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101-413: Summary pages exist for soil erosion , land use status and trends, development of non-federal rural land, and rangeland. This soil science –related article is a stub . You can help Misplaced Pages by expanding it . This article about mining is a stub . You can help Misplaced Pages by expanding it . Soil erosion Soil erosion is the denudation or wearing away of the upper layer of soil . It

202-485: A white noise contribution obtained from the fluctuation-dissipation theorem of statistical mechanics is added to the viscous stress tensor and heat flux . The concept of pressure is central to the study of both fluid statics and fluid dynamics. A pressure can be identified for every point in a body of fluid, regardless of whether the fluid is in motion or not. Pressure can be measured using an aneroid, Bourdon tube, mercury column, or various other methods. Some of

303-400: A considerable depth. Another cause of gully erosion is grazing, which often results in ground compaction. Because the soil is exposed, it loses the ability to absorb excess water, and erosion can develop in susceptible areas. Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward , deepening the valley, and headward , extending

404-455: A continuum, do not contain ionized species, and have flow velocities that are small in relation to the speed of light, the momentum equations for Newtonian fluids are the Navier–Stokes equations —which is a non-linear set of differential equations that describes the flow of a fluid whose stress depends linearly on flow velocity gradients and pressure. The unsimplified equations do not have

505-493: A fluid dynamics problem typically involves the calculation of various properties of the fluid, such as flow velocity , pressure , density , and temperature , as functions of space and time. Before the twentieth century, "hydrodynamics" was synonymous with fluid dynamics. This is still reflected in names of some fluid dynamics topics, like magnetohydrodynamics and hydrodynamic stability , both of which can also be applied to gases. The foundational axioms of fluid dynamics are

606-440: A function of the fluid velocity and have different values in frames of reference with different motion. To avoid potential ambiguity when referring to the properties of the fluid associated with the state of the fluid rather than its motion, the prefix "static" is commonly used (such as static temperature and static enthalpy). Where there is no prefix, the fluid property is the static condition (so "density" and "static density" mean

707-405: A general closed-form solution , so they are primarily of use in computational fluid dynamics . The equations can be simplified in several ways, all of which make them easier to solve. Some of the simplifications allow some simple fluid dynamics problems to be solved in closed form. In addition to the mass, momentum, and energy conservation equations, a thermodynamic equation of state that gives

808-452: A larger amount of surface runoff than less compacted soils. Vegetation acts as an interface between the atmosphere and the soil . It increases the permeability of the soil to rainwater , thus decreasing runoff. It shelters the soil from winds , which results in decreased wind erosion , as well as advantageous changes in microclimate . The roots of the plants bind the soil together, and interweave with other roots, forming

909-536: A model of the effects of the turbulent flow. Such a modelling mainly provides the additional momentum transfer by the Reynolds stresses , although the turbulence also enhances the heat and mass transfer . Another promising methodology is large eddy simulation (LES), especially in the form of detached eddy simulation (DES) — a combination of LES and RANS turbulence modelling. There are a large number of other possible approximations to fluid dynamic problems. Some of

1010-499: A more solid mass that is less susceptible to both water and wind erosion . The removal of vegetation increases the rate of surface erosion . The topography of the land determines the velocity at which surface runoff will flow, which in turn determines the erosivity of the runoff. Longer, steeper slopes (especially those without adequate vegetative cover) are more susceptible to very high rates of erosion during heavy rains than shorter, less steep slopes. Steeper terrain

1111-566: A more vigorous hydrological cycle, including more extreme rainfall events. The rise in sea levels that has occurred as a result of climate change has also greatly increased coastal erosion rates. Studies on soil erosion suggest that increased rainfall amounts and intensities will lead to greater rates of soil erosion. Thus, if rainfall amounts and intensities increase in many parts of the world as expected, erosion will also increase, unless amelioration measures are taken. Soil erosion rates are expected to change in response to changes in climate for

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1212-443: A point in a flow. All fluids are compressible to an extent; that is, changes in pressure or temperature cause changes in density. However, in many situations the changes in pressure and temperature are sufficiently small that the changes in density are negligible. In this case the flow can be modelled as an incompressible flow . Otherwise the more general compressible flow equations must be used. Mathematically, incompressibility

1313-419: A region of the flow called a control volume . A control volume is a discrete volume in space through which fluid is assumed to flow. The integral formulations of the conservation laws are used to describe the change of mass, momentum, or energy within the control volume. Differential formulations of the conservation laws apply Stokes' theorem to yield an expression that may be interpreted as the integral form of

