ELEMENTS OF OCEANOGRAPHY

 

TOPIC 14 - COASTAL ENVIRONMENTS AND BEACH PROCESSES

Overheads (two lectures):

Physical Influences on Coastal Margins
plate tectonics
passive margins - broad shelves, no volcanism, low relief
active margins - narrow shelves, volcanism, high relief
sea level variability (long term)
sea level rise - irregular, submergent coastline
sea level fall - smooth, emergent coastline
ocean/atmospheric circulation + climate

Model for Coastal Processes - I
Three dominant factors define coastal form
Wave Energy
high energy promotes erosion
low energy promotes sediment deposition
Sediment Supply
high supply protects coastline
low supply permits enhanced erosion
Tides - only important in local areas with broad shallow lagoons or estuaries

Model for Coastal Processes - II
There are four dominant types of coastline
Sea Cliffs (headlands)
High Energy Beaches
Low Energy Beaches
Deltas

Beach Zones
Offshore - below wave base
Surf Zone - breaker formation
Transition Zone - reformed breakers
Swash Zone - wave runup
Berm - high tide limit
Backbeach - dunes
Sea Cliff

Seasonality of Seashores
Wave activity and associated longshore currents in many areas is strongly seasonal
The tides also follow a monthly cycle which is superposed on seasonal and longer time scale wave activity
Sediment input to the coastline also varies on seasonal and longer time scales
Sea cliffs become active or inactive depending on the above factors

Sea Cliffs
Sea cliffs represent a net erosional zone
erosion occurs by wave action eroding caves into the base of cliffs
Ultimately, sea caves collapse causing coastal landslides
Sea caves may break through and form sea arches
sea arches may expand and isolate headlands into sea stacks

Global Beach Types
Coastal Plain Beach - most common, typical along passive margins
Barrier Beach - typical wherever hurricanes occur
Pocket Beaches - typical along active margins with resistant bedrock headlands
Spits - localized features dependent on prevailing longshore currents

Human Impact on the Beach Environment
When humans live along beaches they want to preserve the summer beach condition
Seasonal or longer term erosion is a nuisance humans try to control
Build artificial beach structures
groins or jetties
sea walls or revetments

Coastal Deltas
When sediment supply outstrips the ability of wave action to remove it, deltas develop
Most deltas develop on passive margins with large rivers and large drainage areas
Deltas form by extending the river mouth out into the coastal ocean and forming a series of distributory channels
Delta fronts develop where sediment is deposited at the mouths of channels

Estuaries
Semi-enclosed embayments open to the sea at one end and having significant river input
Estuaries may form in a variety of ways
All estuaries have sea water entering at depth, mixing with riverine fresh water, and brackish water leaving at the surface

Lagoons
Semi-enclosed embayments with the sea entering at one end - no significant riverine input
formed behind barrier islands or spits, typically on passive margins
seawater enters at surface, heating and evaporation creates denser water in the restricted lagoon environment,denser water leaves lagoon at depth

 

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