The influence of driftwood debris in accelerating the regional scour near the bridge pier

Flood disasters caused by driftwood debris are increasing and becoming more severe due to rising rainfall, shifting forest conditions, and increased trees along rivers. Driftwood buildup in rivers leads to sedimentation, erosion, and rising water levels, necessitating precise prediction of its behavior to mitigate its harmful effects. This study utilizes a three-dimensional numerical model to investigate the influence of piled-up driftwood debris on the acceleration of local scour development in front of the bridge pier. Depending on their density, driftwood blockages can be deposition types, such as floating, suspended, and submerged materials. The floating driftwood debris types yielded broader and deeper scour holes, while the suspended driftwood debris produced narrower but deeper ones. The submerged driftwood debris obstructed the pier perimeter; consequently, the plunging and spiral horseshoe patterns deteriorated as the flow approached the pier. The horseshoe vortex’s intensity and diving flow are likely lower, producing shallower local erosion around the bridge pier.


Introduction
Driftwood will now apply to any floating wood in a river during a flood event.The accumulation of floating debris, especially wood, may cause flooding and building damage during heavy rain.These effects can be reduced by periodically collecting and removing driftwood from waterways.These accumulations of debris frequently enhance the likelihood of floods and can threaten the structural stability of bridges [1].Driftwood debris can significantly impact bridge construction stability by altering flow characteristics near piers, increasing scour depth and lateral stresses on the piers [2].
Additionally, the accumulation of debris at piers increases the water levels upstream, which may have consequences.Wood accumulation on bridge piers has caused significant damage, leading to bridge collapses in the United States of America, Europe, and Indonesia [3], [4].
Some studies indicate that the accumulation of debris is one of the primary reasons bridges collapse.Wood buildup on bridges can decrease the effective flow area and thus diminish river movement, resulting in a standing water effect that can exacerbate flooding.It may result in bridge failure due to worsened localized erosion and greater hydrodynamic forces produced close to the wood accumulation area [5], [6].The possibility for significant accumulations must be evaluated along with the implications listed above to limit the risk of infrastructure damage and build future river bridge infrastructure to be more resilient to harsh weather events.Many studies have been conducted on large wood debris in the field and lab [7], [8], [9], [10].As computational power grows, numerical analysis is increasingly helpful in analyzing driftwood movement in addition to laboratory testing.Different numerical and analytical models have been developed for situations like the movement of driftwood in rivers and the accumulation of driftwood in catchment areas.To assess the model's ability to reproduce the motions of the driftwood, they were confirmed through laboratory tests.Kimura developed NaysCUBE, a 3D solver for solving Navier-Stokes and continuity equations on a moving-boundary adaptive mesh system accessible on the iRIC river analysis platform [11].The model is capable of simulating the interaction between flow and driftwood-like debris.The motion of driftwood involves sliding and rolling contact with the ground, which impacts the slope.The study investigates the impact of driftwood types on the regional scour levels near bridge piers.

Materials and Methods
We used the International River Interface Cooperative (iRIC) software's 3D solver, NaysCUBE, to calculate the flow [12].The turbulent flow was simulated using the second-order non-linear k-epsilon model on a structured staggered grid with a moving generalized curvilinear coordinate system.[13], [14].The iRIC website (i-ric.org/)provides an in-depth overview of the model [15].

Summary of NaysCUBE
The turbulent-flow field was calculated by NaysCUBE utilizing diffusion terms and the Kappa-Epsilon model.0 w (4) where i x : spatial coordinates; t: time; i U : flow velocity; p: pressure; i u : turbulent velocity; Q : kinematic viscosity coefficient; ρ: fluid density; k: turbulent kinematic energy; H : dissipation rate of turbulent kinematic energy; t Q : turbulent kinematic viscosity coefficient; i G : gravity acceleration; j i u u : Reynolds stress tensor.

