In this article we will discuss about the twelve methods used for controlling stream bank erosion. The methods are: 1. Stream Bank Stabilization—Gabion Method 2. Vegetated Geo-Grid Method 3. Iowa Vanes 4. Vegetative Riprap 5. Stone Riprap 6. Pilings with Wire or Geotextile Fencings 7. Dormant Post Plantings 8. Coconut Fiber Rolls Method 9. Branch Packing Method 10. Live Fascine Method and Few Others.
1. Stream Bank Stabilization—Gabion Method:
The rock-filled gabions are used to stabilize the stream bank; and also to divert the flow away from the eroding stream section. For bank protection the gabions are constructed in the form of mattresses with the help of one layer of rock encased by suitable gage wire. The gabion mattresses are flexible and free draining, as result there is soil filtering and deposition. The gabion mattresses can be used at unstable streambeds and stream banks for preventing soil scouring/washing.
The rock-filled gabions are described as under:
These are found most suitable to use at the bank sections, which are subject to excessive erosion due to very high flow rate. In general, where flows velocity exceeds 2 m/s; and vegetative measures are not effective to check the bank scouring, this measure can be suitably used.
However, the followings are specific points for selecting the gabions:
i. The stability of bank should be sufficient.
ii. There should be moderate to excessive sub-surface water movement that may be creating erosion and damaging other types of non-permeable structures.
iii. An excessively steep stream bank should be stabilized before constructing the gabions. For this purpose the retaining wall or toe wall can be constructed.
Gabions are constructed in such a manner that the sides, ends, lid and diaphragms can be assembled at the construction site in rectangular basket shape of the specified size. Gabion is single-unit construction; the base, the lid, the ends and sides are either woven into a single unit or one edge of these members is connected to the base section in such a manner that the strength and flexibility at the connecting point is at least equal to that of the mesh.
If the length of gabion exceeds 1.5 times of its horizontal width, then it should be divided into various cells, whose length should not exceed the horizontal width. For gabion mattresses construction the size of stone should be from 76 to 127 mm; and 101 to 203 mm for construction of rock-filled gabion.
2. Vegetated Geo-Grid Method:
In this method, vegetated geo-grids are used with erosion control fabrics for covering the bank soil surface. The fabrics are the geo-textile. View of geo-grids is shown in Fig 6.8. The fabric is secured by tucking into the slope.
And live vegetative cuttings are placed between the geo-grids. The length of vegetative cuttings is from 0.5 to 2.5 inches.
From the live cuttings the roots get develop after few days, which bind the bank soil very tightly, within and behind the geo-grids. The toe portion of stream bank is packed by placing the stones. The size of rock pieces varies from 4 to 9 inches (diameter).
Various advantages and disadvantages of this method are as below:
1. It can be used, where bank cannot be reshaped into a gentle slope.
2. It can be used, where bank is severely eroded.
3. As disadvantage, a large amount of soil and rock materials are required for construction.
4. The vegetative growth from live cuttings gets reduce during high flow stages.
The construction or installation of vegetated geo-grid is done under following steps:
i. Dig a trench of 2 to 3 feet depth below the streambed.
ii. Fill the trench with rock and geo-grid-wrapped rock up to the stream level.
iii. At the stream level place the rock and soil; and over it place a layer of live cuttings. The placing of rock should be done in such a way, that each successive layer is slightly lesser than the previous one, to ensure that the layers are not in hanging form over the stream but in contact with the original bank.
iv. Place the fabric on the upper most layer and stake into the original bank; put the soil over it and plant the grasses in the soil.
After few months there gets develop a good vegetative cover over the geo-grids, which is fully capable to prevent the stream bank cutting or erosion. The view of developed vegetated geo-grid to control stream bank erosion is presented in Fig. 6.9.
3. Iowa Vanes:
This is also a structure used for stream bank erosion control (Fig. 6.10) These structures are installed at the eroding stream bank/bed to redirect the flow path, due to which there is deposition of sediments on the eroding bank. Its suitability is very appropriate to use at the locations where stream gets turn. At the turning point of stream there is mainly the problem of undermining. If undermining of stream bank is not properly checked, then there is possibility of collapsing of the bank.
