Tuesday, October 8, 2013

Coastal Flood Processes Along the California Coast

As the Atlantic hurricane season draws to a close and the Pacific storm season picks up, I thought it would be interesting to consider the differences in coastal flood processes between these two coastlines and remind us what's on the way this winter. The post below is based on an article I prepared as part of FEMA's coastal flood mapping along the California coast, currently underway. That article can be found here, but this one has pictures!

Recent media coverage of catastrophic coastal flood events, such as 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 devastate coastal communities. Due to these recent events, the public has seen firsthand the damage wrought by large tropical storm systems along the Gulf and Atlantic coastlines. But what about coastal storms in California? Coastal storm systems and impacts along the California coast differ significantly from the Atlantic and Gulf Coasts due to the characteristics of the Pacific Ocean basin, storm types, and steep coastal topography.

As we know, hurricanes are a relatively common occurrence along the Atlantic and Gulf shorelines. In contrast, 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 reaches 3 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, wave-induced flooding, and coastal erosion.

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 7 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.

A 7 ft King Tide at San Francisco's Embarcadero (January 21, 2012). During the 1983 El Nino, the water level was 1.5 ft higher.

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 stormwater outfalls in tidally influenced areas.

The table below shows the typical tide range and highest observed tide at various points along the California coastline*. As you can see, the tide range increases from south to north. Additionally, the magnitude and frequency of storm surge events increases as well. These factors combine to produce more extreme tides in the north than the south.

Station Daily Tide Range (ft) Highest
Observed (ft)
Port Orford, OR 7.3 11.5
Crescent City, CA 6.9 10.7
North Spit, Humboldt Bay, CA 6.9 9.7
Arena Cove, CA 5.9 8.6
Point Reyes, CA 5.8 8.5
San Francisco, CA 5.8 8.7
Ocean Beach, San Francisco, CA 5.9 8.7
Princeton, Half Moon Bay, CA 5.7 8.5
Ano Nuevo Island, CA 5.4 8.3
Monterey, CA 5.3 7.9
Port San Luis, CA 5.3 7.7
Santa Barbara,  CA 5.4 7.4
Rincon Island, CA 5.5 7.8
Santa Monica, CA 5.4 8.5
Los Angeles, CA 5.5 7.9
La Jolla, CA 5.3 7.7
San Diego, CA 5.7 8.1
*All tide heights reported relative to the MLLW tidal datum.

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 open coast shoreline.

Locally generated wind waves at Point Pinole in San Francisco Bay

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.

Winter storm waves batter the Pacifica shoreline (January 2011)

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. The El Nino cycle oscillates on approximately a 5-7 year timelime. El Ninos correspond to warmer than normal surface ocean temperatures and La Ninas correspond to cooler than normal surface ocean temperatures in the equatorial Pacific. The Oceanic Nino Index quantifies the strength of the El Nino/La Nina based on the magnitude of this temperature anomaly.

The El Nino-Southern Oscillation Index, which indicates the strength of El Nino (red) and La Nina (blue) events. Recent significant El Ninos include 1982-83, 1997-98, and 2009-2010.
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.

Damage at the Crescent City harbor after the March 2011 Japan tsunami (Photo: Nicole Metzger)
Numerical modeling predictions of tsunami wave height and arrival times across the Pacific for the Japan earthquake in 2011. Also of note is the focusing of wave energy in the vicinity of Crescent City in northern California. Source: NOAA Center for Tsunami Research, Pacific Marine Environmental Laboratory