# Field Study on Wood Accumulation at a Bridge Pier

^{*}

## Abstract

**:**

## 1. Introduction

## 2. Methods

#### 2.1. Test Site

^{2}. Its mean (subscript m) discharge at the test site is ${Q}_{m}=$ 4.4 ${\mathrm{m}}^{3}/\mathrm{s}$ [21]. The test site (Figure 2a) is situated 8 km downstream of the Lake Greifensee. A circular concrete bridge pier is located in the river centerline with a diameter of ${d}_{p}=1.2$ m. The River Glatt has fixed banks at the test site and a mobile bed. It is characterized by a channelized geometry with a river width of 12 m. During the field test, the flow conditions remained relatively constant with a discharge $Q=8.4{\mathrm{m}}^{3}/\mathrm{s}$ in the morning and $Q=8.0{\mathrm{m}}^{3}/\mathrm{s}$ in the afternoon. For $Q=8.0{\mathrm{m}}^{3}/\mathrm{s}$, a mean surface flow velocity of ${u}_{m}=0.88\mathrm{m}/\mathrm{s}$ and a mean water depth of ${h}_{m}=0.73\mathrm{m}$ were measured 5 m upstream of the bridge pier ($x=-5\mathrm{m}$). These values correspond to an approach flow Froude number of $\mathsf{F}={u}_{m}/\left(g{h}_{m}\right)=0.3$ with the gravitational acceleration g. Nonuniform flow conditions are present due to a slight left river bend. The surface velocity plot in Figure 3 illustrates that the streamwise surface velocities at the bridge cross section are 43% higher towards the right bank than towards the left bank (${u}_{m,left}=0.72\mathrm{m}/\mathrm{s}$ and ${u}_{m,right}=1.03\mathrm{m}/\mathrm{s}$ at $x=0$ m).

#### 2.2. Large Wood

#### 2.3. Test Procedure

#### 2.4. Video Analysis

#### 2.5. Surface Flow Field

## 3. Results and Discussion

#### 3.1. General Process Description

#### 3.1.1. Impact Phase

**Impact point.**The impact point was defined as the point in time when a free-floating log hit the pier. Figure 7 illustrates the observed log orientation at the impact point (subscript $imp$) by means of the eccentricity and yaw. The impact eccentricity ${e}_{imp}$ is defined as the distance between the log center and the contact point between log and pier along the log axis. Logs with a positive impact eccentricity ${e}_{imp}>0$ hit the pier with their center on the left side of the pier, and logs with a negative eccentricity ${e}_{imp}<0$ hit the pier with their center on the right side (in flow direction). The histogram of the impact eccentricity ${e}_{imp}$ (Figure 7a) shows a Gaussian-shaped distribution with most logs in the range of ${e}_{imp}=[-1.0,1.0]$ $\mathrm{m}$. Only a few logs hit the pier with a higher eccentricity. The highest observed eccentricities were ${e}_{imp}=\pm 1.3\mathrm{m}$. The histogram of the impact yaw ${\phi}_{imp}$ (Figure 7b) illustrates a left skewed distribution with a maximum number of logs n at ${\phi}_{imp}=0{}^{\circ}$. Furthermore, Figure 7 shows that $n=22$ logs were only observed to accumulate given ${e}_{imp}=[-0.3,+0.8]$ $\mathrm{m}$ and ${\phi}_{imp}=[-25,+10]$ ${}^{\circ}$.

**Initial log submergence.**After the log impact on the pier, the log was observed to be pushed under water for a very short time duration of about one second. This subprocess is herein referred to as initial log submergence. Figure 9 shows a series of snapshots of a pronounced initial log submergence. The pictured log hit the pier quite concentrically with ${e}_{imp}=0.17\mathrm{m}$ and perpendicular to the main flow direction with ${\phi}_{imp}=5{}^{\circ}$. As a result of the log’s impact on the pier (at time step $t=0.00{s}$), a water wave was generated and spilled over the log ($t=0.12,0.24{s}$). During the same time, the log changes its vertical position from an emergent position ($t=0.00{s}$) to a partially submerged position ($t=0.36{s}$ and later) with water flowing over it. This process was observed mainly for logs impacting centrically (${e}_{imp}\approx 0\mathrm{m}$). Logs impacting eccentrically (${e}_{imp}>0.5\mathrm{m}$) showed a less pronounced initial submergence.

