Assessing the effects of bed material extraction and channelization on the surface grain size and hydraulic conditions in a Sub-Himalayan river in India

 

The longitudinal variations in grain size of the surface sediments of the sub-Himalayan River in India were studied to ascertain the effect of anthropogenic activity on grain size alteration and hydraulic conditions. The grain size was measured on 49 gravel bars along a 25-km anthropogenically modified river. The channel exhibits four unique segments, which have been delineated through the morphological pattern of the channel. The upper sector-A of the channel features a relatively steeper bed slope, enhancing sediment flux connectivity and promoting abrasion and sorting. This is evidenced by the rapid fining of sediments downstream. The deep channel incision in sector-B is produced from the bed material extraction, led to decreased water surface slope, stream power, and bed shear stress, accompanied by an extensive reduction in grain size. Section-C exhibits the densely populated riverbanks and extensive bank stabilisation, resulting in reduced channel width with increased stream power and transport stage along with a modest rise in granulometric size and drop in sorting. This has led to riverbed erosion and increased channel depth. Additionally, the concentration of flood flows in a narrower and deeper channel may have increased their potential, posing a threat to the foundation walls of bridges. In the terminal sector-D, comparatively substantial bars were noted, where the mining activity again amplified, and as a consequence, the river bed degraded. The findings of this study provide valuable insights into the grain size dynamics of the Himalayan river corridor, with comparisons drawn to similar anthropogenically influenced mountain rivers worldwide.