Weathering, Erosion and Sedimentation Lecture Notes

Introductory Engineering Geology

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4.

4.1 Weathering

Weathering refers to physical and chemical processes that change the characteristics of rock

It is the process

break down and change. Also, weathering may be defined as the process of alteration of

rocks occurring under the direct influence of the hydrosphere and atmosphere. Weathering

creates smaller and smaller pieces of rock called sediment. Examples of sediment are mud,

sand, or silt, which are very fine particles of rock.

Rock weathering is important in geotechnical engineering since it concerns the behaviour of

materials used as embankment fill, concrete, roads, or buildings. It is also concerned with the

behaviour of weathered materials in rock structure.

4.1.1 Weathering processes

The geological work accomplished by weathering is of two kinds:

(a) Physical or mechanical changes: This is the situation whereby materials are

disintegrated by temperature changes, frost action, and organisms and;

(b) Chemical changes: This is the situation whereby minerals are decomposed, dissolved,

and loosened by the water, oxygen, and carbon dioxide of the atmosphere and by organisms

and the products of their decay.

As a result of the changes, we have three types of weathering

(1) Mechanical weathering

(2) Chemical weathering

(3) Biological Weathering

4.1.1.1 Mechanical weathering

This is the process by which rocks break down into smaller sizes without any

weathering. It is highly dependent on the rock type and the time. The following

factors acting for significant period of time may cause mechanical weathering:

(i) Climatic effect: This includes both temperature and rainfall; daily temperature fluctuation

and more importantly freeze thaw-cycle over a long period of time. Temperature plays a

significant role in mechanical weathering. When temperature drops to freezing point of water,

water that fills the cracks of rocks layers freezes, expands and exerts pressure on the rocks

and may cause them to split. Also, when temperature rises, the ice in the cracks of rocks


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melts. The repeated thawing and freezing of water in the cracks of rocks called frost wedging

can split the rocks.

(ii) Abrasion: Whenever there is pushing of large quantity of soil or ice under pressure

across the underlying rock by glaciers, grinding or abrading both materials to smaller size.

(iii) Organic activity: Cracking force exerted by plant roots and roots in crack and crevasse

of rock can break rocks into fragments. As the roots grow and expand they can exert pressure

on the rocks to split.

(iv) Pressure: Bedrock at great depths is under pressure from overlying rock layers. When

the overlying rock layers are removed, the pressure on the bedrock is reduced. The bedrock

surface, formerly buried is then able to expand and long curved cracks can form. These

cracks, also known as joints occur parallel to the surface of the rock. Reduction of the

pressure allows existing cracks in the bedrock to widen. Over time, the outer layers of the

rocks are stripped (exfoliation) away in succession.

The processes most commonly involved in mechanical weathering are listed in Table 4.1

Table 4.1: The processes of mechanical weathering

Mechanical Unloading Vertical expansion due to the reduction of vertical load by

erosion. This will open existing fractures and may permit the

creature of new fractures

Mechanical Loading Impact on rock and abrasion, by sand and silt size windborne

particles in deserts. Impact on soil and weak rocks by rain drops

during intense rainfall storms

Thermal Loading Expansion by freezing of water in pores and fractures in cold

regions, or by the heating of rocks in hot regions. Contraction

by the cooling of rocks and soils in the cold regions.

Wetting and Drying Expansion and contraction associated with the repeated

absorption and loss of water molecules from mineral surfaces

and structures

Crystallization Expansion of pores and fissures by crystallization within them

of minerals that were originally in solution. Note: expansion is

only severe when crystallization occurs within a confined space.

Pneumatic Loading The repeated loading by waves of air trapped at the head of

fractures exposed in the wave zone of a sea cliff


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4.1.1.2 Chemical weathering

The process by which rocks undergo changes in their composition as the result of

chemical reactions is called chemical weathering. It is the decomposition whereby one

mineral species is changed into another through various chemical processes. This

changes happen because of elements in the air or water and the minerals in the rock

interact. The chemical reactions result in the formation of new minerals. The new

minerals have different properties from those of the original rocks. Agents of chemical

weathering include water, oxygen, carbon dioxide and acids.

Some minerals react with oxygen in the air and begin to crumble. Sometimes minerals are

combined with water or carbon dioxide to form weak acids. The acids break down the rock.

Climate has a great affect on rocks. Climates that are warm and moist will produce more

chemical weathering than cool, dry areas will. Rocks in cold and dry or hot and dry areas

generally experience greater mechanical weathering.

