I am going to post my email reply about Hangewa landslide to Badri Vinod Dahal, who is working in Taplejung in the field of Natural Resource Management. Badri jii wanted to know more about the landslide for its long term management. Since, my last field visit was in 2001, my knwoledge on Hangdewa landslidle is not up to date. Nonetheless, I hope this posting would help provide basic information about the Hangdewa-Hireba landslides.
Landslides in both Hangdewa and Hireba rivers are huge scale and very complicated. Types of mass movement are complex. The mass movement type varies in different parts of the slide area. In fact, these landslides are formed by the combination of various individual slides. Therefore, these landslides are very unique in its nature. Most probably, it provides typical example of complicated landslide in the world. An important parameter in landslide control is its depth of slip surface. In both Hangdewa and Hireba landslides, depth of slip surface varies from shallow to very deep. In some areas, it could be exceed 10 meters whereas in some areas, the slip surface simply represents the soil rock interface
Let me first explain the cause of the landslides before going into the control measures. Geologically the area is just south of Main Central Thrust (MCT). MCT is not active recently; however, it affected local ground condition tremendously resulting high degree of metamorphism. The area is primarily composed of highly weathered phyllite with its foliation plane dipping towards north. The orientation of phlyllite also controls the landslides. The soil type is basically fine grained soil with high liquid limit. Therefore, during monsoon unit weight of soil is increased causing the unstable slope and landslides incidences. Soil type, thick soil cover, high intensity rainfall, and terrain are primary cause of the landslides. In my personal opinion, the slides are primarily due to the loss of equilibrium naturally. This is also evident from the existence of similar type of relict landslide in the surrounding area. Those relict landslides can be seen clearly in aerial photo. However, the rate of sliding incidences in Hangdewa and Hireba landslides are aggravated by improper cultivation practice. The major crop of the watershed is cardamom, which requires huge amount of water irrigation. The irrigation practice together with rainfall contributes the increased rate of landslide events. There is no evidence of earthquake-induced landslides in the area.
For the control of the slides, the first step is to reduce amount of water in insitu soil. This will decrease both pore pressure in soil and its unit weight. Surface drainage is recommended in the area above the tension cracks. Diversion of surface is a key step towards landslide control. Subsurface drainage can also be installed if the budget allows doing so. Or, the subsurface drainage can be installed in the area, which requires urgent control of the slides. For example, the landslides growing toward Taplejung airport deserve special attention. For the subsurface drainage, horizontal gravity drains can be drilled and the pipes either perforated P.V.C. (polyvinyl chloride) or steel construction can be installed to drain out the water. Obviously, this is an expensive option.
Bio-engineering method is cheap and easy. It can be implemented depending on the nature of exposed surface and depth of slip surface. If the depth of slip surface shallow, this method could be effective. Plant type and their root depth determine the usefulness of the bio-engineering method. Transpiration capacity of water is another factor to be considered. In deep seated slide, this method is not found to be effective. If the woody vegetation is planted, it would aggravate slide due to increased weight on underlying soil. I recommend you to get literature from Mountain Risk Engineering Course from TU. This course was offered in TU several years ago. It will give general idea of landslide control in Nepal Himalayas. There several publication regarding the choice of vegetation for bio-engineering purposes. You can also check the library of ICIMOD.
River training is also useful in our contest. When, I visited the site in 2001, I saw gabion walls in some areas. However, the placement of most of the gabion walls were not scientific and adequate. These river structures should be placed to reduce channel erosion—both in vertical and bank erosion. Gabion can help reduce toe cutting. I don’t recommend installation of gabion wall or any massive structures for lateral support.
Geosynthetics are good option in landslide control. However, I don’t think that it is possible in Taplejung. Another option is soil nailing. Soil nailing is very effective in small scale slide control. Alternative to the standard soil nailing, we may use timber nailing to control the specific slide in local areas.
Since, the Hangdewa and Hireba landslides are basically deep seated, I don’t think it would be reasonable to try erosion control mat. But, there could be limited use of erosion control mat in Hireba landslides; especially towards west side of the slide where the overlying soil is relative thin. In developed country, shotcreteing is also a common practice but we don’t have this technology in Nepal.
Large scale landslide control is not easy anywhere in the world due to the high cost of the project. We have to evaluate the economic worthiness of the area first, then, formulate corresponding control measure--both short term and long term. In my understanding, the landslide affected area is not giving much economic benefit to the local people. Therefore, I don't think it will be reasonable to invest much money to control it. However, we can use some less expensive method and rational approach to minimize the effect and expansion of the landslide.
In conclusion, the landslide control approach should be planned to deal with specific part of the sliding area. Surface and subsurface water control is strongly recommended. For bio-engineering method, depth of slip surface should be taken into consideration. Not only should the Standards for bio-engineering but the local vegetation and climatic condition also be considered for its successful application. River structures, if placed properly help reduce the slide. For long term control, I don’t see anything other than changing current agricultural pattern. But, changing current agricultural pattern seems to be almost impossible at this time because of the high money market of cardamom. From the current state of knowledge, available tools in the area, and the budget, it would be very difficult to control the landslides. However, the implementation of proper technology could greatly reduce the rate of landslide expansion.
