Modelling of sediment plume associated with the capital dredging for sustainable mangrove ecosystem in the Old Mangalore Port, Karnataka, India

Abstract

Old Mangalore port is located at the Confluence of River Gurpur and Netravathi in the Mangalore District, Karnataka.The Ministry of Shipping has promoted the modernisation of ports, including the Old Mangalore port, under the Sagarmala Project.The Old Mangalore port's navigational channel is approximately 3.2 km long and 80 m wide.It is proposed to deepen the existing approach channel to 7 m to allow medium-capacity vessels access to the port.The expected quantity of dredging is estimated to be 1160000 m3.We investigated the sediment spill plume movement of the dredging/dumping operations at Old Mangalore port using FVCOM (Finite Volume Community Ocean Model).Analysis showed that the spill plume was moved towards the offshore direction during the wet season and is primarily dictated by the prevailing currents and freshwater influx from the rivers.During the dry season (minimal discharge), the plume spreads approximately 8 km to the upstream regions and is mainly controlled by tidal currents.The study also revealed that the dumping point at a distance of 18-20 km offshore (depth > 20 m) is ideal for the optimum settling of the sediments for both seasons.The capital dredging of a navigational channel and construction of a new coastal berth is a one-time operation.Any siltation of the river mouth during the dry season will likely be flushed out in the subsequent wet season.We identified 12 types of mangrove species in the vicinity of the dredging area, and mangroves near the Old Mangalore port may be more exposed to the dredging activities impacts.Spillage during the dredging produces the sediment plume, and its transport to the mangrove region may choke the root system, thereby reducing oxygen availability.Hence, the study recommends that dredging operations can be carried out at any season but restricted to the ebb tide, thereby preventing the movement of sediment plumes to the mangrove region and thus conserving the mangrove ecosystem.