Oceanography 10, T. James Noyes, El Camino College
6A-1
Beaches and Hard Stabilization
Beaches and Shorelines Are Always Changing
When people buy a home or other property by the seashore, they may not realize that nature may
radically alter the value of their investment. In addition to dangers from storms (e.g., Hurricane
Katrina) and other disasters (e.g., tsunami), waves are slowly and inexorably altering the
shoreline, eroding material from some places and carrying it to other places. Each wave has a
small effect, but they keep coming hour after hour, day after day, year after year.
Significant changes in the shoreline (10’s of feet) can occur within a human lifetime. For
example, old maps of Encinitas – located along the coastal cliffs by Interstate 5 on the way down
to San Diego – show that it has lost about a city block of land to the sea in the last century. In
some places along the coast of Alaska, the shoreline has eroded 900 meters in 50 years, an
average of 18 meters (60 feet) per year!
Picture of the end
of Point Arena
from the top of
the Lighthouse.
The point has
eroded 40+ yards
(120+ feet) over
the last 150 years.
The house had to
be moved back
away from the end
of the point which
has now collapsed.
Oceanography 10, T. James Noyes, El Camino College
6A-2
Beach Sediments: Basic Concepts
Beach sediments are composed of
whatever sediments are available
locally: sand, cobbles, gravel, coral
fragments, shell fragments, etc.
Beach sediments are characterized by
the kind of material that they are made
out of, their size, their shape, and
sorting (well sorted: about the same
size and shape, poorly sorted: many
different sizes and shapes).
Lithogenous sediments (“rock
sediments”) are produced from the
“weathering” of the rock of the land:
rocks are broken down into pieces
(sediments) by the physical impact of
water, wind, and other rocks;
chemicals diluted in water; and
repeated heating and cooling (e.g.,
some parts of the rock expand more or
less than other parts 1). Sediments are
then “eroded” (carried, transported):
carried to the shoreline by running
water (rivers 2), and pushed along the
shoreline by waves (longshore
transport). Sediments are also
produced at the shoreline itself by
weathering and erosion of the rock of
the shoreline.
Photographs courtesy of the
National Oceanic and Atmospheric
Administration, Depart. of Commerce
Black Sand Beach, Hawaii (sand made from volcanic rocks)
1
Ancient peoples used to break rocks by heating them in a fire, and then throwing water on them to cool them
down. The outside of the rock will try to contract, but the warm inside stays expanded, so the only way that the rock
on the outside can contract is to break into smaller pieces which contract individually.
2
And, to a lesser extent, winds
Oceanography 10, T. James Noyes, El Camino College
6A-3
“High energy” water (fast-flowing rivers and strong waves) can lift and carry more sediments
and larger, heavier sediments than “low energy” water. Once the water calms, the larger, heavier
sediments are dropped (deposited), but smaller, lighter sediment continue their journey, which
separates (“sorts”) the sediments. The smallest sediments (mud, clay, silt) sink very slowly, so
they are easily carried and only settle in very calm water. Sand can be carried by stronger flows;
even though it drops quickly to the bottom, it is picked up again and again.
As sediments travel, they bump into
one another and the bottom, chipping
away at their surfaces. Typically this
makes their jagged (angular, sharp)
edges more rounded, though impacts
can split a sediment, creating sharp
surfaces as well. Large rocks at the
bottom of rivers are smoothed by
sand washing over them again and
again (like using sand paper to round
the edge of a piece of wood). The
more time rocks spend in high energy
conditions and the farther they travel,
the more rounded they tend to
become.
Some minerals in rocks (e.g., quartz)
River-bottom sediments (center of the river). The quarter was
placed to give you a sense of the size of the sediments. Notice the
are very resistant to being broken
absence of sand. It tends to get carried away, unlike the larger, heavier
down; rivers and waves are just not
sediments shown here.
strong enough to have much of an
effect. Thus, beach sand is made up by a lot of quartz. (It has the same
chemical formula as glass and is used to make glass.) The same is true
Different
of man-made materials like plastic: once they reach the size of sand,
Kinds of
waves have little ability to break them down further. Tiny plastic
Quartz
sediments are making up a larger and larger component of our
3
beaches .
