Dr. Martin Wieland
Published: 13/05/2014at 12:20 AM
At the end of the editorial "Quake gives wake-up call" the following statement is made: "The Chiang Rai quake also put into question many new dam projects that increase the risk of earthquakes because they are sitting right on active fault lines."
For dams located on faults, the seismic hazard comprises ground shaking and fault movements.
The maximum magnitude of earthquakes in the north of Thailand can be estimated as the maximum observed magnitude plus 0.5.
Based on the recent magnitude 6.3 Chiang Rai earthquake, one could estimate a magnitude 6.8 earthquake as the upper limit of that region.
Such earthquakes with a shallow focus create surface ruptures. The fault movements may be in the order of one metre. As concrete dams are vulnerable to fault movements they should not be built there.
But conservatively designed embankment dams can cope with these movements. Therefore, if embankment dams are selected, which make up most dams in Thailand, they can still be built safely.
So these projects are not really in question from the seismic safety point of view. A greater problem may be the older dams.
Here it is recommended to carry out a seismic safety assessment taking into account up-to-date earthquake design and performance criteria.
The unsafe dams may have to be strengthened or their reservoirs may have to be lowered. Decommissioning would be the last resort.
Hardly any dam has been decommissioned due to inadequate earthquake safety.
Finally, it should be pointed out that new dams do not increase the earthquake risk as in tectonically active regions earthquakes will occur sooner or later independent of the presence of a dam.
By impounding large reservoirs, like those in Kanchanaburi province, earthquakes may be triggered but their magnitudes are much smaller than the maximum magnitudes mentioned above, which would be taken into account in designing resilient dams.