Chapter OverviewWhat is Sediment? What is Sediment Transport? What is Sediment Deposition? Why is Sediment Transport and also Deposition Important? where does Sediment Come From? determinants that affect Sediment Transport aftermath of Sediment Transport and also Deposition
What is Sediment?
Sediment refers to the conglomerate that materials, organic and inorganic, that have the right to be brought away by water, wind or ice 3. While the hatchet is often used to show soil-based, mineral matter (e.g. Clay, silt and sand), decomposing necessary substances and inorganic biogenic material are additionally considered sediment 1. Many mineral sediment originates from erosion and weathering, if organic sediment is generally detritus and also decomposing product such as algae 4.
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These particulates are frequently small, with clay identified as particles less than 0.00195 mm in diameter, and coarse sand getting to up just to 1.5 mm in diameter 5. However, throughout a overwhelming or other high circulation event, even big rocks deserve to be classified as sediment as they are brought downstream 6. Sediment is a naturally developing element in countless bodies the water, despite it deserve to be influenced by anthropogenic factors 8.
Suspended or Bedded?
In one aquatic environment, sediment deserve to either it is in suspended (floating in the water column) or bedded (settled ~ above the bottom the a human body of water). Once both floating and worked out particles are monitored, castle are described as SABS: Suspended and also Bedded Sediments 4.
Suspended Sediment vs rely Solids
Fine sediment deserve to be uncovered in virtually any human body of water, brought along by the water flow. When the sediment is floating within the water pillar it is taken into consideration suspended. In this application, the state “suspended sediment” and also “suspended solids” are nearly interchangeable. The main difference between the two is in the technique of measure up 2.
Despite the similarity in meaning, the data detailed by the various measurement techniques are neither interchangeable interchangeable nor similar 2. The suspended sediment concentration (SSC) is in mg/L by filtering and also drying an entire water sample. Full suspended solids (TSS), while also measured in mg/L, are acquired by subsampling. While acceptable for homogenized or well blended samples with really fine sediment, the TSS measurement often excludes larger suspended particles, prefer sand 2. This way that the SSC measurement has tendency to be higher and more representative that a water body as a whole, frequently measuring within 5% that the true fragment concentration 7. As result of the incomparability in between suspended sediment measurements and also total exposed solids measurements, the U.S. Geological inspection recommends SSC analysis over TSS as soon as sampling in surface ar water 2.
What is Sediment Transport?
Sediment deliver is the motion of organic and inorganic corpuscle by water 10. In general, the greater the flow, the more sediment that will be conveyed. Water flow can be strong enough to suspend particles in the water column as they relocate downstream, or simply push them follow me the bottom that a waterway 11. Transported sediment may include mineral matter, chemicals and pollutants, and also organic material.
Another name for sediment move is sediment load. The total load consists of all particles relocating as bedload, rely load, and wash fill 11.
Bedload is the section of sediment carry that rolls, slides or bounces along the bottom that a waterway 12. This sediment is not truly suspended, as it sustains intermittent call with the streambed, and also the movement is neither uniform nor continuous 11. Bedload occurs when the force of the water circulation is solid enough to get over the weight and cohesion that the sediment 12. When the particles are driven along, they frequently do not move as fast as the water about them, as the flow rate is not good enough to completely suspend them 11. Bedload transport deserve to occur during low flows (smaller particles) or at high flows (for larger particles). About 5-20% of full sediment transfer is bedload 10. In instances where the circulation rate is solid enough, several of the smaller bedload particles have the right to be thrust up into the water column and also become suspended.
While there is often overlap, the suspended load and also suspended sediment are not the same thing. Exposed sediment are any type of particles found in the water column, even if it is the water is flow or not. The exposed load, ~ above the other hand, is the quantity of sediment brought downstream within the water shaft by the water circulation 11. Suspended lots require moving water, together the water flow creates small upward currents (turbulence) that save the particles above the bed 13. The size of the corpuscle that have the right to be carried as suspended fill is dependent on the flow rate 11. Bigger particles are more likely to autumn through the upward currents come the bottom, uneven the flow rate increases, raising the disturbance at the streambed. In addition, suspended sediment will certainly not necessarily stay suspended if the circulation rate slows.
