2.A - how are depositional landforms developed? Flashcards
1
Q
why does deposition at coastlines occur?
A
when waves lose or have insufficient energy
- in a bay
- shallower
2
Q
why does deposition at coastlines occur?
A
- material is deposited where there is a loss of energy caused by a decrease in velocity or increase/decrease in volume of water.
- where rate of sediment accumulation exceeds rate of removal (large supply of sediment)
- when waves slow down immediately after breaking
- at the top of the swash where for a brief moment the water is no longer moving
- during the backwash, where water percolates into the beach material
- in low energy environments, such as those sheltered from wind and waves (e.g. estuaries)
3
Q
why do we get beaches in bays whilst erosion is focused on headlands?
A
- wave refraction takes place
- as each wave nears the coastline, it is slowed by friction in the shallower water off the headland.
- at the same time, the part of the wave crest in the deeper water approaching the bay moves faster
- this means waves refract around the headland and the orthogonals converge (show direction of wave travel)
- thus wave energy is focused on the headland and erosion is concentrated there
- in the bays, the orthogonals diverge and energy is dissipated, leading to deposition
- as the waves break on the side of the headland at an angle there is a longshore movement of eroded material into the bays, adding to the build up of beach sediment
4
Q
how is dynamic equilibrium reached on the coast?
A
- even though headland is harder rock (and this is how it is initially formed), wave refraction means erosion is focused on the headland, which protects the bay as the waves are caused to diverge.
- eventually headland will be in line with bay and bay will erode more as it is softer
- cycle repeats = dynamic equilibrium
5
Q
what 3 sources of sediment are there for beaches?
A
- cliff erosion
- offshore
- rivers
6
Q
what sediment type are beaches?
A
consists of sand, pebbles and cobbles
7
Q
how significant is cliff erosion as a source of sediment for beaches?
A
- typically only around 5% (not very significant)
8
Q
how significant is offshore as a source of sediment for beaches?
A
- combed from the sea bed, often during periods of rising sea levels
- about 5% but even less sig than cliff erosion as its not all the time
9
Q
how significant are rivers as a source of sediment for beaches?
A
- the remaining 80% of sediment is carried into coastal system as suspended and bed load through river mouths
- most significant
10
Q
when does a drift aligned beach form?
A
- form when longshore drift moves material down the coast producing a range of partly detached features.
- spits are created in this way
11
Q
when does a swash aligned beach form?
A
- form when waves break parallel to the coast
- bay beaches, bay bars and barrier beaches
12
Q
how does size of sediment affect beach morphology - SAND?
A
- sand produces beaches with a gentle gradient because its small particle size means that it becomes compact when wet, allowing little percolation during backwash
- as little energy is lost to friction and little volume is lost to percolation, material is carried back down the beach rather than being left at the top, resulting in a gentle gradient and the development of ridges and runnels parallel to the shore.
13
Q
how does size of sediment affect beach morphology - SHINGLE?
A
-shingle produces steeper beaches because swash is stronger than backwash so there is a net movement of shingle on shore.
- shingle may back up the upper part of the beach where rapid percolation due to larger air spaces means that little backwash occurs and so material is left at the top of the beach.
14
Q
what is a runnel (and a ridge)?
A
- ridges are areas of the beach that are raised
- and the dips are water filled troughs called runnels
15
Q
what is a berm?
A
- smaller ridges that develop at the position of the mean high tide mark, again resulting from deposition at the top of the swash
16
Q
what is a cusp?
A
- small, semi-circular depressions
- they are temporary features formed by a collection of waves reaching the same point when swash and backwash have similar strength.
- the sides of the cusp channel incoming swash into the centre of the depression.
- further down the beach, ripples may develop in the sand due to the orbital movement of water in the waves.
17
Q
how can beaches be viewed as a system in dynamic equilibrium?
A
- weak backwash and strong swash in summer (constructive waves push sediment forward)
- destructive waves in the winter cause the material to be pulled back.