3
Plastics can be particularly dangerous to the food chain that we are a part of, because they absorb a number of
dangerous man-made chemicals like PCBs.
Oceanography 10, T. James Noyes, El Camino College
6A-4
Example of Beach Erosion: Beach Shape and Rip Currents
The shape of a beach is primarily affected by wave conditions and tide levels. Waves can both
push sand onto the beach from the ocean and drag sand back into the ocean. Typically, a mound
or hill of sand (the “berm”) builds up along the shoreline. At high tide, this is the only part of the
beach that is above water, so the beach slope appears quite steep. As waves push into the
shoreline, they erode a flat area in front of the berm called the “low tide terrace.” At low tide,
this flat area is exposed, and the beach appears less steeply sloped.
During summer, waves tend to be smaller and have a longer period (and wavelength), because
there are fewer, weaker nearby storms and the stronger storms are farther away (in the other
hemisphere where it is winter). Since wave crests arrive at the shoreline less frequently, the
water of the breaking waves has time to soak into the beach sand and can work its way back to
the ocean through the sand. Thus, waves push sand up the beach, and then it remains on the
beach.
During winter, wave crests crash against the beach so frequently that the sand becomes saturated
with water, and more water flows back into the ocean over the sand (and under the incoming
waves, hence the term “undertow”), often dragging more sand back into the ocean than they push
up the beach. Thus, wintertime beaches often have less sand, which sometimes exposes the
larger rocks beneath the sand. The sand, though, is not permanently removed from the beach: it
will be pushed up and out of the ocean again during the spring and summer, only to be removed
again during the fall and winter.
Winter
berm
high-tide level
berm
low-tide terrace
low-tide
level
The sand removed from the beach
piles up offshore, sometimes
forming underwater hills called
“sand bars.” By causing the water
to get shallow very quickly, sand
bars can cause waves to break
quickly, producing the plunging
breakers beloved by surfers.
However, sand bars can also help
create one of the greatest dangers at
recreational beaches: rip currents.
sand
sand bar
End of Ladder
Beach
Winter Beach, North County San Diego. Notice that the ladder
from the lifeguard tower is far above the beach! A lot of sand is
(temporarily) removed by waves during the winter.
Oceanography 10, T. James Noyes, El Camino College
6A-5
Rip currents (also called “rip tides”) are fast-flowing streams of water rushing away from the
shoreline. They may be nearly invisible, particularly when they are starting to form, but they
typically pick up sediments, making the water brown and muddy. They also disrupt the
incoming waves, making them break differently than the
Sand Bar
crest to either side. If a rip current is dragging you out into
He
lp!
the ocean, do not try and fight it; it is too strong, so you will
only waste your energy, increasing the likelihood that you
will drown. Instead, swim out of the rip current by
Beach
swimming up or down the coast (“parallel to the shoreline”).
Once you are out of the rip current, you can safely return to
the beach. (There is a great brochure on rip currents on the
course website. Go to the “websites” section, and page down
to “Beaches.”)
Rip currents can form in several ways, some of
which are not well understood. All involve
waves breaking more strongly in some places
than others along the shoreline. The extra water
rushing up the beach at these locations has to
flow back into the ocean, and it finds it easier to
flow back into the ocean where the waves are
breaking less fiercely, resulting in a stronger
offshore flow at these locations. The best
documented cases of rip currents involve holes
in the ridge of a sand bar, because these rip
currents persist longer and occur in the same
places again and again, allowing them to be
studied. Waves break over the sand bar on
either side of the hole, causing water to surge
over the sand bar. The easiest place for it to
flow back into the ocean is where the water is
deepest: the hole in the sand bar.
