2B.5B Depositional Landforms

Transportation and deposition processes produce distinctive coastal landforms (beaches, recurved and double spits, offshore bars, barrier beaches and bars, tombolos and cuspate forelands), which can be stabilised by plant succession. 
Deposition
  • Deposition occurs when waves no longer have sufficient energy to continue to transport material
  • This loss of energy might be due to:
    • the wind dropping, removing an energy source
    • resistance by obstruction, e.g. a groyne or headland
    • dissipation of energy through refraction
    • friction from extended transport across shallow angled nearshore and foreshore zone
Sediment transported down river systems to the coast or from offshore sources is also important. 

Sediment is deposited when the force transporting the sediment drops. Deposition occurs in two main ways:
  1. Gravity settling occurs when the energy of transporting water becomes too low to move sediment. Large sediment will be deposited first, followed by smaller sediment (pebbles -> sand -> silt)
  2. Flocculuation is a depositional process that is important for very small particles, such as clay, which are so small that they will remain suspended in water. Clay particles clump together through electrical or chemical attration, and become large enough to sink. 

Beaches
  • ​Beaches are accumulations of sand and/or shingle found in the foreshore and backshore zones. 
  • They're produced by material deposited by constructive waves
    • The swash has the strength to carry material up the beach, but the backwash only has enough energy to transport some of the material back down the beach, leaving the remainder deposited

​Bayhead beach
Bayhead beaches are curved beaches found at the back of a bay.
  • They’re common on swash-aligned coastlines where wave refraction disperses wave energy around the bay perimeter.
Waves break at 90' to the shoreline and move sediment into a bay, where a beach forms. Through wave refraction, erosion is concentrated at headlands and the bay is an area of deposition. 

Spits
  • Spits are linear ridges of sand or shingle beach stretching into the sea beyond a turn in the coastline (usually greater than 30') but connected to the land at one end
  • They form on drift-aligned coastlines, where the coastline changes direction, usually by more than 30', e.g. at a bay or a river mouth
    • At the turn, longshore drift continues in the original direction, but its energy is dispersed, lost as the wave refracts and the current spreads, leading to deposition on the sea bed. 
    • Over time, sufficient sediment is deposited to break the surface, extending the beach into the sea as a spit
    • The process continues until equilibrium is reached at the distal end (seaward end) of the spit, between deposition and erosion by waves or the existing river current. 
  • The length of the spit is determined by the existence of secondary currents causing erosion, either the flow of a river or wave action which limits its length. 

​Hooked/Recurved spits
  • A spit whose end is curved landwards, into a bay or inlet.
  • A hook or a recurve may form at the end of the spit. 
    • This is because wave refraction round the distal end transports and deposits sediment for a short distance in the landward direction. 
    • Alternatively, it could be because the wind and wave front are frequently at an opposing angle to the prevailing wind, generating short periods of longshore drift in the landward direction. (These last two can make it more pronounced)
    • Or, a strong incoming tidal current can also create a recurved spit.
Double spits
Double spits are where two spits extend out in opposite directions from both sides of the bay, towards the middle. 
  1. They form where longshore drift is operating in different directions on opposite sides of the bay. 
  • E.g. in Poole Harbour the main longshore drift direction is SW-NE driven round Studland Bay by the prevailing wind, producing a spit from the south. 
  • However, wave refraction around Durlston Head produces wave fronts from NE-SW along the coastline towards the north spit of Poole Harbour, generating a spit from the north. 
      2. They can also form when rising sea levels drive ridges of material onshore from the offshore zone. 
      3. Or, a barrier beach driven across a bay forms a bar (e.g. Haff coastlines) but a strong exiting river current may breach the bar to            form a double spit. 

