8.2. Depositional Landforms

Question 1

Depositional Landforms

Answer 1

• produced by build up of sediment

- occurs where inputs > outputs (where rate of accumulation of sand and shingle exceeds the rate of its removal)

Question 2

Beaches

Answer 2

• most typical of all coastal landforms
• usually made of sand and shingle
• usually form between HWM (high water mark) and the LWM (low water mark) - in inter-tidal zone
• sediment is moved longshore drift
• a dynamic landform like a conveyer belt

Question 3

Beach zones

Answer 3

1) backshore zone
2) foreshore zone
3) nearshore zone
4) offshore zone

Question 4

Backshore Zone

Answer 4

• part of the beach that is only underwater when there are storm waves or during the highest tides, so this zone is usually not under the influence of waves
• lower energy than the foreshore and therefore there is less potential to remove sand
• as a result, a greater amount of sand accumulates in the backshore environment

Question 5

Foreshore Zone

Answer 5

• part of coastal zone that is exposed at low tide and underwater at high tide
• includes majority of wave action
• higher energy removes sand
• berm is the highest point on the beah and forms the foreshore-backshore boundary which changes seasonally
• foreshore gives some protection to backshore against the effects of wave and tidal energy

Question 6

Nearshore Zone

Answer 6

• Seaward of the foreshore
• permanently covered with water, except at extremely low tides
• where waves break

Question 7

Offshore Zone

Answer 7

• further out to sea
• beyond the influence of the waves
• sediment can build-up in bars
• if the bar grows enough to pierce the surface of the water, it can become a barrier island and a barrier beach

Question 8

Role of Tides in the Formation of the beach profile

Answer 8

• ebb and flow of the tides make the beach a transitional zone between land and sea
• width and nature of foreshore and nearshore zones are determined by the tidal range as well as by the amount and type of beach sediment
• spring tides shape the berms at the top of the beach
• on beaches with a high tidal range, each falling tide reveals a new beach environment, washing clean of any sign of human activity

Question 9

Role of waves in the Formation of the beach profile

Answer 9

• waves have energy to change the beach
• constructive waves move material up the beach, contributing to the berms and increasing gradient of the beach
• destructive waves comb material down the beach, contributing the longshore bars and reducing the gradient of the beach
• type of waves that break on any beach will change from day to day and season to season, depending on changing weather and climatic conditions like wind intensity and direction

Question 10

The role of sediment in the formation of the beach profile

Answer 10

• beach sediment is in constant motion and arrives on the beach from a range of sources: cliff collapse, rivers and sea bed
• the larger the particle size, the steeper the beach
• shingle is more permeable than sand so the backwash drains into it, rather than across it
• shingle often makes up the upper part of the beach - thrown up by strong waves
• sand is less permeable than single so there is more backwash
• sand is smoother than shingle and causes less friction with the waves but the beach is gentler, so wider. This means the wave energy is dissipated over a wide area, not concentrated into a small area

Question 11

Berm

Answer 11

terrace of a beach that has formed in the backshore, above the water level at high tide

Question 12

Beach Cusps

Answer 12

semi-circular, scalloped depressions cut into the lower edge of the storm beach
- origins are controversial

Question 13

Draw a Beach Diagram

Answer 13

• dunes
• backshore
• foreshore
• nearshore
• offshore

Question 14

Swash-aligned beaches

Answer 14

• beaches where wave crests are parallel to the beach
• waves come into the shore at 90 degrees, usually due to wave refraction. The waves wash up the beach and then swash back down the beach
• there is therefore no longshore drift
• located in bays
• waves are parallel to the beach so the beach is swash aligned
• rare

Question 15

Drift-aligned beaches

Answer 15

• formed by longshore drift (zig-zag migration of sediment along the beach)
• wave crests approach the beach at an angle
• hence, the beach will migrate laterally with the tide and the beaches are referred to as drift-aligned
• beach may well continue parallel to the drift and detach from the coastline to form spits

Question 16

Spit

Answer 16

an extension of sand and shingle extending from the mainland out into the sea.

