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?
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
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
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.