Presentation on theme: "BASIC FLUVIAL SEDIMENT CONCEPTS Introduction to Sediment Sampling USGS Technical training in Support of Native American Relations (TESNAR) 2011 Klamath,"— Presentation transcript:
BASIC FLUVIAL SEDIMENT CONCEPTS Introduction to Sediment Sampling USGS Technical training in Support of Native American Relations (TESNAR) 2011 Klamath, Warm Springs, Yurok, and Karuk Tribes Chiloquin, OR September 19-23, 2011
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.
FLUVIAL-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
SEDIMENT ORIGINS 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 (throughout Klamath Basin). –Dams (up to 4 PacifiCorp dams in Klamath Basin). –Dredging (Columbia River). –Jetties (Outer Banks, NC). –Stream Corridor Restoration (Wood River near Fort Klamath)
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 Book 5 Chapter C1). –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).
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. 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 –silver10.5 –lead11.4 –mercury13.5 –gold19.3 –??? 22.6 PHYSICAL CHARACTERISTICS OF SEDIMENT
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: V s = particle settling velocity (m/s) (vertically downward if ρ p > ρ f, upwards if ρ p < ρ f ), g = gravitation acceleration (m/s 2 ), ρ p = mass density of the particles (kg/m 3 ), ρ f = mass density of the fluid (kg/m 3 ), μ = fluid's dynamic viscosity (kg/(s·m)), and R = radius of the spherical object (in m). STOKES LAW George Gabriel Stokes, derived 1851
Transformed to: T= (constant) * ( η) * (x) d 2 Where: T = fall time, in seconds η = absolute viscosity, in poises (g/cm. sec) d = fall diameter of particle, in millimeters constant = 0.1113, for spherical particle assumed to have density of 2.65 g.cm 3 and to settle in distilled water under gravitational force of 980 cm/s 2 Used in lab pipette withdrawals of sediment mixtures Fine sand in quart container falls 70 mm in 34 seconds, fine silt in 32 minutes, coarse clay in 2.1 hours, medium clay 8.5 hours STOKES LAW One way to use this equation
PHYSICAL CHARACTERISTICS OF SEDIMENT -- Density August 2006, photos from the yacht 'Maiken' in the South Pacific
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.
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. 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 without 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).
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 (Wood River?).
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. FLUVIAL SEDIMENT TERMINOLOGY
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
RELATIONS AMONG SUSPENDED- SEDIMENT CONCENTRATION AND: 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.
N Santiam Hysteresis: Precip- & Glacial-driven regressions
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). RELATIONS AMONG SUSPENDED- SEDIMENT CONCENTRATION AND:
C mean = ~930 mg/l BC=1.03 BC=~1 Box Coefficient (BC) = C mean /C point BC=~1.1 Culbertson et al., 1964
C mean = ~1,360 mg/l BC=~1.7 Box Coefficient (BC) = C mean /C point BC=~5 BC=~4 BC=~1.5 Mean Values Culbertson et al., 1964
Sampled & Unsampled Zones with an Isokinetic Sampler
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. RELATIONS AMONG SUSPENDED- SEDIMENT CONCENTRATION AND:
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. –Clear as mud! METHODS OF QUANTIFYING FLUVIAL SEDIMENT DISCHARGE