Nagaland is part of a geologically young mountainous and tectonically disturbed terrain. It receives abundant rainfall and falls under the highest seismic hazard category of Zone V. This region also receives abundant rainfall.
Mitigation strategies may not be possible in every landslide prone area due to prohibitive costs and indifferent attitude of the public. Efforts to reduce risk are also made by road construction and maintenance agencies by implementing required treatment measures. There is, however, a need to preempt disaster by making adequate information available in advance before it strikes.
Recommendations cited below are based on literature review, laboratory analysis, knowledge acquired during field surveys and interaction with landowners. The frequency and adverse effects due of landslides can be minimized to a great extent by adopting additional remedial measures. These measures may be useful to the policy makers in decision making, for executives to plan and carry out the right job at the right place and for the common people, to protect their properties and land.
7.2.1 Expectations from the Authority
i) The District Disaster Management Committee which is in existence should be the key authority while dealing with landslide related issues because it comprises the right people in the right places.
ii) Develop clear guidelines to implement community education programs for disaster risk reduction.
iii) Develop a program for landslide experts to monitor areas of concern and the District Disaster Management Committee can act as the conduit for information to community members on safety or safety measures for the area.
iv) Reach out to local communities to promote landslide risk reduction and good land use practices.
v) Teach community members how to identify landslide indicators and report them to the competent authority. Some of the indicators of landslides are -
• Springs, seepages or saturated ground in areas that have not typically been wet before.
• New cracks or unusual bulges in the ground, street pavements or sidewalks.
• Soil moving away from foundations.
• Ancillary structures such as decks and patios tilting and/or moving relative to the main building.
• Tilting or cracking of concrete floors and foundations.
• Broken water lines and other underground utilities.
• Leaning telephone poles, trees, retaining walls or fences.
• Offset fence lines.
• Sunken or down-dropped road beds.
• Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content).
• Sudden decrease in creek water levels though it is still raining or rains have just stopped.
• Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb.
• Faint rumbling sounds that increases in volume is noticeable as landslide nears.
• Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
7.2.2 Recommendations for further research / studies
i) Explore and promote multidisciplinary approach to landslide risk reduction and practice.
ii) Map and document all disasters in the area, assess trends and vulnerabilities and recommend mitigation measures.
iii) The relation between rainfall and landslide in the area needs to be further analyzed and studied.
7.2.3 Environmental
considerations
i) The simplest step towards better slope stability is to avoid ‘view-cut’. The Ao people have a habit of view-cutting the surroundings. View-cutting creates numerous problems because it removes one of the best agents for slope stability. Some species of trees in the study area that are able to stump-sprout include willow and red alder. These trees can grow back even after being cut to a stump. Such trees should be planted as they help stabilize slopes and beauty the area.
7.2.4 Land use planning
i) Barren upper slopes should be afforested with suitable species. Plantation of fast growing, deep rooted trees such as eucalyptus, alder, cedar, willows like Salix tetrasperma, Salix ichnostachya Lindl and Salix sitchensis and fir like Pseudotsuga menziessii in the lower reaches of slide zones will help control the slides.
ii) Biotechnical slope stabilization techniques have been carried out with success in the Varunavat landslide in Uttarkashi, Uttarakhand. A massive landslide took place on 23rd September 2005. Slope stabilization was carried out by the Forest Research Institute, Dehradun and the state Forest Department. The slope was benched and a geo-jute mat laid over it. At the crown of the slide plants such as Pennisetum purpureum I (Napier Grass), Cymbopogon nardus (Java Grass), Cymbopogon citratus (Lemon Grass), Cymbopogon martini (Palma Rosa Grass), Chryosopogon fulvus (Golda Grass) and Dodonea viscose were planted. In the body of the slide and adjoining areas Cymbopogon citratus, Cymbopogon martini, Cymbopogon nardus, Pennisetum purpureum, Panicum maximum (Ginni Grass), Crotalaria sp., Mimosa himalayana, Debregeasia hypoleuca, Vitex negundo (Simalu Shrub), Rumex hasatus (Almoda Shrub), Alnus nepalensis (Uttis Tree), Bauhinia variegate and Trema politoria (Khagshi Tree) were planted. At the toe of the slide Cymbopogon citratus, Cymbopogon martini, Cymbopogon nardus, Pennisetum purpureum, Stylosanthes hamata, Dodonaea viscosa, Rumex hasatus, Vitex negundo, Crotalaria sp., Alnus nepalensis, Trema politoria (Khagshi Tree) and Bauhinia variegate have been planted. These bioengineering measures have been successfully established and helped develop good foliage cover and extensive root networks in the slide area. Similar slope stabilization process can even be adopted in the Alempang slide area. This will increase environmental compatibility and allow the use of locally available natural materials.
iii) Slopes greater than 40° should be left undisturbed.
