Role of riverbed sand mining on planform and cross-sectional morphology of Mayurakshi River, India

Human interventions in the form of riverbed sand mining are escalating worldwide, especially in the humid tropics with excess population pressure exerting an elevated demand for sand as construction materials. Naturally, channel morphological alterations are observed for the tropical fluvial systems to a large extent. The present work examines the riverbed sand mining of the Mayurakshi River (India) during the last fifty years (1970-2020) using topographical maps, satellite images and field-based cross-sectional measurements. Sand mining history exhibits four phases- (1) initiation phase (1970-1980) (2) expanding phase (1980-2000), (3) steady state (2000-2010) and (4) accelerating phase (2010-2020). Though the first three phases depicted a mild impact of sand mining on channel morphology, the accelerating phase vehemently altered the channel morphology. Topographic sinuosity has increased from 72 % to 81 % at the expense of the hydraulic sinuosity resulting in the lower standard sinuosity index (1.06 to 1.04) caused by the sand mining-induced channel straightening during 1970-2020. Though braiding index (BI*), channel count index (BI), and channel length index (Pt) show an increasing trend with a variable rate (31-76 %) till 2010, there has been a rapid fall (1-138 %) in channel braiding due to sand mining and pit formation. The future trend for 2030 (based on 2010-2020 data) indicates a lowering of the channel sinuosity and braiding in the anticipated increase of mining; however, 1970-2020 data-derived ensemble prediction depicts the trend reversal in 2050. Channel depth, area, asymmetry, and hydraulic radius are higher for sand mining cross-sections (CS). The hierarchical clustering shows that few CS have homogenous clusters determined by sand mining; however, few CS are mixed implying no dominant control of sand mining on them. The study has demonstrated how sand mining acts as the catalyst for channel alterations in various spatial and temporal scales by developing a framework that demonstrates geomorphic system destabilization.