#### 3.1.2. Rotation Phase

**Log movement at the pier.**During the rotation phase, different log movements were observed that allow to classify the logs in four classes: NAC

_{1}, NAC

_{2}, AC

_{1}, and AC

_{2}. While the two NAC-classes comprise all $n=33$ not accumulated logs, the two AC-classes comprise all $n=22$ accumulated logs (Table 2 and Figure 10). A detailed list of the classified logs can be found in Appendix A.

_{1}logs. These logs were characterized by a fast and unidirectional rotation, i.e., they rotated around the pier in the horizontal plane without changing their direction of rotation. Due to their high rotation velocity, they separated after short accumulation times ${t}_{acc}=\left[1,30\right]$ $\mathrm{s}$. In contrast to the NAC

_{1}logs, NAC

_{2}logs showed a significantly slower rotation velocity and a bidirectional rotation, i.e., the logs changed their rotation direction at least once during the rotation phase. As this rotation behaviour was mainly observed for accumulated logs, it can be assumed that NAC

_{2}logs were close to being accumulated at the pier. This is also reflected in their rather long accumulation time of ${t}_{acc}=\left[50,80\right]$ $\mathrm{s}$.

_{1}logs. These logs were characterized by a slow and bidirectional rotation. After their impact and initial submergence, they rotated from their impact yaw ${\phi}_{imp}$ towards an equilibrium (subscript $equ$) yaw in the range of $|{\phi}_{equ}|=\left[20,70\right]$ ${}^{\circ}$ (Figure 10c) and kept rotating around ${\phi}_{equ}$. However, $n=5$ accumulated logs showed a distinctively different behavior than the AC

_{1}logs and were classified as AC

_{2}logs. These logs impacted with $|{\phi}_{imp}|=\left[0,15\right]$ ${}^{\circ}$ and remained in an equilibrium yaw close to their impact yaw ${\phi}_{equ}\approx {\phi}_{imp}$. AC

_{2}logs often remained completely submerged after their initial submergence. Due to their submergence, they exhibited stronger hydraulic drag forces than AC

_{1}logs as well as oscillations at the pier, i.e., rotation in the vertical and horizontal direction with abrupt changes in rotation direction and velocity.

_{1}logs. The logs impacted with ${\phi}_{imp}=\left[-20,20\right]$${}^{\circ}$ and subsequently rotated towards more extreme yaws with ${\phi}_{min}=-53{}^{\circ}$ and ${\phi}_{max}=57{}^{\circ}$. Most of the logs reached ${\phi}_{equ}$ after ${t}_{acc}=40\phantom{\rule{0.166667em}{0ex}}\mathrm{s}$. They then kept rotating bidirectionally around ${\phi}_{equ}$ until the end of the observation period at ${t}_{acc}=120\phantom{\rule{0.166667em}{0ex}}\mathrm{s}$.

**Log-induced changes in the flow field.**A log accumulation at a bridge pier reduces the open flow cross section and will affect the flow conditions in the vicinity of the bridge pier. The surface flow field is illustrated in Figure 12 for two logs (log #7 and #22). The logs were accumulated at $\phi \approx 45{}^{\circ}$ (log #7) and $\phi \approx -15{}^{\circ}$ (log #22) and can be characterized as AC

_{1}and AC

_{2}, respectively. Both logs led to small surface velocities in the range of $u=\left[0,0.2\right]$ $\mathrm{m}/\mathrm{s}$ directly up- and downstream of the log. The logs’ influence on the flow field is also reflected in the cross-sectional averaged velocity $\overline{u}$ (Table 3). At the pier cross section ($x=0\mathrm{m}$), both logs reduced the surface velocities compared to the reference flow field with no log accumulated at the bridge pier. Log #22 reduced $\overline{u}$ by 36% due to its orientation almost perpendicular to the main flow direction, thereby blocking a larger flow cross section. In comparison, log #7 reduced $\overline{u}$ by 16% with its yaw of $\phi \approx 45{}^{\circ}$. One meter upstream of the pier ($x=-1\mathrm{m}$), the influence of log #22 was not present anymore (Figure 12c), while log #7 still showed a significantly reduced flow velocity 14% compared to the reference flow field. At $x=-5\mathrm{m}$, the effect of a log accumulation on the surface flow field was not observed anymore. The log accumulation also affected the downstream flow conditions. Two meters downstream of the pier ($x=2\mathrm{m}$), the surface velocities were reduced by 25% (log #7) and 30% (log #22), respectively, compared to the reference flow field. Based on Figure 12a,c in comparison to Figure 3, it can be assumed that the logs affected the flow conditions even further downstream.