National Oceanic and Atmospheric Administration
Oceanography 10, T. James Noyes, El Camino College
6A-6
Sources of Beach Sediments: Coastal Cliffs
Coastal cliffs are one source of beach sediments along the coast of California. Rain water flows
over the top of the cliffs, carving out channels at the top and carrying sediments onto the beach
below 4. Rain water can also soak into the sediments on top of a cliff, adding weight and
“lubricating” them enough for gravity to pull them down onto the beach (a “landslide”). Waves
pound the bottom of the cliffs, eroding a “notch” or “sea cave” at the bottom. As they approach
the cliff, they pick up sediments and fling them at the bottom of the cliff, enhancing waves
ability to erode the cliff. As the cave becomes larger and larger, the rock above becomes too
heavy for the rock below to hold up, and the entire cliff faces collapse.
Before
Erosion
After
Erosion
After
Erosion
Will Fall
Cave
Courtesy of
Dr. Douglas Neves
Rain water carves channels in tops of cliffs.
Landslide. Rainwater soaked into the top of the cliff.
Notice the dangling fence posts: someone lost part of their backyard.
Rainwater can carve canyons all the way down to the shoreline given enough time. Vertical cliffs are created
by waves eroding too much rock from the bottom of the cliffs, so all the rock above tumbles down into the sea.
4
Wind is a smaller, less important factor except in deserts where water plays less of a role in erosion. The science
journalist Robert Krulwich once said “The Grand Canyon bears witness to the magnificent violence hidden in a
torrent of old raindrops.”
Oceanography 10, T. James Noyes, El Camino College
6A-7
fell
from
above
Erosion of the Bottom of Coastal Cliffs by waves at high tide. Notice the debris in front of the cave in the picture
on the right. You can also see where it fell from above when the cave became too deep.
The sandy beach in front
Cliff erosion tends to produce sediments with a variety of sizes
of a cliff or buildings
and shapes (“poorly-sorted sediments”). Both large and small
protects them from erosion
sediments fall out of the cliff. Over time, the larger ones are
by waves at high tide.
ground down into smaller ones, but the beach stays rocky,
because additional landslides add more large sediments to the beach while smaller sediments are
more easily carried away by the waves. On beaches with large waves (“high energy”), few small
sediments may be left behind, leaving the beach comprised of larger rocks.
Initially, when sediments fall out of the cliff, they are typically angular (jagged, sharp), but as
they roll and grind against one another owing to the waves, they begin to become more rounded
(smooth). Thus, sediments that have been on the beach for a long time are rounded, while those
that recently fell from the cliff have sharper edges.
Beach Sediments
Recently Eroded from a Cliff
Oceanography 10, T. James Noyes, El Camino College
6A-8
Sources of Beach Sediments:
Rivers
If you look at the bottom of a natural river,
you will see that it is covered by sediments.
These sediments are being carried down to
the shoreline from the mountains. More
Black Ovals = Large Rocks do not move (much).
Yellow Circles = Sand hopping down the river.
weathering and erosion tends to take place in the
Small
Brown Circles = Mud suspended in the water.
mountains, because their steep slopes lead to
faster-flowing (“high energy”) water. As the rivers leave the mountains, their slope becomes
gentler, the water slows down, and the heaviest sediments are dropped and left behind. Smaller
sediments (mud and sand) are carried down to the shoreline, where the river runs into the ocean,
greatly reducing its speed. The mud particles are very small, so they sink very slowly and are
easily picked up by waves; they stay suspended long enough to drift out into the ocean. The
heavier sand, on the other hand, falls along the shoreline, and is pushed up and down the coast by
waves (longshore transport); the wave effectively spread sand along the coast, covering up
bigger rocks from local erosion and thus creating sandy beaches.
