Oceanography - Chapter 10: Beaches, Shoreline Processes and the Coastal Ocean

Coastal Ocean: A Busy and Increasingly Busy Place

• Humans are drawn to coastal living due to:

  • Mild climate

  • Abundant seafood

  • Easy transportation

• Population distribution:

  • Approximately 50% of the world's population (3.5 billion people) resides along coastlines.

  • 80% of Americans live within an hour's drive of the ocean or the Great Lakes.

• Coastal oceans are rich in marine life:

  • 95% of fish caught are found within 200 miles of the shore.

• Coastal wetlands serve as vital natural cleansers:

  • They filter storm runoff and remove pollutants.

Defining a Coastal Region

• Shore: The zone between the lowest tide level (low tide) and the highest land elevation affected by storm waves.

• Coast: Extends inland from the shore as far as ocean-related features are found.

• Coastline: Marks the boundary between the shore and the coast, representing the landward limit of the highest storm waves.

Coastal Terminology

• Backshore

• Foreshore

• Shoreline

• Nearshore

• Offshore

• Beach

• Wave-cut benches

• These terms define different regions within the coastal area; the following explanations will use a Pacific Coast bias.

Shore Divisions

• The shore is divided into:

  • Foreshore

  • Backshore

• Shoreline: Migrates with the tide

  • High-tide shoreline

  • Low-tide shoreline

Nearshore Zone

• Extends seaward from the low-tide line to the low-tide breaker line.

• Longshore bars: Sandbars accumulating due to tidal action, migrating seasonally.

Offshore Zone

• Lies beyond the low-tide breakers.

Beach Definition

• A deposit of the shore area.

• Composed of wave-worked sediment.

Wave-Cut Cliffs

• Common on the West Coast.

• Often feature coastal highways above them.

Berm

• A dry, gently sloping, elevated portion of the beach.

• Composed of sediment.

• Can be at the foot of coastal cliffs or sand dunes (e.g., on Barrier Islands).

• Commonly used for recreational activities.

Beach Face Area

• A flat area exposed mostly during low tide.

• Composed of compact sand, saturated with water.

• A popular area for runners, especially during low tide.

Longshore Trough

• A low-lying area before longshore bars.

Wave-Cut Bench

• A flat area.

Beach Composition

• Composed of local materials, weathered and eroded.

• Sediment size varies based on travel distance and wave action.

  • Coarse-grained

  • Fine-grained

• Rivers deliver sediment from local or distant sources (e.g., eroding mountains).

Key Sediment Sources

• Local cliffs

• Sand

• Mud from rivers

• Biological material (especially on tropical beaches)

Sediment Transport and Composition

• Coastal material is always in motion due to currents.

• In Florida, sand is largely quartz, resulting in beige or white beaches.

• Beaches in volcanic areas (e.g., Iceland, Hawaii) have black sand due to basalt breakdown.

Sand Movement

• Perpendicular to shoreline:

  • Swash: Waves rush up, bringing sand.

  • Backwash: Water drains back, eroding sand.

• Parallel to shoreline:

  • Longshore current transport

Longshore Current

• Sand movement direction along coast from incoming waves.

• Transports sand along a beach.

Seasonal Beach Changes (California)

• Summer:

  • Light wave activity

  • Wide, sandy berm

  • Steep beach face

  • Swash dominates

  • No longshore bars

• Winter:

  • Heavy wave activity

  • Narrow beach

  • Flattened beach face

  • Longshore bars present

  • Increased backwash

  • Sediment erosion

Longshore Current Dynamics

• Zigzag motion of water along the shore transports sediment.

• Moves at approximately 2.5 miles per hour.

• Created by waves approaching at an angle, causing refraction and bending towards the shore.

Longshore Transport Terminology

• Beach drift

• Littoral drift

• Movement of sediment and direction dictated by the angle of wave approach.

Net Sediment Movement

• Generally southward along the Atlantic and Pacific coasts of the United States.

• East Coast sediment from Appalachian Mountains is delivered by rivers and moves southward.

