Bed load

The term bed load or bedload describes particles in a flowing fluid (usually water) that are transported along the bed. Bed load is complementary to suspended load and wash load.

Bed load moves by rolling, sliding, and/or saltating (hopping).

Generally, bed load downstream will be smaller and more rounded than bed load upstream (a process known as downstream fining). This is due in part to attrition and abrasion which results from the stones colliding with each other and against the river channel, thus removing the rough texture (rounding) and reducing the size of the particles. However, selective transport of sediments also plays a role in relation to downstream fining: smaller-than average particles are more easily entrained than larger-than average particles, since the shear stress required to entrain a grain is linearly proportional to the diameter of the grain. However, the degree of size selectivity is restricted by the hiding effect described by Parker and Klingeman (1982),[1] wherein larger particles protrude from the bed whereas small particles are shielded and hidden by larger particles, with the result that nearly all grain sizes become entrained at nearly the same shear stress.[2][3]

Experimental observations suggest that a uniform free-surface flow over a cohesion-less plane bed is unable to entrain sediments below a critical value of the ratio between measures of hydrodynamic(destabilizing) and gravitational(stabilizing) forces acting on sediment particles, the so-called Shields stress . This quantity reads as:

,

where is the friction velocity, s is the relative particle density, d is an effective particle diameter which is entrained by the flow, and g is gravity. Meyer-Peter-Müller [4] formula for the bed load capacity under equilibrium and uniform flow conditions states that the magnitude of the bed load flux for unit width is proportional to the excess of shear stress with respect to a critical one . Specifically, is a monotonically increasing nonlinear function of the excess Shields stress , typically expressed in the form of a power law. .

Thalweg sediment campbell creek
Bed load sediment in the thalweg of Campbell Creek in Alaska.

References

  1. ^ Parker, Gary; Klingeman, Peter C. (2010). "On why gravel bed streams are paved". Water Resources Research. 18 (5). doi:10.1029/WR018i005p01409.
  2. ^ Ashworth, Philip J.; Ferguson, Robert I. (1989). "Size-selective entrainment of bed load in gravel bed streams". Water Resources Research. 25 (4). doi:10.1029/WR025i004p00627.
  3. ^ Parker, Gary; Toro-Escobar, Carlos M. (2002). "Equal mobility of gravel in streams: The remains of the day". Water Resources Research. 38 (11). doi:10.1029/2001WR000669.
  4. ^ Meyer-Peter, E; Müller, R. (1948). Formulas for bed-load transport. Proceedings of the 2nd Meeting of the International Association for Hydraulic Structures Research. pp. 39–64.
Attrition (erosion)

Attrition is a form of coastal or river erosion, when the bed load is eroded by itself and the bed. As rocks are transported downstream along a riverbed, the regular impacts between the grains themselves and between the grains and the bed cause them to be broken up into smaller fragments. This process also makes them rounder and smoother. Attrition can also occur in glaciated regions, where it is caused by the movement of ice with embedded boulders over surface sediments.

Pebbles are more affected by attrition further upstream, as the rivers' velocity tends to be higher, and therefore its competence (ability to carry sediment) is increased. This means that the load rubs against itself more and with more force when suspended in the river, thus increasing erosion by attrition, though there is a point after transport over a certain distance that pebbles reach a size that is relatively immune to further attrition. Grain-size distribution of sediments produced by attrition will also be controlled by the lithology of the rock from which they are derived.

The effects of attrition can be mistaken for the effects of sorting, in which the grain size of sediments is affected by sediment transport mechanisms e.g. suspension vs. bed load. This affects pebble beaches the most as the pebbles smash into each other, which causes them to smooth.

Bar (river morphology)

A bar in a river is an elevated region of sediment (such as sand or gravel) that has been deposited by the flow. Types of bars include mid-channel bars (also called braid bars, and common in braided rivers), point bars (common in meandering rivers), and mouth bars (common in river deltas). The locations of bars are determined by the geometry of the river and the flow through it. Bars reflect sediment supply conditions, and can show where sediment supply rate is greater than the transport capacity.

