16.3: California Coastal Hazards
- Page ID
- 21576
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)California Coastal Hazards
Many people mistakenly think that a prime hazard in California coastal waters is shark attacks, so let’s take a bite out of this misconception.
The waters of northern California are indeed a major feeding ground for white sharks (Carcharodon carcharias), who enjoy tasty pinnipeds well-fed from the food chain created by upwelling. But the real incidence of shark attacks is much lower than most people realize. According to the International Shark Attack file, in 2022 only five human fatalities occurred worldwide. Only one person died of a shark attack in the entire United States in all of 2022. In the same time period, hippopotamuses are estimated to have killed 500 people worldwide, or one-hundred times the number of fatal shark attacks. So if you must insist upon fearing a wild animal, judge the real dangers and be afraid of hippos.
If you still find yourself skeptical of this information, check out the Instagram account of @scott_fairchild, which hosts scores of drone videos of white sharks approaching unaware California swimmers, surfers, and paddle boarders, getting within a few meters of the humans, and then leaving them alone. Sharks are ubiquitous in California waters, but they are not a major hazard.
Rip Currents
Perhaps the greatest hazard along California coasts, responsible for about 80% of beach rescues, is the phenomenon of rip currents. (They are sometimes mistakenly confused with “rip tides,” but the currents have nothing to do with tidal fluxes.) Rip currents form as temporary, fast-moving, narrow streams of water that move perpendicular to the coast, out to sea. They can quickly drag swimmers away from shore and induce panic in the swimmers, who struggle in vain to swim back to shore directly. This panic, and the potential for leg cramps, can lead to drowning.
Rip currents can form on any beach, but they are particularly likely during periods of vigorous waves, in areas where an underwater sand bar may have a breach that allows water to exit in one spot with speed. There are often only subtle signs, from the perspective of a swimmer, that rip currents have formed at a beach.
A typical scenario for a rip current situation is this: a person wades into shallow water, into an area where a rip current has formed. While the person is standing on sand, all is fine. However, one big wave and suddenly the person is in water too deep to stand. Then the person discovers that the water is surging away from the beach, away from shore, into ever deeper water. A normal instinct is to swim against this flow, directly back to the beach; after all, dry land is right there.
Rip currents are capable of flows exceeding 2.4 meters/second. Michael Phelps is the most decorated Olympic swimmer of all time, winning 28 medals. Phelp’s record for the 100 meter is 47.57 seconds, a rate of 2.1 meters/second. So even the fastest swimmers in the world can’t make progress against a rip current, and such bursts cannot be long sustained, especially for non-Olympic class swimmers. Rip currents simply flow faster than any person can swim, and unlike swimmers, rip currents don’t get tired.
A person in a rip current might look up and gauge that despite vigorous swimming, no progress had been made; in fact, the shoreline is now further away. This is where panic sets in. The swimmer may try to swim full blast. And here tragedy begins; leg muscles under such use easily cramp. Experiencing a hamstring cramp (a “charlie horse”) causes intense pain, akin to a bone break, disabling not only efforts to swim, but even to tread water.
There is a simple solution, and encouragingly, more and more public beaches now display signs informing swimmers of what to do when caught in a rip current. The simple solution is to avoid swimming directly against the flow, but rather to swim parallel to the beach. Rip currents are narrow, a few tens of meters in width. A short swim sideways and one is out of the rip current, and then able to return normally to the dry beach.
Watch the following video to learn more about the dangers of rip currents and how to spot them.
Tsunamis
Not every shoreline hazard is so easily avoided. Any beach exposed to the unrestricted ocean faces the risk of tsunamis. Tsunamis are waves generated not by wind, but by geologic forces, such as earthquakes, submarine landslides, or meteorite impacts. Tsunamis are orders of magnitude larger than wind-generated waves, and can inundate coastal areas with devastating and deadly effects.
In 2004, the Boxing Day earthquake near Indonesia launched a tsunami estimated to have killed a quarter of a million people on both sides of the Indian Ocean. In 2011, the Tōhoku tsunami in Japan killed about twenty thousand people, despite a robust warning system and extensive defensive preparations.
