DST'S initiatives on "Landslide Hazard Mitigation"

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DST'S initiatives on "Landslide Hazard Mitigation"

Shri Bhoop Singh
Director/ Scientist 'F'
Department of Science & Technology
Technology Bahwan, New Delhi-16

Abstract :
Landslides cause extensive damages to roads, bridges, human dwellings, agricultural lands, forests etc. resulting in loss of property as well as human life. The Himalayan region has been the target of intense development activities for the last decade. The natural instabilities are accentuated by the human activities if the development schemes are not properly planned and implemented. The need of the hour is the sustainable development of the hilly regions where the existing instabilities should be taken into consideration while implementing various development projects so that the resultant geo-environmental hazards are kept to a minimum.

Mechanisms causing and controlling the landslides are much more complex in nature because of the interaction of a number of factors. In order to clearly understand the nature of landslide problems of the country, it is essential to identify the "high priority research areas". The focus now is on identification and development of intensive action plan which would be taken up in future studies, keeping in view the risk associated with the landslide occurrences and their impact on different sectors. A major shift in our priority must take place in near future if the losses due to landslides are to be reduced.

1. Introduction
Landslides cause extensive damages to roads, bridges, human dwellings, agricultural lands, forests etc. resulting in loss of property as well as human life. The Himalayan region has been the target of intense development activities for the last decade. The natural instabilities are accentuated by the human activities if the development schemes are not properly planned and implemented. The need of the hour is the sustainable development of the hilly regions where the existing instabilities should be taken into consideration while implementing various development projects so that the resultant geo-environmental hazards are kept to a minimum.

Mechanisms causing and controlling the landslides are much more complex in nature because of the interaction of a number of factors. In order to clearly understand the nature of landslide problems of the country, it is essential to identify the "high priority research areas". The focus now is on identification and development of intensive action plan which would be taken up in future studies, keeping in view the risk associated with the landslide occurrences and their impact on different sectors. A major shift in our priority must take place in near future if the losses due to landslides are to be reduced.

2. International Status
Landslide hazard zonation by cartographic methods has been carried out by several workers since the sixties. If the required information on the terrain characteristics is available, the slope surface can be divided into different classes related to stability.

Blanc and Cleveland (1968) prepared landslide hazard zonation maps of Southern California in which geological formations were divided into different lithologic groups and combined with slope categories, below and above the critical angle. Radbruch and Crowther (1973) in California classified the areas on the basis of lithology and the number of landslides present. In United States, Radbrush-Hall, et al (1976) considered the frequency of slope failure in different groups of geologic units. A smaller

grouping of lithology and mass movements was used by Rodriquez and others (1978) in Southern Spain.

Nilsen, et al (1979) used maps showing geological formations, slope ranges and landslide debris to prepare a landslide zonation map of San Francisco Bay region. Brabb et al (1972) have rated the slope stability of geological units in San Mateo County. California on the basis of percentage of outcrop area of a formation occupied by landslide debris, combined with slope classes.

The French method of hazard mapping called ZERMOS (the term is derived from "Zones exposes auxrisques lies aux movements du sol et du sous sol") has included factors like lithology, structure, slope morphology and hydrology. The area mapped is divided into four zones of different level of hazard with types of movement and direction, activity and sites of erosion by different symbols.

Varnes (1980) prepared a landslide zonation map adopting slope, soil thickness, land use practice and drainage map as the basic factors. Takei (1982) described methods for making debris flow hazard map taking into account the type of rock, fracturing, weathering characteristics, springs, vegetation cover and valley slopes etc.. Hansen (1984) discussed two principal categories of landslide hazard mapping namely direct and indirect mapping. Kawakami and Saito (1984) used valley density, elevation, slope angle and formations for preparing a quantified landslide risk map. Brabb (1984) provided a useful review of development of landslide hazard mapping. Wagner et al (1987) discussed preparation of rock and debris slide risk maps for road alignment purposes, using geological structural, slope and geomorphological factors. Koirala and Watkins (1988) described a slope ranking system mainly for adopting preventive measures during excavations.

