Effect of turbulent structures on the riverbank erosion due to tidal influence: A case study from the Rupnarayan River, eastern India

Abstract

For understanding the sediment removal mechanism from the bank face, field measurements of the dominating turbulent flow structures under the influence of flood and ebb tide were carried out at the middle reach 98 km upstream of the mouth of Rupnarayan River, India where the bank erosion activity was highly dynamic. Measurement of the three-dimensional temporal variation of velocity was carried out using a 16-MHz Micro-ADV during the transition period of flood to ebb tide. Results from the present field study depict that during the transition of the onset of the ebb tide, the velocity field showed reduced values that gradually acquired negative values at the near bank region. This manifested the existence of an anti-clock circulation during flood tide and clock-wise circulation during ebb tide at the near bank flow field. It was also pertinent that the velocity gradient during ebb tide was greater as compared to the velocity gradient during flood tide. Accordingly, the turbulent bursting analysis revealed that the ejection and sweep events were prominent during ebb tide resulting in the dislodgement of sediment from the bank face at a larger rate. Further, the similarity of the wavelet patterns revealed that a good correlation existed between the stream-wise and transverse velocity component during ebb tide that enhanced the erosion process during the ebb tide event.