Onshore Transport of Shelf Sediments into the Netravati-Gurpur Estuary, West Coast of India: Geochemical Evidence and Implications

Abstract

Surface accumulation of a long band of frothy material is frequently noticed in the Netravati-Gurpur and other estuaries along the central west coast of India. This is prominently seen in the flood phase of the tidal cycle, during summer. In order to understand its source and the process of formation, water samples along with frothy material were collected from the Netravati-Gurpur estuarine front during the summer of 1987. Also collected were riverine and estuarine-coastal suspended particulates and surficial sediments from the adjoining inner shelf. Salinity of water samples was measured and the remaining samples were subjected to bulk and partition geochemical analyses. Suspended particulate matter (SPM) concentrations in the estuarine front in the flood tide during summer (February-May) is anomalously higher (4,310 mg/l) than in riverine (51 mg/l) and estuarine-coastal (11 mg/l) waters during monsoon (June-September) and post-monsoon (October-January) seasons, respectively. The frothy material is composed chiefly of fine particles (< 63 microns) with a minor proportion of shells (foraminifers, ostracods and gastropods) and plant debris in the coarse fraction. Geochemically, the frothy material is similar to the Netravati-Gurpur SPM, but has low Al (13.87%), high organic matter (22.62%) and an order of magnitude higher Ca content (2.91 %). All these lines of evidence suggest that the frothy material is detrital and that it is derived from the marine environment by resuspension and landward transport of bottom sediments during the flood phase of the tidal cycle. This inference is in agreement with similar reports from many parts of the west coast of lndia. Implications of this process are discussed in terms of siltation in the navigational channel of the Old Mangalore Port, estuarine morphology and disposal of effluents from the existing and proposed industries along the west coast of India.