Various types of dredging activities are carried out at ports and waterways depending upon the requirement. Capital dredging means new dredging in larger and deeper areas as opposed to maintenance dredging. As per the Dredging Corporation of India, capital dredging projects are primarily port creation and expansion projects which also involve the deepening and/or widening of channels to allow access to larger and deeper draught ships and also the provision of landfill for building port facilities and thereby enhancing port capability. Maintenance dredging consists of restoration of designed depths of waterways and harbours by removing silt, sand and other accumulated sediments due to natural sedimentation. Active channels generally require periodic maintenance dredging, thus creating a continuous source of dredging work that typically must be carried out if navigability of channels is to be maintained. It is also required so that the water carrying capacity of the river is maintained, which further helps in flood protection. Remedial dredging is generally undertaken for the removal of contaminants. Dredging is also undertaken for land reclamation.
Each type of dredging has a varying impact on the environment. Thus, the proper management of dredged materials assumes great importance and there are a wide variety of management options to reduce the impact.
The challenges faced during the management of dredging and dredged materials can be broadly classified into three categories: techno-economic, environmental and socio-economic.
In the techno-economic category, a dredging company faces various challenges such as the selection of the type of dredger to be used. This selection depends on technological, cost and environmental considerations. These include safety, accuracy, turbidity, noise, etc. At present, dredged materials are generally disposed of on the banks of rivers and in char areas or are dumped into stagnant or flowing water.
The environmental issues that come up during dredging are aquatic sensitivity, regulatory compulsions and noise and air pollution. Aquatic sensitivity and analysis are essential to decide the dredging method, its timing and the disposal of materials. With regard to regulatory compulsions, there are no norms and guidelines for the management and disposal of dredged materials. The underwater noise generated during dredging may create behavioural disturbances for aquatic species. Further, the increased sedimentation reduces light penetration and impacts the growth of submerged aquatic flora. The disposal of dredged material on the banks of rivers may impact the breeding and spawning grounds of aquatic fauna.
While undertaking dredging activities, companies face a number of socio-economic challenges such as accommodating cultural and archaeological factors as well as conflict of interest with people living in close proxim-ity to the waterbody. Dredging may also disturb cultural and religious activities undertaken on riverbanks. It may also affects fishing and navigation activities in the area.
Various measures can be taken to reduce the impact of dredging on the environment. Some of these are a reduction in the volume of dredged materials through the adoption of measures such as bandalling, river training works like the closure of secondary channels, storage of water through weirs and barrages for the lean season to reduce dredging requirements and the use of low-draught vessels. Weirs and barrages have other associated impacts and should be constructed after detailed environmental assessment. In order to exclude environmentally sensitive areas and cultural and archaeologically important locations, proper dredging management plans need to be prepared. Further, the use of electrically operated dredgers should be promoted.
Best management practices should be adopted for dredging and disposal of dredged materials, as per Permanent International As sociation of Navigation Congresses guidelines. Some of these are increasing accuracy and efficiency of dredging, increasing slurry density, minimising spillage and adopting environmental suction dredgers such as modified cutter suction dredgers to reduce the turbidity level near the cutter head. The dredged material should be tested for toxicity and contaminated material should be disposed of at approved treatment, storage and disposal facilities only.
To conclude, dredging solutions using best management practices and a scientific approach should be adopted along with a formal material management plan. This will help in reducing the environmental impact of dredging.
Based on a presentation by S.K. Jain, Director, Technical, EQMS India