presentation was provided by Prof W.U Chandrasekara Department of Zoology and Environmental Management For Coastal and Marine resource management course

1. Coastal erosion
Impacts and Management

3. EROSION IS VERY RAPID: Storm erosion of coastal bluffs at Ocean Beach, San Francisco, California

4. Coastal erosion is the wearing away of land and the
removal of beach or dune sediments by wave action,
tidal currents, wave currents, drainage or high winds.
Erosion by windsErosion by waves Erosion by drainage
Erosion by tidal currents and
wave currents
A. Definition of Coastal Erosion
Look at this eroded
coast

5. • The Master Plan for Coast Erosion Management (MPCEM) of
1986 has identified 15 key areas and a further 11 singular
sites where erosion is more localized.
• Of these, 12 key areas and 7 singular sites are in the western,
southwestern and southern coastal stretches where erosion is
most severe.
Alarming sea erosion of the Unawatuna
sea shore
A Tourist resort about to collapse
due to the strong charge of giant
waves.
B. Coastal Erosion in Sri Lanka

6. Coastline in Sri Lanka is 1,85 km long.
Over 50-55% of the coastline is
subjected to or threatened by coastal
erosion.
The erosion is severe in the western,
southwestern and southern coastal
stretches of the country.
Kalpitiya, Matara, the Maha Oya river
mouth, Uswetakeyya and Moratuwa
are the most critically affected areas.
Also it is found that recently Mount
Lavinia, Wadduwa, Beruwala and
Unawatuna are also being affected by
coastal erosion.

7. Main area Local stretches and time periods
(where relevant)
Yearly erosion rate in m/year
Maha Oya –
Lansigama
Waikkal (1988 – 1998)
Gin Oya sand bar (1991 – 1999)
Wellamankara (1994 – 1998)
8-10
10-12
11-13
Colombo N – Dickowita Mutwal to the Kelani River
Palliyawatta – Uswetakeiyawa
0 -1
2-3
Wadduwa - 0-2
Kalu Ganga - Payagala - 1-3
Beruwela – Bentota Beruwela – Bentota
Bentota – Robolgoda Headland
1 - 2
Hikkaduwa Seenigama – Coral Garden Headland
Coral Garden Headland – Dodanduwa
0 - 2

8. Methods of Erosion
Erosion occurs due to;
1. Hydraulic action,
2. Attrition
3. Solution
4. Corrasion (Abrasion)
5. Corrosion
C. Methods of Erosion

9. 1. Hydraulic action: This occurs when waves striking a cliff face compress air in
cracks on the cliff face. This exerts pressure on the surrounding rock, and can
progressively splinter and remove pieces. Over time, the cracks can grow, sometimes
forming a cave. The splinters fall to the sea bed where they are subjected to further
wave action.
2. Attrition: This occurs when waves causes loose pieces of rock debris to collide with
each other, grinding and chipping each other, progressively becoming smaller,
smoother and rounder. Also collides with the base of the cliff face, chipping small pieces
of rock from the cliff or have an abrasion effect, similar to sandpapering.
3. Solution: Acids contained in sea water will dissolve some types of rock such as
chalk or limestone.
4. Corrasion (Abrasion): This occurs when waves break on cliff faces and slowly
erode it. As the sea pounds cliff faces it also uses the scree from other wave actions to
batter and break off pieces of rock from higher up the cliff face which can be used for
this same wave action and attrition.
5. Corrosion (Solution/chemical weathering): This occurs when the sea's pH (anything
below pH 7.0) corrodes rocks on a cliff face. Limestones cliff faces, which have a
moderately high pH, are particularly affected in this way. Wave action also increases
the rate of reaction by removing the reacted material.

10. 1. Natural processes, and
2. Anthropogenic activities
D. Causes of Coastal Erosion

11. 1. Natural processes of coastal erosion
1. Strong waves and high wind play a most
important role in coastal erosion. While the
waves strike the beaches and break fragments
of ground, the wind blows away the sand.
2. Loss of inland sand due to breaking and
wash-over of a sand berm (e.g. the
Uswetakeiyyawa beach).
3. Offshore loss during extreme wave and
storm surge conditions (e.g. southwest coast ).
A Sand berm is
a raised ridge
of pebbles
or sand found
at high tide or
storm tide
marks on a
beach.

12. Uswetakeiyyawa beach
Dickowita area

13. 4. Loss of sand into canyons (e.g. the canyon in the
Trincomalee Bay traps most of the sediment supplied by the
Mahaweli river).
5. Beach rock and reef collapse (e.g. receded coastline north
of Uswetakeyyawa beach in Wattala).
6. Natural variation in sand supply to the coast from rivers.
7. Seasonal and long-term variations in the wind patterns
and climate.
8. Sea level changes.
9. Increase of coral eating organisms
10. High rainfall and floods.
11. Sea floor changes due to seismic activities and the
resultant long waves
Beach rock and reef collapse

14. 2. Anthropogenic processes of coastal erosion

17. 1. Loss of habitat/ beach and landscape quality
2. Loss of buildings, infrastructure facilities such
as roads and power lines.
E. Impacts of Coastal Erosion

18. 3. Degradation of coral reefs due to deposition
of silt.
4. Increased turbidity of water.
5. Reduce the tolerance to face natural hazards
such as Tsunamis

19. 6. Reduce the sand volume to compensate the
natural littoral processes,
7. Reduce scenic beauty of the beach.
8. Collapse the tourism in the area.
Tourists leaving

20. 9. Loss of boat anchorage sites
10. Increase the frequency of
flooding in the lowland areas.
11. Increase the wave energy on beaches
endangering local people.

