Soil Piping near Idukki dam in Kerala- Yet another lesser known threat from large Dams

*Soil piping causes panic in Idukki
**Correspondent : VR Jayaraj | Kochi*

The formation of deep well-shaped pits due to the phenomenon called soil piping in Kerala's Idukki district, home to the biggest arch dam in Asia and several other major reservoirs, has caused panic in the area. Scientists who assessed the phenomenon, noticed last weekend, said there was no need for panic.
Well-shaped pits with circumferences up to ten metres and depth up to 15 metres had appeared all of a sudden in a 20-hectare area in the Udayagiri area, lying near the giant Idukki arch dam, last weekend. But scientists said the phenomenon had been occurring in the area since Year 2000.
The pits had formed in a straight line of about 500 metres from the top of a hill-slope to the valley, where the opening showed sand-mixed water seeping out. Residents in the area said that they could hear water movement beneath the earth's surface.
Soil piping (also called tunnel erosion) takes place beneath the surface of the earth and it normally goes unnoticed. During piping, large quantities of sand and clay beneath the surface get carried away by water, resulting in the formation of tunnel-like, experts say.
Much of the sub-surface soil could get eroded thus leading to formation of deep funnel-shaped tunnels beneath the surface, which is the phenomenon called piping. Indications of piping had been noticed in the Udumbanchola and Udayagiri areas Idukki since August, experts said.
Experts from the Centre for Earth Science Studies (CESS), Thiruvananthapuram, who examined the pits and the soil, said the phenomenon of piping was not rare. The process would go unnoticed as far as it is restricted to the sub-surface but pits appear when the surface becomes unable to bear the burden of the hollowness beneath caused by erosion.
According to scientists in the West, sub-surface erosion as in piping normally occurs when it has an abundant content of loose pumice, volcanic ash and such materials. They also say that piping is a phenomenon associated with factors like broken sewer pipes, storm drains or seepage from reservoirs that trigger water movements.
People in Idukki say that the scientists should examine the phenomenon in relation with the giant reservoirs in the district. The reservoir kept by Idukki arch dam, largest of its kind in Asia, is said to exert a force of two billion tons on its bed when filled to capacity.
"The panic may be unnecessary but there is nothing wrong in having a look at this," Kurien, a teacher from Painavu said. The Idukki hydel project reservoir alone is keeping more than 80 percent of the water held by all artificial reservoirs in the State. According to Western geologists, piping is always a huge concern when it happens near huge dams.
"What we should infer from the scientists' explanation is that the sub-surface soil in the Idukki hills is not as solid as we had thought it to be. If that is the case, the water kept in the reservoirs here could act as an external force that triggers movement of sub-surface flow," the teacher said.
One of the major threats that piping poses is that the vertical pits could widen leading to more earth cave-ins but scientists say that there is no pragmatic method to arrest the process. "Western experts are suggesting installation of filters that could block soil and allow water flow but this need not be practical. It is also expensive," said a scientist.
The most recent incident of piping was reported from Guatemala City in May last when a very deep pit formed right in the middle of a residential area. Guatemala City is situated in a valley covered by up to 600 feet of loose pumice and volcanic ash, materials that can be easily eroded.

SOURCE:
http://www.dailypioneer.com/284670/Soil-piping-causes-panic-in-Idukki.html