1414-642: A slow process that continues relatively unnoticed, or it may occur at an alarming rate causing a serious loss of topsoil . The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality and damaged drainage networks. Soil erosion could also cause sinkholes . Human activities have increased by 10–50 times the rate at which erosion is occurring world-wide. Excessive (or accelerated) erosion causes both "on-site" and "off-site" problems. On-site impacts include decreases in agricultural productivity and (on natural landscapes ) ecological collapse , both because of loss of

1515-517: A soil structure that is more susceptible to erosion and increased runoff due to increased soil surface sealing and crusting; e) a shift of winter precipitation from non-erosive snow to erosive rainfall due to increasing winter temperatures; f) melting of permafrost, which induces an erodible soil state from a previously non-erodible one; and g) shifts in land use made necessary to accommodate new climatic regimes. Studies by Pruski and Nearing indicated that, other factors such as land use unconsidered, it

1616-428: A spoon-shaped isostatic depression , in which the material has begun to slide downhill. In some cases, the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence. Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However,

1717-677: A variety of reasons. The most direct is the change in the erosive power of rainfall. Other reasons include: a) changes in plant canopy caused by shifts in plant biomass production associated with moisture regime; b) changes in litter cover on the ground caused by changes in both plant residue decomposition rates driven by temperature and moisture dependent soil microbial activity as well as plant biomass production rates; c) changes in soil moisture due to shifting precipitation regimes and evapo-transpiration rates, which changes infiltration and runoff ratios; d) soil erodibility changes due to decrease in soil organic matter concentrations in soils that lead to

1818-507: A wide range of applications, including calculating forces and moments on aircraft , determining the mass flow rate of petroleum through pipelines , predicting weather patterns , understanding nebulae in interstellar space and modelling fission weapon detonation . Fluid dynamics offers a systematic structure—which underlies these practical disciplines —that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to

1919-443: Is a form of soil degradation . This natural process is caused by the dynamic activity of erosive agents, that is, water , ice (glaciers), snow , air (wind), plants , and animals (including humans ). In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion , snow erosion, wind (aeolian) erosion , zoogenic erosion and anthropogenic erosion such as tillage erosion . Soil erosion may be

2020-532: Is also more prone to mudslides, landslides, and other forms of gravitational erosion processes. Unsustainable agricultural practices increase rates of erosion by one to two orders of magnitude over the natural rate and far exceed replacement by soil production. The tillage of agricultural lands, which breaks up soil into finer particles, is one of the primary factors. The problem has been exacerbated in modern times, due to mechanized agricultural equipment that allows for deep plowing , which severely increases

2121-606: Is an extensive global data collection effort produced the Global Rainfall Erosivity Database (GloREDa) which includes rainfall erosivity for 3,625 stations and covers 63 countries. This first ever Global Rainfall Erosivity Database was used to develop a global erosivity map at 30 arc-seconds(~1 km) based on sophisticated geostatistical process. According to a new study published in Nature Communications, almost 36 billion tons of soil

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2222-466: Is expressed by saying that the density ρ of a fluid parcel does not change as it moves in the flow field, that is, where ⁠ D / D t ⁠ is the material derivative , which is the sum of local and convective derivatives . This additional constraint simplifies the governing equations, especially in the case when the fluid has a uniform density. For flow of gases, to determine whether to use compressible or incompressible fluid dynamics,

2323-543: Is given a special name—a stagnation point . The static pressure at the stagnation point is of special significance and is given its own name— stagnation pressure . In incompressible flows, the stagnation pressure at a stagnation point is equal to the total pressure throughout the flow field. In a compressible fluid, it is convenient to define the total conditions (also called stagnation conditions) for all thermodynamic state properties (such as total temperature, total enthalpy, total speed of sound). These total flow conditions are

2424-517: Is known as unsteady (also called transient ). Whether a particular flow is steady or unsteady, can depend on the chosen frame of reference. For instance, laminar flow over a sphere is steady in the frame of reference that is stationary with respect to the sphere. In a frame of reference that is stationary with respect to a background flow, the flow is unsteady. Turbulent flows are unsteady by definition. A turbulent flow can, however, be statistically stationary . The random velocity field U ( x , t )

2525-521: Is lost every year because of drought , deforestation and climate change . In Africa , if current trends of soil degradation continue, the continent might be able to feed just 25% of its population by 2025, according to UNU 's Ghana-based Institute for Natural Resources in Africa. Recent modeling developments have quantified rainfall erosivity at global scale using high temporal resolution (<30 min) and high fidelity rainfall recordings. The results

