自然災害科学 J. JSNDS 40 特別号 111 -123（2021 ）Experimental investigation ofsuspended load deposition
on paddy fields during fluvial
inundation
Can DING1, Kenji KAWAIKE2, Hajime NAKAGAWA2 and Kazuki YAMANOI2

Experimental investigation of suspended load deposition
on paddy ﬁelds during ﬂuvial inundation
Can DING1, Kenji KAWAIKE2, Hajime NAKAGAWA2
and Kazuki YAMANOI2
Abstract
In this study, physical experiments were performed to investigate the influence of two
key morphological factors in paddy fields, i.e., paddy ridges and irrigation channels, on
sediment deposition during ﬂuvial inundation. The inlet boundary was given constant waterand sediment discharges to simulate embankment failure conditions. Hydraulic parameters
show that sediment came into the ﬂoodplain in the form of suspended load. By changing theoutlet boundary and channel outlets conditions, three groups of experiments were conducted.
The results show that, under the high-water-level scenario (with a wall at the outlet boundary),
sediment mainly deposited along the main ﬂow path, and the inﬂuence of paddy ridges was
only constrained in its surroundings because of the three-dimensional ﬂow characteristics. The
operation of the channel outlets eﬀectively changed the water depth and main ﬂow direction
and accordingly influenced the sedimentation distribution on the land surface. Under theshallow-water-depth scenario (open outlet boundary), the ridges on the bed retained more
sediment by decreasing the bed-load transport rate. The intersection of the channel and main
ﬂow path was blocked by the bed load from the land surface. Hence, setting ridges close to thechannel could remarkably relieve sedimentation. The experimental results are helpful for better
understanding the mechanism of sediment deposition on paddy ﬁelds with complex topography
and can provide data for numerical model veriﬁcation.
Key words:

1

paddy ﬁeld, sediment deposition, channel, paddy ridge, physical experiment2

Graduate School of Engineering, Kyoto University

111

Disaster Prevention Research Institute, Kyoto University

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Experimental investigation of suspended load deposition on paddy fields during fluvial inundation

1. Introduction

ﬁes the impact of inundation (Kawaike et al., 2017).

Water-related disasters have long been a seri-

A report of the Ministr y of Land, Infrastructure,ous threat to human society and the environment.

Transport, and Tourism (MLIT) of Japan shows

As a countermeasure, several embankments have

that, after the dike breach in Japanʼs Kinu River inbeen constructed along rivers (Wu, 2011). Howev-

2015, serious sedimentation in paddy fields waser, owing to the limited safety level, some trigger-

difﬁcult to remove and posed a difﬁcult problem toing mechanisms, such as piping and overtopping,local farmers.

endanger river embankments and even cause fail-To investigate the sedimentation mechanismure (Foster et al., 2000; Sills et al., 2008). Moreover,on floodplains during fluvial inundation and pro-flood inundation caused by embankment breachvide appropriate countermeasures, researchers

poses a signiﬁcant risk to lives and properties. To

have conducted a series of laboratory experiments

better understand the embankment failure process(Takahashi and Nakagawa, 1987; Islam et al., 1994)

and collect related data for the verification of theand numerics (Zech et al., 2008). However, to thenumerical model, many physical experiments havebest of the authorsʼ knowledge, it is very rare to

been conducted. Most of the tests focused on theconsider paddy ﬁelds as speciﬁc targets in investi-

development and characteristics of embankment

gating sedimentation caused by embankment fail-breaches (Richards and Reddy, 2007; Xu andures. In paddy ﬁelds, two key factors may inﬂuence

Zhang, 2009) and revealed the influence of somesediment deposition during floods: paddy ridges

key factors, such as the shape and material of theand the channel network. During inundation, pad-embankment, on the breach process (Schmocker,

dy ridges play a role similar to small embankments

2011; Frank, 2016). However, for ﬂood inundationand divide the farmland into several parts, and thein downstream areas, most laboratory tests havedrainage function of the channel network changes

been conducted based on dam breaks (Soares-

the water level and velocity distribution. This paper

Frazao and Zech, 2008; Soares-Frazao et al., 2007),

describes the results of the physical experiments

instead of embankment breaches.

that consider the two factors above. The objectives

Another serious but tentatively ignoredof this research are as follows:

problem is that a huge amount of sediment is

1) To study the influence of paddy ridges ontransported and deposited downstream during ﬂu-

suspended load deposition during inundation

vial inundation (Islam et al., 1994; Takahashi andunder different scenarios;

