Landslide - Signature of unfriendly development?

 LANDSLIDES – Nature’s reaction to unfriendly development?

In general, the factors which influence whether a landslide will occur typically include slope angle, climate, weathering, water content, vegetation, overloading, geology, and slope stability. Some of the major types of landfall are given in the Figure.



NATURAL AND HUMAN-INDUCED

A number of elements both natural and human-induced will contribute to a landslide, but often there is one which triggers the movement of material. In Mauritius, it is mostly intense rainfall.

Natural causes include intense or prolonged rainfall and seepage, undercutting by waves or river erosion or vibrations from earthquakes.

Human causes includeremoval of vegetation, changes to natural drainage, modification or overloading of slopes by construction of roads, buildings, heavy traffic or excavation.


LANDSLIDE WARNING SYSTEM

Warning Class 1 – No immediate danger but close watch is essential. Parents are strongly advised to pick up their children from schools.

Warning Class 2 – Inhabitants concerned are advised to take all necessary precautionary    measures.

Warning Class 3 – Inhabitants concerned should be ready to evacuate.

 

THE RECENT SPATE OF LANDSLIDES IN MAURITIUS

Some 37 sites have been identified as susceptible to landslides.
Those with highest level of risk are Chitrakoot, Quatre-Sœurs, La Butte and Vallée Pitot. These sites are equipped with monitoring facilities.

Some recorded events

La Butte
In 1996, following a study by Japanese experts, major protective works were carried out on the flanks of Signal Mountain behind La Butte. The cost was some Rs 600 millions.

Quatre Soeurs
Since 2005, eleven families live under threat of landslide. 

Le Mauricien 12/03/2013

Coromandel – Morcellement Hermitage
In August 2012, a wall several meters long crumbled. Mudslide followed the passage of cyclone Dumilé. Landslide with rockfall occurred over the slopes. The Structural Investigation Report identified a “Landslide Hazard Level 4 “.





Ring Road

Technical faults are considered responsible for the heavy craks on the Ring Road (photo). Les graves défauts techniques décelés sur la Ring Road (photo). et la route Terre-Rouge-Verdun, des projets qui ont coûté des milliards, font sourciller le gouvernement.



Montagne Ory

On 10 March 2013, a section of the road dropped down blocking part of the road at Montagne Ory.



Macondé
On 26 April 2014, a huge rock fell from the cliff standing along the road at Macondé. It disrupted circulation for several days. Now the rock has been stabilised.

 

Rainfall 11 to 16 January 2015 – Cyclone Bansi

Chitracoot
The heavy rains associated with cyclone Bansi that crossed the North of Mauritius from 11 to 16 January have caused a movement of the soil of 3mm in the School yard. On the other side of the road crossing in front of the school, the surface movement is of the order of 10mm, causing cracks in six of the 52 houses in the area. 


Some 21 families are concerned with landslides. To cater for the forest catchment area just above the residential and school area, large drains have been constructed to drain excess rain water. An extensometer has been installed in the school yard. A rain gauge and a siren have been installed.

Kewal-Nagar
A storeyed house built on the flank of a river has lost its enclosure. There is the risk of the house being seriously damaged.

Terre Rouge-Verdun Road
On 18 January 2015 cracks were also observed on Terre-Rouge Road.




Rainfall from 22 to 24 January 2015

Nicolière
The heavy rainfall of the 22 to 24 January 2015 has caused rockfall on the road from Salazie to Ville Bague. The circulation has been closed in the afternoon of 24 January.

Chamarel
On 23.01.15, the Western and Southern areas were seriously affected by flood. The Choisy bridge at Chamarel had caved in.

Rose Hill (23.01.15)
The road at Stanley Rose Hill is impracticable due to heavy rainfall.