2626-462: Is lost every year due to water, and deforestation and other changes in land use make the problem worse. The study investigates global soil erosion dynamics by means of high-resolution spatially distributed modelling (c. 250 × 250 m cell size). The geo-statistical approach allows, for the first time, the thorough incorporation into a global soil erosion model of land use and changes in land use, the extent, types, spatial distribution of global croplands and

2727-463: Is much more severe in arid areas and during times of drought. For example, in the Great Plains , it is estimated that soil loss due to wind erosion can be as much as 6100 times greater in drought years than in wet years. Mass movement is the downward and outward movement of rock and sediments on a sloped surface, mainly due to the force of gravity . Mass movement is an important part of

2828-514: Is naturally sparse. Wind erosion requires strong winds, particularly during times of drought when vegetation is sparse and soil is dry (and so is more erodible). Other climatic factors such as average temperature and temperature range may also affect erosion, via their effects on vegetation and soil properties. In general, given similar vegetation and ecosystems, areas with more precipitation (especially high-intensity rainfall), more wind, or more storms are expected to have more erosion. In some areas of

2929-403: Is of two primary varieties: deflation , where the wind picks up and carries away loose particles; and abrasion , where surfaces are worn down as they are struck by airborne particles carried by wind. Deflation is divided into three categories: (1) surface creep , where larger, heavier particles slide or roll along the ground; (2) saltation , where particles are lifted a short height into

3030-526: Is often referred to in general terms as a landslide . However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. One of the visible topographical manifestations of a very slow form of such activity is a scree slope. Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay that, once released, may move quite rapidly downhill. They will often show

3131-494: Is often represented via a Reynolds decomposition , in which the flow is broken down into the sum of an average component and a perturbation component. It is believed that turbulent flows can be described well through the use of the Navier–Stokes equations . Direct numerical simulation (DNS), based on the Navier–Stokes equations, makes it possible to simulate turbulent flows at moderate Reynolds numbers. Restrictions depend on

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3232-608: Is reasonable to expect approximately a 1.7% change in soil erosion for each 1% change in total precipitation under climate change. In recent studies, there are predicted increases of rainfall erosivity by 17% in the United States, by 18% in Europe, and globally 30 to 66% Due to the severity of its ecological effects, and the scale on which it is occurring, erosion constitutes one of the most significant global environmental problems we face today. Water and wind erosion are now

3333-529: Is reduced, and invertebrates are also unable to survive and reproduce. While the sedimentation event itself might be relatively short-lived, the ecological disruption caused by the mass die off often persists long into the future. One of the most serious and long-running water erosion problems worldwide is in the People's Republic of China , on the middle reaches of the Yellow River and the upper reaches of

3434-429: Is statistically stationary if all statistics are invariant under a shift in time. This roughly means that all statistical properties are constant in time. Often, the mean field is the object of interest, and this is constant too in a statistically stationary flow. Steady flows are often more tractable than otherwise similar unsteady flows. The governing equations of a steady problem have one dimension fewer (time) than

3535-492: Is the Universal Soil Loss Equation (USLE). This was developed in the 1960s and 1970s. It estimates the average annual soil loss A on a plot-sized area as: where R is the rainfall erosivity factor , K is the soil erodibility factor , L and S are topographic factors representing length and slope, C is the cover and management factor and P is the support practices factor. Despite

3636-473: Is the wearing away of the banks of a stream or river . This is distinguished from changes on the bed of the watercourse, which is referred to as scour . Erosion and changes in the form of river banks may be measured by inserting metal rods into the bank and marking the position of the bank surface along the rods at different times. Thermal erosion is the result of melting and weakening permafrost due to moving water. It can occur both along rivers and at

3737-492: Is to use two flow models: the Euler equations away from the body, and boundary layer equations in a region close to the body. The two solutions can then be matched with each other, using the method of matched asymptotic expansions . A flow that is not a function of time is called steady flow . Steady-state flow refers to the condition where the fluid properties at a point in the system do not change over time. Time dependent flow

3838-462: Is treated separately. Reactive flows are flows that are chemically reactive, which finds its applications in many areas, including combustion ( IC engine ), propulsion devices ( rockets , jet engines , and so on), detonations , fire and safety hazards, and astrophysics. In addition to conservation of mass, momentum and energy, conservation of individual species (for example, mass fraction of methane in methane combustion) need to be derived, where