#### 3.1.3. Separation Phase

#### 3.2. Formulation of a Static Accumulation Criterion

**Definition of the acting forces.**The accumulation criterion is based on a simple force system (Figure 13). This force system was first introduced by Schalko [18] for uniform flow velocities $u={u}_{1}={u}_{2}$ and herein adapted for nonuniform flow velocities with ${u}_{1}\ne {u}_{2}$, i.e., different flow velocities on the left and right side of the pier. The flow velocities ${u}_{1}$ and ${u}_{2}$ are defined as the mean average streamwise flow velocity at the pier cross section ($x=0$) at $y=[0,2]$ m for ${u}_{1}$ and at $y=[-2,0]$ m for ${u}_{2}$. Thus, a log with ${L}_{L}=4.0\mathrm{m}$, ${e}_{imp}=$ 0 m, and ${\phi}_{imp}=$ 0° would experience ${u}_{1}$ and ${u}_{2}$. The flow velocities ${u}_{i}$ lead to different hydraulic drag forces on the respective part of the log, which can be defined as ${F}_{i}={\rho}_{w}{C}_{d}{A}_{pr,i}{u}_{i}^{2}/2$, with water density ${\rho}_{w}$, drag coefficient ${C}_{d}$, projected area ${A}_{pr,i}$, and subscript i denoting the respective part of the pier. A log remains accumulated as long as the parallel component of the total hydraulic drag force acting on the log ${F}_{\Vert}={F}_{1,\Vert}+{F}_{2,\Vert}$ is smaller than the friction force between the log and the pier ${F}_{Friction}=\mu {F}_{\perp}$, with the static friction coefficient $\mu $ and the component of the total hydraulic drag force perpendicular to the log ${F}_{\perp}={F}_{1,\perp}+{F}_{2,\perp}$ (Figure 13). Any other processes such as dynamic components of the system, turbulent fluctuations, or a changing flow field are neglected in this approach.

**Equilibrium of forces.**The equilibrium of forces is set up at the contact point between the log and the pier. At this point, ${F}_{Friction}$ must be equal to (or greater than) ${F}_{\Vert}$ to hold the log at the pier.

**Equilibrium of moments.**The equilibrium of moments is also defined at the contact point between the log and the pier. The acting moments can be written as $|{M}_{i}|=\frac{1}{2}{L}_{pr,i}{F}_{i}$ and thus the equilibrium is

**Combination of the equilibria of forces and moments.**The equilibria of forces and moments result in rotation distances around the pier that describe the accumulation process, namely, a required distance $\Delta e$ and two maximum available distances $\Delta {s}^{-}$ and $\Delta {s}^{+}$. If $\Delta e$ is in the range of $\Delta {s}^{-}$ and $\Delta {s}^{+}$, a log can reach its equilibrium eccentricity without separating from the pier. Thus, the log will accumulate if the following criterion is fulfilled

_{1}and AC

_{2}) are within the range of $\Delta {s}^{-}\le \Delta e\le \Delta {s}^{+}$, while 89% ($n=24$ of 27) of the NAC

_{1}logs lie outside the range (given $\mu =0.9$). Thus, the accumulation criterion predicts the accumulation behavior of AC

_{1}, AC

_{2}, and NAC

_{1}logs very well. In contrast, the accumulation behavior of all $n=6$ NAC

_{2}was predicted incorrectly as these logs were not observed to accumulate (given $\mu =0.9$). While the fact that they lie within the range of $\Delta {s}^{-}\le \Delta e\le \Delta {s}^{+}$ explains why they exhibited a very similar behavior as AC

_{1}logs (slow and bidirectional rotation) and comparably long accumulation times ${t}_{acc}=\left[20,70\right]$ $\mathrm{s}$ (Table 2), their separation from the pier cannot be explained by the criterion.