The wash fill is a subset the the suspended fill 13. This fill is made up of the best suspended sediment (typically less than 0.00195 mm in diameter). The wash fill is identified from the rely load due to the fact that it will certainly not settle to the bottom that a waterway throughout a low or no flow duration 11. Instead, these particles remain in permanent suspension together they are little enough to bounce off water molecules and also stay afloat 11. However, during flow periods, the wash load and suspended pack are indistinguishable.
Turbidity in lakes and slow relocating rivers is typically due the wash load 8. Once the circulation rate increases (increasing the exposed load and also overall sediment transport), turbidity also increases. While turbidity cannot be provided to estimate sediment transport, it have the right to approximate exposed sediment concentration at a particular location 14.
What is Sediment Deposition?
Sediment is important to the advance of aquatic ecosystems with nutrient replenishment and the creation of benthic habitat and spawning locations 10. These benefits occur as result of sediment deposition – once suspended particles clear up down to the bottom of a human body of water. This settling often occurs when water flow slows down or stops, and also heavy particles can no much longer be supported by the bed turbulence. Sediment deposition can be uncovered anywhere in a water system, native high hill streams, to rivers, lakes, deltas and also floodplains. However, it should be detailed that while sediment is crucial for aquatic habitat growth, that can reason environmental problems if the deposition prices are also high, or also low.
The suspended corpuscle that fall to the bottom the a water human body are called settleable solids 10. As they are found in riverbeds and also streambeds, these worked out solids are additionally known together bedded sediment 8. The dimension of settleable solids will vary by water device – in high circulation areas, larger, gravel-sized sediment will work out out first. Finer particles, including silt and clay, deserve to be brought all the way out come an estuary or delta 17.
In naval environments, virtually all rely sediment will settle. This is due to the presence of salt ion in the water. Salt ion bond come the rely particles, encouraging them to incorporate with various other particles in the water 15. As the cumulative weight increases, the sediment begins to sink to the seafloor. This is why oceans and also other naval ecosystems tend to have lower turbidity level (greater water clarity) 보다 freshwater environments 15.
While estuaries and also other tidal areas may be considered marine, they room not have to clearer 보다 freshwater. Estuaries room the collection point for suspended sediment coming down river. Furthermore, in a tidal zone, the continuous water movement reasons the bottom sediment come continually resuspend, staying clear of high water clarity during tidal durations 16. The clarity of one estuary will rely on its salinity level, as this will assist with particle deposition 17.
Why are Sediment Transport and also Deposition Important?
Many ecosystems benefit from sediment transport and also deposition, whether directly or indirectly. Sediment build aquatic habitats for spawning and benthic organisms 10. The is also responsible for giving nutrients to aquatic plants, as well vegetation in nearshore ecosystems such together floodplains and marshes 10. There is no sediment deposition, seaside zones can come to be eroded or nonexistent.
Sediment and also Aquatic Life
Sediment deposition creates habitats for aquatic life. While too lot sediment can be detrimental, too tiny sediment can also diminish ecosystem top quality 10. Part aquatic habitats are even grain-size specific. Countless spawning habitats need a specific sediment size (e.g. Gravel) and also too fine of sediment can end up smothering the eggs and also other benthic creatures 8.Sockeye salmon and also other fish require specific sediment products (like gravel) to produce its spawning bed (redd) to defend eggs there is no smothering them. (Photo Credit: Oregon room of Fish and also Wildlife)
Too much sediment deposition can also bury habitats and even physically transform a waterway. Extreme levels the suspended fill tend come have an unfavorable impacts top top aquatic life. Exposed sediment can prevent irradiate from getting to submerged vegetation and also clog fish gills 8. If a body of water is continuous exposed come high levels of sediment transport, it might encourage much more sensitive types to leave the area, if silt-tolerant organisms move in 8.
On the other hand, too tiny sediment transport deserve to lead come nutrient depletion in floodplains and marshes, diminishing the habitat and also vegetative growth 10. When water clarity is frequently heralded together a benchmark the water quality, low quantities of turbidity can protect aquatic types from predation 18. In addition, too tiny sediment deposition have the right to lead to the erosion the riverbanks and coastal areas, resulting in land loss and destroying the nearshore habitats 10, 18.
Where does Sediment Come From?