Tsunami casualties could occur along the California coast, even if the source of the waves was on the other side of the Pacific Ocean. We know this because of an earthquake and tsunami in the year 1700. The giant quake occurred on the Cascadia subduction zone and launched a tsunami across the Pacific, where it hit eastern Japan in what became known as the “orphan tsunami.” This tsunami was an “orphan” because no obvious “parent” quake was felt in Japan prior to the tsunami’s arrival. We do not know if the 1700 event created similar tsunamis in the Pacific Northwest or California because there are no written records from that time.
But we do know of other tsunamis that have affected the California coast. The 1964 Alaskan quake spawned a tsunami that killed 12 people in Crescent City, California. Crescent City was also affected by the 2011 Tōhoku tsunami, killing one and sinking over a dozen boats in harbor.
How future tsunamis will affect the California coast is hard to predict. A major quake far away can send waves, moving at the speed of a jet, across the Pacific. It is also possible that more local fault activity can generate waves, with less warning.
Warnings about the approaching 2011 tsunami provided enough time for panicked, fleeing residents in coastal areas, such as Watsonville and Santa Cruz, to clog roads with so much traffic that roads ground to “a standstill." This does not bode well for future evacuations.
Coastal Erosion
No one drowns from coastal erosion, but people do lose their homes.
The major problem with coastal erosion is that homes have simply been built too close to the shoreline. Often homes are built on cliffs of weak rock, or even mounds of compacted sand. The metastasizing, haphazard building spree of mid-20th century California held little concern for whether or not it was geologically safe to build in unstable areas. Now California faces the consequences of these reckless development decisions.
One case study is the coastal town of Pacifica, just south of San Francisco. The bluffs underlying the northern part of Pacifica are not part of a formal geologic formation, but are described by the USGS as sand sometimes “firm in a few places.” That might be too generous.
The problem of the development of Pacifica can be illustrated by comparing two USGS topographic maps (Figure \(\PageIndex{2}\)). In the first map, from 1950, there is no development on the ocean side of Edgemar, as noted by the 100 foot contour interval. Nineteen-fifty was on the cusp of a postwar building boom and soon everything would change. Everything–except the sandy, weak cliffs rapidly returning to the sea. This second map is from 1980 and shows extensive development on the ocean side of Edgemar. There are several rectangular blocks right at the edge along a road called Esplanade Ave. These pink blocks indicate large apartment buildings.
Of note in both maps is the presence of Mussel Rock, a granitic Salinian Block remnant pushed out away from the coast by right-lateral strike-slip motion along the San Andreas fault. Even in the 1950 map, the location of the San Andreas and its 1906 trace of surface disruption were well-known. Note the presence in the 1980 map of a new public elementary school, the Franklin D. Roosevelt School, built virtually on top of the San Andreas fault trace. That this area was developed at all shows disregard for the obvious geologic hazards, and irresponsibility by public officials that can only be characterized as reckless disregard for human life.
Slightly to the south of the fault, along Esplanade Ave, a dozen single-family homes were built in 1949, near the edge, about 30 meters away, of the steep sandy cliff. Residents could hear the soothing crashing of waves 70 feet below. It was truly a beautiful place to live, with sunsets over the Pacific Ocean, whale watching from one’s living room, and calming views of sailboats plowing the waves. It was the perfect neighborhood–except for where it was built.
This stretch of homes was condemned following the El Niño storms of 1997/1998, which dramatically moved the cliff face back. In some cases, the erosion was so severe that the foundations of these Esplanade Ave. homes were exposed as they projected into open air. The southernmost ten homes were demolished immediately, while two others lingered, uninhabited, until the 2010s.
This loss of homes was hardly a surprise. As Snell et al. 2000 estimated, this area has experienced between 0.5 - 0.6 meters/year of cliff retreat over the last 146 years. So the initial ~30 meter buffer was never expected to last longer than about 60 years. It lasted 49 years.
Slightly northward of the location of these lost homes, a series of large apartment buildings were constructed, on the same weak cliff material, the base of which would be reached on a regular basis by waves. Efforts to delay the impending collapse of the sandy cliffs are shown in Figure \(\PageIndex{3}\).