A mid-channel bar, is also often referred to as a braid bar because they are often found in braided river channels. Braided river channels are broad and shallow and found in areas where sediment is easily eroded like at a glacial outwash, or at a mountain front with high sediment loads. These types of river systems are associated with high slope, sediment supply, stream power, shear stress, and bed load transport rates. Braided rivers have complex and unpredictable channel patterns, and sediment size tends to vary among streams. It is these features that are responsible for the formations of braid bars. Braided streams are often overfed with massive amounts of sediment which creates multiple stream channels within one dominant pair of flood bank plains. These channels are separated by mid-channel or braid bars. Anastomosing river channels also create mid-channel bars, however they are typically vegetated bars, making them more permanent than the bars found in a braided river channel which have high rates of change because of the large amounts of non-cohesive sediment, lack of vegetation, and high stream powers found in braided river channels.Bars can also form mid-channel due to snags or logjams. For example, if a stable log is deposited mid-channel in a stream, this obstructs the flow and creates local flow convergence and divergence. This causes erosion on the upstream side of the obstruction and deposition on the downstream side. The deposition that occurs on the downstream side can create a central bar, and an arcuate bar can be formed as flow diverges upstream of the obstruction. Continuous deposition downstream can build up the central bar to form an island. Eventually the logjam can become partially buried, which protects the island from erosion, allowing for vegetation to begin to grow, and stabilize the area even further. Over time, the bar can eventually attach to one side of the channel bank and merge into the flood plain.A point bar is an area of deposition typically found in meandering rivers. Point bars form on the inside of meander bends in meandering rivers. As the flow moves around the inside of the bend in the river, the water slows down because of the shallow flow and low shear stresses there reduce the amount of material that can be carried there. Point bars are usually crescent shaped and located on the inside curve of the river bend. The excess material falls out of transport and, over time, forms a point bar. Point bars are typically found in the slowest moving, shallowest parts of rivers and streams, and are often parallel to the shore and occupy the area farthest from the thalweg, on the outside curve of the river bend in a meandering river. Here, at the deepest and fastest part of the stream is the cut bank, the area of a meandering river channel that continuously undergoes erosion. The faster the water in a river channel, the better it is able to pick up greater amounts of sediment, and larger pieces of sediment, which increases the river's bed load. Over a long enough period of time, the combination of deposition along point bars, and erosion along cut banks can lead to the formation of an oxbow lake.A mouth bar is an elevated region of sediment typically found at a river delta which is located at the mouth of a river where the river flows out to the ocean. Sediment is transported by the river and deposited, mid channel, at the mouth of the river. This occurs because, as the river widens at the mouth, the flow slows, and sediment settles out and is deposited. After initial formation of a river mouth bar, they have the tendency to prograde. This is caused by the pressure from the flow on the upstream face of the bar. This pressure creates erosion on that face of the bar, allowing the flow to transport this sediment over or around, and re-deposit it farther downstream, closer to the ocean. River mouth bars stagnate, or cease to prograde when the water depth above the flow is shallow enough to create a pressure on the upstream side of the bar strong enough to force the flow around the deposit rather than over the top of the bar. This divergent channel flow around either side of the sediment deposit continuously transports sediment, which over time is deposited on either side of this original mid channel deposit. As more and more sediment accumulates across the mouth of the river, it builds up to eventually create a sand bar that has the potential to extend the entire length of the river mouth and block the flow.

Bed material load

Three components that are included in the load of a river system are the following: dissolved load, wash load and bed material load. The bed material load is the portion of the sediment that is transported by a stream that contains material derived from the bed. Bed material load typically consists of all of the bed load, and the proportion of the suspended load that is represented in the bed sediments. It generally consists of grains coarser than 0.062 mm with the principal source being the channel bed. Its importance lies in that its composition is that of the bed, and the material in transport can therefore be actively interchanged with the bed. For this reason, bed material load exerts a control on river channel morphology. Bed load and wash load (the sediment that rides high in the flow and does not extract non-negligible momentum from it) together constitute the total load of sediment in a stream. The order in which the three components of load have been considered – dissolved, wash, bed material – can be thought of as progression: of increasingly slower transport velocities, so that the load peak lags further and further behind the flow peak during any event.

Clastic rock

Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus, chunks and smaller grains of rock broken off other rocks by physical weathering. Geologists use the term clastic with reference to sedimentary rocks as well as to particles in sediment transport whether in suspension or as bed load, and in sediment deposits.