The alteration and solution of rock material by chemical processes is largely accomplished by

rain water acting as a carrier of dissolved oxygen and carbon dioxide together with various

acids and organic products derived from the soil.

The degree of activity depends on:

(1) the composition and concentration of the solutions so formed

(2) the temperature

(3) the presence of bacteria and

(4) the substances taken into solution from the mineral decomposed.

Water: Water is an important agent in chemical weathering because it can dissolve many

kinds of minerals and rocks. Water has an active role in some reactions, while it simply

serves as a medium through which other reactions occur. The reactions of water with other

substances (hydrolysis) occur in the decomposition of silicate minerals, such as

decomposition of potassium feldspar into kaolinite, a fine-grained clay mioneral common in

soils.

Oxygen: Like water, oxygen combines with other subsatances in a process called oxidation..

For example, Iron in rocks and minerals readily combines with atmospheric oxygen to form

minerals (hematite)

2fe3O4 + O2 3fe2O3

Magnetite Hematite


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Carbon dioxide: Carbon dioxide which is produced by living organisms during the process

of respiration combines with water in the atmosphere, to form a weak carbonic acid (see the

H2O + CO2 H2CO3

Water Carbondioxide Carbonic acid

Carbonic acid reacts with minerals such as calcite in limestone and marble to dissolve rocks.

Carbonic acids can also affects silicate minerals such as mica, and feldspar by reacting with

elements in minerals such as magnesium and calcium.High concentrations of carbonic acid

accumulate in soil, where decaying organic matter and plant respiration produce high levels

of carbon dioxide. When water from precipitation seeps into the ground and combines with

carbon dioxide, large amounts of carbonic acid become available for the process of chemical

weathering.

Acid precipitation: Another agent of chemical weathering which is caused mainly by the

oxidation of sulfur dioxide and nitro oxides that are released by human activities. Sulfur

dioxide forms from the industrial burning of fossil fuels, while nitrogens are emitted from

motor-vehicle exhaust. These two gases combine to form sulfuric and nitric acids

The processes most commonly involved in chemical weathering are listed in Table 4.2.

Table 4.2: Some commonly occurring processes in chemical weathering

Solution Dissociation of minerals into ions greatly aided by the presence of CO2 in the

soil profile, which forms carbonic acid (H2CO3) with percolating rainwater.

Oxidation The combination of oxygen with a mineral to form oxides and hydroxides

and any other reaction in which the oxidation number of the oxidized

elements is increased.

Reduction The release of oxygen from a mineral to its surrounding environment: ions

leave the mineral structure as the oxidation number of the reduced elements

is decreased.

Hydration Absorption of water molecules into the mineral structure. Note: this normally

results in expansion, some clay expand as much as 60%, and by admitting

water hasten the processes of solution, oxidation, reduction and hydrolysis.

Hydrolysis Hydrogen ions in percolating water replace mineral cations: no oxidation-

reduction occurs.

Leaching The migration of ions produced by the above processes. Note: the mobility of

ions depends upon their ionic potential: Ca, Mg, Na, K are easily leached by


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moving water, Fe is more resistant, Si is difficult to leach and Al is almost

immobile.

Cation

Exchange

Absorption onto the surface of negatively charged clay of positively charged

cations in solution, especially Ca, H, K, Mg.

4.1.1.3 Biological weathering

Weathering effects which are small in themselves but noticeable in the aggregate can be

attributed to plants and animals (biotic weathering). Plants retain moisture and any rock

surface on which they grow is kept damp, thus promoting the solvent action of the water. The

chemical decay of rocks is also aided by the formation of vegetative humus, i.e. organic

products derived from plants, and this helped by the action of bacteria and fungi. Organic

acids are thereby added to percolating rain-water and increase its solvent power. Some

bacteria are active I reducing conditions, and contribute to the making of sulphides; others

can convert nitrogen to NH4 compounds which affect the pH value of soils.

The mechanical break down of rocks is brought about when the roots of plants

penetrate into cracks and wedge apart the walls of the crack.

4.1.2 Factors affecting the rate of weathering

The natural weathering of Earth materials occurs very slowly, for example it may take to

weather 1 cm of limestone, and yet most rocks weather at even slower rates. Certain

conditions and interactions can accelerate or slow weathering process. They include climate,

rock type and composition, surface area, and topography.