Sediment originates from geologic, geomorphic, and also organic determinants 10. The amount, material and also size that the transported sediment is a sum of these influences in any certain waterway. Sediment transported in rivers v headwaters indigenous a mountain variety often encompass glacial silt, if a human body of water surrounding by swampland will be inundated v decomposing organic material 23.
Sediment and also Geology
Many sediment particles room mineral-based. The precise nature of the sediment is dependency on location, and also the geology that that location 10. Glacial-type sediment is typical in hill ranges, while low-lying rivers are much more apt to collection soil-based sediment. In high-flow waterways, sediment transfer will incorporate local gravel, pebbles and small rocks. More tough rocks are much less likely to come to be sediment, if soft rocks erode quicker and also are easily carried away by flow water 13. The physical make-up of transported sediment is strongly affected by the geology that the neighboring environment.
Specific geologic aspects are frequently localized, such as basalt close to volcanic bowl boundaries, or limestone in in history shallow maritime regions 21. Sediment transfer is often responsible for intermixing this geologic attributes by carrying mineral particle much away from your origin. Mountains streams full of glacial silt have the right to transport the sediment every the means into a tidal bay 10. Likewise, rivers that run through agricultural regions can bring fertilized soil into the ocean 24.
Millions of year ago, sediment deposition aided to type many of these geologic features20. Sedimentary rocks such as sandstone and limestone, are developed by sediment deposits, which eventually become pressurized into rock 20. As soon as these rocks become re-exposed to water and also air, the sediment transport process can begin again.
Sediment and Geomorphology
Geomorphology refers to both the surface of the planet (terrain), and the processes acting on it (e.g. Wind and rain) 22. As defined earlier – sediment is the repertoire of corpuscle that deserve to be lugged away by wind, water and also ice. These particles can come indigenous the weathering of rocks and also the erosion that surface materials 19. Once wind, rain, glaciers and other aspects scour far a rock face, the particles are lugged away as sediment 10. Runoff can carry away peak soils, advertise the sediment into nearby streams and rivers.
In enhancement to the influence of wind and rain, sediment transport is also impacted by the regional topography 19. The lot of sediment the enters the water and also the distance that it travel is because of the terrain that a waterway runs through 13. Radical streams are less likely to add to the sediment load, together the channel is resistant to rapid erosion 13. These rivers, and also man-made channels with no sediment, are taken into consideration non-alluvial channels.
The majority of rivers however, space alluvial, or self-formed 13. Alluvial rivers and also streams develop their own path by moving sediment away. In an alluvial stream, the depth and breadth the the waterway will depend on the stamin of the water flow and the material that makes-up the channel limits 13. Rivers the run with soft soil commonly have a higher sediment transport load than rivers exposed come bedrock, as lot of the sediment load is taken from the sides and bottom the the channel. In addition to non-erodible bedrock terrains, very vegetated locations are much less subject come runoff erosion throughout flood events, as the roots of the plants organize the soil in ar 19.
In enhancement to the impacts that geomorphology has actually on sediment carry rates, the procedure itself dram a component in developing the terrain. As sediment is transported under stream, the water flow helps to form the planet’s surface by delivering away the eroded material away from some regions and depositing it in others 19.
Sediment and also Organic Factors
In addition to the mineral-based aspect, sediment deserve to be essential in source. Necessary sediment comes from decaying algae, plants, and also other organic material that falls in the water (such as leaves) 4. Bacteria attached come this detritus or other inorganic matter are additionally categorized as organic 18. Organic sediment deliver is will vary by location and season. In one estuarine study, the organic section of the rely load fell from 85% come 18% indigenous February come November as result of seasonal results on sediment deliver 18.
Some phytoplankton can play a unique duty in their donation to sediment loads. In enhancement to the organic element they provide, certain phytoplankton (such together diatoms) can contribute an not natural component also 1. This not natural material originates from diatom frustules and calcium carbonate detritus. If this product is not particularly organic, it is organic in beginning 1.
Factors that influence Sediment Transport
Sediment transfer is no constant. In fact, it is constantly topic to change. In addition to the transforms in sediment load as result of geology, geomorphology and organic elements, sediment transport deserve to be changed by other exterior factors. The alteration to sediment transport have the right to come from alters in water flow, water level, weather events and also human influence.
