Shorelines Chapter 20

The shoreline: A dynamic interface The shoreline is a dynamic interface (common boundary) between air, land, and the ocean The shoreline is constantly being modified by waves Today the coastal zone is experiencing intense human activity

The Outline of Cap Cod Results from the Interplay of Modern Shoreline Processes & Pleistocene Deposition on Soft Sediments Martha’s Vineyard Nantucket

Unlike the East and Gulf Coasts, the Pacific Coast Erodes Very Slowly Because it is Composed of Resistant Rock, Not Sediment

The coastal zone Clarification of terms used to describe the land-sea boundary Shoreline is the line that marks the contact between land and sea The shore is the area that extends between the lowest tide level and the highest elevation on land that is affected by storms

The coastal zone Clarification of terms used to describe the land-sea boundary The coast extends inland from the shore as far as ocean-related features are found Coastline marks the coast’s seaward edge

The coastal zone Shore is divided into the Foreshore – area exposed at low tide Backshore – landward of the high-tide shoreline The nearshore zone lies between the low tide shoreline and the point where waves break at low tide Seaward of the nearshore is the offshore zone

The coastal zone Beach – an accumulation of sediment found along the landward margin of the ocean or a lake The relatively flat platform composed of sand and marked by a change in slope at the seaward edge is a berm Beach face is the wet sloping surface that extends from the berm to the shoreline

The coastal zone consists of several parts The coastal zone consists of several parts. The beach is an accumulation of sediment on the landward margin of the ocean or of a lake. It can be thought of as material in transit along the shore.

Waves Wind-generated waves provide most of the energy that shapes and modifies shorelines Characteristics of Waves Waves derive their energy and motion from the wind

Waves Characteristics of Waves Parts of a wave Measurements of a wave Crest – top of the wave Trough – low area between waves Measurements of a wave Wave height – the distance between a trough and a crest Wavelength – the horizontal distance between crests Wave period – the time interval between the passage of two successive crests

Wave Characteristics Include Height, Length, & Period Wave Characteristics Include Height, Length, & Period. Note That Negligible Water Movement Occurs Below ½ of a Given Wavelength

Waves Characteristics of Waves Height, length, and period of a wave depend on Wind speed Length of time wind has blown Fetch – the distance that the wind has traveled across open water

Waves Types of waves Waves of oscillation Wave energy moves forward, not the water itself Occur in the open sea in deep water

Movement in a Wave of Oscillation The wave energy advances, but the water itself and anything floating on it, moves very slightly from its original position

Waves Types of waves Wave of translation Begins to form in shallower water when the water-depth is about one-half the wavelength and the wave begins to “feel bottom” As the speed and length of the wave diminish, the wave grows higher The steep wave front collapses and the wave breaks along the shore Turbulent water advances up the shore and forms surf

Movement in a Wave of Translation Both the Wave Form & its Energy Advance. As the Wave Form “Feels Bottom”, it Grows Higher and Finally Breaks onto the Shore

Wave erosion Breaking waves exert a great force Wave erosion is caused by Wave impact and pressure Abrasion by rock fragments

Waves Breaking Against the Shore Can Be Powerful and The Erosive Work that They Accomplish Is Great

Undercutting of a Cliff on the Oregon Coast by the Erosive Power of Breaking Waves

Abrasion can also be intense in the surf zone, as these rounded boulders along the California coast demonstrate.

Sandstone cliff undercut by wave erosion in British Columbia, Canada

Sand movement on the beach Movement perpendicular to the shoreline Waves seldom approach the shore straight on, but rather at an angle When waves reach shallow water with a smoothly sloping bottom they are bent and tend to become parallel to the shore

Wave refraction and longshore transport Bending of a wave Causes waves to arrive nearly parallel to the shore

Wave Refraction Causes Waves to Bend Around the End of a Beach at Stinson Beach, California

Wave refraction and longshore transport Consequences of wave refraction Wave energy is concentrated against the sides and ends of headlands Wave energy is spread out in bays and wave attack is weakened Over time, wave erosion straightens an irregular shoreline

Because of Wave Refraction, the Greatest Erosive Power is Concentrated on Headlands. In Bays, the Force of Waves is Much Weaker

Wave refraction and longshore transport Moving sand along the beach Waves that reach the shoreline at an angle cause the sediment to move along a beach in a zigzag pattern called beach drift Oblique waves also produce longshore currents Currents in the surf zone Flow parallel to the coast Easily move fine suspended sand and roll larger sand and gravel along the bottom

Beach Drift and Longshore Currents Move Sediment Obliquely Up the Beach and Directly Down the Beach. These Actions Transport Material Along the Beach.

