Geo-Joint: Sandy Beaches Going with the Flow

A broad, sandy beach, this one near Barcelona, provides plenty of room for recreation, and affords a buffer to coastal erosion.

Who doesn’t love a beach? Some people find the sand to be a bother, and the sun and wind uncomfortable, but they have to be in the minority. How else to explain the huge popularity of a seaside vacation, or the high cost of living coastally due to it being the place everyone wants to move to? The generally milder weather is no doubt a big factor, but having easy access to a sandy shoreline is synonymous with the good life. So it’s generally agreed that those sandy strips are a great place to be, but how do they get there? You don’t see big deposits of sand randomly collecting all over the landscape, except in some desert areas. Obviously, sand is ground-up rock. Wave action at the beach does a pretty good job of wearing down rocks, but to get rocks to grain size, and lots of it, it takes the wearing down of mountains by fracturing and water transport. Long rivers roll and smash rock into ever-finer particles as they flow toward the sea. Very soft rock gets ground into silt, but the more resistant bits of quartz, feldspar, and other hard minerals hold up for the tens or hundreds of miles of tumbling transport from the highlands to the ocean. Some beach material comes from more local sources, such as cliffs along the sea, but the beaches of western North America are mostly fed by rivers.

The sandy beaches of California, fringing the Marine Bioregion on this map, do not line the whole length of the state, thanks to submarine canyons.

As sand is delivered to the sea at a rivermouth, it doesn’t just pile up there. The energetic ocean gets right to work further grinding it in waves and transporting it in what is called the longshore current. Along the Pacific Coast, waves generally come from a westerly or northwesterly direction, due to the swellmaking storms in Alaskan waters. As waves approach a beach, their angle causes one end to contact the shallows before the other, which slows a wave’s movement and forces it to bend to follow the contours of the coastline. As the wave breaks, it releases a surge of energy, which creates this longshore current. Water flowing up onto a beach picks up some sand, and as it returns to sea it arcs downshore, slowly moving the grains along the beach southward. Each swash of incoming water moves only a small amount of sand, but the waves are relentless, and over time they push many thousands of cubic yards of sand downcoast. Sand is also moving in another direction, over a longer time frame. The powerful waves of winter eat into the beach and carry its sand offshore to a depths as great as several tens of feet. There it sits until the calmer waves of the spring and summer push it forward and rebuild the sandy strip. Once back in the wave zone, it resumes its movement downcoast.

Sometimes the swash and backwash create a scalloped pattern along the beach.

Sand movement on an east-facing beach. Longshore current (3) develops from the energy carried by angled waves (4). Swash (5) and backwash (6) move sand along the beach face.

Under stable circumstances, there is a continuous flow of sand, but that sand doesn’t progress all the way down to the tip of South America. At random intervals, submarine canyons can be found, leading to the ocean depths. When sand moving down the coast encounters one of these channels, it flows downhill and into such deep water that it cannot return to land. Even without knowing the bottom contours, these submarine canyons can be inferred because a long line of sandy coastline suddenly changes to a rocky shoreline as one moves south. Moving south of the canyon, beaches will be more sparse or non-existent until other rivers are encountered, delivering their sediment load to the ocean.

The Santa Clara River in Southern California is a fairly dry bed of sand much of the year, but winter storms carry huge amounts of sediment to the beaches of Ventura.

As long as rivers feed the system, the coastline will have beaches in these “littoral cells” between submarine canyons. But there are things that can impede or even shut off the sandy deliveries. One is drought. If there is diminished rainfall, not only is there less erosion in the mountains, but also a less energetic conveyer belt for sediment to be conducted to the rivermouth. That’s a short-term problem for beaches, although California droughts can be multi-year affairs. Still, the real supply-cutters are dams. As river water flows into a reservoir behind a dam, it slows and drops its sediment load. This not only begins to fill the basin with material that decreases storage capacity; it also cuts off a beach’s lifeline for decades. In time, the reservoir will completely fill with sediment, and at that point, sand could be carried over the dam along with the spilling waters. Some no longer useful dams have been taken down, which creates an immediate boon for beach-building once the flow of the newly freed river can reach the sea. Dams are also coming down in order to re-establish an unobstructed path for fish to spawn upstream. With each instance of un-damming, beaches can get back to their natural state. But nearly all rivers and streams in California have some kind of dam, or multiple dams, along their length. The sandy volume we take as normal for our beaches is not the bounty that nature would have intended.

