- Longshore Current
- Barrier Islands
Most beaches occur along the coasts of landmasses. The coast can be defined as the region of a land mass influenced by the presence of the oceans. Those areas of land that are subject to the daily cycle of on-shore and off-shore winds can be considered as coastal regions. One definition for coast is 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."
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. 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. This slope 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.
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.
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. Oceanographers don't really understand how they form.
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 sand bar is a 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.
The beach is dynamic. 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.
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.
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.
Along 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.
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.
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. 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.
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.