Scour Countermeasures Research Articles

Experimental investigation on the mechanism of local scour around a cylindrical coastal pile foundation considering sloping bed conditions

Sea-crossing bridges are often subjected to local scour, significantly impacting the bearing capacity of the foundation system and potentially leading to bridge failures. This study presents experimental investigations of local scour around a cylindrical pile under different sediment slope conditions in unidirectional currents. Unlike previous studies that focused on horizontal sediment beds, this research explores the influence of inclined sediment beds, which reflect coastal and offshore topographies, which have some angles of inclination. Utilizing flume tests, the research investigates the evolution of scour depth, scour range, and the morphology of the sediment bed across various angles of inclination. The study also evaluates the effectiveness of a scour countermeasure involving rip-rap material around the pile on sloping beds. Specifically, a steeper inclination angle induces a faster flow velocity, resulting in increased scour depth at the upstream side of the pile and a reduced scour range at the downstream side due to backfilling. The countermeasure scenario reveals a significant reduction in both scour depth and range. This research provides analysis of the effects of inclined sediment beds, which reflect coastal topographies than horizontal ones, thereby providing insights for the design and protection of bridge foundations in diverse coastal conditions.

Apron and macro roughness as scour countermeasures downstream of block ramps

ABSTRACT A block ramp is a grade-control structure commonly used to stabilize the river bed with a high efficiency of energy dissipation. This study investigates the effects of using macro roughness and a downstream apron as scour countermeasures on the maximum scour depth downstream of block ramps in straight rivers. All the experiments have been carried out in a horizontal channel and in clear water conditions (no sediment transport). A wide range of hydraulic conditions including densimetric Froude numbers, water drop heights, and tailwater depths for three different rough ramp slopes (1 V:3 H, 1 V:5 H, and 1 V:7 H) was tested. The results have been compared with the results from a smooth ramp (no roughness), which has been considered as a reference. Results show that both macro roughness on the ramp and downstream aprons work well as scour countermeasures while as a comparison, using macro roughness (big-size stones) on the ramp is more effective than downstream aprons to reduce the scour depth. The other finding of this study shows that increasing the downstream apron decreases the length of the scour hole, consequently decreasing the volume of eroded material.

Experimental Study on the Optimum Installation Depth and Dimensions of Roughening Elements on Abutment as Scour Countermeasures

The causes of many bridge failures have been reported to be local scour around abutments. This study examines roughening elements as devices with which to intercept the downflow responsible for the formation of the principal vortex, which is what triggers local scour around abutments. Two vertical wall abutments with different widths were examined under four different hydraulic conditions in a clear-water regime. Elements with different thicknesses (t) and protrusions (P) with the same dimensions, (P = t = 0.05 L, 0.1 L, 0.2 L, and 0.3 L, where L is the length of the abutment) and with varying depths of installation (Z) were considered. Elements were installed in two positions: between the sediment surface and water elevation and buried within the sediment. To determine the optimum depth of installation, one element was first installed on the sediment surface, and the number of elements was increased in each subsequent test. The results show that installing elements between water surface elevation and the sediment’s initial level did not show any defined trend on scour depth reduction. However, the optimum installation depth of the elements is 0.6–0.8 L below the initial bed level. Moreover, the roughening elements with thickness and protrusion of P = t = 0.2 L resulted in the most effective protection of the foundation. The best arrangement, (P = t = 0.2 L and Z = >0.6–0.8 L) reduced the maximum scour depth by up to 30.4% and 32.8% for the abutment with smaller and larger widths, respectively.

