Evolution of an Alluvial Tidal Creek – A Case Study

Abstract

The Bhagirathi-Hugli, a major distributor of the Ganga river, traverses a distance of about 550km in the state of West Bengal, India and falls into the Bay of Bengal. The river is the principle drainage artery of the state, and together with its tributaries forms one of the major river systems in the Indo-Gangetic Delta. The river is non- tidal for about 231km and tidal over the remaining 319km. In the tidal part, again, the estuarine portion, i.e. about 180km from Diamond Harbour downwards is very much dominated by tides and waves.The Hugli estuary has braided itself into a number of channels in its estuarine part where it has fanned in an exponential manner. The estuary is dynamically very much active and complicated because of its continued evolutionary changes. A number of islands and submerged sand –bodies within the flow system create the total flow phenomenon complicated and continually changing. It is observed over a short period that a number of islands and sand- bodies disappeared such as Bedford sand and new ones are coming up at different places where the physico-chemical environments are favouring for their formation. The Sagar island is the largest island at the mouth of the Hugli estuary, dividing the total flow into two parts- the major portion flows along the western part leading to Haldia Dock Complex and Kolkata Dock System and minor one flows along the eastern part which is popularly known as the Muriganga river or the Channel Creek. From hydrological observations, it was observed that about 20%-25% of total flows (Flood+ Ebb) would flow along this channel in early nineties but very recent morphological development clearly depicts that a number of submerged sand bodies have developed alarmingly resulting in a drastic fall of capacity of the channel and a reduction of flows to about 8%-10% of the total flows within a span of about 20 years. This channel has its own importance not only due to serving as the main water way connecting Sagar island with main land but also it plays a very important role in this dynamic estuary. For decades, this channel has undergone many changes, and this changing pattern has been studied using Remote Sensing and Geographical Information System (GIS). Change Detection Study has been made using supervised classification of yearly satellite imagery. The ground- truth has been acquired by extensive survey works using HYPACK software and by conventional survey method. All these studies lead to reveal the fact that the entire tidal creek, i.e. Creek Channel area is shoaling alarmingly. From the satellite image analysis, growth of several sand bars of this channel are detected. The total areas of zero-contours are increasing very rapidly, and these are clearly indicated by the infamous phenomenon of sedimentation in the main connecting waterways between Sagar Island and main land i.e. Kachuberia and Lot-8, a lot of bars generated in this very important waterways causing blockage of both the flood and ebb flows in this channel. This phenomenon casts adverse effects in (a) total flow of the main Hugli estuarine regime (b) navigation within Creek (c) drainage in the surroundings and also (d) in the flow –dynamics of the adjacent creek, i.e. Hatania-Doania river.If the present trend of sedimentation in this tidal creek continues, it is apprehended that within few decades it will be silted up.