Nakagawa, 1987). The inﬂuence of sedimentation

2) To investigate the impact of the operation ofis especially serious for paddy ﬁelds. In East Asia,

channel outlets on the sedimentation on the land

due to monsoons, the climate is characterized bysurface as well as in the channel network;

periods of both rain and heat. Hence, summer isvery suitable for crop growth, but flood disasters3) To collect experimental data for numerical model
verification in the future.

are also likely to happen in this period because of
the concentration of annual rainfall. Once an em-

2. Experimental setup

bankment failure occurs, a large quantity of sedi-

Fig. 1 shows the plan view of the experimental

ment carried by water is transported and depositedmodel that was installed in the Ujigawa Open Lab-on paddy ﬁelds, crops are destroyed, and harvests

oratory of Disaster Prevention Research Instituteare affected. Moreover, deposition in the channel(DPRI), Kyoto University. Fig. 1(a) and Fig. 1(b)network disrupts the drainage function and intensi-

shows the layout of paddy ridges and the channel

113

自然災害科学 J. JSNDS 40 特別号（2021 ）Fig. 1

Layout of ridges and channels on the experimental setup

network on the corresponding experimental setup.

wide breach was set on it to connect the ﬂume and

The model consists of a ﬂume, embankment, andinundation ﬂoodplain. The length and width of the

fixed-bed inundation floodplain. The width of theﬂoodplain were 300 and 160 cm, respectively. The

flume was 30 cm, and its downstream boundar yleft and right sides were both closed, and water

was closed so that all the water flowed into the

could only overﬂow from the outlet boundary. By

floodplain through the embankment breach. Thechanging the outlet boundary condition, a series ofslope of the ﬂume was 0, and the ﬂow direction inexperimental cases were performed.

the flume was parallel to the embankment. The

Paddy ridges and the channel network are theheight of the embankment was 15 cm, and a 20-cm-

two key targets of this study. To determine the size

114

Experimental investigation of suspended load deposition on paddy fields during fluvial inundationof the ridges, we visited some practical farmlands.

shear velocity is calculated by Manningʼs equation:

The height of paddy ridges varies from 20 cm to 30（1）cm, and for some main roads, the height is about
50 cm. In addition, the paddy ridges usually havea rectangle cross section and the width is larger

where u＊ is the shear velocity, g is the gravity ac-

than the height. Therefore, to make the scale ofceleration, h is the water depth, n is the roughness

the model conform to the practical situation with

coefﬁcient, and u is the velocity. The shear veloci-an approximate length scale ratio of 1:100, pieces

ty ratio is 0.93 and the Rouse number is 1.53 in theof wood with a height of 0.5 cm and a width of 1 cm

vicinity of the breach point. From the Rouse num-

were set on the inundation ﬂoodplain to represent

ber and considering the strong turbulence there,

paddy ridges. The size varies from 0.5 m to 5 m init was generally assumed that suspended load waspractical farmlands with respect to the irrigation

dominant at the inlet boundary.

channel network in paddy fields. To simplify theThree groups of physical experiments wereexperiment and set the length scale ratio at about

performed: In the ﬁrst and second groups, paddy

1:100, the width and depth of each channel in theridges and the channel network were set on thephysical model were both set at 4 cm. However,inundation platform respectively to investigate

because of the ﬁxed bed, it is impossible to create

their independent influence on sediment deposi-a channel on the inundation ﬂoodplain. Therefore,tion. In the third group, channels and ridges were

12 pieces of wood were placed on the floodplain.

jointly implemented to clarify their joint effect on

The dimensions of the wood were 75 cm long, 50

sedimentation. In each group, three experimental

cm wide, and 4 cm thick, and the distance between

cases were conducted by changing the conditions

each piece of wood was 4 cm. The channel networkof the downstream boundary and channel outlets.

was accordingly formed, and the width and depth

Table 2 lists the related information of each case.

of the channels were the distance between eachpiece of wood and the thickness of each piece of

3. Results and discussion
3. 1

wood, respectively.

