NUMERICAL EXPERIMENT FOR DRIFTWOOD DYNAMICS CONSIDERING ROOTWAD EFFECT AND WOOD COLLISIONTaeun Kang1, Ichiro Kimura2, Shinichiro Onda3
1. JSPS Postdoctoral Fellowship, River System Engineering and Management Laboratory, Kyoto University, Japan
2. Professor, Faculty of Sustainable Design, University of Toyama, Japan
3. Associate Professor, Department of Urban Management, Kyoto University, JapanINTRODUCTIONCOMPUTATIONAL AND EXPERIMENTAL RESULTS• We carry out the flume experiments and numerical simulations to examine the effect of driftwood deposition
patterns and bed deformation• To quantify the bed deformation based on potential hydrodynamic changes by driftwood, we estimate nondimensional stream power index (NDSPI) and bed relief index (BRI)• We compared the simulations with experimental results• This study shows useful data of interaction between the driftwood congestion and bed deformation• This work can suggest the treatments for driftwood and bar formations in the numerical and experimental ways

Time changes in the number of stored wood pieces

Simulation result of driftwood deposition patterns

METHODOLODY

Exp1
Exp2
Sim1
Sim2
Sim3
Sim4
Sim5
Sim6

Experimental and simulation cases
Time interval of wood supply
Random parameter
5s
Absent9s
Absent5s
5s
 Generating position of wood pieces
5swithin the supply zone
9s
 Initial angle of wood piece
9s
9s

REFERENCE• Kang, T. and Kimura, I. (2018). Computational modeling forlarge wood dynamics with root wad and anisotropic bed
friction in shallow flows. Advances in Water Resources,121: 419‐431.
• Kang, T., Kimura, I. and Onda, S. (2019). Computationalmodeling for driftwood collision dynamics in shallow flows
considering projection area, root wad and anisotropic bed
friction. Proceeding of IAHR‐World congress 2019, Panama
City, Panama.
• Kang, T., Kimura, I. and Shimizu, Y. (2020). Numericalsimulation of large wood deposition patterns andresponses of bed morphology in a braided river using large
wood dynamics model. Earth Surf. Processes Landforms,
doi: 10.1002/esp.4789.
• Kimura, I. and Kitazono, K. (2019). Effects of the driftwood
Richardson number and applicability of a 3D–2D model toheavy wood jamming around obstacles. Environmental
Fluid Mechanics, doi.org/10.1007/s10652‐019‐09709‐6.

0.33

700.31600.29

500.270.25

400.23

30

0.21

20

0.19
0.17

100.1502400

0600

1200Time (s)1800nowoodSim1_5s

Sim2_5s

Sim3_5s

Sim4_9s

Sim5_9s

Sim6_9s

Ave_logs_5s

Ave_logs_9s

The number of stored wood pieces (‐)

Non‐dim. streampower (‐)

Time changes in NDSP

0.55

7060

0.5

50
0.45

400.4

30
200.35100.30

1800

02400

600

1200
Time (s)nowoodSim1_5s

Sim2_5s

Sim3_5s

Sim4_9s

Sim5_9s

Sim6_9s

Ave_logs_5s

Ave_logs_9s

The number of stored wood pieces (‐)

Time changes in BRI

BRI x 10‐4 (m2)

Flume domain
Computational and experimental parameters
Initial angle of wood piece 12 m
Parameter
Value
Flow discharge
1.1 liter/s
Target section (5-10 m)
0.4 m
Computational domain size
Flow direction
5 m × 0.4 m(uniform rectangular grid shape)
1.5 m 3.5 m
5m
2m
Bed slope0.005 m/m
Wood piece supply zone
The resolution of gauged bed elevation ( and ) 0.001 m × 0.02 m
Computational domain
The resolution of computational grid length
0.02 m × 0.02 m
5m( and )
Target section (5-10 m)
Manning roughness coefficient0.013 sm‐1/3
0.1 m
0.4 m
Uniform grain size
0.51 m/m•Random position generating within the zone
200 (=100 rootwad +The total input number of driftwood pieces per case
•Random initial angle of wood piece
100 no roowad)
Critical draft for wood motion (trunk particle)
0.0057 m
Conceptual collision modelCritical draft for wood motion (root wad particle)
0.008 mStatic bed friction coefficient
0.5
Kinematic bed friction coefficient
0.2
Rolling bed friction coefficient
0.075
Non-dimensional stream power index
Eddy viscosity for zero equation
0.4
𝟑 𝟎.𝟓
∗
Time for experiment and simulation
40 min,𝒔𝒎
Time interval for wood supply
5 s or 9 s
Range of initial angle of wood piece
0‐360 [degrees]
Diameter of wood pieces generating zone
0.1 m
Poisson ratio
0.3
Young modulus
100000 psi
Time step of driftwood motion in simulation
0.00001 s
Time step of water flow &0.01 s
bed deformation in simulation

Experimental result of bed change

CONCLUSION• In the experiments (Exp1 and 2), the time interval of wood supply may affect the final number of storedwood pieces• This is because the greater number of wood pieces allows more active collisions with floating motion• The simulations (Sim1-6) also partially reproduced the number of stored wood pieces• Along with the experimental results, NDSPI and BRI showed that the driftwood can influence bar formation
through the local deformations on the channel bed• Our results can be useful data to take account of the relationship between bed deformation and driftwood• In particular, this study enhanced the understanding of the totally submerged channel bed, so it can be oneof the important cases for driftwood study• For more detailed discussions, such as the refinement of the wood collision model, we would additionallycompare the experimental results with numerical model refinement in the future works

ACKNOWLEDGMENTS: We are thankful to Japan Society for the Promotion of Science (JSPS) for providing funding for this research.