Sea Level Lecture 4- Where the land and the oceans meet: Sea level and coastal processes Flashcards
Name three features of high energy coastlines.
Waves
Beaches and Dunes
Barriers
Name three features of low energy coastlines.
Estuaries
Intertidal zones
Wetlands
Are high energy coastlines long term depositional environments?
No
Can dune like systems develop on high energy coastlines?
Yes
What is fetch?
Area over which waves are travelling and direction wind is blowing
What controls global wave power?
Wind
Distance
Fetch
What are ‘roaring fourties’?
Southern Hemisphere wave power uninterrupted fetch.
What latitudes have strong global wave power?
Mid latitudes
What happens in deep water? (linear wave theory)
- particles flow in circular motion
- Orbit radius decreases with depth
What happens in intermediate water? (linear wave theory)
Elliptical near sea bed, then to and fro
Disturb sediment at sea floor
What happens in shallow water? (linear wave theory)
Horizontal, to and fro, uniform with depth
Able to move material
Do waves move through the water or does water move?
Waves move through the water
When does shoaling start?
As waves move in when depth is half the wave length.
Why do waves start to steepen and get closer together near the shore?
Friction
What is shoaling?
Wave shoaling is the change in shape and behaviour as waves propagate into water of decreasing depth.
What are the 5 nearshore zone regions?
Deep water
Shoaling zone
Breaker zone
Surf zone
Swash zone
When are rates of transport greatest?
When wave approach is around 45 degrees
Where does longshore current occur?
Anywhere where waves are refracted as they approach the shoreline (i.e. are at an oblique angle).
What happens at gravel beaches?
Transport occurs primarily in the swash zone (uprush of wave onto the beach)
What happens at sandy beaches?
Transport generally occurs in the entire surf zone
What are beaches?
Accumulations of sand or gravel along shorelines deposited by waves and wave induced flows.
Globally what % of coastlines are sandy beaches?
20%
Globally what % of coastlines are gravel beaches?
10%
What identifies a beach?
From low tide mark to next geomorphic feature such as a dune or sea wall.
What 4 zones does a microtidal beach have?
Zone of shoaling waves
Breaker zone
Surf zone
Swash zone
Are zones on beaches variable?
Yes, temporally and spatially variable.
Not al necessarily present
What happens at a macro tidal beach?
Variable wave shoaling depending on tidal cycle.
Where are coastal dunes commonly found?
Temperate climates
What are coastal dunes?
Common features in wave-dominated coastal environments, perceived to mark the landward limit of marine influence.
What are foredunes?
Parallel to the beach
Beach provides sediment
Up to 100m
Significantly affected by wave processes
What are hind dunes/secondary dunes?
Further inland
Orientation controlled by wind direction, water table and topography.
What are coastal barrier systems?
Elongated wave, tide and wind-built ridges
Predominantly unconsolidated sand and gravel
Protect adjacent mainland from open water processes
What % of world’s coastlines do barrier systems constitute?
15%
Where are barrier systems common?
Settings with low gradient continental shelves
Passive margins with abundant sediment supply
What causes sand to accumulate onshore?
wave energy
What do barriers provide quiet water behind for?
Marshes and lagoons
Where are barrier systems not common?
Areas with high tidal range or tidal energy
Do barrier systems interact quite strongly with sea level?
Yes
Where do retrogradational barriers form?
Where sea level rise is greater than sediment supply
Where do vertical accreting barriers form?
Where sea level rise and sediment supply are broadly in equilibrium
Where do progradational barriers form?
Where sea level rise is lower than sediment supply.
What do barrier island processes look like at wave dominated coasts?
Long linear islands, few tidal inlets
Back barrier is open water lagoon.
Marshes on landward side of islands.
Flood-tidal deltas inside tidal inlets.
What do barrier island processes look like at tide dominated coasts?
Low wave energy (high tidal energy) = no barriers
What do barrier island processes look like at mixed energy coasts?
Short barriers
More tidal inlets (tides more important)
Back barrier filled with sediment
What is an estuary?
A semi-enclosed coastal body of water which has free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.
