Dynamic evolution of riverbed potholes in the granitic bedrock of Chota Nagpur Plateau, middle part of Subarnarekha River basin, India

摘要

Potholes are an extraordinary abiotic micro geomorphic feature on the granite and quartzite topography of the Subarnarekha River basin. These erosional features are carved by the running water actions of River Subamarekha and its tributaries. The study has been conducted on 17 sites (n = 1707) which are exemplary for the development of this micro fluvial feature located on the rejuvenated middle part of Subarnarekha River basin. Hydrological and geological factors are imperative for their formation. Joint length, joint density, joint orientation, rock hardness, variations of fluvial hydraulic, bed roughness, river bed slope and hydraulic gradient are conducive factors for the development of this geomorphic feature. Monsoon is the ideal season for carving these features by river turbulence, when abrasive work of river is predominant on the river bed (monsoon = 3000-6500 kg. m(2)s(-3), non-monsoon = 0.50-3.0 kg. m(2)s(-3)). The evolutionary sequence of potholes is from round to oval and then elongated with vertical scouring. The relation between pothole radius and depth has been established to explain the process of its expansion. Joint length shows positive relation (R-2 = 0.71) with pothole area while joint density has a negative relation (R-2 = 0.29) as these are structurally weaker zones, characterised by destruction of these features. Suspended load is also a hindrance in their formation. Sites with high suspended load are delineated as pothole destructive zones. The mean dynamic evolution of circular, oval and elongated shaped potholes occurs at spatio-temporal level (Delta(o)e = 0.005 - 0.60 m(3), Delta(c)e = 0.01 - 0.25 m(3), Delta(e)e = 0.005-0.30 m(3)) and the initiation of this development depends on inner depth and radius of individual pothole.