cross-section, calculate instantaneous bedload discharge. • Welcome to This Course!

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