3. National Status:
Seshagiri and Badrinarayan (1982) have carried out the zonation of Nilgiri areas considering slope, landuse, soil cover and drainage using numerical rating of these factors depending on frequency of landslide present. A similar type of study has been carried out by Gupta and Joshi (1990) in Himalaya using GIS approach where an index value has been given to factors like landuse, lithology, major tectonic features and azimuth of landslides. Choubey and Litoria (1990) have also carried out the land hazard mapping of Garhwal Himalaya considering slope, lithology, structure and earthquake epicenters. Hazard zonation Studies were taken up by the CRRI during 1984 at four locations in the north and east Sikkim (CRRI, 1989). The overall stability of the slope was assessed qualitatively on the basis of the nature and characteristics of the rock and soil materials constituting the slope formation, the slope angle, conditions of the slope surface, hydrological features and toe-erosion. In this study, the overall rating of the slope stability was divided into three categories. Viz. very good, good and fair. Landslide Hazard Potential map of the slopes along the Kathgodam - Nainital Highway was prepared by CRRI (1990) following similar methodology cited above.

Anbalagan (1992) carried out the landslide hazard zonation of Kathgodam - Nainital area, Kumaun Himalaya based on slope, lithology, structure, relative relief, landuse and land cover and ground water conditions. Mehrotra et al (1992) attempted an empirical approach for landslide hazard zonation mapping based on landslide susceptibility index (LSI) considering factors like lithology, slope angle, distance from major thrust/fault, landuse pattern and drainage density in relation to frequency of existing landslides. The technique has been adopted for zonation in Garhwal Himalaya for over 2000 sq. k.m. area.

From the above, it is apparent that various authors have adopted different approaches for landslide hazard mapping. The approaches are mostly qualitative with some quantification.

It may also be noted that over the last several years, many areas in landslide prone terrain - mainly concentrated in the Lesser Himalayas - have been investigated for preparation of Landslide Hazard Zonation maps by Geological Survey of India. DST has funded most of the projects at academic institutions/ universities and research laboratories for carrying out landslide hazard zonation mapping in selected areas. DST's Expert Committee undertook a review of these initiatives and called for a new emphasis on Quantitative Hazard Assessment and control of landslides. Further, it was felt that ongoing landslide hazard zonation studies lacked proper geotechnical engineering, rock mechanics and engineering geology inputs which are so vital for understanding mechanisms driving the movements, appropriate analysis of failed slopes and design of suitable control measures.

4. Need for integrated programme:
Considering the international and national status of landslide investigations and research, it is envisaged that a critical review and synthesis of what has been done so far in the country needs to be organized. In order to put this in a proper perspective it is proposed to

bring together all available data regarding landslide hazard mitigation experiences and practices in countries where extensive efforts have been made to reduce losses from landslide hazards. Some of the countries with notable success in this direction are: Japan, New Zealand, Norway, U.K., U.S.A. and China. It is proposed that the necessary data will be collected in a specially designed questionnaire format from agencies engaged in landslide hazard mitigation in these countries. All this information will then be analyzed and a synthesis will be prepared which may be useful in evolving strategies as relevant to Indian conditions.

5. Proposed action plan:
5.1 Preparation of nation-wide Inventory of landslides: This includes likely causes, probability of occurrence and extent of economic consequences if a landslide occurs in a given community. At present, there is no database available on landslides of any affected area either district level or state level. In order to evolve integrated mitigation measures for landslides, it is felt essential to build-up inventory of landslides at States and District levels of the hilly areas where landslide activities are common. In the area of technological advances i.e. GIS and Remote Sensing, GPS and SAR Inter-ferometry techniques, preparation of spatial databases with surface and sub-surface parameters on landslides would be of great help. These inventories will form the base information for undertaking further detailed investigations and analysis for designing optimal and effective action plans.

