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City of Seattle
Ed Murray, Mayor

SUBJECT: Seismic study: Seattle reservoirs may need work to survive a huge quake

6/19/2014  9:14:00 AM
SPU Customer Service  (206) 684-3000

Seismic study: Seattle reservoirs may need work to survive a huge quake
Even with retrofits, city’s reservoir program expected to be completed at or under budget

SEATTLE — A gigantic earthquake — of the kind that might strike the region only once every 2,500 years — could make four of Seattle’s new covered reservoirs unusable, a two-year investigation being conducted on behalf of Seattle Public Utilities (SPU) has found.

Preventing cracks and leaks, which could occur during a mega-quake, will require retrofits to one or more of the reservoirs. But even with fixes to all four, the city’s program is expected to be completed at or under budget, and utility rates will be unaffected.

So far, a seismic review has been completed for the West Seattle reservoir — which does require retrofits — and similar reviews will be conducted on the other three to determine if they too need fixing.

The total cost of the West Seattle work — including the seismic study, retrofits design, construction work and taxes — is estimated at $7.6 million. If it is found the other three need work, the total cost of the four retrofit projects could range from $25 million to $30 million.

SPU Director Ray Hoffman said the utility has been working collaboratively with the reservoirs’ designer, Denver-based MWH, to recover the costs of the retrofits. The rigorous seismic analysis was set in motion in March 2011 after MWH notified SPU of possible seismic deficiencies in their work.

“MWH has been working with us, voluntarily paying costs incurred and cooperating every step of the way — and we expect that to continue,” said Hoffman. “When the dust settles on these projects, we believe we will still be at or below the original budget, and that utility rates will be unaffected by the retrofits.”

Hoffman emphasized that without the retrofits — even in a monster quake — the reservoirs are in no danger of catastrophic failure, and they pose no risk to public safety. And he said SPU will work to minimize disruption of activities in the city parks that have been built atop the reservoirs.

To date, the overall cost of building and retrofitting the four reservoirs is about $133.5 million* —$15.5 million under the original budget, Hoffman said.

Hoffman said the seismic work “is very important because after earthquakes — and especially very large quakes — the city needs to know its reservoirs will still be working and providing high-quality drinking water as well as water for fire-fighting.”

  • The new Seattle reservoirs are standardized reinforced concrete designs that have performed during many high-seismic events in the past without loss of water or excessive leakage from seismic events — such as the 1994 Northridge Earthquake in California. The sophisticated seismic codes of today generate more robust designs than have ever been completed in the past. The newer codes and seismic designs are state of the art with applications of lessons learned from previous seismic events.
  • There has been no disruption to water service or water quality since the reservoirs were put on line. During the course of the retrofit, SPU will continue to coordinate the work with as little disruption as possible.
  • SPU is now soliciting bids for retrofits to the West Seattle Reservoir that will fix the problems identified in the analysis. That work includes strengthening the floor of the facility, and installing a liner on part of the reservoir wall. CH2M HILL, the engineering firm that is conducting the seismic analysis, is still working on the analyses for the other three reservoirs.

Seattle began installing floating covers at some reservoirs in the late 1990s. The Sept. 11, 2001 terrorist attacks and resulting security concerns, prompted officials to begin replacing the city’s remaining open-air reservoirs with covered concrete vaults. Along with added security and improved water quality, covering the reservoirs created 76 new acres of land for city parks.

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In addition to providing a reliable water supply to about 1.3 million residents in the Seattle metropolitan area, SPU provides essential sewer, drainage, solid waste and engineering services that safeguard public health, maintain the City’s infrastructure and protect, conserve and enhance the region's environmental resources.

* That includes about $3.5 million spent on the seismic study.


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