Barrier Islands

Functions of Barrier Islands• Protect the mainland

from wind and tidal energy

• Provide habitat for plants, birds, fish, shellfish

• Recreation

What is a barrier island?• Forms along the mainland coastline when material accumulates parallel to shore forming

a barrier between the sea and the coast• Provides protection to the mainland from the high

energy of storms• Constantly changing due to wind, waves, tides,

currents• Examples: Dauphin Island (Alabama), Outer

Banks (North Carolina), Chandeleur Islands (Louisiana)

Cross Section of a Barrier Island

GULF OF MEXICO

(high energy)

MISSISSIPPISOUND

(low energy)

PRIMARYDUNE

SECONDARYDUNE

TERTIARYDUNE

MARITIMEFOREST

FRESH WATERPOND

SALT MARSH

BEACH

Cross Section of a Barrier Island

PondBeach

Barrier Island:an island just off the

coast of the mainland;it protects the mainlandduring big storms and

on a daily basis

Overwash Zone:a place on barrier island

where water from the “ocean” washes across to

the sound side during a large storm

Requirements for Barrier Island Formation:

1) sand supply

2) flat surface where sand can accumulate (broad continental shelf)

3) enough wave energy to move the sand

4) rising sea level

Two primary theories: sea level rise and sediment accumulation

Rise in Sea Level

Dune Ridge Theory Barrier islands form as a result of rising sea

levelThere are 3 stages 1. shoreline was further out2. the beach ridge becomes a barrier island backed by a bay 3. the barrier system migrates across the shelf and sea level continues to rise

“Spit’s”

A spit is a length of accumulated sand attached to land at one end, pointing in the direction of the longshore drift

Waves

Longshore Current

Waves come on shore at an angle. -Sand and sediment flow back with the water at this angle-The motion of the backwash causes a longshore current.-Longshore drift occurs as the current moves material down the coast

Longshore Current:current of water parallel tothe shoreline generated by waves hitting the beach at

an angle and flowing back out at an angle

Littoral Drift: (longshore drift)the movement (or current)

of sand along the shoreline being

carried by the longshore current

Approaches to Beach Erosion:1) Hard Stabilization

2) Beach Replenishment

3) Moving Manmade Structures

Source: National Oceanic and Atmospheric Administration (NOAA). 1998 (on-line). “Population: Distribution, Density and Growth” byThomas J. Culliton. NOAA’s State of the Coast Report. Silver Spring, MD: NOAA.

Projected Population Change from 1994-2015: Absolute Numbers

Graphic Credit: National Oceanic and Atmospheric Administration (NOAA). 1998 (on-line). “Population: Distribution, Density and Growth” by Thomas J. Culliton. NOAA’s State of the Coast Report. Silver Spring, MD: NOAA.

Population Density, 1960-2015 (not including Alaska)Sources: U.S. Bureau of the Census, 1997; National Planning Association

Coastal Barrier Resources Act(passed by Congress in 1982):

restricts federal monies to undeveloped barriers and areas

of undeveloped barriers to discourage development

Coastal Barrier Resources Act3 Purposes:

1) to minimize loss of human life2) to reduce wasteful expenditure of federal revenues3) to reduce damage to fish and wildlife habitat & other natural resources of barrier islands

High Tide

Low Tide Supratidal

Intertidal

Subtidal

Submerged, but sand interacts with rest of beach.

Original Map Adapted from GeoMap Interactive GIS Software for Grades K-12, Version 1.4 by H&T Software, Inc., NASA/TSEI & Johnson Controls World Services

What is here today ………May be gone

tomorrow…

BARRIER ISLANDS

H.H. Roberts

Dauphin Island Sand

Grain Concentration quartz greatest abundance _________________________________________

shell fragments (CaCo) 0.2% to 0.6% by weight

__________________________________________heavy minerals (magnetite, 1% or 2% by weight or rutile, ilmenite, zircon, monazite, greater

concentrations spinel, kyanite, staurolite, tournmaline, epidote, amphibole, sillimanite, garnet, leucoxene)

_______________________________________________ organic debris present

WHAT IS A BARRIER ISLAND?

-It is an island that forms along the coast of the mainland. It protects the mainland

from wind and tidal energy during big storms and on a daily basis.

