Bank erosion in case of large rivers and bigger streams need special engineering measures. Some of the simpler erosion control, measures useful on small streams.
Stream bank erosion control may be broadly classified into two groups-direct protection works and indirect protection works. Direct protection includes the works done directly on the bank itself for buffer effect or consolidation effect such as creation of vegetal cover, covering by brush wood, paving with stone or masonry etc.
Indirect protection measures are not constructed directly on the banks, but are constructed in front of them. These measures reduce the erosive force of the current either by diverting the current away from the banks or by inducing deposition of silt in front of the banks.
The various kinds of stream bank erosion control works can be grouped as follows:
1. Direct protection works consisting of –
(i) Stabilizing the stream bank by vegetation,
(ii) Protection of stream banks and slopes, and
(iii) The protection of lower bank.
2. Indirect protection works, consisting of –
(i) Deflection of current and deposition of sediment by installing retards,
(ii) Deflection of current and deposition of sediment by construction of spurs, and
(iii) Diversion of runoff from the stream banks to prevent gully and ravine formation.
Marginal embankments or levees are earthen embankments, nearly parallel to the river at some suitable distance from it. They may be on one or both sides of the river for a distance where protection is needed for specific areas like towns etc.
These embankments contain the flood waters from spreading and are essentially for flood protection rather than river training purpose. Details of river embankments and other river training works are given in Thorn (1966) and Garg (1986).
Direct Protection Works:
i. Stabilizing Stream Banks by Vegetation:
When the vegetative cover over the stream banks is destroyed either by human agencies or by cattle, erosion is likely to take place. Such erosion on the stream banks can be effectively controlled by creation of vegetal cover with suitable species on the eroded banks and protecting them against browsing and grazing.
The planting of the stream banks should be a phased programme and the initial work be covered in a three year period. However, maintenance will be continued even after the third year until the plantations are fully established.
The schedule for plantations on the stream bank may be as follows:
First year –
(i) Survey and demarcation,
(ii) Raising of green fence around the area,
(iii) Digging of pits inside the plantation block, and
(iv) Preparation of nursery for growing of seedlings etc.
Second year –
(i) Refilling of pits, application of manure and fertilizer,
(ii) Planting of seedlings including soil working,
(iii) Maintenance of live fence and gap filling wherever necessary, and
(iv) Watch and ward.
Third year –
(i) Maintenance and repair of live fence with necessary gap filling,
(ii) Casualty replacement and gap filling in the main plantation,
(iii) Weeding, manuring and plant protection measures, and
(iv) Watch and ward.
Fourth and Fifth year –
(i) Maintenance, gap filling etc., and
(ii) Watch and ward.
The extent of plantations on the stream bank depends upon the availability of the area along the stream banks. Cattle passages should be provided in the plantation blocks. The species selected for plantation should be of some economic importance.
ii. Protection of Stream Banks and Slopes:
Where valuable properties extend right up to the edge of the stream bank, the protection of the stream bank by plantation is not possible. In such cases the slopes of the stream bank should be directly protected by suitable methods.
Depending upon the local conditions direct protection of the slopes can be done in one of the following ways:
1. Slope Protection by Sodding or Turfing:
Stream bank slopes if not subject to strong current can be well protected from erosion by a vegetal cover. This vegetal cover may be established either by sodding or turfing of the slopes with local grasses.
However, this is a costly work. Instead establishment of shrubs and creepers can be conveniently done. Bushes like vitex, lantana, ipomea can be usefully adopted for planting the slopes of the stream bank.
In these cases, it will be better if the slopes of the stream bank are eased to a stable slope (preferably 1½: 1) depending upon the soil material with which the bank is made of.
2. Protection of Slopes by Revetment:
When the current near the bank slopes is strong such that ordinary vegetation cannot protect the bank, paving of the slopes that can resist the erosion is necessary. Paving can be done temporarily by brushwood weighted with stones. This serves only for short duration.
In case of strong currents and where permanent protection to the stream bank is required, paving of the slope with stone pitching or masonry, is to be adopted. In all cases before paving is undertaken the stream bank should be eased to a stable slope—usually between 1½ to 1 : 1.
