Essay on Stream Bank Erosion | Soil Management

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Essay on Stream Bank Erosion

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Essay Contents:

1. Essay on the Meaning of Stream Bank Erosion
2. Essay on the Causes of Stream Bank Erosion
3. Essay on the Control of Stream Bank Erosion
4. Essay on the Maintenance of Stream Bank Erosion Control Measures
5. Essay on the Design and Construction Considerations of Stream Bank Erosion Control
6. Essay on the Assessment of Stream Bank Erosion Control Measures

Essay # 1. Meaning of Stream Bank Erosion:

Stream bank erosion is a kind of water erosion, in which soil is removed either by the runoff flowing over the sides of the stream coming from u/s areas or by scouring and undercutting of soil below the water surface from the stream section. The removal of soil from stream bank and scouring from its bed is more concentrated in the rainy seasons when runoff volume and its flow velocity are predominantly increased.

While undercutting of the soil, below the water surface is a continuous and slow process, takes longer time to cut the soil from bank and to collapse the overhanging soil mass into the stream flow. Erosion of soil from stream bank is less serious than the scouring of the soil from its base and banks. It gets increased in the condition when vegetations are removed, either by overgrazing or by cutting of the same from the areas, closer to the stream bank.

Stream bank erosion causes several disadvantages such as – it continuously destroys the productive cropped lands right up the stream bank, and sometimes heavily destroys the lands during flood without any ultimatum. In addition, stream bank erosion also causes several damages to the bridges and other structures located across or near the streams.

Apart from above damages, there is also created further problems; for example the eroded materials from the u/s areas and stream section are carried away by the flow. The eroded materials are deposited over the stream bed after decreasing of flow velocity and thus clogging the stream/channels and welcome to the flood.

A considerable part of the eroded materials (i.e., sediments) is also deposited in the reservoirs and reduces their effective capacity. Loss of agricultural lands either due to undercutting or washing of soils by the stream flow, is also a serious type of problem.

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Essay # 2. Causes of Stream Bank Erosion:

The bank erosion is greatly influenced by the nature of flowing runoff through the streams, rivers etc. A turbulent flow always causes greater erosion than the laminar flow. From the stream section a major part of soil erosion is due to turbulent flow.

In addition, when flood water recedes from the stream, then there starts a reverse flow of water from the bank’s soil into the stream, known as return flow. The return flow builds a differential pressure in the bank soil, which causes sloughing of the stream bank. This phenomenon generally takes place in those banks which contain loose materials.

Scouring, undercutting, sloughing and grazing are the important activities for the stream bank deterioration. These activities are affected by several factors, such as soil characteristics, absence of vegetation, size and character of floods, velocity of flow currents, land use on adjacent and upstream areas, stability of river bank during the flood etc.

However, various causes associated to the stream bank erosion, are described below:

1. Washing of Soil Particles:

The soil particles are washed away by the concentrated runoff from both the banks and bottom of the stream. The washed materials are mixed in the flowing water and moved from one place to another. The washing of particles depends on the size of soil particles, their cohesion and flow velocity of stream water.

The bigger size particles are rolled or creeps, while the fine particles are suspended in and moved through the water. The velocity of stream water significantly affects the washing of soil. When flow velocity is more, then there is greater washing; and vice-versa.

2. Sliding of the Bank’s Material:

The sliding of the bank’s material takes place when bank slope is unstable. The slope un-stability depends on several factors, such as type of materials, bank slope, land use etc. The slide materials fall down into the stream and subsequently carried away by the stream flow.

Sliding of bank material is also caused due to back flow of seepage water, i.e., from stream bank to the stream, after receding of the Hood. The seepage water saturates the bank material, as a result the internal soil strength gets reduce, and thereby the bank becomes unstable and gets slide into the stream.

3. Undermining:

In common course of flow of stream water, the water currents strike the stream bank continuously and erode the soil particles from the bank. After a long period, the lower portion of stream bank gets undermined and overhanging soil mass is ultimately collapsed into the stream water.

The undermining action is pronounced in the streams with larger width and greater depth, where flow velocity is low and development of regular waves. The waves strike the bank and erode the soil materials from there, which results into undermining of the bank section.

4. Sloughing:

Sloughing of stream bank generally takes place, when banks are fully saturated with water. The sloughing tends to produce a small slump or miniature slide. This type of slide leaves a relatively steep face at the bank, saturated with the water. The sloughing of the bank is usually happened during high flood, occurring for a long duration, when constant high level of stream water saturates the bank.

5. Runoff:

The runoff coming from adjoining areas when falls in the stream, also erodes the bank’s material. Sometimes the concentrated runoff, coming from u/s areas develops small gullies on the stream bank which accelerate the stream bank erosion.