Water flow, also called water discharge, is the single most important element of sediment transport. The flow of water is responsible because that picking up, moving and depositing sediment in a waterway 26. There is no flow, sediment might remain rely or resolve out – however it will not relocate downstream. Flow is compelled to begin the transfer 18. There are two straightforward ways to calculation flow. Water discharge can be simplified as area (a cross-section that the waterway) multiply by velocity, or together a volume the water moved over time 25.
Flow (ft3/s) =Area (ft2) * Velocity (ft/s) OR circulation (ft3/s) =Volume (ft3)/ Time (s)
The equations explicate the partnership of water flow and sediment transport are a bit more complex. The intricacy of sediment transport prices are due to a big number that unknowns (e.g. Bed geometry, bit size, shape and also concentration), as well as multiple forces acting upon the sediment (e.g. Loved one inertia, rough eddies, velocity fluctuations in speed and also direction) 11. The sediment transport rate in specific is challenging to measure, as any measurement method will annoy the flow and also thus change the reading. Most circulation rate and sediment transport price equations effort to simplify the script by skip the effects of channel width, shape and also curvature the a channel, sediment cohesion and non-uniform operation 11.
The two main flow factors in sediment transport room the settling rate and the border layer shear anxiety 27. The settling rate (also referred to as Stokes settling) is the rate at i beg your pardon sediment falls through a liquid and it is managed by the drag force (keeping a fragment suspended) and also the gravitational force (a duty of the fragment size) 27. Knowledge this relationship helps to define some of the pressures that sediment transport has to overcome relative to fragment size.
Shear stresses in the border layer of a sediment bed explain how much force is compelled for water circulation to conquer relative inertia and also begin sediment carry (through bedload or suspended load) 27.
In a straightforward freshwater flow system, u∗ can be calculation as:
In the ocean and in various other more complex water systems, this equation is inadequate. Instead, the Von Karman-Prandlt equation should be used. The shear stress is influenced not just by the viscosity the the liquid, however the roughness that the sediment 27. The unstable eddies created at the bottom by water flow must additionally be accounting for. This is likewise known as the regulation of the wall surface 30.
The above equations assist to give a basic understanding of several of the forces acting on sediment in the water. To more understand the conditions required because that sediment transport, the Shields stress and anxiety equation have the right to be used. Shields stress, along with the particle Reynolds number, deserve to be used to predict exactly how much flow is forced for an extensive sediment transfer 27. The Reynolds number is one expression the a particle’s resistance come viscous pressure 28. In other words, the Reynolds number demonstrates whether or not a circulation is viscous sufficient to conquer the loved one inertia of sediment. For sediment transport, the Reynolds number for flow through a sediment bed deserve to be calculated from the boundary layer shear anxiety equation:
The point at i beg your pardon water flow starts to transfer sediment is referred to as the crucial Shields tension 27. This creates an empirical curve to approximate at what circulation rate a sediment particle will relocate (based on particle size) 27.
While this equations help define minimum flow rates because that sediment transportation, they carry out not recognize sediment load and sediment transport prices themselves. One sediment transport rate equation was occurred by valve Rijn, for the bedload deliver of particles between 0.2-2mm.
The suspended fill transport price (still assuming cohesionless sediment and also a sediment size of 0.2-2mm) is even much more complicated:
Other sediment rating curves have actually been developed, however they can not be equally used to all water body 13. This is because in any type of application, there space seven key variables that have actually an result on sediment transport prices 11,31.
The sediment transport rate is a duty of these 7 variables, as well as the size-shape-density distribution (often assumed as a standard deviation the the bit diameter) that the suspended particles 31. In addition, the largest river discharge does not automatically mean that a flow will have actually the largest sediment load. The quantity and also material the the sediment particles, and the location of the neighborhood terrain will certainly still play a contributing duty in the sediment pack 10.
The sediment fill itself is calculated together a depth-integrated sediment mass over a unit area 11. It is variable because that multiple reasons, yet can be approximated with a time-average collected sediment concentration 11. While the is dependency on flow to initiate and continue transport, the is no calculated from flow rates, as the key variables in sediment pack come from setting factors.
Weather Events and also Water Level
Sediment transport relies on water circulation to relocate a load downstream. Water circulation is variable, impacted not only by the neighborhood terrain (e.g. Slope), but by water level which, in turn, is influenced by precipitation (or absence thereof).
