PROBLEM OF BANK SLOUGHING IN THE BORAK VALLEY
The River Borak is the main river of Borak Valley comprising of three Districts namely Cachar, Hailakandi and Karimganj. The River Borak originates from the Naga Hills and Manipur Hill ranges. The river flowing from its sources enters the Cachar District upstream of Lakhipur and flows towards west in Zigzag course till it reaches a place near Haritikar where it bifurcates into two branches namely the Surma and the Kushiyara flows in the western direction forming the International boundary between India and Bangladesh for a length of about 32KM in Karimganj District before entering into the Bangladesh territory near Lafashail. In this reach the Indian territory is on the left bank . The Northern branches Surma initially flows towards north and then north west forming the International boundary between India and Bangladesh for a length of about 22KM in Cachar district before entering into the Bangladesh territory near Natanpur. In this reach the right bank is in the Indian Territory. The river Borak and its branches Surma and Kushiyara are fed by number of tributaries on both banks.
The flood gradient of river Borak is very flat and varied from 1 in 10000 in the upstream to 1 in 21500 in the downstream reaches.
The river Borak and its branches are of highly meandering in nature for which it has changed its course at a number of places. Such process of changing river course is still continuing. The last such change of course of river Borak occurred on 16/10/1992 when the short circuiting of river Borak occurred at Dungripar in Cachar district. Due to such short circuiting at Dungripar the length of the river Borak was reduced by 8KM.
The Valley is subjected to intense rainfall during the monsoon. The highest annual rainfall recorded at Silchar. E & D Colony was 4194MM in the year 1989 and the maximum daily rainfall at the same place was 243MM on 18/6/85. The highest recorded level of the river Borak was 21.84M on 01/08/1989 and the maximum discharge was recorded as 6696 cumec in the year 1966 at Annapurnaghat gauge discharge site (Silcahr). The river carries considerable silt load and highest yearly yield being 7935.68Cm.m in 1985 at Badarpurghat observation site.
Due to gradual siltation the average ground level along the banks of the Borak is generally much higher than the adjoining country side with the ground level falling steeply away from the bank. Throughout the valley there are low- lying areas on either bank of the Borak, the Kushiyara, the Surma and the tributaries. This depressions are locally called Haors and Beels. The principal Haors are the Boila Haor, Chatla Haor, Bakri Haor, Mongalpur Haor and Kaptanpur Haor, Son Beel etc. It is interesting to note that the general ground level at Silchar town which is 6436KM away from the sea has an elevation of about 19.80 M G.T.S. only. Also the bed of the river in many places is well below the mean sea level. This must be ascribed to the fact that the Borak Valley had been a part of the sea before it was built up.
Due to concentration of population along the river banks there is the existence of innumerable house ponds and pools all along the valley. These ponds and pools are mainly used for drinking and domestic use. Due to above characteristics the valley remains water logged. As the ponds are not normally lined there is seepage from the ponds which keeps the soil strata charged all through the year.
The soil formation along the bank consists of a sequence of layers of silty clay, fine sands and highly plastic clay. It is observed that there are silty and sandy layers at different levels with relatively high permeability. Such soils and even the clay layers along the bank have been observed to have dispersive characteristics. In general the soil deposit is unconsolidated and loose.
Due to high meandering nature the river Borak and its tributaries are characterized by scour and erosion on the concave side and shoaling on the convex side of a bend. Whenever a bend is established the deep channel flow of the river gradually shifts towards the concave bank and the process continues from time to time. Causes of bank failures due to erosion in the valley are observed to be as follows :-
- Washing away of soil particulars from the bank by strong current.
- Undermining of the too of the bank by eddies, currents etc. followed by a collapse of overhanging materials which are deprived of support.
