自然災害科学 J. JSNDS 36 特別号 15 -24（2017 ）Experimental study on bridges
over mountainous streams withblocked piers due to woody debris
Kana NAKATANI1, Motoki HIURA2, Yuji HASEGAWA3, Ken’ichiro KOSUGI1 and Yoshifumi SATOFUKA4

Experimental study on bridges over mountainous
streams with blocked piers due to woody debris
Kana NAKATANI1, Motoki HIURA2, Yuji HASEGAWA3,
Ken’ichiro KOSUGI1, and Yoshifumi SATOFUKA4
Abstract
When bridges are blocked with woody debris, a high flow depth leads to overflow and causes
the flow to spread widely, and some bridges become dangerous. Many studies have examinedbridges blocked with woody debris on gentle slopes in the subcritical flow regime. However,there have been few studies on woody debris occurring with debris flows from steep mountainous
areas in the supercritical flow regime. Therefore, we conducted laboratory experiments with a
two-pier bridge model and considered factors for the blockage of bridges by woody debris. We
examined the effects of the bridge clearance, amount of woody debris, and length of the woody
debris on bridge blocking. The results show that the woody debris concentration upstream of thebridge and the ratio of the woody debris length to the bridge pier interval affect bridge blocking.
When the ratio was less than 1, blocking became unstable. When the ratio was larger than 1,
blocking occurred at a lower concentration compared with a ratio of less than 1. When the ratio
was less than 1.5, a smaller bridge clearance had an increased possibility of blocking at the same
concentration. When the ratio was greater than 1.5, the bridge clearance did not seem to affect theblocking possibility.
Key words：Experimental study, bridge, blocking, woody debris, mountainous streams

1. INTRODUCTION

debris, a high flow depth leads to overflow from

When bridges become blocked with woody
12

the bridge, which causes the ﬂow to spread widely.
3

Graduate School of Agriculture, Kyoto University, Kyoto,
Japan
Eastern Construction Office Mihara Branch, Hiroshima
Prefecture, Hiroshima, Japan

4

15

Disaster Prevention Research Institute, Kyoto University,
Kyoto, Japan
Department of Civil Engineering, Ritsumeikan University,
Shiga, Japan16

Experimental study on bridges over mountainous streams with blocked piers due to woody debris

Ishikawa et al. (2014) reported that the Izu Oshima

mitigation has mainly focused on sediment disas-sediment disaster in 2013 caused woody debris toters, such as landslides and debris flows because

block bridges and flooding and deposition in thedamage caused by sediment is much larger thanMotomachi area. Furukawa et al. (2009) reported

that caused by woody debris (Takahashi, 2007).

that the Houfu sediment disaster in 2009 caused

Engineering works and studies have been con-woody debris to block bridges and facilitated sed-

ducted mainly on sediment movement and haveiment deposition on national roads. Therefore,

not focused on the behavior of woody debris orbridges may become critical points in the event

triggering damage as much as on sediment move-of debris flows. However, not all bridges become

ment. However, Ishikawa et al. (2014) reported

blocked with the accumulation of woody debris

that woody debris that occurs from a sedimentbecause bridge conditions such as shape and in-disaster may cause huge damage in residential

stallation slope and woody debris conditions suchareas, and it is important to examine woody debris

as volume, length, and shape are different.

that has occurred in a steep slope area. Several

Many studies have considered bridges

studies have considered woody debris in steep

blocked with woody debris for gentle slopes in theslope areas. Braudrick et al. (1997) reported that

subcritical flow regime. Drift wood studies havethe ratio of the woody debris to the ﬂow discharge

mainly been conducted in reservoir areas because

deﬁnes the accumulation of woody debris and thatit causes problems when ﬂowing in (e.g. Pﬁstter eta higher ratio causes accumulation. Rudolf-Miklau

al. 2013, Sumi and Kantoush, 2016). Also in a gen-and Hübl (2010) reported and classiﬁed the woody

tle slope downstream area, such as a river manage-

debris risk including bridge blocking from steep

ment area, bridge blocking due to the occurrence

mountainous areas and summarized the protec-

of woody debris caused problems, prompting manytion method for risk management considering thestudies to be conducted. Adachi and Daidou (1957)

woody debris source and application area. Someand Matsumoto et al. (2001) conducted channel

studies focused on countermeasures to protect

experiments and focused on the accumulation

against woody debris using check dams (e.g. Bez-mechanism and accumulation rate of woody debris.

zola et al. 2004) and rope nets (Rimböck, 2004).