Just north of the buildings shown above, a series of large apartment buildings were constructed in 1962. These are the five pink rectangles visible in the 1980 topographic map (Figure \(\PageIndex{2}\)). Initially, there were tens of meters of flat ground on the ocean side of these apartments; however, the 20 meter sand cliff between them and the ocean is of such weak material that one can easily carve into the sand with one’s finger. It was just a matter of time and physics before wave erosion pulled back the cliff to the point where it endangered these buildings and the residents within, as illustrated in Figure \(\PageIndex{5}\).
The City of Pacifica began "red-tagging" (forcing evictions) and demolishing these buildings in 2016-2017 after the owner declined to pay for their removal (seeFigure \(\PageIndex{6}\)). This demolition displaced a large number of low-income residents in Pacifica, many of whom lived there using Section 8 housing vouchers, which are frequently not accepted by other Bay Area landlords, leaving few local options for former residents. However, the red-tagging and the removal of these buildings was manifestly necessary given the geologic dangers.
In 2022, a study by the Coastal Process Group at UC San Diego Scripps Institute of Oceanography analyzed rates of coastal cliff erosion along 866 kilometers (538 miles) of California Coastline. Their findings can be seen in the California Coastal Cliff Erosion Viewer. If you zoom in on Pacifica, you will see that indeed rates of cliff retreat in the area surrounding Pacifica are high; some 1 km sections had average erosion rates as high as 1.2 meters per year during an interval in the 2010s, but Pacifica certainly isn’t the only stretch of coastline with erosion rates this high. In fact, when you first open this map, zoomed out to the whole state, a different region pops out. At this scale, the rates shown are averaged over 50 km sections and one section near Cape Mendocino appears to eclipse the rest of California. The average erosion rate for the whole 50 km section is 0.85 meters per year, with some individual 1 km sections reaching rates of nearly one and half meters per year.
This remote region of the California Coast is sometimes called “the Lost Coast.” Much of the “rock” (if it can even be called that) in this region is part of the “Wildcat Formation,” a geologic unit composed of little more than mud. Earlier in this chapter, you learned that sea cliffs result when tectonic uplift outpaces erosion. Even the strongest, most erosion resistant bedrock will eventually be leveled to a beach if there is no driving force to lift it upwards. Conversely, where bedrock is soft, unstable, and easily eroded, tectonic uplift must be extremely rapid to keep pace and maintain a cliff. Perhaps it is no surprise then that the 50 kilometers of coastline with the highest average erosion rate in the state aligns precisely with the Mendocino Triple Junction, an area of significant tectonic activity.
Clifftop erosion along the Lost Coast doesn’t make news for destroying homes the way it does in Pacifica; that’s because there aren’t any homes there to be destroyed, of course. The combination of uplift and erosion makes maintaining even roads in this area a significant challenge, let alone towns or cities. It is the remote nature of this region that makes it a popular destination for backpackers and cyclists.
References
- Bay Area News Group (2011, March 11). Live blog: Tsunami waves hit Bay Area. The Mercury News. https://www.mercurynews.com/2011/03/11/live-blog-tsunami-waves-hit-bay-area/
- International Shark Attack File (n.d.). The ISAF 2022 shark attack report. Florida Museum. Retrieved October 7, 2023, from https://www.floridamuseum.ufl.edu/shark-attacks/yearly-worldwide-summary/
- J. McKinley. (2011, March 16). Sleepy California Town, and a Tsunami Magnet. The New York Times. https://www.nytimes.com/2011/03/17/us/17crescent.html
- National Oceanographic and Atmospheric Administration (2023, July 28). Rip Currents. NOAA. Retrieved October 7, 2023, from https://www.noaa.gov/jetstream/ocean/rip-currents
- Snell, C. B., Lajoie, K. R., & Medley, E. W. (2000). "Sea Cliff Erosion at Pacifica, Ca, Caused by 1997/1998 El Niño Storms, in Proceedings of sessions of Geo-Denver 2000 - Slope stability 2000 (pp. 294-308). https://doi.org/10.1061/40512(289)22