Costal Erosion: “Approximately 25 percent of homes and other structures within 500 feet of the U.S. coastline and the shorelines of the Great Lakes will fall victim to the effects of erosion within the next 60 years, according to a study released by the Federal Emergency Management Agency (FEMA).” http://www.fema.gov/nwz00/erosion.shtm

Coastal erosion and sediment transport

Coastal Evolution

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

Wave and Water Movement

Erosion and movement by WAVES  Wave base: point at which wave action becomes negligible on the sea floor   /2 = wave base   It is the effective lower limit of wave action and erosion

 So, surf is a powerful erosive agent

Waves

 Its energy is expressed by turbulence…  …violent movements of parcels of water which are able to move and suspend sediment

Oscillatory and Translational Motion

 Sediment movement - redistribution by WAVES

Longshore transport:

waves hitting coast obliquely, depends on prevailing winds  waves have both a perpendicular and parallel component relative to the coast  the parallel component represents longshore drift; transport of sediment along the beach

Baymouth Bar - Russian River, CA

Wave Refraction

  Waves change path when they reach shallow water Wave energy is concentrated on headlands and spread out in bays

Wave Refraction off Cape Cod

Tombolo - Santa Cruz, CA

Rips

 When waves break parallel to a beach, rips occur

Rips, Lake Superior

Rip currents: water that flows straight out to sea from the surf zone. Travel at the surface and die out at depth.

Carry fine grained particles out of surf zone to deeper water.

Winter Summer

Sediment movement redistribution by WAVES

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Longshore transport or Beach drift:

oblique waves which move sediment along the beach  Storm transport: very rapid erosion, much larger than normal, which tends to narrow beaches and move sediment offshore

Sea Level Change

1. Glaciers  Glaciers Lower Sea Level Slowly  They Melt quickly – sea level rises 2. Mid Ocean Ridges  Mid Ocean Ridge raises sea level if large and active  Lowers sea level if activity slows becomes smaller

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Sea level rise

linked to global warming, either natural or human-induced  over next century, we anticipate sea level to rise by 30-100 cm  along the east coast where the coastal plain rises gently, this rise translates to a shoreline setback of 100-400 feet.

Shoreline shift

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

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

Effects of the Pleistocene

   Sea level has risen at least 100 meters in the last 18,000 years Most coastlines globally are submergent Primary coastlines are very common

-4 -6 -8 -10 0 -2 4 2

Climate and Greenhouse Gases during the last 650 Kyrs

(From Barnola, 2006)

EPICA Dome C Indermuehle et al (submitted) EPICA project members (2004) Spahni et al (submitted) 1700 ppbv Vostok Pépin et al ( 2001) Petit et al (1999) Delmotte et al (2004) 375 ppmv 300 280 260 240 220 200 180 800 700 600 500 400 700000 600000 500000 400000 300000

Age (yr BP)

200000 100000 0

Nonmarine Processes - Uplift Convergent Margins

Wave-Cut Platform, California

Sediment/Sand Budget

 Dams  Groins

Artificial structures and beach erosion

 Breakwaters  Seawalls  Inlets, dredging, and jetties

Human intervention

Santa Cruz Harbor, California Direction of current

Our efforts to control nature can be a expensive struggle Santa Barbara Harbor in 1931

Source:

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

Santa Barbara Harbor in 1977

Source:

John S. Shelton

Groins

 Designed to trap sediment…  but they instead progressively starve beach of sediment in the direction of longshore-littoral current From Dean (1999)

Groins 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

 Designed to allow sediment to settle  Smaller waves behind, less sediment is transported

Breakwaters

From Dean (1999)

Seawalls

 Various types: revetments, rip rap, concrete walls, piles of rubble, etc.

Loss of beach  Nearly 100% of the time they damage or destroy a beach which is eroding to begin with From Dean (1999) Flooding of beach Destruction of seawall

Beach-Protection Structures Riprap

Source:

Jack Dermid /Photo Researchers, Inc.

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

Source:

Martin Miller

The consequences of NO BEACH

 Hugely costly measures are required to restore beach, e.g., pumping sand, and this is only a temporary solution  Also, houses are now closer to the water and more vulnerable  On either end of the seawall, the beach erodes and is displaced toward the land naturally…so the walled part of the beach is exposed to the ocean and vulnerable to storms

From Dean (1999) Sand bypassing to replenish beach on down-current side of jetty Replenishing sand the old fashioned way

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Inlets

Inlets are created naturally by large storms…they are breaks in a barrier beach  Over time, the inlet will heal naturally  The inlet can heal rapidly, or can linger for years or decades From Dean (1999)

Raccoon Island, Louisiana

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Closing of an inlet

The healing time of an inlet depends on:  -amount of sand transported by littoral drift  -amount of water flowing through with tide Thus, if the tidal flow is low and littoral currents carry abundant sand, the inlet can heal quickly  Flood tide deltas represent a large amount of sand carried inside the inlet’s mouth  This is a natural sink for sand, and the sand remains relatively undisturbed



Inlets and dredging

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

 The dredged sand is often dumped in deep water offshore…the sand is therefore lost  If so, inlet needs to be dredged periodically  If sand were dumped on the beach down current, beach might be stabilized…but not an easy task due to the high-energy surf zone

Inlets and jetties

 Another way to keep inlets open artificially is by building jetties  The jetties serve to “jet” out sand into deeper water…again, a loss  The longer the jetty, the worse the erosion down-current

Jetty in Miami Beach, Florida

Source:

Townsend P. Dickinson

Artificial modification of inlets

 One problem is political…stakeholders may have diametrically opposite viewpoints…  …e.g., tourism operators who need a beach vs. fishing fleets who want easy access to the ocean through the inlet  One solution is sand-bypassing, but this is expensive and inefficient compared to Mother Nature