Survey and investigation works form an important component of a dredging project, both at ports and inland waterways. However, they are often overlooked by project implementing agencies. According to industry experts, not even 1 per cent of the total cost of a dredging project is spent on survey and investigation works.
Given its critical importance in ensuring the availability of proper data for implementing projects, the need for a policy focus on streamlining survey and investigation works has long been felt in the industry. Propositions have been made to incorporate these works as a part of the dredger contract itself, so as to ensure that the dredging agency has an incentive to carry out a proper investigation and collect adequate data. Proper survey and seabed characterisation will help in avoiding possible disputes in the contract which could lead to time and cost overruns.
At present, several technologies exist for undertaking survey and investigation works. One of the most commonly used is that of hydrographic surveys. The surveys can be carried out by using both single-beam and multi-beam bathymetry data acquisition tools. However, the multi-beam bathymetry technology is more advisable for investigation works since it allows the surveying agency to analyse the area under investigation. It allows for a complete scan of the seabed, increasing the coverage to as much as 240 beams (horizontal profile), as against one profile of the conventional echo-sounder. This technology can thus be used by the surveying agency to identify shipwrecks, debris and other materials on the seabed before commencing dredging works.
Meanwhile, side scan sonar surveys can also be carried out. These can be conducted using different frequencies, such as 100 kHz (low frequency) and 400 kHz (high frequency). This technology has been deployed by organisations for surveying works at the Chennai, Vizhinjam, and V.O. Chidambaranar ports.
Besides this, the magnometric/sub-bottom technology can be deployed to identify iron debris along water channels or on seabeds. This technology helps in identifying the quantum of buried iron debris, the type of debris, and its location. Meanwhile, vibro-core or cone penetration test technologies can be used to assess soil characteristics of sediment at dredging locations.
In addition to these technologies, a tidal observatory network system can also be installed along coasts. The system allows the monitoring of tides at coasts, which also helps navigational aids and accurate quantification of dredge volumes.
At the same time, the hydrodynamic pattern also needs to be considered for the disposal of dredged materials. For instance, at Pipavav port, a study was undertaken to simulate the sediment movement from the spoil ground due to tidal flow. The results of the study highlighted the need to identify an ideal location, situated at the shortest distance from the dredged site, to dispose of the dredged material in a way that recirculation of the material due to tidal patterns is avoided.
As a case study, a water injection dredging study was undertaken by National Institute of Ocean Technology (NIOT) on a pilot basis at the Jellingham channel of Haldia port for the Dredging Corporation of India. The study was undertaken over a period of six months, along with monitoring surveys to quantify the extent of dredging required. It deployed several tools such as multi-beam echo sounding systems, an acoustic doppler current profiler, an acoustic seabed classification system, a side scan sonar, a sub-bottom profiler, etc.
Conclusion
In a bid to enhance the efficiency of dredging works, the industry needs to shift its focus to carrying out adequate survey and investigation of the area to be dredged before work commences. This will allow dredging companies to have appropriate information about the sediment to be dredged, the kind of technologies that could be deployed and the associated costs.
Based on a presentation by Basanta Kumar Jena, scientist – F, NIOT, Ministry of Earth Sciences, at a recent India Infrastructure conference