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Sediment transport processes
Sediment transport processes
by A. Gregory
Figure 1. Undercutting Bank
Sediment transport is paramount in considering restoration techniques for both watershed and river restoration. It is responsible for erosion, bank undercutting, sandbar formation, aggradation, gullying, and plugging. However, sediment transport cannot be understood without considering the hydrology, geomorphology, and ecosystem. As Luna Leopold, the father of fluvial geomorphology stated, “So complex are the details of interrelations in this organized system that to describe adequately any single portion tends to make one lose sight of other equally important features,” (Leopold, Wolman, & Miller, 1992). Therefore this paper aims to discuss sediment transport relationships, the necessity of sediment transport in restoration, an overview of how to incorporate sediment transport techniques into practice, and will reiterate its importance through presentation and discussion of case studies.
Sediment transport is a function of slope, velocity, discharge, vegetation, mean sediment inflow rate and channel morphology. When each of these allows a river to become stable, a river is said to have reached dynamic equilibrium. This symbiotic relationship is shown in Lane’s work, Equation 1.
Where Qs is the sediment discharge, d is the median particle diameter, Q is the water flow, and S is the slope of the channel. The equation shows that the sediment discharge and median particle diameter are propor
tional to the stream power (Lane, 1955).
Defining Erosion and Sediment Transport Processes
Figure 2. Sediment plug at the confluence of the
Sediment transport can be defined as the movement of soil particles downstream caused by gravity and the force of moving fluid imparted. The ability of a particle to move is then related to shear stresses, frictional forces, water depth, and specific weight. These components can be classified into two general categories hydraulics and hydrology, and sediment physics.
Fundamentally, erosion of sediment from a watershed begins the process of sediment transport or through human activities. While elements such
as wind and chemical reactions can cause erosion, the main proponent of erosion is water; either in a flowing stream or as precipitation falling on earth’s surface. Once a particle has been eroded, water becomes the “principal vehicle for transport of the eroded material,” (Linsley, Kohler, & Paulhus, 1975).
The effects of human interference with the sediment transport process has resulted in measureable impacts on water quality. One example of the changes to sediment loads relates to dams. Dams have served as a way of trapping sediment and therefore starving streams of their natural sediment load. The result is armoring at the head of the dam, sorting of materials, and incision and widening of the channel. To learn more about the impacts of dams, see
Dams and Other Impoundments
Hydrology and Hydraulics
Hydrology and hydraulics are relevant to sediment transport because they provide the basis for quantifying the amount, depth, and velocity of water at a point whether in a watershed or river, which translates into when and how much sediment will be moved. Other physical characteristics of importance are slope, channel geometry and geomorphology. One of the most prevalent velocity and flow equations used is Manning’s Equation, shown below.
U = Velocity (m/s)
n = roughness coefficient (unitless)
R = Hydraulic Radius (m2/m)
S = slope (m/m)
Another equation that is frequently used is Chezy’s equation. It is empirical in nature
U= Velocity (m/s)
C= Chezy’s coefficient
R = Hydraulic Radius (m2/m)
S = slope (m/m)
Manning’s equation and Chezy’s equation are useful tools when trying to determine average velocity in a stream. In the upcoming sections, velocity will be discussed in relation to shear stress. For more information see
Effective Discharge and Bankfull Discharge
Another fundamental concept of sediment transport in rivers is effective discharge (ED), also called bankfull discharge. ED is the discharge at which a river moves a sufficient amount of sediment to maintain the width, depth and overall dynamic equilibrium. The ED correlates with the point at which overtopping occurs in a stream that is not degraded and generally occurs at regular intervals of 1.5 to 2 years (Rosgen, 1996). See
for more details.
Shear Stress and Friction
The predominate determinates in sediment transport are related to the forces acting on a particle and the shear stresses required to overcome those forces. As can be seen in Figure 3, the weight of the particle must be counterbalanced by shear force in order to resist motion. The equilibrium equation assuming steady and uniform flow is:
The W component can be substituted with γ(D-z)dx. W is the fluid weight, τ is the internal shear stress acting on BC, γ is the specific weight of fluid, D is depth and dx is the length of the channel being considered. If the above equations are rearranged, the equilibrium equation is transformed to:
where it is apparent that shear stress varies linearly (Chang, 1988).
Figure 3: Schematic of Forces on Control Volume
As discussed in the “Shear Stress and Friction” section, sediment must overcome a certain shear stress in order to move. Shields equation (See Equation 6) and diagram (See Figure 4) are often used to determine incipient motion of a uniform sediment on a level bed (Chang, 1988).
Where τc is the critical shear stress,
is the specific weight for the sediment, is the specific weight of water, is the critical shear velocity, d is the particle diameter, and is the kinematic viscosity.
