Costal Erosion:Costal Erosion:

““Approximately Approximately 25 percent25 percent of homes and of homes and other structures within 500 feet of the U.S. other structures within 500 feet of the U.S. coastline and the shorelines of the Great coastline and the shorelines of the Great Lakes will fall victim to the effects of Lakes will fall victim to the effects of erosion within the next 60 years, erosion within the next 60 years, according to a study released by the according to a study released by the Federal Emergency Management Agency Federal Emergency Management Agency (FEMA).”(FEMA).”

http://www.fema.gov/nwz00/erosion.shtm

Coastal erosion and sediment transportCoastal erosion and sediment transport

Coastal EvolutionCoastal Evolution

Balance between:Balance between:•Wave action (energy, geology) Wave action (energy, geology) •Sediment supplySediment supply•Sea levelSea level

Wave and Water MovementWave and Water Movement

Erosion and movement by Erosion and movement by WAVESWAVES

Wave base:Wave base: point point at which wave at which wave action becomes action becomes negligible on the negligible on the sea floorsea floor

It is the effective It is the effective lower limit of wave lower limit of wave action and erosionaction and erosion

/2 = wave base/2 = wave base

WavesWaves So, surf is a powerful So, surf is a powerful

erosive agenterosive agent

Its energy is expressed Its energy is expressed by turbulence…by turbulence…

……violent movements of violent movements of parcels of water which parcels of water which are able to move and are able to move and suspend sedimentsuspend sediment

Oscillatory and Translational MotionOscillatory and Translational Motion

Sediment movement - redistribution by Sediment movement - redistribution by WAVESWAVES

Longshore transport:Longshore transport: waves hitting coast waves hitting coast obliquely, depends on obliquely, depends on prevailing windsprevailing winds

waves have both a waves have both a perpendicular and parallel perpendicular and parallel component relative to the component relative to the coastcoast

the parallel component the parallel component represents represents longshore driftlongshore drift; ; transport of sediment along transport of sediment along the beachthe beach

Baymouth Bar - Russian River, CABaymouth Bar - Russian River, CA

Wave RefractionWave Refraction

Waves change path when they reach Waves change path when they reach shallow watershallow water

Wave energy is concentrated on Wave energy is concentrated on headlands and spread out in baysheadlands and spread out in bays

Wave Refraction off Cape CodWave Refraction off Cape Cod

Tombolo - Santa Cruz, CATombolo - Santa Cruz, CA

RipsRips

When waves break parallel to a When waves break parallel to a beach, rips occurbeach, rips occur

Rips, Lake SuperiorRips, Lake Superior

Rip currents: water that flows straight out to sea from thesurf zone. Travel at the surface and die out at depth.Carry fine grained particles out of surf zone to deeper water.

Winter

Summer

Longshore transport or Beach drift:Longshore transport or Beach drift: oblique waves which move sediment along oblique waves which move sediment along the beachthe beach

Storm transport:Storm transport: very rapid erosion, very rapid erosion, much larger than normal, which tends to much larger than normal, which tends to narrow beaches and move sediment narrow beaches and move sediment offshoreoffshore

Sediment movement - Sediment movement - redistribution by WAVESredistribution by WAVES

Sea Level ChangeSea Level Change1. Glaciers1. Glaciers Glaciers Lower Sea Level SlowlyGlaciers Lower Sea Level Slowly They Melt quickly – sea level risesThey Melt quickly – sea level rises

2. Mid Ocean Ridges2. Mid Ocean Ridges Mid Ocean Ridge raises sea level if Mid Ocean Ridge raises sea level if

large and activelarge and active Lowers sea level if activity slows - Lowers sea level if activity slows -

becomes smallerbecomes smaller

Sea level riseSea level rise linked to global warming, either natural linked to global warming, either natural

or human-inducedor human-induced

over next century, we anticipate sea over next century, we anticipate sea level to rise by level to rise by 30-100 cm30-100 cm

along the east coast where the coastal along the east coast where the coastal plain rises gently, this rise translates to plain rises gently, this rise translates to a shoreline setback of a shoreline setback of 100-400 feet. 100-400 feet.

Shoreline shiftShoreline shift

Impact of sea level rise is greatest in low coastal areasImpact of sea level rise is greatest in low coastal areas

http://pubs.usgs.gov/circ/c1075/contents.html

Effects of the PleistoceneEffects of the Pleistocene

Sea level has risen at least 100 Sea level has risen at least 100 meters in the last 18,000 yearsmeters in the last 18,000 years

Most coastlines globally are Most coastlines globally are submergentsubmergent

Primary coastlines are very commonPrimary coastlines are very common

700000 600000 500000 400000 300000 200000 100000 0Age (yr BP)

400

500

600

700

800

CH

4 (

pp

bv

)

-10

-8

-6

-4

-2

0

2

4

(

°C)

180

200

220

240

260

280

300

CO

2 (p

pm

v)

Climate and Greenhouse Gases during the last 650 Kyrs

375 ppm v

1700 ppbv

EPIC A D om e CInderm uehle et a l (subm itted)EPIC A pro ject m em bers (2004)Spahni et a l (subm itted)

VostokPépin e t a l ( 2001)Petit et a l (1999)Delm otte et a l (2004)

(From Barnola, 2006)

Nonmarine Processes - UpliftNonmarine Processes - Uplift

Convergent Margins

Wave-Cut Platform, CaliforniaWave-Cut Platform, California

Sediment/Sand Budget

Artificial structures and Artificial structures and beach erosionbeach erosion

DamsDams

GroinsGroins

BreakwatersBreakwaters

SeawallsSeawalls

Inlets, dredging, and jettiesInlets, dredging, and jetties

Human intervention

Santa Cruz Harbor, California

Direction of current

Our efforts to control nature can be a expensive struggleOur efforts to control nature can be a expensive struggle

