Turbulent flow structures in alluvial channels with curved cross‐sections under conditions of downward seepage

et al. 2019

Water

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Sand mining in an active alluvial channel can compromise the streambed stability of the hydraulic structures nearby. This experimental study is aimed at investigating the effects of rectangular mining pit on the morphodynamics around circular tandem piers in a movable bed. A rectangular pit is excavated upstream of two circular piers embedded in the sand-bed in a tandem arrangement. The results are then compared to a case having only the piers without any mining pit. Turbulent stresses and mean velocities in the near-bed region rise significantly at the upstream region of the piers in the presence of a pit. Also, stronger flow reversal and horseshoe vortices have been detected at the base of the pier front. Due to these alterations in the nature of turbulence, erosion of channel beds upstream of the piers, increased scour depths, scour volume, and lateral erosion of the scour hole have been observed. Dynamic evolution of the local scour at various time scales has been studied using a wavelet cross-correlation method. Spatial evolution of local scour is found to be faster when a pit is excavated in the channel. Thus, mining activities near the piers can lead to significant changes in the flow-field, causing excessive scour around piers.

Section: Data Collectionmentioning

confidence: 99%

On the Morphodynamic Alterations around Bridge Piers under the Influence of Instream Mining

Lade

Deshpande

et al. 2019

Water

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“…It is observed from Figure that modified log law can be further modified into two different logarithmic laws with k value 0.34 and 0.39, respectively, at Sections A and D, which shows strong momentum exchange between the flows and bed material. Thus, k values for both sections are lower than its universal constant ( k = 0.41), indicating mobile bed condition (Deshpande and Kumar, ).…”

Section: Resultsmentioning

confidence: 97%

“…Velocity profile data has been fitted to the modified logarithmic law in the inner layer ( z / h = 0.2) as discussed by Deshpande and Kumar () for Sections A and D. The logarithmic law is expressed in the non‐dimensional form as follows:

\frac{\overset{true¯}{U}}{U_{*}} = \frac{1}{k} ln ((), z^{+} + Δ z^{+}) - \frac{1}{k} ε^{+}

…”

Section: Resultsmentioning

confidence: 99%

“…The time series velocity data obtained from the SonTek ADV was filtered with the acceleration threshold method for spike removal (Goring and Nikora, 2002). A threshold value of 1 to 1.5 was used based on trial and error method (Deshpande and Kumar, 2016). After spike removal, velocity power spectra should satisfy the Kolmogorov's À5/3 hypothesis in the inertial sub-range (Lacey and Roy, 2008).…”

Section: Experimental Set Up and Data Collectionmentioning

confidence: 99%

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Turbulent flow structures and geomorphic characteristics of a mining affected alluvial channel

Barman

Earth Surf Processes Landf

Sarma

2018

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Sediment mining in rivers may have a major impact on river geomorphology and research is required to quantify these impacts. In this research, experimental studies were conducted to analyse the morphological changes of channel bed and the turbulent characteristics of flow in the presence of mining. The channel bed profile shows erosion at the bank of the pit and that the erosion expands to the whole width of the channel and propagates downstream with time. The deposition of sediment occurs along the upstream edge of the pit and the depth of the pit decreases with time. Velocity reversal occurs at the central bottom of the pit related to a recirculation zone. Reynolds shear stress and the turbulent intensities become higher in the mining pit region and downstream of it as compared to the upstream section, causing a more rapid movement of bed particles. Analysis of the bursting phenomenon shows that the contribution of sweep and ejection events to the total Reynolds shear stress is more dominant over outward and inward interaction events. The dominance of the sweep event over ejection is observed at the near‐bed region for all the sections, but the depth range of dominance of sweep events in the pit and downstream of the pit is found to be more than the upstream. The increase in thickness is responsible for the increase in bed material transport. The increased sediment transport capacity at the mining pit and downstream of it caused the deformation and lowering of channel bed downstream. An empirical formulation of bedload transport for mining induced channels is derived from two different sized uniform bed materials. Copyright © 2018 John Wiley & Sons, Ltd.

“…Seepage, being an integral part of alluvial flow, is needed to understand the morphology and hydrodynamics of fluvial systems. In our previous study, extensive research regarding the influence of seepage on channel morphology and flow turbulence has been pursued (Patel et al ., ; Deshpande and Kumar, ), while the effect of different percentages of seepage on the flow structure and celerity of migrating bedforms have not been discussed over fine sand bed channels. Taking this deficit into consideration, the objectives of this study are to analyse the effect of downward seepage on turbulent flow structure, geometry of bedforms, and celerity of developing bedforms on a curvilinear cross‐section threshold alluvial channel.…”

Section: Introductionmentioning

confidence: 99%

Effect of seepage on flow and bedforms dynamics

Patel

Majumder

Earth Surf Processes Landf

2017

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This study, using an experimental approach, focuses on the effect of downward seepage on a threshold alluvial channel morphology and corresponding turbulent flow characteristics. In all the experiments, we observed that the streamwise time-averaged velocities and Reynolds shear stresses were increased under the influence of downward seepage. Scales of eddy length and eddy turnover time were significantly increased with the application of downward seepage, leading to sediment transport and initiation of bedforms along the channel length. As the amount of seepage discharge increased, eddy length and turnover time were further increased, causing the development of larger bedforms. It was revealed that the geometry of bedforms was linked with the size of eddies. In this work, statistics of bedform dynamics are presented in terms of multi-scalar bedforms in the presence of seepage. These multi-scalar ubiquitous bedforms cast a potential impact on flow turbulence as well as stream bed morphology in channels. We used wavelet to analyse temporally lagged spatial bed elevation profiles that were obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuations at different length scales. A spatial crosscorrelation analysis, based on the wavelet coefficients, was performed on these bed elevation datasets to observe the effect of downward seepage on the dynamic behaviour of bedforms at different length scales. It was found that celerity of bedforms reduced with increase in seepage percentage. Bedform celerity was best approximated by a probability density function such as Rayleigh distribution under varying downward seepage. Further, statistical analysis of physical parameters of bedforms ascertained that the reduction in bedform celerity was a result of increased bedform size.