COASTS:  
Waves, Sand, and Rock

Waves

Definition:  A disturbance in the water column that propagates away from the source.

Wave Parameters

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Wave Length (L):  Distance from crest to crest.
Wave Height (H):  Distance from trough to crest.
Wave Period (T):  Time for two successive crests to pass a fixed point.
Wave Velocity (V):  V = L/T

Wave Generation

¥Wind transfers energy to water (particularly during storms).
¥Controls on wave heights
	- Wind Speed
	- Storm Duration
	- Fetch (length of area over which storm blows)

Wave Propagation

Swell vs. Sea
	- Swell: waves from distant storms; waves have been sorted by period.
	- Sea: waves from local storms; chaotic mix of heights and periods.

Motion of Energy vs. Motion of Water Particles

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	- Wave energy propagates.
	- Water particles make small orbital motions; no net movement.

Wave Shoaling

Deep vs. Shallow Water Waves

	- Deep water waves
		-- Definition: Wave Length < 1/2 Water Depth
		-- Velocity is determined by wave period (T)

	- Shallow water waves
		-- Definition: Wave Length > 1/20 Water Depth
		-- Velocity is determined by water depth (d)

Shoaling Transformations (how waves change as they enter shallow water)

	- Velocity decreases
	- Wave height increases
	- Wave length decreases
	- Period is conserved (remains the same)

	Why?  Because the front of the wave train is in shallower water than
	the back and is therefore moving slower.

Wave Refraction

In shallow water:

	Velocity dependence on water depth 
	+ Uneven water depths near shore
	= Bending of waves

Results from velocity dependence on water depth:  V = sqrt (gd)

Refraction determines the distribution of wave energy at the shoreline

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Headlands:  Energy is concentrated by refraction -> good "point breaks"

Bays:  Energy is spread out by refraction -> good harbors

Refraction causes waves to approach the shoreline at low angles
	-Example:  deep water direction of wave approach locally is
	from the NW;  at the boardwalk beach, waves approaching
	the beach have refracted almost 180 degrees around lighthouse
	point and approach the beach from the south.

Wave Breaking

Waves break when wave height is approximately 3/4 of the water depth:
e.g., a 3-foot wave will break in water 4 feet deep.

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Zones of the nearshore:

	- Breaker Zone
	- Surf Zone
	- Swash Zone

Wave behavior is very different in each zone.
Beaches

Beach Material

	- Chiefly mineral grains and rock fragments weathered out of 	continental rocks

	- Mainly quartz and feldspar grains on our coast and in most 	temporate zones

	- Coral beaches common in tropics

Beach Morphology (form)

Summer Beach:  wide berm, steep beach face

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Winter Beach:  narrow berm, flat beach face, longshore bar

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Seasonal movement of sand offshore to form longshore bars in response to energetic winter waves; return of sand to subaerial beach under gentler waves of summer.

Littoral Drift

Definition:  Alongshore movement of sand due to waves breaking at a angle 
to the shoreline.

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Longshore current:  alongshore current in surf zone produced by waves 
breaking at an angle;  this current acts to transport sand grains put into
suspension by the breaking waves.

Magnitude of the longshore current, and therefore of littoral drift, depends
depends on wave height and wave angle;  an increase in either of these two increases the velocity of the longshore current.

Littoral drift rates can be high:  250,000 cubic meters/year at Santa Cruz 
Harbor.

Sediment Budgets and Littoral Cells

Sediment Sources:
	- Littoral Drift In
	- Streams and Rivers
	- Seacliff Erosion
	- Dune Erosion
	- Onshore Sand Movement
	- Beach Replenishment

Sediment Sinks (places beach sands are lost from the beach):
	- Littoral Drift Out
	- Submarine Canyons
	- Removal to Dunes
	- Offshore Sand Movement
	- Sand Mining

Sediment Budget:  Accounting goes to the beach;  attempts to quantify all sinks and sources for a region and arrive at a balance.

	Sediment Sources - Sediment Losses = Erosion or Growth or Stability


Littoral Cells

Definition:  a geographic region of the coast that is self-contained with
respect to all sosurces and losses of beach sand.

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Seacliffs and Seacliff Erosion

Formation:  Seacliffs formed (at least on west coast of US) by a combination
of tectionic uplift of the land and sea level changes related to coming
and going of ice ages.

	Uplift + Sea Level Changes = Sea Cliffs and Marine Terraces

Marine Terraces:  former sea floors planed flat by the waves then uplifted
and preserved above sea level.

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Erosion

Causes:
	- Wave Action
	- Terrestrial Erosion (surface runoff and groundwater)
	- Chemical Dissolution of Rock Material (e.g. wetting and drying
	by seawater)

Controls of Erosion Rates:
	- Wave Exposure (open coasts vs bays)
	- Climate (storminess)
	- Lithology (granite more resistant than sandstone)
	- Structure (fractures and faulting)
	- Land Use Practices (e.g. lawn watering)

Natural Process:  seacliff erosion is a natural process, not a problem with 
the cliff; vertical slopes are inherently unstable and will always erode; it's
rates and causes of seacliff erosion that differ from place to place and time
to time.


Hazardous Coastal Environments


Coastal Enviroments

East Coast and Gulf Coast:
	- Flat coastal plains
	- Lagoons
	- Barrier islands
	- Large estuaries
	- Wide flat continental shelves

West Coast
	- Coastal mountains
	- Uplifted marine terraces
	- Seacliffs
	- Pocket beaches
	- Narrow steep continental shelves

Differences due to Passive (East and Gulf coasts) vs. Active (Pacific coast)
Continental Margins:
	- Active is on plate boundary with uplift balancing erosion
	- Passive is mid-plate with mainly erosion and high sediment
	production

East Coast Hazards:
	- Beach and Dune Erosion
	- Hurricanes
	- Storm Surge and Coastal Flooding
	- Barrier Island Overwash

West Coast Hazards:
	- Beach and Dune Erosion
	- Seacliff Erosion
	- Landsliding
	- Earthquakes
	- Tsunamis
	- El Nino Events (high tides + storm waves)

Hazardous Coastal Environments in California

Eroding Seacliffs

	- Cliffs are inherently unstable
	- Prone to both wave erosion and landsliding
	due to terrestrial processes
	- Erosion highly episodic

The Beach

	- Unconsolidated material (mobile)
	- Deposited by waves
	- Much too close to sea level
	- Exposed to episodic large storms

The Dunes

	- Unconsolidated material
	- Subject to wave erosion
	- Natural exchange of sand between beach and dune
	- Loss of vegetation can cause dunes to migrate due to wind