While I'm sure most of you have a pretty good idea of what happens on a beach, I suspect there are many subtle features of which you were not aware. Have you ever wondered where all that sand came from? Have you ever wondered why some beaches turn to rock in the winter? Have you ever heard of a beach on the coast of Maryland where wild horses roam? The answers to these questions and more will be revealed today as we hit the beach!
Anatomy of a Beach
Have you ever been down to a beach in the winter and found that all the sand is gone? Has your favorite place in the sand ever turned to cobble? If so, then you have witnessed the not-so-mysterious seasonal cycle of the shifting sands along the shoreline. These seasonal changes in the structure of the beach are primarily the result of how sand is suspended and transported through the actions of the waves and currents. Before we examine the seasonal changes of beaches, let's make sure we are familiar with the different parts of a beach.
Most beaches (except those associated with lakes) occur along the coasts of landmasses (including continents and islands). For the purposes of our study, we will define the coast as the region of a land mass influenced by the presence of the oceans. In other words, those areas of land that are subject to the daily cycle of on-shore and off-shore winds can be considered as coastal regions. Your book defines the coast as a "strip of land of indefinite width that extends from the shore inland to the first major change in terrain that is unaffected by marine processes." Both definitions are sufficiently vague as to just what the coast really is, but we'll leave that question to the philosophers and politicians.
The strip of "land" between the permanent vegetation and the line of the lowest of the low tides is called the beach. The "land" of a beach typically consists of sand, but may also include pebbles, cobble, boulders, logs and chips of wood, or just plain rock. Sandy beaches also come in many different colors and textures, depending on the materials of which they consist. Some beaches in Hawaii are black, being composed of the ground-up particles of black lava rock. Along our coast, the yellow sand consists of the ground-up minerals of quartz and feldspar that we find in the local mountains, which are the source of the sand along our beaches. In Florida, the beaches are made up of bits and fragments of tiny shells. In other regions of the world, beaches may be green or even pink, from minerals or corals, respectively, that get fragmented and deposited there.
The beach is commonly divided into two parts: the backshore and the foreshore. The backshore starts where waves carve out the beach and extends to the uppermost part of the beach where debris is thrown during high waves. Your book considers this area the dry region of the beach that is submerged only during the highest tides and the severest storms. The foreshore is the region where waves wash onshore and extends oceanwards to the low tide terrace, a gently sloping region which causes waves to break. Keep in mind that this division of the beach only serves to define a moving target; that is, the slope of the beach as it is carved and reworked by the seasonal actions of waves.
The region beyond the foreshore is known as the offshore. It typically extends from the low-tide terrace outwards to where the action of the waves no longer affect the bottom. In principle, offshore extends all the way to the edge of the continental shelf, the region that defines the coastal ocean.
One prominent beach feature that results from wave action is the beach scarp. This is an area at the top (landward side) of the foreshore that results from the cutting action of waves. Probably all of us have seen a kind of miniature "cliff" right at the shoreline where the waves break. One of my favorite things to do as a kid was to stand on the top of the beach scarp and make it collapse into an avalanche (Okay, I admit it...it's still one of my favorite things to do). Newport Beach in winter is a good place to observe a beach scarp.
Immediately below the beach scarp is the beach face, which is the gently sloping region on which the waves break. As the beach face descends into the water, it becomes the low tide terrace. This region of the beach face is a popular habitat for sand crabs (sometimes called sand dabs), which are a favorite food of many shorebirds, a popular bait among fishermen, and a sometimes delicious meal for more daring diners.
One other feature of the shore is the beach cusp. These are half-moon indentations in the beach face that appear in a regular fashion all the way down a shoreline. These scalloped faces of the beach can be quite beautiful, even more so since oceanographers don't really understand how they form. The beach at Corona del Mar is a good place to observe beach cusps.
Another interesting feature along the beach face are thin lines of sand left by waves as they return to the sea. These wavy lines of sand are most fascinating to follow, and they appear as the tide ebbs. As a wave spends its last energy on the beach, the sand it was carrying gets deposited at its edge. The result is a line of sand that traces the final shape of the wave washing up on the shore. The patterns created along the beach face can be quite intricate, like a flurry of country roads leading everywhere and nowhere. If you've ever taken the time to observe them, you know what I mean.
