GEOL TEST 2

The Coast: Beaches and Shoreline Processes

Coastal hazards: Coastal Erosion and Hurricanes

why study coastal processes

  • 75% of US population lives in coastal states

  • 50 % lives within 100 miles of coastline

  • Hurricanes

  • Highly dynamic environment

  • Coastal problems will increase with population

  • Global warming

Coast- Area of contact between land and sea - Extend inland until meets a different geographic setting

Shoreline- Precise boundary where water meets adjacent dry land

East Coast

Passive Margin Coastlines

  • Barrier islands and maybe rocky coastlines

West Coast

Active Margin Coastlines

  • Sea cliffs and Rocky coastlines

Forces modifying coasts

Two major processes (forces) shape coastline

  • Waves

    • Waves do erosion (move sediment)

  • Tides

    • Twice daily rise and fall of sea level

    • Changes where waves act

Wave formation

  • Wind blowing over water

  • Size of waves depend on

    • Wind speed

      • faster=bigger waves

    • Wind duration

      • longer=bigger waves

    • Distance wind blows (fetch)

      • further=bigger waves

Wave Characteristics

  • Length (L)

    • Distance between crests or between troughs

  • Height (H)

    • Vertical distance between crest and trough

  • Period (T)

    • Time for successive waves to pass a fixed point

    Types of Waves

    • Waves of Oscillation (unmoving; further from shore)

    • Waves of Translation (moving; closer to shore)

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Wave Movement Near Shore

  • Wave refraction

    • waves bend as they approach the shore

  • Longshore currents

    • Currents parallel to the beach within the surf zone

    • They are the most important to modifying coastlines

  • Longshore (littoral) drift

    • Sediment carried by swash and backwash along the beach

Tides

  • Another force that occurs along every shoreline

  • They are the twice daily rise and fall of the sea, caused by the gravitational attraction of the Moon, and to a lesser degree, the sun

High Water- a water level maximum (high tide)

Low Water- a water level minimum (low tide)

Tidal Range- the difference between high and low tide

Spring Tide- full moon and new moon (14.77 days)

Neap Tide- 1st quarter and 3rd quarter (14.77 days)

The Bay of Fundy: Site of the worlds largest tidal range

  • Tidal energy is focused by shape and shallowness of bay

  • Maximum spring tidal range in Minas Basin

Waves and Tides

  • Waves do the erosion

    • Tides change level of beach that waves act on

  • Erosion takes place when waves interact with the bottom

Factors that impact coastlines

  • Tectonic setting

  • Materials present at the shore

  • Energy of water striking the coast

Coastal Processes

Rifted (Passive continental margins tend to be dominated by depositional features

Active continental margins tend to be dominated by erosional features

Erosional and depositional landforms result of the action of ocean waves

Erosional

  • Sea cliffs

  • Sea stacks

  • Sea arches

  • sea caves

Depositional

  • Beaches

  • Spit

  • Barrier Island

Erosional coasts

Wave energy is focused on headlands: prominent cliffs that jut out into deep water

attack the sides of headlands and form sea caves, sea arches, and sea stacks.

Depositional Coasts

Occurs when amount of sediment exceeds wave/current ability to transport it

Beaches - Relatively narrow strips of sand, pebbles, or cobbles deposited along a shoreline

  • 90% of beach sediment comes from streams that drain to coast distributed by long shore currents

  • wave action erodes, deposits, and moves sediment

  • The beach is different every day

<aside> đź’ˇ Have enough of an understanding of the beach profile to recreate it

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Barrier Islands - Long, low, narrow islands parallel to the coastline made of sand that flank main shoreline and separate bays from open ocean. Created by long shore currents

  • Unstable environments

    • Easily washed over due to low relief

    • Sand migrates constantly with the wind

    • Constantly retreating landward with time

Spit - Narrow strip of sand that grows across the mouth of bay due to long shore current (hooks are hook shaped)

  • Similar to barrier island but it’s connected to mainland

Settlement in the Dynamic Environment

  • Wave - Continues eroding and modifying

  • Stability requires protections from waves

“Controlling” Coastal erosion

Structural approaches

  • Seawalls

  • Groins

  • Jetties

Non-structural approaches

  • Beach nourishment

  • Land use planning

Beach erosion factors

  • Supply e.g. Appalachian weathering

  • Erosion

    • Hurricanes

    • Climate Change

      • Landward push of a beach

  • Human interference

    • Controlling Sand supply

    • Enhance wave action

Human Interference Dam

Impacts

  • Coastal zone short of fresh water

  • Cutoff sand supply

    • erosion > deposition

  • Reduces protection

Groins - walls built along beach extending into water

  • block sand movement along beach

  • cause sand to accumulate on up-drift side of groin

Breakwater - A hard, rocky structure built parallel to shore and attached to the sea floor

  • A few feet above sea level

  • Designed to break waves and make quiet water behind for safe harbor

  • intercept waves and protect boat moorings

  • provide safe harbors

  • radically change shoreline creating sediment deposits

Jetties - Designed to stabilize channel

  • Protect channel from large waves and sand deposition

  • Constructed in pairs at mouth of a river or inlet

Beach Re-nourishment - Adding sand to beach

  • mimics nature but has to be redone

  • Aesthetically preferable

  • Controversial

    • expensive and temporary fix

Hazards in Coastal areas

  • Sea level rise

  • hurricanes

  • Tsunamis

Sea Level Rise

  • Sea level was 120 meters lower during the last ice age

  • About 18,000 years ago, sea level began to rise as the glacial ice melted

  • In NYC

    • Sea level has risen 40 cm since 1850

    • Global warming is predicted to increase the rate of sea level rise

Hurricanes

  • Tropical cyclones (hurricanes, typhoons, etc.)

    • Intense storms that form in tropical waters when ocean is warmest

  • What does a hurricane need?

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  • They uplift water collum

  • High winds

    • Winds up to 200 mph damage buildings

  • Heavy rainfall

    • causes floods in area where storm passes over; also downstream

    • Most damaging if it coincides with high tide