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In 1949 the Coast Protection Act provided legislation that gave a Coastal Protection Authority the power to carry out whatever work they deemed necessary for the protection of any land in their area. In North Norfolk, responsibility for coast protection was with the Urban and Rural District Councils of Wells, Walsingham, Sheringham, Erpingham, Cromer and Smallburgh.

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Professor I. Shennan - Durham University

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This work started in the 1980s, with first paper, published 1987, quantifying the regional-scale pattern of uplift and subsidence around the North Sea. This was followed, in 1989, Journal of Quaternary Science, by the first analysis for 35 years to provide a quantitative estimate of current land uplift and subsidence in mainland Great Britain to be supported by the geological and oceanographic data.

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During and after an ice age, eustatic change takes place. At the beginning of an ice age, the temperature falls and water is frozen and stored in glaciers inland, suspending the hydrological cycle. This results in water being taken out of the sea but not being put back in leading to an overall fall in sea level.

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Rising Seas and Coastal Erosion (SUMMARY)

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Rising Seas and Coastal Erosion (SUMMARY)

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J. Titus

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Rolling easements could allow wetlands and beaches to migrate inland as sea level rises, without seriously harming property owners.

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Zhang

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James Titus

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Global Warming affect on Coastal Erosion

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Global Warming affect on Coastal Erosion

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Climate change and coastal erosion | Climate change | Discovering Geology | British Geological Survey (BGS)

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Weathering, Erosion, and Mass-Wasting Processes - ppt download

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Table of Contents (1 of 2) Definitions: Weathering, Erosion, and Mass-Wasting (slide 4) (Standards: ; 5-3.1) Types of Weathering (slide 5) (Standards: ; 5-3.1) Mechanical Weathering (slide 6) (Standards: ; 5-3.1) Exfoliation (slide 7) (Standards: ; 5-3.1) Frost Wedging (slide 8) (Standards: ; 5-3.1) Temperature Change (slide 9) (Standards: ;

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Chapter Overview

1. What is Sediment?
-Suspended or Bedded?
-Suspended Sediment vs. Suspended Solids

2. What is Sediment Deposition?
-Settleable Solids

3. Why is Sediment Transport and Deposition Important?
-Sediment and Aquatic Life

4. Where does Sediment Come From?
-Sediment and Geology
-Sediment and Geomorphology
-Sediment and Organic Factors

5. Factors that Influence Sediment Transport
-Water Flow
-Weather Events and Water Level
-Human Influence

6. Consequences of Sediment Transport and Deposition
-Too Much Sediment
-Too Little Sediment
-Contaminated Sediment
-Scour

7. Sediment Transport Measurement Methods

8. Scour Monitoring Methods and Equipment

9. Turbidity and Total Suspended Solids Measurement Methods

10. Integrated Monitoring System

11. Applications

What is Sediment?

Suspended or Bedded?

In an aquatic environment, sediment can either be suspended (floating in the water column) or bedded (settled on the bottom of a body of water). When both floating and settled particles are monitored, they are referred to as SABS: Suspended And Bedded Sediments ⁴.

Suspended Sediment vs Suspended Solids

Fine sediment can be found in nearly any body of water, carried along by the water flow. When the sediment is floating within the water column it is considered suspended. In this application, the terms "suspended sediment" and "suspended solids" are nearly interchangeable. The main difference between the two is in the method of measurement ².

Despite the similarity in meaning, the data provided by the different measurement methods are neither interchangeable nor comparable ². The suspended sediment concentration (SSC) is in mg/L by filtering and drying an entire water sample. Total suspended solids (TSS), while also measured in mg/L, are obtained by subsampling. While acceptable for homogenized or well mixed samples with very fine sediment, the TSS measurement often excludes larger suspended particles, like sand ². This means that the SSC measurement tends to be higher and more representative of a water body as a whole, often measuring within 5% of the true particle concentration ⁷. Due to the incomparability between suspended sediment measurements and total suspended solids measurements, the U.S. Geological Survey recommends SSC analysis over TSS when sampling in surface water ².

What is Sediment Transport?

Sediment transport is the movement of organic and inorganic particles 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 move downstream, or simply push them along the bottom of a waterway ¹¹. Transported sediment may include mineral matter, chemicals and pollutants, and organic material.

Another name for sediment transport is sediment load. The total load includes all particles moving as bedload, suspended load, and wash load ¹¹.

