Bed material load

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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. [1] 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. [2] 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. [3]

Contents

Sediment Transport

Bed-material load is composed of larger grains than any of the other loads. The rate in which grains travel is dependent on the transporting capacity of the flow. Particles move by rolling, sliding, or saltation (bouncing or jumping of grains) at velocities less than that of the surrounding flow. Rolling is the primary mode of transport in gravel-bed streams, while saltation in which grains hop over the bed in a series of low trajectories is largely restricted to sands and small gravels. [3] Various equations are used to estimate the rate at which sediments are transported through the fluvial system. Bed-material discharge equations generally are applicable only within the range of flow conditions and sediment sizes for which the equations were derived. [4] Variables used to characterize the bed material load transport as described by Kumar (2012) are as follows: [5]

– Channel geometry: b (width of the channel), y (flow depth) and BF (bed form of the channel)

– Dynamic properties: Q\ (channel discharge), Sf (friction/energy slope), τb (bed shear stress) and τc (critical shear stress or Shields’ shear stress)

– Sediment properties: d (mean size of sediment), σ (gradation coefficient of the sediment particles) and Gs (specific gravity)

– Fluid properties: ν (viscosity)

Bed material load transport (C) is a function of all the above parameters, i.e.:

C = f (b, y, BF, Q, Sf , τb, τc, d, σ,Gs, ν)

Knowledge of sediment transport is important to such endeavors as river restoration, ecosystem protection, navigation, and infrastructure management. [6]

Measurements

Direct and indirect methods are two ways in which bed material can be measured. Direct measurement is done through the use of a physical trap, placing the device in contact with the bed, “allowing the sediment transported as bedload to accumulate (or be trapped) inside the sampler for a certain amount of time, after which the sampler is raised to the surface and the material is emptied and weighed to determine a weight transported per unit time." [6] There are three types of direct samplers, which include a box or basket, pan or tray, and pressure difference as described by Hubbell (1964). [7] Measurements of bedload discharge are rare and frequently of unknown accuracy because no bedload sampler has been extensively tested and calibrated over a wide range of hydraulic conditions. [4] The box sampler has an opening that allows sediment to enter, the pan or tray samplers are placed in front of the open front of a box, and the pressure difference sampler is made to produce a pressure drop at the end of the nozzle. Accurate field measurements are very difficult to make, errors principally associated with the measuring devices themselves and with the extreme temporal variations in transport rate, which are a characteristic feature of bed material movement. [8] [9] Indirect measurements can be performed by a tracer, repeated channel surveys, bedform velocimetry, or velocimetry. No one method is entirely satisfactory, but indirect channel surveys, provided they are detailed enough at the reach scale, can produce reliable results, and have the advantages of minimum disturbance to the flow and time-integrated sampling which averages out short-term fluctuations in the transport rate. [10]

Importance

Bed-material exerts a control on river channel morphology. The bed material load transport in alluvial rivers is the principal link between river hydraulics and river form [11] and is responsible for building and maintaining the channel geometry. [12]

Related Research Articles

<span class="mw-page-title-main">Sediment</span> Particulate solid matter that is deposited on the surface of land

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 through lithification.

<span class="mw-page-title-main">Braided river</span> Network of river channels separated by small, and often temporary, islands

A braided river consists of a network of river channels separated by small, often temporary, islands called braid bars or, in British English usage, aits or eyots.

<span class="mw-page-title-main">Fluvial processes</span> Processes associated with rivers and streams

In geography and geology, fluvial processes are associated with rivers and streams and the deposits and landforms created by them. When the stream or rivers are associated with glaciers, ice sheets, or ice caps, the term glaciofluvial or fluvioglacial is used.

<span class="mw-page-title-main">Stream bed</span> Channel bottom of a stream, river, or creek

A stream bed or streambed is the bottom of a stream or river (bathymetry) or the physical confine of the normal water flow (channel). The lateral confines or channel margins are known as the stream banks or river banks, during all but flood stage. Under certain conditions a river can branch from one stream bed to multiple stream beds. A flood occurs when a stream overflows its banks and flows onto its flood plain. As a general rule, the bed is the part of the channel up to the normal water line, and the banks are that part above the normal water line. However, because water flow varies, this differentiation is subject to local interpretation. Usually, the bed is kept clear of terrestrial vegetation, whereas the banks are subjected to water flow only during unusual or perhaps infrequent high water stages and therefore might support vegetation some or much of the time.

