Tsunami Risks in the City of Pacifica, California
The purpose of this study was to investigate potential tsunami run-up in the City of Pacifica because of a City Hall relocation project located within a designated tsunami flood zone. Historical data used for the study had been collected by the National Geophysical Data Center (NGDC) between 1810 and 2007 in northern California.
The site is bounded by residential use on the north and south, by Beach Boulevard and ocean frontage on the west, and by commercial use on the east. Beach Boulevard is protected from ocean waves by a seawall with a height relative to mean sea level (MSL) varying between 16 and 20 feet. The project site is on relatively flat ground at about 21 feet above MSL, sloping gently toward the ocean. The proposed development would be about 60 ft from the shoreline
Both deterministic and probabilistic analyses were used in this study. The type of analysis used in a given situation is determined by the objective. A deterministic approach is used to prepare tsunami evacuation maps and is based on the worst-case scenario for tsunami occurrence, while a probabilistic approach is used for the engineering design of coastal structures to enable engineers and planners to estimate the risks associated with various scenarios.
The study concluded that the proposed development would be reasonably safe from tsunami hazards for the following reasons:
- Setback distance from shoreline,
- Pre-existing shore protection,
- Planned elevation,
- Protection of the site from existing structures seaward of the proposed development, and
- Anticipated construction quality.
The combination of setback and elevation would place the site mostly out of the influence of a tsunami. Even if 2-3 feet of water were to flow over the top of the existing shore protection, it would lose energy while approaching the parking lot.
This study concluded that although some flooding of the first floor could not be precluded, shore protection would dissipate most of the energy of a tsunami. Thus, tsunami risks at the new City Hall facility would be low in comparison with other locations in northern California. However, due to the possibility that a small tsunami could occur, and some water could reach the facility, the City of Pacifica was advised to store important documents on upper floors as a precaution.