3939-401: Is well beyond the limit of DNS simulation ( Re = 4 million). Transport aircraft wings (such as on an Airbus A300 or Boeing 747 ) have Reynolds numbers of 40 million (based on the wing chord dimension). Solving these real-life flow problems requires turbulence models for the foreseeable future. Reynolds-averaged Navier–Stokes equations (RANS) combined with turbulence modelling provides

4040-589: The Mach number of the flow is evaluated. As a rough guide, compressible effects can be ignored at Mach numbers below approximately 0.3. For liquids, whether the incompressible assumption is valid depends on the fluid properties (specifically the critical pressure and temperature of the fluid) and the flow conditions (how close to the critical pressure the actual flow pressure becomes). Acoustic problems always require allowing compressibility, since sound waves are compression waves involving changes in pressure and density of

4141-549: The Mach numbers , which describe as ratios the relative magnitude of fluid and physical system characteristics, such as density , viscosity , speed of sound , and flow speed . The concepts of total pressure and dynamic pressure arise from Bernoulli's equation and are significant in the study of all fluid flows. (These two pressures are not pressures in the usual sense—they cannot be measured using an aneroid, Bourdon tube or mercury column.) To avoid potential ambiguity when referring to pressure in fluid dynamics, many authors use

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4242-751: The Yangtze River . From the Yellow River , over 1.6 billion tons of sediment flows into the ocean each year. The sediment originates primarily from water erosion in the Loess Plateau region of the northwest. Soil particles picked up during wind erosion of soil are a major source of air pollution , in the form of airborne particulates —"dust". These airborne soil particles are often contaminated with toxic chemicals such as pesticides or petroleum fuels, posing ecological and public health hazards when they later land, or are inhaled/ingested. Dust from erosion acts to suppress rainfall and changes

4343-756: The causes of soil erosion , make predictions of erosion under a range of possible conditions , and plan the implementation of preventative and restorative strategies for erosion . However, the complexity of erosion processes and the number of scientific disciplines that must be considered to understand and model them (e.g. climatology, hydrology, geology, soil science, agriculture, chemistry, physics, etc.) makes accurate modelling challenging. Erosion models are also non-linear, which makes them difficult to work with numerically, and makes it difficult or impossible to scale up to making predictions about large areas from data collected by sampling smaller plots. The most commonly used model for predicting soil loss from water erosion

4444-593: The conservation laws , specifically, conservation of mass , conservation of linear momentum , and conservation of energy (also known as the First Law of Thermodynamics ). These are based on classical mechanics and are modified in quantum mechanics and general relativity . They are expressed using the Reynolds transport theorem . In addition to the above, fluids are assumed to obey the continuum assumption . At small scale, all fluids are composed of molecules that collide with one another and solid objects. However,

4545-547: The erodibility of the soil. These can be measured using geotechnical engineering methods such as the hole erosion test or the jet erosion test . Fluid dynamics In physics , physical chemistry and engineering , fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases . It has several subdisciplines, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of water and other liquids in motion). Fluid dynamics has

4646-501: The impact of a falling raindrop creates a small crater in the soil, ejecting soil particles. The distance these soil particles travel can be as much as 0.6 m (two feet) vertically and 1.5 m (five feet) horizontally on level ground. If the soil is saturated , or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs. If the runoff has sufficient flow energy , it will transport loosened soil particles ( sediment ) down

4747-441: The no-slip condition generates a thin region of large strain rate, the boundary layer , in which viscosity effects dominate and which thus generates vorticity . Therefore, to calculate net forces on bodies (such as wings), viscous flow equations must be used: inviscid flow theory fails to predict drag forces , a limitation known as the d'Alembert's paradox . A commonly used model, especially in computational fluid dynamics ,

4848-555: The sky color from blue to white, which leads to an increase in red sunsets . Dust events have been linked to a decline in the health of coral reefs across the Caribbean and Florida, primarily since the 1970s. Similar dust plumes originate in the Gobi desert , which combined with pollutants, spread large distances downwind, or eastward, into North America. Monitoring and modeling of erosion processes can help people better understand

4949-405: The stress due to these viscous forces is linearly related to the strain rate. Such fluids are called Newtonian fluids . The coefficient of proportionality is called the fluid's viscosity; for Newtonian fluids, it is a fluid property that is independent of the strain rate. Non-Newtonian fluids have a more complicated, non-linear stress-strain behaviour. The sub-discipline of rheology describes

5050-400: The surface runoff which may result from rainfall, produces four main types of soil erosion: splash erosion , sheet erosion , rill erosion , and gully erosion . Splash erosion is generally seen as the first and least severe stage in the soil erosion process, which is followed by sheet erosion, then rill erosion and finally gully erosion (the most severe of the four). In splash erosion ,

5151-551: The 50 years since the introduction of the USLE, many other soil erosion models have been developed. But because of the complexity of soil erosion and its constituent processes, all erosion models can only roughly approximate actual erosion rates when validated i.e. when model predictions are compared with real-world measurements of erosion. Thus new soil erosion models continue to be developed. Some of these remain USLE-based, e.g.