Model for driftwood motion
Driftwood is represented as connected spheres in NaysCUBE, and the Discrete Element Method is used to compute the collisions of driftwood.The computational process for driftwood dynamics is illustrated in Figure 3.A network of connected spheres represents the initial state of a driftwood piece.Without considering the binding force, each sphere is projected individually using the Lagrange-type momentum equation in a generalized curvilinear coordinate [8], [16].4 600 cubic meters for the assumed 50 m wide river section and 2 m wide pier.A 30 m-long piece of driftwood was used as the situational model for the calculations for the numerical study.Figure 4 shows the flat shape of the computing grid.151 x 26 = 3926 grid cells are used to calculate a 300 m-long river section with a resolution of 0.5 m.Through computational testing, the resolution of the 0.5m planar grid is determined to accurately and efficiently represent the main topographic shape of the river bed.Ten layers made up the vertical grid partition.The amount of vertical layers impacts how driftwood moves, mainly if its specific gravity is less than 1.With a specific gravity of about 1.1 t/m 3 , the influence on the capture ratio was negligible in the current situation.Each run included the entry of 130 logs with an input frequency of every 360 seconds.Although irregularly positioned concerning the flow direction, the logs were released at the center of the upstream channel.One run lasted for 720 seconds.Table 1 shows a set of experiments.flow, rotating then slightly rolling, especially the transverse log.Some logs stuck before the pier, while others flowed downstream, as seen in Figure 8.As the flow reaches the obstruction caused by the debris situated at the base of the pier, its velocity diminishes while simultaneously diving down the front face.The flow then moves outside the pier, following the typical horseshoe vortex pattern.The narrow scour at the pier is typically caused by blocky debris masses, influenced by the extent of obstruction at the lower portion, as shown in Figure 10.In contrast to submerged driftwood, Run-2, which simulated the suspended driftwood, shows that the blockage formation was also at the bottom and along the pier height (Figure 11).This blockage formation did not significantly change the bed shear stress distribution compared to Run-0 (Figure 12).The driftwood blockage extended the disturbance of velocity around the pier (Figure 13).Similarly, only the narrow scour at the pier has occurred.Run-3, which simulated the floating driftwood, shows that the blockage formation was mainly at the surface (Figure 14).This blockage formation significantly changed the distribution of bed shear stress and the scouring pattern compared to Run-0 (Figure 15 and Figure 16).

Impact driftwood on pier scour
Through the utilization of a comparative analysis, one may effectively visualize the scouring processes observed in a numerical simulation by contrasting the color and contour lines of local scouring at a pier free of debris with that of a pier obstructed by submerged, suspended, and floating driftwood debris blockages.The bed contour lines depicted in Figure 17 (a) for an unobstructed pier exhibit a remarkable degree of uniformity in the approach region.The flow descends along the frontal facade of the pier and extends in a spiral manner beyond it, conforming to the customary horseshoe vortex configuration.
Figure 17.The contour of local scouring at the bridge pier at 450s (i) no wood debris (ii) submerged driftwood debris (iii) suspended driftwood debris (iv) floating driftwood debris On the other hand, flow at a pier with buried driftwood debris (Figure 17 (b)) was severely slowed.The presence of submerged driftwood debris caused an obstruction that forced the flow around the perimeter.Consequently, the plunging and spiral horseshoe patterns deteriorated as the flow approached the pier.The intensity of the horseshoe vortex and diving flow is likely lower than that of the unblocked case (Run-0), depending on the obstruction near the cross-section of the entire channel.The block caused by submerged driftwood debris is more likely to cause the narrow pier's scour by the end of the simulation period.The pier scour significantly decreases due to a high frontal area of flow obstruction, a significant proportion of the approach channel's cross-sectional area (Figure 18(b)).In the case of a suspended driftwood blockage, the debris blockage's projected area was observed to be more significant at the pier face, with tapering middle and downward points.A notable difference was observed in the scour pattern generated by the suspended debris blockage (as shown in Figure 17 (c)) compared to that of the submerged driftwood blockage.The flow section beneath the jam was weakened at the pier face, resulting in no scour compared to the baseline condition (Run-0).The scour at the pier face was directly linked to the thickness of the debris blockage at the pier face.The pier face experienced less scour due to increased debris thickness, with suspended driftwood debris blocking the surface, as Figure 18 (c) shows.
Like floating driftwood debris (Run-3), the scour created by floating debris blockage exhibited a tapering lateral extent at the surface.The flow caused a severe scour condition near the pier face, resulting in a plunging flow toward the channel bed.The flow slowed down the pier's front face and followed a classic horseshoe vortex pattern, spiraling past it.The massive blocky debris masses at the surface caused the greatest scour at the pier.The total scour at the pier was significantly increased when the entire frontal area of flow blockage was enormous, as depicted in Figure 18.(d).
Figure 18.The final contour of local scouring at the bridge pier after 720s (i) no wood debris (ii) submerged driftwood debris (iii) suspended driftwood debris (iv) floating driftwood debris The development of local scouring near the bridge pier was also observed in Figure 19 and Figure 20.Upon driftwood debris approaching the pier and creating a blockage, the suspended and floating driftwood debris provided the deepest scouring holes (about -0.75 m) in front of the pier compared to other types of driftwood debris.The higher deposition of bed material behind the pier (about 2.5 m) was achieved by floating driftwood debris (Figure 19 and Figure 20).Meanwhile, the developing scour in the transversal direction and the suspended and floating driftwood blockage yielded broader and deeper scour holes (Figure 21 and Figure 22).

