Addressing Landslides

Hidden beneath all the recent headlines about the big freeze in the northern hemisphere have been news items on the occurrence of land/mudslides in both Brazil and the Solomon Islands.


The slides in the Solomon Islands were following an offshore earthquake whilst those in Brazil were the results of very heavy rain. Now whilst freezing weather is clearly not a risk factor in the Seychelles, landslides are. The very steep terrain to be found in areas of the four main granite islands coupled with tropical rainfall provide the basic ingredients for landslide events.

The most famous natural disaster recorded in the human history of Seychelles is the landslide that occurred on the 12th October 1862 known locally as “La Valasse”. Accounts vary as to whether it was very heavy rain, brought on by a tropical cyclone, or a waterspout releasing its contents on the mountainside, either way, a huge landslide resulted. Originating from the east face of the Trois Frères mountains, sweeping down a swathe of land along the St Louis river valley, the boulders, mud and debris battered into the parts of the town of Victoria below. Many lives were lost and much infrastructure engulfed and submerged.

Looking at the photos of the landslide that took place at the beach resort of “Angra Dos Reis” on the island of “Ilha Grande” in Brazil I was struck by the similarity with the Seychelles scenario (see photo in sidebar). The landslide has occurred leaving the bare rock face – or glacis as we would call it locally – behind. Local readers will recall exactly the same thing happening, though on a much smaller scale, on the Beau Vallon Baie by-pass road a few years ago. What happens of course is quite simple, with very heavy rainfall the soil saturates and water actually starts to flow down the rock surface underneath the soil. This serves to lower the friction coefficient between the soil and the mountainside whilst increasing the weight of the soil. If sufficient water flows the soil and boulders can actually begin to slip down over the surface of the rock due to gravitational force.

A more gradual occurrence of this is one of the underlying problems affecting and causing great concern at the Vista-do-Mar estate in recent years.

One of the key activities of the GFDRR project in Seychelles will be a geological survey of the 3 main inhabited islands in order to assess the stability of steep hillsides in particular those that host or are above key residential and infrastructure areas. The findings of this work will enable better guidance to be provided to developers that are building in such areas and also allow the level of risk to be reduced or mitigated in existing developed areas.

These aspects of risk assessment and reduction are a key component of the raison d’etre of the Department of Risk and Disaster Management and the results of this study, when undertaken, should help provide a risk baseline upon which their other activities can be designed and targeted.