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Can we anticipate landslides?

  • Published at 07:18 pm September 2nd, 2020
This file photo shows landslide triggered by heavy rainfall in Kaptai upazila of Rangamati on Saturday, July 13, 2019 Dhaka Tribune

Lessons from the Chittagong Hill Tracts

Right off the coast of Bay of Bengal, Bangladesh is currently experiencing the worst flood since 1998, in which 41 people have died from floods and over 5.5 million are hard hit. 

So far the country has not yet reported any major landslide events resulting from the rainfall, but as the monsoon season is peaking, more rainfall is expected. 

Annual seasonal variations in rainfall fuels landslide susceptibility. With the monsoon season continuing, there is still the potential threat for future downpours furthering current flooding. As Bangladesh continues to provide the required relief to flood victims, how will the country fare in anticipating landslides and minimizing casualties?

Landslides in South Asia

Earlier this year in February NASA ran precipitation data and compared the results with their Global Landslide Catalog. In doing so they identified that extreme precipitation events are likely to be common as the climate warms (Merzdorf, 2020). 

Furthermore, this will result in many regions especially in South Asia experiencing intensified frequencies of landslide activities. NASA anticipated that China and Nepal could see a 30 to 70% increase in landslide activity due to such instances (Merzdorf, 2020). 

Monsoon season rainfall has been unusually severe this year, triggering floods and landslides across the region. Floods and landslides triggered by heavy monsoon rains have killed at least 221 people across South Asia over the past month. 

Since the beginning of the monsoon season in June, thousands of people have lost their lives to floods and landslides. More than one million people have been marooned in Nepal, Bangladesh, India and China, reports Aljazeera. 

Nepal's Ministry of Home Affairs said this year’s vigorous monsoon in Nepal has left at least 116 dead and 50 missing in monsoon-related incidents, including landslides in mountainous areas and flooding in the southern plains over the past month, according to Nepali Times. 

In recent years there has been a change in precipitation patterns. Meanwhile, weather experts have attributed the erratic and heavy rainfall in such a short duration to climate change. “The intensity of rainfall has gone up," said Madhukar Upadhya, a watershed practitioner and climate change expert, when talking to The Kathmandu Post in 2019). 

He further added that “We are experiencing a high intensity of rainfall in short durations." According to the Department of Meteorology and Hydrology, Nepal has received 736mm of rain since the start of the monsoon in mid-June – nearly 150mm more than the average for the same time period. Kaski and the mountains of central Nepal, as well as the eastern districts, had the highest annual precipitation in Nepal with 3,500mm (Nepali Times, 2020). 

In 2020 the South Indian State of Kerala has also incurred monsoon rain-induced landslides, resulting in the death of 29 individuals. The Indian Meteorological department projects more rainfall at the southern and eastern parts of India as low pressure is brewing at the Bay of Bengal. 

Anticipating landslides in Bangladesh

Major recent landslide events have taken place in the Chittagong Hill Tracts (CHT) and were primarily triggered by prolonged rainfall in a short period compared to the monthly average. 

The CHT is an area made up of three hill districts namely Rangamati, Bandarban and Khagrachari. This area has experienced several devastating landslides during the last five decades, with CHT suffering 12 landslides with a death toll of 633 people and damage to thousands of homes between the years 1999 and 2019. 

It is estimated that the economy of Bangladesh has suffered a total loss of Tk249 million due to landslides for the years 2009 to 2014 (BBS, 2016).

Photo: Forewarn, Bangladesh. Landslide Susceptible Zones in CHT (prepared in June 2020 by Forewarn, Bangladesh) 

To identify the landslide susceptible zones in CHT and the vulnerability of these zones to the effects of landslides, a study was conducted by Caritas Bangladesh and FOREWARN Bangladesh titled Landslide Vulnerability Analysis of Bangladesh, A study of Bandarban & Rangamati Districts. 

The analysis determined landslide susceptibility zones in the study location of Rangamati and Bandarban area via the development of a composite vulnerability index. As landslide susceptibility mapping is essential to mitigate landslide disaster, the map was used to identify a range of susceptibility from very low, low, moderate, high, and very high at the study locations. 

