Last reviewed 30 Sept 2026 · 6 min read
Why dredging
Dredging is the excavation of material (sand, silt, clay, gravel, soft rock) from the bottom of a water body, and its transport and disposal. In ports it is required to:
- Create the approach channel, turning basin and berth pockets to the required depth (capital dredging);
- Maintain the depth by removing the silt that settles (maintenance dredging);
- Obtain fill for reclamation and construction (borrow dredging);
- Remove contaminated sediments (environmental dredging);
- Prepare foundations for breakwaters, quays and tunnels (trench dredging).
Dredging is a large part of the cost of many ports, and the volume of siltation governs the recurring maintenance cost.
Dredging equipment
| Dredger | Working principle | Suits |
|---|---|---|
| Cutter suction dredger (CSD) | A rotating cutter head loosens the soil, and a centrifugal pump sucks the mixture and pumps it through a pipeline to the disposal area (usually for reclamation); the dredger swings on spuds (piles) and anchors | Sand, clay, soft to medium rock; large projects, channels and reclamation by pipeline |
| Trailing suction hopper dredger (TSHD) | A self-propelled ship that trails a suction pipe over the seabed while sailing, pumping the mixture into its hopper; then it sails to the disposal site and dumps through bottom doors or pumps ashore (rainbowing) | Sand and silt in open water, navigation channels, maintenance dredging in shipping lanes; mobile, and does not obstruct traffic |
| Grab (clamshell) dredger | A crane with a grab bucket lifts the material into a barge | Small quantities, confined areas near quays, deep pits, and in a trench; accurate |
| Backhoe dredger | A large hydraulic excavator on a pontoon | Hard soil and rock (with breaking), accurate dredging near structures |
| Bucket-ladder (bucket-chain) dredger | A continuous chain of buckets on a ladder digs and lifts the material | Older; hard material; now less common |
| Dipper dredger | A power shovel on a barge | Rock, small jobs |
| Water-injection and sidecasting | Fluidise the sediment so that it flows away, or push it aside | Silt maintenance, special conditions |
The choice depends on the soil type, the quantity, the depth, the distance to the disposal area, the exposure to waves and traffic, and the required accuracy.
Dredging design
- Dredged depth = design ship draught + under-keel clearance + squat and wave allowance + allowance for siltation between dredging campaigns + overdredging allowance (tolerance for dredging accuracy, generally 0.3–0.5 m), referred to the chart datum.
- Side slopes of the channel depend on the soil: about 1:3 to 1:5 in loose sand or silt, and steeper (1:1 to 1:2) in stiff clay or rock.
- Volume: calculated from cross-sections (average end area) and bulking or shrinkage factors between the in-situ, dredged and placed volumes.
- Soil investigation: boreholes, sampling, grain size, density, strength and (for rock) UCS decide the dredger and the production rate.
A channel of length 4,000 m is dredged to a bottom width of 200 m and depth 3.5 m below the existing bed, with side slopes 1:5 (each side). The average cross-section:
Bottom width 200 m; side slope contribution ; central area .
Area ; volume (in-situ). An overdredging allowance of 0.3 m adds about 9 %.