2. INTRODUCTION
Dredging is the term given to digging, gathering, or pulling out material to
deepen waterways, create harbours, channels, locks, docks and berths,
desilt lakes and keep river entrances and approaches to boat ramps clear.
The material removed during dredging can vary greatly and can be any
combination of rocks, clays, silts or sands.
A dredger is a piece of equipment which can dig, transport and dump a
certain amount of under water laying soil in a certain time.
3. Why dredging is done ?
Dredging for navigation
Dredging often is focused on maintaining or increasing the depth of
navigation channels, anchorages, or berthing areas to ensure the safe
passage of boats and ships. Vessels require a certain amount of water in
order to float and not touch bottom
Dredging for construction, reclamation and mining
Dredging is an important way of providing sands and gravels for construction
and reclamation projects ,dredging may be used to recover minerals and
precious metals.
Dredging for the environment
Dredged materials are frequently used to create or restore habitats.
To remove contaminated sediments, thus improving water quality and
restoring the health of aquatic ecosystems. This so-called “remediation” is
used in waterways, lakes, ports and harbours in highly industrialised or
urbanised areas.
4. Dredging is also often undertaken in order to
• create underwater foundations
• facilitate the emplacement of pipelines or immersed tunnel elements
• construct flood control structures such as dams, dikes or levees
• ensure flood defences
• create or maintain storage capacity in water supply reservoirs
5. Dredging required areas
1. Port and Harbours
2. Inland waterways
3. Maritime boards
4. Captive jetties
5. Shipyards
6. Oil and gas
7. Coastal protection
8. Recreation water bodies
Beach in Alicante, Spain
6. Types of Dredging
• Maintenance Dredging
Removal of sediments accumulated in the channel since the previous
dredging project.
• New Work Dredging
Removal of sediments which have not been previously dredged - virgin
sediments - channel deepening.
7. The Process of Dredging Consists of Following
Stages
Excavation
This process involves the dislodgement and removal of sediments or
rocks from the bed of the water body
Transport of excavated material
Transporting materials from the dredging area to the site of utilisation,
disposal or intermediate treatment by Pumping through pipelines,
barges , conveyor belt transport etc
Utilisation or disposal of dredged material
8. Types of Dredgers
Dredging equipment, classified according to the methods of excavation
and operation, can be grouped into the following main categories:
1 .Mechanical Dredger
2 .Hydraulic Dredgers
Mechanical dredging involves removal of material by a bucket,
placement of the material in a scow or on a barge, and towing that
scow or barge to a disposal or transfer area.
Hydraulic dredging uses hydraulic centrifugal pumps to provide the
dislodging and lifting force and remove the material in a liquid slurry
form.
10. Mechanical dredgers
Three main sub-groups of mechanical dredger can be identified
• Bucket-Ladder Dredger
• Dipper and Backhoe Bredge
• Grab Dredger
Bucket ladder Dredge Grab DredgeDipper and Backhoe Dredge
11. 1 .The Bucket Ladder Dredge
The bucket ladder dredge is a stationary
dredger, which has an endless chain of
buckets carried by the so-called ladder.
The buckets filled during their rotation over
the lower tumbler are emptied by the
rotation over upper tumbler.
The size of these vessel is about 60m in
length and 15m in width, having a draft of
about 2.5 to 3m.
The capacity of bucket is about 1cubic meter , The maximum dredging depth
depends on the size of the dredger minimum 8 m and maximum 30 m.
A bucket dredgers can be applied in almost all soils, from soft silt and clays to soft rock
12. 2 .Grab Dredger
A grab dredger picks up seabed material
with a clam shell bucket, which hangs from
an on-board crane or a crane barge, or is
carried by a hydraulic arm.
The opening of the grab is controlled by the
closing and hoisting wire or by hydraulic
cylinders.
The capacity of a grab dredger is expressed
in the volume of the grab. Grab sizes varies
between less than 1 m3 up to 200 m3.
The large grab dredgers are used for bulk dredging. While the smaller ones are
mostly used for special jobs.
13. 3 .Dipper and Backhoe Dredge
Dredgers are available in two models the
backhoe and the front shovel. The first is
used most. The difference between those
two is the working method. The backhoe
pulls the bucket to the dredger, while the
front shovel pushes. The last method is only
used when the water depth is insufficient for
the pontoon
These stationary dredgers are anchored by three spud poles
This type of dredge can dig in depths up to 20m of water
Dipper capacity varies from 1 to 4 cubic meter and heavy capacities of about 12m3 are
used in Panama Canal
14. Advantages of Mechanical Dredges
Limitations of Mechanical Dredges
Mechanical dredging is slower than hydraulic dredging because of
the limited capacity of barges and dump trucks.
Because of the constant movement of the bucket, more turbidity is
created in the water
Weather plays an important role in the success of mechanical
dredging projects.
This type of dredging does not require the building of a spoil basin, so
it is best suited for areas that have little or no open land nearby
The cost to remove material can be lower than hydraulic dredging.
Minimal mobilization requirements allow for smaller projects
17. 1 .Plain Section Dredger
A plain suction dredger is a stationary
dredger that position on one ore more wires,
with at least one dredge pump, which is
connected to the suction pipe and the
delivery pipe.
Most suction dredgers are equipped with jet
water pump(s) to assist the beaching process.
