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.[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]

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]

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. doi:10.1016/0341-8162(79)90027-4.
  3. ^ a b Knighton, David (1998). Fluvial Forms and Processes: A New Perspective. New York: John Wiley and Sons Inc.
  4. ^ a b 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.
  6. ^ a b "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.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: 761–72.
  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.
Chalk stream

Chalk streams are streams that flow through chalk hills towards the sea. They are typically wide and shallow, and due to the filtering effect of the chalk their waters are alkaline and very clear. Chalk streams are popular with fly fishermen who fish for trout on these rivers.

River ecosystem

River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts. River ecosystems are part of larger watershed networks or catchments, where smaller headwater streams drain into mid-size streams, which progressively drain into larger river networks.

River ecosystems are prime examples of lotic ecosystems. Lotic refers to flowing water, from the Latin lotus, meaning washed. Lotic waters range from springs only a few centimeters wide to major rivers kilometers in width. Much of this article applies to lotic ecosystems in general, including related lotic systems such as streams and springs. Lotic ecosystems can be contrasted with lentic ecosystems, which involve relatively still terrestrial waters such as lakes, ponds, and wetlands. Together, these two ecosystems form the more general study area of freshwater or aquatic ecology.

The following unifying characteristics make the ecology of running waters unique among aquatic habitats.

Flow is unidirectional.

There is a state of continuous physical change.

There is a high degree of spatial and temporal heterogeneity at all scales (microhabitats).

Variability between lotic systems is quite high.

The biota is specialized to live with flow conditions.

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.

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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