A class outline on coastal sediments

1. COASTAL SEDIMENTS
GEE-513 Coastal and Marine Environment
Sudipta Roy
M.S. 1st Semester
Geography and Environment

2. COASTAL SEDIMENTS
• Waves, currents and tides are linked to the landforms of the coast by coastal
sediments. Coastal landforms, any landforms, could not exists without breaking
down of the rocks into smaller grains.
• In tropical areas, deposits of the shells and skeletons of marine organisms
contribute to the formation of white sand beaches.
• Deposits of volcanic ash and lava create black sand beaches in some areas.
• In areas near inland deserts, offshore winds can transport sand into the ocean,
where it is later washed up on the shore.

3. COASTAL SEDIMENT SOURCES
• 1. Coastal Erosion
• 2. River erosion
• 3. Glaciers and biogenic sources
• 4. Storm waves or tidal currents causes onshore movement between two areas
produces velocity near bed rock
• 5. Shoaling waves or asymmetric current in shallow water.
• 6. Changes in water level

4. COASTAL SEDIMENT TYPES
• Based on origin, two types of coastal sediments:
• 1. Clastic sediments are from rock weathering and erosion
• 2. Biogenic sediments are the shells and skeletons of marine organisms
• Coastal Sediments can also sub-divided into two more types:
• 1. Cohesionless sediments are made up of solid grains usually bigger than 0.06mm
in diameter and which are held together chiefly by gravitational forces.
• 2. Cohesive sediments are mainly composed of secondary clay minerals which are
held together by electrolytic force.

5. STRENGTH OF A SEDIMENT
Depends on three factors:
• 1. The mean grain size will indicate whether it behaves as a cohesive or cohesionless
mass
• 2. The degree of sorting will influence the packing of packing of the grains. Poorly
sorted sediments pack together much better than well sorted ones.
• 3. The angle of repose or the internal friction

6. COASTAL SEDIMENT TRANSPORT
• Littoral Transport - the movement of sediments in the nearshore zone by waves
and currents.
Littoral transport is divided into two general classes:
• 1. Longshore transport: Transport parallel to the shore.
• 2. Onshore-offshore transport:
Transport perpendicular to the shore.

7. 1. LONGSHORE TRANSPORT
• It is the movement of sediments most often sand, along a coast parallel to its
shoreline - a zig-zag movement of sediment along the beach .
• The largest beach sediment is found where the process begins, updrift, and the
smallest, most easily moved, downdrift.
• Where waves are strong, the coast will be eroded and sediment carried away and
where they are weak sediment will be deposited.
• Longshore drift can have undesirable effects for humans, such as beach erosion. To
prevent this, sea defences such as groynes or gabions may be employed.

8. 1.LONGSHORE TRANSPORT (CONT.)

9. 2. ON-SHORE OFF-SHORE TRANSPORT
• Cross shore transport is a result of the water motions due to the waves and the
undertow.
• Here the sediment transport is perpendicular to the shore.
• Important factors in determining the cross-shore transport of sand are
A. The ratio of wave height to wavelength
B. Physical parameters of the beach;
i.e., grain-size distribution, cohesiveness, beach
slope also play an important role.

10. ON-SHORE OFF-SHORE TRANSPORT

11. LANDFORMS OF LITTORAL DRIFT
• Onshore offshore drift and longshore drift deposit
and shape debris to produce a variety of
landforms. A spit is an elongated deposit that
extends from the land edge out to sea. It may be
shaped to produce a distinctly curved end, as in
Cape Cod. Another landform that attaches an
island to the mainland is known as a tombolo,
while deposits may also accumulate to create
barriers such as a bay barrier.

12. MODE OF SEDIMENT TRANSPORT
• Bed load transport
• The bed load is the part of the total load
that is more or less in contact with the bed
during the transport. It primarily includes
grains that roll, slide or bounce along the
bed.
• experiment and theory suggest that the rate
of bedload transport (qb) is proportional to
the cube of the shear velocity,
• i.e. qb ∝ u *3

13. MODES OF SEDIMENT TRANSPORT
• Suspended load transport.
• It is the part of the total load that is moving
in suspension without continuous contact with
the bed.
• The determination of the rate of suspended load
transport is straightforward by comparison with
measurement of the rate of bedload transport.
• Wash load transport
• It consists of very fine particles transported in
water.