Geomorphological Approach for Natural Disaster Analysis in Karo Highland, North Sumatra Province, Indonesia

Dwi Wahyuni Nurwihastuti, Anik Juli Dwi Astuti, Eni Yuniastuti
Department of Geography, State University of Medan, North Sumatra, Indonesia

Tanah Karo or Karo Lands, has a wide area which spread more than 2 million square kilometer with a population less than 600,000 people. Home to two major volcanoes in Sumatera Island, Karo area suffered a damage due to Mount Sinabung volcano which erupted after 400 years of dormancy in 2010 and series of serious landslides.

The research area is the Karo district of North Sumatra province. Landslides often occur in this area. Since September 2013, a part of the Karo area also suffered damage due to Sinabung volcano eruption. The research objectives are (1) to analyze landform units in the research area based on interpretation of remote sensing imageries (Landsat and SRTM) and (2) to analyze distribution of natural disaster threats using geomorphological approach.

Some enhancement techniques of Landsat imagery were done to obtain the best color composite image that show the clearest object of landform and geomorphological characteristics. Interpretation of landform units performed onscreen digitations using techniques of Geographic Information System (GIS). Geological map analysis also was used to obtain the information of constituent materials of landform. The types of natural disaster threats on every landform unit can be obtained from secondary data and field survey.

The results showed that the landform units derived from interpretation of Landsat and SRTM can be used to describe the distribution of threat of natural disaster types. Levels of natural hazards also can be interpreted based on the existing landform units. Consequently, landform units can be applied as a mapping unit in the mapping of natural disaster threat.

The earth surface have different physical characteristics of land between the one region and other regions. An approach of land characteristics studying is to examine the geomorphological characteristics. Geomorphological characteristics includes four aspects: morphology, morphogenesis, morphochronology, and morphoarransement. Differences in the geomorphological characteristics are influenced by geomorphic force which acted on an area.

One concept in geomorphology is "the present is the key to the past", which indicated that the processes that have occurred in the past will continue until the present and the future despite the different intensity. Thus, the natural disasters that have occurred in the past will happen again in the present and the future. Understanding of landform units will bring to the understanding of the threat of natural disasters in an area. Moreover, the natural disasters are geomorphological processes that occurred in the landform units. Consequently, the understanding of the threat of natural disasters become unseparated characteristics from each different landform units.

Administratively, the research location is in Karo Regency, North Sumatra Province, Indonesia as illustrated in Figure 1. Karo is geographically located at 02o50’ – 03o19’ North Latitude and 97o55’ – 98 o38’ East Longitude. The location is adjacent to research south of Medan with an area of ​​2127.25 Km2. Karo is bordered by Deli Serdang Regency and Langkat Regency in the north, Dairi Regency in the west side, and Lake Toba in the south.

Figure 1. The research location in Karo Regency, North Sumatra Province, Indonesia

Methods

The study of geomorphological characteristics was carried out by satellite image interpretation and field surveys. Image enhancement techniques were applied to clarify geomorphological appearance on Landsat and SRTM. Landform unit delineation and data analysis used Geographic Information Systems (GIS). Geological map analysis also was used to obtain the information of constituent materials of landform. The types of natural disaster threats on every landform unit can be obtained from secondary data and field survey. Analysis of the research results were conducted descriptively and spatially. Descriptive analysis is used to describe the condition of geomorphological characteristics, landform units, and the threat of natural disasters in Karo. While the spatial analysis is used to describe the spatial distribution of landform units and the threat of natural disasters in Karo.

Result and Discussion

Geomorphological Characteristics of Karo

Geomorphological characteristics cover geomorphological aspects that consist of morphology, morphogenesis, morphochronology, and morphoarransement (Van Zuidam, 1985). Morphology is describing a general relief in an area. Morphology consists of morphography and morphometric (Joyosuharto, 1986 in Sartohadi 2001; Gustavsson et al., 2006; Bishop et al., 2012). Morphography is descriptive aspects of the geomorphology of a region such as plains, hills, mountains, and plateaus. Meanwhile, morphometric are quantitative aspects of an area such as slope steepness, height and area of different units. Moreover, the morphometry of a landscape provides a quantitative description of landform shapes, whereas morphography is the mapped description of the configuration (Gustavsson et al., 2006; Fookes et al., 2007 in Bishop et al., 2012). In addition, the morphological aspect becomes very important among other geomorphological aspects because it is a human impression of a specific form on the earth's surface (Sartohadi, 2001).

Morphology in Karo was interpreted based on digital elevation models (DEM) of SRTM (Shuttle Radar Topographic Mapping Mission). SRTM data are useful for large geographic areas (Bishop et al., 2012), such as Karo. DEM techniques also allow a better view of shapes and morphogenetic processes represented in the map (Melelli et al., 2012). The elevation of the Karo area has a range from 250 m to 2451 m above mean sea level. The low point had been located at alluvial plain in Mardingding District. While the highest point is located at the Sinabung Volcano. Morphology of Karo was classified as plain, hill, and mountain. The plain area is located in the middle part of Karo i.e. highland. The hill and mountain areas are located in the west, north, east, and south of Karo.

