Dynamic of channel potholes on Precambrian geological sites of Chhota Nagpur plateau, Indian peninsula: applying fluvio-hydrological and geospatial techniques

Dynamic channel bed potholes or sculpted forms (circular, elliptical, oval, elongated, etc.) on riverbeds are generally formed due to the variation of lithology with structures and dominant fluvio-hydrological processes. Most of the channel bed potholes are confined at Bhakuyadi, Guridih and Ghatsila on Precambrian Chhota Nagpur granitic gneiss and tonalite gneiss of the river Subarnarekha and its tributaries. A total of 215 channel bed potholes are studied at three sites [Site 1 Bhakuyadi (n = 115), Site 2 Guridih (n = 65) and Site 3 Ghatsila (n = 35)]. The main objective is to find out the multi-dimensional growth on Precambrian bed rock channel and analyze the role of fluvio-hydrological factors toward their growth. Geospatial, fluvio-hydrological and statistical techniques have been applied to find out the potholes dynamic on lithological diversity of extended part of the Chhota Nagpur plateau of gneissic complex. Lineament features have been extracted using different SWIR and VNIR bands of two specific images like ETM and ASTER. Arc Hydrology tool (Hydro tool of ArcGIS) has been used to delineate stream networks and watershed of river Subarnarekha. Morphology of potholes like shape–form analysis has been done through morphometric index such as Surface Index, Bottom Index and Vertical Index. Downward and sideward potholes growth rate are calculated using mathematical indices, and depth–radius ratio has been assessed applying descriptive statistics. The results show that the multi-dimensional growth or pothole dynamics are immensely regulated by local condition such as rock types with structural imprints (fractures, joints, etc.). The absence of micro-geological structures on others lithological unit have failed to give birth to multi-dimensional growth of channel potholes along the flow path of river Subarnarekha and its tributaries within the Subarnarekha watershed. Chhota Nagpur granitic gneiss and tonalite gneiss with structural signatures help to form channel bed potholes within the flow path of river Subarnarekha and its tributaries. Statistical analysis reveals that there is a strong positive correlation between depth and radius of potholes. Subsequently, fluvio-hydrological factors (like corrasion, cavitation, hydraulic action, change of channel gradients, monsoonal bankfull discharge, etc.) clearly reflect the dynamic growth of channel bed potholes specifically during monsoonal months.