Stream competency

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Imnaha River, Hells Canyon National Recreation Area, Oregon, example of stream competency. Imnaha River, Hells Canyon National Recreation Area, Oregon.jpg
Imnaha River, Hells Canyon National Recreation Area, Oregon, example of stream competency.

In hydrology stream competency, also known as stream competence, is a measure of the maximum size of particles a stream can transport. [1] 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. [2]

Contents

Stream capacity, while linked to stream competency through velocity, is the total quantity of sediment a stream can carry. Total quantity includes dissolved, suspended, saltation and bed loads. [3]

The movement of sediment is called sediment transport. Initiation of motion involves mass, force, friction and stress. Gravity and friction are the two primary forces in play as water flows through a channel. Gravity acts upon water to move it down slope. Friction exerted on the water by the bed and banks of the channel works to slow the movement of the water. When the force of gravity is equal and opposite to the force of friction the water flows through the channel at a constant velocity. When the force of gravity is greater than the force of friction the water accelerates. [4]

This sediment transport sorts grain sizes based on the velocity. As stream competence increases, the D50 (median grain size) of the stream also increases and can be used to estimate the magnitude of flow which would begin particle transport. [5] Stream competence tends to decrease in the downstream direction, [6] meaning the D50 will increase from mouth to head of the stream.

Importance of Velocity

Stream Power

Stream power is the rate of potential energy loss per unit of channel length. [7] This potential energy is lost moving particles along the stream bed.

Ω = ρw •g•Q•S

where:

Ω = Stream power.

ρw = Density of water.

g = Gravitational acceleration.

S = Channel slope.

Q = the discharge of the stream

Discharge of a stream is the velocity of the stream, U, multiplied by the cross-sectional area, Acs, of the stream channel at that point. As shown by the following equation:

Q = U•Acs

where:

Q = Discharge

U = Average stream velocity

Acs = Cross-sectional area of stream

As velocity increases, so does stream power, and a larger stream power corresponds to an increased ability to move bed load particles.

Shear Stress and Critical Shear Stress

In order for sediment transport to occur in gravel bed channels, flow strength must exceed a critical threshold, called the critical threshold of entrainment, or threshold of mobility. Flow over the surface of a channel and floodplain creates a boundary shear stress field. As discharge increases, shear stress increases above a threshold and starts the process of sediment transport. A comparison of the flow strength available during a given discharge to the critical shear strength needed to mobilize the sediment on the bed of the channel helps us predict whether or not sediment transport is likely to occur, and to some degree, the sediment size likely to move. Although sediment transport in natural rivers varies wildly, relatively simple approximations based on simple flume experiments are commonly used to predict transport. [8] Another way to estimate stream competency is to use the following equation for critical shear stress, τc which is the amount of shear stress required to move a particle of a certain diameter. [9]

τc=τc*•(ρs - ρw)•g•d50

where:

τc* = Shields parameter, a dimensionless value which describes the resistance of the stream bed to gravitational acceleration, also described as roughness or friction,
ρs = Particle density, and ρs – ρw is the effective density of the particle when submerged in water (Archimedes principle). [10]
g = Gravitational acceleration.
d50 = grain diameter, usually measured as d50 which is the median particle diameter when sampling particle diameters in a stream transect.

The shear stress of a stream is represented by the following equation:

τ=ρw•g•D•S

where:

D = average depth

S = stream slope.

If we combine the two equations we get:

ρw•g•D•S = τc*•(ρs – ρw)•g•d50

Solving for particle diameter d we get

d50 = ρw•g•D•S / τc*•(ρs – ρw)•g
d50= ρw•D•S / τc*•(ρs – ρw)

The equation shows particle diameter, d50, is directly proportional to both the depth of water and slope of stream bed (flow and velocity), and inversely proportional to Shield's parameter and the effective density of the particle.

Lift

Velocity differences between the bottom and tops of particles can lead to lift. Water is allowed to flow above the particle but not below resulting in a zero and non-zero velocity at the bottom and top of the particle respectively. The difference in velocities results in a pressure gradient that imparts a lifting force on the particle. If this force is greater than the particle's weight, it will begin transport. [11]

Turbulence

Flows are characterized as either laminar or turbulent. Low-velocity and high-viscosity fluids are associated with laminar flow, while high-velocity and low-viscosity are associated with turbulent flows. Turbulent flows result velocities that vary in both magnitude and direction. These erratic flows help keep particles suspended for longer periods of time. Most natural channels are considered to have turbulent flow. [7]

