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    Is Victoria's new Johnson Street Bridge tsunami proof?


    Information contained in a 2013 AECOM study suggests engineers have inadvertently designed a seismic vulnerability into Victoria's new $110 million bridge

     

    ON APRIL 5, a City of Victoria email sent to residents promised: “Tsunami Preparedness 101–What You Need to Know.” What you need to know, according to the City, is based on a study commissioned by the CRD and carried out by the engineering firm AECOM. That study reported that when the “Big One” occurs off the west coast of Vancouver Island, it will create a tsunami. After the earthquake occurs, the wave will take about 76 minutes to reach Victoria Harbour and, depending on narrowings and widenings of the waterways leading to Portage Inlet, will vary from 2.5 to nearly 4 metres in height.

    The City says you will be safe (from the tsunami) if you are 4 metres (about 12 feet) above sea level. That won’t be much help, though, if you’re buried in a collapsed building on the Songhees shoreline. Because disaster planning requires officials to consider the worst-case scenario, AECOM’s determination of the tsunami inundation zone made the assumption that the earthquake will occur at a high tide. However, they did not factor into their reckoning future sea-level rise attributable to global warming, nor did they factor in storm surge.

    Victoria residents no doubt appreciate such helpful reminders from the City about the risks associated with living above the Cascadia Subduction Zone. But the City’s email should have been sent much earlier, and to the attention of Dwayne Kalynchuk. The AECOM-CRD tsunami study was completed in June 2013. Back then, in the right hands, it might have saved future Victorians a heap of trouble if the City’s own engineering department had considered what 2.5 to 4 metres of water might mean for a project whose design it was finalizing at the time.

    At that moment the City’s head of engineering—Kalynchuk—was overseeing the Johnson Street Bridge Replacement Project. The design accepted by the City was being refined at various engineering offices around North America. A new bridge was needed, it was claimed, because the bridge it would replace had been built in an era when seismic considerations had not been part of the design of bridges. Now, though, engineers are aware that powerful earthquakes around magnitude 9.0 have occurred, on average, every 500 years off the west coast of North America. In fact, seismologists have shown that such earthquakes could occur as often as 300 years apart.

    A potential 300-year return period means another “Big One” could happen at any time. So a bridge that had been designed without any understanding of the seismic forces it could be subjected to needed to be replaced with one designed by modern engineers armed with all the latest insights about seismic vulnerability.

    One of those insights is that the Big One will cause a tsunami. The last one shook Vancouver Island in 1700, and caused a large tsunami. That wave was even observed and recorded in Tokyo’s harbour.

    Apparently, Kalynchuk and other engineers never saw AECOM’s tsunami study. We have to assume that, because the design of the bascule pier the engineers were finalizing at that time had a fatal flaw in terms of tsunami vulnerability. The east side of the bascule pier has three large openings in it (see photo below) that are about two metres above current high tides. The CRD study predicted the height of a tsunami in Victoria’s middle and upper harbours would be about three metres (nine feet). Due to “variability” in the model AECOM used to predict that height, it put the potential maximum height of the tsunami at four metres. Even at three metres, all the electrical equipment and the two 200-horsepower electric motors used to lift and lower the bridge would be submerged and ruined. The bridge would be inoperable.

     

    1421444387_JohnsonStreetBridgebasculepiercutoutswithwaterline.thumb.jpg.dc18b555e2440de546001a46d77c12f5.jpg

    The new $110-million Johnson Street Bridge has three large openings about 2 metres (6 feet) above ordinary high tides (red line). The 2013 AECOM study determined that a tsunami would likely have a magnitude of 2.5 to 4 metres, which, if it occurred at high tide, would put the wave well above the bottom of the openings.

     

    You might well ask, “Why does this matter? After all, what happens to the new bridge will be the least of Victoria’s problems.” Emergency preparedness, however, is a state that’s achieved one project at a time. The hope of emergency planners is that enough weak points in the city’s infrastructure can be strengthened over time so that when a large earthquake strikes our region, many injuries and deaths will be avoided. But if the community doesn’t have a strong process to ensure those weak points are actually being strengthened, public resources will have been wasted and more lives lost than if the city were more seismically prepared.

    The strongest argument for scrapping the old bridge was not the avoidance of casualties caused by shaking of the bridge. The engineers’ actuarial study showed that death and injury on the bridge was a relatively small risk. Far more persuasive was the argument that following a big earthquake, an immediately operable bridge would be needed for emergency vehicles and personnel to access Victoria neighbourhoods on the west side of the bridge. An operable bridge would also be essential for what the City called “post-disaster recovery.” An unimpeded flow of tugs and barges under the bridge would be necessary for the removal of the hundreds of thousands of tonnes of debris a large earthquake would inevitably produce in the city. Those tugs and barges would also bring to Victoria materials needed for rebuilding the city and aiding its economic recovery.

    Anyone who has watched the stunning YouTube videos of the 2011 Tohoku earthquake tsunami hitting Japan’s coast can imagine that as a tsunami enters Victoria Harbour and heads toward the Gorge, it could carry with it large boats, entire buildings, cars—anything that can float—all of which, travelling at three metres per second through the narrows spanned by the bridge (as predicted by the AECOM study), could damage vulnerable machinery and bridge parts inside and outside the bascule pier. The huge cavity inside the pier would likely be filled with debris.

    As mentioned above, the CRD’s study predicts there will be 76 minutes between the time the earthquake occurs and the arrival of the tsunami at Victoria Harbour. That would give the City enough time to raise the bridge so that the superstructure could avoid direct hits from fast-moving floating objects. But since the electric motors would then be flooded and the pier filled with debris, the operator wouldn’t be able to lower the bridge. There would be no immediate access to Victoria West for emergency vehicles to put out fires, extract people from collapsed buildings and so forth—the very same circumstance that was used to condemn the old bridge.

    Why did the engineers put giant openings in the bascule pier just above the current water level? In combing through documents obtained from the City through FOIs, including the one document that specified allowable damage to the bridge as a result of seismic shaking, there’s no evidence that the engineers ever considered how a tsunami would impact the bridge. With PCL suing the City for its bad design, no one will say. I’ll speculate, though, that the bridge’s “iconic” walkway through the rings may have been the underlying reason for those openings. They may have been deemed necessary to prevent a build-up of nitrous oxide from motor vehicles using the bridge. Nitrous oxide is heavier than air and would, over time, fill the huge cavity inside the pier up to the bottom of the lowest opening. So it’s possible the engineers made the pier as open as possible so as not to asphyxiate the occasional tourist that ventures onto the walkway through the rings.

    Ironically, the old bridge didn’t have these vulnerabilities. All electrical equipment was located above the bridge deck, protected from a tsunami.

    David Broadland is the publisher of Focus.

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