Climate

The climate of an area is a major influence on the rate of chemical weathering. Variables of

climates include precipitation, temperature and evaporation. The interaction between

Chemical weathering occurs readily in climates with warm temperature, abundant rainfall and

lush vegetation. These climatic conditions produce thick soils that are rich in organic matter.

When water from heavy rainfalls combines with the carbon dioxide in this organic matter to

produce high level of carbonic acid. Conversely, physical weathering occurs readily in cool,

dry climates. Physical weathering rates are highest in areas where water undergoes repeated

freezing and thawing. Conditions in such climates do not favour chemical weathering as cool

temperatures slow or inhibit chemical reactions.


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Rock type and composition:

The characteristics of rocks including how hard or resistant they are to being broken down

depend on their type and composition. In general, sedimentary rocks are more easily

weathered than igneous or metamorphic rocks.

Surface area

Mechanical weathering breaks up rocks into smaller pieces. As the pieces get smaller, their

surface area increases. This means that more total surface area is available for chemical

weathering. Thus, the greater the surface area, the more weathering occurs.

Topography

Earth materials cover the surfaces of slopes and level areas. Materials on level areas are likely

to remain in place as they undergo changes, whereas materials on slopes have a greater

tendency to move as a result of gravity. As materials move down a slope, it exposes

underlying rock surfaces and thus provides more opportunities for weathering to occur.

4.2 Erosion

Erosion is the process by which the land surface is worn away by the action of wind, water,

ice, or gravity. In simple terms, it is the process where soil particles are dislodged or detached

and put in motion. Erosion can also be defined as the process by which Earth materials are

transported from one place to another. A number of agents transport weathered materials.

These include running water in stream and rivers, glaciers, winds, ocean currents and waves.

Rivers, wind, moving ice ad water waves are capable of loosening, dislodging and carrying

particles of soil, sediment and larger pieces of rocks. Humans, plants and animals also play a

role in erosional process. Weathering prepares the rock surface for erosion, that is, the

removal of decomposed or disintegrated materials by the agents earlier mentioned. At the

point that the movement of transported slows down, the materials are deposited in another

location in a process called Deposition. Erosion reshapes landforms, coastal regions, and

riverbeds and banks.

4.2.1 Erosion Process

There are two types of erosion: geologic and accelerated.

Geologic erosion or natural erosion is the action of the wind, water, ice, and gravity in

wearing away rock to form soil and shape the ground surface. Except for some stream and

shore erosion, it is a relatively slow, continuous process that often goes unnoticed.

Accelerated erosion is a speeding up of erosion due to human activity. Whenever we destroy


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the natural vegetation or alter the contour of the ground without providing some sort of

surface protection, we greatly increase the rate of erosion. Accelerated erosion can be

minimized through careful planning and by implementing appropriate control measures.

Farming, construction, logging, and mining are the principal causes of accelerated erosion.

These activities radically upset the delicate balance that nature has developed between

rainfall and runoff. Although all the sources mentioned above generate sediment, we will

focus on construction. There are two major reasons that erosion is often increased during and

after construction. The first one is the removal of protective natural vegetation. The second is

the placement of impermeable surfaces like paving and rooftops on the soil. This prevents

water infiltration and increases runoff. These two factors increase the likelihood that soil will

be exposed to the erosive forces of water and wind.

4.2.2 Major Categories of Erosion

4.2.2.1 Water Erosion

There are three kinds of eroding action of water erosion. One happens when water flows in a

stream or river. Another form eroding action of water erosion is abrasion. This is the grinding

away of rock by transported particles in the water. A third eroding action of water occurs

when the water dissolves chemical elements in the rock. With the exception of the extremely

strong winds associated with tornadoes and hurricanes. Water has more power to move large

particles of weathered materials than wind. Water erosion is greatest when a large volume of

water is moving rapidly, such as during spring thaws and torrential downpours. Water

flowing down steep slopes also has greater potential to erode Earth materials, because the

steeper the slope, the faster the water flows. Not only does swiftly flowing water have greater

erosional power than wind, but it can also carry more material along with it and over a greater

distance. Runoff causes both stream channel erosion and overland erosion.

Channel erosion occurs both in intermittent and permanent waterways and streams. Three

causes of channel erosion are: increased runoff, removal of natural vegetation along the

waterway, and channel alterations resulting from construction activities. It includes both

stream bank and stream bed erosion. Overland erosion occurs on bare slopes as a result of

rain splash and runoff. It is the predominate type of erosion and source of sediment from

construction sites. Overland erosion is generally separated into three categories: sheet

erosion, rill erosion, and gully erosion.