## 4. Conclusions and Outlook

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## Abbreviations

LW | Large Wood |

PIV | Particle Image Velocimetry |

Notation | |

A | area [${\mathrm{m}}^{2}$] |

${C}_{d}$ | drag coefficient [-] |

d | diameter [$\mathrm{m}$] |

e | eccentricity [$\mathrm{m}$] |

$\Delta e$ | required distance [$\mathrm{m}$] |

f | frequency [$\mathrm{Hz}$] |

${f}_{p}$ | vortex shedding frequency at the pier [$\mathrm{Hz}$] |

${f}_{L}$ | frequency of log rotation [$\mathrm{Hz}$] |

F | Froude number [-] |

F | hydraulic drag force [$\mathrm{N}$] |

${F}_{\Vert}$ | parallel component of hydraulic drag [$\mathrm{N}$] |

${F}_{\perp}$ | perpendicular component of hydraulic drag force [$\mathrm{N}$] |

${F}_{Friction}$ | friction force [$\mathrm{N}$] |

g | gravitational acceleration [$\mathrm{m}/{\mathrm{s}}^{2}$] |

h | water depth [$\mathrm{m}$] |

${I}_{A}$ | moment of inertia [${\mathrm{kgm}}^{2}$] |

${k}_{s}$ | equivalent sand roughness [$\mathrm{m}$] |

L | length [m] |

$L{W}_{p}$ | normalized large wood probability factor [-] |

M | moment of force [$\mathrm{Nm}$] |

n | number of logs [-] |

p | accumulation probability [-] |

Q | discharge [${\mathrm{m}}^{3}/\mathrm{s}$] |

R | Reynolds number [-] |

S | Strouhal number [-] |

$\Delta {s}^{+}$ | rotation distance in positive yaw direction [$\mathrm{m}$] |

$\Delta {s}^{-}$ | rotation distance in negative yaw direction [$\mathrm{m}$] |

t | time [$\mathrm{s}$] |

${t}_{acc}$ | accumulation time [min] |

T | period [$\mathrm{s}$] |

u | flow velocity [$\mathrm{m}/\mathrm{s}$] |

x | width coordinate [$\mathrm{m}$] |

y | length coordinate [$\mathrm{m}$] |

Greek letters | |

$\lambda $ | model scale factor [-] |

$\mu $ | static friction coefficient [-] |

$\nu $ | kinematic viscosity [${\mathrm{m}}^{2}/\mathrm{s}$] |

${\rho}_{w}$ | water density [$\mathrm{kg}/{\mathrm{m}}^{3}$] |

${\rho}_{L}$ | log density [$\mathrm{kg}/{\mathrm{m}}^{3}$] |

$\phi $ | yaw [${}^{\circ}$] |

Subscripts | |

$cr$ | critical |

$equ$ | equilibrium |

i | side of the pier |

$imp$ | impact point |

L | log |

m | mean |

$max$ | maximal |

$min$ | minimal |

o | upstream |

p | pier |

$pr$ | projected |

$sep$ | separation point |

$tot$ | total |

w | water |

## Appendix A. Field Data

**Table A1.**List of all n = 55 logs with the observation parameters and their classification as AC

_{1}, AC

_{2}, NAC

_{1}, or NAC

_{2}log.

Log # | ${\mathit{e}}_{\mathit{imp}}$ (m) | ${\mathit{\phi}}_{\mathit{imp}}$ (°) | ${\mathit{\phi}}_{\mathit{sep}}$ (°) | ${\mathit{\phi}}_{\mathit{equ}}$ (°) | ${\mathit{t}}_{\mathit{acc}}$ (s) | Class |
---|---|---|---|---|---|---|