5.2 Landslide Hazard Zonation and Assessment: Detailed landslide hazard zonation maps (LHZ) will be prepared in areas susceptible to landslides causing heavy losses to life and property;

¢ Focus on landslide hazard zonation on the following scales:

1:50,000 - 1:25,000 for regional planning
1:15,000 -1,10,000 for district level planning
1:5,000-1:2,000 for site specific micro zonation (This includes the following aspects):

Preparation of engineering geologic profile of a landslide indicating slope angle, location of slip surface, nature of soils & rocks along the slip surface, surface and sub surface ground water conditions and weathering profile.
Current landuse practices and the vegetative cover.
Information on meteorological parameters such as; intensity, duration and frequency of significant rain events.

5.3 Field validated zonation methodologies for long term and short term developmental planning: Over the years, landslide hazard zonation mapping has been carried out by various concerned agencies on different scales. Very limited landslide hazard zonation mapping has been carried out on larger scale i.e. 1:2,000 or 1:5,000. In order to use the LHZ maps in undertaking site specific development plans, it is felt essential to validate the LHZ maps and clearly indicate the accuracy percentage along with the associated risk factors. This aspect should essentially form the part of all the R&D projects to be supported under the programme by DST.

5.4 Landslide Hazard Risk Analysis: Risk is the product of hazard and loss in terms of life and property. The following needs to be taken up:

Probability of occurrence of landslides.
Identification of development elements at risk if a landslide occurs.
Estimation of economic loss including loss of life and property, pattern of their spatial spread and their temporal propagation.

5.5. Study of Sub-surface Geology related to Landslides: Efforts should be made to study the geological boundaries of the landslide-affected areas, for which the suitable method is by propagation of artificially created elastic waves, which permits quantitative determination of the depth and angle of dips/ faults of geological strata. The seismic refraction method is a basic one to resolve wide range of geological problems. The complete information of sub-surface geology by refraction upto a depth of 30 - 40 meters would help scientists/ engineers to identify the nature of slope conditions and to formulate future action plan including preventive measures of landslides.

5.6 Monitoring and Analysis of Landslides:

Monitoring:

Monitoring of active landslide in different geologic environments involving landslides of different types viz., rotational/translational slides, flow slides etc. The parameters to be monitored are:
Rate of movement along the slip surface using slope indicator using slope indicator at periodic intervals with increased frequency of observation during rainy season.
Surface movement along the landslide profile at various crosses - sections using precise leveling and other-photogrammetic techniques.
Pore water pressures in the zone of slip movement using quick response piezometers. Changes in pore water pressure with time should be recorded.
Meteorological parameters using a network of raingauges (preferably automatic recording type).
Installation of warning devices in strategically important locations for forecasting landslide occurrences.
Surface movement studies on bench marks located a long different cross-sections of the slopes to infer the likely mode of failure developing in these slopes.
Subsurface monitoring of potential zones for movement using Acoustic Emission techniques especially in rock slopes.
Monitoring of ground water table fluctuation.
Monitoring of tilting of trees and location of water loving plants on the slopes.
Monitoring of development of surface cracks produced by subsurface movement, using extensometers.

Analysis of landslides :

5.7 Control Measures:

Scientific & Technological measures:
- Based on monitored data from active landslides and slopes with high susceptibility to landsliding, it is possible to interpret the mechanism of slope movement which is very essential for designing cost-effective control measures.
- Based on monitored data and the inferred mechanism of movement one or a combination of more than one of the following control measures need to be identified and evaluated for their effectiveness for a given slope failure;
- drainage of surface & sub-surface water.
- Restraint in the form of retaining walls, rows of piles, soil/rock reinforcement, gabions, etc.
- Removal of slide mass/slope modification by cutting and filling.
- Relocation of infrastructure affected
Validation of new technologies as successful landslide control measures
- It is envisaged to bring together, industry, user agencies and R&D institutions to design, execute & monitor full scale trials to demonstrate effectiveness of various currently used landslide control measures under different geological environments.
Legislative and regulatory measures
- Land use pattern & practices.
- Uniform building, land gradation and erosion control codes of practice.
- Insurance against landslide losses to spread cost over large population.
  