Most changes in water level are due to weather occasions such as rainfall 26. Precipitation causes water levels to initially rise, and then go back to previous level (base flow) over the food of hrs or days. Rainfall, whether slight or heavy can impact water flow and sediment transport. The extent to i m sorry a weather occasion will influence sediment deliver is dependence on the lot of sediment available. Snowmelt in a glaciated area will result in a high sediment load because of glacial silt 10. Heavy rainfall over an area of loose soil and also minimal vegetation will develop runoff, carrying loosened particles into the waterway. Likewise, flooding will also pick increase sediment native the local area. In fact, many of an waterway’s sediment load occurs during flood occasions 10.
Increased water level creates extr volume in a channel, and increases the hydraulic radius (cross-sectional area of a waterway). The enhanced hydraulic radius increases the discharge rate, nevertheless of whether or not circulation is uniform or non-uniform 31. Increased flow will rise the anxiety on the bed, making it an ext likely because that water flow to start sediment transport. The greater velocity also increases erosion prices as flow overcomes the shear stress of sediment 13.
Seasonal impacts are also responsible for changes in water level and flow 26. Many seasonal alters are because of precipitation levels and also events such as snowmelt. Throughout low precipitation and also low circulation periods, sediment transfer falls. Throughout the peak of snowmelt, the sediment load have the right to increase through a aspect of 15 or more 13. Climate readjust can likewise play a function in sediment transport, together it affects both the timing and magnitude of floods and also other weather events 10.
Anthropogenic factors, such together dams and also altered land use will impact both the sediment load and sediment transport price 10. Dams influence the water flow through finish detention or limited channels 26. The restricted flow can reason the channel downstream that the dam to end up being “sediment-starved”, if the sediment pack behind the dam builds up. A sediment-starved river will not it is in able to carry out habitats for benthic organisms or spawning fish 35. The very silted reservoir behind the dam might face concerns of too lot sediment, including alters in aquatic life and also the potential because that algal blooms. On the other side that the spectrum, when a dam relax occurs, the circulation rate downstream can dramatically increase. If the relax is controlled, it deserve to refresh the bed material, building bars and other habitat areas. An uncontrolled relax or dam removal can result in flooding, transferring the released sediment more downstream 보다 is necessary 10.
Human land use, together as urban areas, farming farms and also construction web page will impact the sediment load, however not the transport price 10. These impacts are indirect, as they require hefty rainfall or flooding to carry their sediment right into the waterway. However, anthropogenic soil use is among the leading contributors to extreme sedimentation due to erosion and also runoff 33. This increase occurs because “disturbed sites” (logging, mining, construction and farm sites) often expose or loosen peak soil by removing aboriginal vegetation 34. This loose soil is climate easily carried into a adjacent river or present by rainfall and runoff.
Consequences that Sediment Transport and Deposition
While sediment is necessary to develop aquatic habitats and reintroduce nutrients for submerged vegetation, too lot or too small sediment can easily reason ecosystem and also safety issues. Even if it is the involves are brought about by scour, erosion, develop up, or simply too much turbidity, the sediment transport price is an essential environmental variable 35. In addition to the problems reason by pack quantity, sediment can easily introduce pollution and other contaminants into a waterway, spreading the pollutants downstream 40.
Too lot Sediment
Large sediment lots are the most common issue seen with sediment transfer rates. Too lot sediment can cause poor water quality, algal blooms, and also deposition build-up. For aquatic life, extreme suspended sediment can disrupt natural aquatic migrations, as well damage gills and other offal 8, 37.
Diminished water quality occurs with unusually high sediment transport rates. Turbidity can reason water temperature to climb (sediment absorbs more solar warmth than water does) 1. Increasing water temperatures will cause dissolved oxygen levels to drop, as warm water cannot hold as much oxygen as cold water 37. Exposed sediment have the right to block sunlight from reaching submerged plants, to decrease photosynthesis rates and lowering liquified oxygen level still more 38. If the boost in the sediment fill is early out to agricultural and urban runoff, algal blooms can happen from the enhanced nutrient load brought into the water body 36.