Shoreline features Features vary depending on several factors including The rocks along the shore Currents Wave intensity Whether the coast is stable, sinking, or rising

Shoreline features Features caused by wave erosion Wave-cut cliffs Wave-cut platform Features associated with headlands Sea arch Sea stack

An Elevated Wave-Cut Platform on the California Coast An Elevated Wave-Cut Platform on the California Coast. A New Platform is Forming at the Base of the Cliff

A Sea Arch & Sea Stack Produced by Concentrated Wave Attack on a Headland

A Sea Arch & Sea Stack on Mexico’s Baja Peninsula Produced by Concentrated Wave Attack on a Headland

Shoreline features Features related to beach drift and longshore currents Spits Elongated ridges of sand extending from the land into the mouth of an adjacent bay Often the end of a spit hooks landward in response to wave-generated currents

Shoreline features Features related to beach drift and longshore currents Baymouth bar – a sand bar that completely crosses a bay Tombolo – a ridge of sand that connects an island to the mainland or another island

Spit & Baymouth Bar on Coast of Martha’s Vineyard

Tombolo – a Ridge of Sand that Connects an Island to the Mainland

Shoreline features Barrier islands Formed mainly along the Atlantic and Gulf coasts Low ridges of sand that parallel the coast 3 to 30 kilometers offshore Probably form in several ways Modified Spits Storm Generated Sand Ridges Submerged Sand Dunes from Glacial Lowstand

Barrier Islands Are Present Along Most of the Texas Gulf Coast

Barrier islands are also present along much of the Atlantic Coast

Shoreline features If the shoreline remains stable, the result of shoreline erosion and deposition is to eventually produce a straighter coastline

How a Stable Coastline is Straightened

Shoreline erosion problems Shoreline erosion is influenced by several local factors including Proximity to sediment-laden rivers Degree of tectonic activity Topography and composition of the land Prevailing wind and weather patterns Configuration of the coastline and nearshore areas

Wave erosion caused by strong storms forced abandonment of this highly developed shoreline on Long Island, New York. Coastal areas are dynamic places that can change rapidly in response to natural forces.

Shoreline erosion problems Three basic responses to erosion problems Building structures Jetties Usually built in pairs to develop and maintain harbors Extend into the ocean at the entrances to rivers and harbors

Jetties Protect the Harbor from Erosion or Sediment Deposition, But They Create Problems of their Own

Jetties at the Harbor Mouth Protect the Harbor from Erosion Jetties at the Harbor Mouth Protect the Harbor from Erosion. In this Case, They also Create a Shelter for Boats Note Sediment Buildup

Shoreline erosion problems Three basic responses to erosion problems Building structures Groins Built to maintain or widen beaches Constructed at a right angle to the beach to trap sand

Groins Are Built Perpendicular to the Beach to Slow the Erosion of Sand by Longshore Currents. Unfortunately they Often Cause Additional Current Erosion on their Leeward Sides

Shoreline erosion problems Three basic responses to erosion problems Building structures Breakwater Barrier built offshore and parallel to the coast Protects boats from the force of large breaking waves

Breakwater at Santa Monica, California Breakwater at Santa Monica, California. It Provides a Sheltered Harbor for Boats, But Also Traps Sand from Longshore Currents, Creating Beach Growth

Shoreline erosion problems Three basic responses to erosion problems Building structures Seawall Barrier parallel to shore and close to the beach to protect property Stops waves form reaching the beach areas behind the wall Often the building of structures is not an effective means of protection

Sea Wall Built to Protect Homes from Storm Waves Along the Beach Sea Wall Built to Protect Homes from Storm Waves Along the Beach. After its Construction, the Beach Narrowed Dramatically, Destroying the Very Tourist Attraction That Brought People to this Location

Shoreline erosion problems Three basic responses to erosion problems Beach nourishment The addition of large quantities of sand to the beach system Only an economically viable long-range solution for a few areas with dense population, abundant sand & low wave energy Abandonment and relocation of buildings away from the beach

The Results of Beach Nourishment in Miami The Results of Beach Nourishment in Miami. As a Short Term Solution, Nourishment Clearly Works, But Must Be Repeated Frequently

Shoreline erosion problems Contrasting the Atlantic and Pacific Coasts Shoreline erosion problems are different along the opposite coasts Atlantic and Gulf coast Broad, gently sloping coastal plains Tectonically quiet regions

Shoreline erosion problems Contrasting the Atlantic and Pacific Coasts Because the Coastlines Dip Gently & Consist of Loose Sediment, any Shift in Topography or Sea Level Causes Large Changes An Example is the Vanishing Wetlands of Louisiana

Coastal Wetlands Provide Nesting for Birds & Homes for Wildlife and Fish. They Depend on Sediment & Fresh Water From the Mississippi & Storm Protection by Barrier Islands. When these Factors Change, Wetlands are Lost

Shoreline erosion problems Contrasting the Atlantic and Pacific Coasts Atlantic and Gulf coast Development occurs mainly on the barrier islands (also called barrier beaches or coastal barriers) Barrier islands face the open ocean They receive the full force of storms

Shoreline erosion problems Contrasting the Atlantic and Pacific Coasts Hurricanes also have a major impact on the eastern & southern shores of the U.S.

