Geology Lecture Outline

Shorelines, Beaches, and Tsunami (Ch 20)


I. Lecture Content - Topics Covered

Introduction Where Ocean Meets Land

Classifying Coastlines Processes and Features

Erosion-Dominated Coastlines Rugged, Irregular, and Rocky

Deposition-Dominated Coastlines Straight, Regular and Sand-rich

Beaches Where Surf Meets the Sand

Estuaries and Deltas Where Rivers Meet the Sea

Reefs and Atolls Coral Gardens Beneath the Waves

Other Coastal Features From Barrier Islands to Tombolos

U.S. Shorelines West versus East Coasts

Humans Assault on Shorelines - Mother Nature Knows Best

Tsunami When Hell Breaks Loose on a Shoreline


II. Introduction

A. Coastlines are Long, Narrow Geographic Features

1. Occur wherever ocean and land meet at or near sea level


        Margins of continents


        Edges of inner-continental seaways


        Island rims


2. Typically are only up to a kilometer or two wide

        The term coastline or coastal zone has a much broader meaning than shoreline and includes many other habitats and ecosystems associated with terrestrial and marine processes


        The coastal zone includes beaches, wave-cut terraces, offshore bars, bluffs, back beach dunes, deltas, estuaries, lagoons, swamps, and marshes


        A shoreline is more limited to the beach, or littoral zone


3. World Ocean is bounded by roughly 440,000 kilometers (273,000 miles) of


        Equals 1 1/2 times the distance to the Moon


B. Shorelines are the Most Dynamic Regions on Earth

1. Comprises a special geography where atmosphere, land and ocean meet at

a triple interface


2. A geographic region affected by many natural agents







        Sea level fluctuation


        Biologic forces



        Tectonics- Uplift/Subsidence

        Human interference


3. The location and shape of coastlines are always changing

        Location controlled by two factors:

      Tectonics and

      Ocean volume


        Shape is controlled by several factors:



      Eustatic sea level fluctuations





C. Much of World's Population Lives At/Near the Coast

1. Coastal dwellers are exposed to similar dynamic agents that shapes the



2. Coastal dwellers have responded to dynamic change along coastlines by

attempting to stabilize coastal features

        Jetties, seawalls, dikes, breakwater, groins, etc.


III. Classifying Shorelines

A. Classification Schemes for Coastlines are Based on the Dynamic Factors that

Shape Them

1. Common geotectonic origin

        Leading edge coasts - active coasts

      Plate boundary coastlines

      Tectonically active

      Active magmatism


        Trailing edge coasts - passive coasts

      Innerplate coastlines

      Little to no tectonic or magmatic activity


2. Eustatic (worldwide) sea level fluctuations

        Variation in total ocean water volume

        Variation in ocean basin volume

        Variation in water column density


3. Local changes in local sea level

        Tectonic uplift or subsidence

        Isostatic adjustment

        Local ocean surface fluctuations

o       Storm surges

o       Waves

o       Tsunami

o       El Nino and La Nina


4. Coastlines are classified into two distinctive types, based on the source of

dominate influence(s)


        Primary - Dominated by Terrestrial Influences


        Secondary - Dominated by Marine Influences


C. Erosional Coastlines

1. Erosional coasts typically have the following character:

        Rugged Relief




2. Erosional coasts are dominated by erosional agents of change

        Waves and Tides

        Ocean Currents

        River erosion - downcutting of river valleys

        Runoff and Wind

        Glacial erosion - fiords


3. Features found along an erosional coastal zones

        Rocky headlands

o       Sea bluffs, caves, sea arches, and sea stacks

        Small pocket beaches

o       Coarse grained material like, cobbles and gravel

        Exposed wave-cut benches

o       Lack of sediment along shoreline

4. Erosional coastlines are found in many parts of the world

        Example: Big Sur and the Oregon Coast


D. Deposional Coastlines

1. Deposional coasts have a recognizable character:

        Generally straight, low-lying, and rich in loose sediment

o       Example: Texas Gulf Coast

2. Depositional coasts are dominated by depositional agents of change

        River sediment input

        Eroded bluff material input

        Longshore currents

        Tidal action

        Biological activity


3. River and bluff sediments are the primary sources of coastal sediment

        Coral reefs are also important in many tropical regions

        Particularly during storm events


4. Typical features of depositional coasts include:

        Barrier islands

        Sand spits and tombolos

        Bay mouth bars



6. Ocean wave and longshore current action over time will ultimately

straighten the shoreline

        Wave refraction intensifies energy at headlands

        Wave refraction dissipates energy in bays

        See figure 12.13 (page 298)

