Know thy land
- It is time to invest in gathering Geographic Information System-based topographical data
Jun 27, 2015-
Earthquake activity in the Himalayan region is caused by the ongoing collision between the Indian and Eurasian plate. This collision started around 55 million years ago and it has produced the highest topography (the Mount Everest and the Tibetan plateau) on earth. Because of this type of continental collision, thrust faulting (where one plate rides on top of the other) earthquakes are very common in the central Himalayas, including in Nepal. The major earthquake of April 25 (7.9 magnitude) and its May 12 aftershock (7.4 magnitude) in central Nepal occurred when the main detachment fault ruptured at the bottom of central Himalaya. Continuous upliftment of the Himalayas since their birth causes steep slopes on the hill surfaces. As the slopes reach a critical threshold level, it results in exhumation of the hillslope materials in the form of landslides. The upliftment and exhumation of the Himalayas has been active since the collision occured and will continue for millions of years.
In the event of heavy precipitation and earthquake activities, the exhumation of hillslopes accelerates, resulting in huge landslides with the capability to obliterate entire villages.
Landslides and slopes
The coincidence of the recent earthquake and its aftershocks with the onset of the Asian monsoon poses a great threat of heavy landslides in central Nepal. Consequentially, the effect of the earthquake was intense on the lesser Himalayan regions which are densely populated and where the rocks are prone to slope failure. This poses an eminent threat to human lives and properties in Rasuwa, Sindhupalchok, Dolakha, Gorkha and Dhading. To minimise the effects of post-earthquake landslide hazard on human lives, the first task would be to resettle the villagers in safer places. This requires a quick assessment of landslide-prone areas before the geologists and geotechnical engineers are deployed to evaluate the possible impact of landslides on human lives. For this type of analysis, high resolution Geographic Information System (GIS)-based geological and topographical data is required. Unfortunately, the Nepal government does not have GIS-based geological data and trained manpower to perform this type of analysis.
For the past several years, I have been collecting GIS-based geological data on Nepal for my ongoing geological research. The geological data I have collected from central Nepal so far is extremely useful for a quick assessment of post-earthquake landslide hazard. Using the methods developed by various geo-scientists, I have developed slope maps based on the relationship between geological bedding planes and slope aspects by calculating the Topographic Bedding Intersection Angle (Tobia). These slope maps are different than the regular slope maps and the slopes are divided into two categories. If the geological bedding planes are parallel to the topographic slopes they are called cataclinal slopes or dip slopes. If the topographic slopes are perpendicular to the geological bedding planes they are called anaclinal slopes or escarpment.
Rasuwa and Sindhupalchok
In Figure 1, cataclinal or dip slopes are designated by purple, dark red, pink and dark blue colours and anaclinal slopes or escarpments are designated by yellow, green, sky blue colours while the existing landslides are shown as black polygon features. Villages are shown by green stars and not named due to the lack of space. Visual examination of Figure 1 clearly shows a strong spatial relationship among existing landslides and cataclinal or dip slopes. Also the villages are found on threshold hill slopes both on cataclinal and anaclinal slopes. The histogram of landslides and slope type derived from Figure 1 clearly shows that the number of landslides is much higher in cataclinal slopes than in anaclinal slopes. Further, the histogram shows that villages mostly lie on threshold hill slopes (30-40 degrees) where slope failures occur. This weird spatial relationship among villages and slope thresholds clearly indicates that hundreds of villages which may be affected by post-earthquake landslide activities lie mainly in Rasuwa and Sindhupalchok districts.
A closer analysis of data of a small part of Rasuwa district, which is also known as the district of landslides, bordering Nuwakot further indicates that the villages and major landslides are mainly found in cataclinal slopes. Villages like Dhaibunkot, Gairigaun, Jyanlan, Gumbada, Sarsyu, Sarsin, Khalchet, Bhorle in Rasuwa face a huge threats and could be wiped out by landslides at any moment. For this type of quick assessment of landslide hazard, GIS-based high resolution geological database is extremely important. Nearly 60 percent villages fall on the way of deadly slopes and fractured unstable bed rocks of the lesser Himalayas. Without knowing the bedrock characteristic and its relation to topography in detail, it is impossible to make any prediction for geo-hazard management in the event of heavy precipitation, and earthquake activities.
As of today, national agencies as well as international agencies working in Nepal for soil conservation, landslide hazard management/mitigation, hill region development are unaware of a bitter truth that 60 percent or even more villages in the hilly region of Nepal lie in deadly threshold slopes and that they can vanish at any moment. Huge amount of donor dollars is being used for advertisement, placards, workshops, seminars and administrative expenses. But there has been no investment on GIS-based geological data collection, and environmentally friendly land-use planning. This type of work was always been ignored and considered as valueless by national as well as international agencies in Nepal. The April 25 Gorkha earthquake should serve as a wakeup call to change this dismal state of affairs. If not, the ignorance resulting from an absence of information could lead to much devastation in the future. So let us spare a few pennies out of each donor dollar for GIS-based geological database development in Nepal.
Ojha is a research scientist at the Department of Geosciences, University of Arizona, the US
Published: 28-06-2015 07:49