Question 17

Spit Formation

Answer 17

• The formation of a spit begins where the is a sudden change in the shape and direction of the coastline, for example at a headland.
• Normally longshore drift carries sediment around the coast, however, when there is a substantial change in the shape longshore drift continues to deposit material in the same direction rather than following the coastline.
• Overtime the ridge grows in size with material being deposited faster than it can be removed.
• Deposition of sediment can be brought about earlier near estuaries as the flow of water into the sea is stronger than the drift, forcing the sediment to be dropped.
• The shape of a spit can be changed by a couple of things:
• Firstly wave refraction curves around the spit, forming a ‘hook’ on the end known as a recurved spit.
• Change in the prevailing wind can also form the recurved end.
• Sand dunes often form on spits as well as halophytic vegetation colonising the sheltered part of the sea behind the spit creating a salt marsh or mangrove swamp.
• Spits are eroded by the sea and wind but as longshore drift continues along the coast, a constant supply of sediment ensures their continued existence.

Question 18

Features of a Spit

Answer 18

• As spits grow, the water behind them is sheltered
  from wind and waves, and a salt marsh (temperate
  areas) or mangrove swamp (tropical areas) is likely to
  develop.
• The end of a spit attached to land is called the
  proximal end, and the end extending out into water is
  called the distal end.

Question 19

Simple spit

Answer 19

• A simple spit, either straight or recurved, occurs
  without the development of minor spits at its end or
  along its inner side.
• One wind direction is dominant (prevailing wind), preventing the ‘hook’ at the end of the spit.

Question 20

Compound (recurved) spit

Answer 20

If the wave direction change is repeated more than once, hooked features may form along the length of the spit.
- This type of spit is called a compound recurved spit.

Question 21

Bars

Answer 21

• formed as a spit grows across a bay, joining up 2 headlands

Question 22

Bars Formation

Answer 22

• Bars are formed in similar ways to spits.
• As longshore drift transports sediment down the coast, it deposits in low energy zones, such as a bay.
• Bars are particularly noticeable at low tide where they can be exposed in relation to the rest of the beach.
• During high tide, the bar can make the water shallow
  resulting in the waves breaking earlier before they reach
  the shore.
• When a spit or a bar has enough sediment to continue to build up, it is possible that it may reach an island just off the coast. This is then known as a tombolo.

Question 23

Real world example of a spit

Answer 23

Laem Talumphuk in Thailand

• long sandy spit with a hook is located along the coastline of Nakhon Si Thammarat province
• the hook shape protects mangroves from strong waves and erosion of the mud flats
• formed of sand deposited by the mainly northward current interacting with the outflow of the Pak Phanang River

Question 24

Real world example of a tombolo

Answer 24

Nang Yuan tombolo in Southern Thailand

Question 25

Tombolo

Answer 25

When a beach extends outwards to join an offshore island

Question 26

Barrier beaches

Answer 26

long, sandy beaches, detached from but parallel to the coastline - a tidal lagoon forms between the beach and original coastline - salt marshes and mangrove swamps often develop in this lagoon but sometimes strong currents are able to scour the lagoon clear of vegetation and loose sediment

Question 27

Barrier Islands

Answer 27

- similar to bars, but they are detached from the beach and the processes operating on it - they take the form of a ridge of deposits parallel to the shoreline

Question 28

Theories of barrier island formation

Answer 28

1) Emergence Theory 2) Connected Spit Theory 3) Post-glacial Theory

Question 29

Emergence Theory (barrier island formation)

Answer 29

- waves break over a submerged bar - bar emerges above sea level due to deposited sediment - bar develops into barrier island and lagoon

Question 30

Connected Spit Theory (barrier island formation)

Answer 30

- spit starts to grow from point of land - spit is extended along the coast by longshore currents - spit is breached during a storm, forming a tidal inlet and barrier island

Question 31

Post-glacial Theory (barrier island formation)

Answer 31

- dune ridge along the coast - rise in sea level starts to submerge dune and goes to other side of dune (trapping water on the other side) - barrier island and lagoon formed from the ridge