        Overall effect of shoreline straightening

o       Wear away headlands

o       Fill in the bays

o       Build up and accumulate beaches


IV. Beaches and Beach Processes

A. Beaches Defined

1. The beach is a zone of unconsolidated sediment that covers all or part of the



        Beaches extend from beyond the breaker zone to the landward edge of the coastal zone

        Consists of sand and/or pebbles and/or cobbles


        Another term for the beach is the littoral zone


2. Beaches can be divided into three regions:

        Offshore the area seaward from where waves first begin to break, breaker zone


        Nearshore the area from the offshore to where waves wash back and forth across the beach


o       Near shore can be divided into the:

         Breaker zone where the waves begin to break


         Surf zone where the waves expend most of their energy


         Swash zone where waves wash back-and-forth across the beach face


        Backshore the land that adjoins the near shore

o       Also termed the backbeach


3. Position of the divisions of the beach varies with the tides, advancing

landward with high tide and retreating seaward with low tide


B. Beach Profile and Anatomy

1. A beach profile is a cross section of the beach along a line that is

perpendicular to the shoreline


        By comparing a series of beach profiles along the same line made at different times, it is possible to tell if the beach is expanding or eroding


        Beaches display seasonal cycles of expansion and contraction related to wave size


        There is a general relationship between beach material composition and beach slope angle


o       More gradual - the finer the sediment

o       More steeper - the coarser the sediment

o       See Table 12.1 (p. 299) for comparing


        Study text figure showing a beach profile


2. A swell profile is concave upward with a wide, broad berm (relatively flat

backshore) and steep intertidal beach face


        This profile typically develops during summer when the weather is fair and the dominant waves are flat swells, which transport sediment shoreward and enlarge the beach.


3. A storm profile displays erosion of the berm into a beach scarp and a broad

flat intertidal beach face


        This profile typically develops during winter when the weather is more unsettled and the dominant waves are high, steep, and erosive


        Finer sediment is transported seaward, leaving coarser sediment behind on the beach


        Some of the sediment transported seaward forms sets of longshore bars and troughs, which later migrates landward as the swell profile begins to redevelop


C. Ocean Breakers and Currents

1. Ocean waves that meet the shoreline will break

        Breaking waves, or breakers, and the resultant white water is termed surf


        The size of surf is dependent on swell size, coastline shape, and bottom conditions


        The shape of the breaking waves is dependent mainly on bottom conditions


2. Breaking waves turn swell energy into translational kinetic energy

shoreward-moving turbulent water termed "white water"


3. Surf energy goes to work in several ways:

        Generates longshore currents

        Generates rip currents

        Moves beach material perpendicular to shore

o       Combination of saltation and suspension

        Erodes bedrock base - creates wave-cut platform

        Erodes and undermines sea cliffs

4. Waves that approach the shore at an angle will break at an angle to the

beach and result in a peeling wave

        Angled breakers have a translational (white water) component that is parallel to shore


        The shoreline-parallel component of white water over time

generates a nearshore current called a longshore current


        Longshore (parallel-to-shore) currents move in the direction

opposite that of the direction the swell arrives from

o       Northerly swells generate a southward-bound longshore

current along a west or east-facing beach

o       Southerly swells generate a northward-bound longshore

current along a west or east-facing beach


        Beach material moved by a longshore current is termed

longshore transport or drift


        Study Text Figure for longshore transport


5. Waves approaching the shore at an angle will disturb the bottom sediment

prior to breaking

        Sediment will be moved in similar direction to the longshorecurrent


        This is termed offshore transport


6. Longshore currents cause transport (drift) of beach material in the direction

of the current


        The dominant longshore current and transport direction is south, due to the predominant northerly swell directions of winter swell