Sakano (2003) performed channel experiments to

Piton and Recking (2016) conducted a flume ex-determine the woody debris, bridge, and hydraulicperiment focusing on the design of an open checkconditions that lead to a high possibility of woody

dam trap function considering sediment and woody

debris accumulation. Hashimoto et al. (2016)debris in a steep slope area. Recently, studies

performed a flume experiment based on actual

focusing on hazard zoning concerning woody de-disaster cases and bridge conditions and reported

bris occurrence and transportation with GIS havethat the volume of woody debris blocking bridges

been conducted (e.g. Mazzorana et al. 2009 andis influenced by the woody debris and area in a

Ruiz-Villanueva et al. 2014). However, there arechannel shielded by the bridge. Previous studies

few studies focusing on the bridge blocking pro-have shown that bridge piers become blocked withcess or boundary conditions due to woody debris

woody debris accumulation and stagnation, andin a steep slope area. Hasegawa et al. (2015) con-that smaller pier intervals increase bridge block-ducted a ﬂume experiment and showed that bridgeage.

blocking occurs more frequently with a low woody
However, in steep slope mountainous torrent

debris discharge at high velocity under the same

areas in the supercritical flow regime, disasterﬂow depth conditions. Their results imply that the

17

自然災害科学 J. JSNDS 36 特別号（2017 ）supercritical ﬂow condition causes bridge blocking

2. EXPERIMENT CONDITION

more frequently than the subcritical ﬂow condition,

The flume had a length of 4 m, width of 0.2

but their experimental conditions were limited. Form, slope of 2 , water supply of 3.2 L/s, and Froude

example, they only used bridge models with a sin-number of 3.1. The slope of 2 corresponds togle pier. Bridges over mountainous rivers can havethe boundary between a mountainous river and amultiple piers, as shown in Photo 1 (right), andgentle slope river, and commonly becomes a point

the pier interval may inﬂuence the blocking condi-

where ﬂooding occurs. We set the bridge model 0.5

tion as reported for gentle slopes. In this study, wem upstream from the downstream end (see Fig. 1,

conducted laboratory experiments with a two-pierPhoto 2, and Table 1). For the bridge model, we

bridge model in the supercritical ﬂow regime to de-set up two piers with a span length of 0.062 m andtermine the factors that affect the occurrence andconsidered two different clearances of 0.02 and 0.04

phenomena of bridges blocked with woody debris.

m (see Fig. 2). We changed the number of woody

Photo 1. Bridges blocked with woody debris from debris flows: (left) Izu Oshima disaster (PASCOCo.) and (right) Houfu disaster (Furukawa et al. 2009).

Fig. 1.

Overview of experiments.

18

Experimental study on bridges over mountainous streams with blocked piers due to woody debris

Photo 2.

Overview of experimental channel (left) and bridge blocked with woody debris (right).

Fig. 2.

Woody debris model (left) and bridge model (right).

Table 1.

