Levee Break – San Francisco?

Steve's picture

With all the news about Central United States flooding and Mississippi levee breaks, I thought that it might be instructive to run a levee failure simulation that could pique interest of folks in Central California.

The example is hypothetical, but imagine….

(1) A levee spanning the Golden Gate where the Golden Gate Bridge now stands.

(2) San Francisco Bay drained to bottom, and the region developed and cultivated like the lands of Santa Clara Valley further to the south.

Suppose now that levee fails and the sea pours in! 

How fast would area in the once dry Bay be flooded? What would be flow rate past the Golden Gate?  How long would it take for the Bay to refill?   

These are just the types of questions that Hazard Simulations - even hypothetical ones - can address.

 http://es.ucsc.edu/~ward/levee-sfbay.mov shows the progress of the simulated flood over a span of about five hours.  Numbers along the top show the total water volume past the Gate, rate of flow through the Gate, and the area of once dry bottom land covered by water.

Within an hour, 1.7 billion cubic meters of water have invaded the region covering 170 square km.  Total volume and area of the Bay shown in the movie are 5.5 billion cubic meters and 700 square km, so about 1/4 of the Bay is filled already.  Flow rates through the Gate in that hour average about half million cubic meters per second.  That’s three or four times the flow rate seen there in a typical tidal cycle.  Note how the ocean west of the Gate is “drawn down” as water is sucked through the gap.

The inundation rate slows over the next hours as the water levels of the Ocean and Bay equalize.  About six hours after first break, the Bay has nearly refilled and San Francisco becomes a peninsula city once again.

 

 

If you are interested,  http://es.ucsc.edu/~ward/levee-sfbay-flow.mov runs the simulated flood again but with flow directions shown and flow speeds contoured in 1 m/s increments. The ocean draw-down is especially clear in this.

[Full YouTube version of this here....   http://www.youtube.com/watch?v=4gILvSF0NXo    }

This simulation is hypothetical, but the same computer codes can be used to investigate cases of real hazard, like those Mississippi levees.

Steven N. Ward,  Santa Cruz

 

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