7. White water that piles up along a section of beach must find a means to exit

back out to sea to maintain a water mass balance along the seashore

gravity maintained


        This is commonly accomplished by the generation of narrow seaward-bound currents running perpendicular to shore

o       These currents are called rip currents


8. Rip currents are narrow river-like channels of ocean water that move

seaward through the nearshore surf zone and into the offshore region of

the beach where they dissipate

        Rip currents typically have an extended mushroom-like shape (from a

"bird's eye" view), and are often accentuated by rough, foamy water


        Rip currents form by a combination of two phenomena at specific locations along a beach:

o       Convergence zones of inbound white water energy


o       Inshore-bound waters especially accumulate along

low-standing channels along the beach bottom


        Swimmers and surfers caught in outgoing rip currents escape them by moving paddling perpendicular to shore


D. Seasonal Beach Changes

1. Beaches undergo seasonal changes due to differences in marine weather

        Times of the year with persistent large stormy surf results in excessive beach erosion

o       Sand is moved offshore into longshore bars

o       Beach lowers and steepens


        Times of the year with persistently calm conditions results in excess beach deposition

o       Sand is moved shoreward back onto the exposed beach

o       Beach raises and flattens


E. Coastal Beach Cells

1. Beach-lined coastlines are broken into separate unique segments called

coastal cells


2. Each coastal cell acts like a river of sand

        Driven primarily by longshore currents

        "Upstream and "downstream" ends of a cell


3. Each coastal cell has a sand budget

        The sand budget is the balance between sediment added

to and sediment removed from the cell


4. Major input sources of sediment for beach cells:


        Coastal cliffs


5. The boundary between coastal cells is marked by a coastal geographic

barrier that diverts or terminates longshore transport

        Typically the barrier is a submarine canyon or extensive headland


6. Longshore and offshore currents transport beach material both parallel to

the shoreline

        Parallel sand movement is termed longshore transport


        Sediment input to the beach comes from rivers, sea cliff erosion, and on-shore sediment transport


        Beach sediment is removed by longshore current, off-short transport, and wind erosion


        A balance between sediment gains and loss results in a stead-state condition


        If sediment loss is greater than sediment gain, a negative sand budget exists and the beach will begin to erode


        If sediment loss is less than sediment gain, a positive sand budget exists and the beach will begin to expand


7. The typical final output destination for sediment in a coastal sand cell is an

underwater (submarine) canyon

        Sand is funneled down these underwater ravines via turbidity currents


        Sand is permanently removed from the coastal cell


V. Estuaries and Deltas

A. Estuaries

1. Defined:

        Estuaries are semi-enclosed bodies of water wherefresh water from the land mixes with sea water


2. Estuaries originate as:

         Drowned river valleys with the rise in sea level, the lower portions of river valleys have flooded


         Fjords as glaciers have retreated and sea level risen, the lower portions of glacial valleys have flooded

      Fjords are typically long, narrow, and deep with steep

cliff-like sides


      The bottom of fjords frequently are partially blocked byglacial moraines (ridges of sediment deposited at the front of the glacier) which inhibit current flow and can produce hypoxic to anoxic conditions at the bottom.


        Bar-built estuaries spits and sand bars may partially block the entrance of an embayment, thereby restricting tidal flow.


        Tectonic estuaries uplift associated with plate tectonics can partially block the entrance to an embayment.


3. Salinity typically grades from normal marine salinity at the tidal inlet of fresh

water at the mouth of the river.

         In some estuaries, the water is well stratified with a strong halocline separating the dense saline water below from the fresh water above


4. Tidal flow provides the energy for mixing the fresh and saltwater masses.


         If tidal flow is strong, stratification is weak.


5. Estuaries can be subdivided into three types based upon the relative

importance of river inflow and tidal mixing.


