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
Waves



So, surf is a powerful
erosive agent
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.
Summer
Winter
Sediment movement redistribution by WAVES


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
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
Climate and Greenhouse Gases during the last 650 Kyrs
1700 ppbv
(From Barnola, 2006)
375 ppmv
Vostok
EPICA Dome C
Pépin et al ( 2001)
Petit et al (1999)
Delmotte et al (2004)
Indermuehle et al (submitted)
EPICA project members (2004)
Spahni et al (submitted)
300
260
240
4
220
200
0
180
-2
-4
800
-6
700
-8
600
-10
500
400
700000
600000
500000
400000
300000
Age (yr BP)
200000
100000
0
CH4 (ppbv)
 (°C)
2
CO2 (ppmv)
280
Nonmarine Processes - Uplift
Convergent Margins
Wave-Cut Platform, California
Sediment/Sand Budget
Artificial structures and
beach erosion

Dams

Groins

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
Breakwaters


Designed to allow
sediment to settle
Smaller waves
behind, less
sediment is
transported
From Dean (1999)
Seawalls


Various types:
revetments, riprap, concrete
walls, piles of
rubble, etc.
Nearly 100% of
the time they
damage or
destroy a beach
which is eroding to
From Dean (1999)
begin with
Loss of beach
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 oldfashioned way
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
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.
If so, inlet needs to
be dredged
periodically


The dredged sand is
often dumped in
deep water
offshore…the sand is
therefore lost
If sand were dumped
on the beach downcurrent, 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