Suspended Sediment, Bedload, and Substrate Composition Stream
Ecology 1/22/2009
Slide 2
Stream Load Classifications Bed load : Sediments moving
predominantly in contact with or close to the stream bed. 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. 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. Solute load refers to the total amount of dissolved
material (ions) carried in suspension and can only be quantified by
laboratory analytical techniques. Solute load 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. Total load is the total amount of sediment
in motion and is the sum of bed load + suspended load.
Slide 3
Relatively artificial categories. Relatively artificial
categories. The sediment load carrying capacity of a stream
constantly changes both spatially and temporally as flow changes.
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. Flow in any river or stream is never homogenous so
the resulting sediment movement varies greatly.
Slide 4
The majority of material transported by rivers is in
suspension. The majority of material transported by rivers is in
suspension.
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Slide 9
Erosion rates and average suspended sediment load of rivers
depends, in lieu of human activities, on the following factors:
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. Age and morphology of the surfaces.
Composition of the bedrock material. Composition of the bedrock
material. Tectonic uplifting. Tectonic uplifting. Climate and
vegetation cover. Climate and vegetation cover.
Slide 10
Milliman and Meade (1983) estimated the annual suspended load
of the world rivers to be about 15 x 10 9 tons. Milliman and Meade
(1983) estimated the annual suspended load of the world rivers to
be about 15 x 10 9 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. 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.
Slide 11
For every cubic meter of water reaching the ocean, there is (on
average), 0.5 kilogram of sediment carried away from the
continents. For every cubic meter of water reaching the ocean,
there is (on average), 0.5 kilogram of sediment carried away from
the continents.
Slide 12
There is a relationship between climate and erosional rates
and, therefore suspended sediment loads in streams. 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. Relatively modest
shifts in average climate conditions have large impacts on the
behavior of a streams flood response and thus sediment yield.
Slide 13
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. 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. 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.
Slide 14
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.
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. 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.
Slide 15
While rivers may be transporters of pollution, suspended
sediment is the package many pollutants are carried in. 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. 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
quality or overall pollutant load of suspended sediment depends on
the degree of pollution in the watershed.
Slide 16
The load carrying capacity of a stream or river constantly
changes both spatially and temporally as flow changes. 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. Flow in any river or
stream is never homogenous so the resulting sediment movement in
any section of stream or river varies greatly.
Slide 17
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.
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. 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.
Slide 18
Particle stability depends upon individual particle weight and
cohesiveness versus drag and lift forces imposed by flow. 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. 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 () 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
()
Slide 19
Variation in changes with a particles Reynolds Number.
Variation in changes with a particles Reynolds Number. Reynolds
number is the ratio of inertial forces (vs) to viscous forces (/L).
Reynolds number is the ratio of inertial forces (vs) to viscous
forces (/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 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: The
Shields Parameter can be visualized as: = Tc / (sed - fluid ) Dsed)
where Tc = critical sheer stress, sed and fluid are the densities
of sediment and fluids respectively, and Dsed is the diameter of
the particle. = Tc / (sed - fluid ) Dsed) where Tc = critical sheer
stress, sed and fluid are the densities of sediment and fluids
respectively, and Dsed is the diameter of the particle.
Slide 20
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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. 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.
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 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 competence depends on stream
velocity; the faster the current, the heavier the particle that can
be carried.
Slide 22
Stream Capacity refers to the maximum amount of total load (bed
and suspended) a stream can carry. 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. 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. 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.
This is not a linear relationship and doubling discharge and
velocity does not automatically double competence and
capacity.
Slide 23
Stream competence varies as approximately the sixth power of
velocity. Stream competence varies as approximately the sixth power
of velocity. For example, doubling velocity usually results in a
64X increase in competence. 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. 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. 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. This work is in the form of bed scouring (erosion),
sediment transport (bed and suspended loads), and sediment
deposition.
Slide 24
Slide 25
Total Suspended Solids and Turbidity Total suspended solids are
defined as all solids suspended in water that will not pass through
a 2.0 m glass-fiber filter (dissolved solids would be the fraction
that does pass through the same size filter). Total suspended
solids are defined as all solids suspended in water that will not
pass through a 2.0 m 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 105 o C and
weighed. The filter is then dried in an oven between 103 and 105 o
C and weighed. The increase in weight of the filter represents the
amount of TSS. The increase in weight of the filter represents the
amount of TSS.
Slide 26
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. 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.
Slide 27
Related to, but not a proxy for, suspended sediments is
turbidity. 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. 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. 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. 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.
The amount of light scattered or absorbed is proportional to the
concentration of particulates in the sample.
Slide 28
Sediment Sampling Suspended sediment concentration in a stream
varies from the water surface to the streambed and laterally across
the stream. 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. Concentration
generally increases from a minimum at the water surface to a
maximum at or near the streambed.
Slide 29
Depth Integrating Samplers
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Point Integrated Samples
Slide 31
Bedload Samples
Slide 32
Bedload Traps
Slide 33
Substrate The bed material of a stream determines roughness and
subsequent hydraulics. 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. 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.
Slide 34
Substrate Size-Class Composition Relatively simplistic methods
to determine the dominant types of bottom material. 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. The Modified Wentworth Classification
is an easy field technique using marked (every 1 2 meters)
lead-core ropes to determine dominant substrate.
Slide 35
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Embeddedness A field technique to determine the degree to which
larger substrate is surrounded by fine material. 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. High levels of embeddedness is often
associated with impairment and benthic anoxia.
Slide 37
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Modified Wolman Pebble Count Rapid, repeatable technique
superior to visual assessments. 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. Often combined with embeddedness
techniques for a good representation of how land-based activities
affect stream substrate quality.
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Slide 40
Streams and rivers, over geologic time scales, have shaped
continents. Streams and rivers, over geologic time scales, have
shaped continents. Anthropogenic causes have caused both excess and
limitations of sediment delivered to streams. Anthropogenic causes
have caused both excess and limitations of sediment delivered to
streams.