         Salt-wedge estuaries

         Partially-mixed estuaries

         Well-mixed estuaries


6. Salt-wedge estuaries are dominated by the outflow from rivers

         The outflow from rivers is much greater than the inflowfrom the tides


         The water column is highly stratified with a well-defined, strong halocline that inhibits mixing


         Salt water forms a wedge that extends landward below the fresh water wedge that extends seaward


         Strong turbulent currents in the fresh water flow across the halocline and generate internal waves


         As the internal waves steepen and break, they mix salt water into the fresh wand it is swept seaward


         The continual loss of salt water into the fresh water generates a slow current that flows in along the bottom and up along the underside of the fresh water wedge


         The bottom current is too weak to carry much sediment into the estuary from outside the tidal inlet


         Sediment distribution in the estuary consists of river sand at the landward edge of the saltwater wedge and mainly river clays and silts elsewhere


7. Partially-mixed estuaries are dominated by neither river inflow nor tidal


         Tidal currents promote greater mixing and both stratification

and the halocline are greatly weakened


         As more saltwater mixes into the fresh, a stronger bottom

current is generated


        The bottom of the seaward end of the estuary is covered

with sediments from the shelf, whereas the landward end

is dominated by river sediments


8. Well-mixed estuaries are dominated by tidal turbulence which destroys the

halocline and water stratification

         In wide estuaries, Coriolis deflects river outflow to one side and tidal inflow to the other


         A salinity gradient extends across the estuary, but not vertically within the water column


         Seawater flows in and fresh water flows out on opposite sides of the tidal inlet at all depths


9. Since river discharge and tidal inflow vary, the type of estuary can change


10. The widely fluctuating environmental conditions in estuaries make life

stressful for organisms


11. Estuaries are extremely fertile because nutrients are brought in by rivers

and recycled from the bottom because of the turbulence (waves and tides)


         Stressful conditions and abundant nutrients result in low species diversity, but great abundance of the species present


12. The benthic fauna strongly reflects the nature of the substrate and most

fishes are juvenile forms living within the estuary until they mature and

migrate to the ocean.


B. River Deltas

1. Defined:

        A delta is an emergent accumulation of sediment deposited at the mouth of a river as it flows into a standing body of water


2. The three major areas of a delta are:

        Delta plain flat, low-lying area at or below sea level that is drained by a system of distributaries


        Delta front shoreline and broad submerged area of the delta that slopes gently seaward


        Prodelta far off shore area of the inner shelf that receives fine sediment from the river


3. In cross sections, a deltas deposits can be divided into three sets of beds:

        Topset beds flat-lyign beds of sand and mud of the delta plain deposited by the distributaries in their channels and in the inter-channel areas


        Foreset beds thick silts and sands of the delta front that slope gently seaward and form the bulk of the delta


        Bottomset beds flat-lying silts and clays of the prodelta that settle out of suspension far offshore


4. As sediment accumulates the delta expands seaward with forest beds

burying bottomset beds and topset beds covering foreset beds


5. Shape of the delta can be altered by tides, waves, and river deposition

        River dominated deltas form in areas protected from large waves and with a small tidal ranges


o       Delta displays the ideal triangular form

o       Example: the Mississippi River


        Wave dominated deltas are so altered by wave erosion and longshore drift that most of the delta sediment is distributed along the coast and only a slight protrusion exists at the mouth of the river

o       Delta displays only a slight protrusion at the mouth of

the river


o       Example: the Sao Francisco River


        Tide dominated deltas are altered by the ebb and flow of the tides and into the ocean

o       Delta displays long linear submarine ridges

and islands that radiate from the rivers mouth


o       Example: the Ganges River


6. Reduction in the supply of sediment to a delta results in delta erosion and

subsidence as the sediments of the delta compact

        Erosion and Subsidence Mississippi Delta = 1 cm/yr


VII. Reefs, Islands, Atolls, and Guyouts

A. Coral Reefs Defined

1. A coral reef is an organically constructed, wave-resistant, rock-like structure

created by carbonate-secreting organisms


        Most of the reef is composed of loose to well-cemented

organic debris of carbonate shells and skeletons


        The living part of the reef is just a thin veneer on the



        Corals belong to the Animal Order Cnidaris


      The animal is the coral polyp


      The body of the polyp resembles a sac with the open

end surrounded with tentacles


      The corralite is the exoskeleton formed by the polyp.