Santa Barbara Harbor in 1931Santa Barbara Harbor in 1931

Source: Fairchild air photos 0-139 & E-5780, UCLA Department of Geography Aerial Photo Archives

Santa Barbara Harbor in 1977Santa Barbara Harbor in 1977

Source: John S. Shelton

GroinsGroins

Designed to trap Designed to trap sediment…sediment…

but they instead but they instead progressively progressively starvestarve beach of sediment in beach of sediment in the direction of the direction of longshore-littoral longshore-littoral currentcurrent

From Dean (1999)

Groins off Cape May, New JerseyGroins off Cape May, New Jersey

Source: John S. Shelton

Further effects of groins

Rockaway beach, New York

Eroded beach…compare buildings on either side of groin

Highly eroded beach

BreakwatersBreakwaters

Designed to allow Designed to allow sediment to settlesediment to settle

Smaller waves Smaller waves behind, less behind, less sediment is sediment is transportedtransported

From Dean (1999)

SeawallsSeawalls Various types: Various types:

revetments, rip-rap, revetments, rip-rap, concrete walls, piles concrete walls, piles of rubble, etc.of rubble, etc.

Nearly 100% of the Nearly 100% of the time they time they damagedamage or or destroydestroy a beach a beach which is eroding to which is eroding to begin withbegin with

From Dean (1999)

Loss of beach

Flooding of beach

Destruction of seawall

Beach-Protection Structures Beach-Protection Structures - - Riprap Riprap

Source: Jack Dermid /Photo Researchers, Inc.

Beach-Protection Structures Beach-Protection Structures ––Seawall Along the Gulf Coast of LouisianaSeawall Along the Gulf Coast of Louisiana

Source: Martin Miller

The consequences of NO BEACHThe consequences of NO BEACH

Hugely costly measures are required to restore beach, Hugely costly measures are required to restore beach, e.g., pumping sand, and this is only a temporary solutione.g., pumping sand, and this is only a temporary solution

Also, houses are now closer to the water and more Also, houses are now closer to the water and more vulnerablevulnerable

On either end of the seawall, the beach erodes and is On either end of the seawall, the beach erodes and is displaced toward the land naturally…so the walled part of displaced toward the land naturally…so the walled part of the beach is exposed to the ocean and vulnerable to the beach is exposed to the ocean and vulnerable to stormsstorms

Sand bypassing to replenish beach on down-current side of jetty

From Dean (1999)

Replenishing sand the old-fashioned way

InletsInlets Inlets are created naturally Inlets are created naturally

by large storms…they are by large storms…they are breaksbreaks in a barrier beach in a barrier beach

Over time, the inlet will Over time, the inlet will heal heal naturallynaturally

The inlet can heal rapidly, The inlet can heal rapidly, or can linger for years or or can linger for years or decadesdecades

From Dean (1999)

Raccoon Island, Louisiana

Closing of an inletClosing of an inlet The healing time of an inlet depends on:The healing time of an inlet depends on:

-amount of sand transported by littoral drift-amount of sand transported by littoral drift -amount of water flowing through with tide-amount of water flowing through with tide

Thus, if the tidal flow is low and littoral currents carry Thus, if the tidal flow is low and littoral currents carry abundant sand, the inlet can heal quicklyabundant sand, the inlet can heal quickly

Flood tide deltas represent a large amount of sand carried Flood tide deltas represent a large amount of sand carried inside the inlet’s mouthinside the inlet’s mouth

This is a natural This is a natural sinksink for sand, and the sand remains for sand, and the sand remains relatively undisturbedrelatively undisturbed

Inlets and dredgingInlets and dredging Inlets are kept Inlets are kept

open artificially open artificially sometimes, e.g., sometimes, e.g., for a harbour, for a harbour, fishing fleet, etc.fishing fleet, etc.

If so, inlet needs to If so, inlet needs to be be dredgeddredged periodicallyperiodically

The dredged sand is often The dredged sand is often dumpeddumped in deep water in deep water offshore…the sand is offshore…the sand is therefore therefore lostlost

If sand were dumped on If sand were dumped on the beach down-current, the beach down-current, beach might be beach might be stabilized…but not an stabilized…but not an easy task due to the high-easy task due to the high-energy surf zoneenergy surf zone

Inlets and jettiesInlets and jetties Another way to keep inlets open artificially is by Another way to keep inlets open artificially is by

building jettiesbuilding jetties

The jetties serve to “jet” out sand into deeper The jetties serve to “jet” out sand into deeper water…again, a losswater…again, a loss

The longer the jetty, the worse the erosion The longer the jetty, the worse the erosion down-currentdown-current

Jetty in Miami Beach, FloridaJetty in Miami Beach, Florida

Source: Townsend P. Dickinson

Artificial modification of inletsArtificial modification of inlets One problem is political…stakeholders may have One problem is political…stakeholders may have

diametrically opposite viewpoints…diametrically opposite viewpoints…

……e.g., tourism operators who need a beach vs. e.g., tourism operators who need a beach vs. fishing fleets who want easy access to the ocean fishing fleets who want easy access to the ocean through the inletthrough the inlet

One solution is sand-bypassing, but this is One solution is sand-bypassing, but this is expensive and inefficient compared to Mother expensive and inefficient compared to Mother NatureNature