Experimental study on Local Scour around Bridge Pier Models generated by Flash Floods carrying Debris

The analysis of local scouring around bridge piers currently is based on typical river flood discharges. However, extreme events like flash floods with high velocity and discharge, carry wooden debris that accumulates upstream of the bridge pier. This results in additional forces due to flow diversion and reduction in flow area, so impacting hydraulic structures and exacerbating the scouring process. The study objective was to examine the impact of debris transported by flash floods on the depth and distribution patterns of scouring around bridge piers. This research was conducted in a 15.5-m-length, 0.5-m-width, and 1.0-m-height glass flume, layered by 25 cm thick of sand which is d50=0.52 mm and σg=2.327. Flash flood simulations were conducted by flowing the water mixed with debris content of 0, 5, 10, and 15% to hit a sharp nose pier model which is installed in the channel. Scour depth was measured using point gauges at 50 points around the pier. The findings indicated that an irregular distribution pattern of scour is formed around the pier, and scour depth decrease when increasing debris content. It is necessary to corrected in the range of 0.65-1.43 for 0-15% debris content carried by the flash flood compared to normal flood.


Introduction
A river is a naturally formed open channel serving as a flowing water conduit.The flow within a river initiates a process of scouring its bed.Scouring is a natural phenomenon influenced by both the river's morphological conditions and human-made constructions.The presence of these structures impacts the flow, potentially leading to scouring.One such structure built within a river is a bridge pier.A pier constitutes a vital element of a bridge's construction, providing support and bearing the weight of the bridge itself and the loads it carries.The stability of a pier is greatly determined by its resistance to local scouring [1].Many cases of bridge failure are not solely due to construction factors but also arise from scour-related issues around the piers.This is because the continuous scouring process results in a gradual reduction at the base of the pier.
Local scour typically occurs in river channels obstructed by bridge piers, inducing whirlpools.These vortex movements erode sediment particles from their original positions, and if the sediment discharge rate leaving the scour is greater than that entering, scour holes are formed.The magnitude of local scour is influenced by various factors in the riverbed vicinity.Factors leading to local scour around bridge piers include geometric pier shape, flow characteristics, and sediment type [2].Furthermore, local scour around bridge piers in clear-water conditions is extensively considered, encompassing scour geometry and temporal evaluations involving one or several piers under various geometric, hydraulic, and sediment conditions, and even structural protection.Several scour prediction equations have been established, including [1,3,4,5,6,7].
Prior research has been largely focused on clear-water scenarios.However, river flow conditions are often not normal, and extreme events like flash floods can occur unexpectedly.Flash floods swiftly arrive IOP Publishing doi:10.1088/1755-1315/1343/1/012028 2 with significant volumes within a relatively short time due to high rainfall or sudden dam failure and the release of accumulated water, often carrying debris such as wood and trees (referred to as debris).Rapid alterations in water flow patterns can intensify scour depth around piers, posing threats to the river's integrity and the surrounding infrastructure [8].Some studies have indicated that the formation of debris jams is one of the primary causes of bridge failures, contributing to approximately one-third of bridge collapses in the UK, Ireland, and the US.[9].Therefore, it is desirable to evaluate the potential for large accumulations to develop and assess the aforementioned effects.This evaluation is essential for mitigating the risk of infrastructure failure and for designing future river bridge infrastructures that are more resilient to extreme weather events [10].In a previous study conducted by Rizalihadi on flash flood-carrying sediment, he found that the presence of sediment transported by flash floods increased scour depth by 1.02-1.22times compared to normal flood discharges [11].
This study is a continuation of previous research, aiming to investigate the impact of flash floods carrying debris on the depth and distribution patterns of scour around bridge piers.

Definition of Scour
Scour refers to the alteration of a flow accompanied by the movement of materials due to the action of fluid motion.Changes in flow patterns occur due to obstacles within the river channel, such as river structures like bridge piers, abutments, river groins, sluice gates, and the like.He explains that such structures are seen to modify the channel's geometry and flow patterns, subsequently leading to local scour around these structures [2,12].
The different types of scour classifications provided by Raudkivi [6], are as follows: 1. General scour in the river channel, unrelated to the presence or absence of river structures.2. Localized scour in the river channel, occurs due to channel narrowing and concentration of flow.
3. Local scour around structures, arising from local flow patterns around river structures.Types (2) and (3) of scour can further be differentiated into clear-water scour and live-bed scour.Clearwater scour is associated with a situation where the upstream channel bed is quiescent (no sediment transport occurs), or theoretically, τo < τc.On the other hand, live-bed scour occurs when the flow conditions within the channel cause movement of the channel bed material.This phenomenon indicates that the shear force on the channel bed is greater than its critical value, or theoretically, τo > τc [13,14].