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Sheet erosion is the removal of a uniform layer of soil from the land surface as a result of rain

splash and runoff. Rain splash is the impact of raindrops on a soil surface. The splash

dislodges soil particles, making them more susceptible to movement by overland water flow.

The loosened particles that are not washed away can form a muddy slick that clogs pores in

the ground surface. The sealed surface further reduces infiltration and increases runoff. As

runoff water moves down a slope, it increases in velocity and increases the potential for

erosion. The volume of sediment also increases because the transported particles scour and

dislodge more soil particles.

Rilling is another form of overland erosion. Evidence of rill erosion is the development of

small grooves spaced fairly uniformly along the slope. It is caused when runoff is heavy and

water concentrates in rivulets. Individual rills range in depth and width up to several inches

and reflect a tremendous loss of soil. If rilling is not corrected immediately, it will develop

into gully erosion. The depth of erosion defines the difference between rills and gullies.

Although there are no formal definitions for rills and gullies, it is commonly accepted that

rills can be easily obliterated by normal tillage practices, whereas gullies cannot. Gullies do

not always represent the culmination of unchecked rill erosion. Gullies can form wherever

ground or paved surfaces concentrate water into an area that cannot handle the flow. Proper

planning and construction practices prevent this from happening.

4.2.2.2 Wind erosion

Wind is a major erosional agent in areas that experience both limited precipitation and high

temperatures. Such areas typically have little vegetative cover to hold soil in place. In many

ways, wind erosion is similar to water erosion. Wind transports sediment and deposits it in

other locations. Depending on the type of windborn sediment, new landforms may be

produced. Deposits of loess, wind-blown silt and clay sediment that produce very fertile soil,

are found many feet deep in some areas of the world. Wind erosion is common on

agricultural lands and large construction sites. Soil that is piled and left unprotected is

especially vulnerable to wind erosion. In some areas, more soil is lost from wind erosion than

from water erosion. The amount of soil lost from wind erosion may not be realized because

the soil particles disperse over a large area where they are not visible. In an urbanizing area,

the most damaging aspect of wind erosion is dust. It creates traffic hazards, adds to cleaning

costs, is abrasive to plant tissue, and blights the appearance of structures and other surfaces.


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4.2.2.3 Glacial erosion

A glacier is a large, long-lasting mass of ice. Glaciers form in mountainous areas and in

regions that are regularly covered with heavy snowfall and ice. Glaciation is the changing of

landforms by slowly moving glaciers.

landscape. Massive glaciers cut U-shaped valleys in the land. Because they are so dense,

glaciers have the capacity to carry huge rocks and piles of debris over great distances. Glacial

movement scratch and grind some surfaces, while they polish others. On top or within the ice

are other rocks carried by the glacier. When the glacier melts, these rocks are left behind.

Rocks left behind by a glacier may form a ridge or a hill called a moraine.

4.2.2.4 Erosion by plants, animals and humans

Plants and animals living on the surface of Earth also play a role in erosion. As plants and

e to another.

another place. Humans also excavate areas and move soil from one location to another.

Planting a garden, developing a new athletic field and building highways are all examples of

However, the effects of erosion by activities of plants, animals and humans are minimal in

comparison to the other erosional effects of water, wind and glaciers.

4.2.3 Physical Factors Affecting Erosion

Erosion is affected by several physical factors; the common ones are:

4.2.3.1 Climate

The climatic factors that influence erosion are rainfall amount, intensity, and frequency.

Rainfall amount is usually measured in inches. Rainfall intensity is the rate at which the rain

falls. It is measured in inches of water falling in an hour of time. The infiltration rate is the

rate that water is absorbed into the soil. It is also measured in inches per hour. When rainfall

exceeds the infiltration rate, runoff occurs. The frequency of rainfall is the number of separate


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rainfall events occurring during a specific period of time, such as a week or month. During

periods of frequent rainfall a greater percentage of the rainfall will become runoff because of

high soil moisture or saturated soil conditions.

Temperature is another climatic factor influencing erosion. While frozen soil is highly

resistant to erosion, rapid thawing of the soil surface brought on by warm rains can lead to

serious erosion. Temperature also influences the type of precipitation. Falling snow does not

erode. However, heavy snowmelts in the spring can cause considerable runoff damage.

Temperature also influences the amount of organic matter that collects on the ground surface

and incorporates with the topsoil layer. Organic matter is plant and animal residue in various

stages of decomposition. Areas with warmer climates have thinner organic cover on the soil

because decomposition is more rapid. Organic matter protects the soil by shielding it from the

impact of falling rain and by soaking up rainfall that would otherwise become runoff.

