Secondary current and vorticity regime of sand-mined sinuous channels

Sand mining plays a vital role in meeting construction demands and maintaining river sediment balance; however, unregulated extraction can significantly alter river hydrodynamics, morphology, structural and ecological stability. This study investigates the influence of different floodplain sand mining pit configurations on flow velocity, secondary currents and vorticity structures in a sinuous channel using FLOW-3D Hydro simulations validated through laboratory experiments conducted at the IIT Guwahati Fluvial Hydro-Ecological Laboratory. The experimental channel was 18 m long with a sinuosity of 1.1 and operated under subcritical flow conditions. Two mining scenarios were examined: (i) two consecutive floodplain pits and (ii) two consecutive pits with an additional pit on the opposite floodplain. Results indicate that streamwise velocity was consistently higher near the inner bank in both scenarios, while the second configuration produced a narrower high-velocity core. Secondary currents were concentrated near the outer bank and within the mining pits; however, their intensity was stronger in the first scenario, suggesting that it may have a higher potential for pit migration and bank erosion. Vorticity in the streamwise direction was concentrated within the mining pits in both scenarios. Vertical vorticity dominated near the inner bank and within the pits, whereas lateral vorticity prevailed near the bed in the main channel and inside the pits. The combined vorticity field exhibited anticlockwise circulation near the outer bank and clockwise rotation near the inner bank. The findings contribute to sustainable sand mining practices by improving understanding of channel hydrodynamics and reducing potential environmental impacts in sinuous river systems.