Its interior is divided by septa, vertical partitions.


2. Corals share a symbiotic relationship (mutually

beneficial) with the algae called zooxanthallae which live

within the skin of the polyp and can comprise up to 75%

of the polyps body weight

        The coral provides protection for the algae and supplies

them with nutrients and carbon dioxide from the polyps

metabolic wastes


        The algae supply the coral with oxygen and food


        Recycling of nutrients between the polyp and the algae

allows the corals to thrive in the nutrient-poor tropical seas


3. Corals cannot survive in fresh, brackish water or highly

turbid water


4. Corals do best in nutrient poor water because they are

easily out-competed by benthic filter feeders in nutrient-

rich water where phytoplankton are abundant


B. Evolution of Coral Reef Systems

1. As a result of corals growing continuously upward

towards the sunlight as sea level rises and/or land

subsides and, coral reefs pass through three stages of

reef development

        Fringe reefs

        Barrier reefs



2. Fringe reefs form limestone shorelines around islands

or along continents

        Represents the earliest stage of reef development

        Form in areas with low rainfall runoff

        Leeward side of islands

        Many Hawaiian reefs are this type


3. Over time, islands do two important things

        Slowly subside with the underlying ocean crust

        Slowly wear down to sea level by wave erosion


4. Upwards reef growth keep ups with sinking island


        Reefs grow upward at up to 1 cm/year

5. As the land is progressively submerged and the coral

grows upward, an expanding shallow lagoon begins to

separate the fringe reef from the shoreline and the reef is

called a barrier reef (second stage)


         Barrier reefs occur at lower latitudes than fringe reefs


         Australia's Great Barrier Reef is an example


6. In the final stage, the land vanishes below the sea and

the reef forms an island ring or ring or islands, called an

atoll, around a shallow lagoon (final stage)