Organic matter also provides essential nutrients for plant growth.

4.2.3.2 Vegetative Cover

Vegetation is probably the most important physical factor influencing soil erosion. A good

cover of vegetation shields the soil from the impact of raindrops. It also binds the soil

together, making it more resistant to runoff. A vegetative cover provides organic matter,

slows runoff, and filters sediment. On a graded slope, the condition of the vegetative cover

will determine whether erosion will be stopped or only slightly halted. A dense, robust cover

of vegetation is one of the best protections against soil erosion.

4.2.3.3 Soils

Physical characteristics of soil have a bearing on erodibility. Soil properties influencing

erodibility include texture, structure, and cohesion. Texture refers to the size or combination

of sizes of the individual soil particles. Three broad soil size classifications, ranging from

small to large, are clay, silt, and sand. Soils having a large amount of silt-sized particles are

most susceptible to erosion from both wind and water. Soils with clay or sand-sized particles

are less prone to erosion.

Structure refers to the degree to which soil particles are clumped together, forming larger

clumps and pore spaces. Structure influences both the ability of the soil to absorb water and

its physical resistance to erosion. Organic matter influences the structure of most soils. In

clay soils, it loosens the structure and allows more water to infiltrate. In granular structured

sand or silt soils, organic matter tends to bind the soil into clumps that are more resistant to


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erosion. Soils with organic matter absorb and store more water than soils without organic

matter. The last soil property to consider is cohesion. Cohesion refers to the binding force

between soil particles and influences the structure. When moist, the individual soil particles

in a cohesive soil cling together to form a doughy consistency. Clay soils are in this category.

Clay soils are very cohesive, while sand soils are not.

4.2.3.4 Slope Steepness and Length

Slope steepness, length, and roughness affect erodibility. Generally, the longer the slope the

greater the potential for erosion. The greatest erosion potential is at the base of the slope,

where runoff velocity is the greatest and runoff concentrates. To avoid this problem, long

slopes are often "broken up" so that they function as a series of short slopes rather than one

long slope. Runoff is also slowed by using various runoff control structures, including

diversions and terraces. These structures function to intercept runoff and thereby reduce the

flow of water over the lower portion of the slope. Slope steepness, along with surface

roughness, and the rainfall amount and intensity control the speed at which runoff flows

down a slope. The steeper the slope, the faster the water will flow and the greater potential

for erosion. Steepness of slope is expressed in several ways. The most common ways are as a

ratio of the difference in the vertical and horizontal distance or as a percentage. For example,

a slope with a 100-foot horizontal change for every 10 feet of vertical distance would be

called a 10 to 1 or a 10 percent slope. Although we have little control over soil features and

other natural factors, we do have control over how we develop a site and what measures we

use to prevent or minimize erosion. After every effort has been made to prevent erosion,

efforts should then be directed to controlling sediment.

4.2.4 Preventing/Minimizing Erosion

Storm water runoff is rain that does not infiltrate when it comes in contact with the soil.

Runoff can carry several pollutants, including sediment, nutrients, oil, salt, and other toxic

materials. The faster runoff travels, the more soil it erodes and carries.

Without proper planning, construction activities can result in an increase in runoff. This

increased runoff can cause erosion and flooding. One potentially damaging result of

increased runoff is an increase in the amount of sediment.

There are three primary reasons why runoff increases during and after construction:

The first one is that grading removes vegetation. Vegetation is nature's greatest runoff

protector.


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The second reason is that grading compacts the soil, thus reducing the amount of

infiltration.

The third reason is that construction generally results in covering large portions of the

soil surface with concrete, asphalt, roofs, and other impervious surfaces. A small

increase in impervious area can cause a disproportionate increase in runoff during a

rainfall.

4.2.4.1 Construction Practices to Control Runoff

There are several construction practices or control measures that will minimize runoff and

thus control erosion. To be effective, all control measures must be periodically inspected,

maintained, and/or replaced when necessary. All damaged areas must be corrected

immediately. If banks are severely eroded, consider installing slope stabilization. Sediment

should be removed when it accumulates behind check dams or diversions

Scheduling

The first construction practice is scheduling. Scheduling is a planning process that provides a

basis for implementing all control measures in a timely and logical fashion during

construction. It may be necessary to implement control measures sequentially instead of all at

one time. Staging of construction is part of scheduling. Staging is sometimes called phasing.

