Landslide the perceptible downward sliding or falling of a relatively dry mass of earth, rock or combination of the two under the influence of gravity. The cause is often infiltration of water that makes the swelling soils (clays) more fluid. Landslides are a major cause of erosion, causing the strongest degradation known, often exceeding 10,000 tons per sq km in a year. They are common in the hilly areas of southeastern Bangladesh. These areas have a long history of instability. Although written records of landslide incidents are very rare, they have been a hazard to people ever since they have been living there. In fact, every year especially in the rainy season landslides take place in both natural and man-induced slopes. Although, Bangladesh is a densely populated country, the hilly region presents a sharp contrast with the overall demographic pattern. This is partly due to the landslide hazard potential which discourages many people to live there as well as to build infrastructures; however, inaccessibility, dense forest cover and the hilly topography are also discouraging factors.
One of the main problems related to landslides is blocking of roads. This problem is very common in bandarban and rangamati districts. The major roads connecting Bandarban town with the rest of the country are affected by landslides almost every year isolating the town and contiguous areas. Landslides due to the construction of buildings and other infrastructures have been mostly restricted to the urban and semi-urban centres of hill districts town. Many buildings and infrastructures especially those located on steep high slopes usually fail due to landslides causing the loss of property and lives. The effect of jhum cultivation and other forms of cultivation on steep slopes also played a significant role in the occurrence of landslide in the past years.
In spite of all these hassles the hilly regions are important to us because they occupy about 18% of the total area of the country. The region is also rich in natural resources. Recently, many foreign companies are conducting exploration programmes for oil and gas in these regions.
As a developing country well-developed communication system is required to exploit the natural resources. But areas prone to landslide hazard and causes and factors of landsliding were not considered during the construction of these roads. For this reason every year landslides take place along the roads and block communications. Not only roads but also a considerable number of infrastructures are damaged and valuable lives are lost most of which, in oblivion.
Despite its definite adverse impact on human society and economy, studies on landslide began relatively late- towards the end of 1950's. The construction damages of the Mercantile Marine Academy, Chittagong were caused due to the lateral spreading movement in hills formed of Dupi Tila Formation.
On the other hand the steepness of the natural slopes (more than 12), altitude of the bedding, lithology of the rocks, infiltration of rainwater into the barren slopes are the major factors of the hazard around Chittagong University Campus and its contiguous areas.
Three types of landslides and slope failures which are lateral spreading movement, rotational and translational movements in the hills formed of Dupi Tila Formation, planar or block movement in Bokabil Formation and rotational, translational and planar or block failures in Tipam Formation are responsible for the erosional hazards of Chittagong City. The landslide of the Jhagar beel area, Rangamati district is caused due to the huge loads of the embankment resting on the weak stream fill sediments and the high side slopes (1:0.75). In 1997, 160 landslide prone areas in Khagrachhari and Cox's Bazar districts were identified using geographical information systems (GIS). In the years 1998 and 2000, works were concentrated mainly around Bandarban sadar thana and Rangamati town.
Types, causes and nature Major nature and processes that cause landslides in Bangladesh are 1) removal of lateral support: (a) erosion by rivers, (b) previous slope movements such as slumps that create new slopes, (c) human modifications of slopes such as cuts, pits, and canals; 2) addition of weight to the slope: (a) accumulation of rain, (b) increase in vegetation, (c) construction of fill, (d) weight of buildings and other structures, (e) weight of water from leaking pipelines, sewers, canals, and reservoirs; 3) earthquakes; 4) regional tilting; 5) removal of underlying support: (a) undercutting by rivers and waves; (b) swelling of clays; 6) anthropogenic activities as jhum cultivation.
