Introduction 1
Background 1
In the case of an incisive river reach, sediment extraction always has a negative effect, because it causes erosion of the river bed and banks. However, due to ineffective control by regulators and commercialization of sediments, large-scale, uncontrolled and unplanned sediment extraction from rivers is now a major concern.
Types of Alluvial Channel Mining 2
Effects of Channel Mining Activities 3
- Morphological impacts on fluvial system 3
- Ecological, environmental and water quality impacts 5
The kink also migrates in the upstream direction and causes erosion upstream of the well. Channel bed is lowered, and bed slope increases upstream of the mining zone.
Effect of Sand Mining on Bridge Piers 10
The failure and collapse of the Hintze Ribeiro Bridge in Portugal in 2001 claimed the lives of 59 people. According to authorities, the failure is the result of more than two decades of uncontrolled illegal sand mining near the bridge (Sousa et al detected a 20 mm year-1 movement of the structure using multi-temporal interferometric techniques before it collapsed.
Mining Scenario in India 12
Present State of the Art 12
The results showed downstream cave migration as well as downstream bed degradation. The rate of migration was found to increase with decreasing pit depth.
Need for Research 24
When the distance between pits was 14 times greater than the flow depth, the desirable effect was seen in filling the pit downstream. What are the effects of river mining on the hydrodynamics of flow around bridge piers.
Objectives 26
Additional laboratory and field investigations are required to precisely understand the effects and impacts of mine pitting on hydrodynamics and morphology around bridge piers. To study cross section and longitudinal section of scour hole around a pier with and without mine shaft.
Organization of Thesis 27
The average flow velocity profiles at the front and rear of the pier (Location-3 and Location-4, respectively) are consistent with the previous studies (Graf & Istiarto, 2002; Kumar & Kothyari, 2012). The instability of the streambed around the pier was a function of the Reynolds number (Re) and of the distance between the pier and the mining zone (L/d).
Methodology 28
Overview 28
The experimental procedures, data collection methods, and data processing techniques are also illustrated.
Experimental Setup 28
- The Flume 28
- Channel bed material 29
- Flow discharge measurement 30
- Flow depth and bed level measurement 30
- Velocity measurements 32
- Ultrasonic Ranging System (URS) 35
- Bed preparation 36
The flow discharge was measured using a rectangular notch (Figure 2.3) mounted at the end of the collection tank. Before the start of the experiment, the initial sand bed level was set to zero.
Experimental Program 38
In the second case (case 2), a trapezoidal mining pit was excavated in the sand bed on the upper side of the pier. As the current passes past the pier, the wake eddies are shed on the back side of the pier (Location-4). Therefore, excavation of a pit results in higher turbulent shear stresses in the flow downstream of the pit.
The Eulerian length scale also increases in the scour hole area behind the pier in the case of the mine shaft. Therefore, the flow region downstream of the cylinder is more turbulent than upstream. The stream bed upstream of the jetty is subject to higher average near-bed velocities in the presence of a sinkhole.
The streambed in the downstream region of the well is subjected to higher bed shear stress. These increases in shear stress at the bottom of the pier face due to THV are shown in Figure 3.35 for both cases. Filling the scour hole at the face of the pier downstream of a mine pit.
Turbulent Flow Characteristics around a Bridge Pier in a Dredged Channel 41
Effect of a Mining Pit on Turbulent Flow Characteristics around a Circular Pier 43
- Mean streamwise velocity profiles 47
- Bed shear stress 50
- Reynolds shear stress (RSS) 51
- Reynolds normal stress (RNS) 54
- Turbulent kinetic energy, turbulent viscosity and mixing length
- Turbulent kinetic energy flux 59
- Quadrant analysis 62
- Scales of turbulence 66
- Strouhal number 70
- High-order moments of streamwise velocity fluctuations
- Effect of pit shapes on turbulent flow characteristics 77
Reversal of the mean flow velocity can be observed in the lower boundary region (z/h ~ 0.2) in the center of the well (site-1). This results in increased flow depth at the downstream edge of the well and a significant decrease in average velocities, compared to no-well conditions [Figure 3.4(a)]. The shear stresses in the soil (locations 3, 4, 5 and 6) were also higher in the case of a mine pit.
In the presence of a mining pit, the RSS profile in front of the jetty is similar to the case without a pit, with an almost 10% rise in RSS magnitude inside the scour hole. This indicates a predominance of internal and external interactions in the separation zone at the rear of the jetty, in contrast to straight flows. In front of the jetty [Figures 3.8(e) and 3.8(f)], the magnitudes of 𝜍𝑢 are higher in the downstream region in the presence of the pit.
Inside the scour hole at the base of the pier, the 𝜍𝑢 magnitudes have decreased, while the 𝜍𝑤 magnitudes have increased in the presence of a pit. This deviation may be due to the excess momentum transport that results in the flow after passing over the well. Therefore, the scour hole zone at the base of the pier has a higher erosive propensity in the presence of an upstream mine pit.
It describes the intensity of the turbulence field generated around the cylinder due to the fluid-solid interaction in the mobile bearing channel. At the center of the well (Location -1), the flow separation in the well zone is the same for the rectangular and trapezoidal pit.