I should also mention that the beach face and beach scarp are also the area of the beach of which beachcombers are fond. Japanese glass fishing floats, gooseneck barnacles on a light bulb, sea shells of every variety, sea beans (seeds) from Amazon rain forest trees, old bottles, and a fascinating variety of neat stuff are a few of things I've found along the shore. Still, although I have looked for countless days, I have never found a genie or even a message in a bottle. Nonetheless, I'm still searching.
The final feature on our tour of beach anatomy is the sand bar. This ridge of sand that forms just offshore results from summer waves (low-energy) pushing sand from deeper to shallower, or from winter waves carrying sand offshore. Typically, the first breaking waves occur on a sand bar, as the bottom is shallower here. Shoreward of the sand bar is the trough, which can be quite deep and subject to rapid currents.
If there is any take home message upon our cursory examination of the parts of the beach, it is that the beach is dynamic. High-energy waves can occur in summer (although not frequently) and gentle waves can occur in winter. Remember that the beach is a process, not a fixed entity. What you see at the beach is an instantaneous snapshot of all the beach-making and beach-eroding processes that have happened up to that instant in time. Wait a few days and the beach may change again. Like the sea, the beach is an restless and ever-changing creature, not bound to the definitions of men. Looking at the beach in this way can be quite satisfying and make you see the beach perhaps as you never have before.
The Longshore Current
The longshore current results from the action of waves hitting the beach at an angle. This current flows in the region between the breaking waves and the shoreline. Next time you are at the beach, observe the direction of the breaking waves. Are they hitting the beach straight on or at an angle? Except under rare conditions, the waves are probably hitting the beach at an angle. The moving water in these breaking waves hits one end of the beach earlier than the other end; the water piles up at one end. As a result of this "pile up" of water, a current is generated that moves the water down the beach. This is the longshore current. If you've ever been swimming in the waves, and noticed that you moved "downshore" from your starting point, then you have experienced the effects of the longshore current.
Besides people, sand particles and debris are also transported within the longshore current. This process by which the longshore current moves sand and debris is known as longshore transport. Most people do not realize that longshore transport is the singlemost important factor for creating and maintaining sandy beaches in California and just about everywhere else in the world where there's a ocean beach. The movement of the longshore current carries sand down the beach. But where does the sand come from? And where does it go?
To answer the first question, we can examine the sand itself. Try looking at beach sand under a microscope. Here in California, you will see tiny bits of white or clear minerals mixed with tiny bits of cream-colored or yellow minerals. These two minerals are quartz and feldspar, respectively, and these two minerals comprise the majority of all sands found on our beaches and, interestingly enough, make up a good portion of some of the rocks on Mars, as we discovered this summer. These two minerals are derived from our local mountains (or Mars?), the San Gabriel Mountains. As a result of physical and chemical weathering from rain, snow, sun, and wind, the mountains are ground into tiny bits that eventually make their way to the ocean via our local rivers.
With the dry seasons here in California, it is easy to see how rivers carry sand to our local beaches. Except during heavy rainfall, most rivers, like the Santa Ana River are dry river beds. Next time you drive down the 57 freeway, take note of the sand in the Santa Ana River basin (which is a cement drainage basin that channels the river towards the ocean.) The sand in the basin is the same sand you'll find on Newport Beach. Once the sand reaches the shore, it is no longer carried by the river; thereafter the actions of the longshore current carry the sand down the beach.
Now to the second question: where does the sand go? To understand the answer to this question, we must dive beneath the sea at the locations where the beaches appear to disappear. One such location occurs between Newport Beach and Dana Point. At regular intervals along the California coast (and most coasts) a series of submarine canyons can be found. These submarine canyons, some as large as the Grand Canyon, act to funnel sand away from the beaches and towards the bottom of the sea. Most of the sand ends up in a region known as the continental rise. The continental rise receives all the sand and debris and junk that washes off the continental shelf via submarine canyons. In fact, the movement of sand and debris causes a type of current known as a turbidity current, which acts to carve out submarine canyons. Thus, submarine canyons are formed from a type of undersea "weathering" process, whereby submarine trenches (like the Marianas Trench) are formed by plate tectonic processes.
The movement of sand from the mountains through the rivers to the ocean along the beaches to submarine canyons and eventually to the continental rise creates what has been called a "river of sand." This river of sand is in a constant state of motion, always moving, having birth at the tops of the tallest mountains and finally resting at the bottom of the deepest seas. Think about that next time you lay out at the beach!
