Coastal Structures.pdf

Types and Functions of Coastal Structures
Pavan Mohan Neelamraju
pavanmohann.github.io
Indian Institute of Technology Madras
1 Pavan Mohan Neelamraju - pavanmohann.github.io - Indian Institute of Technology Madras 15-11-2023

Overview of Coastal Structures
•Coastal Structures are used to prevent coastal erosion and
flooding of the hinterland
•They also help safeguard the harbour basins and entrances
from
▪Waves
▪Stabilization of navigation channels at the inlets
▪Protection of water intakes and outfalls

a. Sea dikes
• Sea dikes are onshore coastal structures
used to protect low-lying areas from
coastal erosion and flooding, as a result of
waves
• They are usually built as mounds of fine
materials like stones, clay and sand
• Sea dikes consists of sea-ward slope to
prevent erodible effects of the waves
• They are commonly reinforced using
materials like concrete, glass, asphalt etc.

b. Sea walls
• Sea walls are onshore structures typically
used for the prevention of overtopping and
flooding of the hinterland primarily from
storm surges and waves
• They are built parallel to the coastline to
reinforce a particular part of the shoreline,
which is protruding from the natural beach
• They are constructed in the form of vertical
face walls, tied walls of concrete piling or
stone-filled cribwork
• Erosion of the landside is typically reduced.
However, the sea-floor erosion increases due
to the wave reflection.
• Hence, sea walls are in danger of instability by
wave slamming and overtopping
• Therefore, sea walls are commonly used in
combination with beach controls such as
groins and beach nourishment
4
Overtopping effect
Pavan Mohan Neelamraju - pavanmohann.github.io - Indian Institute of Technology Madras 15-11-2023

c. Revetments
• Revetments are onshore structures used to prevent
the erosion of the shoreline
• They typically consist of cladding of stone, asphalt,
and concrete to armour the slopping of the natural
shoreline
• They are mostly similar to sea walls in functionality
• They help in dispersing the wave energy and protect
against overtopping

c. Revetments contd..
• Types of revetments:
• Rock
• Stepped Concrete
• Concrete
• Gabion
• Curved face
• In contrast to sea walls, revetments can
be seamlessly built as they can easily
blend in the public spaces and
surrounding environment
• Public access steps or ramps can be
constructed to maintain the path
towards the beach

d. Bulk heads
• Bulk heads are primarily used to prevent the
landslide
• They appear as vertical walls anchored to the
rods
• Protecting the coastal profile from the wave
effects and hinterland flooding are some of
secondary importance
• Their common applications can be found in the
areas of mooring facilities in harbours and
marinas

e. Groins
• They are narrow structures,
perpendicular to the shoreline.
• They are used to stabilize
naturally or artificially
nourished beach against
erosion
• Its primary aim is to prevent
longshore loss of beach
material
• For a single groin, accumulation
of beach material takes place on
the updrift side and erosion of
material takes place on the
downdrift side.
• A differential in beach level on
either side of the groin takes
place, resulting in a saw-tooth-
shaped shoreline

e. Groins Contd..
• A well-designed groin system can decelerate or arrest the longshore transport, by
building up material in the groin bays
• They also help in deflecting strong tidal currents away from the shoreline
• The orientation, length, height, permeability and spacing of the groins determine the
change in shoreline and the beach level
• Perpendicular groins, designers often consider variations such as curved shapes, T-
heads, and shore-parallel spurs.
• These design adaptations are tailored to specific coastal characteristics, optimizing the
effectiveness of groins in mitigating erosion and maintaining beach stability.
• Sheet-pile groins are often used in less exposed areas, while rubble-mound structures
are preferred for exposed coasts due to their ability to withstand severe wave loads and
reduce wave reflection.

f. Detached breakwaters
• They are non-shore connected near shore breakwaters used in reducing erosion
• They are built parallel to the shoreline seaward in shallow water depths
• They reflect and dissipate some form of the wave energy, reducing the wave heights
and the erosion
• This beach material is deposited in the lower wave energy region, resulting in the
coastal line being formed as a series of tombolo and pocket beaches, which in turn
helps in causing wave refraction and stabilization of the pockets
• They are commonly built as rubble-mount structures with low crest levels that allow
significant overtopping during storm surges
• Optimization of breakwaters is difficult when large water level variations occur (Ex:
Large tidal ranges and Great Lakes)
• Similar to groins, a series of breakwaters can be used to control the distribution of the
beach material
• Just the downdrift side of the breakwater there is a high increased risk of shoreline
erosion and hence it is required to provide a transition section (Ex: breakwaters are
made smaller and kept close to the shoreline or seawall protection)
11
tombolo