In such cases, the Iowa vanes stabilize the stream without affecting the sediment load and flow velocity of other parts of the stream section. For making better effectiveness, the soil bio- engineering should be used with these vanes. These vanes Average are precasted; and can also bed level be obtained from the manufacturers.
Few important points about these vanes for orientation of vane, are mentioned as below:
i. The length of vane should be between 6 and 9 feet. However, it should be such that each vane could be completely submerged in an average flow.
ii. The height of vane should be about 1 foot above the average bed level or between 2 and 3 feet.
iii. The thickness of vane at the top should not be less than 3 inches.
Various advantages of Iowa vanes are given as under:
i. It stabilizes the toe of eroding stream bank.
ii. Once the sediments have been deposited at the bank, then there is development of natural vegetations, which further makes the erosion control, better.
iii. The submerged vanes can be constructed using locally available materials.
Few main disadvantages are given as under:
i. These vanes are impractical for use in narrow streams/channels. For this the width of stream should be from 15 to 20 feet.
ii. Iowa vanes are suitable only for sandy streambeds.
iii. This system requires professional design.
iv. These vanes cannot tolerate ice flows and other debris because they are placed at the bed, submerged in the water.
v. These vanes can damage the boats. Therefore, they require visible labeling. Installation
Take two metal pipes, and insert into the streambed. The pipes should be placed at the interval of 6 to 9 feet at the angle of 20 to 25° towards the bank to align the vane against stream. Fasten the planks between the pipes. For leveling of plank the water surface in the stream can be taken as a reference level.
The installation should be performed as per following points:
i. It should be started at least one channel-width upstream from the turned portion, where erosion is occurring.
ii. Form an array of vanes keeping the view that there should be 2 to 3 vanes in each array; and at least 3 arrays before the protection area of the bank. The spacing of vanes within array should be from 2 to 3 times the height of vane. And between each array the gap should be from 15 to 30 times the height of vane.
However, at u/s side the arrays can be at lesser spacing, while at d/s side there should be greater spacing between them, but not more than 30 times the height of vane. The vanes are not required to install past to the stream turn. Fig. 6.11 presents the view of installation of Iowa vanes.
4. Vegetative Riprap:
Ripraps are used for preventing the stream bank erosion caused by high velocity flows or wave actions. This measure consists of at least one riprap layer, which is mainly a bedding layer formed over the stream bank. At the locations, where big size rock pieces are available in sufficient quantity, then two-layer ripraps can he constructed, in which first layer should be of big size rocks (bedding layer) and second layer of small pieces.
The purpose of coarser rock layer is to prevent migration of particles through riprap. In few cases, the use of non-woven geo-textiles or a thick layer of primary bedding gravel, rather a second bedding layer, is also recommended. The bedding material prevents the underlying or filled materials from being washed out through the voids. Fig. 6.12 illustrates the view of riprap –
The design of bedding layer may be done based on the following criteria:
The following points should also be taken into consideration for design of bedding layer:
i. The bedding material should be well graded; and its grain size curve should be approximately parallel to the protected material, particularly for finer particles.
ii. Thickness of any layer of bedding material should not be less than the maximum particle size of the bedding material or 200 mm, whichever is greater.
iii. In dam where seepage line meets the d/s section, a filter zone should be provided; which can be formed by using above criterion.
Thickness of Riprap:
It varies with the following situations:
High Velocity Flow Area:
At the stream points subject to high flow velocity, the thickness of riprap for erosion control depends on many factors, such as flow velocity or flow turbulence; bank slope; rock characteristics including gradation, shape and specific gravity; and degree of damage that can be tolerated. Usually, the riprap size is determined on the basis of D50.
The riprap size can also be taken from Table 6.1 based on the stream flow. The thickness of riprap also depends on riprap gradation, durability of rock, placement conditions (dry/under-water), exposure conditions, design consideration etc. Overall, the riprap thickness varies between DMAX and 1.5DMAX for high velocity flow areas.
Areas Exposed to Wave Action:
For the stream banks or areas exposed to the wave action, the design of riprap may be done by using the following formula –
W50 = mass of the D50 stone (kg)
Wr = density of the stone (kg/m3)
H = design wave height (m)
SR = specific gravity of the stone
θ = angle of the structure slope from horizontal, and
KRR = dimensionless stability coefficient.