The morphology conditions of Karo can be seen in Figure 2.

Figure 2. The morphology conditions of Karo, North Sumatra

The process of landform forming is closely connected to the genesis or morphogenesis. Morphogenesis can be detailed into passive morphostructure, active morphostructure, and morphodynamic. Passive morphostructure is related to lithology, both rock types and structures. While, the active morphostructure is related to endogenous dynamics that contains tectonic events during the landform formation. The passive morphostructure that was found in the Karo area are volcanic, faults, lineaments, and escarpments.

Morphodynamic is related to exogenous dynamics. Morphodynamic of Karo area is caused by water. The morphodynamic of landforms in the Karo area are mass movements including soil erosion and sedimentation caused by water. Mass movements including soil erosion were occurred in the hills and mountain areas in Karo area. Sedimentation was occurring along rivers and the bank of Toba Lake in Karo area.

Morphochronology is describing absolute and relative dating of landforms and related processes. Morphochronology in the Karo area was analyzed refers the relative dating of landforms and related processes, e.g. the landforms that located close to the river are the younger landforms, because of the active processes of water flow in the river. In addition, morphochronology in the Karo area was shown on the age of surface lithology, e.i. Quaternary, Tertiary, and Permian. Morphochronology is also describing the genesis of each landform.

Morphoarrangement is describing the spatial arrangements and interrelationships of various landforms and processes. Hilly and mountain areas can be divided into peak, the upper slope, middle slope, lower slope, and foot slope plain based on morphoarrangement. Alluvial plains also can be divided into natural levee and flood plain based on morphoarrangement. Natural levees were located along river and they have higher relief compare the surrounding areas. Flood plains were located in the back of the natural levees that have lower relief compare the surrounding areas.

Landform Unit of Karo

The study of geomorphological processes cannot separated from the landform characteristic. It is due to geomorphological processes will be develop following by the landform characteristic (Thornbury, 1959). The geomorphological description of an area cover explanation of four aspects, i.e. morphology, morphostructure, morphochronology, and morphoarrangement. They were reflected on the landform characteristics as unit of geomorphological study in the research area. Based on interpretation of Landsat 7 ETM+ and field survey, the general landform in Karo consist of five main landform, i.e. volcanic, structural, denudational, solutional, and fluvial. They can be detailed into 30 landform units as shown on Table 1 while the landform distribution can be illustrated on Figure 3

Table 1. Landform Units in Karo

Sources: Data Analysis Result (2014)

Figure 3. Landform map in Karo

Threat of Natural Disasters in Karo

Natural disaster analysis was done based on landform units as a result of interpretation of Landsat 7 ETM+ and field survey. The types of natural disasters are different on each landform as presented in Table 2. This is due to a landform is dominated by a certain geomorphological processes that are different from other landforms.

Table 2. Types of natural disasters on each landform unit

Sources: Data Analysis Result (2014)

Based on the Table 2, there are some natural disasters threats on the each landform in the research area. Earthquake threats cover all of landforms in the research area. However, the threat of earthquakes that usually centered around active faults region have different levels of threat.

Landslide threats also cover all of landforms in the research area. Based on the data of landslide events show that denudational and structural landform units have the highest threat. The lower threat of landslide was occurred on solutional and volcanic landform, especially in Sibayak Volcano of Andesit, Dasit, and Pyroclastic. Moreover, the lowest threat of landslide was occurred on fluvial landform. The landslide of fluvial landform was happened on the river bank that was triggered by under-cutting process that erodes the base of the river bank.

Erosion threats cover denudational, structural, and solutional landform units in the research area. The highest threat of erosion was occurred on denudational landform. The lower threat of erosion was occurred on structural landform. Furthermore, the lowest threat of erosion was occurred on solutional landform. Meanwhile, flash flood threats only cover fluvial landform.

Volcano eruption threats, lahar flow threats, and pyroclastic flow threats cover volcanic landform units. There is active volcano in Karo, i.e. Sinabung Volcano. Sinabung Volcano was erupted since September 2013 until now. The volcanic landform units of the Karo area also suffered damage due to Sinabung volcano eruption.

The distribution of the types of natural disasters threats following the distribution of landforms in the research area. Levels of natural hazards can also be interpreted based on the existing landform units. Thus, the landform unit can be used as a mapping unit in the mapping of the threat of natural disasters.

Conclusion

Geomorphological information can be obtain from interpretation of Landsat 7 ETM+ and SRTM. The research area consist of volcanic, structural, denudational, solutional, and fluvial landform. The information of morphogenesis can be used to analyze the potential of natural disasters in the research area. The potential of natural disasters on every landform units in the research area are as follows:

1. Volcanic landform is an area prone to volcanic eruptions, lahar flow, and pyroclastic flow.
2. Denudational, structural, and solutional landform are areas prone to landslide and erosion.
3. Fluvial landform is an area prone to flash flood and landslide in the river bank.
4. All of landforms in the research area are prone to earthquakes.