Friday, February 26, 2016

Tsunami and Straits - the BC Version

The following images are from AECOM (2013) modeling of a Cascadia earthquake event generated tsunami into the Strait of Juan de Fuca. AECOM generated the model using a tsunami model developed by Cheung and others (2011) and utilizing digital elevations and bathymetry across both U.S. and Canadian waters (not so easy given the differences in how the two countries handle this data). The purpose of AECOM's work was to provide tsunami inundation, drawdown and velocity to the BC provincial capital area of Victoria on the southeast end of Vancouver Island. The work provides some localized information for areas of Washington State as well.

Maximum water level estimates

The maximum water level increase has some interesting aspects. Areas along the shore of the Strait at several places water level increases on the order of 4 meters. Just past the southwest tip of Vancouver Island, the maximum water level drops off as the inlet widens into Haro Strait. But past that dampening effect are a few hot spots. One is the build up of water height as the water surges into Rosario Strait and becomes constricted within Rosario Strait, Guemes Channel and Bellingham Channel. The result suggests that low areas along the east side of Lopez Island, southeast side of Decatur Island, South side of Cyprus Island and west shore of Guemes Island and north shore of Fidalgo Island will see much higher water levels from the event than other areas even a short distance away.

Two other red areas of high water that stand out are Port Discovery and Port Townsend, the two bays on the southeast of the image above. Both these bays have deep entrances and then become narrower towards the head of the bays and thus the water levels become enhanced as the water surge from the tsunami enters and floods into the bays. Indeed the head of Port Discovery has been investigated for tsunami deposits and several deposits have been documented (Williams, Hutchinson and Nelson, 2005).

One caution with this is how the elevated water will behave as it comes on shore and across tidal areas would require a significant more detailed analyses. Hence, a 4 meter rise in the strait may translate to a less or more inundation. Part of the AECOM's effort was to look in more detail at areas within the inlets of the Victoria area.  

Drawdown estimates 

The other tsunami impact is how low the water will go as the water is pulled out. The pattern of large drawdown areas is somewhat similar to the maximum water level areas but with a few shifts. The drawdown of water out of harbor areas plays havoc with ports as docks collapse and boats end up grounded and then the water surges into the harbor.

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