• KOUSTUV DEBNATH

Articles written in Journal of Earth System Science

• Cohesive river bank erosion mechanism under wave-current interaction: A flume study

The effect of hydrodynamic forces due to combined action of surface waves and current on the riverbank is critical to understand sediment entrainment, transport and bank line retreatment process. In understanding the temporal effect of turbulent structures under induced wave-current Cow, a series of laboratory experiments were carried out. Micro-Acoustic Doppler Velocimeter (ADV) and Ultrasonic Ranging System (URS) were used simultaneously for the measurement of velocity fluctuations and bank undercut depth increment. Modulation of the turbulent flow characteristics and the benefaction of turbulent bursting structures at the initiation of erosion process and before the failure of the cohesive bank due to undercut progression are discussed. The results show that velocity and Reynolds shear stress have direct dependence on the size and rate of the entrainment of cohesive aggregates from bank face. The effect of wave-current motion leads to an increase in shear stress at the lateral bank giving rise to erosion and transportation of sediment particles/aggregates. Quadrant analysis of the random velocity fluctuation under wave-current flow at the initiation of erosion process shows strong presence of ejection and sweep events. Findings from the present study may provide a better understanding on the design of cohesive bank erosion control measures.

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

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.

$\bf{Highlights}$

$\bullet$ The velocity gradient was greater during ebb tide than during flood tide.

$\bullet$ Flow-circulation is clockwise during ebb tide and anti-clockwise during flood tide.

$\bullet$ Bursting structures reveal the dominance of ejection and sweep events during ebb tide.

$\bullet$ The entrained sediment particles are transported with ebb current.

$\bullet$ The entrained sediment is deposited at the lower reach causing rapid sedimentation.

• Turbulence effect on the mechanics of ripple formation under regular wave

The study reports an experimental investigation on the mechanism of the formation and development of isolated ripple zones over sand bed under unidirectional surface wave propagation. The ripple formation emerges due to the occurrence of vortices at the near-bed region under propagating wave fronts. A number of ripple patches are noticed after 100 cycles of the surface wave. To characterise these patches, Hilbert transform is used. These patches develop as a function of the number of wave cycles following Power law. Further, the rate of development of these patches increases with the increase of surface wave frequency as also reported in the literature. It is revealed that non-isolated turbulence singularity (random phase fractal signal) governs the erosion and deposition of sediment particles at the trough and crest of a progressive ripple front. The anti-phase behaviour of turbulence signal increases the wave excitation energy which may enhance the sediment erosion and transportation rate. However, it is hypothesized that the out-of-phase behaviour with moderate wave excitation energy contributes to the sediment deposition. Due to this, the frontal portion of the ripple does not attain a stable form and develops further.

• # Journal of Earth System Science

Volume 131, 2022
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019