Prem Saddul
4 October 2007
Port Louis — Among the natural hazards, which cause much loss of life and property around the world, landsliding is perhaps the most under recognized and under estimated. In recent decades, man has witnessed great devastations caused by catastrophic failures of mountain slopes involving the sudden and dramatic collapse of large volumes of rocks and soils along mountain slopes.
This problem is very common in volcanic island domes located in tropical areas. Mauritius is a volcanic island dome where mountains and mountain slopes (altitude 300-800 m) and slopes ranging from > 20 º to 60 º cover an area of more than 30,000 hectares i.e 16 % of our total surface area. As far as land suitability and land capability index are concerned, all slopes exceeding 30 º along a mountain becomes dangerous for human activities without appropriate engineering work.
Unfortunately, there are different sectors of mountain slopes in Mauritius along which settlements have escalated higher and higher beyond the zone of high risk and thereby maximizing the risk of landslide hazards. Mauritius is located in a cyclone basin and a few days of excessively high cyclonic rainfall can have dramatic effects along mountain slopes, which are heavily loaded with unconsolidated and loose weathered materials.
A look at what is currently happening along the slopes overlooking the Moka Range near Le Bocage and along the slopes of Candos Hill as well as along the slopes of Tourelle du Tamarin will awe inspire any Earth Scientist . and not the Mauritian authorities.
Chitrakoot is located at an altitude of 215m on a very dynamic upper mountain slope (above 35 °) of Vallée des Prêtres, a completely enclosed amphitheatre valley carved by the Latanier river through the 8 million years old Port Louis mountain range. The processes that have been at work since millions of years and which are still active indicate that Chitrakoot is located in a potentially danger zone, where a slow mass movement of materials down the slope from the vertical Grand Glacis mountain wall towards the village and downwards cannot be written off at any stage.
A slow mass movement can suddenly become a sudden rotational colluvial (rocks and soils) landslide with drastic consequences if appropriate geotechnical works and mitigating measures similar to the one carried out at the La Butte zone are not undertaken to prevent the risk of loss of life and property.
The danger is that a mass movement of materials (rocks and soils) can at any time happen along a zone of discontinuity as shown in the diagram. A classic example is what happened at La Butte located along the 30 º +35 º slopes of the Signal Mountain in 1987 and 1988.
The upper mountain slope of Vallée des Prêtres receives annual rainfall varying from 200 - 250 cm, but torrential outbursts during the passage of a tropical cyclone can cause the loose unconsolidated materials found higher up of Chitrakoot to absorb water, inflate and slide along the zone of discontinuity by a process called solifluction (a technical term used to denote the sudden flushing down of thousands of tonnes of loose materials).
A close look at the topography between Chitrakoot and the mountain wall of the Grand Glacis and Pieter Both show the presence of terraces. The most visible terrace is located at an altitude of 350 meters with other minor balconies lower down at 100 meters. It is most probable that these have been formed by subsidence and slide in steps in the past. Along the western slopes of L'Echelle Rock, located at the entrance of the valley, a stepped topography is easily observed. Hence, the name L'Echelle Rock.
Even through the eyes of a layman, there are clear-cut indications of mass movements in the vicinity of the Chitrakoot primary school. Fissures on the walls, cracks on the roof and walls of buildings, slanting roofs, slanting trees and electric poles, bulging walls, thick ridges of loose materials with convex slopes are visible. In fact, not only the primary school but, all the buildings in the area are at risk at high risk.
In conclusion
Our mountains above the 300 meters contour and above the 30 º slope are areas of risks. There is an urgent need to make an extensive survey of all our mountain slopes with a view to classify them under zones of high, medium and low risks of landslides of any type or kind. From a morphological (scientific) point of view, Chitrakoot is located in a high-risk zone and all buildings, especially a public one such as the primary school are being exposed to stress. We cannot put the life of children at risk, even if the probability is exaggerated and brought down to 10%.
We must not forget that natural disasters tend to strike us at a time least expected and the consequences sometimes may be beyond our imagination.
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