Week 9: Coastlines and barrier islands Flashcards
What are barrier islands?
Variable lengths and widths
Mobile strip of land
Parallel to coast
Separated from mainland by lagoons, marshes and estuaries
Dynamic environment (migrates towards land)
Where are barrier islands?
Form near river deltas in areas w low tectonic (passive margins) activity and flat coastal terrain
Why do barrier islands matter?
Protect mainland against wave attack
Line of defence against sea level rise
Imp ecological communities
Features of barrier islands
Distinctive depositional zones
Lagoon, marsh and tidal flat, barrier flat, dunes, beach
Affected by longshore currents, waves, storms and tidal activity
Barrier islands- humans
As humans populate these islands they are becoming more immobile
**due to concrete and pavement, no sand to pick up
Overwash
Process by which water and sediment flow over the crest of a barrier island or dune; result of storm surge
Washover
Actual sediment deposited inland of a beach by overwash
**sediment moves inland
Beach heather
Anchor sand into places in dunes
Used in restoration of beach dunes
Importance of dunes
Protect homes during hurricane/storm surge
BUT they are removed when people build seaside homes
Beach nourishment
Addition of sand by humans to increase sand width or volume
*sand added either directly on beach or as underwater mounds
Impacts of beach nourishment
Increased turbidity
Increases cloudiness of water and affects sunlight reaching water which impacts photosynthesis and therefore food chains
Change in sand grain size can impact organisms that live btwn them
Smothers coral in sediment
High concentrations of silts and clays may suffocate infauna by clogging their gills
Environmental costs
What shapes the coastlines?
Erosion
Deposition
Transportation
Accretion (adding material in non-natural way)
Sediment movement on a beach
- Dune erosion during storms
- Offshore loss through rips especially during storms
- Wind erosion or wave washover off the beach
- Longshore transport out of area
- Longshore transport into system
Dunes
Mounds/ridges of sand deposited by the wind immediately landward of the beach
**erosion during high-energy events
What makes up longshore transport?
- Longshore current
- Longshore drift (net movement of sand particles)
Longshore transport
Occurs in foreshore and nearshore
Sediment transport by longshore current and by swash and backwash
**moves parallel to coastline
It is why you float away from friends on the beach
How does longshore transport occur?
Waves approach shore at an angle
Change in velocity as part of wave enters shallow water while the deeper part keeps moving
Slower at shore and faster deeper
Longshore transport part 2
Part of wave’s energy is directed perpendicular to shore and part of it is directed parallel to shore
Parallel direction generates longshore current
Swash
Water moving onto beach, carries sediment onshore, increases beach slope
Backwash
Water returning to ocean, can also transport sediment
**travels perpendicular to beach
Lease energy expenditure possible
Can coasts have both depositional and erosional features?
Yes
Erosional coasts- weathering and erosion
Weathering- breaking down rock
Erosion- Broken pieces removed by wave action and currents
How can erosion occur?
Precipitation
Heat
Freezing
Wind
Waves
What causes weathering and erosion?
- Abrasion
- Attrition
- Corrosion
Abrasion
Wave picks up sediment, throw it against cliffs and it wears down cliffs
Attrition
Sediment hits each other, become smaller and smoother
Corrosion
Breaking down of rock naturally by chemical reaction
Hydraulic action
Breaking wave compressing air into a crack
Salt weathering
Salt deposited as saline solutions, solution evaporates and salt crystals left behind
Salt crystals expand w addition of water, temp changes
**creates destructive pressures
Rate of weathering and erosion
Depends on…
- Hardness and rock strength
- Wave energy
- Tidal range
What percentage of present day coasts are rocky?
75-80%
Features of erosional coasts
Blowhole
Headland
Sea stack
Sea cave
Sea cliffs
Exposed beach
Sea arch
Stumps
Wave-cut platforms
Flat area, base of sea cliff
Cliff receedes overtime
Irregular shorelines
Headlands and bays/coves
Result of wave refraction
What are depositional coasts shaped by?
Longshore drift
Wind generates a longshore current moving left to right
Features of depositional coasts
Beach
Spit
Tombolo
Delta
Bay barrier, bay mouth bar
Barrier island
What is a beach?
Accumulation of unconsolidated sand or gravel that extends from average low tide to uppermost exent of wave impact
Types of beach sediment?
- Volcanic source
- Rounded glass fragments (old garbage dump)
- Mix of terrestrial and biological particles
Seasonal beaches
More storms in winter, increase in wave energy moving material onshore and carrying material offshore
Fined grained sediment
Water easily and quickly runs down beach slope
More sediment carried away by backwash
Broader flatter surface
Coarse grain sediment
Water moves easily downward through sediment
Backwash carries less sediment back into ocean
Steeper beach
Beach in winter vs summer
Winter= steeply dipping, narrow berm, sand stored offshore, coaser grains
Summer= Shallow dipping, wide berm, sand stored on beach, finer grains
Spits
Linear accumulation of sediment attached to land at one end
Tombolo
Strip of sediment accumulating btwn an offshore island or sea stack and main shoreline
*forms by change of velocity of longshore current
Baymouth bar/ bar barrier
Spit grows across a bay
Delta
Deposition of sediment at river’s mouth
River-dominated delta
Weak wave activity, low tidal range, RIVER INPUT
EX. missisipi river delta (birds foot)
Tide-dominated delta
Strong tidal influence
Lone narrow sand bodies separated by tidal channels
Parallels tide direction
ex. Ganges river (largest delta in world)
Wave-dominated delta
Wave erosion controls shape
Straight shorelines and well-developed beaches oriented parallel to coast
ex. Nile delta
What is an estuary?
Bodies of water and associated coastal habitats found where rivers meet ocean
Saline water mixies w freshwater (brackish water)
One of the most productive ecosystems on earth
**bullsharks
Ecosystem services of estuaries
Water filtration, habitat and shoreline protection, buffer zones
Classifying estuaries
Either based on geologic origin or based on salinity and mixing patterns
Estuaries origin
- Drowned river mouth
- Fjord
- Bar-built
- Tectonic
Drowned river mouth- coastal plain
Shallow w fluvial sediment input from rivers
ex. East coast of North America
*submergent coast
Fjord (drowned glacial valley)
Formed in deep, U shaped basins carved by glaciers
Steep sides, rock bottoms, underwater sills (underwater ridge of sediment)
*submergent coast
Tectonic
Result of tectonic movements
Faulting causes some sections of crust to subside
*primary coastline
Bar-built (restricted mouth)
Barrier btwn mainland and ocean
Water that remains behind the barrier is partially cut off from ocean
ex. along passive margins (Atlantic coast)
*depositional coastline
Estuaries salinity and mixing
- Salt wedge
- Partially mixed
- Well mixed
- Fjord
Salt wedge
Least mixed (most stratified)
Freshwater on top, saltwater on bottom
ex. St. Lawrence river
How is sediment brought to beaches?
Wave activity, river deposition, cliff/dune erosion
How does sediment withdraw from beaches?
Transport down submarine canyons
Coastal cells
Break coastline up into a number of coastal cells
Complete cycle of sedimentation including source areas, transport paths and sediment sinks
How can we prevent too much deposition?
Build dams
Build jetty
Starved beaches
Not enough sediment being supplied to beaches in coastal cell
Coastline moves inland
Increases effects of sea level rise and storms
