Transcript Slide 1
How Tsunamis Are Generated • Most tsunamis are caused by earthquakes generated in a subduction zone (where oceanic plate is being forced down by tectonic forces) • Others by underwater volcanoes or landslides Wave Propagation SF Bay Treasure Island Development Project SF Bay Vulnerable To Distant Sources > 4 hrs to SF Bay > 5 hrs to SF Bay Less vulnerable to tsunamis from Cascadia or So America Treasure Island Development Project SF Bay’s Local Fault Structure Limits Tsunami Generation • San Andreas is a transform, strike-slip type fault (adjacent plates not subducting below each other) • Little to no vertical displacement Treasure Island Development Project Width of Golden Gate Restricts Tsunami Propagation Into SF Bay Treasure Island Development Project Waves Diffract As They Pass Through Gate 1. Tsunami height reduces due to “energy spreading” as a result of: • • • Restricted Opening (Golden Gate) Shallow Depth of Bay Shape of Bay 2. Tsunami profile changes after passing the Golden Gate due to: • • Wave diffraction Not a “wall of water” Treasure Island Development Project Measured Tsunamis In SF Bay 1960 Chile Presidio, SF 8 6 4 4 2 2 0 0 Predicted -2 Alameda 8 6 1964 Alaska Observed -2 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 0:00 2011 Japan 6:00 12:00 18:00 0:00 6:00 12:00 18:00 0:00 Treasure Island Development Project Tsunamis Included in Design of Treasure Island 1. Analyses and modeling based on: • FEMA Guidance • Tsunami Risk studies conducted by CalEMA for SF Bay Marine Oil Terminals 2. Extreme high water level modeling: • Included coincident events (tides + storm surges + tsunamis + waves) • Used a 157-yr record of tsunamis in SF Bay, extrapolated over a 1008-yr period to determine probability of tsunami occurring jointly with tides and storms • Highest water level including tsunamis over the 1008-yr simulation was +9.5‘ • 100-yr return period high water level was 9.2’ • 100-yr return period wave runup was 10.5’ to 16.5’ Treasure Island Development Project Proposed Design Criteria For Treasure Island 1. Base Flood Elevation = 9.2’ (based on combined 100-yr return period high water level including high tides, wind-waves, ocean swell and tsunamis) Finish floors of buildings will be 3.5’ higher (12.7’ minimum) to accommodate sea level rise 2. Design Wave Runup = 10.5’ to 16.5’ (based on combined 100-yr return period runup elevation including wind-waves, ocean swells, and tsunamis) Perimeter elevations will be 1.33’ higher (11.8’ to 17.8’) as an allowance for sea level rise and more extreme event occurrence, including tsunamis. Treasure Island Development Project Lower Probability Tsunami Events • • • • • • • CalEMA map shows inundation up to 15’ elevation for emergency scenario planning (not for design) Return period for such an event (10’ tsunami occurring at a 5’ tide in SF Bay) is over 1000 years Significantly beyond the standard of practice included in Building Codes (472-yr earthquake, 100-yr flood, etc.) SF Bay will have 4 to 5 hour advance warning of such an event Typical tsunami wave after coming through Golden Gate rises to full height in about 20 minutes (not a wall of water) Structures on TI will be modern, multi-story buildings & built to present codes TI-specific emergency response planning appropriate Treasure Island Development Project