Regular sediment deposition can construct bars because that aquatic habitats, but increased sedimentation can destroy more habitats than it creates. Siltation, the surname for fine sediment deposition, occurs once water circulation rates decrease dramatically. This good sediment deserve to then smother insect larvae, fish eggs and other benthic organisms together it settles the end of the water obelisk 1, 37. Deposition can likewise alter a waterway’s banks and direction as an unusually high sediment load settles out 35. Sediment deposition is responsible for producing alluvial fans and deltas, yet excessive accumulation of sediment can construct up channel plugs and levees. These deposits climate block the river from reaching various other stream object or floodplains 35. Boosted sedimentation is thought about one the the primary causes of habitat destruction 36. Depending upon the regional geology and terrain, sediment accumulation can damage aquatic ecosystems not only in downstream sites, yet in upstream headwaters together the deposits thrive 35.
Sediment deposition is thought about extreme as soon as it above the encourage or established total maximum everyday load (TMDL). A TMDL develops a border for measurable pollutants and also parameters because that a body of water 35. That means that TMDLs have the right to be developed for several different facets of the sediment load, including total suspended solids, nutrient impairment, pathogens and siltation 36.When arising a TMDL report, it is essential to take into consideration whether or no the waterway chin is generating the sediment fill naturally, as an stormy stream channel 36.
Too tiny Sediment
Though too lot sediment is the more common concern, a lack of sediment carry will likewise cause ecological issues. Sediment scarcity is regularly caused by synthetic structures such as dams, despite natural obstacles can additionally limit sediment move 8. There is no sediment transport and deposition, new habitats cannot be formed, and without part nutrient enrichment (carried v sediment right into the water), submerged vegetation might not grow 8. Too tiny sediment can transform an ecosystem to the point that native species cannot survive.
In addition to the impact on aquatic life, the lose of sediment transport and deposition can cause physical transforms to the terrain. Downstream of dammed rivers, the is usual to check out receding riparian zones and wetlands as result of the lose of transported sediment 8. Erosion downstream that a obstacle is common, together is coastline erosion once there is no a large enough sediment load at this time carried by the water 32. The flowing water will certainly pick up brand-new sediment native the bottom and banks the a waterway (eroding rather of update habitats) together it make the efforts to readjust to a uniform flow rate 11.
Contaminated SedimentShipyards and other suggest sources deserve to pollute a human body of water. These contaminants may settle come the bottom and be released slowly over time, or be carried away with other sediment.
Contaminated sediments room the accumulated riverbed materials that contain toxic or hazardous building materials that space detrimental come aquatic, human or eco-friendly health 39. These contaminants often come indigenous point-source pollution (such as industrial wastewater or other effluent sources), though they can also enter the water through runoff end contaminated soils (mine waste, landfills and also urban areas), chemistry spills, or deposits indigenous air pollution 39.
As contaminants carry out not degrade (or degrade very slowly), they can be a source of environmental issues for long periods that time, also if they space not typically resuspended 39. The most problematic contaminants in both bedded and suspended sediment are metals and also persistent bioaccumulative toxics (PBTs), such as pesticides and methyl mercury 39.
Sediment remediation might involve dredging to remove the contaminated sediment indigenous the waterway 40.
When sediment transport clears material indigenous a streambed or bank, the erosion procedure is called scour 41. Scour can occur all over that over there is water flow and also erodible material. Local scour is the design term because that the isolated removed of sediment in ~ one location, such as the basic of underwater structures, consisting of bridge piers and also abutments 42. This localized erosion can cause structural failure, together bridges and overwater constructions count on the bed sediment to assistance them.
While scour can occur anywhere, that is much more likely to take place in alluvial waterways (erodible bed and banks), as opposed come a bedrock-based (nonalluvial) channel 41. Together water circulation is responsible for conducting sediment transport, scour can take place even throughout low flow conditions. However, an important bridge scour conditions typically occur during periods the high flow, together as throughout a flood occasion 41. The greater flow rate have the right to pick up much more sediment, and also turbulence regularly occurs at the base of a pier as it interrupts and speeds up the flow. This turbulence consequently will increase the pressures acting top top a streambed, suspending extr particles and initiating better sediment transport 41. If too lot sediment is removed, the structure deserve to collapse. Scour due to flood-initiated sediment move is the many common reason of bridge failure in the United says 42.
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Cite This Work
smashville247.net Environmental, Inc. “Sediment Transport and also Deposition.” Fundamentals of environmental Measurements. 5 Dec. 2014. Web. .