Control of Left Bank Erosion of River Ganga between Rajmahal and Farakka
FACTORS INFLUENCING RIVER BANK EROSION
River Morphology
Includes from or structure of the river
Channel pattern : straight, meandering, or Braided
Nature of sediment transported
Geology
The geological history of the river basin has a great influence on the behavior of the rivers in the region. This includes the formation of the land form, the types of rocks and history of decomposition, interaction with sea, rivers, climate and other external influence.
Geomorphology
Includes surface features of area around past and present courses of the river. With availability of satellite imageries and GIS, geo-morphological classification can be conveniently done.
Point bar, Channel and back – swamp deposits are prone to erosion.
From geo-morphological classification and Time series bank line shift it is possible to make advance planning for protection measures.
Time series analysis of bank line shift also helps in identifying vulnerable stretches if adequate data is available.
Porcupines
Porcupines are crates made of bamboo pieces which are filled with stones of adequate weight. When these are dropped in the river near the toe of the bank in sufficient numbers, erosion is controlled considerably by promoting siltation.
Bank Erosion
• An alluvial channel particularly on floodplain with imperceptible slope constantly adjusts with the available slope, discharge, length, cross-section geometry and other fluvial and hydrological variables
• In the process, such channels present spectacular spectrums of meander with varying sinuosity index, rotation, translation and migration of meander necks
• All these held essential controls of bank erosion as it involves lateral erosion, channel widening, channel lengthening, changes in competence stability and curvature, in channel deposition and so on.
• At each reach the channel morphology is a result of the interaction of the hydraulics of flow (velocity, discharge, roughness and shear), channel configuration at the reach and immediately upstream,( width, depth, shape, slope and pattern) , load entering the reach (caliber and amount) and bed and bank material.
• Nearly all natural channels exhibit some tendency to develop curves which seem to be proportional to the sizes of the channel. Because most streams are sinuous to some extent, it has often been suggested that the term meandering be restricted to channels exhibiting curves of considerable symmetry.
• Bank cutting and orderly transfers of sediment to its deposition on point-bars were principal requirements for meandering.
• Friedkin’s concept is essentially identical- the only requirement of meandering is bank erosion…… when all outside disturbing influences were eliminated (in the laboratory)…. The development of a series of uniform bends from an initiating bend was positive and capable of duplication.
The forces determined by velocity distribution, including helical circulation, can account for the shape of the cross section in a meander, the depositional and erosional pattern and the down valley migration.
Meandering rivers migrate across their flood plains through deposition on the point-bar at the inner bank matching erosion at the outer bank. As a result meander migration can adversely impact flood plain dwellers and users through erosion of agricultural land by causing damage to riparian infrastructure.
Mechanism of Bank Failure
Bank failure generally are of three types:
Erosional failure Flow slides
Shear failure
Erosional failure occurs
when
sl< cc
&
tc>ss where sl is sediment load and cc is carrying capacity & tc is the tractive force of river current and ss is the shear strength of bank material.
Flow slide occurs during falling season when:
There exists thick beds of sand and silt underlying cohesive overburden
&
Such beds are subjected to seepage under a high hydraulic gradient from bank to river
Shear Failure Occurs, when
• The shear stress along a surface exceeds the shearing resistance along that surface.
• Toe erosion, seepage pressure and many other factors may be responsible for this.
Hydrology & Hydraulic Characteristics:
• Flow pattern : Maximum, Minimum and average flow with direction
• Bed and water surface slopes
• Hydraulic mean depth, width velocity, sediment load concentration and characteristics.
These properties influence channel morphology related to erosion.
River Morphology
• Includes from or structure of the river
• Channel pattern : straight, meandering, or Braided
• Nature of sediment transported
Geology • The geological history of the river basin has a great influence on the behavior of the rivers in the region. This includes the formation of the land form, the types of rocks and history of decomposition, interaction with sea, rivers, climate and other external influence.
Geomorphology
• Includes surface features of area around past and present courses of the river. With availability of satellite imageries and GIS, geo-morphological classification can be conveniently done.
• Point bar, Channel and back – swamp deposits are prone to erosion.
• From geo-morphological classification and Time series bank line shift it is possible to make advance planning for protection measures.
• Time series analysis of bank line shift also helps in identifying vulnerable stretches if adequate data is available.
Technical Intervention
Construction of Barrages and dams – limiting the width of the river downstream
Construction of Circuit Dam Continuous construction of Retired Dams, Marginal Dams to protect land from flooding
Construction of spurs to control the flow
THE GANGES
• IS THE LARGEST PERENNIAL WATER BODY OF THE COUNTRY
• CARRIES NEARLY 33% OF TOTAL DISCHARGE OF WATER IN ALL RIVER SYSTEM OF INDIA
• TOTAL LENGTH 2525 KM
• THE SLOPE AT FARAKKA 1 IN 18700
• AVERAGE ANNUAL DISCHARGE 36,400 MILLION M3
• FLOOD DISCHARGE 59,500 M3 /SEC TO 65,5000 M3 /SEC
• THE RIVER SYSTEM DRAINS 10.68 LAKH K M3 AREA
• CARRIES 30 LAKH CRORE METRIC TONS OF SEDIMENT ANNUALLY.
River Responses When extra water and sediment come to an alluvial channel the normal response of the river is
To overflow its bank through which it increases its cross-sectional area and carry the extra load
If the river can’t overflow its bank and deepens its valley to increase the cross sectional area
Therefore construction of all sorts like retired dams, circuit dams, revetments, spurs etc. ultimately results in increased erosion
Position Of The Main Channel In The Braided Course Of The River
• Far before the construction of Farakka Barrage in the year 1922 or before the main channel was centrally located.
