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49-729: The name could mean 'river by the striking fortress' or 'sweet-flowing river'. The streams of Nant y Bryn, Nant y Fedwen and Nant Hir come together to form the Pyrddin which flows south-eastwards for 3 miles (5 km) to its confluence with the Nedd Fechan (sometimes also known historically in an Englished form "Neath Vaughan"; at one time this would have been a fair indication of the Welsh pronunciation in English spelling, but with changes in English pronunciation it has now diverged considerably)). The Pyrddin

98-539: A confluence (also: conflux ) occurs where two or more watercourses join to form a single channel . A confluence can occur in several configurations: at the point where a tributary joins a larger river ( main stem ); or where two streams meet to become the source of a river of a new name (such as the confluence of the Monongahela and Allegheny rivers, forming the Ohio River ); or where two separated channels of

147-619: A confluence is an industrial site, as in Philadelphia or Mannheim . Often a confluence lies in the shared floodplain of the two rivers and nothing is built on it, for example at Manaus , described below. One other way that confluences may be exploited by humans is as sacred places in religions . Rogers suggests that for the ancient peoples of the Iron Age in northwest Europe, watery locations were often sacred, especially sources and confluences. Pre-Christian Slavic peoples chose confluences as

196-417: A critical angle of repose . Large masses of material are moved in debris flows , hyperconcentrated mixtures of mud, clasts that range up to boulder-size, and water. Debris flows move as granular flows down steep mountain valleys and washes. Because they transport sediment as a granular mixture, their transport mechanisms and capacities scale differently from those of fluvial systems. Sediment transport

245-433: A gradients, cascades and an adequate junction angle which is sympathetic to the direction of the watercourse’s flow to minimise turbulent flow, maximise evacuation velocity and to ultimately maximise hydraulic efficiency. Since rivers often serve as political boundaries, confluences sometimes demarcate three abutting political entities, such as nations, states, or provinces, forming a tripoint . Various examples are found in

294-400: A parabolic concave-up profile, which grades into a convex-up profile around valleys. As hillslopes steepen, however, they become more prone to episodic landslides and other mass wasting events. Therefore, hillslope processes are better described by a nonlinear diffusion equation in which classic diffusion dominates for shallow slopes and erosion rates go to infinity as the hillslope reaches

343-510: A part of the sediment mixture moves, the river bed becomes enriched in large gravel as the smaller sediments are washed away. The smaller sediments present under this layer of large gravel have a lower possibility of movement and total sediment transport decreases. This is called armouring effect. Other forms of armouring of sediment or decreasing rates of sediment erosion can be caused by carpets of microbial mats, under conditions of high organic loading. The Shields diagram empirically shows how

392-531: A reservoir formed by a dam forms a reservoir delta . This delta will fill the basin, and eventually, either the reservoir will need to be dredged or the dam will need to be removed. Knowledge of sediment transport can be used to properly plan to extend the life of a dam. Geologists can use inverse solutions of transport relationships to understand flow depth, velocity, and direction, from sedimentary rocks and young deposits of alluvial materials. Flow in culverts, over dams, and around bridge piers can cause erosion of

441-479: A river (forming a river island ) rejoin at the downstream end. The point of confluence where the channel flows into a larger body of water may be called the river mouth . Confluences are studied in a variety of sciences. Hydrology studies the characteristic flow patterns of confluences and how they give rise to patterns of erosion, bars, and scour pools. The water flows and their consequences are often studied with mathematical models . Confluences are relevant to

490-546: A single-slope infinite channel (as in the depth-slope product , above), the bed shear stress can be locally found by applying the Saint-Venant equations for continuity , which consider accelerations within the flow. The criterion for the initiation of motion, established earlier, states that In this equation, For a particular particle Reynolds number, τ c ∗ {\displaystyle \tau _{c}*} will be an empirical constant given by

539-399: A specific version of the particle Reynolds number, called R e p ∗ {\displaystyle \mathrm {Re} _{p}*} . This can then be solved by using the empirically derived Shields curve to find τ c ∗ {\displaystyle \tau _{c}*} as a function of a specific form of the particle Reynolds number called

