Dredging activities can have adverse consequences for the environment. Typically, the responsibility for identifying a suitable disposal site and scientifically disposing of or reusing the material lies with the contractor. At present, large quantities of dredged material are disposed of at sea, without systematic monitoring. The lack of disposal options not only delays project implementation, but can also incite local protests against dumping as it leads to soil and bank erosion.
Key practices for handling dredged material
The disposal of dredged material is not only challenging but also one of the most costly components of the dredging process. The key steps to be taken while handling dredged material include:
- Preliminary considerations: The scale of the dredging project should be taken into account while undertaking a full assessment of the volume of dredged material and disposal options.
- Sampling: Sampling of dredged material will reveal the size and depth of the area to be dredged and the expected variability in the horizontal and vertical distribution of contaminants. On the basis of the depth of dredging and expected vertical distribution of contaminants, core samples (for projects with heterogeneous sediments) should be taken for studying the dredged material. In all other conditions, grab sampling (homogenous sediments) should be undertaken.
- Dredged material characterisation: This involves studying the physical, chemical and biological characteristics of the sediments.
- Sea deposit site selection: For the selection of the sea deposit site, various environmental considerations and economic and operational feasibility should be taken into account. It is important that the dredged material deposits neither interfere with commercial and economic uses of the marine environment nor result in detrimental effects on marine ecosystems.
- Assessment of potential effects: This assessment should result in a concise statement, called the impact hypothesis, of the expected consequences of a deposit option. It provides a basis for deciding whether to approve or reject the deposit option proposed and for defining environmental monitoring requirements.
- Permits or regulation by other means: If the selected option is sea deposit, then a permit or regulation authorising the deposit at sea must be issued in advance. The deposit operation should be permitted subject to conditions that minimise environmental degradation and maximise benefits.
- Monitoring: The effects of dredged material deposits are expected to be similar in many areas, and it is neither scientifically nor economically feasible to monitor all sites. Thus, it is cost effective and appropriate to conduct detailed investigations at selected sites only.
Alternative uses of dredged material
Owing to increasing difficulties in the disposal of dredged materials, port authorities and dredging contractors are exploring alternative uses of these materials. Most of the material is usually uncontaminated and can be used directly after dewatering either for reclamation or other beneficial uses. Even for contaminated dredged material remedial measures are available. There are two broad categories of alternative uses of dredged materials:
- Engineering uses: These include construction, contaminated site isolation, flood and coastal protection, land improvement and placement on riverbanks.
- Environmental enhancement: Dredged materials can also be used for environmental enhancement after establishing their suitability taking into account the physical and chemical properties and the local legislative requirements. Environmental enhancement using dredged materials includes habitat creation and improvement, water quality improvement and pit filling.
Various constraints prevent the extensive use of dredged material like higher costs as compared to traditional disposal, complex and inconsistent legislation and regulations, and problems in finding suitable schemes for using the material at an appropriate time.
Global best practices for management of dredged materials
- Ocean/Open water placement: Site management and monitoring for ocean/open water placement prevents adverse environmental impacts, recognises and corrects any potential unacceptable conditions, provides a baseline assessment of site conditions, outlines a programme for site monitoring, etc.
- Confined placement: This is a very common option when the dredged material is unsuitable for open water disposal and can be more economical. It is bifurcated into confined disposal facilities and confined aquatic disposal.
Dredging contractors are taking several measures to avoid the negative environmental effects of dredging. Green valves are now being used to reduce air entrainment and turbidity. The side casting of overflows is no longer done to reduce turbidity. Accurate dredging and automated dredging information systems, accurate surveys and mapping of critical environmental resources and predictive modelling tools are among the other measures being undertaken to reduce the environmental impact of dredging.
With inputs from a presentation by L.R. Ranganath, Scientist-D, Central Water and Power Research Station, at a recent India Infrastructure conference