The value of KRR depends on various factors such as stone shape, wave height, exposure condition, slope, degree of tolerable damage etc. The riprap gradation can be predicted based on the following formula –
For the areas subject to wave action the thickness of riprap should be between 1.5D50 and 2D50. For durable rocks to be placed in the dry condition, the minimum thickness of riprap as DMAX is found to be more suitable.
5. Stone Riprap:
The view of stone riprap for stream bank erosion control is shown in Fig. 6.12. In this method of stream bank erosion control, a layer of different size rocks is prepared at the affected stream bank. Normally, the class D or E revetment stones, broken limestone, dolomite, or quartzite etc., are used for this purpose. The rough rocks with angular surfaces and of different sizes are found very suitable for riprap, because such rocks get fit together very tightly; and form a dense layer over the bank face.
The construction of stone riprap is accomplished under following steps:
i. Determine the stream flow velocity. It is required for determining the size of rocks to be used for riprap. The rock size depending on stream flow velocity is presented in Table 6.1.
ii. Select the rocks as per determined flow velocity of the stream. The rock size should be such that the flow velocity cannot remove them during high flow stages. Also, the weight of rock to be used on fast-flowing stream banks should be at least 90 pounds.
The smaller rocks should be used to fill the spaces between the larger rocks. The broken concretes can also be used; but not the broken asphalts because their toxicity affects to the riparian lives. Also, the rubbles of demolished buildings should not be used, if they contain wood chips, wallboard, plaster etc.
iii. Reshape the bank in a stable grade. It should be to a maximum slope of 2H: 1V or flatter.
iv. Use geotextile fabric after shaping the bank slope.
v. Place a 6-inch layer gravel or crushed stone, and then firmly place the riprap. However, if a properly chosen filter is used, then gravel or crushed rock should not be used. The rock size should be in proper range. The largest and heaviest rocks should be placed at the bottom of the bank. The thickness of rock layer should be from 12 to 18 inches.
vi. Extend the riprap across the streambed, if there is soil scouring problem.
Various advantages of stone ripraps are outlined as under:
i. It is very effective to check the stream bank erosion, if the rocks are accurately placed over the stream bank, and also gets stabilize very soon due to growth of vegetations at the junction points of the rocks.
ii. Its construction and repair is very simple.
iii. It has natural appearance, and does not create any harmful effect on environment.
iv. The riprap works well with the combination of plantation of grasses at the junction points of rocks.
6. Pilings with Wire or Geotextile Fencings:
For controlling stream bank erosion, the piling cither with the help of wire or geotextile fencing is done in single or double rows. The pilings are arranged in parallel array to the eroding stream bank. The mesh, wire, or geotextile materials placed in the piling slow down the flow velocity. Due to slow down of flow velocity of stream water, the erosive power of water flow gets dissipated, as result the stream bank becomes protected from the water flow.
The pilings are also done with slotted fencing, either in single row or double row. In double row piling which at the place of wire fencing, the wooden planks are used between the posts. The spaces between bank and the fence or between the two fences are filled with the brush or cut branches of trees. The view of such pilings used for stream bank erosion control is shown in Fig. 6.13.
Its various features are summarized as under:
i. The stream depth should be at least 3 feet.
ii. It is not being suitable to use when piling length exceeds 9 feet.
iii. In stream flow there should not be any kind of debris, because they likely to damage the pilings.
iv. The pilings are not recommended to maintain or enhance riparian wildlife habitat.
It is done as per below:
i. Place the pilings at the interval of 6 to 8 feet; and to the depth of half of the piling length into the eroding stream lied. If double row fencing is sought to be erected, then second row of piling should be made at the distance of 5 feet.
ii. Tie the wire, mesh, or geotextile fabric to the pilings.
iii. In case of double row pilings, fill the space between two rows with the help of rock and brush materials.
The installation procedure of pilings is shown in Fig. 6.14.
7. Dormant Post Plantings:
Dormant post plantings are also done for controlling the stream bank erosion. In this method, the medium-size tree cuttings are placed in the rows along the eroded stream bank. These posts form a kind of pervious barrier and create protection to the stream bank, which is helpful for controlling erosion from there.
The length of post is kept 9 feet and diameter 5 inch. The local tree species that have good root system are preferred for using as dormant posts. For easy insertion of posts into the bank soil, they are tapered by their lower end.