Hurricanes Are Large Tropical Storms that Form During the Summer and Fall Months. They Are Classified on the Basis of Wind Speed & Barametric Pressure

Shoreline erosion problems Hurricanes Can Cause Severe to Catastrophic Damage to Low Lying Coasts by Three Major Factors Storm Surge Wind Damage Inland Flooding

Storm Surge from Hurricane Camille (a Category 5 Storm) Completely Destroyed this Beach Apartment Complex in Mississippi

Storm Surge & Wind Do Extensive Damage Near the Coast; Flooding Is Often the Major Problem Inland

With Hundreds of Miles of Exposed Coastline, Texas Is A Target For Hurricanes From June to November

Paths of Category 3 and Stronger Hurricanes that Struck the Texas Coast A = 1900-1949, B = 1950-1993

Major Hurricanes Along the Texas Gulf Coast

Homes Closest to the Shoreline Were Reduced to Rubble

Lucas Terrace Apts. Along the Beach Lucas Terrace Apts. Along the Beach. Over 50 People Died Here, But 22 People & 1 Cat Survived

Over 6,000 Dead Were Found & Required Burial After The Storm

Many Bodies Were Dumped at Sea, But Washed Up Again on The Beach

Shoreline erosion problems Contrasting the Atlantic and Pacific Coasts Pacific Coast Relatively narrow beaches backed by steep cliffs and mountain ranges A major problem is a significant narrowing of many beaches Shoreline erosion varies considerably from one year to the next largely because of the sporadic occurrence of storms

Emergent and submergent coasts The Atlantic & Gulf Coasts Are Gently Dipping, Primarily Submergent, Coastlines The Pacific Coast Is a Steeply Dipping, Emergent Coastline

The Effects of Sea Level Change on Submergent & Emergent Coastlines

Emergent and submergent coasts Sea Level Fluctuations Occur Naturally For Various Reasons Glacial Waxing & Waning Changes in Ocean Basin Size Tectonics Can Also Be Impacted By Manmade Global Warming

Sea Level Fluctuations Are Natural Events, But They May Be Enhanced by Global Warming, which Produces Glacial Meting & Ocean Water Heating & Expansion

Emergent and submergent coasts Emergent coasts Develop because of uplift of an area or a drop in sea level Features of an emergent coast Wave-cut cliffs Wave-cut platforms

An Elevated Wave-Cut Platform on the California Coast An Elevated Wave-Cut Platform on the California Coast. A New Platform is Forming at the Base of the Cliff

Emergent and submergent coasts Caused by subsidence of land adjacent to the sea or a rise in sea level Features of a submergent coast Highly irregular shoreline Estuaries – drowned river mouths

Chesapeake Bay is a Good Example of a Submergent Coastline, with Drowned Rivers Forming Estuaries Along the Coast

Tides Daily changes in the elevation of the ocean surface Causes of tides Tidal bulges are caused by the gravitational forces of the Moon, and to a lesser extent the Sun

Tidal Bulges Are Caused by the Gravitational Effects of Moon & Sun on the Uniformly Thick Body of Ocean Water

Tides Monthly tidal cycle (spring and neap tides) Spring tides Occur during new and full moons Gravitational forces of the Moon and Sun are added together Especially high and low tides Large daily tidal range

Tides Monthly tidal cycle (spring and neap tides) Neap tides Occur during the first and third quarters of the moon Gravitational forces of the Moon and Sun are offset Daily tidal range is least

The Combined Gravitational Effects of Moon & Sun Can Produce Particularly High Tides (Spring Tides) or Particularly Low Tides (Neap Tides)

Tides Other factors that influence tides Shape of the coastline Configuration of the ocean basin

Tidal Fluctuations in the Bay of Fundy, Nova Scotia, Reach 56 Feet Tidal Fluctuations in the Bay of Fundy, Nova Scotia, Reach 56 Feet. Boats Are Left High and Dry at Low Tide and Extensive Tidal Flats Are Created

Tides Tidal patterns Diurnal tidal pattern = a single high tide and a single low tide each tidal day Semidiurnal tidal pattern = 2 high tides and 2 low tides each tidal day

Tides Tidal patterns Tidal currents Mixed tidal pattern = large inequality in high water heights, low water heights or both Tidal currents Horizontal flow of water accompanying the rise and fall of the tide

Tidal patterns & their occurrence along portions of the coastlines of North & South America. Diurnal tidal patterns shown one high & one low tide each day. Semidiurnal patterns show two highs & two lows of approximately equal height each day. Mixed tidal pattern shows two highs & two lows of unequal height.

Tides Tidal currents Types of tidal currents Flood current – advances into the coastal zone as the tide rises Ebb current – seaward-moving water as the tide falls Areas affected by the tidal currents are called tidal flats Occasionally form tidal deltas

A Flood Tidal Delta Created by Tides Moving into the Bay

Tides Tides and Earth’s rotation Tidal friction against the ocean floor acts as a weak brake that is steadily slowing Earth’s rotation The day is increasing by 0.002 seconds per century This small effect becomes very large over millions of years Length of each day must have been shorter in the geologic past