<span class="mw-page-title-main">Bed load</span> Particles in a flowing fluid that are transported along the bed

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

<span class="mw-page-title-main">Sediment transport</span> Movement of solid particles, typically by gravity and fluid entrainment

Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and the movement of the fluid in which the sediment is entrained. Sediment transport occurs in natural systems where the particles are clastic rocks, 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.

<span class="mw-page-title-main">Abrasion (geology)</span> Process of erosion

Abrasion is a process of erosion which occurs when material being transported wears away at a surface over time. It is the process of friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials. The intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving particles. Abrasion generally occurs in four ways: glaciation slowly grinds rocks picked up by ice against rock surfaces; solid objects transported in river channels make abrasive surface contact with the bed and walls; objects transported in waves breaking on coastlines; and by wind transporting sand or small stones against surface rocks.

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.

Sediments deposited into lakes that have come from glaciers are called glaciolacustrine deposits. In some European geological traditions, the term limnoglacial is used. These lakes include ice margin lakes or other types formed from glacial erosion or deposition. Sediments in the bedload and suspended load are carried into lakes and deposited. The bedload is deposited at the lake margin while the suspended load is deposited all over the lake bed. Glaciolacustrine deposits commonly form varves, which are annually deposited layers of silt and clay, where silt is deposited during the summer, and clay during the winter.

Wash load is similar to a suspended load, but wash load sediment never interacts with the bed load. All of the sediment in the wash load stays suspended in the water throughout the channel. Wash load refers to a river's ability to move sediment through a channel.

Shear velocity, also called friction velocity, is a form by which a shear stress may be re-written in units of velocity. It is useful as a method in fluid mechanics to compare true velocities, such as the velocity of a flow in a stream, to a velocity that relates shear between layers of flow.

<span class="mw-page-title-main">Dissolved load</span>

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.

<span class="mw-page-title-main">Bar (river morphology)</span> Elevated region of sediment in a river that has been deposited by the flow

A bar in a river is an elevated region of sediment that has been deposited by the flow. Types of bars include mid-channel bars, point bars, and mouth bars. 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.

<span class="mw-page-title-main">Stream power</span>

Stream power, originally derived by R. A. Bagnold in the 1960s, is the amount of energy the water in a river or stream is exerting on the sides and bottom of the river. Stream power is the result of multiplying the density of the water, the acceleration of the water due to gravity, the volume of water flowing through the river, and the slope of that water. There are many forms of the stream power formula with varying utilities, such as comparing rivers of various widths or quantifying the energy required to move sediment of a certain size. Stream power is closely related to other criteria such as stream competency and shear stress. Stream power is a valuable measurement for hydrologists and geomorphologists tackling sediment transport issues as well as for civil engineers, who use it in the planning and construction of roads, bridges, dams, and culverts.

Acoustic Doppler velocimetry (ADV) is designed to record instantaneous velocity components at a single-point with a relatively high frequency. Measurements are performed by measuring the velocity of particles in a remote sampling volume based upon the Doppler shift effect.

<span class="mw-page-title-main">Bedrock river</span> Type of river

A bedrock river is a river that has little to no alluvium mantling the bedrock over which it flows. However, most bedrock rivers are not pure forms; they are a combination of a bedrock channel and an alluvial channel. The way one can distinguish between bedrock rivers and alluvial rivers is through the extent of sediment cover.

Channel patterns are found in rivers, streams, and other bodies of water that transport water from one place to another. Systems of branching river channels dissect most of the sub-aerial landscape, each in a valley proportioned to its size. Whether formed by chance or necessity, by headward erosion or downslope convergence, whether inherited or newly formed. Depending on different geological factors such as weathering, erosion, depositional environment, and sediment type, different types of channel patterns can form.

<span class="mw-page-title-main">Stream competency</span> Concept in hydrology

In hydrology stream competency, also known as stream competence, is a measure of the maximum size of particles a stream can transport. The particles are made up of grain sizes ranging from large to small and include boulders, rocks, pebbles, sand, silt, and clay. These particles make up the bed load of the stream. Stream competence was originally simplified by the “sixth-power-law,” which states the mass of a particle that can be moved is proportional to the velocity of the river raised to the sixth power. This refers to the stream bed velocity which is difficult to measure or estimate due to the many factors that cause slight variances in stream velocities.