In case of small streams where there is no water during summer, rough stone dry packing can be adopted. The toe wall is to be taken at least 1 m to 1.2 m below the bed of the stream and connected to the stone paving. The stone paving should also be not less than 60 cm in thickness and well packed.
In case of large streams where there is summer water flow, marginal protection with stone revetment can be done only above the summer water level. To build the stone revetment on the bank slope, a sort of foundation is to be obtained by dumping rough stones to summer water level to form’ a berm of 1.2 m to 2 m in width.
The bank above is to eased as usual and the revetment constructed with, stone packing. Where swift currents are expected stone masonry in cement mortar is to be done. But this is very costly and as such cannot be done on larger lengths of the bank.
Whenever stream banks are protected with permanent revetments, it is necessary to plant grass or some bushes to a certain length immediately above the revetment. This will prevent the erosion at the joints and stabilise the revetment.
3. Protection of Slopes by Brushwood Rollers and Gabions:
This consists of grass or brushwood rollers 20 cm to 30 cm in diameter, with silt inside and tied with coir or any other rope outside. The length of the rollers may be 2 m to 3 m as per convenience.
These rollers are secured to the bank by means of poles driven through these rollers into the bank. This type of work is purely temporary and is to be used where local slopes are to be protected.
This work gives good results when used with stone revetment. Stone revetment may be done upto the height of active erosion and above this the rollers may be used. In this case the rollers prevent the receding flood from eroding the unprotected margin and getting behind the stone revetment.
Stones packed in expended metal or wire net (known as gabions) are also used instead of brush wood rollers. They are more expensive and difficult to install. Care need to be taken regarding the durability of the wire net.
4. Protection of Stream Banks by Brushwood Edging:
Brushwood edging work can be used to protect stream margins from further erosion particularly where the stream takes, a sharp bend and where bed scour is not deep.
Brushwood edging work consists of two rows of sal, casurina or bamboo poles 8 cm to 10 cm in diameter driven into the bed of the river at 1 m intervals and rows being kept about 1 m apart. The top of the edging should not be more than 1 m to 1.5 m above the average bed of the stream at the site.
The poles should be driven into the streambed to at least half of their length in order to obtain stability. The edging should be nearly parallel to the general flow of water in the stream.
Horizontal wallings of split bamboos should be nailed on the outside of each row of the poles. The space between the rows of poles is filled with well pressed brushwood and securely held in position by means of diagonal cross ties fixed on top of the poles.
The area between the edging and the stream bank is filled with earth or brushwood weighted with stones. During floods this brushwood edging prevents the cutting of the bank and helps in accumulation of silt.
This silt accretion should be planted with suitable vegetation as soon as the flood subsides. These plantations protect the stream banks from further erosion and stabilize them.
iii. Toe Protection of the Lower Bank:
The lower portion of the stream bank is constantly under water and is subjected to constant erosion.
For protection of this lower bank the following methods can be adopted:
1. Providing a flexible apron with brushwood or wire mattresses extending to the stream bed as far as needed.
2. Dumping of loose stone, or stones packed in brushwood or wire netting at the toe of the bank.
3. Construction of retaining walls.
Indirect Protection Works:
i. Deflection of Current and Deposition of Sediment by Installing Retards:
For controlling bank erosion for shorter lengths, installation of retards in front of the eroded bank is a cheap and effective way. These retards reduce the erosive force of the water on the stream banks and help in the deposition of the silt on the eroded portion. Subsequently vegetation can be established permanently on this deposited silt, stabilizing the stream bank.
These retards are mainly of two types:
1. Wooden Jack Series:
Where the stream current is not swift, installation of a row of jacks along the eroded stream bank is effective. These jacks consist of three wooden poles of sal or bamboo crossed at the centre and tied by means of wire.
The length of the poles may be between 3 m to 5 m depending upon the depth of the stream which is to be controlled. A series of jacks enough to protect the area where bank cutting occurs should be constructed. They should be spaced as close together as possible, with not over one jack space apart.