Evaluation of angled splitters as scour countermeasure at circular piers

Scour around bridge piers is local phenomenon may cause complete failure for the hydraulic structure. This study explored experimentally using splitter plates at specific angles installed upstream single circular bridge pier to improve the nearby flow field and minimize the local scour. Ninety runs were carried out considering 9 splitter models. Four splitter lengths and 3 vertex angles were used. The splitter lengths were between 0 and 1.5 times of pier diameter, each was tested for 3 vertex angles ranged between 0° and 30°. The tests were done under 9 different hydraulic conditions including 3 discharges of 75, 100, and 125 l/s and 3 tailwater depths of 12.5, 15, and 17.5 cm. The turbulent flow conditions were investigated by plotting velocity profiles at different sections. The bed configurations under clear water conditions were presented. The results of pier without splitter were used as reference. The study outcome that the local scour depth was decreased by the increase of splitter length and vertex angle. The near-bed flow velocity and the corresponding Froude No. downstream of the pier was minimized for 1.5D splitter length at 30° vertex angle and the minimum scour geometry was located. The effectiveness of splitters were remarkable by the decrease of flow discharge and increase of tailwater depth provided that the discontinuity of scour hole length in case of multi-circular piers. Regression analysis was employed to develop empirical formula for the estimation of maximum scour depth around circular pier with splitter.

Numerical modeling of local scour of non-uniform graded sediment for two arrangements of pile groups

A three-dimensional (3D) non-hydrostatic numerical model is established to investigate local scour around four aligned circular piles in uniform and non-uniform sediment mixtures and to provide information for improving scour countermeasures design. In the current study, unsteady Reynolds averaged Navier–Stokes (URANS) equations along with a Re-normalization Group (RNG) k–ε model were applied to simulate the flow field. A non-uniform sediment transport model was applied to estimate the bedload transport. The simulations lasted up to 8 h. The laboratory results of a reference single pile case in uniform and non-uniform sediment were utilized to validate the simulation results of bed elevation changes and time evolution of scour depth and reflected a good agreement. The influence of pile spacing for two arrangements of pile groups, one column (tandem) and two columns (non-tandem) of four aligned piles, on spatial/temporal variations of the bed surface in both sediment bed configurations are discussed in detail. The results reveal the significant effects of the pile spacing on the flow field and patterns of scour hole formation and sediment deposition. In general, lower pile spacing results in more scour upstream of the front pile(s) and shifts deposition more downstream due to the existence of higher energy in the wake region of the piles for both sediment types. Side scouring dominates for non-uniform sediment around each pile due to flow deviating because of the presence of relatively larger particles in front of the pile(s). For non-uniform sediment, deposition begins to develop at the exit area of the scour hole and enhances boundary resistance to scour. The knowledge gained from this study is suitable for the accurate determination of riprap dimensions around the front pile(s) and rear piles in a pile group.

Scour Reduction around Bridge Pier Using the Airfoil-Shaped Collar

Scouring around the bridge pier is a natural and complex phenomenon that results in bridge failure. Failure of bridges have potential devastation and public safety and economic loss, which lead to political consequences and environmental impacts. Therefore, it is essential to countermeasure the scour around the bridge pier. This paper studies the effects of four different airfoil-shaped collars (i.e., bc1 = 1.5b, bc2 = 2.0b, bc3 = 2.5b and bc4 = 3.0b, where bc and b are the diameter of the airfoil-shaped collar and pier, respectively) as a scour countermeasure. All the experiments are conducted under clear water conditions with uniform sediment and a constant water depth (y) of 10 cm. Airfoil-shaped collar is placed at four elevations, i.e., bed level, y/4, y/2 and 3y/4 above the sediment bed level. It is observed that the maximum percentages of scour reduction of 86, 100 and 100% occurred due to protection provided by the collar bc2, bc3 and bc4, respectively, at sediment bed level. So, collars bc2, bc3 and bc4 are efficient at the sediment bed level. The profiles of scour hole show that the length of the transverse scour hole is greater than that of the longitudinal one. Numerical investigation of the morphological changes in sediment bed and scour depth contours is developed using the FLOW-3D for the pier with and without the airfoil-shaped collar.

