Assessing the effects of extensive bed material extraction on the granulometric and fluvial characteristics in a sub-Himalayan River in India

The fluvial characteristics of Lower Balason river have been significantly altered by anthropogenic factors, involving urbanization, embankment construction, and extensive in-channel mining with an annual rate of 6.84 × 106 m3, which is much higher than all other channels of the European and American continents. For assessing the effects of this extraction, we have investigated the surface grain-size distribution of 49 gravel bars by field sampling, and by the hydraulic modeling along a 25-km river reach. We have categorized the channel into four sectors: 'Transport limited' first sector and third sector and 'Supply limited' second and fourth sector. The first sector-A of channel features an incised narrow channel, relatively steeper bed slope, accelerating sediment flux connectivity. In sector B, extensive mining in the channel bed, mid-channel islands, and surrounding riverbanks results in the expansion of the channel, causing a reduction in water surface slope, stream power, and bed shear stress, along with a substantial drop in grain size. Sector-C exhibits the densely populated floodplains and extensive bank stabilization resulting in deep, narrow channel width with increased stream power along with modest rise in granulometric size and drop in sorting. The increased channel flow poses a threat to the stability of the foundation walls of bridges on the channel. 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 observed granulometric patterns suggest that the diminution coefficient of the coarser fraction (D95) is approximately 22 % greater than that of the median size fraction (D50) and 48 % greater than that of the finer size fraction (D5). This difference could be attributed to the outwashing of fine fractions of the bed material due to the extraction of larger particulates from the channel bed surface.