However, the most critical problem of failure of river banks has been observed to be due to sloughing and sliding of bank at number of places in the valley. This is due to seepage flow of water from the near by tanks, ponds, pools (having higher water levels than the river) to the river. In places where there is no nearby tents or ponds but the banks sloughing is occurring the reason of such occurrence can be attributed to relatively higher ground water level than that of the river. Due to dispersive properties of the soil layers as stated earlier. The seepage water carries fine soil particles also. As the fine particles from a soil layer is carried away by the seepage water formation of tunnels takes place in such layers. Such formation of tunnels increases from time to time and ultimately the shearing resistance of the soils along the bank diminishes to a value lesser than the shearing stress induced along the most severely stressed or critical internal surfaces of the bank soil thereby resulting in collapse of soil layers along the bank. It is observed that the process begins when a small amount of material at the low water level is eroded and produces a small slide. It leaves a relatively steep faces which becomes saturated by seepage and slumps again forming a higher and more unstable face. This process is continued until the stability of the slope is achieved and in the process a great width of land mass is eroded away. This process of seepage can be observed at any problematic site in a freshly cut trench along the anticipated seepage water can be observed near the low water level almost all along the river during the draw down stages. From observation it can be said that major failures are found during the rapid recession of floods.
During the sudden recession of the flood level, the pore pressures within the slope cannot adjust themselves quickly to the new water table rendering the bank slope unstable. The problem continues even after the floods till the rise of water level of the river during the next monsoon. This is due to dispersive characteristics of the soil and absence of band between the particles for arresting soil movements due to seepage flow of the charged ground water.
The following are the major problematic sites of Cachar, Hailakandi and Karimganj districts in which the problem of banks sloughing is very acute.
Gobindapur, Bagpur, Jhangerbali, Singirbond, Dhamali, Weavers Colony (Silchar) , Bethukandi (Silchar), Ajur (Silchar),Tarapur(Silchar), Manikpur, Ujangram, Buribail, Malakarbasti, Shantipur, Kathgora Bazar Badarpur site located along the bank of river Borak.
Nagdirgram, Chandpur, Sonaibazar sites located along the bank of river Sonai which is a tributary to river Borak.
Daspalty, Charakuri, Alongjuri, Deopur, Chandsrikona sites etc. located along the bank of river Kushiyara.
Katlicherra bazaar, Mohanpur sites etc. located along with the bank of river Katakhal.
Bilbari, Burunga, Nalibari, Patherkandi bazaar sites etc. located along the bank of river Longei.
The Central Soil and Materials Research Station (CSMRS), New Delhi, carried out field & laboratory investigation in 9 problematic sites in Barak Valley during 1987. "A total number of 65 soil samples (10 core Cutter/Brass tube samples, 8 block samples and 47 representative composite bulk soil samples) have been collected at various locations for taking up preliminary laboratory investigations. However, out of the above mention 65 soil samples, only 56 soil samples have been considered for laboratory tests".
The CSMRS analyzed the test results. The extract of their findings/conclusions & recommendations are reproduced.
Out of 56 soil samples tested for general soil classification, 10 soil samples fall under MI group, 8 fall under CI group, 14 soil samples falls under CL group, 8 soil samples fall under CI group, 12 soil samples fall under SM group & 1 soil sample each fall under MH, CH & SW & SM groups of Indian Standards soil classification respectively.
Barring a few exceptions, the soils at the first 4 sites viz, Baghpur site, Dungripar site, Chandpur site, Sonai Bazar site are predominantly silty/or cohesion less where as soils from the remaining sites show both silty as well as clayey soils. While the bank formations with silty soils are susceptible for erosion & sloughing generally, the clayey soil should be relatively erosion resistant. But as can be seen from the special tests done for dispersibility irrespective of soil classification, the dispersive nature covers the different groups of the soil including CI, CL, and MH etc. It is, therefore, evident that evens this soils which can be classified as cohesive and non-erodible from the general point of view, are erodible in this present case due to their inherent dispersive nature. In such soils , unless meticulous measures are taken to totally avoid movement of colloidal sizes also, these problems of dispersion and sloughing cannot be avoided particularly when extensive flooding and long term water pooling is expected in other words, it is not only drainage measures but the suitability of the particles during drainage should also be totally ensured.
