16.8: Detailed Figure Descriptions

16.1  The endemic California surfer

A wave being plowed by a surfboard, capturing the phenomenon of surface tension in water droplets. This image is decorative in nature, but displays some of the beauty surfers experience.

16.1.1  Dynamic, uplifting, highly-eroded, rocky California coastline

A rocky, highly-eroded beach just north of Bodega, CA, with a large Golden-Bernese dog named Aspen for scale. Clear, foamy water rushes up on a sandy beach, hitting jagged, sculpted rocks, to emphasize how water and waves erode and sculpt coastlines, and how unless those coastline are actively uplifting through tectonics, the coastlines rapidly are flattened. California's coasts tend to have exposed rocky faces and mountains coming right into the surf, while those on the east coast tend to be much flatter. What this emphasizes is the difference with California's coastline being pushed out of the waves by tectonics faster than wave erosion can erode the coast down.

16.1.2 Energetic waves pummeling a rocky coast

Waves crashing on a rocky coastline at Point Lobos, CA. This picture emphasizes the great energy involved with these masses of water striking the rocks 24/7.

16.1.3 The phenomenon of lift illustrated in three circumstances

Three related processes: wave creation in the ocean, the generation of lift on an airplane wing, and the pulling force which moves sailboats forward. The commonality between the three is emphasizes with dark arrows showing the direction of movement generated by hashed areas identifying zones of lower pressure. For waves in the ocean, the hashed area on the face of a swell indicates a zone of uplift created when a smooth, laminar air flow moves over a curved ocean surface. Likewise, in an airplane wing, lift is dependent on laminar air flow moving a greater distance over the top of a wing than below it, creating a difference in pressure that allows airplanes to fly. The hashed area, indicating low pressure and uplift, appears on the back edge of the wing. For sailboats, the combination of curved sails (in the case here, a sloop configuration) creates zones of low pressure on the bow-side of sails, as indicated by the hashed areas. This allows boats to be pulled 45 degrees into the direction of the wind, rather than only being able to sail downwind.

16.1.4 Wavelength, amplitude, and the phenomenon of “feeling bottom.”

A wave cross-section, showing to normal, deep-water wavelength between two crests, and the corresponding wave base at one half of the wavelength. When this wave base intersects with the ocean bottom, we say the wave "feels bottom," and begins to change in the following ways: the waves slow, the amplitude increases, the wavelength shortens, and at 1/7 of the original wavelength, we get a crashing wave. The crashing, spilling waves at the most common type of wave that beach goers are accustomed to seeing, but they actually represent a small fraction of ocean waves because they occur only in water very shallow relative to wavelength.

16.1.5 Longshore current moving parallel to the coastline

Longshore current, showing wave swash and backwash going up the beach, then retreating from the beach, in a zig-zag fashion. The longshore current is the net sum of the movement of the smaller swash/backwash.

16.1.6 A headland causing wave refraction

A headland creating wave refraction, where the waves are initially straight and parallel to each other far from the shore. However, as they approach the shore, the headland creates an area of shallow water relative to the adjacent areas, meaning that the wave base "feels bottom" there first, and consequently slows down. This bends and warps the wave shapes toward the sides of the headland, where there is increased erosion relative to the adjacent areas. The erosional areas are perpendicular to the direction of the waves, and as the wave form warps around the headland, that perpendicular shape makes the erosion seem to move toward the sides of the headland.

16.1.7  Wind and the Coriolis effect

Upwelling occurs where the combination of wind movement parallel to the coast (shown by the large red arrow) over the ocean and the Coriolis effect create under some conditions a "pulling away" from the coastline, moving parallel to the coast and then moving away from it (shown by the curved arrows on the ocean surface) of the upper regions of the sea. This creates a zone along the coast where deeper, colder, nutrient-rich water can rise to the surface, as indicated by the direction of arrows in the cross-section, showing movement pushing toward the coastline, then circling around and moving back to sea.

16.1.8 Typical sea surface temperatures along the California coast

A simple map of the California coast, showing the effects of the phenomenon of upwelling as it produces cold sea surface temperature regions (10^oC) near the Central and Northern California coastline, with a gradation of warmer surface waters (15^oC, 20^oC) further from land, and also in areas of Southern California waters where upwelling does not occur.

16.2.1 Typical coastal features resulting from differential erosion

Coastal features created by differential erosion. Shown here are a sea stack, a sea arch, a sea cave, and a headland surrounded by beaches. Each of these features is created by wave erosion. Because headlands jut out away from the coast, they are subject to greater erosion, as explained in the previous section, and this erosion progresses first aa sea caves, which then form to meet as a sea arch. When enough erosion has occurred that the arch collapses, then a sea stack is created.

16.2.2 A well-developed sea cave

A sea cave in a cliff just north of Santa Cruz, CA. The deep indentation in the cliff was created by wave action.

16.2.3  A sea arch south of Jenner, CA

A sea arch at Jenner, CA, which has formed from a former headland being eroded in such a way that sea caves on either side meet and create a hole in the rock. This sea arch is less stable and will collapse in time, forming an island called a sea stack.

16.2.4 Distant sea stacks, remnants of a former headland

The coastline at Jenner, CA, showing numerous sea stacks both on the beach and in the water. In time these sea stacks will also be eroded into nothingness, as everything along coastlines should be considered as temporary and ephemeral as the latest meme.

16.2.5 A tombolo created from a former sea stack

Another view of the coastline at Jenner, CA, showing a rugged coastline with numerous sea stacks and a tombolo. This is a former sea stack that has created a calm area on the coastal side, allowing accumulation of drifting sand. In this case is so pronounced that it actually reconnects the former sea stack to land, forming a tombolo.