Opening permanent or periodic
Salinity and mixing processes
Seaward portion of a drowned valley system, sediment fluvial and marine sources
Transitional zone of fresh and marine environments
What are the four main types of estuary based upon water mixing?
Salt wedge
Partially mixed
Well mixed
Fjord type
What is an example of a salt wedge estuary?
Mississippi
What is a salt wedge estuary?
Dominated by river that flows into it
Lack of mixing caused by waves or strong currents
Lid of freshwater, often stationary
Vertical salinity profile steep (halocline)
Some mixing by shear stress
Location of lower salt water wedge will vary through time and season
What is a partially mixed estuary?
Moderate tidal range
Friction to sea bed is high (shallow)
Transition zone between fresh and salt water
Weak halocline develops
Null point marks limit of mixing
What are examples of partially mixed estuaries?
Thames
Chesapeake Bay
What are examples of well mixed estuaries?
The Severn
Firth of forth
Bay of Fundy
What is a well mixed estuary?
Often broad and shallow allowing waves to penetrate into estuary
River flow weak compared with tidal flow, or high tidal range to combat freshwater input
Well-mixed homogenised water column
Limited vertical stratification
What is an example of a fjord type estuary?
Nuuk fjord in Greenland
What are fjord type estuaries?
Water has to flow over a sill
Created a freshwater lid on the salt water
Limited mixing
Fjord can be several hundred meters deep
Stagnation can occur and anoxia at the base
What is circulation controlled by?
Freshwater inflow
Tidal amplitude
Wind
Wave induced currents
Secondary currents (mixing salt and fresh/density gradients)
What is the impact on flocculation of sediment?
Freshwater: clay minerals with negative charge
Marine water: clay minerals with positive charge
Mix = encourage attraction = flocculation
Sedimentation/deposition
What are Dalrymple et al. (1992) two idealised models of estuarine sedimentation?
Tide dominated and wave dominated
3 zones within each
Outer zone (marine processes)
Central zone (low energy)
Inner zone (river dominated)
Discuss tide dominated estuaries.
Tidal current energy > wave energy at estuary mouth
Funnel shape; sandbars develop at mouth
Central zone of low energy is not as pronounced; sand bodies may extend the length of the estuary
Name two tide dominated estuaries.
Humber
Erme in south Devon
Discuss wave dominated estuaries.
Protected from offshore wave energy by fronting barrier islands, bars and spits.
Primary drivers: river (behind barrier) and waves (in front)
Tidal energy decreases landward, fluvial energy decreases seaward.
Unlikely to be weel mixed, brackish behind barrier.
Name an example of a wave dominated estuary.
Exe, Devon
What are intertidal environments?
Controlled by tidal inundation, areas of no vegetation, moving into higher evelation means areas of adapted vegetation
What are tidal flats?
Tidal flats are formed in the intertidal and nearshore zones as slackened currents begin depositing silt, clay and organic matter.
Where current starts to weaken, ranges from sands to silts and mud.
Largest where tidal range is high.
Flat to gently sloping unconsolidated sediments.
Fine sediments higher up the estuary. Dissipate wave energy and facilitate nutrient cycling.
What are characteristics of saltmarshes?
Short plants such as grasses, herbs and shurbs
Mostly restricted to mid-high latitude coastline
What are characteristics of mangroves?
trees or shurbs
Low energy tropical and subtropical environments
What is vegetation zonation?
Vertical arrangement ‘zonation’ of species, generally parallel to the shoreline
Succession due to tidal inundation
Often used to distinguish ‘low’ and ‘high’ sections of a saltmarsh
Regarded as high productive ecosystem
What is the model of saltmarsh sedimentation (Allen, 1994)?
Idealised horizontal platform, bounded by deep water body and a physical barrier
Tidal flow velocity decreases abruptly as it enters marsh
Suspended sediment settles out due to velocity decrease as it is carried over the marsh
What is a drowned saltmarsh?
If rate of accretion and surface evelation is smaller than rate of sea level rise.
What is an emerging saltmarsh?
rate of surface elevation is greater than rate of sea level rise