Chronology of major landslides
|1968||At Kaptai-Chandraghona road where the protective vegetation is removed, the soil gets exposed to the monsoon rains and eroded rapidly. This resulted in landslides, and the loose soil washed down the slopes and carried by rivers into the kaptai lake. As a result, the reservoir silted up and the authorities confirmed that in its 30 years existence it had lost about 25% of its volume due to siltation.|
|1970||Similar event along Ghagra-Rangamati road.|
|1990||Occurred on May 30, 1990. Affected the link road embankment at Jhagar beel area of Rangamati district.|
|1997||A major landslide occurred in July 1997 at Charaipada of Bandarban. The total area affected by it was about 90,000-sq m. If such a landslide occurred in Bandarban Town and any other urban or semi-urban centre, the devastation would be tremendous.|
|1999||Two big landslides one in Bandarban and the other one in Chittagong occurred on 11 and 13 August 1999 respectively claiming the life of 17 people. Out of 17 fatalities, 10 were in Chittagong and the rest in Bandarban district. Heavy and incessant rainfall at that time was one of the causes of sliding. This landslide affected Lama thana and the Aziz Nagar union of Bandarban district. Aziz Nagar is almost an inaccessible rugged hilly terrain. Landslide badly affected the villages of Chittaputti, Monargiri, Meounda, Muslimpara, Sonaisari, Bazapara, Kalargiri, Maishkata, Aungratali, Chionipara, Kariungpara. The 11 August landslide was followed again on 15 August at Chittaputti area. At least 50 houses were completely vanished under the solid earth and 300 houses were partly damaged. About 283.50 ha of cultivated land, 810 ha of household garden, and 50 km unmetalled road were crushed. Road communication between Bandarban headquarters and remote thanas became snapped. Especially, Aziznagar-Bazalia road had been closed for traffic due to falling of huge mass of earth over the road at 25 places. Chittagong landslide location was at Gopaipur under Chittagong Kotwali Thana. The slides crushed two thatched houses at the foot of the hill claimed the lives of the inmates of the houses who were asleep.|
|2000||At least 13 people were killed and 20 injured in landslide incidents on the Chittagong University campus and other parts of Chittagong City on Saturday, the 24 June 2000. The incident was caused due to the deluge of mud and water that swamped various part of the port city amid torrential rain. The landslides damaged property worth several lacs of taka in those places.|
Prevention Common engineering techniques for landslide prevention include provision for surface and subsurface drainage, removal of unstable slope materials, construction of retaining walls, or some combination of these. Many of them are currently used in Bangladesh.
A successful integrated bioengineering and drainage control approach for landslide protection and mitigation was carried out at Bandarban. The Soil Conservation and Water Management Centre (SCWMC) of Bandarban is located on a hilltop. The slope of the hill is quite steep. Slope stabilisation for such steep hills require special attention. During heavy rainfall in June 1995, a landslide in an area of 200-sq m on the steep slope necessitated the whole structure of an office building to be demolished for safety reasons. The local engineers of different governmental organisations then suggested the building of an RCC (reinforced concrete) retaining wall for the protection of the building with an estimated cost of Tk 7 million (US$ 129,629). But the Centre was not in a position to bear such an amount of expenditure. Scientists at the Centre took up the matter and used seeds of different species of trees on the affected area and covered up the affected area with Geo-Jute Textile. They then reinforced the Geo-Jute Textile by pegging bamboo posts. They diverted the flow of water from the hilltop away from the affected area by digging up natural drainage facility at the hilltop where the landslide occurred.
In the following year, 1996 the Geo-Jute Textile decomposed and was converted into fertiliser for the plants. The fertilising effect added to plant growth. Ever since the increased vegetation in the affected area of the landslide along with effective diversion of hilltop water was accomplished, the reinforcement so constructed could be sustained along the landslide. Thus against the externally quoted cost of Tk 7 million (US$ 129,629) for a concrete reinforcement, the Centre managed to use the biotechnology effectively at a meagre cost of Tk 4,000 (US$ 75) only. This is a good example of cost saving method of small-scale development with effective management of ecological resource. [Sifatul Quader Chowdhury]
Bibliography DR Greenway, 'Vegetation and slope stability', in MG Anderson and KS Richards ed, Slope Stability, John Wiley and Sons Ltd, London, 1987; MF Karim amd MJ Haider, Erosional hazards of Chittagong city, Bangladesh, UNDP- Regional Training Programme on Erosion and Sedimentation for Asia (RTPESA), 1991; Proceedings of RTPESA: Workshop on soil erosion and debris flow control, Indonesia, 1991; VC Thakur, Landslide hazard management and control in India, ICIMOD, India, 1996.