<span class="mw-page-title-main">River incision</span>

River incision is the narrow erosion caused by a river or stream that is far from its base level. River incision is common after tectonic uplift of the landscape. Incision by multiple rivers result in a dissected landscape, for example a dissected plateau. River incision is the natural process by which a river cuts downward into its bed, deepening the active channel. Though it is a natural process, it can be accelerated rapidly by human factors including land use changes such as timber harvest, mining, agriculture, and road and dam construction. The rate of incision is a function of basal shear-stress. Shear stress is increased by factors such as sediment in the water, which increase its density. Shear stress is proportional to water mass, gravity, and WSS:

Fluvial seismology is the application of seismological methods to understand river processes, such as discharge, erosion, and streambed evolution. Flowing water and the movement of sediments along the streambed generate elastic (seismic) waves that propagate into the surrounding Earth materials. Seismometers can record these signals, which can be analyzed to illuminate different fluvial processes such as turbulent water flow and bedload transport. Seismic methods have been used to observe discharge values that range from single-digits up through tens of thousands of cubic feet per second (cfs).

References

  1. ↑ R.J. Garde; K.G. Ranga Raju. (2000). Mechanics of sediment transportation and alluvial stream problems. New Delhi: New Age International. p. 262. ISBN   978-81-224-1270-3.
  2. ↑ Belperio, A (1979). "The combined use of wash load and bed material load rating curves for the calculation of total load: An example from the Burdekin River, Australia". CATENA. 6 (3–4): 317–329. Bibcode:1979Caten...6..317B. doi:10.1016/0341-8162(79)90027-4.
  3. 1 2 Knighton, David (1998). Fluvial Forms and Processes: A New Perspective. New York: John Wiley and Sons Inc.
  4. 1 2 Andrews, E. D. (1981-02-01). "Measurement and computation of bed-material discharge in a shallow sand-bed stream, Muddy Creek, Wyoming". Water Resources Research. 17 (1): 131–141. Bibcode:1981WRR....17..131A. doi:10.1029/WR017i001p00131. ISSN   1944-7973.
  5. ↑ Kumar, Bimlesh (2012-07-01). "Neural network prediction of bed material load transport". Hydrological Sciences Journal. 57 (5): 956–966. doi: 10.1080/02626667.2012.687108 . ISSN   0262-6667. S2CID   121015519.
  6. 1 2 "Measurement of Bedload Transport in Sand-Bed Rivers: A Look at Two Indirect Sampling Methods" (PDF). webcache.googleusercontent.com. Archived from the original on 2015-10-21. Retrieved 2015-12-17.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
  7. ↑ "Apparatus and Techniques for Measuring Bedload" (PDF). webcache.googleusercontent.com. Archived from the original (PDF) on 2016-11-18. Retrieved 2015-12-17.
  8. ↑ Hubbell, D.W. (1987). Bed load sampling and analysis. Chichestre: Wiley. pp. 89–106.
  9. ↑ Gomez, Basil (August 1991). "Bedload transport". Earth-Science Reviews. 31 (2): 89–132. Bibcode:1991ESRv...31...89G. doi:10.1016/0012-8252(91)90017-A.
  10. ↑ Lane, S.N.; Richards, K.S. & Chandler, J.H. (1995). "Morphological estimation of the time-integrated bedload transport rate". Water Resources Research. 31 (3): 761–72. Bibcode:1995WRR....31..761L. doi:10.1029/94WR01726.
  11. ↑ Gomez, Basil (2006-11-14). "The potential rate of bed-load transport". Proceedings of the National Academy of Sciences. 103 (46): 17170–17173. Bibcode:2006PNAS..10317170G. doi: 10.1073/pnas.0608487103 . ISSN   0027-8424. PMC   1859904 . PMID   17088528.
  12. ↑ Goodwin, Peter (2004-01-01). "Analytical Solutions for Estimating Effective Discharge". Journal of Hydraulic Engineering. 130 (8): 729–738. doi:10.1061/(ASCE)0733-9429(2004)130:8(729). ISSN   0733-9429.
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