Denudation

In geology, denudation involves the processes that cause the wearing away of the Earth's surface by moving water, by ice, by wind and by waves, leading to a reduction in elevation and in relief of landforms and of landscapes. Endogenous processes such as volcanoes, earthquakes, and plate tectonics uplift and expose continental crust to the exogenous processes of weathering, of erosion, and of mass wasting.

Dissolved load

Dissolved load is the portion of a stream's total sediment load that is carried in solution, especially ions from chemical weathering. It is a major contributor to the total amount of material removed from a river's drainage basin, along with suspended load and bed load. The amount of material carried as dissolved load is typically much smaller than the suspended load, though this is not always the case, particularly when the available river flow is mostly harnessed for purposes such as irrigation or industrial uses. Dissolved load comprises a significant portion of the total material flux out of a landscape, and its composition is important in regulating the chemistry and biology of the stream water.

The dissolved load is primarily controlled by the rate of chemical weathering, which depends on climate and weather conditions such as moisture and temperature. Dissolved load has many useful applications within the field of geology, including erosion, denudation, and reconstructing climate in the past.

Glatt (Rhine)

Glatt (German pronunciation: [ˈɡlat]) is the name of a lesser affluent to the High Rhine in the Unterland of the canton of Zurich, Switzerland. It is 35.7 kilometres (22.2 mi) long and flows out from the Greifensee through the Glatt Valley, discharging into the Rhine by Glattfelden. Whereas the upper reaches are only gently inclined, the stream gets steeper beneath, forming banks of bed load.

The earliest mention of the Glatt (fluvii, qui dicitur glat) dates to 1034. The hydronym reflects the (feminine) OHG adjective glat, meaning either "bright, clear" or "plane, smooth". Since the 15th century, the Glatt had been subject to the sovereignty of the city of Zurich, the council of which assigned the custody over the river to two reeves (Glattvögte) in the 16th century.

After a first attempt to regulate the stream in 1593 and a rudimentary project in the early 19th century, the largest reshapings took place during the time from 1878 to 1895. In 1936, another straightening was carried out as a preliminary work to the construction of the Zurich Airport as well as to land improvement and future overbuilding. Due to the last regulation works in 1975 between Niederglatt and the Glatt’s confluence to the Rhine, the hydroelectric power stations built in the late 19th century at the lower course of the stream disappeared.

The Glatt was formerly abounding with fish. Owing to the accelerated growth of Zurich's agglomeration during the 20th century and the insufficiency of the purification plants built in the 1960s, it has been strongly polluted; from 1994 to 2002, the sewage system was restructured by driving a tunnel between the Glatt Valley and the Limmat Valley.

Hans Albert Einstein

Hans Albert Einstein ( eyen-STYNE, -⁠SHTYNE; May 14, 1904 – July 26, 1973) was a Swiss-American engineer and educator, the second child and first son of Albert Einstein and Mileva Marić. Hans A. Einstein was a long-time professor of Hydraulic Engineering at the University of California, Berkeley.Einstein was widely recognized for his research on sediment transport. To honor his outstanding achievement in hydraulic engineering, the American Society of Civil Engineers established the "Hans Albert Einstein Award" in 1988 and the annual award is given to those who have made significant contributions to the field.

Kinetic energy metamorphosis

Kinetic energy metamorphosis (KEM) is a recently discovered tribological process of gradual crystal re-orientation and foliation of component minerals in certain rocks. It is caused by very high, localized application of kinetic energy. The required energy may be provided by prolonged battery of fluvially propelled bed load of cobbles, by glacial abrasion, tectonic deformation, and even by human action. It can result in the formation of laminae on specific metamorphic rocks that, while being chemically similar to the protolith, differ significantly in appearance and in their resistance to weathering or deformation. These tectonite layers are of whitish color and tend to survive granular or mass exfoliation much longer than the surrounding protolith.

Muddy flood

A muddy flood is produced by an accumulation of run-off over agricultural land . Sediments are picked up by the run-off and carried as suspended matter or bed-load. Muddy floods are typically a hill-slope process, and should not be confused with mudflows produced by mass movements.

Muddy floods can damage the road infrastructure and may deposit layers of mud blanket and may also clog sewers and damage private property.

It has been referred to 'muddy floods' since the 1980s. A similar designation appeared in French ('inondations boueuses') during the same period.