Sources of Beach Sediments:
Rivers vs. Coastal Cliffs
The dominant view among
oceanographers is that rivers
provide most of the sand to the
shoreline. However, a recent
study of cliff erosion along the
coast of southern California
Sand is dropped on the
suggests that cliffs may provide
sides of the river where the
more of the sand than previously
water slows down due to
rubbing against the land.
thought (perhaps as much as half
or more). These ideas are not
necessarily contradictory. Humans have dammed
California’s rivers over the last century, and dams hold
back sand as well as water, keeping sand from reaching
the coast (a major problem for dam operators as sand
begins to fill up the reservoir). In the past, sand from
rivers was pushed down the coast by waves and protected
our coastal cliffs, but as our beaches narrow, cliff erosion
is producing more and more of the sand on our beaches.
Another complicating factor is that there has been a lot of
coastal development over the last century; excess
sediments from construction were dumped on the beach
and significantly increased the size of our beaches, so we
may have gotten used to unnaturally wide beaches. I
want you to know the dominant view (and the view bestsupported by the available evidence): that under natural
conditions, most beach sand appears to come from rivers.
NOAA
Oceanography 10, T. James Noyes, El Camino College
Removing Sediments from Beaches:
Submarine Canyons
6A-9
USGS
Many underwater valleys (“submarine
canyons”) get close to the shoreline along
the coast of California. When waves push
sand down the shoreline, some of it falls into
these canyons, and underwater landslides
carry the sand down the canyons onto the
deep sea floor (once enough sand had piled
up). (You can actually see this at the end of
Redondo Canyon on contour maps of ocean
depth.) Beaches farther down the coast get
less sand, so they are narrower and more rocky:
rocks from local erosion do not get covered by sand.
Mountains & Rivers
Beach Compartments / Littoral Cells
Falls into
In southern California, oceanographers have found that
specific rivers feed sand to the beaches south of them
underwater
(waves typically come from the northwest, pushing
canyon.
sand to the south) and that specific submarine canyons
remove sand, keeping beaches south of them from
Redondo Canyon
receiving sand. The route that sand flows down rivers,
to particular beaches, and into the ocean via a
submarine canyon is called a “beach compartment” or
“littoral cell.” For example, sediments washed down
the coastal streams on the north side of Santa Monica
Bluff
Bay (e.g., Malibu) are pushed south down the coast by
Cove
waves 5, adding sand to the beaches of Santa Monica.
The sand continues down the coast until it falls into
Redondo Canyon, which keeps it from reaching the beaches
of Palos Verdes, so Palos Verdes has rocky beaches.
(The shape of the shoreline and human construction are also
important factors in this case.)
Santa
Monica
Redondo
Beach
Cabrillo
Beach
Key Concepts: Sandy beaches are sandy, because lots of sediments are eroded up in the
mountains and carried down to the shoreline by rivers. Waves push sand down the coast
to our sandy beaches. Rocky beaches are rocky, because something keeps sand from
reaching the rocky beach (e.g., underwater canyons) and covering up the larger rocks
from local erosion by rain and waves. Note: Waves push sand down rocky shorelines too
(longshore transport does occur), but there is very little sand to push.
5
prior to the coastal development of the last century
Oceanography 10, T. James Noyes, El Camino College
6A-10
Small sandy
There can be small, sandy
Clearly,
beach along
beaches in the middle of a
erosion
of
a rocky shore.
rocky shoreline, typically
cliffs is not
in the back of coves. All
producing
the sand that is made by
Courtesy of
much sand.
eroding the larger sediPDPhoto.org
ments is pushed into the
back of the coves by waves, making the beach there sandy. I do not consider these small patches of
sand to be good examples of sandy shorelines. They are the exception, not the rule, for these coastlines.
Santa Monica
Courtesy of Faris
Mountains & Rivers
Santa
Monica
Redondo Canyon
Redondo
Beach
Redondo Beach (South)
Bluff
Cove
Cabrillo
Beach
Bluff Cove
Oceanography 10, T. James Noyes, El Camino College
The East and Gulf Coasts
As you will see in topic 6B (“Shorelines”), the east
and Gulf coasts of the United States (and some parts
of the Pacific Northwest) are quite different from
southern California. Among other things, they have
barrier islands – long, thin piles of sand – along the
coast owing to their flatter continental shelves and
more sediments leftover from previous ice ages
(when sea levels where lower, sand traveled farther
out into the ocean). Along the east coast, sand
typically does not leave the coast at submarine
canyons. Instead, it piles up at the ends of barrier
islands where the water is deep.
