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Section 4. D — Environmental Setting and Discussion of Impacts <br />4.9.1.5 Dam and Levee Failure <br />San Mateo County compiled a dam failure inundation hazard map. The project site is not located in a <br />dam failure inundation hazard zone, 26 <br />Although outside the FEMA annual one percent chance flood zone due to protective levees, the site <br />has the potential to flood as a result of overtopping or failure of the levee system. Redwood Shores <br />is located on low-lying bayfront land surrounded by a Ievee system that protects Redwood Shores <br />from high tides. Current levee heights were constructed to withstand a 100 -year high tide. The crest <br />of some levee reaches are at, or a few tenths of a foot lower than, the 100 -year tide elevation. Due to <br />the short duration of that crest, however, flooding would be limited and shallow, provided that the <br />levees do not fail from the overtopping. The City is currently undergoing a process to reaccredit the <br />levees supporting the project area. This process may include strengthening or raising levees, creating <br />new levees, participating in regional mitigation to address the entire Bay, and/or creation of new tidal <br />wetlands. <br />49.1.6 Seiches, Tsunamis, and Mudflows <br />A seiche is an oscillation of the surface of a lake or landlocked sea varying in period from a few <br />minutes to several hours. Seiches are extremely rare in the San Francisco Bay, which generally <br />attenuates such activities due to its irregular shape and shallow shoreline. Therefore, the potential for <br />the project site to be subject to sciches is considered low. <br />A tsunami of tidal wave is a series of water waves caused by the displacement of a large volume of a <br />body of water, such as an ocean, bay, or large lake. Due to the immense volumes of water and <br />energy involved, tsunamis can be devastating to areas along shore lines. The project site is located <br />near the San Francisco Bay, which is subject to Tsunamis. A Tsunami Map prepared by the <br />California Emergency Management Agency, California Geological Survey, and the University of <br />Southern California in 2009 indicates that the site is located just outside the limits of the areas that <br />may be inundated by tsunami waves under mean high water sea -level conditions. 27 <br />A mudflow is the rapid movement of a large mass of mud formed from loose soil and water. The <br />project site and surrounding area are flat. There are no mountains Iocated near the site that in the <br />event of a mudflow would affect the site. <br />21 County of San Mateo. "San Mateo County Hazards: Dam Failure Inundation Areas." Accessed May 31, 2016. <br />Available at: htto://nlanninL.smceov.orv-/docaments/san-mateo-county-hazards-dam-failure-inundation-areas. <br />27 Tsunami Inundation Map for Emergency Planning/Redwood Point Quadrangle/Palo Alto Quadrangle. Accessed <br />September 2, 2014. Available: <br />httu://ww-,v.consrv.ca.gov/cizs/Qeoloaic hazards/Tsunami/Inundation Maas/SanMateo/Documents/Tsunami Inunda <br />tion RedwoodPointPaloAlto Quads SanMateo.odf. <br />City of Redwood City 58 Initial Study/Mitigated Negative Declaration <br />Sandpiper Elementary School Facilities Expansion November 2016 <br />