Bedload

Bedload is the portion of sediment transport that rolls, slides or bounces along the bottom of a waterway ¹². This sediment is not truly suspended, as it sustains intermittent contact with the streambed, and the movement is neither uniform nor continuous ¹¹. Bedload occurs when the force of the water flow is strong enough to overcome the weight and cohesion of the sediment ¹². While the particles are pushed along, they typically do not move as fast as the water around them, as the flow rate is not great enough to fully suspend them ¹¹. Bedload transport can occur during low flows (smaller particles) or at high flows (for larger particles). Approximately 5-20% of total sediment transport is bedload ¹⁰. In situations where the flow rate is strong enough, some of the smaller bedload particles can be pushed up into the water column and become suspended.

Suspended Load

Wash Load

Turbidity in lakes and slow moving rivers is typically due the wash load ⁸. When the flow rate increases (increasing the suspended load and overall sediment transport), turbidity also increases. While turbidity cannot be used to estimate sediment transport, it can approximate suspended sediment concentrations at a specific location ¹⁴.

What is Sediment Deposition?

Sediment is necessary to the development of aquatic ecosystems through nutrient replenishment and the creation of benthic habitat and spawning areas ¹⁰. These benefits occur due to sediment deposition - when suspended particles settle down to the bottom of a body of water. This settling often occurs when water flow slows down or stops, and heavy particles can no longer be supported by the bed turbulence. Sediment deposition can be found anywhere in a water system, from high mountain streams, to rivers, lakes, deltas and floodplains. However, it should be noted that while sediment is important for aquatic habitat growth, it can cause environmental issues if the deposition rates are too high, or too low.

Settleable Solids

In marine environments, nearly all suspended sediment will settle. This is due to the presence of salt ions in the water. Salt ions bond to the suspended particles, encouraging them to combine with other particles in the water ¹⁵. As the collective weight increases, the sediment begins to sink to the seafloor. This is why oceans and other marine ecosystems tend to have lower turbidity levels (greater water clarity) than freshwater environments ¹⁵.

While estuaries and other tidal areas may be considered marine, they are not necessarily clearer than freshwater. Estuaries are the collection point for suspended sediment coming down river. Furthermore, in a tidal zone, the constant water movement causes the bottom sediment to continually resuspend, preventing high water clarity during tidal periods ¹⁶. The clarity of an estuary will depend on its salinity level, as this will assist with particle deposition ¹⁷.

Why are Sediment Transport and Deposition Important?

Many ecosystems benefit from sediment transport and deposition, whether directly or indirectly. Sediment builds aquatic habitats for spawning and benthic organisms ¹⁰. It is also responsible for providing nutrients to aquatic plants, as well vegetation in nearshore ecosystems such as floodplains and marshes ¹⁰. Without sediment deposition, coastal zones can become eroded or nonexistent.

Sediment and Aquatic Life

Sediment deposition creates habitats for aquatic life. While too much sediment can be detrimental, too little sediment can also diminish ecosystem quality ¹⁰. Some aquatic habitats are even grain-size specific. Many spawning habitats require a specific sediment size (e.g. gravel) and too fine of sediment can end up smothering the eggs and other benthic creatures ⁸.

Too much sediment deposition can also bury habitats and even physically alter a waterway. Excessive levels of suspended load tend to have negative impacts on aquatic life. Suspended sediment can prevent light from reaching submerged vegetation and clog fish gills ⁸. If a body of water is continually exposed to high levels of sediment transport, it may encourage more sensitive species to leave the area, while silt-tolerant organisms move in ⁸.

Where Does Sediment Come From?

Sediment comes from geologic, geomorphic, and organic factors ¹⁰. The amount, material and size of the transported sediment is a sum of these influences in any particular waterway. Sediment transported in rivers with headwaters from a mountain range often include glacial silt, while a body of water surrounded by swampland will be inundated with decomposing organic material ²³.

Sediment and Geology

Many sediment particles are mineral-based. The exact nature of the sediment is dependent on location, and the geology of that location ¹⁰. Glacial-type sediment is common in mountain ranges, while low-lying rivers are more apt to collect soil-based sediment. In high-flow waterways, sediment transport will include local gravel, pebbles and small rocks. Harder rocks are less likely to become sediment, while soft rocks erode quicker and are easily carried away by flowing water ¹³. The physical make-up of transported sediment is strongly influenced by the geology of the surrounding environment.

