Analyzing Grain Size Distribution and Depositional Traits at the Ganga River Bend near Varanasi, India

Abstract

Sediment transport within an alluvial channel significantly hinges on the distribution of grain size of the sediment particles. This distribution inherently influences the morphological dynamics of a river system by altering its cross-sectional area and morphological pattern. In this context, statistical matrices like mean, standard deviation, skewness, and kurtosis serve as pivotal indicators, which are used to characterize the grain size and the requisite energy conditions governing the transportation and deposition of the river sediments. The present investigation specifically targets the preliminary assessment of the depositional traits of river sediments. This involves two primary steps: (1) computing the grain size distribution of sand particles, (2) scrutinizing the correlation between grain size and shear stress in the Ganga River at the bend near the city of Varanas, India. To achieve the objectives, samples were meticulously collected from 20 distinct sites with each site containing 3 observation points for sampling. Approximately 2000 grams of samples were collected from each collection site. Subsequently, the soil samples underwent thorough analysis to unveil their statistical parameters linked to the grain size.  The finding reveals that a mean grain size ranging approximately from 1.185 to 1.313 is indicative of medium-grained sand composition. Additionally, other important parameters such as: standard deviation, skewness, and kurtosis were obtained within the range of approximately 1.232 to 1.349, ~ 1.232 to 1.349, ~ 0.2 to 0.7, and ~ 0.096 to 1.236, respectively. These values collectively suggest a fine skewed distribution and a platykurtic nature of collected samples. The comprehensive analysis of these parameters and their intricate relationships provide essential insights into the depositional characteristics of the sand. Examination using CM diagram unveiled that at a specific study location, sand deposition predominantly occurs due to suspension and rolling mechanisms. In the CM diagram, 'C' stands for the coarser one percentile value, and 'M' stands for the median value of sediment samples on a log-probability scale.  Moreover, the plot containing bed shear stress against critical shear stress indicates that the deposition of sand particles is prevalent across a significant portion of the study area. Expanding the analysis to indicate specially/spatially? explicit information on sediment transport capacity by employing geospatial tools such as sediment transport index (STI), revealed noteworthy insights. It was deduced that the study area exhibits minimal erosion while showcasing considerable depositional characteristics at various locations. Keywords: Sediment transport, grain size distribution, river morphology, statistical parameters, depositional characteristics, bed shear stress, sediment transport index (STI).