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588-441: A stream contaminated with acid mine drainage combines with a stream with near-neutral pH water; these reactions happen very rapidly and influence the subsequent transport of metals downstream of the mixing zone." A natural phenomenon at confluences that is obvious even to casual observers is a difference in color between the two streams; see images in this article for several examples. According to Lynch, "the color of each river

637-401: A vast assortment of subjects which concern confluences. In hydraulic civil engineering , where two or more underground culverted / artificially buried watercourses intersect, great attention should be paid to the hydrodynamic aspects of the system to ensure the longevity and efficiency of the structure. Engineers have to design these systems whilst considering a list of factors that ensure

686-551: Is applied to solve many environmental, geotechnical, and geological problems. Measuring or quantifying sediment transport or erosion is therefore important for coastal engineering . Several sediment erosion devices have been designed in order to quantify sediment erosion (e.g., Particle Erosion Simulator (PES)). One such device, also referred to as the BEAST (Benthic Environmental Assessment Sediment Tool) has been calibrated in order to quantify rates of sediment erosion. Movement of sediment

735-421: Is approximately equal to tan ⁡ ( θ ) {\displaystyle \tan(\theta )} , which is given by S {\displaystyle S} , the slope. Rewritten with this: For the steady case, by extrapolating the depth-slope product and the equation for shear velocity: The depth-slope product can be rewritten as: u ∗ {\displaystyle u*}

784-489: Is best known for two spectacular waterfalls, Sgwd Gwladus (sometimes 'Gwladys') and Sgwd Einion Gam, meaning the 'fall of Gwladus' and the 'fall of crooked Einion' respectively. Both Gwladus and Einion are personal names. Sgwd Gwladus is formed where a band of erosion-resistant gritstone known as the ' Twelve Foot Sandstone ' creates a lip over which the river plunges. It is easily accessible by footpath from Pontneddfechan (also known as Pontneathvaughan). A notable feature above

833-494: Is determined by many things: type and amount of vegetation in the watershed, geological properties, dissolved chemicals, sediments and biologic content – usually algae ." Lynch also notes that color differences can persist for miles downstream before they finally blend completely. Hydrodynamic behaviour of flow in a confluence can be divided into six distinct features which are commonly called confluence flow zones (CFZ). These include The broader field of engineering encompasses

882-529: Is important in providing habitat for fish and other organisms in rivers. Therefore, managers of highly regulated rivers, which are often sediment-starved due to dams, are often advised to stage short floods to refresh the bed material and rebuild bars. This is also important, for example, in the Grand Canyon of the Colorado River , to rebuild shoreline habitats also used as campsites. Sediment discharge into

931-601: Is in order to compare the driving forces of particle motion (shear stress) to the resisting forces that would make it stationary (particle density and size). This dimensionless shear stress, τ ∗ {\displaystyle \tau *} , is called the Shields parameter and is defined as: And the new equation to solve becomes: The equations included here describe sediment transport for clastic , or granular sediment. They do not work for clays and muds because these types of floccular sediments do not fit

980-431: Is much greater than its depth, the bed shear stress is given by some momentum considerations stating that the gravity force component in the flow direction equals exactly the friction force. For a wide channel, it yields: For shallow slope angles, which are found in almost all natural lowland streams, the small-angle formula shows that sin ⁡ ( θ ) {\displaystyle \sin(\theta )}

1029-473: Is related to the mean flow velocity, u ¯ {\displaystyle {\bar {u}}} , through the generalized Darcy–Weisbach friction factor , C f {\displaystyle C_{f}} , which is equal to the Darcy-Weisbach friction factor divided by 8 (for mathematical convenience). Inserting this friction factor, For all flows that cannot be simplified as

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1078-413: Is the kinematic viscosity, which is given by the dynamic viscosity, μ {\displaystyle \mu } , divided by the fluid density, ρ f {\displaystyle {\rho _{f}}} . The specific particle Reynolds number of interest is called the boundary Reynolds number, and it is formed by replacing the velocity term in the particle Reynolds number by

1127-481: Is therefore given by: The boundary Reynolds number can be used with the Shields diagram to empirically solve the equation which solves the right-hand side of the equation In order to solve the left-hand side, expanded as the bed shear stress needs to be found, τ b {\displaystyle {\tau _{b}}} . There are several ways to solve for the bed shear stress. The simplest approach

