16.1: Coastal Processes

Changing Shores

The first thing to understand about shoreline processes is that the coastlines move. Not only the water, but also the sand and rock fragments as they are carried along the shoreline; not only the wind, but also the California cliffs themselves, pushed upward out of the cold sea by tectonic motion and uncountable earthquakes. In an emergent shoreline, the land is pushed up faster than it can be brought down by the erosive force of crashing waves. Although not every part of the coast is rising at the same rate, overall California’s emergent shorelines put bare rock directly into waves, with pockets of sand in between, which is a very different situation than the beaches elsewhere in the country, such as the sandy arc of the Gulf of Mexico and the long barrier islands along the East Coast.

How fast is the coast rising? Estimates for the San Diego region for the last million years put the rate at 14 cm/1000 years, with an acceleration in the last 80 ka to 30 cm/1000 years, with one area as much as 45 cm/1000 years. Other areas of the state experience different rates. Because of this tectonic uplift, California’s coast is raw and rocky (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): Dynamic, uplifting, highly-eroded, rocky California coastline, north of Bodega, CA. Note Aspen the dog for scale. "Beach" by Steven Newton is licensed under CC BY-NC 4.0. Access a detailed description.

Query \(\PageIndex{1}\)

Ocean Waves

Waves are a powerful erosive force that incessantly attacks coastlines. The time period between waves is called the wave period. The wave period is measured in seconds, and varies depending on the source of the waves, but is usually within the range of 6 to 16 seconds. If we take as an example a period of one wave every 10 seconds, then a coastline is hit with 6 waves every minute, every day 8,640 waves, and every year 3 million waves. That is an immense, repeated force applied to rocks facing the ocean (see Figure \(\PageIndex{2}\)). The fact that rocky shorelines exist in California at all is a testament to the geologic speed of tectonic uplift relative to the speed of erosion.

Figure \(\PageIndex{2}\): Energetic waves pummeling a rocky coast, Point Lobos, CA. "Waves" by Steven Newton is licensed under CC BY-NC 4.0. Access a detailed description.

Almost all waves are generated by wind, though not in the way most people assume. Wind does not push waves along; rather, the laminar flow of wind over a slightly curved surface creates areas of low pressure (see Figure \(\PageIndex{3}\)). Ocean waves form when low pressure areas lift the ocean upward by the pressure differential, in the same way a curved airplane wing experiencing a fast laminar flow of air creates the lift that allows airplanes to fly. This lifting effect is also what drives sailboats forward, their curved sails creating the lower pressure in front of them that pulls sailboats forward nearly 45^o into the direction of the wind.

Figure \(\PageIndex{3}\): Diagram of the phenomenon of lift illustrated in three circumstances: ocean waves, an airplane wing allowing a plane to fly, a sailboat. "Phenomenon of lift" by Steven Newton is licensed under CC BY-NC 4.0. Access a detailed description.

Wind-created waves can form far offshore, in storms across oceans, producing ocean swells that rapidly approach coastlines. But once these waves reach shallow water, something strange begins to happen: In the deep ocean, waves are relatively fast and flat. As they approach coasts, they slow down and rise up (see Figure \(\PageIndex{4}\)). This happens because in shallow water the wave base, or the bottom of the wave where the wave energy dissipates, begins to interact with the ocean floor creating drag; the term of art is that waves “feel bottom.”

The point at which waves feel bottom is when the water depth decreases to one half of the wavelength of the wave. The wavelength is the distance between two wave crests. If, for example, waves just offshore were 100 meters apart, then one half of this wavelength would be 50 meters. When such incoming waves reach a depth of 50 meters, the waves begin to change.

Figure \(\PageIndex{4}\): Diagram showing wavelength, amplitude, and the phenomenon of “feeling bottom.” "Wavelength and amplitude" by Steven Newton is licensed under CC BY-NC 4.0. Access a detailed description.

One change is that the wavelength decreases. The wave velocity also slows. The height of the wave, also called the amplitude, rises up. And when we reach the critical depth of just 1/7th of the original wavelength, the wave crests and spills over in the familiar breakers that beach visitors see and that surfers seek out.

Next time you’re at the beach, look far out to sea. You’ll see mild swells, pulses of water coming in fast. Then as they near the shore, you’ll see distinct waves, slower than they initially were, start to rise up.

Incoming waves rarely approach the shoreline at exactly 90^o, but rather approach it at some smaller angle. As waves approach the coast and begin to feel bottom, the edge of the wave closest to shore, which is in the shallowest water, slows down. The line of the wave warps. As a consequence of this warping, the angle of wave approach crashes into beaches diagonally, thrusting masses of water up onto the sand and rock (called the swash), whereupon this water recedes back into the sea (called the backwash). The motion creates a kind of zig-zag, with waves pushing up, and retreating waters pulling back.

This zig-zag results in water moving parallel along the coast, a phenomenon termed longshore current (see Figure \(\PageIndex{5}\)). The longshore current sets sediment in motion, and the movement of sediment by a longshore current is termed longshore drift (sometimes also called littoral drift). Both the longshore current and the longshore drift tend to move parallel to the beach.

