Swapan Kumar Maity

Sedimentation under variable shear stress at lower reach of the Rupnarayan River, West Bengal, India

Abstract The lower reach of the Rupnarayan River has been deteriorated and incapacitated due to continuous sedimentation (26.57 million m3 shoaling in last 25 years). Attempts have been made to explain the causes and mechanisms of sedimentation in connection to the seasonal fluctuation of shear stress. River depth and water velocity was measured by echo-sounder and current meter respectively. Textural analysis of grains was done by sieving technique. Available and critical shear stress (N/m2) have been calculated following Du Boys (1879), Shield (1936) and Van Ledden (2003) formula. The lack of available energy to transport a particular grain size during low tide (in dry season) is the main reason behind the rapid sedimentation in this area. Most of the places (>75%) having negative deviation of shear stress (available shear stress lesser than critical shear stress), during low tide are characterized by deposition of sediments. The presence of mud (silt and clay) above the critical limit (15%) in some of the sediment samples generates the cohesive property, restricts sediments entrainment and invites sedimentation.

Identification of the Sediment Sources Using X-Ray Diffraction (XRD) Technique

X-ray diffraction (XRD) technique is used to understand the sources of sediments through identification of mineral composition of sediments in the lower reach of the Rupnarayan River to explain the causes and mechanisms of sedimentation. A total of 21 sediment samples (13 samples from river bed and 8 samples from river banks) have been collected for knowing the sediment mineralogy. Sediment samples are washed by boiled distilled water, dried, disintegrated and scanned at an interval of 7°–45°2θ in XPERT-PRO diffractometer. Diffractograms produced by XRD study indicates that the entire lower reach shows the dominance of the minerals such as quartz, chlorite, illite, anatase, goethite, oligoclase, chloritoid, corundum, sillimanite, which have their origin in the upper and middle catchment area with small contribution from the lower catchment and river banks. Statistical experiment indicates that excluding tourmaline and anatase, all the minerals show steady trend in concentration in sediments. Principal Component Analysis (PCA) indicates that five Eigen values contribute for about 83.154% of the total variation of the distribution of minerals. The minerals discharged from the upper catchment are captured in the estuary and again redistributed towards upstream by stronger flood tide. This leads to a un-conspicuous and hapazard distribution of minerals in the area under study.

Analysis of Bed Load Sediment Texture

Textural analysis of surface sediments is very useful tool to understand the complex interaction between terrestrial and marine environment. Total 180 sediment samples (60 samples in each season) have been collected from the lower reach of the Rupnarayan River and sieving technique is used to calculate different size parameters. Approximately, 63.80% of the sediments are very fine sand, 14.76% are fine sand and 21.44% are coarse silt type. Sediments are coarser in monsoon than in pre-monsoon and post-monsoon seasons due to increase of water volume, stream energy and removal of fine sediments in monsoon. In dry season, >60% sediments are moderately to well sorted but in monsoon season 63.85% sediments are poorly to very poorly sorted. Around 55% of the sediments are of fine and very fine skewed type, 33% of samples are near symmetrical and remaining are of coarse skewed type. The coarser sediments are negatively skewed and finer sediments are positively skewed. In monsoon, >60% of the sediments is platykurtic or leptokurtic in nature which indicates the high energy environment in this season. Proportion of sand, silt and clay in sediments ranges between 38–91%, 4–61% and 1–41% respectively. Nearly, 81.33% of the sediments are silty sand, 7.33% are muddy sand and 6% samples are of sandy silt category.

Sedimentation in the Rupnarayan River

River sedimentation is one of the major water related problem in the world and also in India. Most of the rivers in India are facing the problems of rapid sedimentation and associated socio-environmental hazards. In southern part of West Bengal a large number of rivers like, Damodar, Rupnarayan, Kangsabati and Haldi are being sedimented rapidly and facing the problems of drainage congestion and drainage decay, deterioration of navigability, unavailability of water resources, water storage and resultant flood, river bank erosion, loss of settlements and properties and social dislocation. These problems of sedimentation are more serious and alarming in Rupnarayan River, mainly at the lower reach, from Kolaghat to Geonkhali. During the last 26 years, the area has experienced 28.71 million m shoaling causing the deterioration and incapacitation of the river. The total shoaled up area has increased from 15.41 km to 57.35 km between the years 1973 and 2016. Causes, mechanisms and extent of sedimentation in the lower reach are explained and understood with the detailed study of channel forms and patterns, stream hydraulics, tidal character, sediment load, sediment grain size and related critical and available shear stress and identification of the environment of sediment deposition.

