Extreme rainfall events are more common with climate change some can have devastating impact resulting in landslides, destruction of physical property (homes) and loss of life. Most tropical countries are faced with this problem - this year Brazil has been in the spotlight. The images on this blog show the rehab of one such slope in Rio de Janeiro State. The work was designed and implemented by João Henrique Eboli, a retired telecommunications engineer, and comprised a carefully designed combination of engineering techniques and Vetiver grass. 
The site is shown in the Google Earth image at the top of this blog, Mr. Eboli's land is marked in red and the land slip is marked with a blue X. This image of 2009 shows the initial drain layout undertaken prior to treating with vetiver. The slide on Mr Eboli's land joined with his neighbor's slip and together created major damage. On the far right are raw land slip areas.The treated area was 1840 square meters, and cost just under $16 per square meter. In the three years since implemented in 2008 it has stood up well to extreme rainfall events.
The image at the bottom was taken in mid January 2011 just after 123 mm rainfall in less than 24 hours. The treated slope stood up to the storm magnificently, Read more
Referring to Rio de Janeiro landslides, they occur all along our coastal mountain range called “Serra do Mar” where shallow soils lay right over the rock. The problem is always the water: under heavy rains, the subsurface flows are very strong and once associated to the slippery rock result in landslides. The process begins with subsurface flows transporting soil particles until all friction is gone.
ReplyDeleteIn these areas, Vetiver has influence in many different ways: 1) it reduces runoff speed, thus increasing infiltration what may contribute for the landslide process; 2) It also reduces speed of subsurface flows reducing erosion process thus avoiding the activation of the landslide process; 3) where the rock is fractured Vetiver will work as anchor.
I strongly recommend Vetiver for similar conditions, not only on the plains as remarked by Dale but also for the slopes, under the following concerns:
1) Previous Geological / Geotechnical study to confirm technical solution and identification of risks such as rockfall, for example;
2) Determination of the spacing and position of the hedgerows or plants alone - in case of using Vetiver as a GREEN NAILING as we have been calling the introduction of Vetiver spaced minimum 0.5 m (in reference to SOIL NAILING already used in traditional engineering);
3) MONITORING: as long as water drained in the bottom of slope is clear and free of sediment the structure is working and risk of failure is limited.
We have helped Mr. Joao Eboli in his work at Itaipava, Rio de Janeiro State, one of the major centers of the recent catastrophe and showed to the members of this group by Paul Truong earlier this week. Paul Truong and I visited the place a little over one month prior to the climate event reported. After the rains I called Mr. Eboli to check if his slope had any damage and from his report the most important was that he noted that the water in the lower part of his property was coming out totally clear, free of sediment, while his neighbor was experiencing soil loss and risk.
Rgds,
Eng. Civil Luiz Lucena
Deflor Bioengenharia
Tel.: + 55 31 3284-5622
Cel.: + 55 31 9322-5807
www.deflor.com.br
I would add that vetiver hedge rows spread the water more evenly across potential slip slopes, thus reducing potential high water pressure points/saturation that might might be the point where the slip starts. Also in in the hot tropics vetiver is likely to pump out quite a lot of soil retained water, thus lowering soil water saturation prior to new rainfall events.
Dick Grimshaw