The method which has been used to carry out the analysis for the landslide vulnerability is the graphical overlay method. This method of assessment was first discovered by the famous landscape architect and a regional planner Ian Mcharg in his book ‘Design with Nature’ in 1969.
Later this graphical overlay method of analysis formulated the bases of GIS, which is extensively used as an assessment tool all over the world. In his book, ―Design With Nature‖, McHarg (1969) used semi-transparent map overlays for ―suitability selection processes‖, for a highway route selection. His printed map transparencies represented various physical, social and environmental values around potential routes by degrees of grey shades; from most suitable to least. The overlay resulted in sensitive areas being ―masked‖ through the multiple overlaid maps while more suitable areas remained translucent. To develop an overall composite and comprehensive compatibility, suitable for different land uses he used this method of analysis. Having access to quality and required data helps in making use of available data. Data can be fed and used in information processing cycle for getting accurate forecasts.
Steps for undertaking Grid overlay analysis?
Grid overlay method of analysis – It involves the use of several overlay of maps of different aspects which affects the decision . For example the aspects which affects the landslide vulnerability are slope, land use, forest cover, elevation, soil structure etc. After choosing the overlays to be used, grid cells are carved out each having same dimensions and some information about that overlay, score are given to each grid in terms of high to low suitability .Furthermore after obtaining overlays with scores, theses overlays are also given weightage according to their importance. Thus adding all the scores and the weightage , a composite score is obtained which help in the suitability analysis.
GRID OVERLAY METHOD FOR LANDSLIDE VULNERABILITY ASSESSMENT
For the formulation of landslide hazard zonation through vulnerability mapping , first the study area is divided into small units defined by regular boundaries and later within each small unit the relative influence of combined factors would be studied to evolve the landslide hazard (Carrara et al., 1995). Furthermore the area is divided into regular square grid with dimensions 20*20 m .For overlay analysis the entire study area was overlaid with 20*20 m polygonal grid cells. This grid was prepared by using the AutoCAD software where first the georeferenced study area boundary was imported as a shape file and later grid with 20*20m cells was created. Further, for the overlay analysis it was required that each grid cell must have value 1-5 in the other of their suitability. These values were added in subsequent addition of the other layers to get a composite score. Processing the available data by various means is important and for this it need to follow data processing cycle.
Layers used were – Slope, Elevation, Geology, Prohibited areas and Landuse. Thus, based on the field evidence, judgment ,overlays and logical consideration the landslide hazard zones were classified as; high hazard (HH), medium hazard (MH) and low hazard (LH) .
Five different slope classes have been identified in the area depending on the variation in the surface slope amount. These are as given below:
- 0 – 15 Gentle slope
- 15 – 25 Moderate slope
- 25 – 35 Moderately steep slope
- 35 – 50 Steep slope
- > 50 Escarpment and cliffs
The surface slope in the area; particularly in the catchment of Naini lake is observed to be gentle to moderately steep. Large areas with gentle slope occur along the northwestern and southeastern extremities of the lake. Small areas running almost parallel to both eastern and western boundaries of the Naini lake however show steep slope.