The diameter of this mouth piece is usually 300 millimetre
The suction inlet air speed is much higher than 200 miles per hour
18. 2 .The Cutter Suction Dredger
The cutter suction dredger is a stationary
dredger equipped with a cutter device (cutter
head) which excavate the soil before it is sucked
up by the flow of the dredge pump(s).
The more powerful and very high duty
cutters are employed to excavate harder
rock even without blasting
When the accuracy of the works is important this dredger is very usefull
Cutter dredger can dredge all kinds of soil.
19. 3 .Trailing Suction Hopper Dredger
A Trailing Suction Hopper Dredger (TSHD) is
a self-propelled sea-going or inland vessel
equipped with a hold, called hopper, and a
dredging installation by which it can fill
and/or empty the hopper.
The TSHD is a free sailing vessel and does
not hinder other shipping during dredging
and is therefore ideal for dredging in
harbours and shipping channels inshore as
well as offshore
The normal dredging vary from 10 to 30m
The hopper capacities vary from 300-11000cubic meter
20. Advantages of Hydraulic Dredgers
Hydraulic dredging is a minimally invasive method that does not
require disturbing the shoreline. Material can be placed several
thousand feet or even several miles away.
Material is handled only once when it is pumped through a pipeline
to a settling area. In addition, because no additional handling is
required, hydraulic dredging tends to be faster than mechanical.
Limitations of Hydraulic Dredgers
For very small projects, the cost of removing sediment is more than
mechanical.
An open area is needed to build a settling basin, stage geotextile
tubes or set up mechanical equipment for dewatering of material.
21. Dredged Material Management
Management alternatives for dredged material can be grouped into the
following five main categories:
1. Sustainable relocation
2. Beneficial use
3. Open-water disposal
4. Confined disposal
5. Treatment
Sustainable relocation
Marine or fluvial sediment normally contribute to the sustainability of
natural ecosystems. Their role in river, estuarine and coastal zone
processes should be respected wherever possible. In environmental
assessment therefore as a first option the relocation of a estuary in
the natural environment should be considered
22. Beneficial use
Beneficial use may be defined as “any use which does not regard the
material as a waste”. Dredged material is increasingly regarded as a
resource rather than as a waste. More than 90% of sediments from
navigation dredging comprise unpolluted, natural, undisturbed
sediment. Which is considered acceptable for a wide range of uses.
Open-water disposal
Open-water disposal means that dredged material is placed at
designated sites in oceans, estuaries, rivers and lakes such that it is
not isolated from the adjacent waters during placement. Placement is
generally via release from pipelines, barges or hoppers.
23. Confined disposal
Confined disposal means that the dredged material is placed in an
engineered containment structure, that is, within dikes or bunds, or in
natural or constructed pits, or borrow pits.
Treatment
• Treatment is defined as the processing of contaminated dredged
material to reduce its quantity or to reduce the contamination.
Treatment methods range from separation techniques, in which
contaminated mud is separated from relatively clean sand, to
incineration.
24. Environmental issues
Dredging plays a vital role in the socio-economic development and the
environmental health of many countries and regions in the world. Yet
any infrastructure or development project can have impacts on the
environment, and dredging is no exception.
Effects of the dredging process
Water quality, e.g. increase of suspended solids concentration and
potential release of contaminants during dredging or disposal; leaching
of contaminants from disposal sites;
Habitats and natural areas, e.g. habitat enhancement or creation,
removal or destruction of benthos, smothering;
Local communities, e.g. the effects of noise; increased labour
opportunities;
25. Physical processes, e.g. waves, currents, or drainage, and hence
erosion or deposition
Changes to bathymetry or topography
Construction works, e.g. foundation fill, dikes
Prior to undertaking dredging and/or disposal projects, a careful
assessment to identify potential effects and to determine their
significance is necessary. The “environmental impact assessment” (EIA)
should highlight both positive and negative, short- and long-term
impacts.
26. Case study - The Palm Jumeirah
The Palm Jumeirah is an artificial island
located in Dubai, United Arab Emirates,
created through the process of land
reclamation
The Palm Jumeirah was designed largely to
combat the problem of limited development
space, especially beachfront properties
Extensive dredging and land reclamation was
required to build the two sections: the outer
breakwater and the inner palm shape.
The Palm Jumeirah is one of the largest artificial islands in the world, and is a significant
coastal engineering feat.
27. Conclusion
Summarized it can be stated that every type of dredger has its own
applied working area in which its production is optimal in a technical
way as well as in an economical way.
It will be clear that the boundaries of these applied working areas are
not strictly determined, but are also determined by other working
conditions, which can differ from lob to job.
In the table below the possibilities of the different types are shortly
summarized
28. REFERENCES
• Prof.Ir. W.J.Vlasblom (2003), Introduction to Dredging Equipment,
Pagina 1 van, 27 May 2003.
• Bray, RN, AD Bates and JM Land (1997). Dredging, a Handbook for
Engineers, 2nd edition, Arnold Publishing.
• IADC/VBKO/Port of Rotterdam/ Rotterdam Public Works Dept.
(2001) Construction and Survey Accuracies for the Execution of
Dredging and Stone Dumping Works
• Marsha Cohen (2005) ,International Navigation Association, ISBN 90-
75254-11-3
• Dan McDougal (2007) , Equipment and Methodology