The scouring process at the pier without debris and the effect of debris
The formation of vortex systems, known as horseshoes vortex systems, is induced by flow obstructed by piers or other structures.These vortex systems generate high velocities capable of eroding the bed beneath obstructing structures [2,13].The flow pattern of the horseshoe vortex system can be depicted as shown in Figure 1(a) [15].This system begins upstream of the pier, where flow components emerge in a downward direction.As the incoming flow from upstream encounters the obstruction posed by the pier, it changes direction vertically towards the channel bed and partly redirects toward the front of the pier before being carried downstream.
This vertical flow continues downward towards the bed, subsequently forming a vortex.Near the channel bed, the flow components reverse direction vertically upwards.This event is followed by the transportation of bed material, resulting in a spiral flow that induces bed scour.Due to this scouring process at the foundation of structures, depressions or holes can form, and this process persists until equilibrium is reached.
Previous study states that the scour holes occurring in river channels generally correlate the scour depth with flow velocity, making these scour holes a function of time [16].On the other hand, assert that the maximum scour depth is a function of shear velocity [17].In the case of flood-carrying debris, the accumulation of floating debris around bridge piers further complicates and enlarges the local scour occurrence [18,19,20].Flow resistance caused by the congestion induces accelerated flow in the reduction of water depth beneath the congestion.This upper interface obstruction also prevents the flow field from forming water fluctuation due to stagnation pressure at the upstream surface.The latter, along with the longitudinal triangular profile of clogged debris, forces the flow towards the riverbed, resulting in higher scour [15], as shown in Figure 1(b).

Methods for Estimating Local Scour
Scour depth is influenced by several factors, including flow velocity, flow depth, sediment material roughness, scour time, and the Froude number (Fr) [2,6,7,12,14,21].They stated that based on laboratory data, the maximum scour depth occurs under conditions of sediment-free scour.
There are several methods to estimate scour depth occurring around bridge piers.One common formulation for estimating local scour depth at piers is provided by [9]: where: S = scour depth (m); a = pier width (m); d = water depth (m); FR = Froude Number; K = coefficient value for the pier, significantly influenced by the pier's shape, flow, and sediment.Local Scour at Piers around the pier for lived bed and clear water can be estimated using formula 2 below [22,23].
where: K1 = coefficient due to pier nose shape (as in Table 1); K2 = coefficient due to pier angle of flow of attack (as in Table 2), K3 = coefficient due to bed river conditions (Table as in 3) and K4 = coefficient factor for bed material size (as in Table 4).

The effects of debris accumulation in the local scour around bridge piers
Equation 1 and 2 above is commonly employed to calculate scour depth for normal flood discharges, typically planned discharges with a certain return period (T) and under clear-water conditions.However, in the case of flash floods, which have significantly different flow characteristics compared to normal floods, adjustments are necessary due to the material carried, such as sediment and floating logs (debris), which alter the flow characteristics.
Over the past few decades, new research efforts have been undertaken to understand the effects of debris accumulation in the local scour around bridge piers [18,19,20,24].These studies have provided engineers with guidance on how to estimate scour for given accumulation characteristics.Most of the research in this field is based on laboratory experiments using idealized models of debris accumulation (such as smooth, impermeable, and regular shapes).There are guidelines on the potential increase in scour depth due to the presence of debris jam, which varies significantly.For instance, [25,26] showed based on laboratory data that the effect of simulated floating debris accumulation at bridge pier on the maximum local scour depth increased.According to the experiments, the maximum scour depth in the presence of debris accumulation is 1.49 times greater than the scour depth without accumulation [10], it could increase scour by a factor of 2 to 3 [25], and it could increase from 1.09 to 1.22 for debris content of 5-15% [11].In this study, the dimensions and shape of the model represent the initially assumed debris accumulation, and its influence on scour is tested in the laboratory.Because scour is highly dependent on the dimensions of the accumulated debris, the use of a model that does not represent the size of debris congestion that may occur in field conditions can introduce significant sources of uncertainty.This may explain the widely varying results reported, although other characteristics of each experimental setup may also play a significant role.Several prediction methods have been proposed in the literature based on experimental data.One popular research is the calculation of an effective diameter known as [10,27].Based on this, it is necessary to correct the coefficient value K in Equations 1 and 2 to estimate scour depth under conditions of flash floods carrying debris.