Pendant bar

A pendant bar is a fluvial geomorphology term that is usually applied to large landforms created by large scale flooding events. Pendant bars are thin, sharp-crested deposits, and are typically made up of coarser sediment from the bed load. This type of bar is found on the downstream side of a weathering-resistant protrusion such as a large outcrop of bedrock, and is separated from the protrusion by a depression.

Riffle-pool sequence

In a flowing stream, a riffle-pool sequence (also known as a pool-riffle sequence) develops as a stream's hydrological flow structure alternates from areas of relatively shallow to deeper water. This sequence is present only in streams carrying gravel or coarser sediments. Riffles are formed in shallow areas by coarser materials, such as gravel deposits, over which water flows. Pools are deeper, calmer areas whose bed load (in general) is made up of finer material such as silt. Streams with only sand or silt laden beds do not develop the feature. The sequence within a stream bed commonly occurs at intervals of from 5 to 7 stream widths. Meandering streams with relatively coarse bed load tend to develop a riffle-pool sequence with pools in the outsides of the bends and riffles in the crossovers between one meander to the next on the opposite margin of the stream. The pools are areas of active erosion and the material eroded tends to be deposited in the riffle areas between them.

Rouse number

The Rouse number (P or Z) is a non-dimensional number in fluid dynamics which is used to define a concentration profile of suspended sediment and which also determines how sediment will be transported in a flowing fluid. It is a ratio between the sediment fall velocity and the upwards velocity on the grain as a product of the von Kármán constant and the shear velocity .

Occasionally the factor β is included before the von Kármán constant in the equation, which is a constant which correlates eddy viscosity to eddy diffusivity. This is generally taken to be equal to 1, and therefore is ignored in actual calculation. However, it should not be ignored when considering the full equation.

It is named after the American fluid dynamicist Hunter Rouse. It is a characteristic scale parameter in the Rouse Profile of suspended sediment concentration with depth in a flowing fluid. The concentration of suspended sediment with depth goes as the power of the negative Rouse number. It also is used to determine how the particles will move in the fluid. The required Rouse numbers for transport as bed load, suspended load, and wash load, are given below.

Sediment

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation. If buried, they may eventually become sandstone and siltstone (sedimentary rocks) through lithification.

Sediments are most often transported by water (fluvial processes), but also wind (aeolian processes) and glaciers. Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of aeolian transport and deposition. Glacial moraine deposits and till are ice-transported sediments.

Sediment transport

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks (sand, gravel, boulders, etc.), mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting. Sediment transport due to fluid motion occurs in rivers, oceans, lakes, seas, and other bodies of water due to currents and tides. Transport is also caused by glaciers as they flow, and on terrestrial surfaces under the influence of wind. Sediment transport due only to gravity can occur on sloping surfaces in general, including hillslopes, scarps, cliffs, and the continental shelf—continental slope boundary.

Sediment transport is important in the fields of sedimentary geology, geomorphology, civil engineering and environmental engineering (see applications, below). Knowledge of sediment transport is most often used to determine whether erosion or deposition will occur, the magnitude of this erosion or deposition, and the time and distance over which it will occur.

Stream load

Stream load is a geologic term referring to the solid matter carried by a stream (Strahler and Strahler, 2006). Erosion and bed shear stress continually remove mineral material from the bed and banks of the stream channel, adding this material to the regular flow of water. The amount of solid load that a stream can carry, or stream capacity, is measured in metric tons per day, passing a given location. Stream capacity is dependent upon the stream's velocity, the amount of water flow, and the gradation (because streams that occur on steeper slopes tend to have greater flow and velocity) (Strahler and Strahler, 2006).

Suspended load

The suspended load of a flow of fluid, such as a river, is the portion of its sediment uplifted by the fluid's flow in the process of sediment transportation. It is kept suspended by the fluid's turbulence. The suspended load generally consists of smaller particles, like clay, silt, and fine sands.

Wash load

Wash load as described by Hans Albert Einstein, "is if the sediment is added to the upstream end of a concrete channel and the channel is swept clean, and the sediment has not left any trace in the channel, its rate of transport need not be related to the flow rate." (2) The fine sediments that are in the wash load are generally smaller than .0625 mm, but what determines the wash load in reality is the relationship between the size of the bed load and the size of the particles that never settle in the "fine sediment load" or wash load.

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