- sand bars help decrease energy in the waves, which encourages negative feedback and therefore equilibrium.
18
Q
what does the size of a spit depend on?
A
- the size of the spit depends on the period of time that the LSD has consistently been acting
- but they will typically take 5-10,000 yrs to form
19
Q
how are spits recurved?
A
- secondary wave action and currents from the deeper water will cause the spit to be recurved at the distal end.
- this can happen multiple times along a spit and reflect relict wind and wave pattern
20
Q
why are spits often heavily protected for environmental reasons?
A
- over time, halophytic (salt tolerant) plants such as sea samphire will colonise the mudflats and in turn trap more sediment to form a salt marsh.
- these are populated with wading bird species, meaning spits are often heavily protected for environmental reasons
21
Q
how do salt marshes form behind spits?
A
over time, halophytic (salt tolerant) plants such as sea samphire will colonise the mudflats and in turn trap more sediment to form a salt marsh.
22
Q
why is fluvial sediment deposited behind spits?
A
- e.g hurst spit, hampshire
- clay and silt carried by the river Test and Itchen are deposited behind the spit
- this is because flocculation occurs when the river water meets salt water in the estuary
23
Q
what wave type will dominate around spits?
A
- constructive waves will dominate in this depositional region.
- a steady supply of sediment from up stream is needed.
24
Q
where do spits usually form?
A
- spits usually form over river estuaries where river sediment gets deposited as it is a low energy environment
- the combination of fresh water and salt water creates brackish environments, where salt marshes form.
25
why will spits continue to grow?
- spits will continue to grow as long as there is LSD acting
- often because they develop across an estuary, their size is limited by the river water forcing its way throughout the ocean
26
what is a spit?
- spits are long narrow beaches of sand/shingle that are attached to the land at one end and extend across a bay, estuary or indentation in a coastline
27
how are spits formed?
- generally formed by longshore drift occuring in 1 dominant direction which carries beach material to the end of the beach and then beyond into the open water
- as storms build up more + larger material they make the feature more substantial and permanent
28
what happens to spits over time?
- the end of the spit often becomes recurved as a result of wave refraction around the end of the spit and, possibly, the presence of a secondary wave/wind direction
- over time spits may continue to grow and a number of recurves/hooked ends may develop.
- if a spit forms across an estuary its length may be limited by the actions of the river current
- in the sheltered area behind the spit, deposition will occur as wave energy is decreased.
- the silt and mud deposited build up and eventually salt tolerant vegetation may colonise, leading to the formation of a salt marsh
29
what happens in the sheltered area behind a spit?
- in the sheltered area behind the spit, deposition will occur as wave energy is decreased.
- the silt and mud deposited build up and eventually salt tolerant vegetation may colonise, leading to the formation of a salt marsh
30
what is a tombolo?
- beaches that connect the mainland to an offshore island
- e.g. chesil beach, dorset
31
how do tombolos form?
1. from spits that have continued to grow seaward until they join an island
- LSD- sand build up between beach and island = bar
- influenced by wind direction
2. wave diffraction and refraction
- waves slowed as enter coastline => picks up more sediment, sediment deposited around the island
32
what is an onshore bar?
- linear ridges of sand/shingle extending across a bay that connects land on both sides
- e.g. slapton sands
33
what has happened to sea levels over the last 10000 yrs?
- particularly 6000 yrs ago
- post glacial sea level rise of 120 m
34
how does an onshore bar differ to a spit?
spit - proximal end connected, distal end is not
onshore bar - both ends connected
35
give 3 ways an onshore bar could form
1. spit grows across an indentation (cove/bay) until it joins onto land on the other side
2. increased sea-level means sediment is pushed up beach (flandrian transgression)
3. increased sea-level causes constructive waves to drive a ridge of sediment onshore
36
what is flandrian transgression?
a period of fairly rapid sea level rise
