GSC 1530: Chapter 20

Shorelines

Shorelines

Shorelines, both ocean and lake, can be very beautiful settingsHowever, shorelines are some of the most geologically dynamic settings on EarthProcesses like shoreline erosion, sea level increases and deadly coastal storms (e.g., hurricanes) threaten the lives and property of billions of people worldwideAs of 2008 about 165 million people in the U.S. live within 50 miles of a marine coast

Coastal Processes Terminology

Current ( ) - unidirectional flow of water and energy caused by wind, water density differences and water temperature differences

Oceanic currents redistribute heat through the ocean and therefore play a critical role in climate (see figures)

Warm, less salty surface current

Cold, salty deep current

Gul

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Gulf Stream:50-90 miles wide;maximum discharge:540 billion tons/hour

Coastal Process TerminologyTides ( ) - periodic, rhythmic rise and fall of water along coastlines due to the gravitational tug of the Sun and Moon on the EarthMost oceanic coastlines experience two high tides and two low tides each day

Spring tide

Neap tide

Tidal Cycle

Coastal Process TerminologyTsunami ( ) – omni directional flow of water away from a central point caused by any event (e.g., seafloor earthquake, submarine landslide) that vertically displaces water from its equilibrium position

Don’t call tsunamis “tidal waves”

earthquake triggered tsunami

Japan Tsunami2011 Japanese Tsunami

Photograph

Wave Terminology

Wave – undulation of the water surfaceCurrents, tides and tsunamis generate wavesWavelength – the horizontal distance between two adjacent wave crests or two adjacent wave troughs (see figure)Wave frequency – the number of waveforms that pass a fixed point per unit of timeThe higher the wave frequency the greater the wave’s impact or penetration energy

Wave frequency – the number of crests, or troughs, of a waveformthat pass a fixed point per unit of timeCommon units – cps (cycles per second); Hertz (Hz); 1 Hz = 1cpsWave base – the maximum depth to which energy from a surface wave is transmitted; equal to the wave’s wavelength/2

As a wave approaches a shoreline, waves of oscillation transformto waves of translation – the predominant water flow is horizontal

Note “breaking” waves; they can perform substantial shoreline erosion

Major Factors Controlling Shoreline Erosion

Topography (e.g., steep or shallow) of shorelineResistance of shoreline rocks or sediments to erosionErosive force of daily currents, tidesFrequency and intensity of high-energy storm events (e.g., hurricanes) or abnormal events like tsunamisFrequency of tectonic events (e.g., earthquakes)

Seaarch

Waveundercutting

Sea arch

Sea stack

Importance of recognizing the rates of shoreline erosion?

Evidence for long-term fluctuations in sea-level is found in wave-cut platforms in rocky coastlines (see figures)

What long-term cyclic process could cause significant fluctuations in sea level?

wave-cut platforms

Source: Geology of Michigan and the Great Lakes, 2008

Emergent and Submergent Coasts

Emergent coast – coastline rises relative to sea level as a consequence of sea level decline or land upliftSubmergent coast – coastline is lowered (covered by water) relative to sea level as a consequence of sea level increase or land subsidenceGlobal warming and cooling and plate tectonic motion can obviously affect long-term sea levels; what rapid, high energy process could quickly create emergent or submergent coasts? (see slide)

Coastal Processes

Shoreline erosion and sediment movement are also influenced by wave refraction

Wave refraction – change in shape of a waveform as it approaches shore; occurs when portions of the wave pass into waters of unequal depth (see figure)

Coastal straightening due to long-term wave erosion

Wave Refraction

Wave refraction is also responsible for establishing the longshore current and initiating littoral drift (beach drift) (see slides)

= littoral driftor beach drift

Dynamic Coastal Settings

Mainland beaches, barrier islands, spits and estuaries are some of the most dynamic settings on Earth

Let’s examine the geological characteristics of these settings and the hazards associated with their development

BarrierIslands

Mainland Beach

Spit

Beach – an accumulation of unconsolidated sediment along the landward margin of an ocean or lake

Dynamic Coastal Environments

An elongate, narrow, low-elevation island, composed of unconsolidatedsediment, that parallels the mainland

Aerial view of the NC “OuterBanks” barrier islands

Spit – an elongate ridgeon unconsolidated fine-grainedsediment that extends fromthe mainland into themouth of an adjacent bay

Estuary – a coastalembayment formed bythe flooding of a river’s mouth; often result from the rise insea level that accompanies themelting of Ice Ageice sheets – anexample of a submergent coast

Earth DIGIT CD

Estuary

Human Impacts

Let’s examine some of the impacts and consequences of human interaction with these dynamic coastal environments

Question to ponder: why do we develop land in these dynamic coastal environments? What ongoing controversy surrounds this development?

Long Island, New York

eye

eye

1969 Hurricane Camille Storm Surge Effects, MS Coast

Storm Surge – one or more domes (walls) of water that sweeps

across the coast near where the eye of the hurricane makes

landfall; can be 40-50 mi wide and have

heights of 10-30 ft above the mean high tide!

Before storm surge

After storm surge

Shoreline developmentcontroversy?

An elongate, narrow, low-elevation island, composed of unconsolidatedsediment, that parallels the mainland

Sand

Galveston Island

Gulf of Mexico

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1900 Galveston Island Hurricane Damage

An estimated 8000 -12000 people on the island lost their lives from storm surge waves of 15 – 20 feet height

Status of Galveston Island today?

Human Intervention

Humans have tried multiple methods to minimize coastal erosion and storm impact; none of these approaches are failsafe or permanent and none can protect the coast from massive stormsMany of these approaches benefit some coastal properties while concurrently harming othersLet’s examine some of the common approaches

Soft Stabilization

Hard Stabilization

Hard Stabilization

Possible negative consequences?

Bea

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Groin

Longshore currentand littoral drift

depositedsediment

Waves

acceleratederosion

Fig. 7.17

Breakwaters may also increase coastal erosion rates at specific sites

Effects of beach nourishment along Miami Beach, FATypical costs: $2-10 million per mile; 3-9 year average renourishment frequency!

Beach Nourishment

Beach Nourishment

Great Lakes Shores

Always investigate shoreline processes before purchasing property in the Great Lakes basin.

Lake Michigan

Lake Erie

Lake Michigan