Millions of years ago, sediment deposition helped to form many of these geologic features ²⁰. Sedimentary rocks such as sandstone and limestone, are created by sediment deposits, which eventually become pressurized into stone ²⁰. Once these rocks become re-exposed to water and air, the sediment transport process can begin again.

Sediment and Geomorphology

In addition to the effects that geomorphology has on sediment transport rates, the process itself plays a part in creating the terrain. As sediment is transported down stream, the water flow helps to shape the planet's surface by carrying away the eroded material away from some regions and depositing it in others ¹⁹.

Sediment and Organic Factors

Some phytoplankton can play a unique role in their contribution to sediment loads. In addition to the organic factor they provide, specific phytoplankton (such as diatoms) can contribute an inorganic component as well ¹. This inorganic material comes from diatom frustules and calcium carbonate detritus. While this material is not specifically organic, it is organic in origin ¹.

Factors that Influence Sediment Transport

Sediment transport is not constant. In fact, it is constantly subject to change. In addition to the changes in sediment load due to geology, geomorphology and organic elements, sediment transport can be altered by other external factors. The alteration to sediment transport can come from changes in water flow, water level, weather events and human influence.

Water Flow

The equations describing the relationship of water flow and sediment transport are a bit more complex. The complexity of sediment transport rates are due to a large number of unknowns (e.g. bed geometry, particle size, shape and concentration), as well as multiple forces acting upon the sediment (e.g. relative inertia, turbulent eddies, velocity fluctuations in speed and direction) ¹¹. The sediment transport rate in particular is difficult to measure, as any measurement method will disturb the flow and thus alter the reading. Most flow rate and sediment transport rate equations attempt to simplify the scenario by ignoring the effects of channel width, shape and curvature of a channel, sediment cohesion and non-uniform flows ¹¹.

The two main flow factors in sediment transport are the settling rate and the boundary layer shear stress ²⁷. The settling rate (also called Stokes settling) is the rate at which sediment falls through a liquid and it is controlled by the drag force (keeping a particle suspended) and the gravitational force (a function of the particle size) ²⁷. Understanding this relationship helps to define some of the forces that sediment transport has to overcome relative to particle size.

vs = (g * (ρp - ρf) * Dp ²) / 18μ
v s = settling velocity
g = gravitational constant
ρ p = particle density
ρ f = fluid density
D p = particle diameter
μ= fluid viscosity ²⁹

Shear stresses in the boundary layer of a sediment bed explain how much force is required for water flow to overcome relative inertia and begin sediment transport (through bedload or suspended load) ²⁷.

τ = ρf * u∗²
τ = shear stress
ρ f = fluid density
u∗ = characteristic velocity of turbulent flow (shear velocity) (see following equations) ²⁷

In a basic freshwater river system, u∗ can be calculated as:

u∗ = Sqrt(g * h * S)
u∗ = shear velocity
g = gravitational constant
h = river depth
S = river slope ²⁷

In the ocean and in other more complex water systems, this equation is inadequate. Instead, the Von Karman-Prandlt equation should be used. The shear stress is influenced not only by the viscosity of the liquid, but the roughness of the sediment ²⁷. The turbulent eddies created at the bottom by water flow must also be accounted for. This is also known as the Law of the Wall ³⁰.

u/u∗ = (1/κ) * ln(z/z₀)
u = averaged flow velocity
u∗ =shear velocity
κ = Von Karman's constant (0.4)
z = roughness height above the bed
z ₀ = roughness height as flow velocity approaches zero ³⁰

The above equations help to give a basic understanding of some of the forces acting on sediment in the water. To further understand the conditions required for sediment transport, the Shields stress equation can be used. Shields stress, along with the particle Reynolds number, can be used to predict how much flow is required for substantial sediment transport ²⁷. The Reynolds number is an expression of a particle's resistance to viscous force ²⁸. In other words, the Reynolds number demonstrates whether or not a flow is viscous enough to overcome the relative inertia of sediment. For sediment transport, the Reynolds number for flow through a sediment bed can be calculated from the boundary layer shear stress equation:

Rep = (u∗ * Dp) / ν
Re p = Reynolds number of the particle
u∗ = characteristic velocity of turbulent flow (shear velocity)
D p = particle diameter
ν = kinematic viscosity (viscosity/ fluid density, (μ/ρ f)) ²⁷

The point at which water flow begins to transport sediment is called the critical Shields stress ²⁷. This creates an empirical curve to approximate at what flow rate a sediment particle will move (based on particle size) ²⁷.