1176-417: Is to assume the flow is steady and uniform, using the reach-averaged depth and slope. because it is difficult to measure shear stress in situ , this method is also one of the most-commonly used. The method is known as the depth-slope product . For a river undergoing approximately steady, uniform equilibrium flow, of approximately constant depth h and slope angle θ over the reach of interest, and whose width

1225-735: Is used to describe the meeting of tidal or other non-riverine bodies of water, such as two canals or a canal and a lake. A one-mile (1.6 km) portion of the Industrial Canal in New Orleans accommodates the Gulf Intracoastal Waterway and the Mississippi River-Gulf Outlet Canal ; therefore those three waterways are confluent there. The term confluence can also apply to the process of merging or flowing together of other substance. For example, it may refer to

1274-407: The continental shelf —continental slope boundary. Sediment transport is important in the fields of sedimentary geology , geomorphology , civil engineering , hydraulic 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

1323-418: The deposits and landforms created by sediments . It can result in the formation of ripples and dunes , in fractal -shaped patterns of erosion, in complex patterns of natural river systems, and in the development of floodplains and the occurrence of flash floods . Sediment moved by water can be larger than sediment moved by air because water has both a higher density and viscosity . In typical rivers

1372-413: The shear velocity , u ∗ {\displaystyle u_{*}} , which is a way of rewriting shear stress in terms of velocity. where τ b {\displaystyle \tau _{b}} is the bed shear stress (described below), and κ {\displaystyle \kappa } is the von Kármán constant , where The particle Reynolds number

1421-528: The ' Farewell Rock ' up against softer mudstones. The steep and wooded gorge of the Pyrddin – like that of its neighbours, the Nedd Fechan , Afon Mellte , and Afon Hepste – is home to many rare bryophytes which enjoy the cool humid conditions which exist here. Accordingly, it has been designated as both a site of special scientific interest and a special area of conservation . 51°46′N 3°36′W  /  51.767°N 3.600°W  / 51.767; -3.600 Confluence In geography ,

1470-457: The Shields Curve or by another set of empirical data (depending on whether or not the grain size is uniform). Therefore, the final equation to solve is: Some assumptions allow the solution of the above equation. The first assumption is that a good approximation of reach-averaged shear stress is given by the depth-slope product. The equation then can be rewritten as: Moving and re-combining

1519-411: The bed. This basic criterion for the initiation of motion can be written as: This is typically represented by a comparison between a dimensionless shear stress τ b ∗ {\displaystyle \tau _{b}*} and a dimensionless critical shear stress τ c ∗ {\displaystyle \tau _{c}*} . The nondimensionalization

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1568-413: The bed. This erosion can damage the environment and expose or unsettle the foundations of the structure. Therefore, good knowledge of the mechanics of sediment transport in a built environment are important for civil and hydraulic engineers. When suspended sediment transport is increased due to human activities, causing environmental problems including the filling of channels, it is called siltation after

1617-412: The boundary Reynolds number. The mathematical solution of the equation was given by Dey . In general, a particle Reynolds number has the form: Where U p {\displaystyle U_{p}} is a characteristic particle velocity, D {\displaystyle D} is the grain diameter (a characteristic particle size), and ν {\displaystyle \nu }

1666-411: The dimensionless critical shear stress (i.e. the dimensionless shear stress required for the initiation of motion) is a function of a particular form of the particle Reynolds number , R e p {\displaystyle \mathrm {Re} _{p}} or Reynolds number related to the particle. This allows the criterion for the initiation of motion to be rewritten in terms of a solution for

1715-440: The discharge point is structurally stable as the entrance of the lateral culvert into the main structure may compromise the stability of the structure due to the lack of support at the discharge, this often constitutes additional supports in the form of structural bracing. The velocities and hydraulic efficiencies should be meticulously calculated and can be altered by integrating different combinations of geometries, components such

1764-499: The distribution of living organisms (i.e., ecology ) as well; "the general pattern [downstream of confluences] of increasing stream flow and decreasing slopes drives a corresponding shift in habitat characteristics." Another science relevant to the study of confluences is chemistry , because sometimes the mixing of the waters of two streams triggers a chemical reaction, particularly in a polluted stream. The United States Geological Survey gives an example: "chemical changes occur when