1 | 0.4 | −11 | −3 | 120 | AC_{2} | |

2 | 0.0 | 7 | 54 | 120 | AC_{1} | |

3 | 0.6 | −42 | −73 | 45 | NAC_{2} | |

4 | −1.0 | −28 | −65 | 0 | NAC_{1} | |

5 | −1.3 | 12 | −75 | 7 | NAC_{1} | |

6 | 0.0 | 9 | −51 | 31 | NAC_{2} | |

7 | 0.4 | 3 | 48 | 120 | AC_{1} | |

8 | −0.5 | −4 | −80 | 7 | NAC_{1} | |

9 | 1.3 | −1 | 59 | 4 | NAC_{1} | |

10 | 0.4 | −4 | 62 | 11 | NAC_{1} | |

11 | 0.6 | −23 | −29 | 120 | AC_{1} | |

12 | −1.0 | −27 | −47 | 0 | NAC_{1} | |

13 | −0.8 | −13 | −72 | 2 | NAC_{1} | |

14 | 0.7 | −7 | 55 | 14 | NAC_{1} | |

15 | −0.2 | −22 | −75 | 7 | NAC_{1} | |

16 | 0.2 | −12 | −23 | 120 | AC_{1} | |

17 | −0.3 | −8 | −73 | 10 | NAC_{1} | |

18 | 0.5 | 2 | 51 | 70 | NAC_{2} | |

19 | 0.7 | 9 | 55 | 8 | NAC_{1} | |

20 | 0.7 | −1 | 56 | 5 | NAC_{1} | |

21 | 0.1 | 8 | −45 | 120 | AC_{1} | |

22 | 0.5 | −19 | −14 | 120 | AC_{2} | |

23 | 0.8 | −2 | 51 | 120 | AC_{1} | |

24 | 0.1 | −29 | −74 | 27 | NAC_{2} | |

25 | 0.5 | 8 | 58 | 14 | NAC_{1} | |

26 | 0.6 | 0 | 61 | 31 | NAC_{1} | |

27 | 0.2 | 5 | 5 | 120 | AC_{2} | |

28 | −0.2 | 8 | 0 | 120 | AC_{2} | |

29 | 0.4 | 18 | 62 | 19 | NAC_{1} | |

30 | 0.9 | −16 | 58 | 12 | NAC_{1} | |

31 | −0.3 | 9 | −50 | 120 | AC_{1} | |

32 | 0.0 | 5 | −50 | 120 | AC_{1} | |

33 | 0.7 | −8 | 61 | 15 | NAC_{1} | |

34 | −0.2 | −8 | −1 | 120 | AC_{2} | |

35 | 0.5 | 6 | 58 | 120 | AC_{1} | |

36 | 0.5 | −15 | 53 | 120 | AC_{1} | |

37 | 0.1 | 0 | −25 | 120 | AC_{1} | |

38 | −0.6 | −4 | −72 | 9 | NAC_{1} | |

39 | 0.3 | −2 | 24 | 120 | AC_{1} | |

40 | −0.2 | −15 | −74 | 8 | NAC_{1} | |

41 | 0.4 | 13 | 62 | 23 | NAC_{1} | |

42 | 0.3 | −25 | −70 | 120 | AC_{1} | |

43 | −0.6 | −2 | −80 | 17 | NAC_{1} | |

44 | 0.6 | −4 | 51 | 120 | AC_{1} | |

45 | 0.2 | −31 | −72 | 21 | NAC_{2} | |

46 | 0.3 | −11 | 32 | 120 | AC_{1} | |

47 | 0.6 | −3 | 64 | 11 | NAC_{1} | |

48 | 1.3 | −41 | 59 | 13 | NAC_{1} | |

49 | −0.9 | −20 | −69 | 1 | NAC_{1} | |

50 | 0.5 | 7 | 61 | 18 | NAC_{1} | |

51 | 0.0 | −3 | −40 | 120 | AC_{1} | |

52 | −0.5 | 5 | -74 | 15 | NAC_{1} | |

53 | −0.5 | 2 | −74 | 8 | NAC_{1} | |

54 | −0.1 | −1 | −81 | 42 | NAC_{2} | |

55 | 0.1 | 4 | −37 | 120 | AC_{1} |

## References

- Lucía, A.; Comiti, F.; Borga, M.; Cavalli, M.; Marchi, L. Dynamics of large wood during a flash flood in two mountain catchments. Nat. Hazards Earth Syst. Sci. Discuss.
**2015**, 3, 1643–1680. [Google Scholar] [CrossRef] - Keller, E.A.; Swanson, F.J. Effects of large organic material on channel form and fluvial processes. Earth Surf. Process.
**1979**, 4, 361–380. [Google Scholar] [CrossRef] - Bezzola, G.; Hegg, C. Ereignisanalyse Hochwasser 2005, Teil 1—Prozesse, Schäden und erste Einordnung (“Analysis of 2005 Flood, Part 1—Processes, Damages, and Classification”); Technical Report 0707; Federal Office for the Environment (FOEN), Eidgenössische Forschungsanstalt WSL: Bern, Switzerland; Birmensdorf, Switzerland, 2007. [Google Scholar]
- Ravazzolo, D.; Mao, L.; Mazzorana, B.; Ruiz-Villanueva, V. Brief communication: The curious case of the large wood-laden flow event in the Pocuro stream (Chile). Nat. Hazards Earth Syst. Sci.
**2017**, 17, 2053–2058. [Google Scholar] [CrossRef][Green Version] - Diehl, T.H. Potential Drift Accumulation at Bridges; US Department of Transportation, Federal Highway Administration, Research and Development, Turner-Fairbank Highway Research Center: McLean, VA, USA, 1997.
- Lyn, D.; Cooper, T.; Yi, Y.; Sinha, R.; Rao, A. Debris Accumulation at Bridge Crossings: Laboratory and Field Studies; Joint Transportation Research Program; Indiana Department of Transportation and Purdue University: West Lafayette, Indiana, 2003; p. 48. [Google Scholar]
- Bocchiola, D.; Rulli, M.; Rosso, R. Transport of large woody debris in the presence of obstacles. Geomorphology
