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2021 Multijurisdictional Local Hazard Mitigation Plan islands, and flood control channels may alter the level of damage. Offshore canyons can focus tsunami wave energy, and islands can filter the energy. A tsunami wave entering a flood control channel could reach a mile or more inland, especially if it enters at high tide. The orientation of the coastline determines whether the waves strike head-on or are refracted from other parts of the coastline. A wave may be small at one point and much larger at others. The inundation area for a tsunami event is often described as runup as illustrated in Figure 15-2. Source: UNESCO, Retrieved from Different Directions: Tsunami, n.d. Figure 15-2. Runup Distance and Height in Relation to the Datum and Shoreline 15.1.2 Secondary Hazards Seiches are a potential secondary hazard from tsunamis. Seiches are standing waves oscillating in a body of water, and they can form in any enclosed or semi-enclosed body of water, including San Francisco Bay. They typically result from strong winds and rapid changes in atmosphere pressure, which push the water from one end of the enclosure to the other. When the wind stops, the water rebounds to the other side and then continues to oscillate for hours or days. Tsunamis, earthquakes, and severe storm fronts can also cause seiches. The destructive potential associated with seiches is exemplified through one from 1844, where a 22-foot seiche in Lake Erie breached a 14-foot sea wall, killed 78 people, and dammed the ice to the extent that the Niagara Falls temporarily stopped flowing (NOAA, 2021a). While seiches are not as common in the San Francisco Bay as they are in the Great Lakes, bayside communities should still be mindful of this potential hazard and recommend community members avoid close proximity to the bay for several days after a tsunami. 15-2