Experiments were performed in a sand bed channel to investigate the effects of a mining pit on the hydrodynamics around circular bridge piers. Mean velocity profiles, Reynolds stresses, kinetic energy fluxes and scales of turbulence were analysed at critical locations along the channel bed as well as in the proximity of the pier. At the approach location where flow had passed over the pit and was approaching the pier, substantial increments in the near-bed velocities, bed shear stress and Reynolds stresses were observed. Dredging of the pit increased the strength of the horseshoe vortex in the scour hole region and also amplified the shedding frequency of trailing vortices at the rear of the pier. These effects may be instrumental in the alteration of local scour as well as erosion and deposition patterns around bridge piers.

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.

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We conducted flume experiments to study the morphological behaviour around a circular pier located downstream of a mining pit. Severe transverse erosion at the pier base as well as lowering of channel bed upstream of the pier was observed because of channel dredging. Streambed instability was analysed based on two non‐dimensional parameters, namely, pier exposure factor and normalized upstream incision depth. The maximum values of both these parameters were observed when the mining pit was dredged closest to the pier. We also observed growth in streambed instability with an increase in flow Reynolds number.

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Extraction of sand from riverbed has catastrophic repercussions on aquatic animalia habitat, water quality, and the environment. Alongside, physical alterations in the fluvial hydraulics arising on account of sand mining are also worthy of attention. Flows passing over the pits excavated in a channel have enhanced erosive propensity, which can be a cause of concern for the downstream hydraulic structures. The complex nature of flow interacting with the bridge piers after passing over a mining pit is not fully understood. Experiments were conducted to apprehend the effects of a dredged pit on the turbulence flow-field around an oblong pier. Flow was passed in an erodible sand bed rectangular channel having an oblong pier for the first case. In the second case, a pit was dredged in the mobile bed to replicate a mined channel, and the pier was subjected to the same discharge. The streambed at the approach of the pier experiences greater mean bed shear because of dredging. The amplification of the instantaneous bed shear beneath the turbulent horseshoe vortex (THSV) zone at the pier front is almost twice due to channel dredging. The findings can be useful in understanding the streambed instabilities around bridge piers in mining-infested channels.

Channel dredging has become a common phenomenon across several fluvial systems. Pits dredged in the riverbank can influence the downstream turbulence and affect the bank slope and central channel morphodynamics. Erodible bed experiments were conducted in a laboratory flume having a riverbank cross section with three different bank slopes, i.e., 25°, 31°, and 40° with and without a mining pit. Flow over the bank slope and near the bank toe was most affected by the pit excavation at the channel upstream. Turbulence levels were amplified within the flow over the slope and near the bank toe. The logarithmic scaling range of higher-order fluctuations at the bank toe is greater for with pit case. The complex interaction of simultaneous processes like pit migration, sliding failure and bank erosion, and fluvial erosion was studied at multiple length scales and time scales. A wavelet cross correlation analysis was used to calculate the celerity of bedform migration at the bank toe along the flow. Statistical celerity of bedforms with smaller scales (up to 15 mm length scales) is significantly higher due to pit interaction. The study reveals that instream mining has notable effects on the inherent nature of higher-order turbulence statistics, especially near the bank slope and toe, as well as the multiscale morphological structures.

Flow in streams and rivers typically erodes the banks, causing channel bank migration laterally, resulting in loss of nearby land, modification in channel morphology, excessive sediment transport, and water quality degradation. A spur dike is a hydraulic structure placed at the channel bank projecting outward to guide or divert the flow away from the bank, thus protecting it from erosion. The stability of the riverbed and banks is influenced by turbulent characteristics such as three-dimensional velocity distribution, turbulent kinetic energy, Reynolds shear stress, turbulent intensity, and bed shear stress. The researchers found that these turbulence parameters are instrumental in sediment movement along the channel's bed and from its banks. Spurs dikes are a significant river training structure provided along the river bank to protect from erosion. Several aspects related to spur dikes, such as their geometry, physical features, design considerations, flow and scour patterns, etc., are critically reviewed in this paper. Despite the numbers of literature in the field of turbulent characteristics and scour depth around spur dike, the role of vegetation and the effect of seepage around spur dike still remains an unexplored area. These knowledge gaps of spur dikes in field conditions are discussed for future studies.