**2006**, 76, 166–178. [Google Scholar] [CrossRef] - Schmocker, L.; Hager, W.H. Probability of Drift Blockage at Bridge Decks. J. Hydraul. Eng.
**2011**, 137, 470–479. [Google Scholar] [CrossRef] - Gschnitzer, T.; Gems, B.; Mazzorana, B.; Aufleger, M. Towards a robust assessment of bridge clogging processes in flood risk management. Geomorphology
**2017**, 279, 128–140. [Google Scholar] [CrossRef] - De Cicco, P.N.; Paris, E.; Ruiz-Villanueva, V.; Solari, L.; Stoffel, M. In-channel wood-related hazards at bridges: A review: In-channel wood-related hazards at bridges: A review. River Res. Appl.
**2018**, 34, 617–628. [Google Scholar] [CrossRef] - De Cicco, P.N.; Paris, E.; Solari, L.; Ruiz-Villanueva, V. Bridge pier shape influence on wood accumulation: Outcomes from flume experiments and numerical modelling. J. Flood Risk Manag.
**2020**, 13. [Google Scholar] [CrossRef] - Panici, D.; de Almeida, G.A.M. Formation, Growth, and Failure of Debris Jams at Bridge Piers. Water Resour. Res.
**2018**, 54, 6226–6241. [Google Scholar] [CrossRef] - Panici, D.; de Almeida, G.A.M. Influence of Pier Geometry and Debris Characteristics on Wood Debris Accumulations at Bridge Piers. J. Hydraul. Eng.
**2020**, 146, 04020041. [Google Scholar] [CrossRef] - Panici, D.; Kripakaran, P.; Djordjević, S.; Dentith, K. A practical method to assess risks from large wood debris accumulations at bridge piers. Sci. Total Environ.
**2020**, 728, 138575. [Google Scholar] [CrossRef] [PubMed] - Panici, D.; de Almeida, G.A.M. A theoretical analysis of the fluid–solid interactions governing the removal of woody debris jams from cylindrical bridge piers. J. Fluid Mech.
**2020**, 886, A19. [Google Scholar] [CrossRef] - Schalko, I. Large Wood Accumulation Probability at a Single Bridge Pier. In Proceedings of the 37th IAHR World Congress, Kuala Lumpur, Malaysia, 13–18 August 2017. [Google Scholar] [CrossRef]
- Schalko, I.; Schmocker, L.; Weitbrecht, V.; Boes, R.M. Risk reduction measures of large wood accumulations at bridges. Environ. Fluid Mech.
**2020**, 20, 485–502. [Google Scholar] [CrossRef] - Schalko, I. Modeling Hazards Related to Large Wood in Rivers. Ph.D. Thesis, Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie (VAW), ETH Zurich, Zurich, Switzerland, 2018. [Google Scholar] [CrossRef]
- Braudrick, C.A.; Grant, G.E.; Ishikawa, Y.; Ikeda, H. Dynamics of wood transport in streams: A flume experiment. Earth Surf. Process. Landforms
**1997**, 22, 669–683. [Google Scholar] [CrossRef] - Schalko, I.; Schmocker, L.; Weitbrecht, V.; Boes, R.M. Laboratory study on wood accumulation probability at bridge piers. J. Hydraul. Res.
**2020**, 58, 566–581. [Google Scholar] [CrossRef] - AWEL. Discharge data sheet of the River Glatt at Dübendorf, Switzerland. 2020. Available online: https://tinyurl.com/ym9f5r7w (accessed on 20 August 2021).
- Swain, M.J.; Ballard, D.H. Indexing via color histograms. In Active Perception and Robot Vision; Springer: Berlin/Heidelberg, Germany, 1992; pp. 261–273. [Google Scholar]
- Thielicke, W.; Stamhuis, E.J. PIVlab—Towards User-friendly, Affordable and Accurate Digital Particle Image Velocimetry in MATLAB. J. Open Res. Softw.
**2014**, 2, 30. [Google Scholar] [CrossRef][Green Version] - Achenbach, E.; Heinecke, E. On vortex shedding from smooth and rough cylinders in the range of Reynolds numbers 6× 103 to 5× 106. J. Fluid Mech.
**1981**, 109, 239–251. [Google Scholar] [CrossRef] - Möhler, K.; Herröder, W. Obere und untere Reibbeiwerte von sägerauhem Fichtenholz (“Upper and lower friction coefficients of spruce wood”). Holz als Roh-und Werkstoff
**1979**, 37, 27–32. [Google Scholar] [CrossRef] - Jaaranen, J.; Fink, G. Frictional behaviour of timber-concrete contact pairs. Constr. Build. Mater.
**2020**, 243, 118273. [Google Scholar] [CrossRef]

