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Coastal Flood Processes Along the California Coast
Justin Vandever, P.E., Coastal Engineer, BakerAECOM
Media coverage of catastrophic coastal flood events, including Hurricanes Katrina (2005) and Sandy (2012), has increased the public’s awareness of coastal flood vulnerabilities along the nation’s shorelines. These large storm systems, with their powerful winds and overwhelming storm surge and rainfall, can and have devastated coastal communities.  The aftermath of such has graphically shown  the public the damage wrought by large tropical storm systems along the Gulf and Atlantic coastlines. However, coastal storm systems and impacts along the California coast differ significantly due to the characteristics of the Pacific Ocean basin, storm types, and steep coastal topography.
 
The likelihood for a hurricane to make direct landfall along the California coast is very remote, although offshore tropical storms can affect coastal communities through wind, rain, and remote swell impacts. Due to the oceanographic conditions in the northeast Pacific Ocean and the narrow continental shelf, it is not possible to generate the large storm surges seen in the warmer and shallower Gulf and Atlantic waters. Instead, coastal flooding along the California coast is typically a result of the combination of high tides, modest storm surge, and moderate to high wave energy. Unlike the Gulf and Atlantic coasts, where storm surge in excess of 20 feet is possible, storm surge along the California coast rarely exceeds three feet and is typically on the order of 1–2 feet during winter storm events. Instead, wave effects, such as wave setup and runup, typically dominate flood levels at the shoreline. The majority of coastal flood events in California occur during the late fall through early spring and are the result of extratropical storm systems that originate offshore in the northeast Pacific Ocean. During El Niño winters, tides are further elevated along the coastline and storms follow a more southerly track, exposing the California coast to abnormally high tides and wave-induced flooding.
 
The summary below explains various types of coastal flood processes along the California coast that are typically responsible for flood impacts, ranging from King Tides to extratropical storms to tsunamis:
  • King Tide – Abnormally high, but predictable, astronomical tides that occur approximately twice per year, typically during the winter months. King Tides are the highest tides that occur each year and typically exceed seven feet (relative to mean lower low water). Coastal flood impacts include nuisance flooding and inundation of low-lying roads and paths. High tides can exacerbate coastal and riverine flooding, especially in inland bay areas such as San Francisco and Newport Bays.
  • Extreme High Tide – When Pacific Ocean storms coincide with high astronomical tides, storm surge due to meteorological effects can further elevate water levels along the coast to produce extreme tides. El Niño conditions along the coast can also contribute to storm surge and produce extraordinarily high water levels (for example, January 1983 and February 1998). Extreme high tides can exceed 7.5 to 8.5 feet in southern and central California and 10 feet in Northern California. Impacts include severe inundation of inland bay shorelines, intensification of upstream riverine flooding, and inhibited drainage from storm water outfalls in tidally influenced areas.
  • Wind Wave Event – Pacific Ocean storms or strong thermal gradients can produce strong winds that blow across sheltered water bodies and inland bays (for example, San Francisco Bay, Tomales Bay, etc.). When the wind blows over long reaches of open water, large waves can be generated that impact the shoreline and cause damage to coastal structures such as levees, docks and piers, wharfs, and revetments. Locally generated wind waves in the Southern California bight can also cause flood and erosion issues along the shoreline. 
  • Pacific Winter Storm – During the winter, storm systems from the Aleutian Islands, Hawaii (“Pineapple Express”), and other parts of the North Pacific impact the California coastline. Storms generally approach from the west or northwest, although “southeaster” events can also occur in southern California. These low pressure systems generate large waves and elevated tide levels along the coast. Impacts include beach and bluff erosion and damage to homes and coastal structures.
  • El Niño Winter Storm – During El Niño winters, atmospheric and oceanographic conditions in the Pacific Ocean produce severe extratropical winter storms that impact the California coast. Storms follow a more southerly track and bring intense rainfall and storm conditions. Rainfall and elevated tide levels persist through the winter and often coincide to produce upstream riverine flooding. Impacts are widespread but sheltered south facing beaches are particularly vulnerable.
  • Remote Swell – Remote swell is generated by storms in the Pacific Ocean and from other regions such as Baja California and more distant areas such as New Zealand. Storm types include offshore extratropical storms, tropical storms, hurricanes, and southern hemisphere storms. Remote swell events can be difficult to predict since waves travel from distant source regions. Impacts include wave damage and overtopping along the shoreline, particularly to coastal structures such as breakwaters, piers, and revetments. Wave overtopping can also cause inundation and ponding of water in backshore areas, such as low-lying roads and parking lots.
  • Tsunami – Tsunamis are extremely long period waves generated primarily by earthquakes, but can also be caused by volcanic eruptions or landslides. Tsunamis can be generated from far-field source regions such as Chile, Alaska, or Japan and from near-field source regions along the Pacific coast. Impacts include strong currents and long lasting water level oscillations in harbors which can damage docks, piers, and boats moored within the harbor. For larger tsunami events, impacts could include shoreline inundation and overland flow of water that damages structures in low-lying areas.
 

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