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Essay # 3. Control of Stream Bank Erosion:

The control of stream bank erosion requires the information on physical conditions of the runoff contri­buting watershed and channel, both. It also needs the study of soil characteristics of the stream section and nature of stream flow. The selection of most appropriate methods for protecting the stream bank requires a careful consideration of the hydraulic principles regarding location, design and installation of the structures, used.

Stream bank protection works are classified into following two categories:

1. Direct method, and

2. Indirect method

1. Direct Method:

Direct protection work of stream bank includes those practices which are done directly on the bank, itself. These practices are the formation of vegetal cover. Covering of the bank with brushwood edging, construction of rip-rap using stones and masonry etc. These activities dissipate the impact of water currents, striking on the stream bank and slope, both.

Direct protection of stream bank is performed by following three ways:

i. Stabilization of stream banks by vegetation

ii. Protection of stream bank and side slopes, and

iii. Protection of the lower bank by stone rip-rap.

i. Stabilization of Stream Bank by Vegetation:

It is generally observed that, when vegetative cover of the stream bank is destroyed either by human beings or by animals, then the soil of that part becomes unclothed, as result the erosion is likely to set in at the banks. Under such condition, the growing of vegetation on stream banks plays a significant role in controlling the soil erosion, both from its banks and upstream areas.

The vegetative method is one of most cheapest and effective method for controlling the soil erosion. Its effectiveness depends on the species of vegetation, used. Along small streams, a good sod of grass often provides an adequate result, except in the condition of severe gouging by ice. Sometimes, plantation along the stream is also recommended, but there must be made some preventive measures against grazing of the plants. For this purpose the planted land should be fenced.

ii. Protection of Stream Bank and Slopes:

The stream bank protection by plantation is not technically feasible particularly where valuable properties are extended along the stream bank, as normally found in big ‘cities. In such condition, stream banks are directly protected by using some different techniques rather plantation. It can be done by sodding or turfing, stone or masonry revetment etc.

a. Sodding or Turfing:

This method is suitable for protecting the stream bank especially, when bank is not subjected by strong water currents. The sodding or turfing may be established by using the local available grasses. At the place of grasses, the shrubs, creepers and bushes such as ipomea, vitex, lantana etc. can also be grown on the stream bank’s slope.

The bushes should have very fast vegetative growth and cover a large area within a short duration. They act as a permeable barrier along the stream bank, which dissipates the stream currents to reach them on the bank and also forces to deposit some silts near the bank. In this case of bank protection, proper shaping of the bank slope gives a better result.

b. Stone Revetment:

The extra-vulnerable points of stream bank, such as sharp bends are subject to swift currents. At such points the control of bank erosion, using vegetation, sometimes become impossible. Under such condition, the stone paving or revetment of the vulnerable point becomes necessary. The revetment can also be done by using the brushwood materials weighted by stones, but it acts as a temporary arrangement and effective for a short duration.

When erosion from highly valuable properties takes place due to wave action, the erosion can be stopped immediately by using stones or masonry materials. In all the cases before starting the paving work, the bank face should be shaped in a suitable slope, between 1 – ½: to 1:1.

In case of small size streams having no water in summer season, the dry and rough stone packing is recommended. The packing should be wall packed. The toe of the packing wall should be at least 1 to 2 m below the toe of stream bed and must be connected to the stone revetment. The thickness of revetment should be at least 60 cm.

Similarly, in case of large streams with summer water flow, the stone packing should be done up to the summer water level. And stone revetment should be performed by making some short of foundation on the stream bank, to provide stability to the revetted stones. In addition, where swift currents are expected to strike on stream bank section, the stone pitching should be done with the help of cement and mortar.

However, it involves expenditure of money and not feasible for longer stream lengths. This type of revetment is known as permanent revetment. Permanent type revetments require planting of grasses in the spaces of stones, where root system binds the soil very tightly and thus stabilizes the revetment.

c. Brushwood Rollers:

The brushwood rollers are constructed by using local available grasses or brush­wood materials in the form of rollers with circular in cross-section. The grasses or brush woods with silt inside are tied with coir rope from outside. The diameter of these rollers varies from 20 to 30 cm and length from 2 to 3 m as per requirement and length of vegetative materials available for construction.

These brushwood rollers are secured on the stream bank with the help of wooden poles, driven through them into the bank. For better result the stream bank should be shaped, if necessary. This technique is completely temporary and is suitable for local slopes only. The stream bank protection from erosion, using this method yields better result, when it is used in combination of stone revetment. The revetment for this purpose should be done above the erosion point.

d. Brushwood Edging:

Brushwood edging is most successful method for controlling the stream bank erosion, parti­cularly where stream takes a sharp turn and bed scour is not deep.

