Landslide zonation for hill area development

Landslide zonation for hill area development

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Dr.S.S. Ramakrishnan
Sr. Lecturer

V.Sanjeevi Kumar
Sr. Project Associate

M.G.S.M. Zaffar Sadiq
Research Scholar

Dr. K.Venugopal
Director

Institute of Remote Sensing, Anna University,
Chennai- 600 025,Tamil Nadu, INDIA.
Ph : 91-44-2352189,Fax:91-44-2352166
Email : [email protected]
[email protected]

Introduction
Landslides are frequent and annually recurring phenomenon in the Nilgiri district. Outward and downward movement of mass, consisting or rock and soils, due to natural or man-made causes is termed as landslide. High intensity rainfall triggered most of the landslides in the Nilgiri district. As long as landslides occur in remote, unpopulated regions, they are treated as just another denudation process sculpting the landscape, but when occur in populated regions, they become subjects of serious study. Most of the landslides occur due to exhaustive deforestation for the development of urbanization and plantation. In these areas rainwater directly penetrates into the soil and cause landslides.

  1. Classification of Landslides
    Based on the mass movement, landslides are divided into four major groups.
    1. Slow Flowage: Rock Creep and Soil Creep
    2. Rapid Flowage: Earth movements, Mudflows, Debris Avalanche
    3. Sliding: Slumps, Rock Slides, Rock falls, and Landslips
    4. Subsidence: Sinking of mass
  2. Causes of Landslides
    1. Internal factors
      The steeping of the slope, water content of the stratum and mineralogical composition and structural features, which are tending to reduce the shearing strength of the rocks.
    2. External factors
      A slight vibration or jerk to the mass would greatly add up against the frictional resistance and the mass would become unstable. The heavy traffic on hill roads is of great contributing factors towards causing the imbalance of the masses.

Objectives

  • To create spatial database in GIS environment.
  • To delineate the comprehensive landslide prone areas in various zonation level.
  • To visualize landslide prone areas in 3D view for hill area planning.

Study area
Study area is located in Nilgiri district, which is a mountainous terrain in the North West part of the TamilNadu, and the study area covers a part of the Kothagiri Taluk. It is geographically located between 76° 14′ 00″ and 77° 02′ 00″ E longitude and 11° 10′ 00″ and 11° 42′ 00″ N latitude. The study area falls under the Survey Of India toposheet No: 58A/15 on scale 1:50,000 scale.

Methodology

  1. Landslide Hazard Zonation Mapping
    Orthophotos were prepared form the aerial photos and then the contour maps with 2m interval are generated from the orthophotos. The digitized contour maps are given as input into the GIS – ArcView 3D analyst. Here, the 3d visualization of the study area is created. Various thematic layers (slope, landuse, soil) are visually interpreted from the orthophotos. The delineated thematic layers were digitized. The landslide model is created and the ranks and weights are assigned to each category. Based on themes and its impacts different zones were delineated.
  1. Data Base Generation For Landslide Vulnerable Zonation Mapping
    The landslide investigation and hazard zonation mapping study involves preparation of number of thematic databases such as slope, soil, rainfall, geomorphology, geology, and landuse of the area. As the main purpose of the study is to prepare landslide hazard zonation map and collection of these information being carried out for several decades through the traditional, conventional- ground based surveying and mapping methods, which had taken a lot of time, more man power and cost. But nowadays the emerging satellite based remote-sensing techniques has become more efficient tool for obtaining such information with less cost and time. In the present study, three thematic maps have been prepared by interpreting orthophotos.

    The different thematic maps are,

    1. Slope Map
    2. Soil Map
    3. Landuse Map

    1. Slope Map
      Slope map is very important factor for landslide study. If the slope is higher then there is a chance of occurrence of landslide. Contour maps have been used for the preparation of slope map. Using the Arc view 3D analyst, the tin model (figure-2) for the study area is created from the digitized contour map (figure-1). From the tin model, slope map was generated using “derive slope” command.
    2. Soil Map
      By interpreting the orthophotos with photo interpretation keys like tone, texture, associated feature and with help of agricultural engineering soil maps, soil types present in the study area have been delineated and mapped. Eight soil series are identified in the study area.
      1. Soil Texture
        The following soil series are present in the study area