Numerical study on residual rate of fluidized solidified slurry in pumping for scour repair around monopile

A new scour countermeasure for monopile foundation using solidified slurry have been proposed recently. The fluidized solidified slurry is pumped to the seabed area around pile for scour protection or into the developed scour holes for scour repair as the fluidized material solidifies gradually. During this complex process with pumping and solidification, the flow and diffusion of solidified slurry can result in the material loss and deterioration. However, there is still not a comprehensive and reasonable analysis on the state of retention of the fluidized solidified slurry in this process. Thus, the effect of scour protection can't be evaluated effectively. The current work focuses on the residual rate of fluidized solidified slurry during the pumping operation, which is the ratio between volume of the solidified slurry remaining in the scour pit and that pumped into. A series of multiphase flow (the ambient seawater and the solidified slurry) numerical simulations using a computational fluid dynamics (CFD) approach were conducted to investigate the effects of pumping location, current velocity, pumping velocity and material property of solidified slurry on the residual rate of fluidized solidified slurry. It was found that the pumping location near and at the downstream of the monopile with a lower pumping pipe nozzle is optimal for a higher residual rate of fluidized solidified slurry. Solidified slurry with smaller slump flow value pumped at a lower current velocity and higher pumping velocity contributes to a higher residual rate. Based on a comprehensive parametric study, an empirical formula is established to calculate the final residual rate of the solidified slurry for theoretical reference and practical application. This work provides an important insight in understanding the residual rate of solidified slurry in application to scour protection of monopile.

Effects of collars on local scour around semi-circular end bridge abutments

The occurrence of scour around bridge elements due to the transportation of bed material during flood events can cause serious structural damage and loss of life. Increased uncertainties in precipitation and runoff predictions due to climate change make this phenomenon more complex and dangerous. Bridge scour countermeasures should thus be more focused on decreasing scour formation around bridge elements. In this study, abutment scour under clear-water conditions with constant flow intensity was conducted and collars were tested as scour countermeasures around semi-circular end bridge abutments. The experimental study was performed in a rectangular channel with an almost uniform cohesionless bed material for 3 h with and without collars. Collars of various lengths located at different elevations around the abutments were tested to investigate the effect of collars on scour development. The results of the study showed that the scour depth decreased with increasing collar width and when the collar was placed below the bed level for a given abutment length. The results were compared with those of similar earlier studies to show the effect of abutment shape, size of the bed sediment and test durations on the development of scour depth around abutments.

Investigation of using sill under gate as a scour countermeasure downstream of a sluice gate

Investigation of using sill under gate as a scour countermeasure downstream of a sluice gate

Combination of Riprap and Submerged Vane as an Abutment Scour Countermeasure

Scour is one of the main causes of hydraulic structural failures. The present experimental study examines the use of riprap, submerged vanes, and a combination of these for scour reduction around vertical walls and spill-through abutments under clear-water conditions. Specifically, the influence of placing riprap stones with different apron shapes (geometry) and/or a group of submerged vanes of constant height and length on abutment scour was examined. The main aim is to propose the optimum apron geometry and placement of submerged vanes to (1) reduce edge failure at vertical walls and spill-through abutments; and (2) prevent shear failure at the spill-through abutment (no shear failure is observed around the vertical wall abutment). The results show that using ripraps for scour protection is more effective than submerged vanes. However, the highest reduction in scour depth was achieved when a combination of riprap and submerged vanes was used together. This arrangement can reduce the maximum clear-water scour depth by up to 54% and 39% with vertical walls and spill-through abutments, respectively. Furthermore, selecting appropriate apron scale ratios reduces the required riprap volume by up to 46% and 31% for the vertical wall and spill-through abutment, respectively. In addition, the installation of vanes increased the riprap stability and reduced edge failure in both abutments tested. Finally, using riprap aprons with proper scales ratios at the downstream side of the spill-through abutment also prevents shear failure in this zone.