Effect of a Mining Pit on Turbulent Flow Characteristics around an
- Experimental cases and data recording 80
- Mean Velocities 82
- Bed shear stress and turbulent viscosity of the incoming flow 84
- Reynolds shear stresses (RSS) 85
- Quadrant analysis of the Reynolds shear stresses 87
- Instantaneous bed shear stress induced by the
- Flow oscillations and Strouhal number 93
- Summary 94
Also, aft of the jetty, near-bed downstream velocities are about 15% greater in the presence of a mine pit. Excavation of a mining pit on the upstream side of the pier may change the characteristics of the inflow, which interacts with the pier. At the upstream pier, the momentum transport is in a plane parallel to the flow (X-Y plane) due to the downward and unsteady vortex motions in the return zone.
The vertical distribution of the Reynolds shear stresses (RSS) upstream and downstream of the pier is shown in Figure 3.30. At the base of the pier, RSS due to ejection and sweeping decreased compared to no pit case. The turbulent horseshoe vortex (THV) formed at the base of the pier exerts high shear stress, causing erosion in the local scour area.
It is formed within the flow reversal zone (below z/h~0.1) detected inside the scour hole in front of the pier (location-2). At the back of the pier, the mean current velocities inside the scour hole zone increase due to pit excavation. In both cases, momentum transport over depth occurs at the rear flow region of the jetty due to inward and outward interaction type bursting events.
Effect of a Mining Pit on Turbulent Flow Characteristics Around
- Experimental cases and data recording 95
- Time averaged flow field 96
- Summary 101
Therefore, the excavation of a mining pit in the channel leads to a greater depth of excavation and a greater amount of sediment extraction at the bottom of the pier face. The stream bed in the approach lane of the jetty has also been eroded, which has resulted in a lowering of the bed height in front of the jetty. The filling of the cleaning hole directly depends on the distance of the pit from the pier.
A strong possibility for this behavior could be the difference in the width of the well. Fear of a pit caused the scour hole at the front of the pier to be filled. In the second experimental case, a rectangular well was excavated 1.0 m upstream of the pier front, as mentioned above.
Increased turbulence in the flow caused heavy erosion and lowering of channel bed upstream of the piers. Careful observation shows that fluctuations in the scour depth are greater in the case of the mine shaft. In-stream mining in the vicinity of the pier can lead to exposure of bridge piers, especially during flood conditions.
Streambed Instabilities around Bridge Piers in a Dredged Channel 102
Introduction 101
Excavation of a mining pit significantly changes the flow field around the bridge piers present on the downstream side. These changes can affect the morphological features around the bridge piers such as local scour, dune formation, etc. The effects of two parameters of a mine pit on the instabilities of the stream bed around a circular bridge pier, namely the distance of the pit from the pier and the shape of the pit, are studied.
Finally, the morphological behavior around two circular tandem piers in the presence of a mine pit is also reproduced in the end.
Mining Induced Streambed instabilities Around a Circular Pier 102
- Experimental cases and data recording 102
- Effect of distance of pit from the pier on the streambed morphology 106
- Effect of shape of the pit on the streambed morphology 113
- Summary 118
Plots of the bed morphology around the circular jetty at different Reynolds numbers are shown in Figures 4.3 – 4.7. It can be seen from Figure 4.9(a) that the exposure factor strongly depends on the Reynolds number and also on the distance of the pit from the pier. A lowering of the bed upstream of the jetty or upstream cut by L/d =13.33 was also found.
At L/d≥ 26.66, the excavation of the mining pit does not critically affect the morphology of the bed upstream of the jetty. The fill of the leach hole increased and consequently the maximum excavation depth decreased when mining took place near the jetty. Plots of the morphology of the bed around the round pier for all three types of mining pit are shown in the figure.
The general pattern of erosion around the circular pier was virtually consistent for all three mine pit shapes. The extension of the erosion zone on the downstream side of the pier was also consistent in all three cases. The pit resulted in excessive erosion and exposure of the pier, as well as the lowering of the bottom in the pier approach zone.
Effect of a Mining Pit on the Morphodynamics around
- Morphology around tandem piers 118
- Wavelet-based statistical evolution of local scour 122
Accordingly, in the present case, reattachment of the turbulent boundary layer occurred in the region between both pillars. The statistical rate of the spatial evolution of local scours at different time scales was evaluated using cross-correlation analysis of the wavelet coefficients, and the results are presented in the next section. The evolution of spatial excavation is approximately 15% faster due to the excavation of a mine pit (Table 4.3).
Excavation of pit in the channel caused higher exposure of the piers due to lateral erosion of the scour hole. The morphological response of the stream bed around a circular pier under the influence of a mine shaft was also studied. Greater flux of the streamwise turbulent kinetic energy moving in the streamwise direction in the scour hole zone due to well excavation.
Excessive erosion and exposure of the pier as well as bed lowering in the approach zone of the pier, due to pit excavation. Similar morphological response of the streambed around a circular pier was in the case of rectangular and trapezoidal wells with the same widths. River mining: assessing the ecological effects of river mining in the Rio Minho and Yallahs Rivers, Jamaica.