τ∗ = τ /(g * (ρp - ρf) * Dp)
τ∗ = Shields stress
τ = shear stress
g = gravitational constant
ρ p = particle density
ρ f = density of fluid
D p = particle diameter ¹³

While these equations help define minimum flow rates for sediment transportation, they do not determine sediment load and sediment transport rates themselves. One sediment transport rate equation was developed by van Rijn, for the bedload transport of particles between 0.2-2mm.

qb = 0.053 * [(s-1)*g]0.5 * d₅₀ 1.5 * [T∗2.1 / D∗0.3]
q b = bedload transport rate
s= specific density of sediment
g = gravitational constant
d ₅₀ = median particle diameter
T∗ = transport stage parameter
D∗ = dimensionless grain size ¹⁸

The suspended load transport rate (still assuming cohesionless sediment and a sediment size of 0.2-2mm) is even more complicated:

qs = u * h * ca * [ ((a/h)Z' - (a/h)1.2) / ((1-a/h)Z' * (1.2-Z')) ]
q s= suspended load transport rate
u = average flow velocity
h = average flow depth
c a= reference concentration
a = height above the bed, relative to particle size
Z' = suspension number ¹⁸

Other sediment rating curves have been developed, but they cannot be equally applied to all water bodies ¹³. This is because in any application, there are seven main variables that have an effect on sediment transport rates 11,31.

qs = f (τ, h, D, ρp, ρf, μ, g)
q s = sediment transport rate per unit width
τ = shear stress
h = depth
D = particle diameter
ρ p = particle density
ρ f = fluid density
μ = water viscosity
g = gravitational constant

The sediment transport rate is a function of these seven variables, as well as the size-shape-density distribution (often assumed as a standard deviation of the particle diameter) of the suspended particles ³¹. In addition, the largest river discharge does not automatically mean that a river will have the largest sediment load. The quantity and material of the sediment particles, as well as the geography of the local terrain will still play a contributing role in the sediment load ¹⁰.

The sediment load itself is calculated as a depth-integrated sediment mass above a unit area ¹¹. It is variable for multiple reasons, but can be estimated with a time-average collected sediment concentration ¹¹. While it is dependent on flow to initiate and continue transport, it is not calculated from flow rates, as the main variables in sediment load come from environment factors.

Weather Events and Water Level

Sediment transport relies on water flow to move a load downstream. Water flow is variable, affected not only by the local terrain (e.g. slope), but by water level which, in turn, is influenced by precipitation (or lack thereof).

Increased water level creates additional volume in a channel, and increases the hydraulic radius (cross-sectional area of a waterway). The increased hydraulic radius increases the discharge rate, regardless of whether or not flow is uniform or non-uniform ³¹. Increased flow will increase the stress on the bed, making it more likely for water flow to initiate sediment transport. The higher velocity also increases erosion rates as flow overcomes the shear stress of sediment ¹³.

Human Influence

Anthropogenic factors, such as dams and altered land use will affect both the sediment load and sediment transport rate ¹⁰. Dams affect the water flow through complete detention or restricted channels ²⁶. The restricted flow can cause the channel downstream of the dam to become "sediment-starved", while the sediment load behind the dam builds up. A sediment-starved river will not be able to provide habitats for benthic organisms or spawning fish ³⁵. The highly silted reservoir behind the dam may face issues of too much sediment, including changes in aquatic life and the potential for algal blooms. On the other side of the spectrum, when a dam release occurs, the flow rate downstream can dramatically increase. If the release is controlled, it can refresh the bed material, building bars and other habitat areas. An uncontrolled release or dam removal can result in flooding, carrying the released sediment further downstream than is needed ¹⁰.

Consequences of Sediment Transport and Deposition

While sediment is needed to build aquatic habitats and reintroduce nutrients for submerged vegetation, too much or too little sediment can easily cause ecosystem and safety issues. Whether the concerns are caused by scour, erosion, build up, or simply excessive turbidity, the sediment transport rate is an important environmental factor ³⁵. In addition to the problems cause by load quantity, sediment can easily introduce pollution and other contaminants into a waterway, spreading the pollutants downstream ⁴⁰.

Too Much Sediment

Large sediment loads are the most common issue seen with sediment transport rates. Too much sediment can cause poor water quality, algal blooms, and deposition build-up. For aquatic life, excessive suspended sediment can disrupt natural aquatic migrations, as well damage gills and other organs 8, 37.