g. Reef breakwaters
• They are coast-parallel structures submerged to reduce the
impact of wave action, by enforcing wave breakage over the
reef
• They consist of homogeneous pile of stone and concrete
armour units
• The breakwater can be designed to be stable or it may be
allowed to reshape under wave action
• They are narrow-crested in shallow waters and wide-crested
in deep waters with lower crest elevation and a wide rim
parallel to the coastline
• Besides triggering wave breaking and subsequent energy
dissipation, reef breakwaters can be used to regulate wave
action by refraction and diffraction
• They can be an invisible hazard to swimmers and boats and
hence could be risky.

h. Submerged sills
• It is a special version of reef breakwater used to retard the
offshore sand movements as a structural barrier (Ex: Mouth
of a river, estuary or harbour)
• This may also interrupt the onshore sand movement
• The sill introduces a discontinuity into the beach profile so
that the beach behind it becomes a perched beach as it is at
higher elevation
• Submerged sills are also used to retain beach material
artificially placed on the beach profile behind the sill, just
like a submerged breakwater
• They can also be used to create a milder wave climate by
provoking wave breaking and ensuring some wave
transmission
• Submerged sills are usually built as rock armoured, rubble-
mound structures or commercially available prefabricated
units.

i. Beach drains
• They enhance the accumulation of the beach
material in the drained part of the beach
• The drains are positioned at an elevation just
beneath the lowest seasonal elevation of the beach
profile
• Pumping from drains reduces groundwater table,
decreasing backwash speed and outflow
• This allows more sand to settle on the foreshore,
enhancing the beach material.
• Stable granular filters are placed around perforated
pipes.
• Avoid replacing granular filters with geotextiles to
prevent drainage system clogging.

j. Beach Nourishment and Dune Construction
Beach Nourishment
• It is a soft solution for the prevention of shore erosion
• Artificial material of the same or larger grain size is placed on the eroded
part to compensate for the lack of a natural supply of beach material
• This not only protects the eroded part of the beach, but also the downdrift
stretches by providing an updrift source of beach material
Dune Construction
• Dune construction is a protective measure to pile up beach-quality sand to
replace those washed away during severe storms
• It includes placement of vegetation and netting to retain and trap wind-
blown sand
• Storm over-wash are viable source of material for dune construction (When
storm-induced waves exceed the height of the dune, sand is transported over
top of the dune and deposited inland. This process, known as over-wash)

k. Breakwaters
• They are utilized to reduce the wave action and ensure the wave breakage through a
combination of reflection and dissipation of incoming wave energy.
• This is important to create sufficiently calm waters at harbours for safe mooring and loading
operations
• They also help in regulating the longshore transport of beach material by directing the
currents
• They help in creating areas with different levels of wave disturbance
• Other applications include water intakes for power stations and protection from tsunamis
• When used for shore protection, they are built parallel to the shoreline similar to the
detached breakwaters
• Layout of breakwaters is determined by – the shape of the area, amount of area, the direction
of storm waves, littoral drift (movement of sand, sediment, and other materials along the
coastline) and manoeuvrability of the vessels in the harbour
• Cost was determined by – water depth, climate severity and foundation conditions
16
1. Wave Action and Wave Breakage
2. Dissipate or reflect wave energy
3. Milder wave conditions for mooring
4. Parallel for coastal erosion
prevention
5. 2 types and describe them

k. Breakwaters Contd…
• These are of two different types – sloping-front and
vertical-front structures
• Slopping-front structures - These are mostly rubble-
mound structures with rock or concrete armour, with
or without wave-walls
• Vertical-front structures – These are sand-filled
concrete caissons or massive stacks of concrete
blocks on a rubble-stone bedding layer
• In deep waters, they are mostly placed on high
mound quarry rock. These breakwaters are called
composite structures
• The upper part of the concrete structure is slopped
seaward to reduce the wave force, and the front wall
is perforated with a wave chamber to dissipate the
wave energy

k. Breakwaters Contd…
Reef breakwaters are in
principle designed as rubble
mound structures which are
homogeneous piles of stone
or concrete
Vertical front structures are a
major class of breakwaters,
which are made of sand-filled
caissons or massive concrete
blocks
Piled breakwaters contain an
inclined or vertical curtain on
the pile walk. This type of
breakwater is applicable in
milder wave climates.
Rubble mound structures
(slopped front structures) are
the most commonly used
structures. It is large enough
to resist any displacement
due to wave forces.
18
Toe protection
is necessary
for all
breakwater
types

