Beaches constantly change. They may be high and wide during the summer, then
disappear during winter storms. Barrier islands "move like curtains in a
breeze," shifting location from year to year and decade to decade. Inlets
come and go. What processes lead to these changes?
Beaches need waves. No waves, no beaches. This is why lakes rarely have
beaches. Waves breaking offshore of the beach lose energy and create
turbulence which suspends sediments (mostly sand, but sometimes pebbles
or even boulders). Small waves carry sand up higher on the beach, building
high wide summer beaches. Storm waves erode the beach and carry sand
offshore. Erosion by storm waves is episodic. It occurs mostly in rare
but severe storms, hurricanes in the south and east coasts, nor' easters
in New England, and during El Niño years
in California. More information is in the chapter on Coastal
Breaking waves pour water into the surf
zone. The water then moves parallel to the shore as an longshore current,
carrying sand and other sediments along the coast,
changing the shape of the coast, and forming and eroding barrier islands
Waves coming ashore first break on offshore bars. As the wave breaks,
it pours water into the surf zone between the bar and the beach.
Currents, shown by arrows, carry water into the surf zone as the wave
Bottom: After the wave breaks, very
little water returns back to sea over the bar. Instead, it feeds an
longshore current into the paper (shown by the circle with crossed
Three waves coming ashore at Bondi Beach,
Sidney, Australia on 28 March 2006, are bringing water into the surf
zone as walls of white water. The wave in the background is just starting
to break on an offshore bar. The two waves in the foreground have broken
and are coming towards the bathers as walls of white water. Click on
the image for a zoom.
& Jules photographs on Flickr.
When the longshore current becomes sufficiently strong, it turns offshore
in a fast, narrow current called a rip current. Rips are strongest and
most dangerous on days with high surf. Rip currents are dangerous.
caught in a rip sometimes panic as they are carried offshore. Trying
desperately to swim back to shallower water, they can tire and drown.
The safest action is to swim parallel to the beach until out of the
rip, then swim in.
Rip current at a beach (marked by the red arrow). This fast current
is only a few meters wide, extending maybe 100 meters offshore.
From NOAA web page on Rip
Sediments carried along the coast by longshore currents change the shape
of the coast. Where storm waves erode the beach, the sediments are carried
away, to be deposited elsewhere, building new land.
Sand carried by longshore currents has built
up the sandy beach at the bottom of this image of the coast Revel Island,
Virginia. The current carrying sediments tends to flow from the top
to the bottom of the image. Notice the clear water between the white
water caused by breaking waves, and the beach. This indicates the waves
are breaking on an offshore bar. Click on the image for a zoom of
the breaking waves.
From Google Maps.
Tides raise and lower sea level from hour to hour and modulate the influence
of waves and currents.
Beaches are composed mostly of sand. Wind carries the sand inland, creating
dunes. Four conditions are needed for wind to create dunes:
- The sand must be dry. Wet sand near
the waterline is held tightly by capillary forces. When the sand
dries, wind can move the sand grains. Very flat beaches, and beaches
of very fine sand or mud tend not to dry out. Mud and fine sediments
running of land close to the beach keeps the beach moist, which eventually
leads to loss of dunes. Bacteria fed by nutrient rich pollutants also
- There must be tides. Dry beaches require tides that
expose sand to drying by the sun during low tide, and the absence of
groundwater that keeps the beach wet. Seas with weak tides tend to have
- There must be an onshore wind. The wind must be strong at the sand,
and blowing sand higher onto the beach. Beaches with cliffs and buildings
that block the onshore wind will not have dunes. Strong wind blowing
over a dry beach blows sand in windrows, the lighter areas in the
- There must be low obstacles that slow the wind, causing sand to be
deposited on the surface near the obstacle. Grass and other vegetation
in dunes slow the wind, causing growth of the
Thus wind, tides, dry beaches, and beach grass are
essential for formation of dunes.
Sand blowing along a beach on Cumberland Island, Georgia. Click on the
image for a zoom.
Changing Sea Level
Rising sea level due to climate change, and changes in the height
of the land due to geological processes, cause sea level to slowly rise
or fall. Throughout most of the USA, sea level is rising, slowly inundating
the coast, causing the sea to move landward and the land to move seaward.
Along some coasts, especially in Alaska, sea level is
Map of changes in sea level around the United States. Click on the image
for a zoom.
From NOAA Sea Levels
A century of sea-level change at Galveston Island measured by a tide
gauge on Pier 21. Click on the image for a zoom.
From NOAA Sea Levels
80 years of sea level change at Seward Alaska measured by a tide gauge.
The vertical line marks the time of the very large Valdez earthquake
which shifted sea level at Seward. Click on the image for a zoom.
From NOAA Sea
Sand on beaches comes from the erosion of rocks on land.
Rivers carry the sediments to the coast. The construction of dams has
slowed the flow of sand to coastal regions.
Humans have simultaneously increased the sediment transport
by global rivers through soil erosion (by 2.3 ± 0.6 billion metric tons per year), yet reduced
the flux of sediment reaching the world's coasts (by 1.4 ± 0.3 billion metric tons per year)
because of retention within reservoirs. Over 100 billion metric tons of sediment and 1 to 3 billion metric tons
of carbon are now sequestered in reservoirs constructed largely within the past 50 years.
et al (2005).
If you know little about waves, tides, and currents, and
I recommend you begin with Pamela
Gore's Shoreline and Coastal Processes web page.
To gain a deep understanding of beach processes, I recommend
you read Dunes
and Beaches by J. Floor Anthoni, who has spent a lifetime studying
beaches. This is one section of his online Oceanography
Book published by Seafriends
Marine Conservation and Education Centre. His pages are full of wisdom
learned from first-hand experience.
Syvitski, J. P. M., C. J. Vorosmarty, et al. (2005). "Impact of
Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean." Science
308 (5720): 376-380.
8 January, 2009