7.2.5 Engineering techniques
i) The stability of a slope can be increased by removing all or part of a landslide mass, or by adding earth buttresses at the toe of potentially weak slopes.
ii) Drainage works should include both the surface and subsurface. Surface drainage improvement works are implemented to minimize the infiltration of rainwater that builds up pore pressure. This includes two major components such as drainage collection works and drainage channel works. Surface drainage measures comprise lined catch-water drains above the crown of a slide, lined contour drains at different levels of the slide mass and lined cascading chutes to intercept and divert rainwater from the upslope and slide surfaces. Such measures help check infiltration and reduce pore water pressure substantially. Subsurface drainage improvement works help remove groundwater from within the landslide mass and lowers the groundwater table. These include shallow and deep subsurface drainage control works depending upon the nature of the slide. Subsurface drainage works may include intercept under drains, interceptor trench drains, horizontal gravity drains and drainage wells.
iii) Impermeable material such as mortar or asphalt mulch may be sprayed at the crown and head regions to seal tension cracks and other permeable zones that provide avenues for excessive water infiltration.
iv) Retaining walls efficiently c heck potential localized landslide hazards where cuts into slopes are required for construction of buildings or roads. However, they have to be used with precaution because retaining walls may also increase the hazard when the soil is not allowed to drain properly. Such retaining structures should have deep foundations with weep holes for smooth passage of excess groundwater. Retaining walls should penetrate below slip surfaces as such walls should not cause excessive loading on soils.
v) Even structurally safe buildings may collapse if their foundations sink, tilt, uplift or move down a slope. Failure of the foundation may also be caused by liquefaction of underlying soil deposits, inadequate foundation, subsidence due to collapse of soil structure of filled-up areas, inappropriate choice of foundation and shoddy foundation construction. Thus the strengthening of a building (retrofitting), that is, upgrading it to the required design level to prevent its collapse aims at improving the design of buildings.
7.2.6 Public awareness
i) Human settlements must be viewed not only from the perspective of their landslide vulnerability but also from the perspective of the hazards that they may create. Mitigation measures, particularly in ecologically fragile hilly areas such as Mokokchung, will become much more expensive if new settlements continue to be built without recourse to proper slope investigation and timely protective action, ignoring well known professional practices in landslide risk management. Thus, awareness should be created amongst the public about landslides and its causes and subsequent remedial measures.
ii) Public education and publicity campaigns should be launched for public awareness of landslide risk and to promote proper understanding of the nature of risk.
(The writer is a Senior Geologist ONGC, Ltd.
PhD Scholar Department of Geology
Nagaland University)
Mitigation strategies may not be possible in every landslide prone area due to prohibitive costs and indifferent attitude of the public. Efforts to reduce risk are also made by road construction and maintenance agencies by implementing required treatment measures. There is, however, a need to preempt disaster by making adequate information available in advance before it strikes.
Recommendations cited below are based on literature review, laboratory analysis, knowledge acquired during field surveys and interaction with landowners. The frequency and adverse effects due of landslides can be minimized to a great extent by adopting additional remedial measures. These measures may be useful to the policy makers in decision making, for executives to plan and carry out the right job at the right place and for the common people, to protect their properties and land.
7.2.1 Expectations from the Authority
i) The District Disaster Management Committee which is in existence should be the key authority while dealing with landslide related issues because it comprises the right people in the right places.
ii) Develop clear guidelines to implement community education programs for disaster risk reduction.
iii) Develop a program for landslide experts to monitor areas of concern and the District Disaster Management Committee can act as the conduit for information to community members on safety or safety measures for the area.
iv) Reach out to local communities to promote landslide risk reduction and good land use practices.
v) Teach community members how to identify landslide indicators and report them to the competent authority. Some of the indicators of landslides are -
• Springs, seepages or saturated ground in areas that have not typically been wet before.
• New cracks or unusual bulges in the ground, street pavements or sidewalks.
• Soil moving away from foundations.
• Ancillary structures such as decks and patios tilting and/or moving relative to the main building.
• Tilting or cracking of concrete floors and foundations.
• Broken water lines and other underground utilities.
• Leaning telephone poles, trees, retaining walls or fences.
• Offset fence lines.
• Sunken or down-dropped road beds.
• Rapid increase in creek water levels, possibly accompanied by increased turbidity (soil content).
• Sudden decrease in creek water levels though it is still raining or rains have just stopped.
• Sticking doors and windows, and visible open spaces indicating jambs and frames out of plumb.
• Faint rumbling sounds that increases in volume is noticeable as landslide nears.
• Unusual sounds, such as trees cracking or boulders knocking together, might indicate moving debris.
7.2.2 Recommendations for further research / studies
i) Explore and promote multidisciplinary approach to landslide risk reduction and practice.
ii) Map and document all disasters in the area, assess trends and vulnerabilities and recommend mitigation measures.
iii) The relation between rainfall and landslide in the area needs to be further analyzed and studied.
