Coastal Environments Test 2

Set Up/Set Down

Change in water surface elevation due to wave breaking

Requires mass and energy balance

Longshore current

a water current that travels at an angle and is near parallel to the shoreline

Undertow

Wave breaking uniform along shore then returns offshore along the seafloor

Low Velocity 0.05-0.2 m/s

Important for sediment transport

Rip Current

A strong, narrow current that flows briefly from the shore back toward the ocean through a narrow opening

Set up by a wavy beach

Caused by converging longshore transport that is directed offshore upon colliding

Wind & Breaking Waves push surface water towards land, causes a flow back in path of least resistance

Beach cusps

Semi-circular, scalloped depressions cut into the lower edge of the storm beach.

Tied to waves and swash

No one is quite sure why they form (tidal? longshore transport? beach sediment?)

Weathering and erosion of rock, Activity of living organisms, Volcanic eruption, Chemical reactions, Wind, Rivers

Coastal Sediment Sources

Sediment

Particles of inorganic and organic material that accumulate in a loose, unconsolidated form

Pebbles

4-64 mm sediment

Granules

2-4 mm sediment

Coarse Sand

0.5-2 mm sediment

Medium Sand

0.25-0.5 mm sediment

Fine Sand

0.06-0.25 mm sediment

Silt

0.004-0.06 mm sediment

Clay

<0.004 mm sediment

Sediment Sorting

If the grain size consistency is similar, it is well-sorted (<0.50 phi)

If it is different, it is poorly sorted (>1.00 phi)

Grain Size Analysis

Sieves**

Examine the fractions of sediment sizes

Determine the percentile of each size class (% fine sands, % very fine sands, % coarse sands, etc.)

Calculate grainsize parameters (Mean, Median, Sorting, Skewness, Kurtosis)

If phi is higher, the grain size is smaller (max 14)

If phi is lower, the grain size is larger (min -12)

Sediment Skewness

Whether a given sediment sample has more fine particles, coarse particles, or an even split

Form (equant to platy), Roundness (round to angular), Surface Texture (pitted to smooth)

Particle Shape Factors

Basalt (Volcanic sands)

What causes black beaches?

Iron (oxides into a red color)

What causes red beaches?

Olivine (volcanic sands)

What causes green beaches?

Larger grain size causes steeper slopes, smaller grain size causes shallower slopes

How are beach slope and grain size correlated?

Terrigenous sediment

Sediments that originate from Erosion of land, Volcanic Eruption, Blown Dust

45% of ocean sediments

Ex. Quartz

Coarser Grains - More Rounded - Well Sorted

Biogenous sediment

Sediment originating from the accumulation of the hard parts of marine organisms (calcium carbonate from shells)

55% of ocean sediments

Hydrogenous sediment

Sediment originating from the precipitation of dissolved minerals from the water

1% of ocean sediments

Ex. Ooids (Carbonate forming around a nucleus)

Cosmogenous sediment

Sediment originating from outer space (meteorites)

Wave Orbitals (Swash/Backwash), Longshore Transport, Rip Tides, Tidal Currents, Rivers, Wind

How can sediment be moved naturally?

Delta Discharge (Fluvial), Sand Dune Migration (Eolian), Longshore Current, Rip Currents, Tidal Currents (Ocean)

Unidirectional Flow Examples

Ocean Waves

Oscillatory Flow Example

Fluid

substance that deforms under own weight

Viscosity

force / unit velocity; how well a fluid flows or how much resistance it faces while moving

Laminar Flow

The movement of water particles in straight-line paths that are parallel to the channel. The water particles move downstream without mixing.

Turbulent Flow

an irregular, mixing flow pattern where molecules go in all directions but eventually flow through one place

Reynold's Number

Defines laminar or turbulent flow

<500 : Laminar

500 - 2000 : Transitional

>2000 : Turbulent

Re = UL/v

Re = (velocity*length)/kinematic viscosity

Bed Shear Stress

Frictional force exerted by flow per unit area of bed. Controls entrainment and transport of sediment. Has three components. Tau = rho[(u*/k)ln(z/z0)]^2

Poorly Consolidated Clastic Sedimentary Rock (Shale/Sandstone)

Weakest type of Rock Cliffs

Carbonate Sedimentary Rock (Chalk/Limestone)

Moderate strength type of Rock Cliffs

Metamorphic/Igneous Rock (Granite/Basal)

Strongest type of Rock Cliffs

Consolidated Rocks

Rocks that have cemented or crystallized together to become more resistant to erosion (wave erosion)

Cliffed Coasts

Shoreline: Cohesive Material

Resistant to Wave Erosion Limited Sediment

Small or absent beaches

Cliff (Slope>40°)

No Progradation (different than beaches)

Cliff

Steep slope above water (>40°)

Not overtopped by waves.

Bank

Steep slope above water (>40°)

Commonly overtopped by waves.

Bluff

Steep slope above water (>40°)

Composed of poorly consolidated material (sand/ silt/ clay)

Plunging Cliff

Cliff face extends below the waterline (even at low tide) No beach or platform. Waves break directly against cliff face.

Usually in resistant bedrock (very strong rock)

Usually in locations with rapid SLR or subsidence

Very slow erosional rates

a) Lacking toe b) Subaerial Erosion MINOR c) Depth deep enough for no breaking

Type A Cliff

Most common type of cliffed coast

Erosion at toe leads to horizontal recession

Gentle slope in the intertidal zone to deeper water

a) MIN & MAX water lines at toe b) Plunging Waves

Type B Cliff

More rapid horizontal recession than vertical erosion at toe.

