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Maintenance Dredging

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Maintenance Dredging Overview

What is Maintenance Dredging?

Maintenance dredging is the process of excavating unwanted materials that are carried out underwater or partially underwater.

What Problems It Solves:

Maintenance dredging removes deposits from the bottom of ports, rivers, lakes, and other bodies of water. This way, it keeps the ports and waterways operational for ships to pass and helps in creating adequate space to construct dams, bridges, and dykes.

Advantages & Disadvantages of Maintenance Dredging

Advantages:

Maintenance dredging does a lot for happiness in our everyday life. These benefits include:

  • It maintains harbors and shipping routes through the removal of silts from lakes.
  • It also reduces the effects of shoreline erosion for both humans and wildlife.
  • Dredging restores the aquatic environment. It helps create more significant marine habitats for plants and animals by carving out the sides of reservoirs, lakes, and ponds.
  • Following mining activity carried out in an area, dredging helps restore the land to support animals and plants. Also, it helps provide materials to restore the natural landscapes.

Disadvantages:

But such radical transformation of delicate environments carries some risks as well. Costs of maintenance dredging include:

  • Dredging alters the pre-disposed composition of the soil deposits in any body of water. As a result, the existing habitat may die out because of the resulting changes. These creatures and organisms in this habitat depend on the original composition of the soil deposits.
  • It may transfer unwanted organisms to other areas, leading to a broader spread of contamination when soil particles mix with water.
  • If the water is contaminated, maintenance dredging can cause contamination to spread, further affecting the aquatic environment adversely.
  • The dredging process can affect local communities due to noise and pollution.

Maintenance Dredging Process

Process

The maintenance dredging process features three key elements:

Excavation

This part involves the removal of the deposits from the bed of a water body. A dredger removes the sediments, either mechanically, hydraulically, or by a combination of both.

Transport of dredged material 

The transport of dredged materials normally uses one of the following methods. The method of transportation depends on the type of dredger and the specific conditions of the project.

  • Hoppers of the dredgers
  • Barges
  • Pumping through pipelines
  • Natural forces such as waves and currents.

Disposal or recycling of dredged material

Nowadays, construction projects are driving the demand for dredging for its dredged materials such as sand, gravel, silt, and clay. These materials are potentially valuable resources in the construction industry.

Different Types of Maintenance Dredging

There are three well-known methods used to extract debris from a site.

Mechanical Dredging

The first method is mechanical dredging which is usually carried out near the shoreline. It uses mechanical tools such as grabs and buckets to pick up sediments and transfer them to nearby land, water, or into a barge.

Hydraulic Dredging

Hydraulic dredging uses pumps, such as a centrifugal pump, to remove sediments from dredged sites. Because the sediments mix with water, which results in a slurry mixture, the pump can quickly suck and transfer them for proper disposal.

Hydrodynamic Dredging

The third method is hydrodynamic dredging which uses water injection technology. This process uses nozzles attached to a horizontal jet powered by pumps to inject large amounts of water into a water bed. When sprayed into a channel bed, the pressure loosen up the sediments, making them flow down to the deeper areas.

Because this process uses natural flow in sediment transport, hydrodynamic dredging is generally the most cost-effective and environmentally friendly among the three methods.

Alternative Solutions to Maintenance Dredging

There are numerous alternative solutions to maintenance dredging, each of which has various levels of efficiency and cost.

  • Circulation Improvements: This system follows the keep sediments moving (KSM) methodology, which removes all solid walls from port facilities to create a flow-through structure.
  • Mechanical Agitation: This process also follows the KSM method using mechanical agitators to maintain the velocity in a water column. A maintained velocity won’t allow sediments to accumulate.
  • Rounding Corners: This process rounds the sharp corners in ports to reduce low circulation areas, which is a leading cause of deposition. It is the most cost-effective alternative method since there is no slip reduction.
  • Current Deflecting Wall: One way to avoid sediment-laden waters from entering a port is by installing a current deflecting wall. This allows water to deflect outward while the upper waters move into the port. A current deflecting wall is typically a more long-term solution than the other methods. 

Service vs ServiceAdvantagesDisadvantages
Maintenance Dredging vs. Circulation Improvements

Maintenance dredging, compared to circulation improvements:

  • is less expensive
  • doesn’t affect the safety of the structure during large storm events

Maintenance dredging, compared to circulation improvements:

  • requires more maintenance
  • is less natural
Maintenance Dredging vs. Mechanical Agitation

Maintenance dredging, compared to mechanical agitation:

  • is less expensive
  • requires less maintenance

Maintenance dredging, compared to mechanical agitation:

  • a more challenging solution to implement
  • more damaging to the predisposed composition of the soil
  • is less natural
Maintenance Dredging vs. Rounding Corners

Maintenance dredging, compared to rounding corners:

  • is less expensive
  • requires less maintenance

Maintenance dredging, compared to rounding corners:

  • a more challenging solution to implement
  • more damaging to the predisposed composition of the soil
  • is less natural
Maintenance Dredging vs. Current Deflecting Wall

Maintenance dredging, compared to current deflecting wall:

  • is less expensive
  • is not affected by weather and storm events

Maintenance dredging, compared to current deflecting wall:

  • is less natural
  • more damaging to the predisposed composition of the soil
  • requires more maintenance

Costs of Maintenance Dredging

Dredging costs usually include the equipment costs, such as the excavator, tugboat, hydro survey boat, disposal scows, and labor. The total costs are computed based on the number of hours of excavation, transportation, and disposal.

Typically, dredging contractors are paid based on a cubic yard of sediment removed. The total costs also vary significantly from $5 to $20 per cubic yard for hydraulic dredging and $10 to $30 per cubic yard for mechanical dredging. This already includes the engineering design and construction. The best practice is to get quotes from several experienced contractors to better understand the costs involved in a particular project.

Frequently Asked Questions

How often does maintenance dredging take place?

The demand for maintenance dredging continues to rise due to increasing climate change and damage from weather conditions. Typically, maintenance dredging is carried out twice a year.

What happens if the port or waterway is not dredged?

If there is no regular maintenance dredging, sediments continue to build up, preventing vessels from entering a channel. This might cause more severe impacts on the local economy.

Where does the sediment go?

The sediments are transferred to an approved placement area. These may be used later to provide construction materials such as sand, gravel or providing landfills. There are cases where dredged materials are used to construct industrial and residential buildings, airports, highways, and even habitats for wildlife.

Who pays for dredging?

Private owners typically fund the dredging projects from their private docks. On the other hand, government agencies support many dredging projects to create or maintain navigational channels to keep them open. They also use public-private partnerships to fund the expansion projects of harbors and terminal facilities.

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