With staging, grading and stabilization are finished in one area before proceeding to the next.

Staging allows you to take advantage of the existing vegetation on the site. Plan the stages or

phases of development so that only areas which are actively under construction are exposed.

All other areas should have a good cover of vegetation or mulch.

Seeding and Mulching

The second practice is to seed and mulch all areas that have no vegetative cover. If it is not

feasible to permanently seed, establish a quick-growing temporary grass cover. Mulch should

always be placed on bare soil to protect it from rain or wind, whether or not it has been

seeded.

Preserve Vegetative Buffers

Preserve vegetated buffer areas above and below the graded area. The buffer above will slow

the runoff before it has a chance to erode. The buffer below slows runoff and will filter some

of the sediment before the runoff leaves the site ( see Figure 4.1).


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Figure 4.1: Vegetative buffers

Surface Roughening

The rate of runoff can be reduced by surface roughening. It is an easy and economical

method that simply creates an uneven or bumpy condition on the soil surface. Horizontal

grooves tend to spread runoff over the slope, slowing it down and allowing more of it to

infiltrate into the soil (Figure 4.2). Scarification is one way to roughen the soil surface. It can

be easily accomplished with a drag, cultivator, or by back blading perpendicular to the slope.

Roughening also produces a soil surface more suitable for the growth of vegetation because it

will hold the seed and retain moisture.

Figure 4.2: Surface roughening (Horizontal grooves)

Diversions


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Diversions can be used to intercept runoff that would otherwise flow across the exposed soil

(Figure 4.3). Care must be taken to divert runoff to an area where it can infiltrate or be safely

discharged. A diversion is generally constructed as a channel with a ridge on the lower side.

Often the excavated material from the channel is used to construct the ridge. The channel and

ridge can be bare compacted soil or vegetated. When the anticipated runoff velocities exceed

1.5 to 2.0 feet per second, diversions should be vegetated. Soil reinforcement measures, such

as erosion control blankets, may be necessary while establishing vegetation in the channel or

on the ridge.

Figure 4.3: Intercepting runoff using diversion

Grade Stabilization Structures

Grade stabilization structures are used to carry runoff from one level to another (Figure 4.4).

Figure 4.4: Grade Stabilization

All grade stabilization structures must be designed to carry the anticipated runoff from the

site and constructed in such a manner to prevent "piping." Piping occurs when water erodes

small channels under or along the side of the water conveyance structure. The potential for


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piping can be minimized by using flared metal inlets and compacting the soil around the inlet

section.

No matter what grade stabilization structure is used, care must be taken to prevent scouring or

erosion at the outlet. Scouring can be prevented by using one or more of the following: place

large rocks on geotextile material downstream of the outlet, use flared end sections, or place

large rocks or concrete blocks in the flume channel.

Check Dams

Check dams may be necessary to reduce the velocity of flow in roadside ditches or in other

concentrated flow areas. Check dams can reduce the potential for erosion and protect

vegetation in early stages of growth. In some situations, vegetation may not become

established without the help of check dams. The primary purpose of check dams is to reduce

water flow to non-erosive velocities. In some situations, the water velocity will be slowed

sufficiently to allow large-sized particles to settle out of the water and be deposited upstream

of the check dam. The deposition of sediment can be increased by excavating sumps

upstream of the check dams (Figure 4.5).

Figure 4.5: Check dam

Check dams are generally constructed of rock. In low flow situations, pea-stone or gravel-

filled bags may be used instead of rock. Sandbags should never be used in flowing water

because water will not pass through the bags. When constructing check dams, place the rock

in the ditch and up the sides to a level above that of the anticipated flow. The middle of the

dam should be nine inches lower than the outer edges. This allows water to flow over the

depression in the center of the check dam, as opposed to around the sides where it could

erode the banks. Check dams are usually used in a series. They should be located or spaced

so that the toe of the upstream check dam is at the same elevation as the lowest point of the


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top of the downstream check dam. Therefore, the steeper the slope, the closer the check dams

should be.

Channel and Slope Stabilization

Check dams are not always capable of reducing water velocities to levels that will prevent

erosion. When this occurs, additional measures must be used for stabilization. Anticipated

velocities, and to lesser extent aesthetics, will dictate what stabilization measures will be

used. For example, unvegetated bare channels (Figure 4.6) can generally only sustain

velocities up to 1.5 to 2 feet per second without eroding. Established grassed lined channels

can accommodate velocities up to approximately 4 to 5 feet per second (Figure 4.7). Until

grass is established, runoff may have to be diverted away from the exposed area to protect the

seedlings and the channel itself from erosion. Under extreme conditions, channel velocities

can reach 15 feet per second, and extreme measures will be needed for stabilization.