Conditions of the channel experiments.
2 degrees

Slope gradientHydraulic conditions

Model of bridge

Water discharge

3.2 L/s

Flow depth

0.014 mFroude number

3.1

Piers

2

Clearance

0.02 m0.04 mLength: supplied number at intervals of 10 pieces

0.05 m: 170-300 pieces
0.07 m: 10-160 pieces
0.1 m: 10-80 pieces

Diameter

0.005 mSpeciﬁc gravity

0.8

Models of woody debris

debris and length of the woody debris (0.05, 0.07,

at the downstream end to check how much woodand 0.1 m; see Table 1) to observe the difference

passed through the bridge. We collected the woodin blockage. To supply the woody debris, we gath-from the downstream end. When woody debris oc-

ered the wood, put it into a dustpan, and dropped

curs with debris ﬂow, it moves a long distance in ait en masse in the channel upstream. We set video

steep slope area colliding with wood and sediment,

cameras at three angles: the side of the channel toand most branches are torn off and the length of

see the rise in the water level due to blocking, toproots and timber are shor tened or broken intoof the channel to see the blockage process, andpieces (e.g. Nakamura and Swanson, 1993). The

19

自然災害科学 J. JSNDS 36 特別号（2017 ）Fig. 3.

Method for calculating the woody debris discharge.

condition whereby there are no branches and roots

discharge of woody debris qlog [cm3/s].

is more dangerous because blocking is less likely（1）

to occur compared to when there are branches androots, and therefore we used woody debris without
branches and roots. Furthermore, the condition

Nlog represents the number of pieces of woody

explains the use of the woody debris supplied fromdebris, and V log [cm 3] shows the volume of one

timber cut and abandoned in natural forests. We

piece of woody debris. We defined the blocked

conducted 10 trials for each woody debris supply

condition as woody debris remaining at the bridge

condition.

and causing dam-up at the upstream side.

3. EXPERIMENT RESULTS
3.1 WOODY DEBRIS DISCHARGE
Even when the conditions for the amount ofwoody debris and discharge are the same, the

3.2

BLOCKING FREQUENCY, DAM-UPDISTANCE, WOODY DEBRIS CONCENTRATION ON CASES OF 2-CM
CLEARANCE

woody debris transportation process differs with

Here, we show the experimental results of thethe occurrence of phenomena such as spreading

blocking frequency, dam-up distance, and woodyor accumulation. Moreover, if there are variations

debris concentration on cases of 2-cm clearance.

in the woody debris input volume or procedure,Fig. 4 shows the relationship between thethe amount of driftwood flowing to the bridge

discharge of woody debris and frequency of block-changes. Therefore, we evaluated the amount ofing, temporar y blocking, and non-blocking forwoody debris as a time series with the discharge

each length of woody debris. Temporary blocking

0.2 m upstream of the bridge as the baseline (seemeans that woody debris blocked the bridge andFig. 3) to avoid the effect of reproducibility due tocaused dam-up but became disentangled andthe input procedure. We checked the time from theflowed down within 120 s. We used 120 s as thefront end of the woody debris to pass the baseline

index because we found that blocked woody debris(i.e., tlog first [s]) and the back end to do the same (i.e.,at the bridge did not flow down after 120 s fromtlog last [s]) and then divided the number of pieces

the preliminary experiment. We did not considerof wood by this time. We deﬁned this value as thethe condition whereby woody debris blocked the20

Experimental study on bridges over mountainous streams with blocked piers due to woody debris

Fig. 4.

Frequency of blocking, temporary blocking and non-blocking conditions according to theexperimental results (2-cm clearance).

bridge within 120 s but did not cause dam-up as

length is slightly longer than the interval. There-

temporary blocking. Since dam-up leads to small

fore, it can span the piers with the single wood

velocity and deposition, the risk at the bridge

model but even with slight movement, it loses one

becomes larger due to the blocking even in tempo-

support point easily. Therefore, the blocking thatrary phenomenon. Therefore, we considered tem-occurred in 7-cm cases seems to be unstable andporary blocking as part of the blocking condition.

collapses, indicating a higher frequency of tempo-

For all lengths, a larger discharge of woody debris

rary blocking compared to 10-cm wood cases.

led to blocking, and a smaller discharge led toFig. 5 shows the woody debris concentration

non-blocking. Blocking seemed to occur at a small

and occurrence of blocking. Concentration Clog wasdischarge with longer pieces of woody debris. Lon-calculated from the woody debris discharge qlog

ger pieces of wood reduced the frequency of tem-and supplied water discharge qwater.

porar y blocking. At a length of 5 cm, temporar y
blocking was more frequent than at other lengths.