Other influencing factors

Hjulstrom curve Hjulstroms diagram en.PNG
Hjulström curve

Cohesion

Another important property comes into play when discussing stream competency, and that is the intrinsic quality of the material. In 1935 Filip Hjulström published his curve, which takes into account the cohesiveness of clay and some silt. This diagram illustrates stream competency as a function of velocity. [12]

By observing the size of boulders, rocks, pebbles, sand, silt, and clay in and around streams, one can understand the forces at work shaping the landscape. Ultimately these forces are determined by the amount of precipitation, the drainage density, relief ratio and sediment parent material. [7] They shape depth and slope of the stream, velocity and discharge, channel and floodplain, and determine the amount and kind of sediment observed. This is how the power of water moves and shapes the landscape through erosion, transport, and deposition, and it can be understood by observing stream competency.

Bedrock

Stream competence does not rely solely on velocity. The bedrock of the stream influences the stream competence. Differences in bedrock will affect the general slope and particle sizes in the channel. Stream beds that have sandstone bedrock tend to have steeper slopes and larger bed material, while shale and limestone stream beds tend to be shallower with smaller grain size. [6] Slight variations in underlying material will affect erosion rates, cohesion, and soil composition.

Vegetation [13]

Vegetation has a known impact on a stream's flow, but its influence is hard to isolate. A disruption in flow will result in lower velocities, leading to a lower stream competence. Vegetation has a 4-fold effect on stream flow: resistance to flow, bank strength, nucleus for bar sedimentation, and construction and breaching of log-jams.

Resistance to flow

Cowan method for estimating Manning's n.

n = (n0 + n1 + n2 + n3 + n4)m5

Manning's n considers a vegetation correction factor. Even stream beds with minimal vegetation will have flow resistance.

Bank strength

Vegetation growing in the stream bed and channel helps bind sediment and reduce erosion in a stream bed. A high root density will result in a reinforced stream channel.

Nucleus for Bar Sedimentation

Vegetation-sediment interaction. Vegetation that gets caught in the middle of a stream will disrupt flow and lead to sedimentation in the resulting low velocity eddies. As the sedimentation continues, the island grows, and flow is further impacted.

Construction and Breaching of Log-jams

Vegetation-vegetation interaction. Build-up of vegetation carried by streams eventually cuts off-flow completely to side or main channels of a stream. When these channels are closed, or opened in the case of a breach, the flow characteristics of the stream are disrupted.

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">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">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">Soil mechanics</span> Branch of soil physics and applied mechanics that describes the behavior of soils

Soil mechanics is a branch of soil physics and applied mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids and other matter. Along with rock mechanics, soil mechanics provides the theoretical basis for analysis in geotechnical engineering, a subdiscipline of civil engineering, and engineering geology, a subdiscipline of geology. Soil mechanics is used to analyze the deformations of and flow of fluids within natural and man-made structures that are supported on or made of soil, or structures that are buried in soils. Example applications are building and bridge foundations, retaining walls, dams, and buried pipeline systems. Principles of soil mechanics are also used in related disciplines such as geophysical engineering, coastal engineering, agricultural engineering, hydrology and soil physics.

The Manning formula or Manning's equation is an empirical formula estimating the average velocity of a liquid flowing in a conduit that does not completely enclose the liquid, i.e., open channel flow. However, this equation is also used for calculation of flow variables in case of flow in partially full conduits, as they also possess a free surface like that of open channel flow. All flow in so-called open channels is driven by gravity.

Drainage density is a quantity used to describe physical parameters of a drainage basin. First described by Robert E. Horton, drainage density is defined as the total length of channel in a drainage basin divided by the total area, represented by the following equation:

<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/or 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.

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.

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

Stream load is a geologic term referring to the solid matter carried by a stream. 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.

<span class="mw-page-title-main">Ice-sheet dynamics</span> Technical explanation of ice motion within large bodies of ice

Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable factors: the temperature and the strength of their bases. A number of processes alter these two factors, resulting in cyclic surges of activity interspersed with longer periods of inactivity, on both hourly and centennial time scales. Ice-sheet dynamics are of interest in modelling future sea level rise.

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">Bridge scour</span> Removal of sediment from around bridge abutments or piers by the movement of water

Bridge scour is the removal of sediment such as sand and gravel from around bridge abutments or piers. Hydrodynamic scour, caused by fast flowing water, can carve out scour holes, compromising the integrity of a structure.

<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 quantify the energy required to move sediment of a certain size. Stream power is closely related to various other criterion such as stream competency and shear stress. Stream power is a valuable measurement for hydrologists and geomorphologist tackling sediment transport issues as well as for civil engineers using it in the planning and construction of roads, bridges, dams, and culverts.

