The Nisqually earthquake in Olympia on 28 February 2001 shook the Pacific Northwest and caused billions of dollars worth of damage throughout Washington state. State governor Gary Locke immediately declared a state of emergency and President Bush sent Joe Allbaugh, head of the Federal Emergency Management Agency, to visit the area and assess the damage.
No dams were affected by the 6.8 quake, but it served as a reminder to the US Bureau of Reclamation (USBR) of the importance of emergency procedures at the 33 facilities it owns within a 450km zone of the impact.
The USBR oversees the operation of more than 70 dams in the Pacific Northwest, a region encompassing Idaho, Oregon, Montana, Wash-ington and Wyoming. When the Teton dam failed in 1976, the Bureau responded with a rigorous safety review of all its facilities. Each structure is periodically examined for overtopping, internal stability, physical deterioration and, importantly, seismic stability.
The February quake triggered onsite visual inspections of all 33 USBR dams in the area, which were undertaken by staff from local irrigation districts: now responsible for the operation and maintenance of all the facilities. This took place according to the rigid procedures outlined in the Bureau’s emergency action plan.
Inspectors looked for obvious signs of cracking, such as cloudy seepage, and used peizeometers to measure changes in the level of the water table. Initial daily measurements were followed up with weekly readings. The seepage from neighbouring weirs was also monitored. Some dams are located in high country, where snow made it difficult to ascertain whether there had been any damage. These facilities were checked in the first 24 hours after the quake and will be examined in more detail when snow-melt begins to fill the reservoirs.
‘Visual inspections revealed no apparent damage and instrument readings appear to be normal,’ said Jim Mumford, regional safety of dams programme manager.
‘Fortunately, none of the dams in the area affected by the earthquake had the potential for liquification,’ Larry Wolf, one of the Bureau’s safety of dams engineers, added. ‘Although tremors were felt as far away as Grand Coulee, most of our structures were quite a long way from the epicentre and the quake itself was 52km deep,’ he added.
All the facilities will continue to be carefully monitored as the snow melts and fills the reservoirs, although a low snow pack means that many are unlikely to fill to capacity.