16.2.6 Three marine terraces

The phenomenon of marine terraces. The area under water is currently flattening the land through wave action, but if tectonics lifts it up out of the water, this flat area then forms a terrace. There are three dry terraces to the left of the active erosional zone, indicating long periods of tectonics uplift interrupting wave erosion.

16.2.7 A marine terrace with “fossil” sea stacks

"Fossil" sea stacks on a flat marine terrace. The flat land is pocked with these large rocky outcrops, which represent former sea stacks that have been lifted out of the ocean by tectonic movement. Of note is the fact that the San Andreas resides shortly out of frame to the left; because of this proximity, quake here may be quite violent and capable of several meters of uplift in one major quake. These rocks now form a popular bouldering site. Also of note are smooth rubbings 4-5 meters off the ground; the prevailing theory is that Columbian mammoths (Mammuthus columbi) used these rocks to groom their long tusks, and it is not a stretch of the imagination to see this scene twelve thousand years ago as these giant proboscideans roamed this marine terrace in family groups, waiting their turn for clean their tusks. (Wooly mammoths may be more familiar to readers, but they did not live at these latitudes.)

16.3.1 Aerial photograph of a green dye exposing a rip current

Rip current on a beach exposed by releasing dye into the ocean. The dye is drawn out in the rip current perpendicular to the shore, in tight flow; this illustrates how narrow rip currents are, and emphasizes that the best way to escape them is to swim parallel to the beach a few tens of meters. Once out of the rip current, swimmers may easily return to shore, though it is impossible to swim directly against the fast flow of a rip current.

16.3.2  Comparison of two topographic maps of Esplanade Avenue, Pacifica

Two topographic maps showing the same area of Esplanade Avenue, Pacifica, CA, as it developed from 1950 to 1980. In that thirty year period, a huge development was built very closet to the Pacifica cliffs, which are highly unstable. Many of these structures have since been condemned and removed.

Left picture: Detail of the 1950 South San Francisco USGS 7.5 minute topographic map. Note that there is no development on the ocean side of the Edgemar neighborhood. Also of note: a sea stack known as Mussel Rock lies at the ocean entry point of the San Andreas fault. In 1950 this area was not developed.

Right picture: Detail of 1980 South San Francisco USGS 7.5 minute topographic map. Note that there is now extensive development along the coast side of the Edgemar neighborhood. In addition, a Daly City neighborhood has been developed near Mussel Rock.

Also of note the position of the San Andreas fault, which cuts across this heavily populated area. All of these postwar developments were put in without regard to either the hazard of eroding cliffs or the seismic violence of the future.

16.3.3  Dubious attempts to “armor” the coastline against wave erosion

Esplanade Ave, Pacifica, detailing the construction mentioned in 16.3.2. In the surf zone is the placement of numerous large boulders ("rip rap") which are intended to absorb wave energy and slow cliff erosion. However, given the nature of these cliffs as unconsolidated sand, such efforts at armoring the coast may only delay the inevitable wave erosion.

16.3.4 A closer look at attempts to “armor” the coastline

The beach below the armoring attempts from 16.3.3. Note the large chunks of concrete that have fallen down the cliff, suggesting the cliff is not successfully stabilized by this artificial hardware. Also of note are the layers of sand in the cliff; if one walks up to these layers, the sand comes apart easily in one's hand. There is no bedrock here; this is all perched on a pillar of loose sand.

16.3.5 Coastal erosion threatening an apartment complex

The cliff-side edge of a large apartment building on Esplanade Ave, Pacifica, now demolished and removed. When this building was constructed, there was a considerable amount of land, but the high rates of erosion have pulled the cliff toward the building with alarming speed. When this photograph was taken, residents were protected from the cliff edge by a wooden fence and netting. Only a few yard separates the building from the sea.

16.3.6 Cliff erosion resulted in the dreaded “red tag”

Official notices affixed to a residential door indicating that occupants may not longer legally live in this residence. One notice reads, "This building is deemed unsafe for human occupancy." The other emphasizes the danger: "Danger! This building is deemed unsafe for human occupancy. It is unlawful for any person to occupy or reside in this building." It gives the address as 329 Esplanade, number 46, and is dated July 13, 2015. Both notices were issued by the City of Pacifica Building Inspection Department.

16.3.7 A photo taken just south of Centerville Beach in 2023

A collapsed section of beach along the Lost Coast at Centerville Beach. This photograph shows a sign, at the headscarp of a recent landslide, warning hikers, "Hazardous Cliff, Stay Away." Now that the sign has nearly fallen that seems to be a bit redundant.

16.4.1 A seawall protecting the house behind the wall

The effects of a sea wall. Incoming waves are reflected by the sea wall with almost full energy, meaning that the waves carry away beach sand (the particles moving away from the wall). In the best case scenario, this eliminates the sand that made the beach enjoyable, and in the worst case, undermines the sea wall itself, causing possible collapse and endangerment of the house behind the sea wall.

16.4.2 Before and after human alteration to the shoreline

The unaltered example shows a waterway entering the ocean, with a smooth coastline and a longshore current parallel to the coast.

After jetties and groins were built the coastline, the pre-existing longshore current began eroding/depositing sand and altering the shoreline. This current is shown moving from left to right in the diagram.The waterway entering the ocean is now protected by rocky jetties that push out into the water. Water is pulled offshore as the current pushes against the first jetty, depositing more sand. Just beyond the second jetty the sand erodes away as water pushes onshore. Similar forces are at play where groins are built out into the water. 

16.4.3  Before and after the installation of a breakwater

The unaltered example shows a smooth coastline with a longshore current parallel to the coast.

installation of a breakwater creates sand accumulation behind the breakwater. This alters the shoreline so that the sand eventually reaches the breakwater. In addition, water moves toward the coast on either side of the breakwater, eroding away the sand in those areas.