Additionally, various factors, including rainfall, soil characteristics, local geology, topography, climate, and land use/land cover type, were looked at to find the major triggering factors of a landslide. A Frequency Ratio Model method was used to identify the weightage of these factors resulting in a landslide event.

Upon determining the triggering factors required to anticipate landslides, the study also looked into methods of reducing the loss of lives and property from such an event by designing early action protocols.

To distinguish the most landslide vulnerable locations that need early action intervention, socio-economic indicators were considered. Education (literacy rate), income and population density of the study locations were overlaid with a landslide susceptibility map. 

The study’s findings pointed out that a combination of attributes contributes to several Upazilas of Rangamati and Bandarban being highly vulnerable to the impacts of the landslide. This process identified 43 locations vulnerable to landslides within the 8 Upazilas of the study site. 

“It is estimated that the economy of Bangladesh has suffered a total loss of Tk249 million due to landslides for the years 2009 to 2014”

Of this, the most vulnerable site to the impacts of landslides is Ruma Upazila in Bandarban (as shown in the map). The contributing factors included greater population density, low-income level and poor literacy rate. Rowangchari Upazila follows behind Ruma Upazila in being the most vulnerable to landslides. 

By contrast, Lama Upazilla in Bandarban, is highly susceptible to landslides, but it is not as vulnerable. Better economic and education attributes of Lama makes the Upazila less vulnerable to the impact of landslides. 

Kaptai, Nainarcchar and Kawkhali Upazilas in Rangamati are the most landslide susceptible zones as well as the most vulnerable spots. 

When to raise alert and prepare for landslides

Climate change has been a major player in the variation of rainfall patterns in the CHT. Event-based rainfall data of the area indicates that Rangamati is more vulnerable compared to Bandarban in terms of withstanding heavy rainfall within a small period. 

Upon further comparing rainfall data of the two locations, it was identified that 50mm-100mm of rainfall within 24 hours can create landslide favourable conditions in Rangamati in contrast to 100mm of rainfall within the same period in Bandarban. 

Bangladesh Meteorological Department (BMD) projects that in coming years and considering climate change, extreme rainfall events will be higher. Therefore, increasing rainfall, as well as the amount of rain in a single time, will be higher than current trends and is expected to compound the effects, causing more landslides during the monsoon season in coming years. 

Intense rainfall followed with unplanned deforestation and a change in land use pattern increases the vulnerability of a landslide event in Rangamati and Bandarban. 

Therefore, the study analyses that an alert should be raised when the precipitation forecasts of the locations are close to the identified threshold level. Once the alert is raised, a set of early warning protocols should be initiated. 

Apart from anticipatory actions, there are some effective non-structural and structural mitigation approaches that can be undertaken to retrofit the impending risks of a landslide. Upon assessing risk, uncertainty, possible consequences, constructability, environmental impacts and costs of such approaches, improvement of drainage conditions, controlled hill cutting, landslide mapping and geophysical analysis should be considered. Structural measures considering the safety measures, building codes are also inevitable for mitigation of landslide disaster risk.

The study provides locations where stringent landslide prevention interventions have to be followed to reduce the vulnerabilities stimulated by landslide susceptibility. Responsible stakeholders need to put forth the effort to reduce the risks and prepare to take actions before the monsoon season starts to initiate delirious landslide consequences in CHT. 

With monsoon downpours resulting in serious casualties in other South Asian counterparts, it is imperative for Bangladesh to prepare itself in each monsoon season. Caritas Bangladesh and Forewarn have laid the groundwork with their study for responsible stakeholders to venture in precautionary measures at vulnerable sites of Rangamati and Bandarban. The findings from this study can be used as a tool by civil authorities and local stakeholders to better equip themselves to predict and prepare for future landslide scenarios.  

Rukhsar Sultana is a Researcher at ICCCAD. Her research interest lies in exploring urban resilience aspects as well as climate change-induced health risks. She can be reached at [email protected]

Sumaiya Binte Anwar is a Research Officer at ICCCAD working in the Urban Resilience Programme. She is a Civil Engineer and a climate enthusiast. She can be reached at [email protected]

Nafis Fuad is a Research Officer at ICCCAD. His main research interest is in urban climate resilience and development. He can be reached at [email protected]

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