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5252-641: The G2 model. Other soil erosion models have largely (e.g. the Water Erosion Prediction Project model ) or wholly (e.g. RHEM, the Rangeland Hydrology and Erosion Model ) abandoned usage of USLE elements. Global studies continue to be based on the USLE. On a smaller scale (e.g. for individual channels , dams , or spillways ), there are erosion rate models available based on the critical shear stress of erosion as well as

5353-435: The USLE's plot-scale spatial basis, the model has often been used to estimate soil erosion on much larger areas, such as watersheds , continents , and globally. One major problem is that the USLE cannot simulate gully erosion, and so erosion from gullies is ignored in any USLE-based assessment of erosion. Yet erosion from gullies can be a substantial proportion (10–80%) of total erosion on cultivated and grazed land. During

5454-476: The air, and bounce and saltate across the surface of the soil; and (3) suspension , where very small and light particles are lifted into the air by the wind, and are often carried for long distances. Saltation is responsible for the majority (50–70%) of wind erosion, followed by suspension (30–40%), and then surface creep (5–25%). Silty soils tend to be the most affected by wind erosion; silt particles are relatively easily detached and carried away. Wind erosion

5555-472: The amount of soil that is available for transport by water erosion. Others include monocropping , farming on steep slopes, pesticide and chemical fertilizer usage (which kill organisms that bind soil together), row-cropping, and the use of surface irrigation . A complex overall situation with respect to defining nutrient losses from soils, could arise as a result of the size selective nature of soil erosion events. Loss of total phosphorus , for instance, in

5656-515: The canopy. However, the intact forest floor, with its layers of leaf litter and organic matter, is still able to absorb the impact of the rainfall. Deforestation causes increased erosion rates due to exposure of mineral soil by removing the humus and litter layers from the soil surface, removing the vegetative cover that binds soil together, and causing heavy soil compaction from logging equipment. Once trees have been removed by fire or logging, infiltration rates become high and erosion low to

5757-402: The clay helps bind soil particles together. Soil containing high levels of organic materials are often more resistant to erosion, because the organic materials coagulate soil colloids and create a stronger, more stable soil structure. The amount of water present in the soil before the precipitation also plays an important role, because it sets limits on the amount of water that can be absorbed by

5858-526: The coast. Rapid river channel migration observed in the Lena River of Siberia is due to thermal erosion , as these portions of the banks are composed of permafrost-cemented non-cohesive materials. Much of this erosion occurs as the weakened banks fail in large slumps. Thermal erosion also affects the Arctic coast, where wave action and near-shore temperatures combine to undercut permafrost bluffs along

5959-409: The continuum assumption assumes that fluids are continuous, rather than discrete. Consequently, it is assumed that properties such as density, pressure, temperature, and flow velocity are well-defined at infinitesimally small points in space and vary continuously from one point to another. The fact that the fluid is made up of discrete molecules is ignored. For fluids that are sufficiently dense to be

6060-557: The degree the forest floor remains intact. Severe fires can lead to significant further erosion if followed by heavy rainfall. Globally one of the largest contributors to erosive soil loss in the year 2006 is the slash and burn treatment of tropical forests . In a number of regions of the earth, entire sectors of a country have been rendered unproductive. For example, on the Madagascar high central plateau , comprising approximately ten percent of that country's land area, virtually

6161-415: The effects of different regional cropping systems. The loss of soil fertility due to erosion is further problematic because the response is often to apply chemical fertilizers, which leads to further water and soil pollution , rather than to allow the land to regenerate. Soil erosion (especially from agricultural activity) is considered to be the leading global cause of diffuse water pollution , due to

