Dredging

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Dredging

Introduction

Dredging refers to the removal of sediment from the bottom of freshwater bodies such as a river, lake, or delta (the region where a river empties into a larger water body such as an ocean), or from coastal saltwater areas.

The excavated sediment is discarded elsewhere. Dredging typically keeps rivers, bays, and coastal waters clear for the navigation of boats, and often needs to be accomplished at regular intervals because sediment will continue to accumulate as a river empties into a larger body of water, or due to deposition of eroded soil in the water.

If the dredged sediment is not contaminated, it can be recycled for other purposes including creating new land and providing foundations in the construction of buildings. Contaminated sediment must be treated to remove or degrade its pollutants, or contained in storage in order to avoid further harm to the environment.

Dredging is performed using specialized vessels. The sediment is typically scraped or suctioned from the bottom. If not done in a controlled manner, dredging can be damaging to life in the area and can disperse excess sediment in the overlying water.

Historical Background and Scientific Foundations

Dredging has been done for millennia. For example, archeological evidence indicates that dredging of the Tiber River was done during the reign of Emperor Nero in the first century AD. Then as now, the intent was to maintain shipping lanes. In Nero’s time, protective structures were also built along the coastal shoreline near Roman coastal ports in an effort to reduce erosion and so reduce the need for frequent dredging.

In ancient Rome, dredging involved sweeping a large bag attached to a shipboard boom along the bottom of the water body. The sediment-filled bag was then raised up and the contents dumped into an adjacent barge for transport elsewhere.

By the seventeenth century, dredging had become more sophisticated. Rotating wheels with attached buckets churned through the sediment, picking up the sediment and dumping it into a container as the buckets upended.

With the settlement of the East Coast and Gulf of Mexico regions of the United States and the development of sea-going trading, dredging had by the nineteenth century became a vital activity in U.S. seaports including Baltimore, Maryland, and New Orleans, Louisiana.

Dredging is accomplished now using several techniques. The most common is by sucking or vacuuming the bottom sediment into a tube that is positioned on a specialized vessel called a dredger. The sediment empties into the cargo hold of the dredger. When full, the sediment is off-loaded from the dredger either through doors that open and allow access to the cargo hold, or by reversing the action of the suction tube to pump the material out.

Typically, the sediment is vacuumed directly off the bottom of the water body. However, the suction hose can be equipped with a cutting device or a rotating screw-like device that loosens the sediment before it is suctioned up, similar to the operation of some designs of vacuum cleaner.

Other designs of dredgers include buckets attached to a rotating chain, a grabbing device similar to a front-end loader that is used in surface construction sites, a blade that is dragged across the bottom, and an injector device that gently applies a jet of water to the sediment. The blade and water jet systems are intended to move the sediment to another location on the bottom rather than to actually remove the material to the surface.

Impacts and Issues

Without dredging, the build-up of sediment would restrict boat traffic on rivers such as the Mississippi and Nile, and impede commercial shipping in coastal regions. As another example, the Fraser River that empties into the Pacific Ocean at Vancouver, British Columbia, Canada, carries over two million cubic meters of silt and sand downriver in the spring. This sediment must be dredged from the river’s mouth each year if commercial shipping into Vancouver is to continue.

Dredging is also an important means of minimizing flooding, since the build-up of sediment decreases the volume of water that can be contained in the area of the river. As well, the decreased capacity for water can cause the river to flow faster in that region. When more water is added such as runoff while the snow melts in the spring in northern climates, this decreased capacity combined with increased river flow can be too much to be contained by the river bank.

In areas where pollutants have accumulated in the sediment, dredging represents the only means of removing the toxic compounds.

These advantages of dredging come with the possibility of damage to the dredged region of the water course, which can adversely affect life in the immediate area and, if stirred up sediment is washed downriver, life farther removed from the dredge site. It is also conceiv-

WORDS TO KNOW

EROSION: As related to water, the wearing away of the surface material by running water.

RUNOFF: Water that falls as precipitation and then runs over the surface of the land rather than sinking into the ground.

SEDIMENT: Solid unconsolidated rock and mineral fragments that come from the weathering of rocks and are transported by water, air, or ice and form layers on Earth’s surface. Sediments can also result from chemical precipitation or secretion by organisms.

WETLANDS: Areas that are wet or covered with water for at least part of the year.

able that pollutants such as heavy metals that have accumulated deeper in the sediment and which persist for a long time can be stirred up and released into the water during dredging.

Thus, dredging must be done in a controlled manner and with knowledge of past activities in the particular region of the water course, and is often avoided in ecologically sensitive areas.

In September 2004, Hurricane Ivan deposited enough sediment to reduce the depth of the channel that serves as the port of New Orleans’s outlet to the Gulf of Mexico to 24 ft (7 m) in some areas, preventing deep-

draft shipping vessels from using the channel. Emergency dredging operations that followed topped $17 million. The following year, Hurricane Katrina obliterated more than 30 mi (48 km) of marshlands off the coast of Mississippi and Louisiana, transforming them into open water. Efforts to reclaim these valuable wetlands that serve as a buffer for storms include specialized dredging techniques that use an amphibious dredge, which can remove excess sediments from sensitive marshlands with minimal damage to the environment.