2. JETTIES
• A structure of wood or stone extended into
the sea to influence the current or tide, or to
protect a harbor or beach.
3. Purpose of a jetty
• In rivers , the jetties divert the current away from the river bank and thus, scouring action is
prevented. As the current is diverted into deep waters navigation is also controlled
• In the sea, jetties are provided at places where harbour entrance is affected by littoral drift or the
sea is shallow for a long distance. Thus they extend from shore to the deep sea to receive the ships.
4. TYPES OF JETTIES
• Type of a jetty depends upon:
i. Physical characteristics of site.
ii. Exposure to wind, waves,
current, tides.
iii. Possibility of ice damage.
iv. Meteorological conditions and
their effects on water bodies.
• Types:
1. Random stone
2. Stone and concrete
3. Caisson type
4. Crib
5. Asphaltic material
5. 1. Random stone jetty
• A rubble-mound structure 1s in fact a long mound of random
stone. The larger pieces are placed on the outer face to afford
protection from destructive waves, and the smaller sized stones
are placed in the interior of the structure.
• This type is adaptable to any depth, may be placed on any kind of
bottom, and absorbs the wave energy with little reflected wave
action. This type requires relatively large amounts of material.
• If not carried high enough, storm waves may sweep entirely over
the Jetty and cause a secondary wave action in the protected area,
and if the voids between the stone are too large a considerable
portion of the wave energy may pass through the structure.
6. 2.Stone and concrete
• Is a combination of rubblestone and concrete.
• This type ranges from a rubble-mound structure, in which the voids in the
upper portion of the rubble are filled with concrete, to massive concrete
superstructure on rubble-mound substructure.
• The mound is used either as a foundation for a high concrete
superstructure or as the main structure surmounted by a concrete cap with
vertical, stepped, or inclined face.
• This type requires less material, and 1sused where the foundation is soft
or subject to scour. The superstructure may be undermined by wave recoil
down the face; rubble foundations require time to become permanently
stable and should be placed year before the superstructure.
• This type of Jetty, when properly designed and constructed, gives very
satisfactory service. Cross-sections of stone and concrete jetties are shown
in Figs.
7.
8.
9. 3. Caisson
type jetty
• The first caissons were built of iron but today they are
built of reinforced concrete, floated into position.
settled upon a prepared foundation, filled with stone to
give stability, then capped with cap stones or concrete
slab, and, occasionally, parapet walls are added.
• Some caissons have a reinforced concrete bottom which
is an integral part of the caisson, while others, such as
the ones used in constructing the Welland Ship Canal,
are bottomless and are closed with a temporary
wooden bottom which is removed after the caisson is
placed on the foundation.
• Caissons are suitable for depths up to 35 ft.
Foundations are either rubblestone alone or piling and
rubllestone. Riprap of heavy stone is used along-side to
prevent scour, to provide resistance against sliding, and
to prevent weaving under wave action. On sand bottom,
considerable riprap is required
10. Contd..
• The top of the foundation rubble is dressed with crushed stone and
leveled by a diver before the caisson is placed. Periods of calm water are
necessary to float the caisson into position and sink on the foundation.
caissons are satisfactory. If properly designed and placed, Cross-sections
of concrete caisson structures are shown in Fig.
11. 4. Crib
• They are built of timber, and some of the compartments are floored. The
cribs are floated into position, settled upon a prepared foundation by
loading the floored compartments, after which all compartments are
filled with stone.
• The structure 1s then capped with a timber superstructure which is
usually replaced by concrete when the timber decays. Stone-filled cribs
can withstand considerable settlement and racking without rupture.
• Such structures are suitable for depths up to 50 ft. or more. Foundations
are the same as for caissons but do not require such careful dressing.
• Timber structures are not suitable for saltwater where marine borers may
occur. In Fresh water, timber-crib structures give long and satisfactory
service.
12. 5. Asphalt jetty
• Asphaltic materials have been used to fill the voids of rubblestone
structures above the low-water line.
13. breakwater
• • Breakwaters are structures constructed on coasts as part of
coastal defense or to protect an anchorage from the effects of
both weather and long shore drift.
• • A structure protecting a shore area, harbor, anchorage or
basin from wave disturbance.
• • A barrier that breaks the force of waves, as before a harbor.
• • Breakwater are the structures constructed to enclose the
harbours to protect them from the effect of wind generated
waves by reflecting and dissipating their force or energy. Such a
construction makes it possible to use the area thus enclosed as
a safe anchorage for ships and to facilitate loading and
unloading of water by means of wave breakers.
15. Detached
Breakwater
• Breakwater without any
constructed connection to the
shore. This type of system
detached breakwaters are
constructed away from the
shoreline, usually a slight
distance offshore .they are
designed to promote beach
deposition. Appropriate in areas
of large sediment transport
16. Head land
breakwater
• A series of breakwaters
constructed in an “Attached”
fashion to the shoreline & angled
in the direction of predominant
waves - the shoreline behind the
structures evolves into a natural
“crenulate” or log spiral
embayment.
17. Nearshore breakwater
• • Nearshore breakwaters are
detached, generally shore-parallel
• • structures that reduce the amount
of wave energy reaching a protected
area. They are similar to natural
bars,reefs or nearshore islands that
dissipate wave energy. The reduction
in wave energy slows the littoral drift,
produces sediment deposition and a
shoreline bulge or salient feature in
the sheltered area behind the
breakwater. Some longshore
sediment transport may continue
along the coast behind the nearshore
breakwater
18. Rubble
mound
breakwater
• • Rubble mounds are frequently used structures.
• • Rubble mound breakwater consists of armour layer, a filter layer
&core.
• • It is a structure, built up of core of quarry run rock overlain by
one or two layers of large rocks. Armour stone or precast
elements are used for outer armour layer to protect the structure
against wave attack. Crown wall is constructed on top of mound
to prevent or to reduce wave
• • A breakwater constructed by a heterogeneous assemblage of
natural rubble or undressed stone.• When water depths are large
RBW may be uneconomical in view of huge volume of rocks
required.
• • Built upto water depth of 50m.• Not suitable when space is a
problem. If the harbor side may have to be used for berthing of
ships, the RBW with its sloping faces is no suitable for berthing.
• • These type of breakwaters dissipate the incident wave energy by
forcing them to break on a slope and thus do not produce
appreciable reflection.
19.
20. Vertical
breakwater
• • A breakwater formed by the construction in a
regular and systematic manner of a vertical wall of
masonry concrete blocks or mass concrete, with
vertical and seaward face.
• • Reflect the incident waves without dissipating
much wave energy
• .• Wave protection in port/channel
• • Protection from siltation, currents
• • Tsunami protection
• • Berthing facilities
• • Access/transport facility
• • Normally it is constructed in locations where the
depth of the sea is greater than twice the design
wave height.