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6262-570: The effects of the excess sediments flowing into the world's waterways. The sediments themselves act as pollutants, as well as being carriers for other pollutants, such as attached pesticide molecules or heavy metals. The effect of increased sediments loads on aquatic ecosystems can be catastrophic. Silt can smother the spawning beds of fish, by filling in the space between gravel on the stream bed. It also reduces their food supply, and causes major respiratory issues for them as sediment enters their gills . The biodiversity of aquatic plant and algal life

6363-442: The entire landscape is sterile of vegetation , with gully erosive furrows typically in excess of 50 metres (160 ft) deep and 1 kilometre (0.6 miles) wide. Shifting cultivation is a farming system which sometimes incorporates the slash and burn method in some regions of the world. This degrades the soil and causes the soil to become less and less fertile. Human Impact has major effects on erosion processes—first by denuding

6464-549: The erosional process, and is often the first stage in the breakdown and transport of weathered materials in mountainous areas. It moves material from higher elevations to lower elevations where other eroding agents such as streams and glaciers can then pick up the material and move it to even lower elevations. Mass-movement processes are always occurring continuously on all slopes; some mass-movement processes act very slowly; others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment

6565-413: The farm. The amount and intensity of precipitation is the main climatic factor governing soil erosion by water. The relationship is particularly strong if heavy rainfall occurs at times when, or in locations where, the soil's surface is not well protected by vegetation . This might be during periods when agricultural activities leave the soil bare, or in semi-arid regions where vegetation

6666-433: The finer eroded fraction is greater relative to the whole soil. Extrapolating this evidence to predict subsequent behaviour within receiving aquatic systems, the reason is that this more easily transported material may support a lower solution P concentration compared to coarser sized fractions. Tillage also increases wind erosion rates, by dehydrating the soil and breaking it up into smaller particles that can be picked up by

6767-582: The flood regions result from glacial Lake Missoula , which created the channeled scablands in the Columbia Basin region of eastern Washington . Wind erosion is a major geomorphological force, especially in arid and semi-arid regions. It is also a major source of land degradation, evaporation, desertification, harmful airborne dust, and crop damage—especially after being increased far above natural rates by human activities such as deforestation , urbanization , and agriculture . Wind erosion

6868-527: The flow is irrotational everywhere, Bernoulli's equation can completely describe the flow everywhere. Such flows are called potential flows , because the velocity field may be expressed as the gradient of a potential energy expression. This idea can work fairly well when the Reynolds number is high. However, problems such as those involving solid boundaries may require that the viscosity be included. Viscosity cannot be neglected near solid boundaries because

6969-416: The forest floor. These two layers form a protective mat over the soil that absorbs the impact of rain drops. They are porous and highly permeable to rainfall, and allow rainwater to slow percolate into the soil below, instead of flowing over the surface as runoff . The roots of the trees and plants hold together soil particles, preventing them from being washed away. The vegetative cover acts to reduce

7070-436: The governing equations of the same problem without taking advantage of the steadiness of the flow field. Turbulence is flow characterized by recirculation, eddies , and apparent randomness . Flow in which turbulence is not exhibited is called laminar . The presence of eddies or recirculation alone does not necessarily indicate turbulent flow—these phenomena may be present in laminar flow as well. Mathematically, turbulent flow

7171-455: The land of vegetative cover, altering drainage patterns, and compacting the soil during construction; and next by covering the land in an impermeable layer of asphalt or concrete that increases the amount of surface runoff and increases surface wind speeds. Much of the sediment carried in runoff from urban areas (especially roads) is highly contaminated with fuel, oil, and other chemicals. This increased runoff, in addition to eroding and degrading

7272-410: The land that it flows over, also causes major disruption to surrounding watersheds by altering the volume and rate of water that flows through them, and filling them with chemically polluted sedimentation. The increased flow of water through local waterways also causes a large increase in the rate of bank erosion. The warmer atmospheric temperatures observed over the past decades are expected to lead to

7373-403: The law applied to an infinitesimally small volume (at a point) within the flow. In the above integral formulation of this equation, the term on the left is the net change of momentum within the volume. The first term on the right is the net rate at which momentum is convected into the volume. The second term on the right is the force due to pressure on the volume's surfaces. The first two terms on

7474-580: The macroscopic and microscopic fluid motion at large velocities comparable to the velocity of light . This branch of fluid dynamics accounts for the relativistic effects both from the special theory of relativity and the general theory of relativity . The governing equations are derived in Riemannian geometry for Minkowski spacetime . This branch of fluid dynamics augments the standard hydrodynamic equations with stochastic fluxes that model thermal fluctuations. As formulated by Landau and Lifshitz ,