The centre point of each jack is tied with the help of a main cable which in turn is connected to one or two heavy logs serving the purpose of an anchor. The logs used for the purpose of anchor should be about 2 m in length and about 20 cm to 25 cm in diameter. Each jack in turn should be weighted by rocks which can be wired on to the poles.
2. Groynes or Spurs:
Where the stream takes to meandering and eroding the banks and where the cuts are vertical, spurs can be effectively used for controlling such erosion. Groynes/spurs are structures constructed transverse to the river flow and extend from the bank into the river. These are also known as spur-dikes or transverse dikes.
The spurs are constructed to serve one or more of the following purposes:
1. Training the river along a desired course;
2. .Creating a slack flow for silting up the area;
3. Protecting the river bank from erosion by keeping the flow away from the bank; and
4. Contracting a wide river channel usually for improvement of depth for navigation.
Classification of Spurs:
Spurs are classified as under:
1. Classification according to materials of construction, i.e., permeable or impermeable;
2. Classification according to height of the spurs, i.e., submerged or non-submerged;
3. Classification according to the characteristics of flow, e.g., attracting, deflecting and repelling; and.
4. Classification according to shape of nose, i.e., Hockey, T-headed, etc.
The choice of the type and design of the groynes will depend on the particular situation that is considered. In case of large rivers, the designs are based on hydraulic model studies. In general, permeable groynes are used for silt laden rivers, deflecting groynes to form deep water channels for navigation and repelling and attracting groynes for bank protection and encouraging sedimentation near the banks.
Dry Boulder Spurs:
Dry boulder spurs are permanent structures and will effectively protect the stream bank. These can be advantageously constructed where large size boulders are available.
These spurs consist in closely packed stones in the form of a trapezium or to a nearby shape and starting from the bank extending into the stream.
The following points may be observed while constructing these spurs:
1. Fairly large size stones should be used and they should be well packed.
2. The structure should be well keyed into the stream bank.
3. The spurs should be extended into the stream at an angle of 45° for a distance of about 30 per cent of the channel width.
4. On small streams (width less than 30 m), the spacing of the spurs may be made equal to the stream width and length 25 per cent of the channel width.
5. On larger streams a method of locating the spurs is illustrated in Fig. 25.6. Point A, location of the first spur is the intersection of the centre line of flow and the eroding banks. Spur C is located by drawing EB parallel to the flow line and across the toe of spur A.
Length AC is taken as twice AB. Spur D is located by projecting a line across the toes of A and C. The remaining spurs are located in the same way as D is located. Supplementary spur K is located at a distance of AB upstream from A and the size of the spur is to be half the size of the other spurs.
Sometimes the spurs are spaced based on the energy loss principle. Referring to Fig. 25.7 the spacing between the spurs, L, is governed by the factor (2 gL/c2h) where c is the coefficient in the Chezy’s equation). This factor must be less than unity.
This is under the assumption that the back water flow in a confined eddy is only possible if the level of the water surface in the stagnation point of the downstream spur is higher than the water level at point A of the upstream spur. This implies that the energy loss iL of the stream flow between two spurs is less than the velocity head U2/2g.
Permeable Spurs:
Permeable spurs are useful in areas where deep scours are not expected. Permeable spurs consist of two rows of wooden poles driven into bed of the stream and the space in- between the poles is filled with brush wood, casuarina or good bamboos depending upon the local availability.
A type design of a permeable spur is illustrated in Fig. 25.8. The design of the permeable spur may be varied depending upon the materials available at the site. The length and spacing may be followed as per those mentioned for dry boulder spurs.
Loose brushwood if available at the site can be utilised for the control of bank erosion. This has to be properly arranged and secured both to the bed and bank of the stream.
ii. Diversion of Runoff:
At some places along the stream bank, runoff from the adjacent areas enters the stream and in that process gullies and ravines are formed on the stream bank. If this is left unchecked the erosion is accelerated by the stream also.
In such cases usual gully control measures like diversions, construction of check dams etc. are to be adopted. In most cases diversion of runoff and disposing the same through a protected outlet will control such erosion.