Eos asked Wapda, the lead authority overseeing the construction of Diamer Basha dam, about the seismic design of the project, and whether they have factored in the heightened risk of earthquakes in the region when drawing up the design parameters. Specifically two questions were posed to the authority, both asking for the range of ground shaking that the dam has been designed to withstand.
Earthquakes involve two kinds of ground shaking that have to be taken into consideration when designing structures in hazardous zones. There is vertical acceleration, the speed with which the ground is expected to rise and fall in a possible earthquake, and the horizontal acceleration, which is the speed with which the ground is expected shake from side to side in an earthquake. Eos asked Wapda what range of vertical and horizontal acceleration they have used in the design parameters of Diamer Basha dam.
In vertical acceleration, Wapda replied saying the range they have factored is 0.146g as the operating basis for an earthquake, and 0.247g as the maximum design earthquake that the structure will be designed to withstand. In horizontal shaking, the operating earthquake assumption used in the structure’s design is 0.22g and the maximum design earthquake parameter is 0.37g.
The water and power development authority’s seismic design parameters indicate they may be woefully inadequate
Wapda explained that they arrived at the values by measuring the level of shaking that was observed at the dam site during the October 2005 earthquake in Kashmir. They say that two strong motion accelerographs have been installed at the dam site since 2002, and after August 2007, they have 10 additional micro seismic stations within a 50km-diameter of the dam site to also measure the small, almost imperceptible earthquakes that occur in that region with some regularity. Their response says that the accelerographs at the dam site measured horizontal ground shaking of up to 0.11g on rock and 0.015g on alluvium during that event. Their response adds that “[s]eismic criteria after 2005 earthquake have been duly considered in the design of the dam and its structures.”
But the data raises very important questions. The ranges identified by Wapda are more or less consistent with the seismic tolerance ranges given in the Building Code of Pakistan 2007. In that code, the peak horizontal ground acceleration that should be accounted for in Zone 3 of Pakistan (where the dam site is located) is 0.24g to 0.32g. The Wapda range for the dam structure is very similar to this.
There are two big problems that this data has. First is that the Building Code of Pakistan 2007 specifically says that its tolerance ranges are not applicable to dams as well as a host of other structures that are larger than small houses, including power stations and transmission lines, military installations, tunnels and pipelines and more. In fact, the code specifically says that its provisions are for “buildings and building-like structures” only.
The second big problem is that the ground shaking parameters are taken from the October 2005 earthquake, which occurred hundreds of miles away from the dam site. The question naturally arises: what if the next earthquake is closer to the dam site? The ground-shaking in an earthquake decreases by the square of the distance as one moves away from the epicentre. If done properly, the seismic design parameters should assume that the epicentre of the next mega earthquake will be directly under the dam itself, then draw up a tolerance range and multiply it by five, at least, to arrive at a safe provision for the seismic hazard that the dam faces.
At the moment, the seismic design parameters that Wapda has derived for the dam and its associated structures (power house and tunnels, transmission lines, etc) is grossly inadequate to the seismic risk that the structure is exposed to.
The writer is a member of staff
Published in Dawn, EOS, April 7th, 2019