21. 12. Reduction of coral reef sites.
13. Decrease water quality
14. Marine and associated ecosystems are made
more fragile
15. Reduce primary production and fish
productivity.
16. considerable annual expenditure for damage
mitigation, control and disaster relief.
eg. Approximately SLRs 1,520 million has been invested
on erosion management in the Coastal Zone during the
period 1985 to 1999.

22. Three globally adopted erosion prevention
/Control strategies;
1. Soft erosion control methods,
2. Hard erosion control methods,
3. Monitoring only method (wait and see
method).
F. Prevention/Control of coastal erosion

23. 1. Soft erosion control methods
(Soft stabilization)
Soft erosion strategies refer to temporary options
of slowing the effects of erosion
Eg;
1. Beach Replenishment
2. Sand dune Management

24. 1. Beach replenishment
This is one of the most popular soft engineering
techniques of coastal defense management
schemes. This involves importing sand off the
beach and piling it on top of the existing sand.
Sand pumping from offshore areas Sandbags as seawalls
Beach bulldozing for
the creation of an
artificial dune in
front of a building

25. 2. Sand dune Management
Sand dune stabilisation or sand dune management works
using a number of different methods in order to prevent
the loss of sediment on the beach. Eg;
1. Introduction of public amenities such as car parks,
footpaths, Dutch ladders and broad walks, to stop the
removal of sediment by humans.
2. Education of visitors with noticeboards, leaflets and
beach wardens explain to visitors how to avoid damaging
the area.
3. Construct fences using simple materials such as wood,
sand traps to create blowouts.
4. Promoting dune vegetation to prevent sand loss. Also,
areas of the beach can be simply closed to the public to
allow rejuvenation to occur.

26. Dutch ladders Broadwalks

27. 2. Hard erosion control methods (Hard
stabilization).
1. Construction of Groynes
Groynes are barriers or walls
perpendicular to the sea, often made of
concrete, rock or wood. Groynes are
extremely cost-effective coastal defense
measures, requiring little maintenance,
and are one of the most common coastal
defense structures.

28. A view of Groynes in Christchurch Harbour, Dorset, UK

29. 2. Construction of Sea walls
Walls of concrete or rock, built at the base of a cliff or at the back
of a beach. Essentially, a seawall acts as a layer between the
vulnerable coastline and the ocean. Wave action can beat at the
seawall without eroding the coast, although the seawall itself
will eventually break down and require repair or replacement.
Identify this photo.

30. 3. Construction of Revetments
Revetments are slanted or upright wooden
blockade structures built parallel to the sea on
the coast in such a way as to absorb the energy
of incoming water.

31. 4. Use of rock armour
Large rocks are piled or placed at the foot of dunes or cliffs, which are
placed with native stones of the beach: — also known as rip rap,
rubble or shot rock. When rock is placed on the beach it is arranged so
that large void spaces are left. When waves hit the rock they are only
partially reflected, and can flow round the rocks, expending their
energy less quickly which reduces scour.

32. 5. Construction of Gabions
Boulders and rocks are wired into mesh cages and usually placed in front of
areas vulnerable to heavy to moderate erosion. Gabions are relatively cheap
and easy to construct but they are not suitable for use in locations exposed to
wave action because they are too flexible and are easily destroyed.

33. 6. Construction of offshore breakwaters.
Breakwaters are structures constructed on coasts as part of coastal defense
or to protect an anchorage from the effects of weather and long shore drift.
Offshore breakwaters, also called bulkheads, reduce the intensity of wave
action in inshore waters and thereby reduce coastal erosion. They are
constructed some distance away from the coast or built with one end linked
to the coast.

34. 7. Cliff stabilization
Cliff stabilization can be accomplished through drainage of
excess rainwater of through terracing, planting, and wiring to
hold cliffs in place. Cliff drainage is used to hold a cliff together
using plants, fences and terracing, this is used to help prevent
landslides and other localized damage. This is most suitable for
softer or less stable cliffs.

35. 8. Construction of Entrance training walls
Rock or concrete walls built to constrain a river or creek
discharging across a sandy coastline. The walls help to
stabilise and deepen the channel which benefits navigation,
flood management, river erosion and water quality.

36. 9. Construction of Floodgates
Floodgates or Storm surge barriers were introduced after the North Sea Flood
of 1953 and are a prophylactic method to prevent damage from storm surges
or any other type of natural disaster that could harm the area they "protect".
They are habitually open and allow free passage, but close when the land is
under threat of a storm surge. The Thames Barrier is an example of such a
structure.

37. • Shoreline mapping
• Remote sensing
• Video analysis
• Beach profiling surveys
• Aerial photographs
• Historical maps
G. Monitoring coastal erosion

39. References
• https://en.wikipedia.org/wiki/Coastal_manag
ement
• https://en.wikipedia.org/wiki/Coastal_erosion
• http://www.coastal.gov.lk/index.php?option=
com_content&view=article&id=109&Itemid=5
7&lang=en