Requirements For Barrier Island Formation

1. Sand supply2. Flat surface where sand can accumulate

(continental shelf)3. Enough wave energy to move sand around4. Rising sea level

Theories of Barrier Island Formation

Barrier Islands are Dynamic

-They form along the mainland-High energy area-Constantly change due to wind, waves, tides, and currents – not stable bodies of land-DI’s east end is shrinking while the west end is stretching out…

Forces Moving Barrier Islands:

1. On a Daily Basis – longshore currents and windLongshore current = current of water parallel to

shoreline generated by waves hitting the beach at an angle and flowing back out at an angle

Littoral drift = the movement (or current) of sand along the shoreline carried by the longshore current

Movement of sand and water in the Gulf of Mexico is from East to West (FL to TX) due to typical wind patterns. Now this isn’t everyday, water and sand flow depend greatly on wind direction…

So, DI is traveling westward

2. Catastrophic Forces – HurricanesThe largest storms on Earth can make or break an island…

Hurricane Camille – 1969Hurricane Frederic – 1979

(no bridge until 1982)Hurricane Elena – 1985Hurricane Opal – 1995

Hurricane Danny – 1997Hurricane Georges – 1998

Tropical Storm = winds 39-74 mph (distinct CCW) -It is now namedHurricane = winds 74+

Dauphin Island through the years

Pre Camille 1952

Post Camille 1996

Ship Island, MS

Ship Island after Georges 1998

The Georges cut on the right spans a little more than a mile.Depending on tide, the depth at this cut may be as shallow as only 4-5 ft deep.Overwash zone = area where storm waves wash over beach between or around dunes ***

SAND CYCLE-Sandy beach = the area from the edge of the dunes to about 30 ft underwater.-“Recreational Beach” – the sand above the water (according to Orrin Pilkey). *But all of the sandy beach to 30 ft underwater interacts and is in almost constant motionThe Sand Sharing System involves sand being moved between the continental shelf and the island. -storm waves pull sand away from dunes to beachfront or farther out to continental shelf -regular waves return sand to beach, sea breezes blow sand from beach up to dunesWhat one area of the island loses another area gains along the length of the barrier. Sand travels by Littoral Drift and the Longshore Current, sometimes from one island to another.

GLOBAL WARMING ISSUES

Greenhouse gases are increasing the world’s temperature, causing glaciers to melt, with sea level rising as a result. This has a major impact on Barrier Islands and coastlines.

Approaches to Beach Erosion1. Hard Stabilization – putting hard structures on the beach to

slow erosionGroins – rocks put on the beach perpendicular to the

shorelineJetties – rocks put on either side of an inlet or boat

channel, parallel to the channel and perpendicular to the shoreline.Groins and Jetties are not perfect solutions, upstream may work okay in trapping sand, but on the downstream beach it can cause erosion to happen even faster.

Seawalls – are built parallel to the shoreline and reflect waves at an angle downward. This is the most drastic action taken – rigid masses of rock or concrete, considered irreversible because beach in front of wall will disappear, eventually another wall will have to be built behind it when the first one collapses and is left as rubble in the surf – unsightly and hazardous! Vital processes of sand sharing, overwash sedimentaion, and island migration are brought to an abrupt halt!

2. Beach Replenishment – putting sand back on the recreational beach.-This has a short life span since the sand on the recreational beach will be taken back out to the continental shelf becoming part of the “underwater beach system”-$$$ Expensive process-Continuous process

3. Moving – move houses back as the shoreline approaches!

NO EASY ANSWERS!!!

Values of Barrier Islands-It is a barrier to the mainland against storms, erosion, etc.-Nesting sites for many shore birds and turtles-Recreational

C.C. Lockwood

Pressures on Barrier Islands-Development – destruction of dunes, erosion, severe hurricane damage and destruction of nesting grounds for shorebirds and sea turtles-Coastal Population growth – some positives – economic growth, jobs, tax profits; negatives – less habitat for wildlife, less nesting ground, decreased water quality, more trash, nonpoint source pollution, etc.

I.A. Mendelssohn

MANAGEMENT EFFORTS-Coastal Barrier Resources Act of 1982

-designed to discourage overdevelopment of coastal barriers along the Atlantic and Gulf Coasts

-the PURPOSE –1. minimize loss of human life

2. reduce wasteful expenditure of federal revenues

3. reduce damage to fish and wildlife habitat and other natural resources-This act states that federal flood insurance and federal assistance for construction of sewer systems, highways, water supply systems, airports, bridges, and jetties will not be provided for coastal barrier islands.

1. Sand moving through the system follows the natural path along the beach to the edge of the ebb tidal delta, destined for Dauphin Island beaches.

The sand trapThe movement of sand has been interrupted by the deepened shipping channel that serves Mobile Bay. Because of that, some scientists say parts of Dauphin Island are suffering from chronic erosion

2

2. But the trip is cut short when the sand falls into the man-made shipping channel, trapped in water too deep for the wave-driven system to reach it.