Mass Wasting Events in Indonesia

Mass Wasting and Landslides in Indonesia

A landslide is defined as the movement of a mass of rock, debris, or earth down a slope. Landslides are a type of "mass wasting," which denotes any down-slope movement of soil and rock under the direct influence of gravity. The term "landslide" encompasses five modes of slope movement: falls, topples, slides, spreads, and flows. These are further subdivided by the type of geologic material (bedrock, debris, or earth). Debris flows (commonly referred to as mudflows or mudslides) and rock falls are examples of common landslide types.

Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope. Causes include factors that increase the effects of down-slope forces and factors that contribute to low or reduced strength. Landslides can be initiated in slopes already on the verge of movement by rainfall, snowmelt, changes in water level, stream erosion, changes in groundwater, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors. Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.

Indonesia is frequently affected by landslides induced by both rainfall and earthquakes. An annual frequency of 49 landslides per year is reported by Chrisanto et al. (2008) during the period 1981-2007. The DesInventar inventory for Indonesia covering the period 1998-2009, contains 890 landslide events that killed 1280 persons. According to the Geological Agency of Indonesia (Geological Agency, 2006, 2007 & 2008), within the period 2003-2007, rapid landslides caused an average of 32 casualties per event. The majority of victims due to landslides in that period were in the Islands of Java (52%), Sulawesi (24%) and Sumatra (18%). In addition to the impact in terms of loss of lives and damage to buildings, landslides in Indonesia produce significant damage to agricultural land and roads, with the subsequent economic disruption (Kuncoro and Resosudarmo, 2006).

Figure 1: Map of precipitation-triggered landslides

hazard distribution for Indonesia.

Figure 2: Map of earthquake-triggered landslides hazard

distribution for Indonesia.

Notable Landslides in Indonesia

A catastrophic eruption occurred on Mount Kelud in East Java, Indonesia on May 19-20, 1919. This major eruption, one of the most deadly of the 20th century, is estimated to have killed 5,160 people. The vast majority of those perished in lahars or volcanic debris flows triggered by the eruption. This was also one of the worst landslide disasters of the 20th century. Authorities pointed out that this was a particularly interesting lahar event in part because of the detailed accounts of the impacts. The underlying problem at Mount Kelud was the presence of a crater lake, which at the time had an estimated volume of 40 million cubic meters. The eruption displaced this lake, which entrained large volumes of ash, generating the enormous lahars.

Figure 3: The lahars generated by the

Mount Kelud eruption in 1919.

On January 10, 2021 two landslides set off by heavy rainfall and unstable soil killed at least 12 people on Java, Indonesia’s most populous island, and left rescue workers searching for survivors, disaster officials said Sunday. Among those killed in the landslides in West Java Province were the head of a local disaster relief agency and an Indonesian Army captain who had gone to help rescue survivors from the first landslide on Saturday afternoon. They were caught in a second landslide that evening.

On April 29, 2021, at least five people died and eight were reported missing after landslides hit the Batang Toru hydropower plant area in Marancar, South Tapanuli District, North Sumatra Province, Indonesia. Indonesia's disaster agency BNPB said the landslides were caused by heavy rainfall in the area.

Search and rescue operations after a landslide in South Tapanuli Regency.

Photo: Regional Disaster Management Agency (BPBD) of South Tapanuli Regency

Landslide Mitigation

Acknowledging the frequency and the economic, environmental and social impacts of mass wasting or landslides, Indonesian authorities use the following mitigation measures to ensure the safety of Indonesians:

Landslide Hazard Susceptibility Mapping and Landslide Early Warning System

Landslide Monitoring

Officials monitor the activity of landslides based on the direction and velocity of their movement in order to reduce the socio-economic impact of landslides .

Monitoring by using GPS (Global Positioning System)

Socialization

The aims of landslide socialization are to inform the population of the landslide hazard early warning system; to inform residents of susceptibility to landslide zone; to calm down the community in the landslide area; to improve understanding and awareness on geohazard phenomena as well as on the importance of local community, local governments, decision makers and public education; and to develop appropriate strategies and programs for public education.