**Figure 1.**Naturally formed LW accumulation at the test site (River Glatt) in February, 2021. Flow direction from bottom to top.

**Figure 2.**(

**a**) Test site at the River Glatt with a log being placed upstream of the bridge pier by a truck crane with discharge Q, log length ${L}_{L}$, and pier diameter ${d}_{p}$, and (

**b**) log storage prior to the log placement in the river.

**Figure 3.**(

**a**) Streamwise surface velocity u at the test site. Note that u is generally higher towards the right bank than towards the left bank due to the slight left bend of the River Glatt. Vertical lines illustrate location of lateral profiles. (

**b**) Lateral profiles of u at distances of $x=-5\mathrm{m}$, $x=-3\mathrm{m}$ and $x=0\mathrm{m}$ from the upper edge of the pier.

**Figure 4.**Manual detection of log orientation with (

**a**) snapshot of log at impact point with reference points P1 to P4, (

**b**) perspective transformation of the snapshot, and (

**c**) cropped section of the perspective transformation with manually detected log endpoints (P5 and P6).

**Figure 5.**Automatic detection of log orientation with (

**a**) cropped section of a perspectively transformed snapshot, (

**b**) log detection using histogram backprojection, and (

**c**) image dilation and fitting of a straight line to determine the yaw $\phi $. Note that the color of the log should be easily distinguishable from the color of the water that surrounds the log. Therefore, the automatic detection was conducted for selected logs only.

**Figure 6.**Schematic overview of the accumulation process of a single log at a bridge pier with the three phases impact, rotation, separation, and their subprocesses. The values in brackets indicate an approximate time-scale for the process duration based on video observations.

**Figure 8.**Accumulation time ${t}_{acc}$ versus (

**a**) impact eccentricity ${e}_{imp}$ and (

**b**) impact yaw ${\phi}_{imp}$ for accumulated and not accumulated logs.

**Figure 9.**Snapshot series of an initial log submergence. After the log impact on the pier ($t=0.00{s}$), a water wave was generated and spilled over the log ($t=0.12,0.24{s}$) while the log changes its vertical position to a partially submerged position ($t=0.36{s}$ and later).

**Figure 10.**Histograms of (

**a**) impact eccentricity ${e}_{imp}$, (

**b**) impact yaw ${\phi}_{imp}$, and (

**c**) final yaws at separation point ${\phi}_{sep}$ (NAC logs) or in equilibrium ${\phi}_{equ}$ (AC logs); the dotted lines indicate the critical yaw ${\phi}_{cr}=\pm 42{}^{\circ}$.

**Figure 12.**Streamwise surface velocity u in the vicinity of the pier for (

**a**) log #7 with $\phi \approx 45{}^{\circ}$, (

**c**) log #22 with $\phi \approx -15{}^{\circ}$ accumulated at the pier, and snapshots of (

**b**) log #7 and (

**d**) log #22 during PIV measurement.

**Figure 14.**(

**a**) Sketch of the critical yaw for ${\phi}_{cr}=\pm 42{}^{\circ}$. Logs can only accumulate if they impact with a yaw between $-{\phi}_{cr}<{\phi}_{imp}<+{\phi}_{cr}$ (blue area) and do not rotate further than $\Delta {s}^{-}$ or $\Delta {s}^{+}$, respectively. (

**b**) Illustration of the rotation of a log from its eccentricity ${e}_{imp}$ at the impact point to the equilibrium eccentricity ${e}_{equ}$ with $\Delta e={e}_{imp}-{e}_{equ}$.