Environment of Sediment Deposition

Grain size of sediments relates to the physical characteristics of the depositional environments. The distribution of sediment grain size is affected by the variations of wave energy and turbulent conditions of depositing environment. Linear Discriminate Analysis (LDA) technique and bi-variate plotting of grain size parameters are used to explain and understand the environment of sediment deposition. Hydrodynamic processes working during the deposition of sediments have been identified by C-M plotting. Sedimentation, in this reach is the result of the spatial and seasonal variation of the interaction between fluvial and marine processes. In non-monsoon season more than 60% of the sediment samples fall under marine environment, but in monsoon season more than 65% of the sediment samples fall under riverine environment. Nearly, 58% of the sediments are deposited under fluvial action and 42% samples are deposited by turbidity action mainly in low to moderate energy condition. Deflection of sluggish riverine discharge towards right by Coriolis force causes more sedimentation towards right bank of the river mainly in non-monsoon season. About 88% sediments are transported by suspension with rolling and graded suspension and are deposited in moderate to lesser violent hydrodynamic condition. The clustering of sediments in PQR segment in C-M plotting indicates the estuarine characteristics of the region.

Channel Forms and Patterns

Channel forms and patterns are important determinants of ease of movement of water and sediment and immediate clearance of materials from up slope. Twenty five (25) cross-sections have been drawn along the lower reach of the Rupnarayan River, by measuring river depth using Echo Sounder and collecting stuff readings using leveling instrument, to understand the channel forms and patterns. Most of the cross-sections, except two (at Kolaghat-AA′ and Geonkhali-YY′) are asymmetrical in nature. Depth is more near Kolaghat (maximum 10.5 m) and Geonkhali (maximum 11.5 m) than the middle of these two extremes (<8 m). Most of the portions near two banks and around the mid channel bar have the depth less than 2 m. Width-depth ratio is less near Kolaghat (38.09) and Geonkhali (130.43), but in the middle portion width-depth ratios are more than 300 and becomes 1111.10 in cross-section QQ′. Sudden expansion and widening of the channel near Kolaghat (width is 400 m at AA′, 1650 m at BB′, 2900 m at CC′) leads to flow separation , reduction of energy and deposition of sediment (during low tide) and near Geonkhali sudden constriction (bottle neck shape) of the channel (width-depth ratio—130.43) hinders free draining of ebb tide water leading to ponding effect and reduction of velocity and stream energy to drain the sediments and initiates sedimentation.

Available and Critical Shear Stress for Sediment Entrainment

Causes and mechanisms of sedimentation are explained in connection to the seasonal fluctuation of shear stress. Available and Critical shear stress have been calculated following DuBoys and Shield formula. Critical shear stress of sediment entrainment varies from 0.031 to 0.147 N/m2 in pre-monsoon, 0.041 to 0.169 N/m2 in monsoon and 0.034 to 0.148 N/m2 in post-monsoon season. Available shear stress varies from 0.271 to 0.923 N/m2 in high tide and 0.014 to 0.683 N/m2 in low tide during pre-monsoon season. In monsoon, it varies from 0.275 to 0.965 N/m2 and 0.237 to 0.907 N/m2 during high tide and low tide respectively. It varies from 0.259 to 0.889 N/m2 and 0.022 to 0.521 N/m2 during high tide and low tide in post-monsoon. Lack of available energy to transport the sediment during low tide (in dry season) is the main reason behind the rapid sedimentation in this area. Most of the places (>75%) having deficiency of energy (available shear stress is lower than critical shear stress), during low tide are characterized by deposition of sediments. Presence of mud above the critical limit (30%) in some of the sediment samples generates the cohesive property, restricts sediment entrainment and invites sedimentation. Sheltering of fine grains by coarse grains, biological activity and organic content increase the critical shear stress of sediment entrainment and causes sedimentation.

Sediment Load: Concentration and Transport

Amount of suspended sediment in water is measured by collecting water samples from different depths during high and low tide . Rate of transportation of bed load has been computed using well accepted empirical equations. Suspended sediment amount ranges between 3.1 and 5.05 gm/l in non-monsoon season and 4.97 and 6.5 gm/l in monsoon season. During non-monsoon period, upstream penetration of suspended sediment is more during high tide (3.71 × 107 to 1.29 × 108 metric tons/year) than that is discharged towards downstream during low tide (2.5 × 107 to 1.0 × 108 metric tons/year) and accelerates the rate of sedimentation . But in monsoon, the transport of suspended sediment during high tide (7.5 × 107 to 2.45 × 108 metric tons/year) and low tide (7.3 × 107 to 2.3 × 108 metric tons/year) is almost equal which restricts the sedimentation rate. Rate of suspended sediment transport is strongly affected by variation of water discharge (r = 0.908) and moderately affected by sediment concentration (r = 0.553). Increasing tendency of water velocity and discharge towards downstream leads to the increase of bed load transport rate from Kolaghat to Geonkhali. It varies from 0.1905 to 6.52985 kg/m/sec, 0.5008 to 14.74893 kg/m/sec and 0.2318 to 6.31764 kg/m/sec in pre-monsoon, monsoon and post-monsoon season respectively. In non-monsoon season, the transport of bed load is more during high tide than during low tide, but the transport of bed load is almost equal in both the tidal phases in monsoon season.