7. See Figure 12.27 in text (page 307)

C. Atolls and Guyouts are Modified Oceanic Islands

1. Island eventually worn down to below sea level, with

only the growing reef able to maintain at sea level


        This stage of an island is termed an atoll


2. Eventually reef growth lags behind rate of atoll

subsidence, and entire atoll structure becomes

permanently submerged


        The submerged atoll is termed a guyout


3. See Figure 12.27 in text (page 307)


V. Other Geographic Features of Coastlines

A. Coastal Bluffs and Cliffs

1. Defined:

        A sea cliff is an abrupt rise of the land from sea level


2. A sea cliff is most vulnerable to erosion at its base


        As waves slam against the cliff, air is compressed inside

cracks and then expands violently as the wave recedes

o       This can eventually shatter the rock


        Sediment is hurled against the cliff by the waves


        Sea water can dissolve some rock types


        When sufficient rock at the base of the cliff has been

removed, the upper part of the cliff collapses


3. Collapsed material protects the base of the sea cliff from

additional erosion until it is destroyed and removed


4. The rate at which the cliff recedes is dependent upon:

        Composition and durability of the cliff material

        Joints, fractures, faults and other weaknesses in the

cliff material

        Amount of precipitation

        Steepness of the cliff


5. The wave-cut platform is the gentle sloping area in front

of the sea cliff that was produced by sea-cliff retreat


B. Coastal Dunes

1. Sand dunes are formed by onshore winds blowing sand

landward from the dry part of the beach


2. Well developed dunes typically have a sinusoidal profile

with the primary dune at the landward edge of the beach

and possible secondary dunes located farther inland

        Dunes can extend up to 10 km into the interior

        The area between adjacent dunes is called a valley or swale

        Small, irregular foredunes commonly occur a the upper

edge of the beach


2. Vegetation on the dunes traps windblown sand on their

downwind side and promotes dune growth and stability


3. Blowouts are wind-scoured breaks In the dune or

depressions in the dune ridge and commonly occur if

vegetation is destroyed

        With time blowout can enlarge and destroy the dune


4. Dunes are best developed in the following conditions:

        Sand is abundant

        Onshore winds are strong and persistent

        The tidal range is large

        The beach is wide and gently sloping


5. Wave erosion of sand dunes transports sand offshore

and creates a steep scarp at the base of the dune

        The scarp reflects the wave energy and lessens additional

erosion of the dune by the breakers


7. Dunes act as a natural barrier and prevent inland



8. Human activity that damages vegetation leads to dune

destruction by blowouts and washover by storm waves

        Washover forms a washover fan on the landward side of

the dune


C. Barrier Islands

1. Barrier islands are islands composed of sediment that

parallel the coast and form where sand supply is

abundant and a broad sea floor slopes gently seaward


2. The islands are separated from the mainland by shallow

bodies of water which are connected to the ocean

through tidal inlets


3. A series of distinct environments develop across the

island parallel to the beach and include the nearshore

zone, dune field, back-island flats, and salt marshes

        The back island flats are washover fans deposited during

storms as water flooded across the lower parts of the island


        Salt mashes are protected areas on the back side of the

island where mud collects


4. Barrier islands are created in many ways including:

        Sand ridges on the coastal plain which paralleled the coast

and were later isolated as lowlands were submerged by

rising sea level


        Sand spits that were breached during a storm and remained

separated from the mainland by a tidal inlet


        Vertical growth and emergence of alongshore sand bars


        As sea level rises, barrier islands migrate landward as

washover transports sediments from the seaward side

of the island to the landward side


D. Sand spits

1. Sand spit defined:

        linear feature made up of unconsolidated sediment that

grows downcurrent of the longshore current


2. Sand spits are anchored to coast off of natural or man-

made projections, such as sea stacks or jetties


3. Usually, the youngest end of the spit is wide and/or


E. Tombolos

1. Tombolo defined:

        A sand spit that grows from the shore to an offshore



2. Tombolos are common features of sandy shorelines that

have abundant seastacks and islands in close proximity

to shore

F. Bay Mouth Bars

1. Bay mouth bar defined:

         Form when spits grow across and block the entrance to a bay


2. If waves and tides are strong enough, the bar will be

breached. Otherwise, a lagoon will develop

G. Lagoons

1. Lagoons defined:

        Lagoons are isolated to semi-enclosed, shallow, coastal

bodies of water that receive little if any fresh water inflow


2. Some lagoons are now polluted; were once productive,

now are wastelands


H. Salt Marshes

1. Salt marshes defined:

        Salt marshes are intertidal flats covered by grassy vegetation


2. Mashes are most commonly found in protected areas

with a moderate tidal range, such as the landward side of

barrier islands


3. Marshes flood daily at high tide and then drain through a

series of channels with the ebb tide


4. They are one of the most productive environments,

despite harsh conditions


5. Marshes can be divided into two parts:

        Low salt marshes extend from the low tide mark to neap

high tide

o       Along the Atlantic and Gulf Coasts, these areas are

dominated by a knee-high cordgrass


o       Low marshes are the more productive area with

productivity of 800-2600 gm C/m2/yr