Barrier Island. National Oceanic & Atmospheric
Administration, Dept. of Commerce
6A-11
Oceanography 10, T. James Noyes, El Camino College
6A-12
Hard Stabilization
Hard stabilization refers to large, heavy, and/or strong objects that humans build in an attempt to
resist nature and keep the present shoreline from changing 6. Below, I discuss 4 examples of hard
stabilization: groins, jetties, breakwaters, and seawalls.
Groins
I kid thee not. They are really called “groins.”
LST
Groins are long, thin walls that extend out into the ocean. They
are built to try to hold onto a sandy beach, to keep waves from
carrying the sand away, and perhaps even build it up. In this, they
tend to be successful. Waves push sand down the coast, but the
groin blocks the flow of sand, so sand piles up on one side.
However, waves continue to push sand down the coast on the
other side of the groin, so the beach there erodes: it is “starved” of
sand because the lost sand is not replaced from farther up the
coast. Eventually, so much sand piles up on the one side that sand
begins to leak around the edge and the shoreline stabilizes. Thus,
the main effect of the groin is to change the shape of the shoreline,
not to add more sand: it merely makes one person’s beach wider at
the expense of other people’s beaches down the coast.
Groin
Beach
Groin
LST
sand piling up
sand being removed
Groin. Longshore transport is represented by the red arrows. Sand is being pushed down the coast by waves to the
left in the picture. The sand runs into the right side of the groin and cannot get past it, so the sand stops and piles up.
On the left side of the groin, waves push sand farther down the coast, so there is less and less sand on the beach.
Groin field. A series of groins along
a coast. Notice how sand builds up on
the right side of each groin, and there
is less sand on the left side of each
groin. LST is going to the left in the
picture (red arrow). Courtesy of
North Caroline Division of Coastal
Management.
6
Typically expensive – hundreds of thousands or millions of dollars – and ultimately fruitless.
Oceanography 10, T. James Noyes, El Camino College
6A-13
Jetties
LST
Jetties resemble groins, but are typically longer and come in
pairs. They are built at the entrances of harbors to keep
Beach
sand from being pushed into the mouth of the harbor by
waves, making the mouth of the harbor shallower (ships
Jetty
could run aground) and eventually blocking it. (If the
entrance to the harbor is narrow, jetties also block waves,
Entrance to Harbor
keeping the water in the entrance calmer.) Like groins,
Jetty
sand piles up along one of the jetties, and sand erodes from
the opposite side of the other jetty. Because jetties are so
long, sand cannot get around the jetty, because they reach
LST
out into deep water where waves cannot reach the bottom
where the sand is; instead the sand keeps building up or
slides off deeper into the ocean. As a result, the shoreline
never stabilizes, because sand never reaches the beaches on
the other side of the jetties. On the east coast of the United States, there are documented cases of
the construction of jetties resulting in entire barrier islands shifting hundreds of feet in a few
decades (say bye-bye to a bunch to people’s homes and businesses), a process that the builders
argued would take a thousand years.
In spite of jetties, sand still builds up in the entrance of harbors. About 15 years passed between the last
two “dredges” (removing sand) of the entrance to King Harbor in Redondo Beach (the removed sand was
put on shrinking beaches south of the harbor which do not get sand anymore owing to the jetties). The
cost for the last dredge was $560,000. So even with the jetties, it costs about $37,000 a year to keep the
entrance to King Harbor safe for boats.
Jetties
Jetties.
Left: Courtesy of the National Park Service. Notice the sand built up one side, and the lack of sand on the other side.
Right: Courtesy of Rick Crawford, NOAA. The jetties keep the entrance to a harbor (an estuary) from being blocked
by sand.
