Rutuja Chavan and Bimlesh Kumar- Prediction of Scour Depth and Dune Morphology Around Circular Bridge Piles in Seepage Affected Alluvial Channel Environmental Fluid Mechanics (Accepted for Publication). Empirical relationships have been developed for estimating scour depth and dune morphology around single piers in the presence of downward seepage.
Introduction
State of art
- Scour at Bridge Piers
- Mechanism of Local Scour
- Factors Affecting Scour Depth
- Estimation of Scour depth
- Flow around Bridge Piers
Many researchers have focused on estimating the maximum scour depth around bridge piers (Breusers, Nicollet and Shen, 1977; Raudkivi and Ettema, 1983; Chiew and Melville, 1987; Melville and Coleman, 2000). Ettema (1980) reported that the equilibrium scratch depth on bridge piers is reached after 14 days.
Problem Descriptions
In the drainage area, it was found that the bed shear stress increases as the downward drainage velocity increases. They found that the velocity, bed shear stress and hence sediment transport rate increase with the application of downward drainage.
Objectives
A schematic representation of the objectives described in this research work is shown below in Figure 1.4.
Thesis Organisation
In this chapter, various empirical approaches for predicting scour depth are discussed and further, new empirical relationships are developed for predicting scour depth and dune geometry in alluvial channels of non-uniform sand bed taking into account the effect of downward drainage. . The various references that were reviewed for this thesis are listed at the end of the dissertation.
Experimental Setup and Methodology
- Overview
- Bed Material
- Tilting Flume
- Test Section
- Main Channel Discharge
- Seepage Discharge
- Flow Depth
- Bed slope
- Velocity and Turbulence Measurement
- Measurements of the Geometry
- Uncertainily Analysis
- Pier Characteristics
- Experimental Program
At downstream (C) of the jetty, flow separation occurs, resulting in increasing turbulence; The Reynolds shear stress is more than upstream (A) of the pier and fluctuates strongly due to wake eddies. Downward seepage leads to decreasing Reynolds stresses near the bed upstream of the jetty.
Effect of Downward Seepage on Turbulent Flow Field and Bed Morphology
Introduction
The flow field around bridge piers is one of the most important topics for hydraulic engineers. They stated that studying the hydrodynamics of the flow around the piers provides a correct understanding of scour and helps predict the scour depth accurately.
Velocity
Much less information is available on the deposition of bed material or dune-like beds at the rear of the pier. Reverse flow can be seen near the bed within the scour hole, hence the negative streamwise velocity component upstream of the pier.
Reynolds Shear Stress (RSS)
The RSS increases with an increase in distance from the free surface, reaches a maximum value somewhere between 0.15 < z/h < 0.25 and then decreases again towards the bottom due to the presence of a roughness sublayer. At the upstream (A) and downstream (C) of the pier, the RSS fluctuates sharply due to the continuous development of the vortex region (Figures 3.3 and 3.4). At the upstream (A) of the pier, the magnitude of the Reynolds shear stress is higher at no seepage than at 10% and at 15% seepage due to the reversal of the flow in the scour hole.
Downstream (C) of the pier near the bottom, the Reynolds shear stress increases at 10% seepage then no seepage and increases again at 15% seepage, leading to more erosion due to an increase in momentum exchange with respect to the seepage conditions .
Quadrant Analysis
By systematically varying the hole size H, the contribution of events to the total Reynolds shear stress can be investigated, regardless of whether it is large, small or frequent. Figure 3.5 shows that the contribution of events Q1 and Q3 is greater than that of Q2 and Q4 within the area of the scour pits (z/h<0.1) upstream of the pier. The dominance of the sweep results in the arrival of high-velocity fluid particles that contribute a larger +u' in the flow direction and a larger –w' in the vertical direction, causing the gradual deepening of the scour hole upstream of the pier.
The magnitude of the events is stronger downstream of the pier than upstream due to the higher turbulence production downstream of the pier.
Morphological features around bridge piers
- Development of a new Scour depth prediction method
- Morphological Dune Evolution
- Statistical Criteria
Figure 3.7 shows that the change in scour depth is initially substantial, both under no seepage and under seepage conditions, and gradually decreases over time. This can be deduced from Figure 3.7; the rate of formation of scour depth decreases by 50% in 12 hours. Close observation of Figure 3.8 shows that the scour depth upstream of the pier is greater when there is no seepage and decreases as the seepage rate increases.
The dimensional analysis indicated that the non-dimensional scour depth (ds/D) is a function of four dimensionless.
Discussions
Because of the characteristic dimensions of laboratory channels and the lower limit of sediment size, it is difficult to relate channel spacing to river spacing. Distorted models are those in which one or more members of the model are not properly identical to its prototype. The warped models for surfing around piers are very helpful in getting a proper understanding of the likely performance of the prototype.
Sensitivity analysis is useful to determine the impact of the actual result of a particular variable when it differs from the previously assumed value and the analyst can determine how changes in one variable will affect the target variable.
Conclusion
Downstream of the rear pier, a reverse flow is seen near the free surface and the velocity gradually increases as one moves towards the bottom. A significant decrease in the Reynolds shear stress near the bed upstream of the piers results in a lower scour depth in case of seepage runs. The turbulent kinetic energy is higher near the edge of the scour hole and decreases with downward seepage.