Diminished water quality occurs with unusually high sediment transport rates. Turbidity can cause water temperatures to rise (sediment absorbs more solar heat than water does) ¹. Rising water temperatures will cause dissolved oxygen levels to drop, as warm water cannot hold as much oxygen as cold water ³⁷. Suspended sediment can block sunlight from reaching submerged plants, decreasing photosynthesis rates and lowering dissolved oxygen levels still further ³⁸. If the increase in the sediment load is due to agricultural and urban runoff, algal blooms can occur from the increased nutrient load carried into the water body ³⁶.

Regular sediment deposition can build bars for aquatic habitats, but increased sedimentation can destroy more habitats than it creates. Siltation, the name for fine sediment deposition, occurs when water flow rates decrease dramatically. This fine sediment can then smother insect larvae, fish eggs and other benthic organisms as it settles out of the water column 1, 37. Deposition can also alter a waterway's banks and direction as an unusually high sediment load settles out ³⁵. Sediment deposition is responsible for creating alluvial fans and deltas, but excessive accumulation of sediment can build up channel plugs and levees. These deposits then block the river from reaching other stream threads or floodplains ³⁵. Increased sedimentation is considered one of the primary causes of habitat degradation ³⁶. Depending on the local geology and terrain, sediment build-up can damage aquatic ecosystems not only in downstream sites, but in upstream headwaters as the deposits grow ³⁵.

Too Little Sediment

Though too much sediment is the more common concern, a lack of sediment transport will also cause environmental issues. Sediment starvation is often caused by man-made structures such as dams, though natural barriers can also limit sediment transport ⁸. Without sediment transport and deposition, new habitats cannot be formed, and without some nutrient enrichment (carried with sediment into the water), submerged vegetation could not grow ⁸. Too little sediment can alter an ecosystem to the point that native species cannot survive.

Contaminated Sediment

Contaminated sediments are the accumulated riverbed materials that contain toxic or hazardous substances that are detrimental to aquatic, human or environmental health ³⁹. These contaminants often come from point-source pollution (such as industrial wastewater or other effluent sources), though they can also enter the water through runoff over contaminated soils (mine waste, landfills and urban areas), chemical spills, or deposits from air pollution ³⁹.

As contaminants do not degrade (or degrade very slowly), they can be a source of environmental issues for long periods of time, even if they are not frequently resuspended ³⁹. The most problematic contaminants in both bedded and suspended sediment are metals and persistent bioaccumulative toxics (PBTs), such as pesticides and methyl mercury ³⁹.

Sediment remediation may involve dredging to remove the contaminated sediment from the waterway ⁴⁰.

Scour

When sediment transport removes material from a streambed or bank, the erosion process is called scour ⁴¹. Scour can occur anywhere that there is water flow and erodible material. Local scour is the engineering term for the isolated removal of sediment at one location, such as the base of underwater structures, including bridge piers and abutments ⁴². This localized erosion can cause structural failure, as bridges and overwater constructions rely on the bed sediment to support them.

While scour can occur anywhere, it is more likely to occur in alluvial waterways (erodible bed and banks), as opposed to a bedrock-based (nonalluvial) channel ⁴¹. As water flow is responsible for conducting sediment transport, scour can occur even during low flow conditions. However, critical bridge scour conditions typically occur during periods of high flow, such as during a flood event ⁴¹. The higher flow rate can pick up more sediment, and turbulence often occurs at the base of a pier as it interrupts and accelerates the flow. This turbulence in turn will increase the forces acting on a streambed, suspending additional particles and initiating greater sediment transport ⁴¹. If too much sediment is removed, the structure can collapse. Scour due to flood-initiated sediment transport is the most common cause of bridge failure in the United States ⁴².

Cite this work:
Fondriest Environmental, Inc. "Sediment Transport and Deposition." Fundamentals of Environmental Measurements. 5 Dec. 2014. Web. < http://www.fondriest.com/environmental-measurements/parameters/hydrology/sediment-transport-deposition/ >.

Additional information:

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Sediment Transport and Deposition - Environmental Measurement Systems

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Chapter Overview What is Sediment? -Suspended or Bedded? -Suspended Sediment vs. Suspended Solids What is Sediment Deposition? -Settleable Solids Why is Sediment Transport and Deposition Important? -Sediment and Aquatic Life Where does Sediment Come From?