7575-404: The magnitude of inertial effects compared to the magnitude of viscous effects. A low Reynolds number ( Re ≪ 1 ) indicates that viscous forces are very strong compared to inertial forces. In such cases, inertial forces are sometimes neglected; this flow regime is called Stokes or creeping flow . In contrast, high Reynolds numbers ( Re ≫ 1 ) indicate that the inertial effects have more effect on

7676-403: The medium through which they propagate. All fluids, except superfluids , are viscous, meaning that they exert some resistance to deformation: neighbouring parcels of fluid moving at different velocities exert viscous forces on each other. The velocity gradient is referred to as a strain rate ; it has dimensions T . Isaac Newton showed that for many familiar fluids such as water and air ,

7777-566: The more commonly used are listed below. While many flows (such as flow of water through a pipe) occur at low Mach numbers ( subsonic flows), many flows of practical interest in aerodynamics or in turbomachines occur at high fractions of M = 1 ( transonic flows ) or in excess of it ( supersonic or even hypersonic flows ). New phenomena occur at these regimes such as instabilities in transonic flow, shock waves for supersonic flow, or non-equilibrium chemical behaviour due to ionization in hypersonic flows. In practice, each of those flow regimes

7878-413: The most significant environmental problems worldwide. Intensive agriculture , deforestation , roads , acid rains , anthropogenic climate change and urban sprawl are amongst the most significant human activities in regard to their effect on stimulating erosion. However, there are many prevention and remediation practices that can curtail or limit erosion of vulnerable soils. Rainfall , and

7979-432: The nutrient-rich upper soil layers . In some cases, the eventual result is desertification . Off-site effects include sedimentation of waterways and eutrophication of water bodies, as well as sediment-related damage to roads and houses. Water and wind erosion are the two primary causes of land degradation ; combined, they are responsible for about 84% of the global extent of degraded land, making excessive erosion one of

8080-409: The order of a few centimeters (about an inch) or less and along-channel slopes may be quite steep. This means that rills exhibit hydraulic physics very different from water flowing through the deeper wider channels of streams and rivers. Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow, removing soil to

8181-453: The power of the computer used and the efficiency of the solution algorithm. The results of DNS have been found to agree well with experimental data for some flows. Most flows of interest have Reynolds numbers much too high for DNS to be a viable option, given the state of computational power for the next few decades. Any flight vehicle large enough to carry a human ( L > 3 m), moving faster than 20 m/s (72 km/h; 45 mph)

8282-551: The pressure as a function of other thermodynamic variables is required to completely describe the problem. An example of this would be the perfect gas equation of state : where p is pressure , ρ is density , and T is the absolute temperature , while R u is the gas constant and M is molar mass for a particular gas. A constitutive relation may also be useful. Three conservation laws are used to solve fluid dynamics problems, and may be written in integral or differential form. The conservation laws may be applied to

8383-467: The production/depletion rate of any species are obtained by simultaneously solving the equations of chemical kinetics . Magnetohydrodynamics is the multidisciplinary study of the flow of electrically conducting fluids in electromagnetic fields. Examples of such fluids include plasmas , liquid metals, and salt water . The fluid flow equations are solved simultaneously with Maxwell's equations of electromagnetism. Relativistic fluid dynamics studies

8484-432: The right are negated since momentum entering the system is accounted as positive, and the normal is opposite the direction of the velocity u and pressure forces. The third term on the right is the net acceleration of the mass within the volume due to any body forces (here represented by f body ). Surface forces , such as viscous forces, are represented by F surf , the net force due to shear forces acting on

8585-502: The shoreline and cause them to fail. Annual erosion rates along a 100-kilometre (62-mile) segment of the Beaufort Sea shoreline averaged 5.6 metres (18 feet) per year from 1955 to 2002. At extremely high flows, kolks , or vortices are formed by large volumes of rapidly rushing water. Kolks cause extreme local erosion, plucking bedrock and creating pothole-type geographical features called rock-cut basins . Examples can be seen in

8686-416: The slope. Sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small, ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hillslopes. Generally, where water erosion rates on disturbed upland areas are greatest, rills are active. Flow depths in rills are typically of

8787-405: The soil (and hence prevented from flowing on the surface as erosive runoff). Wet, saturated soils will not be able to absorb as much rainwater, leading to higher levels of surface runoff and thus higher erosivity for a given volume of rainfall. Soil compaction also affects the permeability of the soil to water, and hence the amount of water that flows away as runoff. More compacted soils will have

