Chapter 7, Spr

Chapter 7

Coastal Zones and Processes

POPULATION-DENSITY IN THE COASTAL/NONCOASTAL COUNTIES OF THE U.S.

Nature of the Coastline • Factors that impact coastlines: – Tectonic setting – Materials present at the shore – Energy of water striking the coast

• Active margin – associated with active plate tectonics – Often cliffs found near active margins

• Passive margin – far from active plate tectonics – Often a broad shelf, beaches and with sandy offshore beaches

ACTIVE AND PASSIVE MARGINS ACTIVE MARGINS HAVE EARTHQUAKES, PASSIVES HAVE NONE OR FEW

ACTIVE AND PASSIVE MARGINS ACTIVE MARGINS HAVE LITTLE OR NO CONTINENTAL SHELF (IN LIGHT BLUE COLOR, BY COASTLINES), PASSIVES HAVE A LOT

Nature of the Coastline •

Beach – a gently sloping surface washed over by the waves and covered by sediment – Sediments are carried by longshore currents and placed on the beach by littoral drift

•

Waves – generated by wind are small undulations on the ocean surface – – – –

Waves move along an ocean and water molecules move only in orbits Waves disappear below wave base Orbits are larger nearest the surface and become smaller with depth Once a wave ‘feels’ the bottom the orbits are interfered with and breaking occurs – Erosion of solid rock along a coast takes place because of milling

•

Ocean surfaces appear to move in and out relative to the shore because of tides – Tides occur because of the gravitational pull on the oceans by the moon and sun

A CALIFORNIA COAST MARKED BY CLIFFS, TYPICAL OF ACTIVE MARGINS

Figures 7.3 A. A TYPICAL BEACH-PROFILE (A CLIFF MAY BE PRESENT AT THE BACK OF A BEACH, INSTEAD OF SAND DUNES)

B. A HAWAIIAN BEACH

WHERE THE SEDIMENT IS COARSER, AS ON THIS ALASKA SHORE, THE BEACH MAY BE STEEPER

Nature of the Coastline •

Beach – a gently sloping surface washed over by the waves and covered by sediment – Sediments are carried by longshore currents and placed on the beach by littoral drift

•

Waves – generated by wind are small undulations on the ocean surface – – – –

Waves move along an ocean and water molecules move only in orbits Waves disappear below wave base Orbits are larger nearest the surface and become smaller with depth Once a wave ‘feels’ the bottom the orbits are interfered with and breaking occurs – Erosion of solid rock along a coast takes place because of milling

•

Ocean surfaces appear to move in and out relative to the shore because of tides – Tides occur because of the gravitational pull on the oceans by the moon and sun

A. SEA WAVES AWAY FROM A COAST LINE (VARIOUS TERMS USED TO DESCRIBE WAVES) B. SEA WAVES APPROACHING A COAST LINE

EVENLY SPACED BREAKERS ON A GENTLY SLPOPING SHORE, AUSTRALIA

BOTH PHOTOS FROM CALIFORNIA

A. SEA ARCH FORMED BY WAVE ACTION (“HOLE” IN THE ROCK, BACKGROUND) B. SEA-STACKS, FORMED BY CONTINUED EROSION AND COLLAPSE OF SEA ARCHES

A. SPRING TIDE (RESULTS IN A HIGH DEGREE OF TIDAL EXTREMES) B. NEAP TIDE (DIFFERENCE BETWEEN HIGH/LOW TIDES IS MINIMUM)

LONGSHORE CURRENTS AND THEIR EFFECT ON SAND MOVEMENT

A. WAVE-CUT PLATFORMS (FORMED WHEN LAND IS ELEVATED OR SEA LEVEL FALLS) B. WAVE-CUT PLATFORMS AT MIKHAIL POINT, ALASKA

Storms and Coastal Dynamics • Storms are caused by low pressure systems – Low pressure cause a bulge of water to form – Onshore winds, associated with storms, approach the shore with unusually high tides called a surge – The exceptional force of the wind driven waves and surge combine to erode beach faces, dunes above the beach, and sometimes cliffs behind the beach – Hurricanes are power examples of these storms

Coastal Erosion and Stabilization • A rise in sea level will cause coastal erosion and will require stabilization – Sand may be needed, from dredging, to replace sand lost to long shore currents – Breakwaters, jetties, or groins act to stabilize beaches • These artificial barriers have erosion and deposition problems associated with them also

• Projects to stabilize coastlines and beaches are required because of changes in sea level

A. RELATIVE VULNERABILTY OF COASTAL REGIONS B CYPRESS TREES DIED IN A SWAMP DUE TO FLOODING BY SALTWATER

THE SAME COASTLINE LOOKS DIFFERENT AFTER 3 DIFFERENT STORMS, N. C.