        KG1 – sandy loam – sandy clay loam.
        KG2 – sandy clay loam
        KG3 – sandy clay
        KG4 – sandy loam – sandy clay loam
        KG5 – sandy loam – sandy clay loam
        KG6 – sandy clay loam
        KG7 – sandy clay – sandy clay loam.
        KG8 – sandy clay loam – sandy loam
        KG9 – sandy loam

    3. Landuse Map
      Landuse refers to “man’s activities and various use which carried on land”. Land cover refers to “natural vegetation, water bodies, rock, soil, artificial cover and other resulted due to land transformation”. In the present study, landuse / landcover map has been prepared on 1:2500 scale using Orthophotos with the help of visual interpretation keys such as tone, texture, size, shape, pattern and associated features.

GIS analysis
In order to generate the landslide prone areas map for Kothagiri area, a model has been developed in a GIS environment. Data in the form of thematic maps such as slope, soil and landuse were input in to GIS. The detailed method of assigning weights has been discussed below. Finally the landslide prone areas map has been prepared.

  1. Slope
    Slope is a very important parameter in any landslide hazard zonation mapping. In the study area slope varies from 0 to grater than 54 deg. The entire slope contour map was divided in to four categories as follows:

    > 54 deg – very steeply sloping
    36 – 54 deg – steeply sloping
    18 – 36 deg – moderately sloping
    0 – 18 deg – gently sloping
    Thus, the slope contour map has got four categories and suitable weights are assigned. (Figure – 5)

  2. Soil
    The occurrence of landslide is mainly due to the presence of huge thickness of loose soils when mixed with water, it triggers the landslide. In the study area, based on the soils erodable nature, it is divided in to four categories as follows:
    • Very highly erodable
    • Highly erodable
    • Moderately erodable
    • Poorly erodable.

    Thus, the Soil map has got four categories and suitable weights are assigned. (Figure- 4)

  3. Geology
    In the Nilgiri district, the lithology is the charnockite group of bedrocks, covered by the ubiquitous red laterite or lateritic soil. This is taken as a common factor for the whole district. Because of its uniform nature, it is not taken in to account for our study.
  4. Landuse
    Landuse / land cover map of an area under investigation has got direct or indirect influence in triggering the landslides. Different types of landuse /land cover features are identified in the study area are such as tea, tea+tree, agriculture, grassland, road, settlement and forest. Suitable weights are assigned. (Figure – 3)

  1. Criterion Table
    THEME RANK1 RANK2 RANK3 RANK4
    (4 * weight) (3*weight) (2*weight) (1*weight)
    Landuse Grass land, Agriculture Tea, Tea+Tree Forest Settlement
    Soil KG4, KG5 KG3, KG6 KG2, KG7, KG8 KG1, KG9
    Slope 36-54 deg 18-36 deg > 54 deg 0-18 deg
  2. Landslide Hazard Zonation Mapping
    Using union command, Landslide hazard zonation map was prepared by integrating the effect of various triggering factors. The zonation map divides the study area into four zones of landslide vulnerability viz., very high, high, moderate and poor. Thus, the landslide prone areas having 4 zones were obtained as shown in figure-6.

Conclusion
Remote sensing and GIS techniques play a significant role in landslide zonation mapping. Landslide identification, which is a crucial parameter for any regional landslide hazard assessment, can be very well done particularly with aerial photographs. Coupled with aerial photos, GIS is an excellent tool to display the spatial distribution of landslides along with their attributes. However, the landslide map so prepared should be validated with ground checks.

Acknowledgement
Authors are grateful to the Director, IRS for his kind permission to publish the work.

References:

  • “The Nilgiri Landslides” – Geological Survey of India – Publication no: 57, 1982.
  • Quido Zaruba and Vojtech Mencl – “Landslides and their control” Czechoslovakia, 1982.
  • Paul R.Wolf, – “Elements of Photogrammetry” McGraw-hill Inc. New York, 1983.
  • Lillesand, J.M and Kiefer, R.W., – “Remote Sensing and Image Interpretation”,
  • P.A.Burrough – ” Principles of Geographical Information Systems”
  • Mohanadoss and Sunitha – “landslide hazard zonation map of Marappalam using Remote sensing technique and GIS”,Anna University, Chennai-25.