**Figure 15.**Comparison of the measured log orientation (${\phi}_{imp}$ and ${e}_{imp}$) with the accumulation criterion $\Delta {s}^{-}\le \Delta e\le \Delta {s}^{+}$. A total of 95% of the observed accumulated logs (AC

_{1}and AC

_{2}) lie within the predicted range, while 89% of the NAC

_{1}logs lie outside the range. The $n=6$ NAC

_{2}logs are poorly predicted by the criterion. The criterion was evaluated with $\mu =0.9$ and the gray areas reflect the uncertainty in $\mu $ ranging from 0.5 to 1.1 [26]. Error bars of $\pm 0.05{e}_{imp}+0.1\mathrm{m}$ (position accuracy of P3) for ${e}_{imp}$ and $\pm 5{}^{\circ}$ for ${\phi}_{imp}$ are plotted in gray.

**Table 1.**Mean, standard deviation, and extreme values of log length ${L}_{L}$, log diameter ${d}_{L}$, and log density ${\rho}_{L}$ of all tested logs ($n=55$).

Mean | Std. Dev. | Minimum | Maximum | |
---|---|---|---|---|

${L}_{L}$ [m] | 4.0 | 0.4 | 3.0 | 5.5 |

${d}_{L}$ [m] | 0.20 | 0.05 | 0.12 | 0.42 |

${\rho}_{L}$ [kg/m^{3}] | 560 | 110 | 320 | 750 |

**Table 2.**Observed movements of accumulated (AC

_{1}, AC

_{2}) and not accumulated logs (NAC

_{1}, NAC

_{2}) during rotation phase.

Not Accumulated Logs | Accumulated Logs | |||
---|---|---|---|---|

Class | NAC_{1} | NAC_{2} | AC_{1} | AC_{2} |

n[-] | 27 | 6 | 17 | 5 |

Rotation | Fast, unidirectional | Slow, bidirectional | Slow, bidirectional | No |

Oscillations | No | No | No | Yes |

Submergence | in rare cases | in rare cases | partially | often completely |

$|{\phi}_{sep}|$ [${}^{\circ}$] | $\left[45,80\right]$ | $\left[50,80\right]$ | - | - |

$|{\phi}_{equ}|$ [${}^{\circ}$] | - | - | $\left[20,70\right]$ | $\left[0,15\right]$ |

${t}_{acc}$ [$\mathrm{s}$] | $\left[1,30\right]$ | $\left[20,70\right]$ | ≥120 | ≥120 |

$\mathit{x}=-5\phantom{\rule{0.166667em}{0ex}}\mathbf{m}$ | $\mathit{x}=-1\phantom{\rule{0.166667em}{0ex}}\mathbf{m}$ | $\mathit{x}=0\phantom{\rule{0.166667em}{0ex}}\mathbf{m}$ | $\mathit{x}=2\phantom{\rule{0.166667em}{0ex}}\mathbf{m}$ | |
---|---|---|---|---|

No log (reference) | 0.88 | 0.81 | 0.73 | 0.77 |

Log #7 with $\phi \approx 45{}^{\circ}$ | 0.83 | 0.70 ^{a} | 0.61 ^{a} | 0.58 |

Log #22 with $\phi \approx 0{}^{\circ}$ | 0.86 | 0.77 | 0.47 ^{a} | 0.54 |

^{a}Note that the log intersects with the cross-section.

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**MDPI and ACS Style**

Wyss, A.; Schalko, I.; Weitbrecht, V. Field Study on Wood Accumulation at a Bridge Pier. *Water* **2021**, *13*, 2475.
https://doi.org/10.3390/w13182475

**AMA Style**

Wyss A, Schalko I, Weitbrecht V. Field Study on Wood Accumulation at a Bridge Pier. *Water*. 2021; 13(18):2475.
https://doi.org/10.3390/w13182475

**Chicago/Turabian Style**

Wyss, Andris, Isabella Schalko, and Volker Weitbrecht. 2021. "Field Study on Wood Accumulation at a Bridge Pier" *Water* 13, no. 18: 2475.
https://doi.org/10.3390/w13182475