• About hundred years before that it was hugging the right bank
• In the available maps of 1948/50 the river shows a tendency to shift its main channel to the left
• Construction of Farakka Barrage was a shock to entire natural system leading to changes in bedfrom and planform of the river
• Bhutni Diyara circuit dam and deterioration of river Kosi aggravated the left ward shift in Manikchak and Kaliachak sub-division
• Recent images show a gradual development of a central channel and a in the right
RATES OF EROSION IN DIFFERENT YEARS
VALUATION OF PROPERTY DAMAGED
Homestead Land @ 4,50,000 per Hect Rs 1,437.75 Lakhs
Orchard land @ 22,40,000 per Hect Rs 776.80 Lakhs
Agricultural Land @ 2,20,000 per Hect Rs 2,811.60 Lakhs
Waste land @ 60.000 per Hect Rs 127.86 Lakhs
Total : Rs 5,144.10 Lakhs Net average due to erosion per year = 5144.10 = 514.40 Lakhs
10
Measures suggested by different committees
• In 1980 Preetam Shing Committee suggested construction of two spurs at 28 and 29km upstream from Farakka to divert the flow again to right to Rajmahal
• In 1996 Expert Committee also suggested the same
• Recent suggestion is to dredge the right channel to divert the main flow
• In 2007 near Kaliachak the river is showing a tendency to leave the left bank
Channel shifting and bank erosion is a natural process and any attempt to mitigate it atleast in case of the Ganges in Malda has failed leading destruction of eight retired dams and nineteen spurs one by one
RESULT IS LOSS OF PROPERTY AND LAND
CHANGES IN DRAINAGE PATTERN
• BETWEEN 1937 AND 1963 THE RIVER BETWEEN RAJMAHAL AND FARAKKA WAS STRAIGHT
• SINCE 1963 THERE WAS A TENDENCY OF SHIFT TOWARDS THE LEFT BANK
• 1971 INCEPTION OF FARRAKA BARRAGE
• THE TENDENCY WAS AGGRAVETED SINCE 1971 AND SEVERE EROSION TOOK PLACE IN 1971-73
• BETWEEN 1974-1996 BANK EROSION AND CHANNEL SHIFTING WAS MODERATE
• SINCE THEN SEVERE BANK EROSION CONTINUED TILL DATE
THE AREA COMPRISES OF HOMESTEAD, MANGO ,ORCHARD AGRICULTURAL LAND AND WASTE LAND
HOMESTEAD LAND………319.50 Hect
ORCHARD LAND ……….. 319.50 Hect
AGRICULTURAL LAND …1278.00 Hect
WASTE LAND …………....213.10 Hect TOTAL: 2130.10 Hect
River Rupnarayan
• The Rupnarayan river is a vast tidal basin that receives the tidal influx through the Ganga estuary at the Hooghly Point
• The river does devastating bank erosion at several places in the Dainan- Geonkhali segment where the hazard enhances every year during monsoon destroying vast stretches of rich, fertile and valuable land resources.
• The fluvial hydraulics of this channel represents the resultants of three mutually interacting elements, the tidal influx and outflux, the alluvial material matrux and the catchment hydrology
River Hogol
River Hogol is a tidal link channel between Matla river and Hana river. The average width of the river 200 to 250mtrs. The average depth is at the order of 25 to 18 mtrs.
A subsidence in the bank line of about 150mtr long occurred at Basanti Bazar endangering the protection of the Basanti Bazar itself approximately since 1985. moreover, a good number of places on both the banks of the river have been showing sign of subsidence concerning to security of the entire area.
• The sector of the Ganga river between Mayapu and Nabadwip, the Ichamati and Bidyadhari river at Taki and Basirhat. The Bhagarathi River in Murshidabad and Nadia along all their distributaries doing enormous erosion.
In conclusion it may be said that we needy and greedy people and are interrupting the natural flow of the river to fulfill our own need so we have to face the consequence
PREVENTIVE MEASURES
• SPUR is a construction transverse to river which is built to divert the flow of water. It’s one end is attached to the bank and other end faces the river. Spurs are of three types:
1. Rippling Spur : 75 degree to the direction of flow
pointing toward upstream
1/3rd of effective waterway
the spacing of the spurs is 2 ti 3 times to the length of spur
built all along the affected area
in areas of severe erosion small spurs of 125 m length are built at a gap of 250 – 300m Attracting Spur: is built to maintain the depth of navigation channel
pointing to down stream
length equals to 1/3rd of effective waterway
3. Deflecting Spur: are vertical or built at 90 degree
mouth semicircular
nose solid
radius 16.5m
time of persistence depends on rate of bank erosion, shifting of scoured depth etc
• Aprons : are layer of boulders and stones covering the beach to protect it against direct erosive force
are of two types:
1. Launching Apron : boulders are scattered on bank area
extent depends on the scoured depth
when land gradually subsides this will automatically forms a slope of boulders and prevent erosion
2. Revetments: a type of apron
building depends on profile of the bank
boulders are placed from boats or vessels
50 to 60m width length depends on the effective erosion of channel
• Embankments:
1. Marginal embankments are built away from the bank line on high land
2. Retired embankments are built on lowland beside river
purpose is to protect yearly flooding
distance of construction from the river is not fixed depend on as per availability of land
Porcupines • Porcupines are crates made of bamboo pieces which are filled with stones of adequate weight. When these are dropped in the river near the toe of the bank in sufficient numbers, erosion is controlled considerably by promoting siltation.