1813-454: The fall is an erratic block of gritstone which was balanced so that it rocked - at least until Victorian times when it was displaced. The ruins of a small building close to the falls are thought to be those of a corn-mill. Sgwd Einion Gam, which is a further half mile, upstream can be accessed only by crossing and re-crossing the river itself. This fall is caused by the presence of a geological fault which brings another hard sandstone known as

1862-521: 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

1911-443: The geometric simplifications in these equations, and also interact thorough electrostatic forces. The equations were also designed for fluvial sediment transport of particles carried along in a liquid flow, such as that in a river, canal, or other open channel. Only one size of particle is considered in this equation. However, river beds are often formed by a mixture of sediment of various sizes. In case of partial motion where only

1960-679: The globe. Dust from the Sahara deposits on the Canary Islands and islands in the Caribbean , and dust from the Gobi Desert has deposited on the western United States . This sediment is important to the soil budget and ecology of several islands. Deposits of fine-grained wind-blown glacial sediment are called loess . In geography and geology , fluvial sediment processes or fluvial sediment transport are associated with rivers and streams and

2009-420: The grain-size fraction dominating the process. For a fluid to begin transporting sediment that is currently at rest on a surface, the boundary (or bed) shear stress τ b {\displaystyle \tau _{b}} exerted by the fluid must exceed the critical shear stress τ c {\displaystyle \tau _{c}} for the initiation of motion of grains at

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2058-583: The largest carried sediment is of sand and gravel size, but larger floods can carry cobbles and even boulders . Coastal sediment transport takes place in near-shore environments due to the motions of waves and currents. At the mouths of rivers, coastal sediment and fluvial sediment transport processes mesh to create river deltas . Coastal sediment transport results in the formation of characteristic coastal landforms such as beaches , barrier islands , and capes. As glaciers move over their beds, they entrain and move material of all sizes. Glaciers can carry

2107-431: The largest sediment, and areas of glacial deposition often contain a large number of glacial erratics , many of which are several metres in diameter. Glaciers also pulverize rock into " glacial flour ", which is so fine that it is often carried away by winds to create loess deposits thousands of kilometres afield. Sediment entrained in glaciers often moves approximately along the glacial flowlines , causing it to appear at

2156-553: The list below. A number of major cities, such as Chongqing , St. Louis , and Khartoum , arose at confluences; further examples appear in the list. Within a city, a confluence often forms a visually prominent point, so that confluences are sometimes chosen as the site of prominent public buildings or monuments, as in Koblenz , Lyon , and Winnipeg . Cities also often build parks at confluences, sometimes as projects of municipal improvement, as at Portland and Pittsburgh . In other cases,

2205-417: The merger of the flow of two glaciers . Sediment transport 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 ( sand , gravel , boulders , etc.), mud , or clay ;

2254-448: The sites for fortified triangular temples, where they practiced human sacrifice and other sacred rites. In Hinduism , the confluence of two sacred rivers often is a pilgrimage site for ritual bathing. In Pittsburgh, a number of adherents to Mayanism consider their city's confluence to be sacred. Mississippi basin Atlantic watersheds Pacific watersheds Occasionally, "confluence"

2303-422: The surface in the ablation zone . In hillslope sediment transport, a variety of processes move regolith downslope. These include: These processes generally combine to give the hillslope a profile that looks like a solution to the diffusion equation , where the diffusivity is a parameter that relates to the ease of sediment transport on the particular hillslope. For this reason, the tops of hills generally have

2352-427: The terrestrial near-surface environment. Ripples and dunes form as a natural self-organizing response to sediment transport. Aeolian sediment transport is common on beaches and in the arid regions of the world, because it is in these environments that vegetation does not prevent the presence and motion of fields of sand. Wind-blown very fine-grained dust is capable of entering the upper atmosphere and moving across

2401-491: The time and distance over which it will occur. Aeolian or eolian (depending on the parsing of æ ) is the term for sediment transport by wind . This process results in the formation of ripples and sand dunes . Typically, the size of the transported sediment is fine sand (<1 mm) and smaller, because air is a fluid with low density and viscosity , and can therefore not exert very much shear on its bed. Bedforms are generated by aeolian sediment transport in

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