Limited tests conducted for liquefaction potential of the river bank soil indicate that possibility of liquefaction problems during earthquakes is not ruled out.
The treatment in such circumstances are guided by specific site conditions of physiography , flood levels, velocity etc. the efficacy of any treatment should also be verified on trial sections in the field and even if certain ineffectiveness of such measures are noticed, they can throw further light on the approaches. The problem is very complex and a tailor-made solution that too based on this limited investigations cannot be thought of. However, in general, the dispersive nature of the soils can be mitigated with addition of 2-4% of lime which can be tried for economic permits. Use of geo-filter, geo-fabrics consisting of natural and indigenous material can be tried.
Provision of suitable trenches fixed with well graded granular materials going down up to silt sizes may also work effectively. But there are several other treatments like provision of diapharm walls, plastic concrete, mulching of phosphate and bentonite slurries etc. have also helped in some cases to deal with dispersive soils. However, the efficacy and economics of all such treatment need to be validated particularly under field circumstances. The problem being extensive and complex, calls for in-depth field oriented observational studies and an approach of conservative surgery as otherwise, a total solution would be totally uneconomical.
Use geo-fabric (nailing of geo-fabrics with proper backing/ filter wick using jute fabrics etc) may also be tried in some selected problematic reaches.
The above suggestions in combination with suitable river training measures (in order to control the high degree of meandering of the Barak river) in consultation with the designers may be more effective.
It is observed that the annual rate of loss of width of land mass due to bank sloughing and erosion varies from 3M to as high as 40M or so.
To counteract the problem of bank sloughing longitudinal filter drains running along the bank with transverse inclined filter drains originating from the longitudinal drains and ending at the low water level of the river at suitable intervals are provided. This is also as per recommendation of the Technical Advisory Committee. These inverted type of filter drains consist of four layers of graded filter materials arranged with the smallest particle size at the bottom and largest at the top. Typical plan and cross-section of the arrangement of the above system are shown below. The longitudinal filter drain is provided at requisite depth below ground so as to intercept the seepage flow and divert it through the transverse drain towards the river. However, the transverse drains also arrests seepage flow if however prevailing in the intermediate zone between longitudinal drains and low water level. The aforesaid arrangements along with bank revetment and provision of apron where there is securing is found to be most useful in this valley. The stability of the affected bank can further be improved by providing counterfort type of transverse drains which in addition to its action as a filtration medium also induces frictional resistance to the sliding of bank. In Borak valley the Water Resources Department have so far successfully implemented more than one hundred such protection measures against erosion & sloughing. Names of some important and most successful work are " Dudhpatil, Ghantagram, Gobindpur, Joypur Natunbazar, Umarpur (Badarpur), New Colony (Silchar), Ghaniwala (Silchar), Uttarkrishnapur (Silchar), Bambidyapith (Sonai), Katlicherabazar (Katlichere), malugram (Silchar), Madhurbond (Silchar), Padrighat and Khadiman (Badarpur), Sutkipatty (Karimganj) , Daspatty (Karimganj), Chandpur (Sonai).
However, in some case it is observed that the drains get gradually chocked after 3 to 5 years of services. This is due to deposition of fine soil particle in the drains by the arrested seepage water. This is one of the causes of failures of filter drain type preventive measures for the prevention of chocking filter drains in critical areas. The graded filter materials are wrapped with geo synthetic woven fabrics which prevents entry of fine soil particles in the graded materials. Another cause of failure is due to dislocation of the drains. As stated earlier, the soil formation along the bank is generally unconsolidated and loose for which sometimes a part of the drains settles down resulting in discontinuity of the drains. Proper care needs to be exercised for compaction of the foundation of the excavated drains as called for before laying the filter medium.
Though the filter drain system of preventive measure against bank sloughing is most useful. It is at the same time very expensive for which protection measures can be provided to important areas only in this valley. In view of the above the department is looking for alternative economic measures such as use of Geo-Mat, Geo-tube walls, etc. It is expected such as use of Geo-Mat, Geo-tube walls, etc. It is expected some of such pilot projects will be implemented in near future.