o       Nitrate is commonly the limiting nutrient


o       Plants die in autumn, partially decompose and supply

abundant detritus which becomes food for the

detritivores or accumulates and eventually forms peat


        High salt marshes extend form neap high tide to highest

spring tide

o       This area is flooded only at the highest spring tide or

during a storm surge


o       It is more terrestrial than marine in nature and has a

more diverse fauna and flora


         Distribution and density of organisms in salt marshes strongly

reflects availability of food, need for protection, and frequency

of flooding


         Salt marshes serve as nursery and shelter for juvenile organisms


         Many salt marshes damaged by man filled in


I. Mangrove Swamps

1. Mangroves defined:

        Mangroves are large woody trees with a dense, complex

root system that grows downward from the branches


2. Mangroves are the dominant plant of the tropical and

subtropical intertidal area


3. Distribution of the trees is largely controlled by air

temperature, exposure to wave and current attack, tidal

range, substrate, and sea water chemistry


4. Detritus from the mangrove forms the base of the food



J. Effects of Storm Surges on Coastline Features

1. Storm surge defined:

        Storm surge is the high water created by the accumulation

of wind-blow water against the shore and the mound of

water generated by the low atmospheric pressure of the



2. The elevated water level allows waves to reach much

farther inland than usual, especially if the storm surge

coincides with high tide


3. During a storm surge, ocean waves more easily breach

the island and wash over lower areas


4. New tidal channels may form during a storm surge

        Most tidal inlets are eroded from the landward side of the

barrier island seaward


        The bay becomes swollen with rainwater, runoff, and inflow

from the sea


        The onshore wind piles the water against the mainland and

after the storm has passed, a seiche can develop which

then raises the water level against the landward side of the

barrier island


        Storm winds may also reverse direction on the rear of the

storm and blow offshore, piling additional water against

the barrier island


        If water breaches the island, its seaward flow may create a

new inlet


        Most inlets are quickly filed with sediment because of

longshore drift


VI. Coastlines of the U.S.A.

A. Pacific Coastlines

1. Tectonically active coastline

        Dominated by uplift

2. Typically rugged and irregular with abundant sea cliffs

3. Sediment sources from nearby granitic and volcanic

mountains via rivers far exceed local bluff input

4. Deltas tend not to form due to high energy shorelines

B. Atlantic Coastlines

1. Tectonically inactive coastline

        Dominated by subsidence


2. Abundance of barrier islands and submerged river



3. Sediment sources are mainly from offshore deposits


4. Deltas are rare or absent


C. Gulf Coast

1. Tectonically inactive coastline

        Dominated by extreme subsidence


2. Typically very low-lying and straight with abundant

broad beaches and barrier islands


3. Absence of large waves (except infrequent hurricanes)

and submarine canyons


4. Formation of large deltas

        Excessive sediment input from rivers


VII. Humans Assault on Coastlines

A. Beaches are Systems that Exist in a Natural Balance

Between Erosion and Deposition

1. Natural input of material from rivers and sea cliffs


2. Natural movement of material in longshore transport


3. Natural seasonal changes in beach sand budget


B. Humans Build Structures That Oppose Coastal

Processes and Typically Cause Cell Imbalances

1. Breakwaters

        Eliminate or reduce wave influence


        Disrupt longshore current and transport


        Sand piles up on beach behind breakwater


2. Groins

        Disrupts and intersects longshore transport


        Designed to trap and hold sand on beach


        Excess sand buildup on "upstream" side of groin


        Excess sand erosion on "downstream" side of groin


3. Jetties

        Disrupts and intersects longshore transport


        Excess sand buildup on "upstream" side of jetty


        Designed to prevent sand from blocking harbor

channel entrance


4. Seawalls

        Blocks wave erosion of sea cliff or bluff


        Reflected wave energy increases beach erosion in front

of seawall


        Blocking of bluff erosion reduces sediment input fro beach



        Designed to protect bluff and/or structures behind seawall


IV. Tsunami!

A. Defined

1. Tsunami are very long wavelength shallow water

progressive (gravity) waves caused by the rapid

displacement of ocean water

        Tsunami is mistakenly called a tidal wave

        Seismic sea waves are tsunami

        Not all tsunamis are seismic sea waves


B. The Nature of Tsunami

1. Tsunami are shallow water waves because they always

travel in water depths shallower than their wavelength

        Tsunami wavelengths are up to 200 kilometers


5. Tsunami waves travel very fast

        Calculated by the shallow water wave equation: C = √gd


where C is speed, g is acceleration due to gravity, and d is water depth

(typically 15,000 feet in the Pacific)

        up to 500 miles per hour


        Can cross the Pacific Ocean basin in 10 hours


4. Tsunami waves in open ocean only 1-2 meter in height


        Ocean vessels on the high seas wouldnt notice one

5. Tsunami resemble a swiftly rising tide (a tidal bore) rather than a breaking

wave when they make shore


        Picture a super gigantic mush burger wave


        Unlike a normal sea wave, the tsunami wave keeps driving onshore for minutes


C. How are Tsunami Generated

1. Tsunami are generated by several water disturbing forces

which acts to displace surface water

        Earthquake/Faulting (sea bottom displacement)