7.2.3 Environmental
considerations
i) The simplest step towards better slope stability is to avoid ‘view-cut’. The Ao people have a habit of view-cutting the surroundings. View-cutting creates numerous problems because it removes one of the best agents for slope stability. Some species of trees in the study area that are able to stump-sprout include willow and red alder. These trees can grow back even after being cut to a stump. Such trees should be planted as they help stabilize slopes and beauty the area.
7.2.4 Land use planning
i) Barren upper slopes should be afforested with suitable species. Plantation of fast growing, deep rooted trees such as eucalyptus, alder, cedar, willows like Salix tetrasperma, Salix ichnostachya Lindl and Salix sitchensis and fir like Pseudotsuga menziessii in the lower reaches of slide zones will help control the slides.
ii) Biotechnical slope stabilization techniques have been carried out with success in the Varunavat landslide in Uttarkashi, Uttarakhand. A massive landslide took place on 23rd September 2005. Slope stabilization was carried out by the Forest Research Institute, Dehradun and the state Forest Department. The slope was benched and a geo-jute mat laid over it. At the crown of the slide plants such as Pennisetum purpureum I (Napier Grass), Cymbopogon nardus (Java Grass), Cymbopogon citratus (Lemon Grass), Cymbopogon martini (Palma Rosa Grass), Chryosopogon fulvus (Golda Grass) and Dodonea viscose were planted. In the body of the slide and adjoining areas Cymbopogon citratus, Cymbopogon martini, Cymbopogon nardus, Pennisetum purpureum, Panicum maximum (Ginni Grass), Crotalaria sp., Mimosa himalayana, Debregeasia hypoleuca, Vitex negundo (Simalu Shrub), Rumex hasatus (Almoda Shrub), Alnus nepalensis (Uttis Tree), Bauhinia variegate and Trema politoria (Khagshi Tree) were planted. At the toe of the slide Cymbopogon citratus, Cymbopogon martini, Cymbopogon nardus, Pennisetum purpureum, Stylosanthes hamata, Dodonaea viscosa, Rumex hasatus, Vitex negundo, Crotalaria sp., Alnus nepalensis, Trema politoria (Khagshi Tree) and Bauhinia variegate have been planted. These bioengineering measures have been successfully established and helped develop good foliage cover and extensive root networks in the slide area. Similar slope stabilization process can even be adopted in the Alempang slide area. This will increase environmental compatibility and allow the use of locally available natural materials.
iii) Slopes greater than 40° should be left undisturbed.
7.2.5 Engineering techniques
i) The stability of a slope can be increased by removing all or part of a landslide mass, or by adding earth buttresses at the toe of potentially weak slopes.
ii) Drainage works should include both the surface and subsurface. Surface drainage improvement works are implemented to minimize the infiltration of rainwater that builds up pore pressure. This includes two major components such as drainage collection works and drainage channel works. Surface drainage measures comprise lined catch-water drains above the crown of a slide, lined contour drains at different levels of the slide mass and lined cascading chutes to intercept and divert rainwater from the upslope and slide surfaces. Such measures help check infiltration and reduce pore water pressure substantially. Subsurface drainage improvement works help remove groundwater from within the landslide mass and lowers the groundwater table. These include shallow and deep subsurface drainage control works depending upon the nature of the slide. Subsurface drainage works may include intercept under drains, interceptor trench drains, horizontal gravity drains and drainage wells.
iii) Impermeable material such as mortar or asphalt mulch may be sprayed at the crown and head regions to seal tension cracks and other permeable zones that provide avenues for excessive water infiltration.
iv) Retaining walls efficiently c heck potential localized landslide hazards where cuts into slopes are required for construction of buildings or roads. However, they have to be used with precaution because retaining walls may also increase the hazard when the soil is not allowed to drain properly. Such retaining structures should have deep foundations with weep holes for smooth passage of excess groundwater. Retaining walls should penetrate below slip surfaces as such walls should not cause excessive loading on soils.
v) Even structurally safe buildings may collapse if their foundations sink, tilt, uplift or move down a slope. Failure of the foundation may also be caused by liquefaction of underlying soil deposits, inadequate foundation, subsidence due to collapse of soil structure of filled-up areas, inappropriate choice of foundation and shoddy foundation construction. Thus the strengthening of a building (retrofitting), that is, upgrading it to the required design level to prevent its collapse aims at improving the design of buildings.
7.2.6 Public awareness
i) Human settlements must be viewed not only from the perspective of their landslide vulnerability but also from the perspective of the hazards that they may create. Mitigation measures, particularly in ecologically fragile hilly areas such as Mokokchung, will become much more expensive if new settlements continue to be built without recourse to proper slope investigation and timely protective action, ignoring well known professional practices in landslide risk management. Thus, awareness should be created amongst the public about landslides and its causes and subsequent remedial measures.
ii) Public education and publicity campaigns should be launched for public awareness of landslide risk and to promote proper understanding of the nature of risk.
(The writer is a Senior Geologist ONGC, Ltd.
PhD Scholar Department of Geology
Nagaland University)
