Dredging is defined as excavation of bed below water.
- This technique is often used to keep waterways navigable.
- It is also used as a way to replenish sand on some beaches, where too much sand has been lost because of coastal erosion.
- Dredge or dredger, is the equipment that carries out dredging work.
- To know where dredging is necessary, hydrographic survey is performed in the entire basin.
- If the dredged materials are to be removed only once, it is known as primary dredging.
- Dredged materials are removed only at the start or opening of harbour.
- Initial removal of material to obtain required depth.
- This depth acquired is maintained regularly to have a convenient harbour.
- Materials removed includes hard rocks and even silt.
It is a dredging to deepen or maintain navigable waterways , which are threatened to become silted with the passage of time.
- Materials removed will be in the form of fresh deposits (sand and silt) only.
- It is carried out periodically to maintain the navigable depth.
Use of dredging
1.For ensuring proper depth at the harbour or ship channel
2.For the maintenance of the harbour
3.Land reclamation- dredged materials used to construct new land.
5.Seabed mining- recovering natural metal ores.
6.Construction materials- dredged sand and gravel can be used in construction industry.
7.Contaminant remediation- to reclaim areas affected by chemical spills,etc
- Earth moving machine to remove bed material from under water, to increase the water depth.
- i.e. for dredging
- Dredgers can be classified into two:
Dredger which excavate the material by physically lifting it
The main types of mechanical dredgers are:
1.Bucket ladder dredger
2.grab or grapple dredger
- They suck the material
- Loosening the material by cutter or jet if required.
- Transporting it using water as the carrier.
Comparison between Mechanical and Hydraulic dredgers
Removes the material by physically lifting it.
Advantageous in getting high density of dredged spoil.
Removes the material by sucking.
Used to handle loose strata.
Great variation in type, size and mode of operation.
consists of a large suction pipe which is mounted on a hull and supported and moved about by a boom
Has a mechanical agitator or cutter head which churns up earth in front of the pipe
centrifugal pumps mounted on a dredge sucks up water and loose solids.
BUCKET LADDER DREDGER
- It consists of an endless chain of buckets mounted & running round a ladder,formed in the middle of the bow of a floating vessel
- The ladder could be lowered or raised.
- Chain of buckets is operated manually or mechanically by a big wheel
- Suitable for handling coarse gravel or sand, hard clays and soft or broken stone
- Each bucket cuts and brings up material to the top of ladder as the chain moves round
- It then discharges its contents into special holds
- The vessel moves forward on completion of work at each section
- Its capacity is 6000 cu.m to 7650 cu.m per day
- Suitable for handling coarse gravel or sand, hard clays and even soft or broken stone at fairly good depths.
- The bed dredged is the least disturbed
- A backhoe/dipper dredge has a backhoe like on some excavators
For sand, clay or mud, not for hard diggings