This is a new segment in our course this semester, inspired by an article in National Geographic (August 1997) and fond memories of camping with wild horses on the barrier islands off the coast of Maryland. Barrier islands are an important result of the longshore current along the eastern and Gulf coasts of the US, and their characteristics are worth reviewing here.
While California and the west coast of the US do not have barrier islands, the Atlantic and Gulf shores of the US exhibit some of the most beautiful barrier islands in the world. From Cape Cod National Seashore in Maine to Key West in Florida, a long, thin barrier of sand protects the Atlantic coastline. These barrier islands continue along the western coast of Florida and all along the Gulf coasts of Mississippi, Louisiana, and Texas.
Like the sand along our beaches, barrier islands are formed from the erosion of inland mountains and the migration of these materials (mostly quartz and feldspar) through the actions of rivers to the coasts. Sand deposited along the coasts is carried southward by the longshore current, but prevailing easterly winds and predominantly gentle waves push the sand back towards the land. If you think about the kinds of waves that are typical along the east coast (i.e. crummy surf), it is easy to understand why much of the sand tends to push towards the continent. Add to this the westward continental drift of the North American plate along the east coast (a phenomenon known as a trailing continent), then we have conditions which are perfect for the formation of barrier islands.
Barrier islands are literally dunes of sand that roll over themselves and push towards the continent. Behind these islands, great expanses of marshland are formed. During periods of heavy seas, such as during a hurricane (another feature prominent on the east coast but missing in California), water breaks through the dunes and push the sand towards the continent, sweeping away the shoreline sand dunes, but creating new areas for sand deposition. Thus, the cycle continues: sand is piled up by the longshore current, waves, and winds, and eroded by storms and hurricanes.
Consider also the ingenious mechanism by which barrier islands act to protect themselves during severe storms. As large waves carry sand seaward, sand bars are formed. These offshore sandbars act to reduce the energy of powerful waves. The coarse sand present in these sandbars can absorb tons of pressure and prevent wholesale destruction of the island.
However, man has altered much of the natural cycle in the life of a barrier island and here's where problems occur. People love living by the sea and barrier islands are no exception. Perhaps you've seen pictures of houses falling in the sea along the east coast, especially during hurricanes. That's because strong waves act to carry away sand and anything attached to the sand.
To combat this, man has built seawalls and jetties. As we learned in class (and as pointed out in your book), jetties form a barrier to the natural river of sand created by the longshore current. Thus, sand piles up at the jetty; downstream beaches are robbed of sand as a result. Seawalls have a different but equally destructive effect. Because they are rigid, seawalls tend to reflect all of the energy of waves, increasing erosion and accelerating the speed of the longshore current. As a result, sand depletes rapidly from areas in front of seawalls. Consequently, the shore in front of seawalls gets deeper, waves get larger, and, eventually, during storms, the seawall is demolished. In effect, the seawall cripples the natural protective response of the barrier island. The end result is no beach.
As a result of man's interference with the natural cycle of the barrier islands, many of the islands and much of the accompanying marshlands are being threatened. Areas such as Assateague Island National Seashore, home of wild horses brought there by sailors some decades ago, are eroding at rates of up to 30 feet per year. The jetties at Ocean City in New Jersey are the culprit in stealing sand from Assateague's shoreline. Other areas, such as Miami Beach, a barrier island along the east coast of Florida, require monumental efforts to resupply the sand that is restricted by jetties. Beach nourishment programs run in the tens of millions of dollars annually and unless the problem is solved, the sand must continue to be replenished. Beach nourishment programs have been described as "like applying a Band-Aid to a hemorrhage." One researcher describes the communities' cries for beach replenishment as similar to an addict's need for a fix.
Similar beach nourishment programs are underway in California, for the exact same reasons. Seawalls, jetties, and damming of rivers all stop the natural flow of sand; the end result is no beach. The beach replenishment project at Seal Beach is costing hundreds of thousands of dollars. Other cities are spending similar sums. What will happen to all that sand the next time a series of vicious winter storms hit southern California? Well, you know the answer to that.
So, the longshore current and the processes that contribute to the natural migration of sand from the mountains to our beaches to the deep sea has vast importance to our lives. It affects our recreational activities, our homes, and our tax-paying pocketbooks. Think about that the next time you stand on the beach and look down the shoreline at all the jetties and beachfront houses. Are they worth spending millions of dollars to protect? Can't we find a better way to live in harmony with the longshore current? If you think of a good answer, let me know.