Offshore bars (a.k.a. breakpoint bars)
Offshore bars are ridges of sand or shingle running parallel to the coast in an offshore zone. 
They form from sediment eroded by destructive waves and carried seawards by backwash. 
The sediment is deposited at the boundary of the offshore and nearshore zone, where the orbit of water particles ceases to reach the seabed, halting the transport offshore. 
They are also called breakpoint bars because the offshore/nearshore boundary is where waves first begin to break. 
They can sometimes be exposed by neap tide. 
They are used:
  1. to construct wind farms
    1. e.g. Scroby Sands in Norfolk
  2. as a source of sand for beach nourishment
  3. for shingle dredging for construction material

Bars and Barrier Beaches (they're the same thing)
Bars are linear ridges of sand/shingle extending across a bay and are connected to land on both sides. 
It traps a body of seawater behind it, forming a lagoon. 
They can form in two ways:
  1. On drift-aligned coastlines, when longshore drift extends a spit across the entire width of the bay. 
  2. When rising sea levels cause constructive waves to drive a ridge of sediment onshore to coastlines with a gently sloping shallow sea bed. (barrier beach)
    1. E.g. there's a 9 km barrier beach that extends across Start Bay in Devon, slapping Slapton Ley lagoon behind it

Another definition of a barrier beach is a ridge of material emerging just offshore to form a chain of beaches parallel to the coast (barrier islands). 
  • E.g. the Freisian islands off the North Sea coast of the Netherlands and Germany

Tombolos
Tomobolos are linear ridges (or bar) of sand and shingle connecting an offshore island to the coastline of the mainland. 
Two ways they can form are:
  1. On drift aligned coastlines, when longshore drift builds a spit out from land until it contacts with an offshore island. 
  2. On swash aligned coasts when there is wave refraction around both sides of the island. 
    1. This causes a collision of wave fronts on the landward side, cancelling each other out and producing a zone of still, calm water where deposition occurs, between the island and the coast.
    2. Oppositional longshore currents may play a role, in which case the depositional feature is similar to a spit. 
  • E.g. St Ninian's tombolo on the Shetland Islands
  • The tombolo connecting Portland Bill to the mainland in Dorset. 

Cuspate Forelands
Cuspate forelands are low lying triangular shaped headlands, extending our from a shoreline, formed from deposited sediment. 
Formation (there is debate about this):
  • When longshore drift currents from opposing directions converge at the boundary of two sediment cells. 
  • The sediment is deposited out into the sea by both currents creating a triangular shaped area of deposited material. 
They can extend from a few metres to several kilometres. 
  • An example is at Dungeness in Kent. It extends for 11 km in a south-easterly direction  where the main west-east longshore drift meets north-south longshore drift currents produced by swell waves travelling down the North Sea into the English Channel. 
    • Dungeness foreland is thought to have been two spits converging at distal ends with a lagoon between that infilled through salt marsh succession, wind deposition and storm beach material being thrown up during storms. 


Plant Succession (I don't know why this is here, there's already a section on it...)

Depositional landforms are unstable because:
  • They are made of unconsolidated material
  • They are dynamic as they loose material transported by waves, tides, currents and wind.
They are stabilised by plant succession, which binds the loose sediment together and encourages further deposition. 

Plant succession on sand dunes is psammosere. 
Plant succession on salt marsh is halosere. 

Plants can stabilise unconsolidated material because:
  1. Plant roots hold sediment together
  2. ​Their leaves/stems slow water and wind flow reducing erosion and encouraging further deposition

Land is built up through the addition of organic matter and increased deposition. 
In shallow water, deposition occurs further out and land extends as an outbuilding coastline.


​Extra

A swash aligned coastline:
  • directly faces prevailing wind
  • wave fronts approach it aligned parallel to the coast
  • swash aligned beaches often exhibit well-defined berms
A drift aligned coastline:
  • ​aligned at an angle to the prevailing wind direction
  • wave fronts approach the coast at an angle, meaning there is transport by longshore drift
  • may exhibit some sorting of sediment, with smaller, more rounded sediment at the furthest end
    • this is because smaller particles are transported by even low energy waves and are carried greater distances up the beach by each swash
    • more frequent movement leads to greater rounding by attrition
  • drift aligned beaches:
    • are linear beaches stretching along drift-aligned coastlines
    • They’re dynamic as material is in constant motion along beach due to longshore drift
    • they're part of the sediment cell
    • they need a constant input of sediment from an area of coastal erosion or river mouth
      ​
​In Slapton Ley, salt marsh plant succession behind Slapton sands barrier beach threatens to infill lagoon. Coastal management required to prevent marsh loss to a land ecosystem.