A. BERTHA, 7-16-96

B. FRAN, 9-7-96

C. BONNIE, 8-28-98

Figures 7.17

A. THE CAPE HATTERAS LIGHTHOUSE THREATENED (1996) C. THE LIGHTHOUSE HAD TO BE MOVED (COST: $11.8 MILLION)

Figure 7.17 b B. THEY DID TRY TO SAVE THE CAPE HATTERAS LIGHTHOUSE, BEFORE MOVING IT

Figure 7.18 WHEN GROINS (OR JETTIES), PERPENDICULAR TO THE SHORELINE, DISRUPT LONGSHORE CURRENTS, DEPOSITION OCCURS UP-CURRENT, EROSION BELOW (OCEAN CITY, NJ)

Figures 7.19 A. SEDIMENT IS DPOSITED BEHIND A BREAKWATER (RUNNING PARALLEL TO THE SHORELINE), EROSION OCCURS DOWN-CURRENT B. BREAKWATERS AND GROINS TOGETHER COMBAT BEACH EROSION (PRESQUE ISLE STATE PARK, PA.)

Emergent and Submergent Coastlines • Long-term sea-level changes often are associated with plate tectonics – Continental margins may uplift and produce an emergent coastline – Large rivers drain water and sediments from the continental interior and form large deltas in adjacent oceans – Loading of the crust produces submergent coastlines

• Signs of changing relative sea level – wave-cut platform will form at coastlines • The number of wave cut platforms is an indication of uplifting land or dropping sea level

– Drowned valley records changes in sea level also

CHESAPEAKE BAY IS A DROWNED VALLEY

EFFECTS OF THE SAME BUT SMALL RISE IN THE SEA LEVEL A. STEEP SHORE LINE B. GENTLE SHORE LINE

Cliff Erosion • Waves and currents act to erode beaches and eventually cliffs • Cliffs are undercut – Cliff face then slumps or slides off into the sea – Cliff faces show landward retreat of the shoreline

• Wave refraction works to erode points of land jutting out into the sea – Wave base interaction with the ocean bottom slow the progress of the wave – Wave base encounters the bottom near a jutting point before the coastline

A. ENERGY WAVES FOCUS ON A JUTTING POINT B. WAVE REFRACTION, BY A CORAL REEF, MARKED BY “WHITE” WATERS

Figure 7.21 PROJECTING ROCKS ARE SUBJECT TO EROSION; SAND DEPOSITS IN THE LOW-ENERGY RECESSED BAY (OREGON COAST)

A. RIP-RAP, A SHORE PROTECTION MEASURE B. SEAWALL, AN EXPENSIVE MEASURE

Barrier Islands • Long, low, narrow islands parallel to the coastline – Unstable environments • Easily washed over because of low relief • Sand migrates constantly with the wind • Constantly retreating landward with time

BARRIER ISLANDS, N.C.

DEVELOPED, BUT VULNERABLE, BARRIER ISLAND (S. DADE CO, FL)

Estuaries • Body of water along a coastline, open to the sea, in which tides rise and fall – Often rivers bring freshwater into a mix with sea water producing a brackish water environment

• Water circulation is limited – Estuaries do not tolerate major changes in salinity or chemistry of the water – Heavily impacted by pollution

PARTIALLY FILLED (RECLAIMED) ESTUARY, THE NETHERLANDS

Construction and Reconstruction in High-Energy Environments • Coastal areas easily damaged by major storms that frequent our oceans/seas • People have traditionally flocked to the coast to live/work • Property values are high along coastlines and at risk • Governments spend large amounts of money to maintain coastal areas

BUILDING ABOUT TO BE LOST BY CLIFF EROSION, SAN MATEO CO, CALIF.

AFTER THE 1999 HURRICANE, RODANTHE , N.C., & DUNES/DEPOSITS, IN RED

DAMAGE AFTER THE 1999 HURRICANE RODANTHE , N.C.