Coastal water and estuaries - 10 Flashcards
What does it mean when waves ‘shoal’?
When waves touch the bottom of the floor
What is wave set up?
A process whereby waves pile water against the short
What are longshore currents?
Currents generated by a pressure gradient (the sea surface slopes from areas of larger waves down to areas of smaller waves) that runs parallel to the shore
What are rip tides?
A swift, narrow, sea ward flow perpendicular to the coastline generated by the convergence of longshore currents
What the angle of wave approach?
The acute angle between the wave crest and the beach (it’s rarely larger than 10º)
How is sediment moved on the beach and the nearshore?
Currents and breakers
How can you divide the beach up?
- Offshore
- Nearshore
What is offshore?
The area seaward of where waves first being to break
What is nearshore?
The area from offshore to where waves wash back and forth across the beach … it can be divided into the breaker zone, the surf zone, the swash zone and the backshore
What is the breaker zone?
Where waves begin to break on the nearshore
What is the surf zone?
Where the waves expend most of their energy
What is the swash zone?
Where waves wash back and forth
What is the back shore?
The land that adjoins the nearshore
Zones of the beach change with high and low tide. How?
At high tide, zones advance landward. At low tide, zones advance seaward
What is a beach profile?
A cross section of a beach taken perpendicular to the shoreline that can be compared at different times to see if the beach is expanding or eroding
What 2 different types of beach profile exist?
A swell profile and a storm profile
What are the characteristics of a swell profile?
- wide, broad berm
- steep intertidal beach face
- develops during the summer
- enlarges the volume of the beach
What are the characteristics of a storm profile?
- erosion of the berm
- broad, flat intertidal beach face
- develop during winter
- coarse sediment left on the beach
What is the sand budget?
The balance between sediment added to and sediment eroded from the beach
Where does beach sediment input come from?
River, sea-cliff erosion and onshore sediment transport
How is beach sediment removed?
Longshore currents, offshore transport and wind erosion
How are coastal cells formed?
When sand transported by long shore drift is permanently lost in submarine canyons
What are the best conditions for sand dune formation?
- Abundant sand
- Strong persistent onshore winds
- Large tidal range
- Beach is wide and gently sloping
How far inland can sand dunes extend?
Up to 10km
What are blowouts?
Wind-scoured breaks in the dune or depressions in the dune ridge, commonly caused by destruction of vegetation
What is dune migration?
The movement of dunes. Sand salutes (bounces) up the windward side of the dune, collects in the wind-shadow and periodically slides down the leeward face of the dune
What impact does wave erosion have on sand dunes?
Creates a steep scarp at the base of the dune which then reflects wave energy, reducing erosion
What are barrier islands?
Landforms composed of sediment that parallel the coast and form where sand supply is abundant and a broad seafloor slopes gently seaward
What environments develop on barrier islands?
Nearshore zone, dune fields, back-island flats, forests and salt marshes
How are barrier islands created?
- Sand ridges on the coastal plain that become isolated due to sea level rise
- Sand spits that become breached during a storm and remained separate by a tidal inlet
- Vertical growth and emergence of longshore sand bars
What is rollover?
The migration of barrier islands landward due to sea level rise, which causes the washover of sediments from the seaward died to the landward side
How are sea cliffs eroded?
- Waves slam into cliffs … water compressed inside cracks … expands violently when the wave recedes … shatters the rock
- Sea water can dissolve some types of rock
- Evaporation leaving behind salt crystals in faults … chemically breaks down rocks
What does cliff recession depend on?
- Composition and durability of cliff material
- Presence of joints, faults and cracks
- Amount of precipitation
- Steepness of the cliff
- Wave energy
What is a wave-cut platform?
The gentle sloping area in front of the sea cliff that was produced by erosion and sea-cliff retreat
What is a delta?
An emergent accumulation of sediment deposited at the mouth of a river
What are the major areas of a delta?
- Delta plain
- Delta front
- Prodelta
What is the delta plain?
The flat, low-lying area at or below sea level that is drained by a system is distributaries
What is the delta front?
The shoreline and broad submerged area of the delta that slopes gently seaward
What is the pro delta?
The far offshore area of the inner shelf that receives fine sediment from the river
How can we divide up a delta?
- Topset bed
- Forest bed
- Bottomset bed
What are topset beds?
Flat-lying beds of sand and mud of the delta plain and deposited by the distributaries in their channels
What are foreset beds?
Thick silts and sands of the delta front that slope gently seaward and form the bulk of the delta
What are bottomset beds?
Flat-lying silts and clays of the pro delta that settle out of suspension far offshore
How does a delta expand?
Sediment accumulation (foreset beds bury bottomset beds and topset beds blanket foreset beds)
What is the shape of a delta dependent on?
The relative power of tides, waves and rivers
Where do river-dominated deltas form?
- Areas protected from large waves
- Small tidal range
What are the different types of delta?
- River dominated
- Wave dominated
- Tide dominated
What are wave dominated deltas altered by?
Wave erosion and longshore drift (most of the sediment if distributed along the coast and only a slight protrusion exists)
What are tide-dominated deltas altered by?
The ebb and flow of the tides into long, linear submarine ridges and islands
What are estuaries?
Semi-enclosed bodies of water where freshwater from the land mixes with sea water
How are estuaries created?
- Drowned river valleys (rise in sea level floods lower portions)
- Fjords (glaciers retreat, sea level rises, lower portions flooded)
- Bar built
- Tectonic estuaries
What is the typical salinity gradient in an estuary?
Normal marine salinity at the tidal inlet to freshwater at he river mouth
What provides the energy for mixing of water in estuaries?
Tidal flow
How are estuaries categorised?
Based upon the relative importance of river inflow and tidal mixing
What are the three types of estuaries?
- Salt-wedge
- Partially mixed
- Well imxed
What is a salt wedge estuary?
An estuary dominated by the outflow from rivers (greater than inflow from tides)
What are the characteristics of a salt-wedge estuary?
- Well stratified
- Sharp halocline
- salt water form a wedge that lifts freshwater outflow off the bottom
- internal waves propagate along the halocline
- land-ward directed bottom current
- sea ward directed surface current
What is sediment like in a salt wedge estuary?
River sand near the landward edge and river clays and silts elsewhere
What is a partially mixed estuary?
An estuary equally affected by river discharge and tidal mixing
What are the characteristics of a partially mixed estuary?
- Weaker halocline
- strong landward directed bottom current due to upward mixing of seawater
- beach and shelf sediment blanket the lower half of the estuary
- river sand and mud blanket the upper landward half
What is flocculation?
When clay particles stick together in sea water forming a larger aggregate which sinks more rapidly
Where do mud shoals form?
Where currents are weak in the estuary
What is a well mixed estuary?
When tidal turbulence destroys the halocline, meaning water does not change with water depth
What effect does Coriolis have on wide estuaries?
Deflects river outflow to one side and tidal inflow to the other … creates a lateral salinity gradient
What is sediment like in a well mixed estuary?
Abundant marine sediments that dominate throughout the estuary