This technique consists of two rows of wooden poles (Sal or bamboo) of 8 to 10 cm in diameter, are driven into the stream bed at an interval of 1 m from each other. The driven depth of poles should be such that, they are stable and erected on the bank, properly. Generally, half of the total pole length is sufficient to drive into the bank. The top edging should not be kept more than 1 to 1.5 m above the average stream bed. Similarly, another row is also constructed at 1m apart from the first row, in the same way.

The edging should be kept parallel to the stream flow. In order to provide a greater stability, the outer edges of these two rows should be nailed with split wooden poles. At last, the gap between the rows is also filled with brushwood materials, pressed and secured in position by tieing them diagonally from the top of the edge. The area left between the stream bank and brushwood edging should also be filled with the earth or brushwood materials and weighted with the help of stones.

During monsoon season, when flood occurs, these brushwood edgings prevent erosive water currents to come across the bank and thus control the cutting of the stream bank. In addition, they also reduce the flow velocity near the bank and cause siltation, there. Sometimes, for making the structure more effective, the silted areas are also planted with the vegetations like ‘Ipomea’, as soon as flood water recedes. It covers the entire silted area very soon and protects the bank from further erosion.

iii. Protection of Lower Bank:

The lower portions of stream bank are constantly under submerged con­dition; and are threatened by water currents and thus subject to constant erosion. This phenomena can result the development of undermining problem, which leads to the mass removal of soil from the stream bank.

The protection of lower bank can be achieved by using the following techniques:

a. The lower portion of the stream bank can be protected by providing a flexible apron, made of brushwood or wire mattresses. It should be extended up to the stream bed for better result.

b. By dumping the loose rocks or stones at the toe of the stream bank, and

c. It can also be done by constructing retaining wall along toe of the bank. But this involves a heavy expenditure of money, hence not economical for construction to protect the stream bank.

2. Indirect Protection Measures:

Indirect stream bank protection work refers to those activities, which are not constructed on the stream bank, but at some distance before the bank. The working procedure of these measures is based on the principle of making ineffective the incoming erosive currents by diverting them away from the bank and also depositing the sediments near the bank.

Indirect protection work includes the following measures to control the stream bank erosion:

i. Deflecting the stream currents away from the bank and causing deposition of sediment,

a. By installing retards, and

b. By constructing the spurs.

ii. Diversion of runoff from the stream bank to control the formation of small gullies or rills on the bank

Deflection of Current away from the Bank and Deposition of Sediments:

In this mode of stream bank erosion control, the erosive power of currents striking the bank is made ineffective by deflecting them, using some structures in front of bank. This is achieved by installing the retards in front of stream bank having some projection towards the stream bed.

These retards dissipate the erosive force of the stream currents, before their striking on the bank, which causes deposition of silts near the bank. In order to make the retards more effective, the silted areas are also stabilized by growing vegetations, such as ‘Ipomea’. The selection of vegetation species should be such that, which can cover the entire area very rapidly in a short time.

Deflection of current is performed by the following retards:

a. Wooden Jacks:

This type of retards are more effective to control the stream bank erosion, particularly where stream currents are not swift. Wooden Jacks are made of three wooden poles (sal or bamboo) crossed at centre and tied with the help of wire. Construction details of wooden jack is shown in Fig 6.4, from which we can be have an idea about it. The length of Wooden Jack to be used, depends on the length of stream bank, sought to be protected.

However, it ranges from 3 to 5 m. Several such wooden jacks are installed in a series along the eroded portion of the bank. The spacing of jacks is kept small, but should not be more than the length of one Jack. The centres of each Jack are tied with the help of cable which is connected to heavy logs. These logs act as anchor. The length of log ranges up to 2 m and diameter from 20 to 25 cm. These jacks create obstruction in front of stream bank for incoming water currents and dissipate their energy, causing reduction of soil detachment from the bank.

b. Spur:

A vertical or inclined projection (structure) from the stream bank, facing towards stream bed or centre of the stream, is known as spur. The main purpose of spur is to create the obstruction for water currents and keep them away from the bank.

Location of Spur:

Spurs are located along the eroded portion of stream bank. Generally, two or more spurs are additionally installed towards upstream side of the eroded portion for checking the advance of stream currents to come across the bank.

Angle of Projection:

Angle of projection refers to the inclination of spur towards stream bed from the bank. It mainly depends on the nature of stream currents striking the bank and types of spur, used. The projection of spur should be such that, it can obstructs the water current effectively. Generally, inclination angle of spur varies from 30° to 60° from the vertical (Fig. 6.6).