Creation of River and Pier Models
Investigating scouring in a laboratory model, often referred to as flume or hydraulic model testing, serves several important purposes.The primary reasons for conducting scouring investigations in a laboratory model are as follows: Laboratory models enable researchers to simulate river flow and study the movement of river water and sediment transport, which can be complex and is crucial for understanding erosion, deposition, and the evolution of river channels.Thus, using laboratory models provides the opportunity for controlled experiments to study sediment transport, which has implications for riverbed erosion, scour and sedimentation.This is the reason to study the scour in the bridge pier due to flash flood is conducted.This sharp-nose pier model was positioned 5 meters from the upstream end of the flume.The riverbed material was fine sand with d50 = 0.52 mm and σg = 2.327.This sand was evenly spread along the flume area with a thickness of 25 cm.For a clearer understanding of the design of the channel and pier, the model can be seen in Figure 2. Furthermore, the debris material carried by the flash flood consisted of branches and leaves, ranging in length from 0.5 cm to 15 cm and in diameter from 0.2 cm to 0.9 cm.
It is necessary to carefully consider the potential effects of scaling in a model.The use of dimensional analysis, and evaluation of dimensionless numbers like Reynolds and Froude numbers is a must.This is meant to ensure geometric and kinematic similarity, adjust material properties, replicate boundary conditions, and perform sensitivity analyses.The validation against field data is conducted to verify that the scaled model accurately represents real-world conditions.This comprehensive approach is essential for the model's reliability and its ability to provide meaningful insights and predictions.However, in this research, the model scale is not applied since the constructed model is not based on the specific location of the river prototype.

Measurement and Data Analysis
The experiment was conducted by releasing water from a storage tank into the flume.Each experiment was repeated three times for each variation of debris content in the flash flood flow, as well as for cases without sediment.The distribution of scour was obtained by measuring scour contour data around the pier using a point gauge after each run.Scour contour data were collected using a point gauge at 50 observation points according to the predefined grid.Following measurements, the sand bed was leveled again for subsequent runs with different debris contents.The experimental series included three repetitions, and the collected data were processed using software like Surfer to obtain scour distribution and scour depth around the bridge pier.This allowed the establishment of the relationship between debris content in the flash flood discharge and local scour depth.

Results and Discussions
The results and discussions encompass sediment grain size analysis, scour distribution around the pier, and the relationship between scour depth and the carried debris content due to the flash flood.These aspects will be comprehensively discussed in the following sections.

Sediment Grain Size Analysis
The sediment gradation analysis was conducted at the Soil Mechanics Laboratory of the Civil Engineering Department, Universitas Syiah Kuala.The sand used as the riverbed material in this research was sand that passed through an ASTM sieve no. 4 and was retained on no.200.The relationship between the grain diameter and the percentage of passing is illustrated in Figure 3. From the graph, it's evident that the average diameter (d50) is 0.52 mm, and σg = 2.327.

Analysis of live-bed scour
In the live bed of the channel, scour occurs when the flow conditions within the channel cause movement of the channel bed material.This phenomenon indicates that the shear force on the channel bed is greater than its critical value, or τo > τc, and if τo < τc is scour occurs.The calculation of both shear stress in normal and critical shear stress as follow; From measurement when running the model with rectangle channel (flume) of = 0.50 m width of channel (b), in normal flood condition with discharge (Q) = 0.018 m 3 /sec, Manning's coefficient for sand bed (n) = 0.022, slope (I) = 0.005 and, it is recording the Normal depth (yo) = 7,5 cm.The shear stress that works on the bottom (with a normal uniform flow along a slope) [25,27] is obtained as, Using equation 3, the shear stress normal (τo) is obtained as follows: ߬ = 1000.9,81.0,075.0,005= 3.68 ܰ ݉ ଶ then the shear stress velocity is found using equation 4, as follows; The critical shear stress (V*C) can be obtained using equation 6, where dimensionless ratio of the Shield's parameter (Tc) is determined using Shield's graph as stated in [25,27].
From Shield's diagram for Re* = 24.9 and Shield's parameter Tc) = 0.038, then, the critical shear stress (V*C) is found as follows:

Scour distribution and depth around pier due to normal flood.
Picture 5 illustrates the distribution of scour around the pillar under normal flow conditions.The scour pattern observed is more regular compared to flash floods and resembles a horseshoe shape due to the presence of a vortex flow system.It is evident that the front and rear parts of the pillar experience the most scour.From the above image, it can be seen that the maximum scour depth on the pillar is 28 mm, while sediment accumulation occurs on the right and left sides of the pillar.