Local Scour Patterns around a Bridge Pier with Cable-Wrapping

The performance of cable flow-altering bed scour countermeasures was experimentally evaluated based on the scour reduction, bed morphology, and the effects on the flow field. An unprotected 40 mm diameter pier was compared to piers protected with spiral cables (2, 4, 6, 8 and 10 mm diameters) wrapped at a 15-degree angle for two-bed sediment sizes with median grain sizes of 0.86 and 1.83 mm, for a cylinder Reynolds number of 7120. The scour depth was reduced by the cables by up to 52 percent compared to the unprotected pier case, a reduction that increased with increasing cable diameter for both sediment beds. Scour depth and sediment deposition varied by sediment size, where the scour hole was up to 45 percent deeper for the finer sediment bed than that of the coarser bed. Velocity and turbulence statistics showed that cables attenuated the flow within the scour hole by diminishing the downflow and horseshoe vortex, whereas in the case of finer sediment, spatially averaged turbulent kinetic energy and Reynolds shear stresses were respectively up to 1.4 and 1.8 times higher for the unprotected pier than the protected pier, resulting in scour depth reduction. The presence of the cable also reduced the vortex shedding frequency in the pier wake as indicated by a Strouhal number of around 0.175. The results demonstrate the potential of cable threading as a flow-altering scour countermeasure to reduce bridge pier scour.

Dam-Break Energy of Porous Structure for Scour Countermeasure at Bridge Abutment

The aim of the study is to determine the structure for energy absorption in order to countermeasure the scouring on the bridge abutment. Consider a porous structure for energy absorption, which can reduce flow velocity and depth of scouring due to its porosity. The energy absorber plate demonstrated in triangular shape with several porous as submerged barrier. The investigation was conducted in laboratory and placed the abutment in the middle of the channel with a distance of 3Lb, 5Lb, 7Lb and 9Lb. The plate area consists of 0% (MP1), 5% (MP2), and 10% (MP3). The scour depth measurement (ds) is carried out at 6 crucial points in the abutment area. Comparisons between experimental measurements and a numerical prediction model are presented. The experimental results show that the percentage of frictional velocity in the inhibition area for each pore opening before the obstacle, 31.42% (decreasing), - 9.27% (increasing), and -32.92% (increasing), respectively. Furthermore, the optimum position of the porous energy absorber at 9Lb to the abutment. The magnitude decreases of scour depth obtained from MP2. It can be concluded that the placement of energy absorbers can lead to damping forces. It also found that the porous structures could be beneficial for motion damping and absorber of the scouring. Doi: 10.28991/CEJ-2022-08-12-019 Full Text: PDF

Investigation of using mangrove-inspired skirt pile group as a scour countermeasure

Mangrove forests are able to protect the coastline because their dense prop root system reduces the flow velocity and stabilizes sediments against erosion. Inspired by this unique character of the mangrove root structure, a ring of skirt piles is proposed as a measure to mitigate local scour around monopile foundations. Numerical simulations using computational fluid dynamics (CFD) modeling as well as laboratory flume experiments were conducted to evaluate the scour mitigation effect in clear water current. The simulation results reveal that the presence of skirt piles reduces the near-bed tangential velocity and affects the vertical flow field around the monopile. The experimental results indicate that if all other parameters are held constant, the scour depth and volume increase with skirt pile spacing, decreases with the number of skirt pile rings. With a single ring of skirt piles, the scour potential reaches the lowest with an intermediate critical submerged height of the skirt piles; the scour depth and volume increase when the submerged height deviates from the critical value. The comparison between the numerical and experimental results implies that the installation of the skirt piles has not only hydraulic but also geotechnical benefits for scour mitigation.

Joint Modelling of Flood Hydrograph Peak, Volume and Duration Using Copulas—Case Study of Sava and Drava River in Croatia, Europe