Length of Spur:

The length of spur depends on the types of spur to be used and nature of stream bank material. The length is determined by considering that ⅓ to ½ of the total spur length is embedded into the bank and remaining portion is left above, facing towards the stream bed. However, spur length generally varies from 5 to 10 m.

Spacing and Number of Spurs:

Spacing of spur is governed by its angle of projection from the vertical.

The number of spurs can be determined by using the following formula –

N = (L/S) + 1 … (6.1)

Where,

N = number of spurs

L = total length of eroded stream bank at one side

S = spacing of spurs

= 7h in which, h is the vertical projection of the spur. If θ is the angle of inclination from vertical, then h = l’ cos θ.

l’ = effective length of spur over the stream bed.

Generally, 2 or more spurs are additionally installed towards upstream side of the eroded bank, depending on the nature of stream currents.

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Essay # 4. Maintenance of Stream Bank Erosion Control Measures:

All the measures of stream bank erosion control require a careful maintenance over time. The observations regarding maintenance should be made on a regular basis before and after major stream flow events.

In general, the maintenance is done in following respects:

1. If there is debris or other materials which likely to damage the bank materials, then they must be removed from there.

2. If there is missing or erosion control materials have been removed during high stream flow, then they must be replaced during low flow condition.

3. There must be application of fertilizers to the planted vegetations, for better growth.

4. If there are buffer vegetation strips, then there should be applied fertilizers for proper growth, and weed removal practices should also be followed.

5. The livestock should be restricted to go on the steep stream banks, and also in the area under protection.

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Essay # 5. Design and Construction Considerations of Stream Bank Erosion Control:

The main parameters taken into consideration for design and construction of stream bank erosion control structures/measures are given as under:

i. Velocity of stream flow

ii. Depth of flow at bank point

iii. Slope and height of the bank, and

iv. Bank soils.

These are described below:

i. Stream Flow Velocity:

Although, the bank erosion takes place at any flow velocity, but it becomes evident only when flow velocities are greater than 3 fps. At the flow velocity greater than 3 fps, it becomes essential to follow safety measures. Also, for implementation of erosion control work, there requires a special skill and equipment for construction of structures.

ii. Depth of Stream Flow:

The stream flow depth affects the design and construction of stream bank erosion control measures, significantly. Normally, up to the flow depth of 3 feet, the work can be carried out at the water’s edge without special equipment. On the other hand, when flow depth is more than 3 feet and velocity is more than 3 fps, then manual method becomes impossible. In this condition, the work should preferably be carried out with the help of machines through contractors.

iii. Bank Slope:

The slope of stream bank is measured for every 1-foot drop in elevation. In general, at the slope of 6H: IV or flatter the control works can be performed with the help of ordinary machines, provided that the bank height is less than 4 feet. At the bank slope steeper than 6H: IV the stream bank control work should only be performed by using special equipments.

iv. Bank Height:

The difference in elevation between the top of the bank and the water surface in the stream refers to the bank height. The height of stream bank to be protected is very important, especially when it is of a particular owner. In this condition, the consideration must be on protection of stream bank for its maximum height. For protecting greater bank height, the design and construction of structure should be done with great care, and by means of special equipments.

v. Soils:

The bank soil is also very important parameter in design and construction of stream bank erosion control structures. If the soil is stable and slope is within permissible limit, then use of machineries is easily possible; otherwise, there is required to take care about selection of suitable machines depending on the soil conditions.

Apart from above the following points are also taken into consideration:

i. The construction works should be conducted during low flow condition.

ii. The construction equipments and other materials should be kept at the safe points, near stream bank.

iii. The debris during construction should be disposed to such a place, from where it could not return back into the stream.

iv. Care should also be taken to prevent any petroleum products, chemicals etc. from their entering the stream.

v. If any vegetations, including tree etc. are at the site, then they should be cleared from there.

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Essay # 6. Assessment of Stream Bank Erosion Control Measures:

It is carried out based on the change in cross section of the stream bank. The stream cross section is deter­mined by taking several measurements, perpendicular to the stream flow direction. By analyzing the measure­ment of vertical cross sectional area of stream bank, the conclusion whether there is recovery (sedimentation) or deterioration (erosion) in the stream bank due to use of control measure or not. A stream showing deep incision indicates about increased runoff towards upstream.

In general, the following informations are collected, prior to carry out the exercises for assessment of stream bank stabilization works:

i. Position of bedrock.

ii. Whether the lenses of fine sand have been exposed at the eroding bank or not.

iii. Is the stream channel is stable at upstream of erosion site.

iv. Depth of stream flow along the eroding bank.

v. Width of stream at the erosion sites; it is determined in comparison to the upstream/ downstream stable sections.

vi. Status of vegetation close to the site.

vii. Status of site protection measure.