Tuesday, July 1, 2014

Geology Ignored In The Planning And Building Of Himalayan Dams

In Current Science K.S Valdiya explains (Open Access):

It will be obvious from the distribution of dam locations (Figure 1; Tables 3–6) based on information culled from reports of Central Electricity Authority, Uttara-khand Hydroelectricity Nigam, Uttarakhand Renewable Energy Development Authority, etc. that the existing hydroelectric projects and those that are under construction or planned are sited close to the terrane-defining thrusts known to be active. The sites were chosen presumably in the narrowest stretches of the river valleys, little realizing that the otherwise wide valleys with gentle valley sides become narrow with steep to nearly vertical walls due to uplift of the ground and attendant accelerated riverbed erosion as explained earlier. The ground rises as a conse-quence of upward movement on active faults/thrusts (Figure6). Moreover, the belts of active faults are made up of deformed rocks –many-times folded, sheared, shattered an even crushed rocks. These rocks understandably easily break-up, fall -off, creep and slide or slump down when excavated or shaken by earthquakes and explosions,and sink under loads. These incidences are bound to pose a threat to the various structures built in the project areas.

The development of hydroelectric projects not only entails excavations for the head race dams and associated coffer dams, diversion tunnels, main tunnels for carrying water to turbines, and multitudes of adits, but also for thenetwork of roads, for residential colonies for work force,and for power generators. Obviously, a dam site–nomatter if it is just a small one–is excessively subject to tampering with the natural balance in a zone of very weakened rocks.

Reactivation of the active thrusts is bound to impact the stability of the engineering structures. One of the impacts could be the displacement or disruption of the structures due to sudden release of stress that the thrust movements entail. The effects on the tunnels associated with dams would be far more severe – there would be dis-ruption or offsetting of tunnel, roof collapse, sudden on-rush of interstitial groundwater with crushed material,and severe damage to tunnel lining. The very making of a tunnel is like opening an underground drainage and thus altering the groundwater regimes of the mountains, resulting in drastic lowering of groundwater table and at tendant drying up of springs and dwindling of surface flow in streams.

Figure 1 is self-explanatory. Needless to state that a large number of existing and planned hydroelectric projects are bound to encounter serious problems, particularly if and when movements take place on the thrusts in the proximity of the project locations.

Uttarakhand has plans for 180 big and small hydroelectric projects with 95 dams in the middle and upper reaches of the Alaknanda and Bhagirathi rivers in the vicinity south of the Main Central Thrust. K.S Valdiya suggests that the sites should be chosen preferably north of the Main Central Thrust in regions with much lower population density which will lead to less environmental, social and economic problems. The current government has indicated that it will lean towards rapid environmental clearances for infrastructure projects, so just how much attention will be given to warnings like this one?

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