Scour distribution and depth around pier due to flashflood without carrying debris.
The results of the scour distribution analysis are presented in contour form, allowing for a clearer view of the scour pattern around the pillar and the location of maximum scour depth, as depicted in Figure 6.From the image, it can be observed that the scour distribution around the pillar due to flash floods without debris transport is somewhat regular and resembles a horseshoe shape.This phenomenon occurs due to the presence of a vortex flow system.The flow impeded by the pillar leads to turbulence, resulting in scour around the pillar.Sediment accumulation is also evident on the right and left sides of the pillar, while scour is primarily concentrated in the immediate vicinity of the pillar.Notably, the maximum scour depth is 40 mm occurred on the right side of the pillar.This scour pattern occurs because the flash flood flow is obstructed by the pillar, leading to scour around the pillar and sediment accumulation on the right, left, and rear sides of the pillar.

Scour distribution and depth around pier due to flashflood carry debris.
Figure 7 illustrates the scour distribution in the vicinity of the pillar with a 5% debris ratio, where the scour pattern that occurs is irregular and significantly different from Figure 6.This disparity arises due to the presence of debris retained by the pillar, causing the flash flood flow to tend to bypass areas without debris entrapment.The quantity of debris trapped on the pillar cannot be predicted.It is evident that, in each run, the number of debris retained on the pillar varies, resulting in random scour patterns.The maximum scour depth occurs on the left side of the pillar, while scour on the right and left sides of the pillar occurs randomly.There is a noticeable accumulation of sediment on the right side of the pillar and scour on the left side of the pillar.This phenomenon is attributed to the presence of debris retained by the pillar.Consequently, the flash flood flow seeks openings free from debris entrapment.Nevertheless, the maximum scour depth observed, at 31 mm, is still smaller than that in the absence of debris, although it remains larger than the scour depth resulting from a normal flood Figure 8 shows that the erosion distribution in the vicinity of the pillar with a 10% debris ratio becomes increasingly irregular.This random erosion pattern is a result of a larger number of debris being trapped by the pillar, causing the flow to tend to bypass areas without debris entrapment, particularly on the right and left sides of the pillar.Additionally, the accumulation of debris in front of the pillar hinders the flow, leading to backwater conditions.This situation can weaken the flow's erosive ability, resulting in a reduced maximum erosion depth of 21.67 mm compared to the erosion depth in the absence of debris or with 5% debris, and even smaller than the erosion depth caused by a normal flood.
. Figure 9 reveals the scour distribution in the vicinity of the pillar with a 15% debris ratio.The increasing amount of debris results in a more irregular and random scour pattern.In the front part of the pillar, nearly all of it is blocked by debris, leading to a more pronounced backwater effect while reducing the flow velocity at the pillar.Observations indicate that the retained debris shields the front of the pillar, causing the flash flood flow to tend to bypass areas without debris entrapment, specifically on the right and left sides of the pillar.Although scour still occurs in front of the pillar, measuring 18.33 mm in depth, it is significantly reduced compared to situations without debris, with 5% debris, and slightly lower than with 10% debris.Furthermore, it is considerably smaller than the scour depth caused by a normal flood.

The effect of flash flood carrying various debris content on scour depth
During the occurrence of flash flood events, flow patterns change, resulting in scour around the bridge piers.The dispersed scour around the piers exhibits varying depths at each point.From the contour sections of Figures 5-9 above, the location of maximum scour depth becomes more evident.The results of measuring scour depth for each condition of debris ratio are comprehensively summarized in Table 5.The analysis shows that flow-carrying debris affects the scour around the bridge piers, mainly higher in front of the pier than the back or right and left side of the pier.The scour depth tends to decrease when the debris content in the flow is increased, as can be seen in Figure 10.This is due to the debris carried by the flash flood causing an increase in flow resistance and a reduction in flow velocity, as shown in Figure 11, resulting in reduced scour, compared to flash flood without carrying debris, there is a decrease in scour depth of 0-54.17%.Another phenomenon that emerges from this research is that as the debris trapped at the front of the pier increases, the scour depth on the sides of the pier tend to decrease.This is attributed to the alteration of flow patterns caused by the accumulation of trapped debris.Meanwhile, the maximum scour occurs during a flash flood without debris, yet the scour pattern resembles that of a normal flood.The formed scour pattern is centered near the pier and appears relatively uniform.The analysis results experiment that is the presence of debris carried by the flash flood leads to a reduction in scour depth by 0-54.17%,accompanied by an increase in the number of transported debris compared to the maximum scour that occurs in flash flood without carrying debris.However, when compared to normal flood discharge, there is an increase in scour depth by 1.11-1.43times when flashflood carrying debris less than 5%, and decrease up to 0.65 when carrying more than 5%, as can been seen in Figure 12.These findings align closely with earlier research involving sediment carried by the flashflood, where scour depth reduction was found to be between 0-16.53% compared to maximum scour, or an increase by 1.22-1.44times when compared to normal flood discharge [11].However, the patterns and distribution of the scour are significantly different due to the distinct characteristics of sediment and debris.Therefore, IOP Publishing doi:10.1088/1755-1315/1343/1/01202813 additional considerations are necessary for bridge pier design in flash flood-prone areas by including the correction coefficient due to the present of debris.
However, it is necessary to conduct critical analysis and validation through comparison with field data to ensure accurate representation in the model.These steps are crucial to ensure that the research results can be relied upon and are beneficial in understanding the actual conditions of scour events.