Shore Types

• Erosional shores

  • Well-developed cliffs

  • Associated with tectonic uplift (e.g., Pacific Coast)

• Depositional shores

  • Gradually subsiding shorelines (e.g., East Coast)

  • Made up of unconsolidated sediment deposited by rivers

  • Features include barrier islands and sand deposits

Erosional Shoreline Features

• Headlands: Protruding land that absorbs wave energy

• Sea arches

• Coves: Sand deposits where wave energy dissipates

• Sea caves

• Sea stacks: Remnants of headlands after sea arch collapse

• Blowholes: Water shoots up due to flood currents

• Sea cliffs

• Marine terraces

Marine Terraces

• Land uplift results in former wave-cut benches becoming marine terraces.

• Show multiple flat areas indicating past uplift events.

Depositional Shoreline Characteristics

• Barrier islands: Large piles of sand parallel to the coastline

• Bay barriers: Barrier islands covering a bay

• Lagoons: Bodies of water sealed off by barrier islands

• Spits: Curved barrier islands hooking into a bay

• Tumbelos: Areas exposed only during low tide, often behind a sea island

Barrier Islands

• Long offshore sand deposits parallel to the coastline.

• Common on depositional coastlines but not on erosional shores.

• Serve as a first line of defense against storms.

• Developed at the end of the last ice age.

• Separated from the mainland by a lagoon.

Features of a Barrier Island

• Ocean beach: Recreation area

• Dune: Protects the beach, stabilized by dune grasses

• Barrier flat: Grassy area behind the dunes

• Salt marshes: Low and high marsh areas, productive ecosystems, can contain peat deposits.

Barrier Island Movement

• Barrier islands move due to erosion and rising sea levels and migrate toward the coastline.

• Peat deposits from lagoons can erode, appearing on the ocean beach.

Deltas

• Deposits forming where rivers deposit sediment into the ocean.

• Mississippi River Delta (Birdsfoot Delta):

  • Sediment from Midwest is deposited into the Gulf of Mexico.

• Distributaries: Branching channels carrying sediment.

• Nile River Delta:

  • Sediment delivery is dispersed by longshore currents, leading to erosion.

  • Dams impede sediment flow.

California Beach Compartments

• Components:

  • Rivers: Supply sediment

  • Beaches: Accumulate sediment

  • Submarine canyons: Transport sediment offshore

• Beach starvation: Occurs when damming rivers blocks sediment supply.

Shoreline Changes

• Emerging shorelines: Shorelines above current sea level, creating marine terraces

• Submerging shorelines: Shorelines at or below current sea level, with drowned beaches

Eustatic Sea Level Changes

• Worldwide changes in sea level.

• Caused by tectonic movements and climatic changes.

  • Increased ocean crust creation

  • Ice age to interglacial period transitions

Isostatic Adjustments

• Local changes in sea level due to the weight of glaciers.

• Continental crust subsides under ice weight and rebounds when ice melts.

• Currently happening in Norway, where crustal rebound exceeds sea level rise.

Ways Sea Levels Change

• Destruction of inland lakes

• Seafloor spreading rates

• Formation or melting of glaciers

• Thermal expansion of water

• All affect sea levels.

The Pleistocene Epoch

• Time of many ice ages (approximately every 100,000 years).

• Sea level fluctuates.

• Last glacial maximum: Sea level was 400 feet lower than today.

• Melting all ice on Earth would raise sea level by 230 feet.

Sea Level Fluctuations (Past 40,000 Years)

• Last glacial maximum (18,000 years ago): Sea level was 400 feet below present value.

• Over the past 18,000 years, sea level has been rising.

Hard Stabilization: A Response to Rising Seas

• Structures that reduce coastal erosion.

• Often exacerbate erosion problems.

Groins

• Barriers built perpendicular to the coastline.

• Interrupt longshore current, causing sand deposition on one side and erosion on the other.

Stabilization Structures

• Groins: Perpendicular to the coast

• Groin fields: Multiple groins

• Jetties: Groins blocking inlets

• Breakwaters: Parallel to the shoreline

• Seawalls: Built in front of the beach

Jetties Functionality

• Built in pairs to protect harbors.

• Cause sand buildup on one side and erosion on the other.

Breakwaters and Coastlines

• Built parallel to the shoreline to diminish wave activity.