Morphodynamic changes in the riverbed may be accelerated by the climate change-induced effects, mostly through the increase of the frequency of extreme climatic events such as floods. This can lead to scouring of the riverbed around the bridge substructure and consequently reduces its overall stability. In order to better understand hydromorphological processes at the local scale, the influence of floods on bridge scour requires a detailed analysis of several interacting flood hydrograph characteristics. This paper presents a multivariate analysis of the annual maximum (AM) flood discharge data at four gauging stations on the Drava and Sava Rivers in Croatia (Europe). As part of the hydrograph analysis, multiple baseflow separation methods were tested. Flood volumes and durations were derived after extracting the baseflow from measured discharge data. Suitable marginal distribution functions were fitted to the peak discharge (Q), flood volume (V) and duration (D) data. Bivariate copula analyses were conducted for the next pairs: peak discharge and volume (Q–V), hydrograph volume and duration (V–D) and peak discharge and hydrograph duration (Q–D). The results of the bivariate copula analyses were used to derive joint return periods for different flood variable combinations, which may serve as a preliminary analysis for the pilot bridges of the R3PEAT project where the aim is to investigate the influences on the riverbed erosion around bridges with installed scour countermeasures. Hence, a design hydrograph was derived that could be used as input data in the hydraulic model for the investigation of the bridge scour dynamics within the project and a preliminary methodology is proposed to be applied. The results indicate that bivariate frequency analysis can be very sensitive to the selected baseflow separation methodology. Therefore, future studies should test multiple baseflow separation methods and visually inspect the performance.

Near-Nature Ecological Technique for Pier Scour Countermeasure in a Submerged Overfall

This paper proposes a near-nature ecological technique, which can consist of a wide range of materials, to protect against pier scouring. The proposed technique can involve the use of many long strips that behave like water weeds. This paper studied a protection method against pier scour by using long strips in a submerged overfall, particularly for a pier located at the maximum depth area of overfall scour. The length and size of the strips were chosen as factors to study their protective effect. Our results showed that this approach slowed the flow velocity between the installed strips and bed. The sediment in flow might accordingly move slowly or even settle down. Thus, the pier could be kept stable and safe by the installation of those strips. Experimental results show that the protective effect is more efficient when the strip length is closer to the pier and has a small diameter. Moreover, the maximum protective effect reached 45.5% scour reduction. Varied lengths provided different protective effects against overfall scour. These findings show that this near-nature ecological technique could be a good and economical solution for pier protection in submerged overfalls.

Using a Bed Sill as a Countermeasure for Clear-Water Scour at a Complex Pier with Inclined Columns Footed on Capped Piles

River bridge piers may collapse due to the local scour around their foundations. It is known that local scour is an effect of the three-dimensional flow field that develops near the pier and that the geometric complexity of a non-cylindrical pier may, correspondingly, increase the complexity of the process. It is also known that various devices may be used as scour countermeasures. This manuscript explores the use of a bed sill as a countermeasure for local scour at a complex bridge pier compound of an array of piles, a pile cap, and two inclined columns with the rectangular sections above the cap. This pier geometry, never studied before in combination with a scour countermeasure, was stimulated by an existing bridge. Different sill placements were tested (at the upstream or downstream edges of the pier, or in an intermediate position) for various values of the pile diameter and number, cap thickness and cap elevation. The results of a wide experimental campaign consistently showed that the most effective placement of the transverse sill was at the upstream edge of the pier, for which scour reductions of up to 30–40% could be obtained for the long-term scour depth. The countermeasure performance decreased to about 10% when the sill was placed at the downstream edge of the pier. Furthermore, the installation of a transverse sill upstream of the pier also changed the shape of the scour hole because the pier was then located in an area prone to sill scour; however, for the present experiments, the combination of the effects was beneficial in terms of the resulting scour depth. Although the investigation of a single hydro-dynamic condition prevents the experimental findings from being generalized, the promising results stimulate further consideration of a transverse sill as a countermeasure for local scour at a complex pier.