        Shoreline or underwater landslide event

        Volcanic eruption

        Bolide impact

2. Seismic sea waves are generated when the ocean bottom is rapidly raised, or lowered, along an underwater fault zone

during a large earthquake, i.e. large fault rupture


        Up-lifted sea bottom causes an initial bump in the ocean surface

      Crest of tsunami wave forms first


        Down-dropped sea bottom causes an initial dimple in the ocean surface

      Trough of tsunami wave forms first


3. After a tsunami is generated, the wave typically disperses

into multiple crests

        The first wave often is not the largest

        Because of the long wavelength, the crests may be separated by 10s of minutes or even hours.


D. Tsunami Are Classified Into Two Categories

1.      Based on their proximity to origin


        Far traveled


2.   Local tsunamis primarily affect a small area and are usually caused by landslides (often underwater) which are triggered by earthquakes or volcanic eruptions.


        These are sometimes very severe and occur with

virtually no warning.


3.      The largest local tsunami on record occurred in Lituya Bay, Alaska due to a landslide.

        From damage to trees, it is estimated to have reached more than 1500 feet up the mountainside


        A wave about 150 feet high swept down the bay and out to sea


        Four of six people aboard three boats anchored in the bay survived.

4.      Tsunami can hit coastlines that are thousands of kilometers from the point of tsunami generation


5.      They are free gravity waves like ocean swell


6.      They do lose energy the further they travel


7.      The Pacific basin is notorious for abundant far-traveled tsunami


8.      Tsunami can be predicted after an earthquake


D. When Tsunami Meet the Shoreline

1.      Fast-moving tsunami waves change radically when they

encounter the shoreline very

        Slows down


        Wavelength shortens dramatically


        Tremendous increase in wave height


        First wave encountered may be either the trough or the crest


      Trough Appears like a Super low low tide


      Crest - Looks like a humungous tidal bore


2.      Low-lying areas along coastlines are at serious risk when a

tsunami hits

        A very rapid onslaught of sea water rushes onshore

        The driving surge pushes inland as Sea level


3. Examples of devastating far-traveled tsunami events

        Hilo, Hawaii 1946

        Alaska, 1964

        Japan 1703, 1960

        Lisbon, Portugal 1755

        Indonesia 1883

        Flores Island - 1992

        New Guinea 1998


E. Tsunami Prediction and Warning Systems

1) Coastal Tidal Gauge Network

2) The DART Open Ocean Network


F. Important Tsunami Safety Tips

1. If you are in a coastal community less than 50 feet above

sea level, and you feel a severe earthquake (one that

makes it almost impossible to stand up, which is

causing substantial damage to buildings, or is opening

cracks in the ground), RUN for the highest point you

can reach within minutes.

        Once you see the wave, you cannot outrun it. If all else fails, some people have survived by climbing trees.


2. Even if you have felt no earthquake, or only a mild one, a

sudden recession of water is always a danger sign.

        Run away from the water to high ground.


3.      Remember - more severe waves can follow for hours.

        Do not return to low-lying areas for 24 hours.


2.         Ships at anchor should weigh anchor and head to sea.


3.         Ships at dock should also, if there is a warning due to a distant earthquake.

        However, if at dock during a severe earthquake, it is questionable whether the best choice is to jump ashore and run inland, or to try to ride it out aboard (loose mooring lines if possible.)


4.         Tsunami warnings for distant sites are still inexact.

        They can warn that a tsunami might occur, and approximately when, but the danger at a particular location depends on topography, the particular characteristics of the wave, and other factors


        This results in many false alarms, leading people to disregard alarms when they occur



XI. Coastline and Beaches Vocabulary - Chapter 12


Active coast





Barrier Island

Barrier reef

Bay Mouth Bar


Beach scarp


Berm crest




Coastal cell




Eustatic sea level change



Fringing reef



High-energy coast


Longshore bar

Longshore current

Longshore transport (drift)

Low-energy coast

Primary coast


Rip current

Sand spit

Sea cave

Sea cliff



Secondary coast


Submarine canyon




Turbidity current

Wave-cut platform