English Free Article | Water | VOL. 15, ISSUE 92, September-October 2015 |

Dredging in Shallow Waters

Inland water transport is considered safe, economical, efficient and environment friendly worldwide. Development and maintenance of inland water transport for flood control, passengers, recreation, sports and tourism can further the economic and social development of a region, besides providing an alternative mean of livelihood to the local population.

For efficient inland water transport, dredging is an essential activity. Depending upon the length, width, draught and turning radius of the largest size of vessels, a waterway can be classified for safe operation. According to the National Oceanic and Atmospheric Administration (NOAA) “Dredging is the removal of sediment and debris from the bottom of lakes, rivers, harbours, and other water bodies. It is a routine necessity in waterways around the world because sedimentation, the natural process of sand and silt washing downstream, gradually fills channels and harbours”. In other words, excavation of bed material by means of mechanical, hydrological or hydrodynamic methods carried out by dredgers and disposed of to another location through barges, pipeline or for other useful purposes is called dredging.

Dredging may be classified into capital and maintenance dredging. Capital dredging is carried out to plan a new port, terminal, turning circle and fairway for safe navigation and is mainly carried out by using a cutter suction dredger (CSD). Maintenance dredging is regularly done to maintain a port, channel or waterway at a desired depth by using trailing suction hopper dredger (TSHD).

Traditional and conventional methods of diverting the flow of a river from a secondary to a navigable channel by erecting ‘bandals’ is also another conservancy measure to increase the depth during lean seasons for safe navigation. In addition, there are mechanical dredgers (backhoe, bucket ladder, grab), hydraulic dredgers (stationary suction, cutter suction, trailing suction hopper) and hydrodynamic dredgers (water inject, underwater plough, TSHD with overflow) that can be deployed in shallow waters.

However, dredging in shallow waters involve many constraints that need to be considered before planning an operation. Furthermore, selection of the right dredger, dredging techniques and the methods of sediment disposal (through barges or pipelines) depend on various geographical, physical, and environmental factors, as also the nature and type of sediment to be dredged, besides its transportation, disposal and dispersal.

Dredged sediment can be useful for land reclamation, building material, beach nourishment or protection of flood banks. However, dredging can have an adverse impact on the environment, fisheries, the flora and fauna of shallow waters. It can also contribute to the creation of a spoil area, a secondary channel, siltation and river bank erosion. Therefore, a technical investigation involving hydrological studies and environment impact assessment is imperative before dredging operations are carried out on any inland waterway. Besides, the material dredged from inland waters, rivers, estuary and lakes should be regarded as a resource rather than waste as it comprises of unpolluted, natural and undisturbed sediments acceptable for many uses. Better utilisation of this resource can be achieved through disposal in the open or confined area.

Dredging in shallow waters through water inject dredging using amphibious and portable dredgers can be an economical method for cleaning and maintenance of small waterway stretches, in keeping with the aims of the Swachh Bharat Abhiyaan. However, CSDs and TSHDs will need to be continued for capital and maintenance dredging of ports and waterways. In keeping with the growing demand for dredging in shallow waters, especially in view of the government’s resolve to develop inland waterways all over the country, there is a need for appropriate training and certification of controllers, operators, and maintenance engineers for dredging operations using simulation methods.

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