Temporal evolution of scour at bridge abutments in compound channels

Bridge abutment-induced sediment scour in compound river channels is time-sensitive during floods, and, thus, estimating the scour rate has significance in design. In the current study, large-scale laboratory experiments were done to examine the influence of multiple factors on scour evolution, including channel geometry, bed mobility (clear-water or live-bed), and lateral (embankment intrusion) and vertical (deck submergence) flow contractions. A new method is applied for scour measurement and determination of characteristic scour trends. Results show that the non-dimensional time scale is related to flow intensity and may peak before reaching the clear-water scour threshold; the corresponding equations also are derived. It is found that the use of scour countermeasures has a significant impact on the scour rate, and the existing equations for unprotected abutments have limitations. Under clear-water conditions, the scour rate is closely related to the overall flow contraction, which can be better reflected by the ratio of the approaching unit discharge to that at the bridge section. Specifically, the effect of vertical flow contraction on scour rate is more dominant than other factors. Two scour rate curves are derived to suit different structural and flow contraction types for clear-water scour. In contrast, the live-bed scour rate data are generally consistent and fall in a narrow band, showing less dependence on other factors. Finally, a new design approach is established to estimate the time scale and scour rate using the new equations proposed in this paper.

Particle image velocimetry evaluation of flow-altering countermeasures for local scour around a submerged circular cylinder

The effect of scour countermeasures on the mechanism of local scour around a cylinder requires clarification in order to develop design methodology for use in practice. Previous investigations on countermeasure performance, though useful, have not provided adequate measurements to support this understanding. In the present investigation, particle image velocimetry (PIV) measurements were acquired at several streamwise-vertical planes in the flow field surrounding a submerged circular cylinder with a vertical splitter plate and with a rectangular collar (a horizontal base plate). Analysis of the distribution of mean streamwise velocity and Reynolds shear stress as well as the scour formation indicated that the splitter plate disrupted shear layer interaction in the wake region, reducing the width of scour downstream of the cylinder by 3%. The collar eliminated scour upstream of the cylinder entirely by providing protection at the sides of the cylinder where the shear stress exceeds the critical bed shear stress, and the streamwise velocity distribution in the region between the cylinder and the channel sidewall indicated that installation of a rectangular collar causes flow separation at a point further upstream on the cylinder surface.

Lifetime Resilience Migration Quantification Using Nonparametric Distance Metrics and Application for River-Crossing Bridges

Quantitative resilience assessment is an essential to the lifetime management of civil structures. With the uncertainties arising from both physical and socioeconomic dimensions, resilience needs to be measured probabilistically. Although existing resilience frameworks have addressed this need, none statistically characterize how much a system’s resilience migrates and in what direction it migrates. This paper proposes a lifetime resilience migration quantification (LRMQ) framework based on nonparametric distance metrics. In this framework, system resilience positioning is proposed by comparing the resilience of a variable system against two reference systems. Two decision-making analytics, a binary resilience classification (BRC) diagram and a lifetime resilience attenuation (LRA) model, are proposed. The proposed method is evaluated with success by performing numerical experimentation over a representative river-crossing bridge. It is observed that the bridge’s system resilience attenuates progressively in its lifetime; with the consideration of a scour countermeasure, resilience is effectively enhanced via visualizing the proposed BRC diagram and LRA model.

Experimental study of debris-induced scour around a slotted bridge pier

One of the most common problems for river engineers is the accumulation of waterborne debris upstream of the bridge piers. In addition to reducing the cross-sectional flow area, debris increases the drag force exerted to the pier and contributes to scour. Several studies have been carried out by previous researchers to examine the usefulness of different types of countermeasures. The effectiveness of these countermeasures is not well understood when debris accumulation occurs. In this study, the effect of debris accumulation on the efficiency of a bridge pier slot, as scour countermeasure, is investigated experimentally. A total of 54 experiments were carried out under different hydraulic and debris geometrical conditions. The results showed that slots were effective in protecting bridge piers against scouring in presence of debris. Depending on the debris shape, the reduction efficiency may increase or decrease for a slotted pier in presence of debris accumulation when compared to the standard pier conditions without debris accumulation. Except for the inverse pyramid shape, the maximum scour is generally more reduced due to sheltering effect when the debris is located on the bed. While debris accumulation can lead to a reduction of the slot efficiency, the slot can be considered a reliable countermeasure against scouring. The outcome of this study can help the design of new bridges affected by large wood debris accumulations.