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This book explains the causes, mechanisms and dimension of sedimentation in the

Rupnarayan River, which offers a representative example of a tidal river combining fluvial and marine processes. The book is unique in addressing all the hydrodynamic characteristics of the river, especially the tidal impact, sediment load and textural characteristics of the sediments.

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The data presented here was gathered through continuous field monitoring using state-of-the-art techniques and robust laboratory analysis, and will help readers develop a systematic understanding of sedimentation processes. As such, the book offers a valuable resource for all students and researchers conducting research on fluvial geomorphology and sedimentology. In addition, the outcomes it presents will benefit engineers, hydrologists, planners and other authorities affected by a number of aspects related to sedimentation.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract
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 m3 shoaling causing the deterioration and incapacitation of the river. The total shoaled up area has increased from 15.41 km2 to 57.35 km2 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.
Swapan Kumar Maity, Ramkrishna Maiti

Chapter 2. Channel Forms and Patterns

Abstract
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.
Swapan Kumar Maity, Ramkrishna Maiti

Chapter 3. Stream Hydraulics

Abstract
Seasonal variation of stream hydraulics was monitored to understand and explain the mechanism of sedimentation. Water velocity is measured by floating method and using digital current meter. Density of water is measured by testing the collected water samples in the laboratory. The pattern and nature of flow is identified by Reynolds Number and Froude Number. Spatial and temporal variation of stream depth and velocity indicates the unsteady and non-uniform nature of flow. Values of Reynolds Number are >6248.66 in all the cross-sections, which indicates the turbulent pattern of flow. Values of Froude Number (0.2493–0.5238), indicates the sub-critical nature of flow. Maximum surface water velocity is measured as 1.22–2.02 m/s in pre-monsoon, 1.42–2.25 m/s in monsoon and 1.33–2.05 m/s in post-monsoon season. High tide water velocity is more (0.87–2.25 m/s) than the velocity in low tide (0.65–2.05 m/s). Stronger high tide transport more sediment towards upstream than that discharged towards downstream during low tide, causing sedimentation. During dry season paucity of rainfall causes less discharge of water (850–4160 m3/s), reduction of stream energy and sediment transporting capacity which allows sedimentation. But in monsoon season occurrence of huge rainfall increases the water discharge (3455–9050 m3/s), stream energy, sediment transporting capacity and reduces sedimentation rate.
Swapan Kumar Maity, Ramkrishna Maiti

Chapter 4. Tidal Character

Abstract
Being a part of Hoogly estuary, the lower reach of the Rupnarayan River is affected by semi-diurnal tide of Bay of Bengal. Tidal range, tidal prism, tidal asymmetry and variation of tidal velocity in different phases play dominant role to control the mechanism and rate of sedimentation in the lower reach. Tidal gauge data have been collected at an interval of one hour at different gauge stations. Tidal range increases from upstream to downstream (2.8 m at Kolaghat, 3.75 m at Soyadighi, 3.85 m at Anantapur, 4.05 m at Pyratungi, 4.3 m at Dhanipur and 4.4 m at Geonkhali). Extensive intertidal areas towards downstream receive more water and sediment during high tide than areas towards upstream and enhances the sedimentation rate towards downstream. Mid-tide water velocity is more (1.31–2.15 m/s) in all the places than full-tide water velocity (1.25–2.08 m/s). The mudflats and marshy areas which lie above mid-tide level are flooded over 700 times every year during flood tide and getting sedimented. High tide duration is shorter by 2–6 h than that of low tide and this tidal asymmetry results swifter flow with greater energy during high tide leading to landward transport of sediment. The sluggish low tide discharge over longer duration (8–9 h) allows sufficient opportunity for settlement of sediments.
Swapan Kumar Maity, Ramkrishna Maiti

Chapter 5. Sediment Load: Concentration and Transport

Abstract
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.
Swapan Kumar Maity, Ramkrishna Maiti

Chapter 6. Analysis of Bed Load Sediment Texture

Abstract
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.
Swapan Kumar Maity, Ramkrishna Maiti

Chapter 7. Conclusion

Abstract
The lower reach of the Rupnarayan River, from Kolaghat to Geonkhali (40 km) has been showing signs of rapid deterioration and incapacitation due to sedimentation that creates a series of inter-connected problems. Asymmetry of the cross-sections leads to channel dynamism due to concentration of energy near the bank. Channel widening and separation of flow and associated variation in energy distribution across the channel lead to sedimentation. Seasonal variation of water discharge is very important to control the stream energy, sediment transporting capacity and rate of sedimentation. Due to high tidal asymmetry (high tide is shorter than low tide), the sluggish low tide discharge over longer duration (8–9 h) allows sufficient opportunity for settlement of sediments. The mechanisms of sedimentation, the rate of sediment transport and textural characteristics of sediments in the lower reach of the Rupnarayan River are the result of the interactions between fluvial and marine processes and the seasonal variation of the intensity of their influences.
Swapan Kumar Maity, Ramkrishna Maiti

Backmatter

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