Dredging is essential for smooth navigation at ports and harbours, development of port facilities, remediation, flood management and maintenance of the carrying capacity of marine and coastal systems. In recent times, dredged materials are increasingly being regarded as a resource.
Alternative methods of managing dredging materials
Dredging material can be used as an alternative to land-based resources. This practice is common in many engineering projects worldwide, such as land reclamation, beach nourishment, coastal protection works and road construction. In this way, an equivalent amount of fresh earthwork can be avoided.
Dredging materials are used as a resource and have the potential to improve the environment when managed in a sustainable manner, as in the case of habitat creation or sediment cell maintenance. Suitable dredging materials are being used to form useful products, especially in the agricultural sector. The physical, chemical and biological properties of products, such as manufactured topsoil, landfill cover or ceramics, bricks and concrete, must comply with the appropriate industry standards.
The materials chosen for use in land reclamation are available locally, and can be transported economically. The most ideal material is well graded, free draining sand, with a particle size in the range of 0.1 mm to 0.6 mm. Sand and gravel mixtures are also suitable, but materials with a significant content coarser than 0.6 mm is likely to cause problems, especially if they are to be pumped over long distances requiring higher energy for pumping. Materials that are finer than 0.1 mm are likely to cause excessive losses during dredging and placement.
Disposal of dredging materials
There are various opportunities for the reuse of dredging materials. More than 60 per cent of the dredged material from the outer harbour at Visakhapatnam port is being used for beach nourishment while the remaining is disposed of in the open sea. At Paradip Port, the maintenance dredging material is disposed of in the open sea, whereas the capital dredging material is used for shore nourishment. A few ports have tried to utilise the dredged material for engineering applications, whereas the majority of the material is disposed of in the sea. Other domains where dredged material can be used directly or with minor treatments need to be explored. In order to minimise the overall impact of the dredged material on the ecosystem, sustained efforts are required to reuse it.
Assessing the average cost of dredged material
The operational cost of a dredger remains an important factor in identifying the optimum hopper capacity of the dredger that can be deployed at the port. Thus, it is necessary to assess the average cost incurred per cubic metre of dredged material at different levels of utilisation of the dredgers. The cost can be calculated based on the average dredging cost per cubic metre of dredged material at different levels of utilisation of dredgers. It will depend on various parameters such as fuel consumption, crew cost and power consumption. The level of utilisation for a dredger is the amount of time it was dredging out of the total days available for dredging after consideration of annual repairs and maintenance days, dry dock days and the voyage period.
The way forward
Going forward, the uses of dredged material have to be adequately explored. Further, the dredged material needs to be tested for contamination. In case of contamination, the material needs to be processed in dredging pits before use. The contaminated dredged material can be separated into a usable sand portion and a contaminated silt portion in separation fields using different methods. After that, the resulting contaminants need to be reduced, removed or immobilised, and then the dredged material can potentially be considered as a resource.
The dredged material may be directly applied or may need further processing before application, depending on the material type and the purpose of use. Further, it is necessary that the suitability of land-based or water-based systems for sediment washing/ pre-treatment be analysed based on the technical feasibility of transporting the dredged material to shore, storage space and treatment, as well as the transportation, operational and management costs. To improve the environmental and economic impact of dredging operations, dredged materials can be relocated and reused.