Conclusion
An experimental study on local scour around bridge pier models generated by flash floods carrying debris had been done with conclusions as follows: the distribution pattern of scour around the pier is significantly influenced by the debris content.A higher debris content leads to an increasingly irregular distribution pattern of scour around the pier, and the presence of debris carried by the flash flood leads to a reduction in scour depth by 0-54.17%compared to the maximum scour that occurs in flash flood without carrying debris, or 0.65-1.43times compared to normal floods.Therefore, Additional correction coefficients due to the presence of debris are necessary for bridge pier design in flash flood-prone areas.However, it is necessary to consider the potential effects of scaling in a model by validating against field data in order to verify the scaled model accurately represents real conditions.This comprehensive approach is essential for the model's reliability and its ability to provide meaningful insights and predictions.

Figure 2 .
Figure 2. Layout of the channel and pier model for scour experiment.This research was conducted at the Hydro Technics Laboratory of the Faculty of Engineering, Syiah Kuala University, Banda Aceh.Preparatory tasks included procuring materials, obtaining and preparing equipment, cleaning the flume channel, and designing the sharp-nose bridge pier model.The channel used was constructed from waterproof fiberglass, with a steel frame.The channel had dimensions of 15.46 meters in length, 0.5 meters in width, and 1.0 meters in height.The bridge pier model employed was a sharp-nose type constructed from wood, with dimensions of 12 cm in length, 4.5 cm in thickness, and 80 cm in height.This sharp-nose pier model was positioned 5 meters from the upstream end of the flume.The riverbed material was fine sand with d50 = 0.52 mm and σg = 2.327.This sand was evenly spread along the flume area with a thickness of 25 cm.For a clearer understanding of the design of the channel and pier, the model Reynolds number (Re* ) in equation 5, with mean diameter of grain (d50) = 0.52 mm = 0.52x10 -3 m and kinematic viscousity of water (Q x m 2 /sec at 20 o C is found as follows:

8 4. 3 .
Based on calculation above the normal shear stress (τo) = 3.68 N/m 2 and the critical shear stress (τc) = 3.19 N/m 2 .As τo > τc, fine sand grains could move during the occurrence of the flash flood flow, meaning that the scour is occur in the live-bed.IOP Publishing doi:10.1088/1755-1315/1343/1/012028Scour Distribution Around the Pier due to Flash Flood Flow The patterns and depths of scour observed around the piers from each data point indicate irregular and varying characteristics.This irregularity is attributed to the influence of flash floods that carry debris with varying volumes.Some of the debris becomes trapped upstream of the piers, causing the flow to seek new openings and thus creating new flow patterns.This, in turn, affects the non-uniformity in both the patterns and depths of scour.Figure 4 provides a visual representation of the flow patterns, trapped debris, and scour patterns that occur around the piers.The results of measuring scour patterns and depths for normal and flash floods carry various debris contents are clearly illustrated in Figures 5-9 below.

Figure 4 .
Figure 4. Scour patterns around the piers during flash floods carry debris.

9 Figure 5 .
Figure 5. Scour distribution and depth around the pier in 2D and 3D due to normal flood.

Figure 6 .
Figure 6.Scour distribution and depth around pier in 2D and 3D due to flashflood: without debris.

Figure 10 .
Figure 10.Scour depth and reduce with respect to flashflood carrying debris.

Figure 11 .
Figure 11.Flow velocity with respect to flashflood carrying debris.

Figure 12 .
Figure 12.Scour depth with respect to flash floods carrying debris.

Table 1 .
Coefficient due to pier nose shape.

Table 2 .
Coefficient due to pier angle of flow of attack.

Table 3 .
Coefficient due to bed river conditions.

Table 4 .
Coefficient due to sediment diameter.

Table 5 .
Scour depth analysis for various debris contents and normal flow