The left side of Equation 6 is referred to as the critical Shields stress and the right hand side is referred to as the critical boundary Reynolds number. For more information on Reynolds number
. In the instance of the Shields Diagram, see Figure 4, Reynolds number relates particle size to and flow region (laminar, transition, and turbulent).
Knowing the point of incipient motion is important to stream restoration for several reasons. If specific objectives have been set such as bank stabilization or improving flood plain connectivity, it will be critical to know if the materials used will potentially be moved downstream under the design criteria. Furthermore, if the sediment gradation is known for a particular reach, then it can be understood how the channel may respond to changes and how sediment will travel downstream.
Figure 4: Shields Diagram for Incipient Motion
Sediment Size and Channel Formations
Whether a channel forms dunes and antidunes or pools and riffles is a function of sediment size. For an alluvial stream with sand sized particles and smaller, it is expected that in low flow conditions, the stream bed will be composed of dunes and antidunes. In larger storm events, an alluvial system will become a flat bed channel and erode materials downstream. On the Rio Grande, dune and antidune formations can be seen in shallow riffle areas. In a gravel bed stream, the formation of pools, runs and riffles is expected. The formation and approximate geometry of pools, runs, and riffles can be estimated.
In the instance of implementing a restoration project where a stream is being placed back in a remnant channel, the use of a reference stream would be used. Reference streams have similar geomorphological and hydrologic conditions as the stream to be restored as well as being near pristine. Typically, several different reference reaches are analyzed to find the closest match before proceeding with the design stage. The chosen reference reach is then used to design the features in the remnant channel. Although there is a remnant channel, it should not expected that feature spacing and channel geometry will be the same as it was when the remnant channel was still a segment of the stream. Understanding how sediment size and channel geometry in a reference reach apply to the remnant channel restoration are useful tools, because they provide the designer an opportunity to check the design of a project.
Whether the stream sediment is sand or cobble, being able to predict the success of a particular restoration method can be supported by understanding the fundamental idea that channel shape will vary with its sediment size.
To take the previously expressed knowledge a step further, two typical approaches exist for classifying sediment loads of streams (Chang, 1988). The first classification differentiates bed load, suspended load, and saltation. Suspended sediment particles are those that are carried in suspension by flowing water. Bed load are material that are transported by sliding and rolling along the bottom of a channel. Finally, saltation refers to those particles that bounce along the bottom of a riverbed.
The second is wash load and bed-material load (Chang, 1988). Bed- material load relates to the material of a stream bed. Finally, wash load is composed primarily of silt and clay.
Sediment Discharge Equations
Sediment transport is much like hydrology in that there is not a “one size fits all” equation. Indeed, there are a plethora of sediment transport equations and algorithms that are based on different premise and that try to predict a variety of parameters. For the purposes of this paper, discussion will begin with sediment yield in watersheds and then will be limited to sediment discharge in rivers. Discussion of sediment discharge equations will be based on the sediment classifications: bed load, suspended sediment load, bed material load, and wash load.
According to Schumm, a fluvial system can be divided into three components. First there is the watershed, where a majority of the sediment and water in a river system originates. The middle reach is where a river channel is most stable. The last portion is near the outlet, where variations in the channel occur due to variations in tides, and base level (ie. at the inlet of reservoirs), (Chang, 1988).
Sediments from watersheds can be correlated to many factors such as climate, soil type, land use, and topography (Linsley, Kohler, & Paulhus, 1975). It is undoubtedly difficult to relate all of the factors contributing to sediment yield to one specific equation and yield accurate results. Existing efforts include those of Langbein and Schumm, and Fleming which relate sediment yield to watershed characteristics.
Bed Load and Shields Equation
Shields equation, Equation 7, is a dimensionless formula that relies on overabundance of shear stress to determine sediment discharge for bed load (Chang, 1988) and (Shields, 1936).
is the water discharge per unit width,
is the bed load discharge per unit width, and d is the particle diameter. The left hand side of Equation 7 represents the bed load discharge and the right hand side contains both excess shear stress and the submerged weight of sediment particles (Chang, 1988).
Shields equation can be used to evaluate sediment discharge in gravel bed channels. An example where Shields equation might be used would be in a river where hydraulic mining has been completed upstream. If a reach of that river was to be restored, it would be useful to evaluate the reduction of sediment discharge before and after the project and to set goals in reaches suffering the same impairment.
Suspended Load and Einstein’s Suspended Load Method
Suspended sediment concentrations and velocity vary within the water column. To evaluate suspended sediment Einstein integrated the following equation:
is the suspended sediment discharge, D is the depth, a is the lowest point at which suspension occurs, C is the concentration, and u is the velocity. This equation is then permutated with Rouse’s equation, shown in Equation 9.
