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Suspended Sediment, Bedload, and Substrate Composition. Stream Ecology 1/22/2009. Stream Load Classifications. Bed load : Sediments moving predominantly in contact with or close to the stream bed.
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Suspended Sediment, Bedload, and Substrate Composition Stream Ecology 1/22/2009
Stream Load Classifications • Bed load : Sediments moving predominantly in contact with or close to the stream bed. • Suspended load : Sediments that move primarily suspended in fluid flow but may interact with bed load and is comprised primarily of silts and clays and sometimes sand. Suspended load has a continual exchange between sediment in fluid flow and on the bed as it is constantly being entrained from the bed and suspended while heavier particles settle out from the flow to the bed. • Soluteload refers to the total amount of dissolved material (ions) carried in suspension and can only be quantified by laboratory analytical techniques. • Total load is the total amount of sediment in motion and is the sum of bed load + suspended load.
Relatively artificial categories. • The sediment load carrying capacity of a stream constantly changes both spatially and temporally as flow changes. • Flow in any river or stream is never homogenous so the resulting sediment movement varies greatly.
The majority of material transported by rivers is in suspension.
Erosion rates and average suspended sediment load of rivers depends, in lieu of human activities, on the following factors: • Age and morphology of the surfaces. • Composition of the bedrock material. • Tectonic uplifting. • Climate and vegetation cover .
Milliman and Meade (1983) estimated the annual suspended load of the world rivers to be about 15 x 109 tons. • The distribution of the sediment loads of different areas is very heterogeneous, and high sediment transport rates are concentrated in regions near the Equator and in Arctic regions.
For every cubic meter of water reaching the ocean, there is (on average), 0.5 kilogram of sediment carried away from the continents.
There is a relationship between climate and erosional rates and, therefore suspended sediment loads in streams. • Relatively modest shifts in average climate conditions have large impacts on the behavior of a streams flood response and thus sediment yield.
A few classic examples of anthropogenic activity known to increase sedimentation are: • Logging, deforestation, wildfire, vegetative change. Specific types of logging activity can increase sediment yield by 100X for short periods. Fire suppression and drought can combine to create catastrophic wildfire which can have devastating impacts on receiving waters from these areas. • Overgrazing by domestic animals. Sedimentation can increase not only due to decreased vegetative interception of precipitation enhancing erosion, but also through direct trampling of the streambed and channel.
Urbanization and road construction. Road construction commonly results in a 5 - 20 fold increase in suspended sediment yield. Impervious materials such as pavement, parking lots, rooftops, etc. will increase the velocity of storm water runoff which will increase erosion once this water comes in contact with soil. • Mining operations. Mines, particularly strip mines, can lead to extraordinarily high levels of erosion and subsequent sedimentation in rivers. Some coaling strip mines in Kentucky have shown 1000X increases in suspended sediment yields.
While rivers may be transporters of pollution, suspended sediment is the “package” many pollutants are carried in. • Heavy metals, organic pollutants, pathogens, and nutrients responsible for eutrophication can all be found attached to sediments in flowing water. • The “quality” or overall pollutant load of suspended sediment depends on the degree of pollution in the watershed.
The load carrying capacity of a stream or river constantly changes both spatially and temporally as flow changes. • Flow in any river or stream is never homogenous so the resulting sediment movement in any section of stream or river varies greatly.
Sediment Transport Kinetics • The quantification and predicted rates of transport of sediment are based upon the assumption that for any given flow and sediment, there is a unique transport rate. • Estimates of sediment transport rates are based upon measures of flow (including velocity, depth, shear velocity, viscosity, and fluid density) and both sediment size and density.
Particle stability depends upon individual particle weight and cohesiveness versus drag and lift forces imposed by flow. • Variables interfering with this simplistic relationship include fluctuations in current speed, variation in particle shape, density, and degree of exposure to flow. • The equilibrium that exists between the sheer stress promoting entrainment and forces resisting entrainment such as density and gravity is referred to as the Shields Parameter (Ø)
Variation in Øchanges with a particles Reynolds Number. • Reynolds number is the ratio of inertial forces (vsρ) to viscous forces (μ/L). • The Shields Parameter predicts when a particle with a given Reynolds Number (Re#) will be transported into either bed or suspended load. • The Shields Parameter can be visualized as: • θ = Tc / (ρsed - ρfluid ) Dsed) where Tc = critical sheer stress, ρsed and ρfluid are the densities of sediment and fluids respectively, and Dsed is the diameter of the particle.
Particles that are too heavy to be fully suspended may roll or slide along the bed (traction load) or hop as they rebound on impact with the bed. • In the latter case, ballistic trajectories occur and the particle is said to move by saltation. • Stream competence refers to the heaviest particles a stream can carry. • Stream competence depends on stream velocity; the faster the current, the heavier the particle that can be carried.
Stream Capacity refers to the maximum amount of total load (bed and suspended) a stream can carry. • It depends on both discharge and velocity since velocity affects the competence and therefore the range of particle sizes that can be transported. • Note that as stream volume and discharge increase so do competence and capacity. • This is not a linear relationship and doubling discharge and velocity does not automatically double competence and capacity.
Stream competence varies as approximately the sixth power of velocity. • For example, doubling velocity usually results in a 64X increase in competence. • Tripling the discharge usually results in a 9 – 27 times increase in capacity. • Most of the work of streams is accomplished during floods when stream velocity and discharge (and therefore competence and capacity) are many times their level compared to periods of quiescent flow. • This work is in the form of bed scouring (erosion), sediment transport (bed and suspended loads), and sediment deposition.
Total Suspended Solids and Turbidity • Total suspended solids are defined as all solids suspended in water that will not pass through a 2.0 μm glass-fiber filter (dissolved solids would be the fraction that does pass through the same size filter). • The filter is then dried in an oven between 103 and 105o C and weighed. • The increase in weight of the filter represents the amount of TSS.
Generally, water with less than 20 mg/L is considered relatively “clear” , levels between 40 and 80 mg/L tends to be “cloudy”, while levels over 150 mg/L appears to be “dirty” or “muddy”.
Related to, but not a proxy for, suspended sediments is turbidity. • Specifically, turbidity is the quantification of the light that is scattered or absorbed rather than transmitted through a water sample. • Turbidity is another measure of water clarity but is not a measure of dissolved substances that can add color to water. • Particulates are what add turbidity to water and can include such things as silt, clay, organic matter, algae and other microorganisms, or any other particulate matter that can scatter or absorb light. • The amount of light scattered or absorbed is proportional to the concentration of particulates in the sample.
Sediment Sampling • Suspended sediment concentration in a stream varies from the water surface to the streambed and laterally across the stream. • Concentration generally increases from a minimum at the water surface to a maximum at or near the streambed.
Substrate • The bed material of a stream determines roughness and subsequent hydraulics. • Of primary importance to aquatic life and provides refuge for many different species of aquatic macroinvertebrates and is one of the most important aspects of fish habitat.
Substrate Size-Class Composition • Relatively simplistic methods to determine the dominant types of bottom material. • The Modified Wentworth Classification is an easy field technique using marked (every 1 – 2 meters) lead-core ropes to determine dominant substrate.
Embeddedness • A field technique to determine the degree to which larger substrate is surrounded by fine material. • High levels of embeddedness is often associated with impairment and benthic anoxia.
Modified Wolman Pebble Count • Rapid, repeatable technique superior to visual assessments. • Often combined with embeddedness techniques for a good representation of how land-based activities affect stream substrate quality.
Streams and rivers, over geologic time scales, have shaped continents. • Anthropogenic causes have caused both excess and limitations of sediment delivered to streams.