The other features of this measure are given as under:
i. In this measure a good vegetative cover is developed which reduces the bank erosion.
ii. Dormant post planting slows down the stream flow velocity, especially during high water flows, as result there is deposition of sediments near the stream bank.
iii. In small and non-gravel streams, this measure is found very suitable.
iv. If the stream flow contains ice then this measure (dormant post plantings) is likely to get damage by the ice mass.
v. Since, the planted plants take water from the bank soil, therefore, there is lowering of moisture contents in the bank soil, which causes better stability of the bank area.
It is done under following guidelines:
i. The installation should be started from the point of normal water flow level in the stream; and proceed upward.
ii. The length of post to be inserted into the soil should be from 0.5 to 0.66 time of the total post length. But it should always be kept in mind that about 12 inches of the post should always be in the touch of saturated soil, so that for proper vegetative growth the moisture content could be available in the bank soil.
iii. The posts should be inserted in rows, either in square or triangular array.
8. Coconut Fiber Rolls Method:
The coconut fiber rolls are made of coconut husk fibers by binding together with the help of coconut husk twine. The shape of roll is cylindrical. The diameter of rolls is about 12 inch and length is up to 20 feet. For stream bank erosion control, these rolls are staked at the toe of the slope. Before staking the rolls, a trench is dug at the toe of the slope for the depth slightly below the base flow in the stream.
The following guidelines are followed for its installation:
i. The placing of coconut rolls in the trench should be done in such a way that the stream flow is above the roll.
ii. The stakes are inserted by passing through the coconut roll at about 2 to 4 feet interval.
iii. In the stream having fast-flowing water, more number of stakes should be used so that the coconut rolls get retain at their position in firm way.
iv. In the rest portion of stream bank the vegetations should be allowed to get grow.
The other features of this measure are outlined as under:
i. This structure checks the shallow earth slips.
ii. Undermining at the toe of the stream bank is also checked.
iii. Plants can also grow in the roll, which makes the measure more effective for erosion control.
iv. A flexible roll is effective to cover the bank area very well.
v. There is limited disturbance to the stream bank.
vi. The sediments are easily trapped by the coconut fiber.
vii. This structure remains effective for controlling the stream bank erosion for the period of 6 to 10 years.
9. Branch Packing Method:
Stream banks are also likely to get affected, if there are holes in the bank area. If the size of hole is large then stream water directly enters them and detach the soil mass from there. In this condition the soil mass above hole gets settle down.
The striking water washes the settled soils from there. Sometimes, during high flow there generates waves, which directly enter the hole; and wash the soil particles from there. For controlling such type of happening, the branch packing in the bank holes plays very significant role. It is done by using live branch cuttings to cover the affected portion of bank.
For better result the live branches should be covered with soil and thoroughly compacted to seal the bank hole completely. Fig. 6.15 illustrates the view of branch packing for stream bank erosion control. This measure is not very effective for the eroded bank with the holes greater than 4 feet in depth and 4 feet in width.
The rootable live cuttings are preferred. The length of cuttings should be from 3 to 5 feet and diameter one-half to two inches.
For construction the following guidelines can be followed:
i. The bottom of hole should be clean to ensure a good foundation.
ii. The bottom portion of hole should be sloped towards rear of the bank.
iii. To stabilize the fill at the toe, the rocks can be used.
Fig. 6.15 presents the view of branch packing procedure for stream bank erosion control.
Various advantages of this measure are outlined as under:
i. It can be used to fill the holes or gullies of 2 to 4 feet depth.
ii. The root system of branches stabilizes the soil of the bank, strongly.
iii. It is an effective and inexpensive method for small projects.
iv. The vegetations get established, naturally, which make the erosion control more effective.
v. It can also be used for preventing the soil scouring.
The following steps are followed for its installation:
i. Drive the stakes vertically for the depth ranging from 3 to 4 feet in the bank soil. The spacing should be about 1 foot.
ii. Spread the branches between the stakes; and press them using rocks.
iii. Place the soil above the first layer of branches; and compact it thoroughly.
iv. Again place the branches; and repeat the same process.
v. Continue layering of the branches, and stake up to the level of stream bank.
