5A Sediment Transportation

Sediment transportation is influenced by the angle of wave attack, tides and currents and the process of longshore drift. 

Eroded material is transported by four processes:
  1. Traction
    1. Where large, heavy load items are rolled along the sea bed,
    2. e.g. boulders, cobbles and pebbles
  2. Saltation
    1. Where lighter sediment bounces along
    2. Sand particles are usually transported this way
    3. Sand can be saltated by wind as well as waves
      1. On a dry, windy day, there can be a layer of saltating sand 2-10 cm above the beach
  3. Suspension
    1. Where very light sediment is carried aloft within a body of water or air
    2. E.g. silt or clay particles
    3. Suspended clay particles, give the sea a cloudy, muddy brown colour on soft-rock coasts, e.g. Holderness
  4. Solution
    1. Where sediment is carried dissolved within the water

Direction (angle) of wave attack
​This is the main determinant of the direction of sediment transport (in the foreshore zone).
Where the wind is blowing directly onshore, the incoming swash transports the material direction up the beach at 90' to the coastline. 
Backwash then transports sediment perpendicularly back down to the beach to its original starting position. 
Sediment is moved up and down the beach, but there is no net lateral movement. 

Longshore drift
This is the net lateral transport of material along the coastline when waves approach the coast at an angle. 
  1. Incoming swash transports sediment up the beach at an angle (movement contains an up-beach component and a lateral component)
  2. Gravitational backwash then transports sediment back down the beach at 90' to the coastline. 
  3. A sediment particle comes to rest some distance along the beach from the original starting point due to net lateral movement. 
  4. Particle moves in a zig-zag fashion along the beach with each incoming wave.

A wave angle 30' to the coastline produces the strongest longshore drift movement. 
On most coastlines there is a dominant prevailing wind, so over time there is a dominant direction of longshore drift. 

Current
This is the flow of water in a particular direction, and they can transport sediment in the nearshore and offshore zones. 
They can be driven by winds, or initiated by differences in water density, temperature or salinity. 
​Currents transport sediment over a variety of spatial and temporal scales:
  • The global thermohaline circulation connects four oceans and takes 500 years for one complete circuit. 
  • Rip currents on the beach transport sediment a few metres out to sea for a few hours when the wind is blowing direcly onshore with the right strength. 

Tides
Tides are changes in sea level produced by the gravitational pull of the moon and the Sun. 
  • The incoming and ebbing tide can create tidal currents in the nearshore and offshore zones that transport sediment. 


Superfluous

  • Prevailing wind = most common direction
  • Dominant wind = strongest wind direction
    • ​On the south coast of the UK wind from the south west is both prevailing and dominant, though on the east cast the prevailing wind is from the south east but the dominant wind is from the north east. 
  • The dominant direction of longshore drift forms part of the sediment cell, with material eroded from the source area, transported along the coast by longshore drift and deposited in the sink area. 
  • Predominant direction of sediment transport usually determined by the direction of the prevailing wind, however wave refraction can alter this - in the UK south coast the prevailing wind from the SW and the longshore drift from west to east, however refraction of waves round the Isle of Wight produces longshore drift from east to west up the Solent.  
  • The Earth's rotation combined with the gravitational pull of the Moon and Sun causes bulges in the water surface to shift position within each ocean or sea basin. 
  • The tidal bulge in the water surface rotates around a location called a amphidromic point, which is determined by the morphology of the sea bed.
  • The sea may contain several separate amphidromic points. 
  • Tidal range is the distance between high tide and low tide.
  • Tidal range is determined by the distance from an amphidromic point (range decreases with distance) and the shape of the coastline. 
  • Bays and estuarine coastlines act like funnels, concentrating tidal flow and increasing tidal range. A large tidal range will produce a strong tidal current - and may be sufficient to create a tidal bore, which produces a wave that can transport sediment in the foreshore zone of an estuary.