Lesson Outline

1
Lesson Outline
I. Beach Sediments
II. Sediment Transport Longshore Currents
III. Rip Currents
IV. Human Interruption of Sand Transport
V. Productivity of Shoreline Communities
2
Rivers of Sand

• Sediments are carried to the ocean by wind,
  water, and ice.

The wind and water deposit the sand-sized
particles at the point where the sea meets the
land, forming a beach.
Once deposited on a beach, sand is picked up by
wave action and moved up or down the coastline by
currents.
A beach is a slow moving river of sand.
3
Beach Sediments

• A mature sediment is one that has traveled a long
  time or long distance from its point of origin to
  the beach.

An immature sediment is one that has only
traveled a short distance or time from its point
of origin to the beach.
Three factors to determine the maturity of a
sediment

• the degree of rounding of the particles

• the degree of sorting of the particles by size

• the amount of clay in the sediment

4
Degree of Rounding

• The rounding of individual sediment particles is
  the end result of those particles bouncing down
  miles and miles of stream bed and coastline.

Mature beach sand consists of rounded sediment
particles angular particles are prevalent in
immature sediments.
5
Degree of Sorting

• The sorting of sediment is simply how well the
  sedimentary grains are separated out by size.

If all the grains in a sand sample are very
nearly the same size, the sample is
well-sorted.
If the sample contains pieces of gravel, as well
as sand and silt, it would be a poorly-sorted.
Mature sediments are characterized by well-sorted
particles of nearly equal size immature
sediments are poorly-sorted by size.
6
Clay

• Mature sediments, which have been transported a
  long distance since erosion, have little no clay
  mixed in with them.

Immature sediments will always have some clay
size particles mixed in with the sand and other
size sediments.
7
Wave Energy

• The composition of beach sand is a function of
  the sediment source and the wave energy and
  current patterns at the shoreline.

Recall that as a wave approaches a beach, its
length shortens and height increases until it
breaks and delivers its energy to the surf
zone.
Wind waves expend a great amount of energy on a
beach, and much of the energy is used to
transport sand up and down the coast.
8
Wave Refraction

• As a wave enters shallow water, friction with the
  bottom causes it to slow.

When a wave approaches the beach at an angle, one
end of the wave train will drag before the other,
causing the wave to refract, or bend, toward the
shore.
9
Longshore Currents

• Refraction is never complete, so waves typically
  surge onto the beach at an angle.

When they break, waves rush up the beach at one
angle, then return down the beach slope in a
slightly different direction.
The net result is a movement of water down the
beach called a longshore current.
The current is stronger with larger waves.
10
Rivers of Sand

• The movement of sediment along a shoreline by
  wave action and longshore currents is referred to
  as longshore transport.

11
Seasonal Changes in Beach Topography

• During winter, high winds and large waves carve a
  steep beach face and build sand bars just off the
  beach that run parallel to the shore.

In some areas the beach face will lose much of
its sand during the winter from this heavy wave
action.
The smaller waves of summer move the sand back
from the offshore bars onto the beach in a smooth
gently sloping beach.
12
Transport Along the Coastal US

• Larger storm waves are generated in the North
  Atlantic and North Pacific, so along the US
  coastline the transport of sand is generally
  southward.

Along the east coast, sand moves from the New
England coast down to the tip of Florida. When
it reaches southeast Florida, where the
continental shelf narrows, sand runs down the
slope to the sea floor in the Florida Straits.
Along the west coast, a series of submarine
canyons off California drain the sand off to the
sea floor at irregular intervals.
13
Beach Erosion

• Most beaches in the US are eroding.

14
The Gulf of Mexico

• In the Gulf of Mexico, there is no dominant
  direction for the wind waves.

The result is that the sand transport can occur
in either direction.
Along Pinellas beaches the sand may move north or
south depending on the weather patterns, which
are always changing.
In general, larger ocean basins have larger waves
and greater longshore transport of sand.
Consequently, along the Florida coast, a greater
amount of sand moves along the Atlantic beaches
than those in the Gulf of Mexico.
15
Rip Currents

• Rip currents occur when waves approach the beach
  in parallel lines or strong winds push water
  onshore.

Water rushes straight up the beach and the
undertow returns it straight back down the beach
slope.
The undertow is unable to return all the water
before the next wave (or wind) pushes it back up
the beach.
The piling up of water cannot continue, so the
excess water will flow up or down the beach until
it comes to some imperfection, at which point the
excess water flows back to the sea in a narrow,
concentrated rip current.
16
Swimming Out of a Rip Current

• A swimmer that steps into the channel eroded by a
  rip current is quickly pulled out by the rip
  under the incoming waves.