Figure \(\PageIndex{5}\): Diagram showing longshore current moving parallel to the coastline. "Longshore current" by Steven Newton is licensed under CC BY-NC 4.0. Access a detailed description.

But what if the shoreline is not a straight line? What if some part of the shore juts out away from the rest of the coast, a promontory that meets the incoming waves first?

In such instances, the waves warp around the promontory. A headland, which is the proper name for a promontory, meets the waves before the normal coastline, and hence the waves slow down around the headland. The waves bend. This phenomenon is called wave refraction (see Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): A diagram showing a headland causing wave refraction, with the waves warping around the headland and increasing erosion along the headland sides. "Diagram of Headland and Wave Refraction" by Steven Newton is licensed under CC-BY-NC 4.0. Access a detailed description.

Wave refraction changes where waves focus their erosive energy; the waves concentrate on the sides of headlands. The sides of a headland therefore erode faster than other areas, such as beaches, or even the tip of the headland. This means that the headland will, in time, be cut off from the shore.

Wave refraction accounts for many of the shoreline features–bays, headland, arches, sea stacks–on display all along the California coast. It is no exaggeration to say that the major reason our California coast looks the way it does is because of the phenomenon of wave refraction.

Query \(\PageIndex{2}\)

Larger Forces Influencing Waves

Another major process along California coasts is less visible. As winds kick up waves, as waves move water, all of these movements are affected by the rotation of the Earth. From the perspective of looking from space at the north pole, Earth rotates in a counter-clockwise fashion. This rotation means that winds and water flows are not moving on a stable platform, but rather on a shifting base. A good analogy would be trying to walk a straight line on a merry-go-round; no matter how one tries, one’s straight path would appear curved because the merry-go-round itself is rotating.

This curving is called the Coriolis effect. Wind seems to be blowing in a straight line, directly from areas of high pressure to areas of low pressure. But in fact the wind bends because of the Coriolis effect, which is a result of the rotation of the Earth.

For the purposes of this book, we do not need to go into this somewhat confusing phenomenon further than to say that in the Northern Hemisphere, the Coriolis effect creates deflections to the right. Oceanography textbooks often make an odd analogy of a pirate’s cannon firing a cannon ball, so following this tradition, if you were a pirate (and really, who doesn’t at some point envy the corsair lifestyle?) and you fired a cannon ball anywhere in the vicinity of California, its path would deflect to the right. It doesn’t matter which direction the cannon is pointing.

Here’s where this fits with our story: oceanic wind directions off the coast of California tend to move from the northwest to the southeast, roughly following the edge of California from north to south. As a consequence of the Coriolis effect, and a by-product called Ekman transport, surface water tends to pull away from the California coast. And as nature abhors a vacuum, colder, deeper, nutrient-rich water rushes in to replace the displaced surface water. This phenomenon is called upwelling (Figure \(\PageIndex{7}\) and Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{7}\): A diagram showing wind and the Coriolis effect creating the phenomenon of upwelling along the California coast, allowing deeper, colder, nutrient-rich water to rise to the surface. "Diagram of Upwelling" by Steven Newton is licensed under CC-BY-NC 4.0. Access a detailed description.

Figure \(\PageIndex{8}\): A diagram of typical sea surface temperatures along the California coast, showing the strong cooling effect of coastal upwelling. "Diagram of Sea Surface Temperature from NOAA data" by Steven Newton is licensed under CC-BY-NC 4.0. Access a detailed description.

Upwelling brings to the surface cold, deep water teeming with nutrients, which are vital to the ecosystem off California’s coastline. These nutrients feed phytoplankton, who then feed zooplankton, who then feed little fish, then bass and salmon, then seals and sea lions, then white sharks. This complex trophic chain hinges upon upwelling and the nutrients upwelling delivers.

Small changes to upwelling can produce big effects on California marine life. Climate change is thought to influence the amount of upwelling. ENSO warm events (El Niño) reduce the amount of upwelling. In recent years, deficits of upwelling combined with inland drought have prompted regulatory authorities to curtail ocean fishing; in 2023, the entire commercial and recreational salmon season was canceled.

Swimmers in Santa Monica generally do not require wetsuits, but if one enters the water north of Santa Barbara without neoprene protection, the water is so cold that hypothermia can set in within minutes. The reason so much of California’s coastal waters is so cold, and why surfers, small craft sailors, and swimmers require wetsuits, is directly because of the phenomenon of frigid, upwelling waters.

Query \(\PageIndex{3}\)

References

1. Abbott, P. L. (1999). The Rise and Fall of San Diego (p. 195). Sunbelt Publications.
2. Einhorn, C. (2023, April 3). California Salmon Stocks Are Crashing. A Fishing Ban Looks Certain. The New York Times. https://www.nytimes.com/2023/04/03/c...alifornia.html
3. Jacox, M.G., J Fiechter, A.M. Moore, and C.A. Edwards (2015). ENSO and the California Current coastal upwelling response. Journal of Geophysical Research: Oceans, 120, doi:10.1002/2014JCo10650
4. National Oceanographic and Atmospheric Administration (2023, January 5). Oceanography of the Northern California Current Study Area. NOAA.gov. Retrieved October 5, 2023, from https://www.fisheries.noaa.gov/west-...ent-study-area