（2）

Temporary blocking also occurred more frequently when the discharge condition was increased

Considering the temporary blocking case as

while the length was ﬁxed at 5 cm. This appeared

the blocking condition, the results show that a

to be because of the ratio between the woody de-higher concentration and longer pieces of woody

bris length and bridge pier inter val: 0.8 at 5 cm,

debris caused blocking more frequently. However,

1.1 at 7 cm, and 1.6 at 10 cm. When the length isunder the same concentration and length condi-

shorter than the bridge pier interval, in the 5-cm

tions, both blocking and non-blocking processes

wood model cases, single pieces of woody debris

showed a wide range of results, especially at small-

could not block the bridge. Even though blocking

er ratios between the woody debris length andoccurred from several pieces once, collapse due tobridge pier interval (e.g., 0.8). Based on the experi-

ﬂow turbulence or wood model movement seemedmental results, blocking seems to be inﬂuenced byto occur because the length is short. When theseveral factors of woody debris dynamics. For ex-

length is longer than the interval, in 10-cm cases,

ample, the woody debris ﬂow angle to the bridge,

single pieces were sufficiently long to span two

ﬂow course to the bridge, collision and accumula-piers. Once woody debris caused blocking, thetion of wood, and especially the behavior upstream

woody debris would not be dislodged easily evenof the bridge seem to affect the blocking process.

if some of the pieces moved because blocking is

Fig. 6 shows the relationship between the ac-

stable from two supporting points. Wood 7 cm incumulation volume of woody debris and the dam-

21

自然災害科学 J. JSNDS 36 特別号（2017 ）Fig. 5.

Occur rence of blocking in theexperimental results (2-cm clearance).

Fig. 6.

Relationship between the accumulationof woody debris and the dam-up
distance (2-cm clearance).

up distance in the event of blocking. The damup distance indicates the slope distance from thecondition using only the experimental results.

bridge upstream to the hydraulic jump. Usually

In addition, the results may be influenced by thewe use an ultrasonic sensor to observe ﬂow depth

number of experiments. Thus, we used a statisticalbut it was difﬁcult in our experiment because sev-

method. We applied logistic regression analysis us-

eral pieces of the woody debris protruded froming the woody debris concentration as the explan-

the water surface. Therefore, we used the dam-atory variable and the occurrence of blocking as

up distance instead of the dam-up depth because it

the response variable. We compared the blocking

was easy to obtain and showed us a more accurate

probability P based on the woody debris concentra-

value from video. The distance increased withtion x (see Fig. 7).

dense woody debris blocking and decreased under（3）

the sparse condition. The results show that longer
pieces of woody debris caused sparse blocking,and shorter lengths caused dense blocking. This

Here, e indicates the natural logarithm, andis due to the void difference; shorter length woodya and b are coefficients. Figs. 8 and 9 show thedebris blocking includes smaller voids comparedresults of the logistic regression analysis. The

with longer length woody debris blocking evenconditions under which both non-blocking andthough the volume of wood is the same. This oc-

blocking occur, as shown in yellow in Fig. 7, cancurs because shorter length cases, especially 5-cm

be explained by the shape and inclination of thecases, require more wood pieces to form larger

regression curve. When the inclination was steep

accumulation and cross over two piers or support-

and the concentration range for P＝0.1-0.9 was

ing points and cause stable blocking. As shown innarrow, the yellow area with both blocking con-Figs. 4 and 5, increasing the length caused block-ditions became smaller. Therefore, in both clear-ing to be more frequent, but the dam-up distanceance cases, pieces of woody debris 5 cm in length

due to blocking increased with shorter pieces ofshowed a larger yellow range. This indicates thatwoody debris.

the transition from non-blocking to blocking waslarger than that for the 7- and 10-cm lengths.