  Effectiveness of each of the landslide control strategy needs to be contrasted with the following guidelines:
- Degree of mobility of the existing landslides i.e., its current margin of safety.
- Desired level of improvement of stability.
- Location and sequence of construction of remedial measures employed.
- Time of the year best suited to carry out construction of control measures, and
- Monitoring and maintenance of programmes for the control measures employed.

5.8 Dissemination of knowledge on landslide hazard mitigation:

Raise awareness among policy makers & planners at state/district and user institution level through conducting training programmes/workshops.
Raise awareness among community leaders and general public affected by landslide hazards about the cost-effectiveness and benefits of taking landslide hazard mitigation measures.
Bring out a quarterly/ half yearly journal dedicated to general awareness & concerns about landslide hazards & their mitigation.
Produce trained technical manpower through training courses, for effective analysis, design, construction and maintenance of engineered slopes and the control measures provided.

6. Impact of landslides in local area management:
Landslides have wide ranging impact on the people of the affected area in terms of the devastation caused to material and human resources. The magnitude of a destruction depends on the location of the landslide area. In the context of India it is a painful truth that most, if not all, the areas susceptible to landslide hazards are inhabited by the economically weaker section of the population who have neither the resources nor the expertise to organize rehabilitation measures out of their own. Necessarily, therefore, administrative assistance is called for to organize remedial measures - both short term and long term. Such administrative action is to be controlled and managed by appropriate technological support if optimum benefit is to be derived out of the efforts put in all front. There are two significant aspects of this. First, it is necessary to have a hazard zonation map of the area so that the local area management can take pre- emptive action to meet an impending challenge rather than to wait for the disaster to happen. Secondly, the relief and long term rehabilitation measures are to be worked out once the disaster has struck - whatever be its magnitude.

It is, therefore, necessary to bring the desired education and awareness not only to the managers of the society but to the people at large so that a less than severe landslide does not become a cause for major human calamity. Measures of self help should be sufficient to avert the minor disasters.

To summarize, the following approach may be followed:

The Municipalities and local authorities should attempt to identify potential landslide zones on the basis of available zonation studies and prepare population maps of the vulnerable areas. The engineering department of the State Government e.g., PWD, Irrigation, Public health etc. should help the local authority in this respect. Waste disposal system to be systematized.
A comprehensive study of the natural resources of the area should be made to evaluate the impact of landslides on these natural resources in terms of their economic values to the livelihood of the people. Pressure due to waste disposal habits of people need to be relooked.
Short-term contingency plan should be prepared along with identification of the natural resources, including materials and equipment, so that relief measures call be initiated without loss of time. Too often we wait for a calamity to happen before gearing up for action. Valuable time is lost in the process. This need not be so in case of landslides because landslides, though not entirely predictable, have got their preferred time of occurrence, in terms of the climate and rainfall and with a little awareness warning of the impending disaster can be given.
Long-term remedial measures have to be worked out primarily by engineers in association with scientists working in related areas. While individual landslides may need detailed analytical studies a number of centers devoted to disaster prevention studies will help to evolve a scientific approach to landslide investigation. While it is true that number of factors, such as hill movement, rainfall, river flow and local and regional geology have important bearing on the landslide, at the time of actual movement it is basically the interplay of the forces of instability with the strength of the materials that controls the ultimate slide. Geotechnical investigation should, therefore, be an integral part of the landslide management programme.
Proper norms and guidelines may be developed for land use and appropriate environmental management in landslide prone areas.