1. Marine Transportation & Deposition
Coastal Processes (4)
Marine Transportation & Deposition
Sediment Sources & Characteristics, Sediment Transportation Types, Longshore Drift, Sediment Cells, Deposition

2. Marine Transportation & Deposition
Sediment Sources & Characteristics
Sediment Transportation Types
Longshore Drift
Sediment Cells
Deposition
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3. Sediment Sources & Characteristics
Sediment outputs from erosional landforms such as cliffs and wave-cut platforms are transported by waves and currents. As long as they are not removed offshore they become inputs to depositional landforms. Additional sediment supply may be delivered by rivers and nearshore sandbanks. Under natural conditions, there is always a balance between the energy levels (wind, waves, tidal currents) and the sediment movements so that erosion in one location is balanced by deposition taking place somewhere else.
Sediment Inputs into the Coastal System
Erosion of Cliffs – rock falls and slumps produce large amounts of material for beach building, especially coarse sand and shingle
Rivers – bring fine grained silts, clays and sands. May result in salt marshes and deltas. Usually the largest source of coastal sediment
The Sea – tides and constructive waves may bring to the shore the huge volumes of sand and clay deposited on the sea bed during the last Ice Age. May result in offshore bars or simply be added to beaches
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4. Sediment Transportation Types
Swash carries material up the beach, while backwash carries it back down. Therefore, waves can sort the beach material. Constructive waves move material up the beach, forming berms (ridges or terraces), while destructive waves comb it downwards, forming offshore (longshore) bars. Onshore winds can blow sand up the beach and deposit it at the top of the beach, forming sand dunes.
Coastal sediment comes in a range of sizes – cobbles, pebbles, shingle, sand and mud. The larger particles usually form a storm beach at the top of the beach, while the smaller particles form the wider, more gently- sloping lower beach.
The main processes of marine transportation are similar to transportation by rivers:
Traction - larger pebbles and cobbles are rolled along the sea bed
Saltation – small pebbles are moved when one pebble hits another, causing it to bounce. This bouncing can set up a chain reaction
Suspension – fine sediment is carried as a suspension in the water, making it look muddy or murky
Solution – dissolved material is carried along in solution
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5. Longshore Drift
Sediment is moved along the coastline by the process of longshore drift (littoral drift). Waves usually arrive at an oblique angle to the beach, despite refraction trying to align their course to the shore. A completely parallel approach is rare as the seabed can only influence the direction of wave travel in the last few hundred metres before the shore. The wave angle (direction of the swash) is therefore determined more by the wind direction than wave refraction. The backwash runs almost straight back down the beach under the greater influence of gravity. This means that the swash and backwash operate to cause a zigzag movement of material, the overall effect of which is to move material laterally along the beach. The beach, therefore, acts like a conveyor belt for the transportation of sediment. Man-made obstacles, such as groynes, interfere with the movement (see below), by trapping material on the updrift side to build a wider beach. However, groynes starve the beach further along the coast (downdrift).
The prevailing south-westerly winds along the south coast of England ensure that longshore drift is mainly from west to east. On the east coast of England the winds tend to be from the north and north-east, so longshore drift is from north to south
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6. Sediment Cells
Research suggests that coastlines can be divided up into a series of sediment cells (littoral cells), with smaller sub-cells within them. Sediment movement is relatively self-contained within each sediment cell. The UK has 11 such cells recognised by scientists (see map). Within each sediment cell, there are inputs of sediment (e.g. from rivers, cliff erosion and from the sea bed), movement of sediment (e.g. by constructive waves, destructive waves and longshore drift) and outputs of sediment in the form of depositional landforms or to deeper water). Most sediment is moved by longshore drift, longshore currents and tidal currents. Because of this, sediment cell boundaries usually occur at headlands and estuaries (see map)
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7. Sediment Cells (continued)
Sediment cannot cross the cell boundaries so it can only be lost out to sea. Tidal currents are especially important here. They surge up an estuary with the rising tide (flood tide) and surge out with the falling tide (ebb tide). The seaward ebb currents are usually the strongest, since they are enhanced by river flow. The result is the removal of large amounts of mud, silt and sand by suspension into deeper water where sediment is lost from the cell.
Sediment cells and sub-cells form ‘coastal systems’ and are the natural units on which shoreline management plans (SMPs) can be based. SMPs are an essential step towards integrated coastal management by which coastal management strategies can be devised to protect one stretch of coastline without creating and adverse effect on another stretch of coastline further downdrift.
Shoreline Management Plans (SMPs) in Sediment Cell 5 & part of Cell 6
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8. Deposition
Deposition takes place whenever the movement of coastal sediment slows down or stops. This happens updrift of a groyne, or in sheltered locations such as an estuary or bay. Deposition also occurs where the coastline changes direction abruptly. At such locations, spits can develop as the beach is built out across the inlet, estuary or bay. Wave refraction around an island that is close to the coast can lead to the deposition of sediment between the island and the coast, known as a cuspate tombolo.
‘Angel Road’, Tonosho, Shodoshima, Japan
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9. Assessment Time: Tasks
1. Complete the following systems diagram using the words and phrases listed to the right.
Inputs
Processes
Outputs
Sediment moved by tidal currents; longshore drift; storm beach; sediment from coastal erosion; seaward ebb currents; alluvium from rivers; suspension; spits & tombolos; sediment moved by constructive waves
2. Study the Christchurch Bay sub-cell on the right.
a) Name the coastal features that form the edge of the sub-cell.
b) Identify the inputs of sediment to this sub-cell.
c) Deposits of sediment are called ‘sediment sinks’. Identify the sediment sinks on this map.
d) Hurst Castle Spit neds some shingle to nourish its beach and help protect it from coastal erosion. Suggest where this shingle might come from and why?
e) Cliff erosion has generally ceased at Barton, following the construction of coastal defences. What effects will this have on the sediment cell?
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10. Assessment Time: Past Exam Questions on Marine Transportation & Deposition
ANSWER ANY 2 EXAM QUESTIONS
Nov 2017 (22) Describe how sediment is transported and deposited along coastal areas. Explain how these processes lead to the formation of two coastal landforms. [10] June 2016 (23) Describe the nature of coastal sediment cells. How do processes of sediment movement contribute to the formation of coastal landforms? (10) June 2015 (21) Explain how coastal sediment cells operate. To what extent can the disruption of sediment cells affect coastal landforms? (15) June 2015 (22) Describe the sources of coastal sediments and explain how waves transport and deposit sediments. (10)
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