GIS for landslide hazard zone mapping

Modeling of geomorphology

One of the key factors of activating landslides is the geomorphology. Therefore terrain related features were important parameters for hazard zone mapping. GIS was used to extract such parameters.
Digital Elevation Model (DEM) is a powerful tool associated with GIS. The easiest way to construct DEM was to use a Triangular Irregular Network (TIN). TIN was constructed by using contour layer which has been developed by the Department of Survey, Sri Lanka at the scale of 1: 50,000. Degree slope, percent slope and elevation surfaces were extracted from the TIN model and were converted into 25 m square raster layers. The procedure is shown in the Fig. 3.2

Fig. 3.2: Modeling of DEM

Modeling of watershed and stream order

Terrain, stream network and boundary of study maps are used as the input coverage layers. The topogrid function in Arc Info software which is an interpolation method, specially designed to generate hydrologically corrected DEM [Graves, 2001], was used to produce digital elevation model as a grid with spacing 25 m. This DEM was corrected by sink filling and it was used to generate flow direction grid and flow accumulation grid. The procedure is shown in the Fig. 3.3. Finally constructed a watershed cover and it was cleaned and build in arc info for generate the geometrical parameters. This coverage was used to create the final watershed grid for hazard mapping.

Modeling of land use pattern

A major part of the hill country of Sri Lanka forms important watersheds and their degradation would cause irreparable damage to land and water recourses including the occurrence of major landslides. Therefore it is an important area to use for landslides hazard zone mapping. The land use map developed in 1: 50,000 scales was used to model land use pattern in study area. The systematic categorization of land use type is shown in Table 3.1 and used to prepare the derived map of land use pattern in study area.

Fig. 3.3: Modeling of watershed

Table 3.1: Classification of land use

Classification Category
River 3
Streams 3
Canal 3
Irrigation canal 3
Reservoir 3
Tank 3
Inland water bodies 3
Built-up land 1
Associated non-agricultural area 2
Homesteads 2
Tea 2
Rubber 2
Coconut 1
Other crop land 3
Paddy 1
Sparsely used crop 3
Chena 3
Mixed tree and other crops 1
Dense forest 1
Open forest 2
Forest plantation 1
Scrubland 2
Grassland 3
Marsh & swamps 1
Barren land 3
Abandoned paddy 1
Aquatic farms 1

Modeling of reservoir effect

The risk of landslide increases in areas around reservoirs. Therefore it is a significant factor to consider hazard zone mapping. Reservoir layer buffer for 500 m and 1000 m to get influenced area of reservoir and converted to grid layer.

Modeling of stream density

Stream density is also affecting the instability of slopes. Stream density value was derived for each 25 m cell from stream network layer.

Application of GIS for study of landslides

1.1 Introduction to GIS

GISIn the strictest sense, a GIS system is a computer system capable of assembling, storing, manipulating and displaying geographically referenced information, i.e. data identified according to their locations. It is best tool that uses the power of the computer to pose and answer geographic questions [Edirisinghe et al, 2003]. Practitioners also regard the total GIS as including operating persons and the data that go into the system. GIS has been growing during the last few years in almost all different spatial applications. The GIS model is an integration of five basic components that are People, Software, Hardware, Procedures and Data [Padma Kumara et al, 2003]. Fig. 3.1 shows the GIS model with its components.

Fig. 3.1: Integration of basic components in GIS

GIS software provides the functions and tools needed to store, analyze, and display information about places [Edirisinghe et al, 2003]. The key components of GIS software are;
# Tools for entering and manipulating geographic information. such as addresses or political boundaries
# Tools that create intelligent digital maps you can analyze, query for more information, or print for presentation
# An easy-to-use graphical user interface (GUI)
GIS careers typically include positions such as project manager, computer programmer, database administrator, system administrator, cartographic designer, business development, and related managerial and administrative roles. GIS has become such an important part of many organizations' information management strategy that it has resulted in the creation of a new position: Geographic Information Officer.
GIS has six fundamental operations in order to be useful for finding solutions to real world problems. Those are capturing data, storing data, querying data, analyzing data, displaying data and output data to hard copies.
The user guides the program to arrange and display data about places on the planet in a variety of ways- including maps, charts and tables. The hardware and software allows the users to see and interact with data in new ways by blending electronic maps and databases to generate color-coded displays. There are two types of data;
# Geographic, because data collected is associated with some location in space.
#Informational, because attributes, or the characteristics (data), about the space is what we want to learn about.

1.2 Structure of GIS Database

The database structure is very important part of database development. The overall database consists of spatial, temporal and attributes data. All database elements have spatial information and attribute data. But the few elements consist of temporal information. Fig. C.1 in Appendix C shows the outline of GIS database structure for the mail input layers. The spatial, temporal and attribute data related to each layer are given in Table C.1. Any geodetic coordinate system (eg. WGS 84) used in GPS converted to Everest 1830 geodetic coordinate system and after that projected with Transverse Mercator projection. The information presents in GIS database presents with the Transverse Mercator projection i.e. the formal projection system used in Sri Lanka. For the each and every layer required the good metadata system. The used software provides good metadata system. Fig. C.2 and Fig. C.3 shows the metadata relevant to the spatial information for Arc info coverage and shape files. It consists of all the data of map projection and scale. Fig. C.4a to Fig. C.4e shows the layers included in GIS database.