Oceanography 10, T. James Noyes, El Camino College
6A-14
Breakwaters
LST
Beach
Breakwater
Breakwaters are long, thin walls built along (“parallel to”), but separated
from, the coast. Like jetties, breakwaters are also built for harbors, but
their job is to stop waves from entering the harbor, making the harbor
calmer for working on your ship, loading/unloading cargo, and so on.
(Sometimes a jetty and breakwater are combined into one structure.)
Since breakwaters block the waves, they also block the flow of sand
down the coast, so sand tends to pile up behind them, beginning to fill in
the harbor and making it useless. A famous example comes from Santa
Monica; the remains of the breakwater 7 can still be seen.
Edge of a Breakwater at the entrance to a harbor.
Ex-Breakwaters.
Waves refracted around the
breakwaters and pushed sand
behind them from both sides until
the sand reached all the way out
to the breakwater. This is how a
natural feature called a
“tomobolo” forms (see reading
6B).
Breakwater. Notice the small lighthouse at the end to mark the entrance to the harbor.
7
See the pictures on page 314 of your textbook. It was torn down once the problem was realized: a lot of wasted
money…
Oceanography 10, T. James Noyes, El Camino College
6A-15
Seawalls
Seawall
Seawall
Wave Crests
Seawalls are walls built along the coast to keep waves from eroding it, typically to protect a
building. Waves eventually erode the seawall (just like they erode the land), so it needs constant
maintenance. If funds run out, then debris from the seawall litters the beach, including rusty
pieces of iron that were used to bind the seawall together. The shoreline on either side of the
seawall continues eroding, of course. The seawall can actually help the shoreline on each side
erode, because the seawall’s ends reflect waves towards the land
Before Erosion
on either side. As the shoreline to the side of the seawall erodes,
the seawall has to be extended (costs more $), because more of the
building’s property is exposed to the sea. This will never end,
Land
costing more and more money. Worse yet, seawalls can cause the
beach in front of the seawall to erode. Wave energy is reflected
from (“bounces off”) the seawall towards the sand in front of the
seawall, pushing it out into the ocean. As the sand is removed
from the base of the seawall, the land that the seawall is built on is
exposed to the waves. The waves erode the land beneath the
After Erosion
seawall, causing they seawall to collapse.
Seawall
Seawall
Seawall
at low tide
(upper left)
and high tide
(lower right).
Land
Oceanography 10, T. James Noyes, El Camino College
6A-16
More erosion
near the end
of a seawall.
Seawall
After Erosion
Beach
What can we do about beach erosion?
As we have seen, hard stabilization can be used to maintain the shoreline, but this is both
expensive and can have negative consequences (often unforeseen).
Dams (which block the flow of sediments to the shoreline) can be removed, but the dams
themselves provide a variety of benefits that we would have to forego.
Millions of dollars are spent by cities on a regular basis to
replace lost sand. Sand for “beach renourishment” can be
trucked in from deserts or behind dams, or dredged from the
ocean floor. It may only stay on the beach for as little as a
year, so this is an ongoing expense. Careful research is needed
to estimate how long it will remain. When possible, the best
strategy is to help the sand “bypass” obstacles like long jetties:
remove the sand from one side and carry it to the other side.
Places that regularly replenish
their beaches using dredged
sand are finding that they must
go farther and farther out into
the ocean to find sand, because
we have already dredged the
sand closest to shore. This
make dredging more and more
expensive over time.
Another option is to do nothing, let nature takes its course, and
plan for the changes (to adapt). For example, shoreline buildings can be
built so that they can be moved when necessary. People who live on
shifting barrier islands are building these kinds of homes more and more
often.
Perhaps we could
grind up re-cycled
glass bottles
to make sand?
All and all, I would argue that there is no one right answer (certainly one right answer that
applies in all circumstances). Each choice involves expensive trade-offs; some people will
“win” (benefit), and some people will “lose” (suffer).
Dredging Sediments. NOAA, Dept. of Commerce.