• Result in sand deposition behind the structure and erosion beyond it.

Seawalls

• Protect human developments

• Concentrate wave energy, leading to beach erosion and eventual collapse of the seawall.

Alternatives to Stabilization

• Construction restrictions on barrier islands

• Beach replenishment (adding sand to beaches)

• Relocation of buildings away from the coastline

• All offer alternatives to hard stabilization efforts.

Beach Replenishment

• Adding sand to the beach from offshore bars.

• Expensive and temporary solution.

• Dredging sand from offshore is often used to add sand back to the beach.

Relocation

• Physically removing structures and moving them more inland.

• Allows for more of a natural balance with beach processes.

Types of Ocean Waters

• Open Ocean Waters

• Coastal Waters have drastic changes (salinity and temperature)

Salinity Variations in Coastal Waters

• Affected by river runoff, resulting in stratification of freshwater and seawater.

• Halocline: Spot in the water column with a drastic change in salinity.

• Heavy evaporation increases salinity here.

• When well-mixed = isohaline

Coastal Water Temperatures

• Dependent on latitude and season.

  • High latitudes see uniform temperatures near freezing.

  • Low latitudes see uniform high temperatures.

  • Mid latitudes have variation depending on seasons and thermocline develops.

• During the Winter, develop a thermocline where there are very cold surface waters.

• Isothermal water is well-mixed.

Geostrophic Currents

• Influence the Coriolis effect and gravity and affect these currents.

• Davidson Current: Off the coasts of Washington and Oregon, a northward flowing coastal geostrophic current.

• Opposite direction of the California Current.

Estuaries: Transition Zone for Salt and Fresh Water

• Partially enclosed body of water where freshwater mixes with saltwater.

• pH, salinity, temperature, and water levels vary.

• Most formed after the last glacial maximum.

Estuary Types

• Coastal plain estuaries: Sea level rises and floods existing river valleys (e.g., Chesapeake Bay).

• Fjords: Sea level rises with glacier valleys (e.g., Canada, New Zealand, Chile, and Norway).

• Bar-built estuaries: Barrier islands with lagoons and sand deposits (e.g., Gulf Coast and East Coast).

• Tectonic estuaries: Faulting and folding creates a down-dropped area (e.g., San Francisco Bay).

Cross Section:

• Drowned river valley versus a hanging-valley.

• Fjords were typically U-Shaped.

Water Mixing in Estuaries

• Vertically mixed: Net flow from head to mouth with well-mixed water.

• Slightly stratified and highly stratified: Salinity increases in distinct layers with distinct change.

• Salt wedge: A wedge of salty water from the ocean moves underneath the river depending on density.

Threats to Estuaries

• Coastal pollution from fertilizers, pollutants, and sewage.

• Harmful algal blooms.

• Breeding grounds for marine animals.

The Chesapeake Bay

• Slightly stratified estuary with seasonal changes in salinity, temperature, and dissolved oxygen.

• Anoxic waters: Oxygen-depleted due to decomposition of algal blooms.

• Harmful algal blooms are a hot environmental issue.

Brackish Lagoons

• Shallow water bodies landward of barrier islands that contain 3-zones: transition, freshwater, and saltwater zone.

• Rainy and dry seasons affect salinity levels.

• They are home to marine organisms.

Estuaries as Wetlands

• Ecosystems where the water table is close to the surface.

• Coastal wetlands are along many zones and that include swamps, tidal flats, coastal marshes, and bayous.

Salt Marshes

• Salt-tolerant grasses ranging from 30 and 65 degrees latitude.

• Salt-tolerant plants deal with excess salt.

Mangroves

• Restricted to tropical regions.

• Salt-tolerant mangrove trees grow roots into mud or sediment.

• Shrubs and palms

• Found in the Caribbean, Florida, and Southeast Asia.

Coastal Wetland Benefits

• Serve as nurseries for commercially important fishes.

• Stopping points for migrating birds.

• Soak up nutrients and protect inland areas from erosion.

• Very important marine functions when left undisturbed.

Wetland Threats

• More than half of US wetlands have vanished due to development.

• The EPA established the Wetlands Protection Program in 1986.

• There are movements to conserve these wetlands.