Owners of beachfront structures like these in Florida often call for seawalls or rock groins for protection. It’s a short-term and beach-destructive reaction as the sea moves shoreward.

The coastal palisades at Pacifica, California, are eaten away by the sea, providing beach material but making life precarious for blufftop residents.

Fortunately for the beaches of the continental U.S., the worst threat to a beach’s existence—hauling it away for industry—is a small and vanishing concern. The last coastal sand mine, near Monterey, California, will be closed by the end of 2020. (Surprisingly, there is still a beach sand mining operation on the island of Maui in Hawai’i, but it is hotly contested.) Beaches serve far too great a service as natural habitat, for scenic beauty, and for coastal protection, to sacrifice to the concrete industry. But in other parts of the world, the gouging of beaches continues apace. One might wonder why anyone would carve up a beach when countless tons of sand dunes are easily accessible in the deserts of the world. Deserts have their ecological balance to maintain just as beaches do, but it’s not out of concern for the environment that sand miners don’t head inland for hot, dry sand. The small rocks that bounce around on their trip down the river, grinding into smaller pieces, eventually reach a grain size where the water surrounding each particle acts as a shield against further collisions, thus retaining its texture. In the desert, sand is wind-blown, and the grains constantly collide, becoming far more rounded in shape. As it turns out, the more angular surface of river and beach sand offers a better gripping opportunity for cement, making for stronger concrete. Concrete made with desert sand does not hold up to the stresses required of it.

Another looming problem for sandy beaches is one we can’t legislate away, at least not directly. That is the inexorable rise in sea level. In completely natural locales, such rises will merely push the beach complex inland. Sand will march forward as the ocean pushes it farther, except where narrow beaches are backed by cliffs—those beaches may be drowned, depending upon the amount of rise. Clifftop development will then face more erosion. Where the coastline is flatter, development often sits right behind the sand. Homes, businesses, and roadways are in jeopardy. Cities and individuals will not gladly accept that their infrastructure is now on no-man’s land, and will call for the emplacement of bulky, unsightly, and disruptive seawalls to save their investments. These can be built—they often perform their task of property protection well for some time. But hard structures built on beaches ultimately lead to the increased erosion caused by deflected wave energy. Beaches in front of seawalls become thinner, and in the face of sea level rise, will disappear, creating nothing but a churning salty basin in front of the “saved” buildings. And given enough sea level rise, these places will become surrounded, or inundated. Trying to hold the sea back is a foolish short-term pursuit, and eventually impossible.

And so it is that we must either learn to live with the rising sea, or suffer the loss of a lot of the beaches that we so obviously cherish. Watching the inevitable clash of money and engineering against the immutable laws of nature and physics will tell us a lot about how humans will deal with the myriad other effects of a changing climate yet to come.


At this scale, it’s pretty hard to see the narrow strip of sand that borders much of California’s coast, which is a testament to its rare and fragile nature. But this satellite wall map shows off the whole state in its many landforms and would make a great addition to anyone’s home. Made by and available from Maps.com.

 

CLICK HERE TO VIEW THE MAP!


PHOTO CREDITS:

caption: The sandy beaches of California, fringing the Marine Bioregion on this map, do not line the whole length of the state, thanks to submarine canyons.
source: Flickr: California Department of Fish and Wildlife (CC by 2.0)

caption: A broad, sandy beach, this one near Barcelona, provides plenty of room for recreation, and affords a buffer to coastal erosion.
source: PublicDomainPictures.net: Unknown (CC 0)

caption: Sand movement on an east-facing beach. Longshore current (3) develops from the energy carried by angled waves (4). Swash (5) and backwash (6) move sand along the beach face.
source: Wikimedia Commons: USGS, modified by Eurico Zimbres (Public domain)

caption: Sometimes the swash and backwash create a scalloped pattern along the beach.
source: Wikimedia Commons: Boomer Depp (CC by 2.0)

caption: The Santa Clara River in Southern California is a fairly dry bed of sand much of the year, but winter storms carry huge amounts of sediment to the beaches of Ventura.
source: Flickr: brewbooks (CC by SA 2.0 Generic)

caption: Owners of beachfront structures like these in Florida often call for seawalls or rock groins for protection. It’s a short-term and beach-destructive reaction as the sea moves shoreward.
source: PublicDomainPIctures.net: Unknown (CC 0)

caption: The coastal palisades at Pacifica, California, are eaten away by the sea, providing beach material but making life precarious for blufftop residents.
source: Wikimedia Commons: Brocken Inaglory (CC by SA 3.0 Unported)

 

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