8888-430: The stream meanders across the valley floor. In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster-moving water is available to carry a larger sediment load. In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles and boulders can also act erosively as they traverse a surface , in a process known as traction . Bank erosion

8989-440: The stress-strain behaviours of such fluids, which include emulsions and slurries , some viscoelastic materials such as blood and some polymers , and sticky liquids such as latex , honey and lubricants . The dynamic of fluid parcels is described with the help of Newton's second law . An accelerating parcel of fluid is subject to inertial effects. The Reynolds number is a dimensionless quantity which characterises

9090-405: The term static pressure to distinguish it from total pressure and dynamic pressure. Static pressure is identical to pressure and can be identified for every point in a fluid flow field. A point in a fluid flow where the flow has come to rest (that is to say, speed is equal to zero adjacent to some solid body immersed in the fluid flow) is of special significance. It is of such importance that it

9191-436: The term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm (0.02 to 0.04 in) in diameter by wind along the soil surface. Tillage erosion is a form of soil erosion occurring in cultivated fields due to the movement of soil by tillage . There is growing evidence that tillage erosion is a major soil erosion process in agricultural lands, surpassing water and wind erosion in many fields all around

9292-414: The terminology that is necessary in the study of fluid dynamics is not found in other similar areas of study. In particular, some of the terminology used in fluid dynamics is not used in fluid statics . Dimensionless numbers (or characteristic numbers ) have an important role in analyzing the behavior of fluids and their flow as well as in other transport phenomena . They include the Reynolds and

9393-464: The two primary causes of land degradation ; combined, they are responsible for 84% of degraded acreage. Each year, about 75 billion tons of soil is eroded from the land—a rate that is about 13–40 times as fast as the natural rate of erosion. Approximately 40% of the world's agricultural land is seriously degraded. According to the United Nations , an area of fertile soil the size of Ukraine

9494-489: The valley into the hillside, creating head cuts and steep banks. In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep. When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as

9595-497: The velocity field than the viscous (friction) effects. In high Reynolds number flows, the flow is often modeled as an inviscid flow , an approximation in which viscosity is completely neglected. Eliminating viscosity allows the Navier–Stokes equations to be simplified into the Euler equations . The integration of the Euler equations along a streamline in an inviscid flow yields Bernoulli's equation . When, in addition to being inviscid,

9696-407: The velocity of the raindrops that strike the foliage and stems before hitting the ground, reducing their kinetic energy . However it is the forest floor, more than the canopy, that prevents surface erosion. The terminal velocity of rain drops is reached in about 8 metres (26 feet). Because forest canopies are usually higher than this, rain drops can often regain terminal velocity even after striking

9797-423: The volume surface. The momentum balance can also be written for a moving control volume. The following is the differential form of the momentum conservation equation. Here, the volume is reduced to an infinitesimally small point, and both surface and body forces are accounted for in one total force, F . For example, F may be expanded into an expression for the frictional and gravitational forces acting at

9898-416: The wind. Exacerbating this is the fact that most of the trees are generally removed from agricultural fields, allowing winds to have long, open runs to travel over at higher speeds. Heavy grazing reduces vegetative cover and causes severe soil compaction, both of which increase erosion rates. In an undisturbed forest , the mineral soil is protected by a layer of leaf litter and an humus that cover

9999-510: The world (e.g. western Europe ), runoff and erosion result from relatively low intensities of stratiform rainfall falling onto previously saturated soil. In such situations, rainfall amount rather than intensity is the main factor determining the severity of soil erosion by water. The composition, moisture, and compaction of soil are all major factors in determining the erosivity of rainfall. Sediments containing more clay tend to be more resistant to erosion than those with sand or silt, because

10100-631: The world (e.g. the Midwestern United States and the Amazon Rainforest ), rainfall intensity is the primary determinant of erosivity, with higher intensity rainfall generally resulting in more soil erosion by water. The size and velocity of rain drops is also an important factor. Larger and higher-velocity rain drops have greater kinetic energy , and thus their impact will displace soil particles by larger distances than smaller, slower-moving rain drops. In other regions of

10201-458: The world, especially on sloping and hilly lands A signature spatial pattern of soil erosion shown in many water erosion handbooks and pamphlets, the eroded hilltops, is actually caused by tillage erosion as water erosion mainly causes soil losses in the midslope and lowerslope segments of a slope, not the hilltops. Tillage erosion results in soil degradation, which can lead to significant reduction in crop yield and, therefore, economic losses for

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