Indian Journal of Geo-Marine Sciences Vol. 45 (10), October 2016, pp. 1349-1356 Tidal impact leading to sedimentation at lower reach of Rupnarayan River, West Bengal, India 1Swapan Kumar Maity & 2Ramkrishna Maiti 1Department of Geography, Haldia Govt. College, West Bengal, India 2Department of Geography and E. M., Vidyasagar University, Midnapur, West Bengal, India [E-mail: [email protected] ; 2 [email protected]] Received: 07 December 2012; revised: 07 July 2013 During the last 20 years the lower reach of the Rupnarayan River has experienced a net shoaling of 26.57 million m3 (survey made by Kolkata Port Trust, 1992-2012) with 42.04 million m3 shoaling and 15.47 million m3 scouring. Bathymetric close grid survey of the study area was made using Leveling instrument and Echo sounder interfaced with Position Fixing System (GPS). Simultaneous tidal observations were available at an interval of one hour, velocity of high tide and low tide, tidal asymmetry and discharge of water etc. are measured at different Gauge Stations in the field. Swifter and stronger high tide results more available energy during high tide than as low tide. The incoming high tide brings a lot of sediments from the downstream and deposits it on the bed of the studied river in absence of any upland discharge during the non-monsoon months. During the freshet (July, August and September) the impact of high tide is felt less due to the voluminous anti-directional flow of the upland discharge. Tidal range is more near Geonkhali (4.5m) and decreases towards upstream, being lowest near Kolaghat (2.9m). Tidal asymmetry increases from downstream (2 hours at Geonkhali) to upstream (6 hours at Kolaghat). Tidal prism (volume of water that enters a tidal region during flood flow) is very high in that region. [Key words: Sedimentation, transport power, bathymetry, tidal asymmetry, shoaling and scouring, tidal prism.] Introduction sediment during high tide is the main cause of Sedimentation is the main cause of rapid and continuous sedimentation at lower reach deterioration of stream channels and navigation of the Rupnarayan River. In the present study difficulties. The settling velocity of a particle of attempts have been made to explain the causes certain size is the threshold of deposition for that grain. Thus the flow velocity lowers than the and mechanism of sedimentation in connection to threshold causes deposition of that particle1. Flow the tidal mechanisms, i.e., the role of tidal range, separation due to channel widening causes tidal prism, tidal velocity and tidal asymmetry deceleration of flow and invites deposition. If a leading to sedimentation. huge amount of sediment is discharged from upstream area, the river response by aggrading in Materials and Methods order to steep the gradient and increase its The Rupnarayan River with its network of velocity to become more competent2-3, 1. sub-tributaries, including the Dwarakeswar, At present Rupnarayan river has been Silabati, Kangsabati and Damodar Rivers, is the showing signs of rapid deterioration and most important tributary of the River Hoogly. incapacitation due to sedimentation. Rapid River Silabati and Dwarakeswar meet at Bandar, sedimentation and development of shoal area and the combined flow is named as Rupnarayan, creates a variety of detrimental problems and which joins river Hoogly at Geonkhali covering a impacts on society, economies and the distance of 78 km. The whole Trans- Damodar environment including the shifting of river course area is ultimately drained through this river. The and thalweg position, shortage of water during entire Rupnarayan River has been divided into neap tide for utilization in different purposes, three reaches. Upper reach having 28 km length hindrance of easy discharge of water and is extended between Bandar and Jasar. The upstream flood, navigation difficulties, river bank middle reach is 10 km long extending between erosion and loss of settlements and properties. Jasar and Dainan (Kolaghat). The lower reach Being a tidal river Rupnarayan is affected by both (study area) extends from Kolaghat to Geonkhali riverine and maritime processes. That’s why the with a length of 40 km and is bounded by causes and mechanism of sedimentation on that 22 12 N to 22 26 N and 87 53 E to 88 03 E 2 river bed is different from other normal streams. (Fig 1), approximately 10930 km catchment area The landward intrusion of marine water and of the Rupnarayan River. The study area has a 1350 INDIAN J. MAR. SCI., VOL. 45 NO. 10 OCTOBER 2016 typical tropical monsoonal type of climate with an wooden tray on river bed for few days. Some bed average rainfall of 1320mm to 1630mm and samples was collected and grain size analysis was annual temperature ranging from 11oC to 45oC. done in the laboratory of Indian Institute of Lower reach is divided into four sub-reaches