The result is
Where Ca is the concentration at depth the lower limit of U’*where suspension begins, and U’* is the velocity as a result of grain roughness (Chang, 1988). Finally, substitute and z for the respective values of A =a/z and
. The results are shown in Equation 10, Equation 11, and Equation 12.
Einstein’s suspended sediment equation would be particularly useful in the southwestern part of the U.S., where in a large number of streams sand particles are prevalent. This equation can be used to determine how suspended sediment discharge might change with time. Like the example used in the bed load section, determining the suspended sediment can be used to set goals. It can also be useful in reducing effects on fisheries that are sensitive to suspended sediment and have a clear threshold.
As discussed earlier, bed-material load is simply the total of the bed load and suspended load, excluding wash load. Several relationships between bed-material load, stream power and shear stress exist. The Colby relations relate bed-material per unit channel width in terms of temperature, mean flow velocity, depth, sediment size and fine particle concentration (Chang, 1988).
Figure 5: Yellow River at Houkou Falls, China
Wash load has been accepted as not being linked to flow hydraulics with the exception of rain events (Yang & Simoes, 2005). It is considered a function of supply from a watershed and does not interact within the same bounds as other sediment types. However, Yang found a strong correlation between bed-material load and wash load on the Yellow River in China and that wash load can effect bed-material transport rates in the Yellow River’s sediment laden system. The relationship of wash load and bed-material load on the Yellow River were related through development of an algorithm. See Figure 5 (crienglish.com) to view picture of Yellow River at Hukou Falls.
When wash load is an issue in a reach, developing an algorithm would be a good solution if properly developed. The use of a reasonable algorithm could aid in identifying areas that would benefit from restoration. This not only holds true for wash load but also suspended load and bed load.
The Big Picture
As mentioned before, sediment transport processes are intricate. Sediment is only one component of an elaborate system that includes: biota and fauna, water quality, geomorphology, and much more. Table 1 illustrates how variables change spatially and are dependent on changes within the watershed. In terms of evaluating geologic time, no predictions can be made about how interactions will transpire and this effects a slew of variables.
Table 1: River Variables for Different Time Spans (After Schumm, 1971)
Status of Variable
Valley slope, width and depth
Vegetation (type and density)
Mean water discharge
Mean sediment inflow rate
Observed discharge and load
Hydraulics of flow
I*, independent variable; D, dependent variable; X, indeterminate
To get a general idea of how these are tied together, an overview is presented that relates each system to sediment transport.
Collecting Data and Application
Figure 6: Wolman Pebble Counter
So, how does one incorporate sediment transport processes into a robust watershed or stream restoration project? To begin, a plan and project should be defined. Next, field reconnaissance should be completed that mindfully incorporates as many variables into the design as can feasibly be allowed. In terms of sediment transport, this means assessing whether a site’s stream composition is primarily alluvial or gravel in nature.
The collection of bed-material particle size from rivers is an important part of the reconnaissance process. To start, transects are selected at regular intervals, depending on the variation of sediment size, of either equal distance or based on riffle formations (Bunte & Abt, 2001). For a stream composed of mainly fine particles, a bed material sampler can be used to obtain a representative sample of the streambed. The sample is then filtered, dried, and sent through a set of standard sieves to determine percent by weight of particle sizes (USDA). In the instance of larger particles, a Wolman pebble counter is used to determine the size of 100 pebbles or cobbles at minimum.
Once the collected data has been evaluated, it is generally evaluated for a particular particle size. For instance, an engineer may decide that the d50 is the most useful particle size to use in Shields equation. Other equations are specific about the particle size required to determine critical shear stress.
Figure 7: Elwha Dam Removal, WA
The Elwha Dam, see Figure 7, in Washington is one of the largest dams removed to date. The dam removal process began in 2011 and management of the project is based on adaptive management principles. The purpose of its removal was to provide increased spawning habitat to anadromous fish and to restore the natural ecosystem. The Elwha River is a steep gradient river that is 45 miles long on its main stem and has over 100 miles of tributaries. Each year the Elwha carries between 120,000 and 290,000 cubic meters of sediment downstream (Czuba & Randle, 2011). In order to understand the potential outcomes of dam removal, data from the 1994 Lake Mills Drawdown Experiment were used. The predicted sediment load partway through dam removal process is an estimated 21 million yd3.and the expected length of impact is 3 to 5 years.
The managing agency, the Bureau of Reclamation has seven objectives to monitor by to ensure the that best outcome prevails (USBR, 2010).
1. Dam removal should occur fast enough to minimize the duration of sediment impacts to the aquatic environment, but slow enough so that impacts to downstream water users and property owners are not substantial or they can be mitigated.
