Beaches Unit (Topic 6A) – page 1
Name:
Beaches Unit
Section:
Beaches and Shorelines Are Always Changing
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!
Beach Sediments
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 their 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). Sediments are then “eroded” (carried, transported): carried to the shoreline
by running water (rivers), and pushed along the shoreline by waves. Sediments are also produced
at the shoreline itself by weathering and erosion of the rock of the shoreline.
1. How are well-sorted sediments different from poorly-sorted sediments?
2. What is weathering?
3. What is erosion?
4. What carry sediments from farther inland to the shoreline?
5. What both carry sediments down the coast and “weather” the coast (push the shoreline
back)?
Beaches Unit (Topic 6A) – page 2
Sediment Transportation (Erosion)
“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) are very resistant to being broken down; rivers and waves
are just not 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
of man-made materials like plastic: once they reach the size of sand, waves have little ability to
break them down further. Tiny plastic sediments are making up a larger and larger component of
our beaches.
6. What carry more sediments, big waves or small waves?
7. Which sediments are carried more easily by waves, large sediments or smaller ones?
8. Which is larger, sand or mud?
9. Which is carried more easily and in larger quantities by rivers, sand or mud?
10. As sediments travel, how does their size and shape change?
11. True or false? “Given enough time, sand grains exposed to waves will get smaller and
smaller until they become mud.”
Beaches Unit (Topic 6A) – page 3
Winter and Summertime Beaches
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.
Winter
berm
high-tide level
berm
low-tide terrace
low-tide
level
sand
sand bar
During summer, waves tend to be smaller and have a longer period, 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.
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.
12. When is there more sand on beaches, during the summer or the winter?
13. What pushes sand onto the beaches during this season? Where does the sand come from?
14. When is there less sand on beaches, during the summer or the winter?
15. What pulls sand off beaches during this season? Where does the sand go?
Beaches Unit (Topic 6A) – page 4
Rip Currents
Sand Bar
Rip currents (also called “rip tides”) are fast-flowing streams
He
of water rushing away from the shoreline. They may be
lp!
nearly invisible, particularly when they are starting to form,
but they typically pick up sediments, making the water brown
Beach
and muddy. They also disrupt the incoming waves, making
them break differently than the crest to either side. If a rip
current is dragging you out into 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 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.
16. If a swimmer tries to swim against a rip current,
who will win, the swimmer or the rip current?
Beach
17. Sketch arrows into the picture on the right
to show a path that the swimmer should take
to escape from the rip current.
!!
lp!
e
H
Rip C
urre
nt
Swim this way?
Beaches Unit (Topic 6A) – page 5
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. 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.
Rain water carves channels in tops of cliffs. 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.
Cliff erosion tends to produce sediments with a variety of sizes and shapes (“poorly-sorted
sediments”). Both large and small sediments fall out of the cliff. Over time, the larger ones are
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.
18. What is the main cause of erosion at the top of coastal cliffs?
In other words, what breaks down most of the rock at the top?
19. What is the main cause of erosion at the bottom of coastal cliffs?
In other words, what breaks down most of the rock at the bottom?
20. Describe the size and shape of the sediments that fall down from cliffs.
In other words, are they large, small (sand and mud), large, or a mixture of large and
small? Are they angular (sharp), rounded (smooth), or a mixture of angular and smooth?
21. Describe how the size and shape of sediments changes as they spend time on a beach.
In other words, do they get larger or smaller? Do they get sharper or smoother?
Beaches Unit (Topic 6A) – page 6
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 weathering and
erosion tends to take place in the 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
suggests that cliffs may provide more of the sand than previously 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 best-supported by the available evidence):
that under natural conditions, most beach sand appears to come from rivers.
22. Where does most sand on naturally sandy beaches come from? In other words, is it
brought to the coast by rivers, or does it come from waves breaking down the land along
the coast (e.g., cliff erosion)?
23. What take sand away from the ends of rivers and spread the sand along the coast,
creating sandy beaches?