Landslides in both Hangdewa and Hireba rivers are huge scale and very complicated. Types of mass movement are complex. The mass movement type varies in different parts of the slide area. In fact, these landslides are formed by the combination of various individual slides. Therefore, these landslides are very unique in its nature. Most probably, it provides typical example of complicated landslide in the world. An important parameter in landslide control is its depth of slip surface. In both Hangdewa and Hireba landslides, depth of slip surface varies from shallow to very deep. In some areas, it could be exceed 10 meters whereas in some areas, the slip surface simply represents the soil rock interface
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| Figure: Landslide distribution map of Hangdewa - Hireba watershed (2001) |
Let me first explain the cause of the landslides before going into the control measures. Geologically the area is just south of Main Central Thrust (MCT). MCT is not active recently; however, it affected local ground condition tremendously resulting high degree of metamorphism. The area is primarily composed of highly weathered phyllite with its foliation plane dipping towards north. The orientation of phlyllite also controls the landslides. The soil type is basically fine grained soil with high liquid limit. Therefore, during monsoon unit weight of soil is increased causing the unstable slope and landslides incidences. Soil type, thick soil cover, high intensity rainfall, and terrain are primary cause of the landslides. In my personal opinion, the slides are primarily due to the loss of equilibrium naturally. This is also evident from the existence of similar type of relict landslide in the surrounding area. Those relict landslides can be seen clearly in aerial photo. However, the rate of sliding incidences in Hangdewa and Hireba landslides are aggravated by improper cultivation practice. The major crop of the watershed is cardamom, which requires huge amount of water irrigation. The irrigation practice together with rainfall contributes the increased rate of landslide events. There is no evidence of earthquake-induced landslides in the area.
For the control of the slides, the first step is to reduce amount of water in insitu soil. This will decrease both pore pressure in soil and its unit weight. Surface drainage is recommended in the area above the tension cracks. Diversion of surface is a key step towards landslide control. Subsurface drainage can also be installed if the budget allows doing so. Or, the subsurface drainage can be installed in the area, which requires urgent control of the slides. For example, the landslides growing toward Taplejung airport deserve special attention. For the subsurface drainage, horizontal gravity drains can be drilled and the pipes either perforated P.V.C. (polyvinyl chloride) or steel construction can be installed to drain out the water. Obviously, this is an expensive option.
Bio-engineering method is cheap and easy. It can be implemented depending on the nature of exposed surface and depth of slip surface. If the depth of slip surface shallow, this method could be effective. Plant type and their root depth determine the usefulness of the bio-engineering method. Transpiration capacity of water is another factor to be considered. In deep seated slide, this method is not found to be effective. If the woody vegetation is planted, it would aggravate slide due to increased weight on underlying soil. I recommend you to get literature from Mountain Risk Engineering Course from TU. This course was offered in TU several years ago. It will give general idea of landslide control in Nepal Himalayas. There several publication regarding the choice of vegetation for bio-engineering purposes. You can also check the library of ICIMOD.
River training is also useful in our contest. When, I visited the site in 2001, I saw gabion walls in some areas. However, the placement of most of the gabion walls were not scientific and adequate. These river structures should be placed to reduce channel erosion—both in vertical and bank erosion. Gabion can help reduce toe cutting. I don’t recommend installation of gabion wall or any massive structures for lateral support.
Geosynthetics are good option in landslide control. However, I don’t think that it is possible in Taplejung. Another option is soil nailing. Soil nailing is very effective in small scale slide control. Alternative to the standard soil nailing, we may use timber nailing to control the specific slide in local areas.
Since, the Hangdewa and Hireba landslides are basically deep seated, I don’t think it would be reasonable to try erosion control mat. But, there could be limited use of erosion control mat in Hireba landslides; especially towards west side of the slide where the overlying soil is relative thin. In developed country, shotcreteing is also a common practice but we don’t have this technology in Nepal.
Large scale landslide control is not easy anywhere in the world due to the high cost of the project. We have to evaluate the economic worthiness of the area first, then, formulate corresponding control measure--both short term and long term. In my understanding, the landslide affected area is not giving much economic benefit to the local people. Therefore, I don't think it will be reasonable to invest much money to control it. However, we can use some less expensive method and rational approach to minimize the effect and expansion of the landslide.
In conclusion, the landslide control approach should be planned to deal with specific part of the sliding area. Surface and subsurface water control is strongly recommended. For bio-engineering method, depth of slip surface should be taken into consideration. Not only should the Standards for bio-engineering but the local vegetation and climatic condition also be considered for its successful application. River structures, if placed properly help reduce the slide. For long term control, I don’t see anything other than changing current agricultural pattern. But, changing current agricultural pattern seems to be almost impossible at this time because of the high money market of cardamom. From the current state of knowledge, available tools in the area, and the budget, it would be very difficult to control the landslides. However, the implementation of proper technology could greatly reduce the rate of landslide expansion.
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