Spits and Bars, sand dunes Flashcards
What do you need for sand dunes to form?
Large flat beach
Requires a large amount of sand
Large tidal range (The tidal range is the difference between the high tide and the following low tide) - to allow time for sand to dry
Onshore wind (Onshore wind is the one that blows from the sea towards the land) to move sand to back of beach (backshore)
An obstacle e.g. driftwood (Driftwood is wood that has been washed onto a shore or beach of a sea, lake, or river by the action of winds, tides or waves) - for the dune to form against
Sand dunes: forms on a dry backshore (dry under normal conditions, waves don’t reach unless in extreme conditions) of a sandy beach
3 types of wind transport of sand:
suspension - 1% of total wind movement is where sand is carried within the wind
Saltation - 95% of total wind movement is where grains of sand bounce along in the wind as they are alternatively raised and dropped
creep - 4% of total ind movement is where sand grains collide with each other and push other grains along
How do sand dunes form?
Heaviest grains of sand settle agains obstacles e.g driftwood to form small ridge. Lighter grains may be transported and settle on the other side of the obstacle.
2) area facing the wind (windward slope) reaches a crest because the pile of sand becomes so steep, it becomes unstable and begins to collapse under its own weight
3) lighter grains if sand drop down on the other side on lee face (aka. slip face). Sand stops sliding down on lee face once it recaches a stable angle of 30 to 34 degrees
4) repeated cycle of wind blowing sand up the windward side and then sand slips down the leeward slope, causing the sand dune to migrate inland (away from the sea) over time
5) sand dune itself become an obstacle and so more sand dunes can dorm in front of it
Height of dunes depends of strength of wind, stronger winds create higher dunes
define the following: embryo dunes marram grass saltation crest water table dune slack leeward slope windward slope
Embryo dunes: a newly formed sand dune closest to the sea
Marram grass: a plant found in sand dunes that has long, binding roots
Saltation: how sand is bounced along by the wind
crest: the top of the sand dune
water table: the upper horizontal limit of wet sand (everything below the water table is damp)
A level below which the sand/rock is saturated with water.
Dune slack: where there is a trough or low point in a line of dunes
leeward slope: the slope that faces away from the wind
Windward slope: the slope that faces the wind
How do Sand dunes change inland ?
Parallel lines of dunes
Several lines of dunes may run parallel to the shore
Dune succession
there is a change in vegetation with increased distance inland, this is called dune succession.
Grow taller:
embryo dunes are only a few metres high whereas mature dunes are 15m high
Size
Size increases as you go inland (further away from the coast)
This is because marram grass grows quickly and binds the sand together (its long roots bind with sand, preventing further migration and help build up height of dunes) and helps sand accretion (build up).
Sand colour and vegetation
dunes closest to the beach have a yellow, sandy colour and not much vegetation. Dunes further back look grey and less sand-like.
Colonisation from vegetation
Inland dunes get increasingly colonised by vegetation
Slack formation
Each line of dunes is separated by a trough (like a valley) called a slack. Slacks are formed when there is ongoing removal of sediment (sand) from the leeward base and travels up the windward side of the next dune line.
Salty ponds
Slacks can be eroded so much (go down) that they reach the water table, resulting in salty ponds.
Blowout
sometimes, a dune may develop a huge depression called a blowout, this is when strong winds remove sand from an area that has lost its protective vegetation cover. If the vegetation cover is gone, strong winds might destroy the dune
what is longshore drift?
Load (sand etc.) is transported along the shore by longshore drift (is the movement of sand in a zig-zag pattern along the beach). The direction of longshore drift is determined by the prevailing wind direction. (Prevailing winds are winds that blow consistently in a given direction over a particular region on Earth).
The Southwest wind (from SW) of the Dorset Coast causes longshore drift in eastward direction, making the swash surge up at an angle and come back down parallel to the shore (called the backwash) due to gravity. Suspended load (load carried by waves) is carried in an easterly (towards east) direction in a zig-zag manner.
This movement of material means that over time beaches can change shape and sometimes disappear completely so, people build groins on beaches. They trap sand moving in from the longshore currents and prevent it from moving away
how are spits formed ?
Explain how a spit forms
A spit is formed when sand or shingle sediment is deposited out from land at a place where the coast changes shape, e.g. at the mouth of an estuary where the river meets the sea.
This occurs when the longshore drift transports load from the beach out into sea.
Direction of LSD is determined by the prevailing wind.
Waves carry sand/shingle with them
The swash refers to the waves moving up the beach, carrying sediment with them. The direction of the swash is in the same direction as the prevailing wind.
The backwash refers to the wave moving down and away from the beach. The direction of the backwash is perpendicular to the beach.
Sand/shingle is transported in a zig zag pattern along the beach
It ends up out in the sea where it is deposited
The load nearest to the land (proximal end of spit) has larger pebbles than the distal end (end of spit far from land) because rocks are broken down by attrition by the time they are transported to the distal end.
Since the wind direction can change shape out at sea this can cause a hook to form in the spit
The spit will stop extending out when the water gets too deep, or, sediment is removed by waves faster than it is deposited
characteristics of spits where does it form, when they stop growing, why does it have a hook at the end ?
A spit is a shingle or sand beach that is joined to the land but projects downdrift into the sea.
Spits form when the coastline changes shape or at the mouth of an estuary (mouth of river were it meets the sea) a spit is an unstable landform (shape can change).
It will continue to grow until
the water is too deep at distal end of spit
or, until material is removed (from waves) faster than it is deposited.
- Proximal end is closer to the land (see figure 10.33 on pg 136)
- A hooked end can form if there is a change in wind direction.
Hurst castle spit in Hampshire is growing in length, but losing shingle from its main ridge.
Sandbanks spit in Poole, Dorset is not growing due to tidal currents (water movement generated by tides) and dredging
CHARACTERISTICS
Proximal end (closer to land) has large pebbles
Has a hooked distal end which has small pebbles, a hooked end can form towards the land if there is a change in wind direction.
On a shingle or pebble spit the pebbles become smaller and more rounded towards the distal end due to attrition. They become smaller as the longshore drift gets weaker because weak longshore drift can only move small pebbles not large ones.
Spit can be approx. 4km long
Recurves
Lagoon (A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs)
Large hook
narrow, steep beach with berms ( and large pebbles at the back of beach
mudflats and salt marshes sheltered by the spit
how is a Tombolo formed ?
A tombolo is formed when a spit connects the mainland coast to an island. A spit is a feature that is formed through deposition of material at coastlines. The process of longshore drift occurs and this moves material along the coastline.
Material is pushed up onto beaches at an angle when the swash brings it onto the coastline at a 45 degree angle. The backwash takes it back out towards the sea at a right angle to the coast.
Through this process material is constantly moved along the coastline. When the coastline changes direction or there is a river estuary the process of longshore drift continues. This causes material to be deposited in a long thin strip that is not attached to the coast and is known as a spit. If this feature moves in the direction of island and connects it to the mainland then it becomes a tombolo.