Wave cut platform in intertidal zone

a) Waves break BEFORE reaching the toe

Waves/Wave-Induced Currents --> Debris/Toe Erosion --> Mass Movement --> Cliff Recession

Process of Cliffed Coast Erosion

Wave Hammering (Air compression in fractures and pore space, Widening of cracks) Seismic motion/Shear stress (Wave Orbitals, Swash, Vertical Jet directed up cliff face)

Hydraulic Forces on Cliffed Coasts

Root wedging, Gastropods (Limpets rock scraping), Boring organisms, Acid dissolution, Digestion/Waste/Decomposition, Chelation (Lichen)

Biological Forces on Cliffed Coasts

Sea Stack

An isolated mass of rock standing just offshore, produced by wave erosion of a headland

Sea Arch

A natural opening (Upside down U-shaped) eroded out of a cliff face by marine processes

Wave cut platform

A wide, gently sloping rocky surface at the foot of a cliff

Often formed after a cliff collapses and the remaining rock is eroded away

Sea Cave

Indention created by waves undercutting the base of a sea cliff

Blow Hole

Openings in layers of coastal rock, where the softer level of rock has been eroded away and there is an opening in a sea cave

All the water gets pushed into a smaller and smaller area, and so it is forced upward

Beach

Are composed of unconsolidated sediment

Sediment transported by waves (+LST, + wind)

Composed of sand, pebbles, and cobbles (other sediment sizes too large or small)

Dynamic relationship between sediment deposition and sediment erosion

To exist there needs to be:

More sediment deposited than eroded

A platform for the sediment to rest upon

Cross shore sediment transport

The cumulative movement of Beach and Nearshore sand perpendicular to the shore by the combined action of tides, wind and waves, and the shore-perpendicular currents produced by them.

Swash and Backwash alternation back and forth movement (vertical)

Long shore sediment transport

The cumulative movement of Beach and Nearshore sand parallel to the shore by the combined action of tides, wind, and waves and the shore-parallel currents produced by them

Swash and Backwash alternation horizontal movement

Sand Ripples

Series of small, linear ridges of coarse sand aligned perpendicular to prevailing wind direction

Relationship between sediment size and wave energy with height and spacing

velocity of flow slows due to friction closer to shore

Asymmetrical

When the waves have yet to break, what are the sand ripples like?

Lunate Megaripple (Crescents pointing towards land)

When the waves are beginning to break, what are the sand ripples like?

Tuning Fork Shaped

When the wave direction is constant, what are the sand ripples like?

Ladder Back Shaped

When the wave direction is changing, what are the sand ripples like?

Rolling (Smaller)

When the waves are low energy, what are the sand ripples like?

Aeolian sediment transport

Movement of sediment by wind

Affected by organic materials

Debris

Moisture (wet sand is harder to move and get heavy)

Salt crust created by evaporating saltwater is also hard to move

Vortex (Bigger)

When wave energy is higher, what are the sand ripples like?

Dune Vegetation

Have to be able to endure dry and salt spray environments

Abundant light/less competition

High nutrients

Often have long spindly leaves and hairs that create a rough surface

Create a barrier for wind to catch sand which accumulates over time

Causes dunes to grow as leaf surface area increases

Positive Feedback Loop (Dune creation)

Elaborate Root systems that keep the dunes in place

Makes them larger

How does coarser beach sediment affect ripples size?

Sea Oats

Dune species in the Southeast

Beach Grass

Dune species in the Northeast/West Coast

Barrier System

Emergent, generally linear in shape, depositional landforms which are separated form the mainland coast by a lagoon, bay, or marsh.

Comprised of Sand or Gravel

Minor clay/silt from marsh or remnant terrestrial sediments

Associated with strong LST

Trailing Edge Coasts (Amero-trailing)

Common along US

Tombolo, Bay-mouth, Mid-bay

Barrier Systems with no free ends

Spits (Baymouth, Constrained, Continuation, Flying)

Cuspate Foreland

Barrier systems with one free end

Barrier Island

Barrier systems with two free ends

Offshore Bar/Shoal Aggredation Hypothesis

Wave shoaling builds sediment pile

Pile becomes a nucleus "on which flotsam can accumulate"

Overtime and with abundant sediment, it grows

Beaumont 1845

Stratigraphy = marine sediments throughout

Spit Elongation/Spit Detachment Hypothesis

Spit elongates across bay

Shallow bay upholds sediment

Overwash detaches spit

Tidal inlets form

Maintain a channel that perpetually separates the detached spit (will not reattach)

Gilbert 1858 (North Carolina Outer Banks, Penland 1988 Louisiana Chandeleur Is.)

Stratigraphy: Expect lagoonal sediments overtopped with beach sediments

Submergence/Mainland Beach Detachment Hypothesis

Sea level transgression

Floods low-lying areas

Former dune ridges become island(s)

Hoyt (1967) - Georgia Sea Islands

Stratigraphy: expect no marine sediments, Terrestrial sediments overtopped by lagoon then beach sediments Biogeography = barrier island biota is the same as mainland (Few endemic species)

Overwash

Barrier Island breached by storm waves, which causes sediment to both be deposited and pulled away from shore. Contributes to barrier system movement

Regressive sea level

Causes barrier islands to build up and march out to sea

Progradation occurs

Depositional

Rooster Tail shaped islands

Transgressive sea level

Causes barrier islands to roll landward

Erosion occurs

Development cannot readily adapt

Black seashells uncovered