(The Seattle Times, 2011)
2. Reservoir delta sediments should be eroded and redistributed so that the resulting topography is stable and consistent with natural landscapes. In addition, reservoir sediment erosion and redistribution should keep pace with the rate of reservoir drawdown so that sediments are not left behind in an unstable condition following dam removal which could cause large and uncontrolled releases of sediment to the river.
3. High suspended sediment concentrations should be mitigated for water users by the efficient operation of downstream water treatment facilities.
4. Sediment deposition along the river channel should not increase the 100-year flood stage beyond 2.5 feet for the lower river (downstream from Elwha Dam) and 1.5 feet for the middle river reach (between the two reservoirs).
5. Sediment concentrations can be treated at the Elwha Water Treatment Plant to meet water quality standards.
6. Adequate water clarity should be maintained in the Elwha River and estuary for fish migration during critical migration periods.
7. Large landslides along the reservoirs or the release of flood waves are avoided by limiting the rates of reservoir drawdown.
The management objectives of the Elwha Dam Removal project illustrate the importance of understanding and modelling sediment transport through a system. While each system is different, there are broader consequences of restoration actions that need to be considered. Ensuring the safety of the public whether it be drinking water supply or physical safety are important to consider. Dam removal is one extreme version of stream restoration, but it sets the example of what needs to be considered in smaller restoration projects.
Red Clover Creek Project
The Red Clover Creek project in Plumas County, CA was an attempt to repair one mile of river located in a meadowland that had been degraded by overgrazing. In 1880, Red Clover Creek indicated was described as a narrow channel with a well developed riparian zone and the goal of the project was to achieve similar results. The creek had since transformed into a gully that was 60 to 60 feet wide with banks that were in some cases 10 ft. high. Along with the gully channel features, Red Clover Creek had also been disconnected from its floodplain and the riparian vegetation that once existed had been replaced by drier climate species.
To restore Red Clover Creek the Feather River Coordinated Resource Management (CRM) set three main objectives (Wild Fish Habitat Initiative, 2007):
1) Obliterate the gully created by the actively eroding stream channel
2) Reconnect the channel to the floodplain and raise the water table to restore riparian and wet meadow vegetation adjacent the creek
3) Monitor fish populations in the demonstration project area and at a downstream control site to assess the effects of restoration measures on fish and fish habitat.
In 1985 the Red Clover Restoration Project began construction. Four check dams were installed, the banks were revegetated and the area was fenced to limit grazing by local ranchers. Monitoring occured through 1995, and as shown below Red Clover Valley did show signs of recovering. However, in 2005 the restoration was declared not sustainable due to development of a new actively eroding channel similar to the one shown in 1985. The energy gradient at the bottom of the increased the stream power at that point and propagated a new channel upstream.
Figure 8: Red Clover Creek before Restoration in 1985
Figure 9: Red Clover Creek after Restoration in 1999
The failure of the Red Clover Creek project is an example of why it is important to not only evaluate the sediment transport processes that will occur but also what the long tern expectations of the restoration project are. It is also an example of why it is important to take a holistic approach to restoration and a significant challenge to fully restore a system to previous conditions. The CRM has since developed a new technique for treating gullied systems. They treat gully's be creating a series of pond and plugs and connecting the ponds with remnant channels. Time will tell whether the new technique will be sufficient in maintaining the life of restoration projects. However, the new technique is not significantly significantly different than the use of check dams and the reported success is more likely caused by strong root systems of vegetation since the area that the CRM treats receives sufficient rainfall and snowfall to support high densities of vegetation.
Assessment of sediment transport in stream restoration is of great importance. The processes within river systems are married to dynamic equilibrium and it is inlikely that a restoration project will meet objectives without sediment transport being considered. On the Elwha River , the management objectives not only applied to the restoration of the river and habitat but to the safety of people in surrounding communities. Furthermore, the planning component of the dam removal enabled the Bureau of Reclamation to use adaptive management to modify operations and management of the Elwha River. This tactic will not only reduce the risk of further impairing the ecosystem but will also reduce the potential for flooding, land slides, drinking water contamination
On Red Clover Creek, understanding sediment transport processes as part of the system would help set realistic expectations of the life and the achievable goals of the project. It is a perfect example that even if sediment transport was considered on a microscale, there are more complex relationships that coexist and it is important to look at upstream and downstream reaches as well as the watershed itself for clues to its response.
Also, there are a variety of equations available for evaluating sediment discharge loads and it is important to choose equations carefully. No single equation will provide exact outcomes of sediment transport, aggradation and scour. Evaluation of the sediment transport processes must include the sediment gradation, the geomorphology of the system, and understanding of the relationship between hydraulics and channel geometry and what that means for sediment discharge. Investigating sediment characteristics and movement is beneficial in supporting fisheries, life of a restoration project, and setting goals for reducing sediment load. It is an essential component of analysis and design process and should not be dismissed when considering the effects on a watershed.
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