Figure 4.6: Unvegetated bare channel

Figure 4.7: Grass lined channel


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Another option is to line the channels with erosion control blankets or turf reinforcement

mats. Blankets and mats are manufactured by several companies, each of which has specific

applications. Primary differences between blankets and mats are in the materials that are used

and how they are constructed. Some are designed for low velocity situations while others are

capable of accommodating higher velocities.

4.2.4.2 Practices to Control Wind Erosion

Sandy and organic soils tend to be the most susceptible to wind erosion. Soil may start

moving, or eroding, when wind speed exceeds 13 miles per hour measured at one foot off the

ground. Similar to rainfall-induced erosion, the best way to protect against wind erosion is to

keep the area covered with vegetation or with securely anchored mulch. Also, in areas

subjected to strong winds, such as along the Great Lake shorelines, soil should never be

placed in piles and left unprotected.

Windbreaks

Leave trees or other tall vegetation along the perimeter and intermittently across the site to

serve as wind barriers (Figure 4.8). When trees or vegetation must be removed, snow fence

can be used to form mini wind barriers. The snow fence must be placed perpendicular to the

prevailing wind direction at evenly spaced intervals across the site. Most barriers will protect

the soil downwind for a distance of about 10 times the height of the barrier. Therefore, place

rows of snow fence about every 40 to 50 feet (Figure 4.9). Although the primary purpose of

fencing or other barriers is to reduce the erosive velocity of wind, they also create barriers to

stop wind-born soil. Thus they help keep wind-generated sediment on the site.

Figure


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Figure 4.9: Snow fence

Watering

Another temporary measure for controlling wind erosion is to keep the bare soil moist by

watering. A readily accessible water source is required. Water should be applied to the site

whenever moderate to high winds are anticipated. Haul roads may have to be watered

continuously

Chemical Binders

In addition to watering, chemical binders can be sprayed on the soil surface. The chemical

penetrates into the soil and bonds the individual soil particles, making them resistant to the

forces of wind.

4.3 Sedimentation

Sedimentation is the process whereby the detached particles generated by erosion are

deposited elsewhere on the land or in our lakes, streams, and wetlands. Together, the two

processes (erosion and sedimentation) result in soil being detached, carried away, and

eventually deposited elsewhere.

4.3.1 The Sedimentation Process

As previously explained, sedimentation is the process whereby eroded soil particles settle out

or are deposited. Deposition of sediment occurs when soil-laden water slows enough to allow

the different sized particles to settle out. Sediment deposition may result in one or more of the

following:


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estroyed terrestrial habitats

4.3.2 Factors affecting sedimentation process

There are important physical factors that influence the sedimentation process. The

interactions of these factors will determine how sediment is transported and deposited.

The velocity and turbulence: The velocity and turbulence of the runoff water are key factors

in determining the fate of sediment. The greater the velocity and turbulence of flow, the

greater will be the amount of sediment transported in suspension in the water or carried along

the stream bottom as bedload. The lesser the velocity and turbulence of flow, the greater the

amount of sediment deposited.

The size, shape, and density of the transported particles: These factors influence the rate at

which they settle out. Smaller, lighter particles, such as clay-sized particles, are more easily

transported. They stay suspended and are slow to settle out. Larger, heavier particles, such as

sand, are harder to transport and thus are more quickly deposited.

4.3.3 Principles and Strategies

The goal of erosion and sediment control is to protect land and water resources by

minimizing erosion and off-site sedimentation, using the best practical combination of

procedures, practices, and people.

Protect land and water resource:. Responsible people seek to be stewards of all our natural

resources, including land and water. A balance must be met between resource protection and

the other activities of the construction project.

Minimizing erosion and off-site sedimentation: During construction activities, everything

possible should be done to prevent the erosion of soil from the site and its deposition off-site

and into surface waters and wetlands.

Using the best practical combination of procedures, practices, and people: To control

erosion and sediment we need workable laws, regulations, and procedures; up-to-date

practices and techniques; and responsible people working together.


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The effective control of erosion and sedimentation requires the application of the following

five principles of erosion and sediment control:

Plan the development to fit the particular topography, soils, waterways, and natural

vegetation at a site. Think stewardship or a partnership with nature. When structures

and grading are designed to fit the site less soil is exposed to erosive forces. The result

can be both reduced environmental damage and savings in project costs.