4. STATISTICAL METHODAt different clearances, the regression curves

It is difficult to determine the increased ten-

moved to the higher concentration side, and thedency to change from the non-blocking to blocking

curve shape became gentler with a 4-cm clearance

22

Experimental study on bridges over mountainous streams with blocked piers due to woody debris

Fig. 7.

Method of logistic regression analysis.

Fig. 8.

Results of logistic regression analysis for a clearance of 2 cm.

Fig. 9.

Results of logistic regression analysis for a clearance of 4 cm.

23

自然災害科学 J. JSNDS 36 特別号（2017 ）for woody debris with lengths of 5 and 7 cm. Thisa smaller bridge clearance showed a higher possi-

indicates that larger clearances required higher

bility of blocking at the same concentration. When

woody debris concentrations to increase the block-the ratio was greater than 1.5, the bridge clearance

ing probability P for woody debris pieces with

did not seem to affect the blocking possibility. The

lengths of 5 and 7 cm. This seems to be because

woody debris length and pier interval data can be

woody debris with lengths of 5 and 7 cm could not

obtained from ﬁeld observations. However, woody

induce stable blocking compared to debris withdebris discharge and concentration data are difﬁ-a length of 10 cm owing to the ratio with the piercult to obtain. Therefore, information on the woodyinter val, as shown in Figs. 4 and 6. At smaller

debris concentration and proposed estimation

clearances, the ﬂow surface and the bridge beam

methods are required. Furthermore, this study

distance became smaller, and the contact frequen-

focused on woody debris shape without branches

cy between each piece of woody debris and theand roots. The actual possibility of blocking willbridge beam or piers increased. Therefore, thebe higher because some of the roots may remain

2-cm clearance case required a lower woody debris

with woody debris and become entangled moreconcentration for the same blocking probability P

easily. In future, we will conduct further experi-with woody debris lengths of 5 and 7 cm comparedments under different conditions. For example, wewith the 4-cm clearance case.

can change the bridge model to one without piers,

However, the shape and inclination of the re-which is used frequently for steep mountainous

gression curve did not show remarkable changes

channels, and use pieces of woody debris that in-with 10 cm long woody debris. This length seemed

clude roots, which seem to increase blocking.

to be sufﬁcient to cross the pier interval and cause
both accumulation and blocking, as shown in Figs.
4 and 6. Thus, the clearance had a smaller effectcompared to shorter pieces of woody debris.

ACKNOWLEDGMENT
The study was supported by JSPS KAKENHI
Grant No. 15K16312, Grant-in-Aid for Young Scientists (B).

5. CONCLUSION
We conducted a laboratory experiment with a
two-pier bridge model and considered factors that
affect the occurrence and phenomena of bridges
being blocked with woody debris. The results
show that the important factors were the woody
debris concentration upstream of the bridge andthe ratio of the woody debris length to the bridge
pier interval. A higher concentration increased thefrequency and possibility of blocking. When theratio of the woody debris length to the bridge pierinterval was smaller than 1, the blocking became
unstable compared to larger ratios. A larger ratio(or longer woody debris length) required a lower
concentration for blocking than a smaller ratio (orshorter length). When the ratio was less than 1.5,

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（投 稿 受 理：平成29年 4 月 7 日
訂正稿受理：平成29年 7 月13日）

旨

流木により橋が閉塞すると，水位上昇による氾濫・堆積が発生して危険になる場合がある。既往研究では常流の緩勾配領域について検討されてきた。しかし，射流の急勾配領域の山間部で発生する流木による橋の閉塞の検討は少ない。本研究では，橋脚 2 本の橋模型を用いて，橋のクリアランス，流木量，流木長さを変えて水理実験で検討を行った。結果から，橋の上流での流木濃度と流木長さに対する橋脚間の距離の比が閉塞に影響することを示した。比が 1 よりも小さいと不安定な閉塞が生じ， 1 を超えると低い流木濃度で閉塞が発生した。比が1-1.5の条件では小さいクリアランスで閉塞確率が高いが，1.5を超えるとクリアランスの違いは閉塞確率に影響しなかった。