In this situation, the correct response is to
swim parallel to shore.
The swimmer does not have to go far to escape the
rip current and find shallower water where the
swim to shore is easier.
17
NWS Marine Warning Statement
MARINE WEATHER STATEMENT HIGH SURF ADVISORY
NATIONAL WEATHER SERVICE TAMPA BAY AREA - RUSKIN
FL 446 AM EST SUN FEB 15 2004 THE NATIONAL
WEATHER SERVICE IN RUSKIN HAS ISSUED A HIGH SURF
ADVISORY UNTIL 439 PM EST SUNDAY AFTERNOON FOR
THE FOLLOWING COUNTIES ...MANATEE ...CHARLOTTE
...LEE ...PINELLAS ...HERNANDO ...SARASOTA ...
HILLSBOROUGH ...CITRUS ...LEVY AND PASCO. A HIGH
SURF ADVISORY IS ISSUED WHEN DANGEROUS WATER
ACTION IS EXPECTED ALONG THE COAST. THIS INCLUDES
ROUGH SURF...LARGE BREAKING WAVES...STRONG
UNDERTOW...AND RIP CURRENTS. STRONG WEST WINDS
WILL MOVE INTO THE AREA LATER THIS MORNING. WITH
THE STRONG WINDS WILL COME VERY ROUGH
SEAS...BEACH EROSION AND RIP CURRENTS. BEACHGOERS
ARE URGED TO HEED LIFEGUARD WARNINGS. A RIP
CURRENT IS A 10 TO 30 YARD WIDE CHANNEL OF WATER
THAT CAN PULL EVEN A STRONG SWIMMER INTO DEEPER
WATER. TO AVOID GETTING CAUGHT IN A RIP
CURRENT...SWIM AT GUARDED BEACHES AND HEED THE
ADVICE OF THE BEACH PATROL. AT UNGUARDED BEACHES
AND NEAR PIERS...JETTIES...AND INLETS...DO NOT GO
INTO THE SURF MUCH ABOVE YOUR KNEES. IF CAUGHT IN
THE SEAWARD PULL OF A RIP CURRENT...DO NOT
ATTEMPT TO MOVE DIRECTLY TOWARD SHORE.
INSTEAD...MOVE SIDEWAYS ACROSS THE RIP CURRENT
UNTIL THE PULL EASES. ANOTHER METHOD OF ESCAPE IS
TO LET THE RIP CURRENT PULL YOU SEAWARD 50 TO 100
YARDS...WHERE THE FORCE WEAKENS. THEN...SWIM
TOWARD THE BEACH AT AN ANGLE AWAY FROM THE
CURRENT.
18
Stemming the River of Sand

• There are several man-made structures that will
  interrupt the movement of sand along a coastline.

• Groins

• Jetties

• Breakwaters

• River Dams

19
Groins

• A groin is a structure built of large rocks
  placed out in the water at right angles to the
  shoreline.

Groins are used along a beach that is having
erosion problems. A groin will trap sand on one
side because it will block the normal longshore
transport along the beach.
Once a single groin is built, then the entire
beach eventually ends up with a series of groins.
20
Jetties

• A jetty is built from large rocks placed out at
  right angles to the beach, but this time they are
  placed on one or both sides of an inlet or pass
  out to the open sea.

The purpose of a jetty is to keep sand from
filling in the pass or inlet.
Eventually, sand spills over the end of the jetty
into the pass. When this happens, the pass and
jetty must be dredged or the jetty extended.
21
Breakwaters

• A breakwater is built along parallel to the
  beach, again with large rocks.

Since the breakwater stops the wave action, it
also stops longshore transport for the portion of
the beach behind the structure.
Sand will accumulate behind the breakwater, while
the beach down stream from the breakwater
experiences erosion.
22
River Dams

• Normally rivers flow down to the sea with a large
  load of sediment, mainly sand, silt, and clay
  size particles.

The sand will be deposited on the beach at the
mouth of the river or stream, and the silt and
clay will be transported offshore.
When a dam is built along a river or stream, the
sudden drop in water velocity in the reservoir
behind the dam will cause all the sediment
carried by the river to fall out of suspension
and land on the bottom of the reservoir behind
the dam.
All dams are stopping the flow of sediment down
to the sea, which can eventually lead to a loss
of beaches in some areas.
23
Classification of Shorelines by Wave Energy

• When the waves from the open sea come in and
  break for the first time along a beach, it is
  called a high energy shoreline.

In a low energy shoreline, ocean waves are broken
by a barrier island, offshore sand bars, or
offshore reefs, so the wave energy is dissipated
before it reaches the shore.
24
High Energy Shorelines

• There are only two type of high energy shorelines.

On a high energy rocky shore, the ocean waves
break onto rocks and boulders.
This type of shore always occurs when there is
not a source of sand to supply the beach. With
high wave energy, if there is not a constant
supply of sand, the results is always a rocky
shore.
25
High Energy Sand Beach

• If there is a supply of sand, then a high energy
  sand beach will occur.

The sand on this type of beach is kept in
constant motion by longshore transport.
26
Low Energy Shorelines

• Low energy shorelines are always dominated by
  soft sediment.

The lack of wave action allows silt particles to
settle down to the bottom and make a soft shore.
In areas of rigorous tidal flow, the bottom will
be a mixture of sand and silt particles, which
are often stabilized by submerged grasses or
mangroves.
27
Shoreline Productivity

• The most abundant marine life is found along the
  high energy rocky shore, where there numerous
  niches for plants and animals to find shelter and
  habitat.

The high energy sand beach is the opposite in
regard to abundance of marine life Few plants
and animals live on a sand beach.
Since the sand is in constant motion, there is no
place to hide, and only a few very specialized
animals can live on a high energy sand beach.
Low energy shorelines are variable in abundance
of marine life.