Expose the smallest practical area of land for the shortest possible time, by scheduling

and staging project activities. This means that the soil surfaces exposed during the

first phase of the project are stabilized before beginning construction on the next

phase. Daily seeding and mulching with permanent or temporary seeding mixtures is

recommended.

Apply soil erosion prevention practices as a first line of defense against onsite

damage. Use practices that minimize erosion on a site to prevent sediment from being

produced and the need for costly controls to trap and control sediment.

Examples of erosion control practices include:

uctures

vegetation

Apply sediment control practices as a perimeter protection to prevent sediment from

leaving the site. Use practices that control sediment once it is produced, and prevent it

from getting off-site. Examples of sediment control are: Silt fences; Interceptor dikes

and ditches; Sediment traps; Vegetative filters; and Sedimentation basins.

Implement a thorough inspection, maintenance, and follow-up program. Erosion and

sedimentation cannot be effectively controlled without a thorough, periodic check of

the site and continued maintenance of the control measures. An example of applying

this principle would be a routine end-of-day check to be sure all control practices are

working properly.

4.3.4 Controlling Sediment

Vegetative and structural practices are used to control sediment. It is important to remember

that sediment control should not be used as a substitution for erosion control. Both should be


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part of a coordinated strategy. Erosion control is the first line of defense. It reduces the

amount of sediment that will be generated. Sediment control is the second line of defense. It

prevents much of the sediment created by uncontrolled soil erosion from leaving the

construction site or entering surface waters within the site.

4.3.4.1 Vegetative Sediment Control

Vegetative sediment control involves using existing or newly planted vegetation to trap or

filter sediment from runoff (Figure 4.10).

Figure 4.10: Controlling sediment using vegetation

The most common method is to use vegetative filter strips. Vegetative filters are one of the

more effective and economical methods for removing sediment. Natural vegetation can be

preserved for the filter strip or it can be established before grading the site (Figure 4.11).

Figure 4.11: Natural vegetation

The effectiveness of filter strips depends on flow patterns, strip width, vegetation type, and

density. To be the most effective, water should pass through the strip of vegetation as sheet

flow and not as concentrated flow (Figure 4.12).


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Figure 4.12: Concentrated and Sheet flow

The required length of the strip will vary depending on the slope length and steepness of the

disturbed area. However, a 20-25 foot strip of dense grass should be considered the

minimum. Except for essential roadway crossings, no vehicles or construction should be

allowed within the filter strip. Dense grass provides the best filtration.

Brushy and wooded areas are less desirable filters. However, they can provide some degree

of filtering action as well as aiding in the absorption of runoff. Care must be exercised when

directing flow to wooded areas because the deposition of a few inches of sediment around a

tree can kill it (Figure 4.13). Use woodland areas only for filtering sheet flow with minor

sediment concentrations.

Figure 4.13: Deposition of sediment around trees

4.3.4.2 Structural Sediment Control

Structural practices must be used where vegetative practices are not practical or sufficient to

control sediment. Structural practices to control sediment include silt fences, straw bales,

diversions, interceptor dikes, sediment traps and basins, rock construction exits, and storm

drain inlet filters.

Perimeter Barriers: Silt fences and straw bales are commonly used along the perimeter of

small graded sites. Silt fences are far superior to straw bales because they are less expensive,

easier to install, longer lasting, more effective, and can be re-used. Silt fencing must be


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installed correctly and trenched (Figure 4.14) or it will be ineffective in trapping sediment

(Figure 4.15). Silt fences must be trenched in a minimum of six inches.

Figure 4.14: Effective silt fence

Figure 4.15: Ineffective silt fence

Silt fences must also be installed on the same elevation contour across the slope. If the

elevation contour is not followed, the fence will act as a diversion and the concentrated water

will flow around the end or over the top of the fence. The effectiveness of silt fencing can be

increased by placing it beyond the toe of the slope (Figure 4.16). This will enhance sediment

deposition by allowing more area for the water to pond. A very limited amount of water can

pass through a silt fence.

Figure 4.16: Silt fence place beyond the toe of slope

For long slopes or large areas, silt fence should be installed parallel to each other in a series at

approximately 200-foot increments along equal contour lines and drain no more than one-half

acre per 100 feet of silt fence. When it is necessary to dredge in lakes, marinas, and other

Weathering, Erosion and Sedimentation Lecture Notes

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