Conclusions
Understanding driftwood dynamics can prevent bridge damage or collapse, as it significantly impacts erosion and deposition, particularly scouring, due to driftwood jams.Driftwood accumulation near the pier intensifies erosion, flooding, and potential bridge collapse.Debris accelerates the regional scour development, causing a plunging flow that follows a horseshoe vortex pattern.The blockages of driftwood debris in the pier can be categorized into floating, suspended, and submerged types.The study shows that floating driftwood debris produces deeper holes, while suspended driftwood debris creates

Figure 1 .Figure 2 .
Figure 1.Sketch of physics of the scouring process at piers: a) no wood debris, b) affected by wood debris jam [3]

Figure 3 .
Figure 3. Schematic representation of the driftwood motion simulation model [16]2.3.Computational conditionsUsing numerical modeling, we examined the behavior of driftwood as floating debris, suspended and accumulated in the background.According to field investigations, most of the driftwood debris in the river basin is generated by severe floods.The hypothetical flood flow is simulated with a discharge of

Figure 4 .
Figure 4. Grid divisions and the computational area in a plan view with a pier in the center

3. Result and Discussion 3 . 1 .Figure 5 .Figure 6 .Figure 7 .
Figure 5. Velocity vectors and colored contours of the bottom shear stress along the channel without the presence of driftwood debris

Figure 8 .
Figure 8. Velocity vectors and colored contours of the bottom shear stress along the channel by the presence of submerged driftwoodDue to the buildup of wood debris at the bottom, bed shear stress in front of the pier is decelerating and moving backward toward the upstream flow.According to the formation and level of obstruction, the pattern of local scouring tends to differ, as seen in Figure9.

Figure 9 .
Figure 9. Velocity vectors and color contour of local scouring around the pier affected by trapped submerged driftwood

7 Figure 10 .Figure 11 .
Figure 10.Vertical distribution of velocity vectors and color contour of local scouring affected by trapped submerged driftwood

Figure 12 .
Figure 12.Vectors of velocity and color contours of bed shear stress along the channel affected by trapped suspended driftwood

Figure 13 .
Figure 13.Vectors of velocity and color contours of local scouring around the pier resulted from the trapped suspended driftwood

Figure 14 . 9 Figure 16 .
Figure 14.Vertical distribution of velocity vectors and the color contour of local scouring affected by trapped floating driftwood

11 Figure 19 .
Figure 19.The scatter point of longitudinal bed elevation changes at 450s.

Figure 20 .Figure 21 .
Figure 20.The scatter point of transversal bed elevation changes at 450s.

Figure 22 .
Figure 22.The scatter point of transversal bed elevation changes at 720s.

Table 1 .
Computational conditions