One of the key elements of fairway development is dredging. There are a number of inevitable processes involved in dredging inland waterways. The first step is to undertake a survey to identify the shoal that requires dredging, followed by a detailed survey to establish the extent of the shoal patch. Thereafter, a pre-dredging survey is conducted as part of the dredging process to estimate the dredging quantity and time required for the activity. For dredging to begin, the dredger unit which consists of a dredger, tug, houseboat and pipeline is mobilised to the site. After the logistics are in place, a post-dredging survey is conducted to ascertain the shoal clearance status and assess the quantity of dredging material. The final step is to demobilise the dredger unit from the site. Typically, cutter suction and grab dredgers are used for dredging. While cutter suction dredgers are used for wider river portions and for instant dredging at sections with less current, grab dredgers are used for sections with shallow depth of cut and for sections close to structures.
Operational models for dredging
There are a number of operational models available for dredging projects. The deployment of in-house (departmental) dredgers and manpower ensures accountability and discretion with respect to deployment and avoids any sort of contract-related issues. However, it involves a huge capital investment for the procurement and maintenance of the dredger, tug and accommodation boat. The departmental dredger with contract crew model is similar to departmental dredging in terms of responsibilities. Its only advantage is that the government’s liabilities are greatly reduced for the employment of crew for dredger operations as they are sourced through contract agencies that work under operation and maintenance contracts. The agency supplies the required manpower and is paid on a monthly basis. The payment for dredging is made on a running-hour basis. Besides, major repairs with due approval by the concerned department are carried out by the agency.
Another model used is contract dredging with private dredgers which is relatively easier to execute. Under this model, areas to be dredged are marked and a pre-dredging survey is carried out before taking up works in any area of the waterway. The post-dredging survey is carried out after dredging to the required depth. Here, the quantity dredged is calculated in cubic metres and the payment is made per cubic metre. More recently, a new model – the assured depth contract – has been introduced, wherein the contractor pledges and maintains the assured depth for a predefined period of time. This model was used in the recent World Bank-aided JalMarg Vikas Project.
The method used for measurement of the quantity dredged is an important part of the contract. In the pre-post dredging survey method, the payment is done based on per cubic metre by comparing the pre-dredging and post-dredging depths. Another method involves the measurement of the dredged material based on the disposal pit or reclamation site. Finally, the production meter method involves the use of radioactive materials. However, getting approvals for this method is a lengthy process.
Key issues and challenges
River dredging is different from sea dredging as it is undertaken at protracted locations and fragmented sites. Sea dredging, on the other hand, is undertaken at concentrated locations and on continuous stretches. There is regulated access in river dredging as compared to free access in the case of sea dredging, and this limits dredging activity on the riverfront. Other issues faced while undertaking river dredging are limited availability of draught for tug and pipeline anchorages. Besides, there is an issue of availability of supply points in the case of river dredging while ports already have these facilities available. River dredging is often undertaken in remote areas and this results in a loss of time due to supply chain constraints, repairs and shifting of cutter anchorages and pipelines. Mobilisation and demobilisation of dredgers also takes time. Further, the work period is restricted due to tides, limited machinery size, lower depth of cut, frequent relocation, lack of dumping and disposal grounds, etc.
Based on a presentation by S.V.K. Reddy, Chief Engineer, Technical, Inland Waterways Authority of India, at a recent India Infrastructure conference