Technology, Kharagpur. Satellite data was (Geonkhali to Dhanipur, Dhanipur to Anantapur, procured from National Remote Sensing Agency Anantapur to Gopinathpur and Gopinathpur to (NRSA) and required analysis (digital image Dainan reach) based on the necessity of the study. processing and visual image interpretation.) was Topographical maps from Survey of India, made. ERDAS IMAGINE 8.5, ArcGIS 9.2 and District Planning Maps from National Atlas and Microsoft Office Excel etc. have been used for the preparation of maps and diagrams. Amount of water discharge is calculated following Maddock4 Q= WDV …………… (1) Where, Q = discharge, W= width of channel, D= depth, V= mean velocity. Analysis of mean grain size is done following the equation of Folk and Ward5 Graphic mean = (φ16 +φ50 +φ84)/3 ……………… (2) Amount of energy required to transport the sediments of uniform size (critical shear stress) during all the seasons are calculated using Shield6 formula τcr= ηg(ρS-ρ)π/6 Dtanθ……………(3) Where, ρ= fluid density, ρS= sediment density, g= gravity constant, D= grain diameter, η= nD2, a measure of grain packing. Fluid density (ρ) and sediment density (ρS) is measured in the laboratory by collecting water and sediment samples. The available stream energy passing through open channel is expressed as energy equation following Fig.1- Study area the equation- 2 Thematic Mapping Organization (NATMO), Soil H= Z + dcosθ +V /2g …………….. (4) and Land use Maps from National Bureau of Soil Where, d= depth, θ is the slope of river bed, Z= Survey and Land Use planning have been elevation above datum, v= mean velocity. collected. Bathymetric close grid survey of the study area was made using Leveling instrument Result and Discussion and Echo sounder interfaced with Position Fixing The lower reach, from Dainan to Geonkhali System (GPS). Simultaneous tidal observations (40km) of the Rupnarayan River has been were available at an interval of one hour at deteriorated and has lost its carrying capacity due different Gauge Stations in the river Rupnarayan to continuous sedimentation and development and and discharge of water is measured in the field. shifting of shoal area. Throughout this stretch the Tidal gauge and discharge related data were also river is interspaced with inter tidal flats which collected from the Sectional Office at Ghatal and keeps on shifting, decaying and growing, Tamluk, under the Irrigation and Waterways contributing to the frequent changes of the river Department of Government of West Bengal. morphology. During the last 20 years net shoaling Bathymetric, tidal, sediment and discharge data area is 26.57 million m3 (survey made by Kolkata have also been collected from the concerned Port Trust, 1992-2012) in this region with 42.04 authority of Kolaghat Thermal Power Project and million m3 shoaling and 15.47 million m3 Kolkata Port Trust. Velocity is measured by scouring (Fig 2 and Table 1). Four cross-sections Digital water current meter and floating method at have been drawn at different stretches to analyze different sections along the river. Water samples the shoaling and scouring pattern in that region during high and low tide were collected to and in shown in Fig 3. measure the suspended load concentration. Rate In Geonkhali to Dhanipur stretch of 10 km of sedimentation was measured by keeping the 75% of the area has been shoaled up. Maximum MAITY & MAITI: TIDAL IMPACT LEADING TO SEDIMENTATION 1351 shoaling of around of 5 m is observed near the confluence at Geonkhali reach. During the period of last 20 years the right bank between confluence and 3 km upstream has encroached in to the river to the tune of 200-500 m. The incidence of local scour area is observed to be restricted within the lower half of this reach. It is observed that a deep pocket area just downstream of Dhanipur (22013’N and 87059’ to 880E) has not only survived but also improved with a maximum scour of 307m during the last 20 years. The upper half of the reach has been shoaled up almost completely with a maximum deposition of 2.1m. During the last 20 years, Dhanipur to Anantapur stretch of about 12 km has narrowed down to the extent of 45% of its width prevailing in the year 1992. Though the right bank at the lower stretch of around 5 km has got eroded to the extent of about 500m, the remaining 7 km lengths has encroached into the river to the tune of 750m. However this stretch could manage to maintain a narrow scouring channel in its entire length. Anantapur to Gopinathpur stretch of about 10 km has undergone severe transformation with respect to the year of 1992. Main channel, which used to flow hugging the left bank in its upper part in 1992, has completely shifted towards right bank to the extent of around 1.5 km. Severe deposition along the left bank in the upper part of the reach has induced sedimentation in the central portion of the lower part also.