<span class="mw-page-title-main">Reynolds number</span> Dimensionless quantity in fluid mechanics

The Reynolds number helps predict flow patterns in different fluid flow situations by measuring the ratio between inertial and viscous forces. At low Reynolds numbers, flows tend to be dominated by laminar (sheet-like) flow, while at high Reynolds numbers flows tend to be turbulent. The turbulence results from differences in the fluid's speed and direction, which may sometimes intersect or even move counter to the overall direction of the flow. These eddy currents begin to churn the flow, using up energy in the process, which for liquids increases the chances of cavitation. Reynolds numbers are an important dimensionless quantity in fluid mechanics.

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 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.

<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.

Hydraulic roughness is the measure of the amount of frictional resistance water experiences when passing over land and channel features. One roughness coefficient is Manning's n-value. Manning’s n is used extensively around the world to predict the degree of roughness in channels. Flow velocity is strongly dependent on the resistance to flow. An increase in this n value will cause a decrease in the velocity of water flowing across a surface.

<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:

The Izbash formula is a formula for the stability calculation of armourstone in running water.

<span class="mw-page-title-main">Shields formula</span> Parameter (and formula) to describe stability of grains in flowing water

The Shields formula is a formula for the stability calculation of granular material in running water.

References

  1. ↑ WILCOCK, DAVID N. (1971). "Investigation into the Relations between Bedload Transport and Channel Shape". Geological Society of America Bulletin. 82 (8): 2159. Bibcode:1971GSAB...82.2159W. doi:10.1130/0016-7606(1971)82[2159:iitrbb]2.0.co;2. ISSN   0016-7606.
  2. ↑ Rubey, W. W. (1938). The force required to move particles on a stream bed (No. 189-E). USGS.
  3. ↑ Cara, Karyth (30 January 2014). "What are the differences between stream capacity and stream competency? How does it relate to "suspended load?" . Retrieved 21 April 2018.
  4. ↑ Leopold, L.B., M.G. Wolman, and J.P. Miller. (1964). Fluvial Processes in Geomorphology. San Francisco: W.H. Freeman and Co. ISBN   0486685888.{{cite book}}: CS1 maint: multiple names: authors list (link)
  5. ↑ Whitaker, Andrew C.; Potts, Donald F. (July 2007). "Analysis of flow competence in an alluvial gravel bed stream, Dupuyer Creek, Montana". Water Resources Research. 43 (7): W07433. Bibcode:2007WRR....43.7433W. doi:10.1029/2006wr005289. ISSN   0043-1397. S2CID   41201225.
  6. 1 2 Brush, Lucien M. (1961). Drainage Basins, Channels, and Flow Characteristics of Selected Streams in Central Pennsylvania. U.S. Government Printing Office.
  7. 1 2 3 R., Bierman, Paul (2013-12-27). Key concepts in geomorphology. Montgomery, David R., 1961–, University of Vermont., University of Washington. New York, NY. ISBN   9781429238601. OCLC   868029499.
  8. ↑ Shilling, F., S. Sommarstrom, R. Kattelmann, B. Washburn, J. Florsheim, and R. Henly. (May 2007). "California Watershed Assessment Manual: Volume II Chapter 3, May 2007. Prepared for the California Resources Agency and the California Bay-Delta Authority" . Retrieved 21 April 2018.{{cite web}}: CS1 maint: multiple names: authors list (link)
  9. ↑ Knighton, D. (1998). Fluvial Forms and Processes: A New Perspective. New York: Oxford University Press Inc. ISBN   0340663138.
  10. ↑ Heath, T.L., Editor (1897). The Works of Archimedes. Cambridge: Cambridge University Press. p. 258. ISBN   0486420841.{{cite book}}: |last= has generic name (help)CS1 maint: multiple names: authors list (link)
  11. ↑ J., Garde, R. (2000). Mechanics of sediment transportation and alluvial stream problems. Ranga Raju, K. G. (3rd ed.). New Delhi: New Age International. ISBN   812241270X. OCLC   45845211.
  12. ↑ Hjulstrom, F. (1935). "Studies of the morphological activity of rivers as illustrated by the River Fyris". Bulletin. Geological Institute Upsalsa. 25: 221–527.
  13. ↑ Hickin, Edward J. (June 1984). "Vegetation and River Channel Dynamics". The Canadian Geographer. 28 (2): 111–126. doi:10.1111/j.1541-0064.1984.tb00779.x. ISSN   0008-3658.
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