RSS is found to be negative near the bottom upstream of the forward pier and near the free surface downstream of the rear pier.
Hydrodynamics and Morphology around Tandem Piers in Alluvial Channel with
Introduction
As a result, the flow rate is less near the bed at upstream of the forward pier (section A) and near the surface at section C in the case of seepage run. Downstream of the rear pier, section C, negative Reynolds stresses turn to positive as they move toward the bed. The strength of vorticity is more downstream of the circular pier than the elongated pier.
The wake eddies downstream of the elongated jetty are weaker than the wake eddies at the circular jetty.
Tandem Piers of Same Diameters
- Velocity
- Reynolds Shear Stress
- Quadrant Analysis
- Moment Analysis
- Flux of the Turbulent Kinetic Energy
- Power Spectra Analysis
- Turbulent Kinetic Energy
- Scour around Piers
- Conclusions
Tandem Piers with Different Diameters
- Turbulence Intensity
- Moment Analysis
- Turbulent Kinetic Energy
- Flux of the Turbulent Kinetic Energy
- Power Spectra Analysis
- Scour around Piers
- Conclusions
In the wake region downstream of the back pier, negative velocity occurs near the free surface and turns positive as it moves toward the bed. From Figure 5.12, it can be clearly seen that dune-like bedforms are occurring downstream of the piers due to sediment deposition. However, for both scaffold shapes the scour depth is decreasing with the application of downward drainage.
TKE fluctuations and TKE magnitude are significantly higher downstream of the circular jetty.
Suction Effects on Dynamics of Migrating Scour Depth and Dune- Like Bedforms
Introduction
Some of the researchers have investigated the morphodynamic characteristics of bedforms from the initial stage to equilibrium to understand the multiple separation and merging of bedforms and stated that the change in bedform geometry affects the hydrodynamics of the channel (Defina, 2003; Jerolmack and Mohrig, 2005). In this study, we have focused on the dynamic properties of scour depth migration on piers and the pdf of scour depth growth around piers for different time intervals for drained and undrained conditions. Therefore, the dynamic properties of the scouring process are understood by the scour rate-dependent agility around piers and scouring is a time-dependent phenomenon, we performed wavelet analysis on the time series of bed height.
The scoured bed material is deposited along the centerline downstream of piers and dune-like beds are formed, the dynamic properties of migrating bedforms forming downstream of single and tandem piers are also studied.
Results
- Velocity
- Reynolds Shear Stress
- Scale dependent scour depth migration around tandem piers
- Wavelet Analysis
- Pdf of scour depth increments
- Scale dependant bedfom migration at downstream of tandem piers
Furthermore, Figure 5.3 reveals that the scratch depth is higher in the absence of seepage; decreases with It can be seen from Figure 5.4 that for smaller scales (< 60 s) small values of cross-correlation coefficients (0.5–0.2) were obtained. Figure 5.10 shows that initially (2h) the pdfs have heavier tails than the Gaussian pdf for all three conditions, but as time increases the bottom height increases become thinner (12h).
The celerity with different length scales for the different time intervals is shown in figure 5.13.
Conclusions
Conclusions and Recommendations
Effect of Downward Seepage on Turbulent Flow Field and Bed Morphology
Reducing the reverse flow upstream of piers by seepage downwards results in a shallower scour depth under seepage than under no-seepage conditions. Towards the beginning of the scratch, the rate of expansion of the scratch depth is progressive and gradually decreases with the development in time. The height of the dunes at the rear of the embankment is lower in the absence of seepage and increases with increasing percentage of seepage.
Finally, new empirical relationships are developed for predicting scour depth upstream of piers and predicting dune morphology behind piers.
Hydrodynamics and Morphology around Tandem Piers in Alluvial Channel with
In the wake region of the pier, particles remain in suspensions and are transported along the flow due to the dominance of ejection events. With tandem pillars with the same diameter, the excavation depth at the front pillars is twice as great as the excavation depth at the rear pillars. However, in the case of tandem piers with different diameters, when installing a 75 mm front and 90 mm rear pier, and a 90 mm front and 75 mm rear pier, the scour depth at the rear pier is reduced by almost 40% and 60% than the excavation depth. at the front pier, respectively.
Suction Effects on Dynamics of Migrating Scour Depth and Dune- Like
Recommendations of Future Research
Diameter of the circular pier was 75 mm, while elongated pier with width 75 mm, length 225 mm and 270 mm was used. Careful observation of figure 5 shows that the scour depth upstream of the pier is smaller for elongated pier than circular pier in all experimental conditions. The abraded bed material is deposited further downstream of the pier along and to the sides of the centerline and produces hump at the rear of the pier.
Vertical profile of turbulent kinetic energy (TKE) at upstream (A) and downstream (B) of the elongated pier and circular for no seepage, 10% seepage and 15% seepage runs are depicted in figure 9. From table 2 it can be observed that the Strouhal number is less in the case of the elongated pier and more for the circular pier. When this streamlined flow is achieved downstream of the pier, it creates less turbulence, thereby reducing the strength of the eddies downstream of the elongated pier.