10. Live Fascine Method:
The live fascine refers to the placing of bundles of tree branches in the trench constructed at the stream bank, to check the soil erosion across the bank and establish the soil stability. For this purpose, the live stakes, branches, rocks etc. are used. The stream bank is sloped to the tune of at least 2 feet horizontal to 1 foot vertical (2H: IV); or tiered with 3 to 4-5 feet steps.
The long and straight bundles of native branches are constructed in 6 to 8 inch diameter; and are bound with untreated twine. The length of bundles should be from 5 to 10 feet. It is taken care that within the bundles all the growing tips of branches must be oriented in the same direction.
The installation procedure is described as under:
It is done under following steps:
i. Dig the trench on the bank slope.
ii. Place the erosion control fabric in and between the trench.
iii. Place the fascines in the trench.
the dead stout stakes directly through the fascine at 2 to 3 feet intervals; and flush with the top of the fascines and at the connections of bundles.
v. Drive the live stakes between dead stout stakes, keeping 3 inch above the fascine.
vi. Place moist soil along the side of fascine.
vii. Riprap the toe of the slope to prevent it from being scoured, and also to secure the fabric.
Although, the fascine causes site disturbance but offers various advantages, outlined as under:
i. It is most effective to control stream bank erosion, when combined with the live staking and rock riprap.
ii. Since, the bank slope is modified into tiered form, therefore, there is significant reduction in the velocity of flow.
iii. Also, the angled fascines create a kind of drainage path in the bank slope.
11. Brush Mattresses Method:
In this method the brush materials are placed over the affected bank; and are staked and pressed with the help of rocks. The length of branch should be from 6 to 9 feet and diameter 1 inch. For making more effective, these brushes are secured with the help of wire mesh.
For its construction the following points are taken into consideration:
i. Prior to place the brushes, the stream bank should be graded to a maximum slope of 2:1 (2H: IV).
ii. The stakes should be prepared before starting the installation work.
The structural view of brash mattress is shown in Fig. 6.16.
It accomplishes following steps:
i. After grading the bank soil, place the live fascine in trench at the base of the slope.
ii. Place the dead stakes at 1-foot depth in the graded slope at the rate of 2 square feet per stake.
iii. Place the branches with basal ends in the trench of the fascine, in the layer of 1 to 2 branch thickness.
12. Seeding of Stream Bank:
It is very simple and common method to control the soil erosion from stream bank. As for as stream bank erosion control is concerned, it is the simplest way to stabilize the bank soil and check the erosion from there.
However, it is only suitable for very low erosion rates. Normally, seeding is done of suitable grasses and legume species. Sometimes, for better result the live tree cuttings are also inserted in the soil to provide greater bank protection for longer period.
For seeding, the stream bank is prepared with following guidelines:
i. Grade the bank slope not steeper than 6 feel horizontal to 1 foot vertical (6H: IV). It should be done by removing the foreign materials away from the stream bank.
ii. Keep preserve the soil material obtained from the stream bank grading provided that the soil is fertile. If the graded bank does not contain adequate topsoil to support good vegetative growth, then topsoil should be replaced.
iii. If required, additional bank stabilization should also be done by using buffer strips of trees, shrubs and grasses adjacent to the stream bank.
It is carried out as per below:
i. After grading the bank surface, broadcast the seeds of desired grass/legume species, uniformly. And cover the seeds with the help of soil in the thickness of about 0.5 inch.
ii. Lightly tamped the soil to ensure a good contact between the seeds and soil.
iii. If required, the straw or other suitable mulching materials should also be used, to provide a better environment for seed germination and good growth of germinated seeds.
iv. Application of nitrogenous fertilizer should also be done for making good vegetative growth.
The followings are the main advantages of this method:
i. It is very effective when topsoil is good.
ii. It creates natural appearance of stream bank.
iii. In comparison to structural methods, it is low cost technology.
Apart from above mentioned advantages, there are few disadvantages of this technique, given as under:
i. It is not adequate to control severe bank erosion.
ii. Seeding must be done during low flow conditions; and must be available sufficient time to grow before high flow to take place. Otherwise, the germinated plants will not develop a good root system, as result due to high flow the plants will likely to get destroy due to washing away of soil.
iii. Seeding is not suitable for stabilizing the toe of the stream bank.
Table 6.2 summarizes the criterion for selection of suitable stream bank stabilization methods.