l. Floating breakwaters
• They are used in protecting the
regions which have mild wave
climates and for very short periods.
• They are a series of pontoons
anchored to the seabed with the
help of mooring lines
• Ex
• box-shaped concrete structures are
used to protect marinas
• Floating docks used in marinas

m. Jetties
• They are used for the stabilization of navigation
channels
• They are shore-connected and are generally built on
either one or both sides of the navigation channel
perpendicular to the shore, and extending into the
ocean
• They help in reducing channel shoaling (shoaling
refers to the process of sediment accumulation in
shallow areas near the coastline – clogs channels,
infrastructure damage) and dredging costs (the act
of removing silt and other material from the bottom
of bodies of water)
• They also help in reducing the cross-current and
direct it across the deep waters (makes it easier to
navigate)
• They also provide shelter to manoeuvring ships
shelter during storm surges as they are constructed
similarly to breakwaters

n. Training walls
• They are used to direct
the flow and prevent
unwanted
sedimentation
• Helps to improve the
mooring conditions,
particularly in estuaries
• Most of the are
constructed using sheet
piles

o. Storm surge barriers
• They protect estuaries against storm surge flooding, wave attack
and excessive saltwater intrusion
• They contain a series of movable gates which are normally open
to let the flow pass, which are eventually closed when storm
surges beyond a particular limit
• The gates can be either sliding or rotating based on a concrete
foundation
• Scour protection on either side of the barrier is an important
part of the structure (scour protection: measures to prevent loss
of seabed sediment around any structure placed in or on the
seabed by use of protective aprons)

p. Pipelines
• They are used as outlets of treated sewage; for transportation
of oil and gas from the offshore fields; and for water supply
between islands and mainland
• Types – small-diameter flexible PVC pipes, large low-pressure
sewage outfalls, semi-flexible concrete-covered steel pipelines
• Pipelines are generally buried with or without surface
protection depending on – the risk of damage, flow-induced
instability, damage from surface loads, collision with ships etc
• If significant changes in the seabed are expected (Ex: eroding
beaches), it is common to bury the pipelines below the
maximum erodible profile
• Scour protection is generally required in case of an ambiguity
in detecting the maximum erodible profile

q. Pile structures
• Commonly, they are the bridge piers which extend
from the shore to the water
• They provide open coast mooring for vessels
• They provide recreational facilities such as fishing,
outlook platforms etc
• Depending on the bearing capacity and settlement
characteristics of the seabed, supporting pile
structures might consist of wood, steel or concrete
pillars or piles
• Large diameter piles would mostly consist of
concrete blockwork or concrete-filled steel piles

r. Scour protection
• They prevent the instability of coastal structures with
foundations that rely on stable seabed
• Scour protection generally contains a rock bed or
geotextile filter
• They are commonly used in the toe-levels of sea dikes,
sea walls, piles, and pillars
• It is mostly required for structures that face a
concentration of currents such as breakwaters
• Highly reflective structures (Ex: Vertical face walls and
storm surge barriers) are susceptible to more scour
than seaward slope structures like sea dikes.

s. Rip Raps
• Rip raps are utilized to safeguard and stabilize
shorelines, slopes and bridges against erosion and
scour
• Their construction depends on the slope gradient of
the land and the water velocity
• They consist of stones, cobbles or boulders, which
are placed on geotextile fibres, which can help
mitigate the surface irregularities
• Advantages – protects from erosion, dissipates wave
energy, is effective for longer durations and cost-
effective
• Disadvantages – visually unattractive, beach can
become inaccessible and inefficient during storm
surges

t. Geo tubes
• They are geotextile tubes which are used to
control the wave action and reduce erosion, beach
nourishment and sediment management
• These tubes are filled with sand, soil or other
materials to create effective barriers against
erosion, wave action and flooding
• They have the advantages of cost-effectiveness,
ease of installation, adaptability to various
scenarios and resilience to coastal engineering.
• Disadvantages – Less durable, high maintenance
cost and geotextile degradation.

References
• Chapter 2, TYPES AND FUNCTIONS OF COASTAL STRUCTURES, Hans F. Burcharth and Steven A. Hughes,
2003.
• Images: Expertly Googled/ Collected from above for Impact.
15-11-2023
Pavan Mohan Neelamraju - pavanmohann.github.io - Indian Institute of Technology Madras
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