Placement, use or disposal
Worldwide millions of cubic metres of material are removed each year from ports, harbours and waterways in order to optimise navigation, remediation and flood management. The destination of this dredged material is often one of the greatest challenges facing a dredging project.
Over the last few decades, however, research and experience have shown that dredged material can be reused and is not inevitably a waste to be disposed. When dredged material has a purpose, it can be used, for instance, for environmental improvements, giving the material added value and creating a win-win situation.
When a use purpose is not an option, disposal alternatives must be considered and their destination on land or at sea may become controversial. Since 90 percent of dredged material is clean, use options can usually be found if all parties think about this during the planning of the project.
Finding a win-win destination
Contractors and clients now focus on finding uses for dredged material and for coordinating the supply of dredged material with a concurrent demand. This can be a win-win outcome. For instance, if a harbour is dredged, and a nearby beach needs replenishment, then the newly retrieved sediment may be suitable for beach nourishment and coastal protection.
Not all dredged material is suitable as a resource, but more often the old adage ‘seek and ye shall find’ applies. In some countries, like Japan, more than 90 percent of dredged material is ultimately put to good use.
This may require treatment of the sediments, but generally speaking, dredged material such as rock, gravel and sand, consolidated clay, silt or soft clay and a mixture of rock, can to varying degrees be used as a resource.
A variety of uses of dredged material:
- Rock may range from soft marl like sandstone and coral to hard rock like granite and basalt. Depending on size and quantity, rock can be a valuable construction material.
- Gravel and sand are perhaps the most valuable resource and are routinely used for beach nourishment, wetland restoration and coastal protection.
- Consolidated clay, if the water content is low, can be used for engineering purposes.
- Silt and soft clay usually come from maintenance dredging, are rich in nutrients and thus are good for agricultural purposes such as topsoil and for wildlife habitat development.
- Mixed materials are somewhat more restricted in use options but may still be used for fill, land improvement and topsoil.
Contaminated sediments
When dredged materials are lightly contaminated, direct use may still be an option if the environmental risks are low, taking site-specific conditions into account.
A relatively small percentage of dredged material is heavily contaminated, but even for these materials, treatment is an option. Contaminated dredged material can be separated into a usable sand portion and a contaminated silt portion in separation fields or by dewatering or in hydrocylones. The resulting contaminants then need to be reduced, removed or immobilised, after which even this dredged material can potentially be considered as a resource.