24. How will damming a river affect beaches? Will the beaches at the coast have more sand
or less sand if dams are built in rivers?
Beaches Unit (Topic 6A) – page 7
Removing Sediments from Beaches: Submarine Canyons
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.
Beach Compartments / Littoral Cells
Mountains & Rivers
In southern California, oceanographers have found that
specific rivers feed sand to the beaches south of them
Santa
(waves typically come from the northwest, pushing
Monica
Falls into
sand to the south) and that specific submarine canyons
underwater
remove sand, keeping beaches south of them from
canyon.
receiving sand. The route that sand flows down rivers,
to particular beaches, and into the ocean via a
Redondo Canyon
Redondo
submarine canyon is called a “beach compartment” or
Beach
“littoral cell.” For example, sediments washed down
the coastal streams on the north side of Santa Monica
Bay (e.g., Malibu) are pushed south down the coast by
Cabrillo
waves , adding sand to the beaches of Santa Monica.
Bluff
Beach
Cove
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.)
25. What typically causes sand to be permanently lost from the shoreline?
In other words, how and why does sand leave the shoreline and never return?
26. What direction does sand typically move along the coast of California, towards the north
or towards the south?
Beaches Unit (Topic 6A) – page 8
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.
Rocky Shores and Beaches in Coves
There can be small, sandy beaches in the middle of a rocky shoreline, typically in the back of
coves. All the sand that is made by eroding the larger sediments 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.
The East and Gulf Coasts of the United States
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.
27. Rocky beaches are rocky because the sand and mud that is eroded from the cliff is taken
away and not replaced by sand from other places. Why does the sand get taken away but
the larger rocks remain? What might be preventing lots of sand from reaching the beach
and covering up the rocks?
28. True or false? “Rocky shorelines have small stretches of shoreline that are sandy.”
29. What are barrier islands?
30. Where are barrier islands common places to find sandy beaches along the coast?
Beaches Unit (Topic 6A) – page 9
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. Below, I discuss 4 examples of hard
stabilization: groins, jetties, breakwaters, and seawalls.
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: sand is taken away and the lost sand is not
replaced by sand 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.
Jetties resemble groins, but are typically
longer and come in pairs. They are built at
the entrances of harbors to keep sand from
being pushed into the mouth of the harbor
by waves, making the mouth of the harbor
shallower (ships could run aground) and
eventually blocking it. (If the entrance to
the harbor is narrow, jetties also block
Jetties. Courtesy of the Army Corp of Engineers.
waves, keeping the water in the entrance
calmer.) Like groins, 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 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.
Beaches Unit (Topic 6A) – page 10
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
breakwater can still be seen.
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 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, 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
seawall, causing they
seawall to collapse.
Seawall
Beaches Unit (Topic 6A) – page 11
31. Label each kind of hard stabilization. In other words, write the name of each kind of hard
stabilization next to it in the map below.
32. Where will the sand pile up in the picture below? In other words, where will the beach
grow larger? Write “Deposition” at this location.
What pushes the sand up against the groin at this location?
33. Where will sand erode in the picture above? In other words, where will the beach get
smaller? Write “Erosion” at this location.
What carries sand away from the beach at this location?
Beaches Unit (Topic 6A) – page 12
34. What is the purpose of each kind of hard stabilization?
In other words, what is the primary reason each one is built?
Built to block waves, making it easier to load and unload boats
_________________
Built to capture sand and build up a beach in a specific location
_________________
Build to keep sand from blocking a harbor
_________________
Built to protect the shoreline from erosion by waves
_________________
Possible Answers: breakwaters, groins, jetties, seawalls
35. What effect can a seawall have on the public beach in front of the seawall? For example,
what effect did seawalls have on the beaches of New Jersey?
36. Circle all the true statements about seawalls:
“Seawalls can cause the beach in front of the seawall to erode.”
“Seawalls can cause the land on either side of the seawall to erode faster.”
“Seawalls erode over time, and eventually collapse if not maintained.”
“Seawalls protect the land behind them from eroding.”