Results of group 1

In the experiments, a constant discharge of 2

Fig. 2 shows the deposition results of theL/s was given as the inlet discharge. The sediment

three experimental cases. In cases 1 and 2, most of

feeder over the breach supplied uniform sediment

the deposition occurred along the centerline andinto the water at a rate of 1 g/s, and the sedimentdownstream boundary of the inundation platform.

diameter was 0.01 cm. Table 1 shows the hydrau-

The measured data show that the deposition thick-lic condition in the vicinity of the inlet boundary ofness presented a decreasing trend from the breachthe case with a lower Froude number and Rouseof the embankment to the downstream boundary,number. The critical shear velocity is calculated byand the highest thickness was approximately 7 mm

using the Iwagaki formula (Iwagaki, 1956) and thein the vicinity of the inlet boundary. Moreover, theTable 1

Hydraulic parameters near the inlet boundary

Inﬂow
discharge

Meanvelocity

Flow depth

Shearvelocity

Criticalshear
velocity

Shearvelocity
ratio

Fr

Rouse
numberSettling
velocity

Sediment
density

2 L/s

19.2 cm/s

5.2 cm1.23 cm/s

1.31 cm/s

0.93

0.27

1.53

0.75 cm/s

2.65 g/cm3

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自然災害科学 J. JSNDS 40 特別号（2021 ）Table 2
With/withoutridges

With/without
channels

Outlet boundary
heightDuration

Case 1

Without

Without

4.5 cm45 minCase 2With

Without

4.5 cm45 minCase 3With

Without

0.0 cm45 minCase 4

Without

With

4.5 cm45 minOutlets 1 to 3
openedCase 5

Without

With

4.5 cm45 minOutlets 4 to 5
openedCase 6

Without

With

4.5 cm45 minAll outlets closed

Experimental cases
Group 1

Group 2

Group 3

Information of each experimental case
RemarkCase 7WithWith

0.0 cm45 minOutlets 1 to 5
openedRidges are in the
paddy center

Case 8WithWith

0.0 cm45 minOutlets 1 to 5
openedRidges are close to
the channels

Case 9WithoutWith

0.0 cm45 minOutlets 1 to 5
openedNo ridges

Fig. 2

Experimental results of cases 1, 2, and 3

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Experimental investigation of suspended load deposition on paddy fields during fluvial inundation

total quantity of the deposited sediment in the twovelocity suddenly decreased when the water ﬂowed

cases was also similar. The difference between thethrough the ridges. Moreover, at the point near

two cases was only in the area near the ridges. In

the downstream boundary, the value of the bottomcase 2, the deposition occurred at the backside ofvelocity was negative in both cases, which means

the ridges, but at the front side and in the down-that the water of the bottom layer ﬂows back when

stream area, no deposition was observed. To de-it meets a wall. However, this phenomenon was

termine the reason for this difference, an L-shaped

not observed near the ridges. This condition is at-

electromagnetic current meter was employed totributed to the scale effect (the height of the ridges

measure the velocity along the setup centerline

is too small, only 5 mm). However, in practicalunder a clear water scenario. The probe was posi-

situations, this phenomenon would be reproduced

tioned at the desired position inside the water bodynear ridges. The measurement also recorded thein different water depths to record the velocity. At

velocity in the vertical direction of each measuring

each measuring point, the velocity was measured

point in the layer z = 1 cm (Fig. 4). The verticalat two layers (1 cm and 3 cm from the bed). The

velocity at the back side of the ridges is negative,

ﬂow velocities at the layers of z = 1 cm and z = 3 cm

which means that the water has a downward veloc-

were measured in both cases (z = 1 cm represents

ity after passing the ridges. Based on the measured

the near-bed velocity and z = 3 cm represents theresults, the three-dimensional characteristics ofaverage velocity; water depth h =5.2 cm; and z ≈the ﬂow causes the sedimentation difference near

0.6 h). Fig. 3 shows the measured results of thethe ridges. Owing to the ridges, the water in thevelocity along the centerline. According to the law

bottom layer is obstr ucted and flows back, soof vertical velocity distribution, the velocity of theerosion may occur at the front side of the ridges.

layer z = 3 cm should be larger than that of the

Furthermore, after the surface layer water flows

layer z = 1 cm, and the measured results agree wellthrough the ridges, it has a downward velocity, andwith this. In the ﬁgure, the velocity of the layer z =then some of the water ﬂows back with sediment,

3 cm was almost the same in those of cases 1 andwhich results in erosion in the downstream area

2, but the bottom velocity of case 2 was obviously

and deposition at the back side of the ridges.

lower than that of case 1. In addition, the bottom

Fig. 3

The results of case 3 were entirely different

Velocity distribution along the centerline

117

自然災害科学 J. JSNDS 40 特別号（2021 ）from those of the previous two cases. The figure

bed-load transportation played an important role inshows that no deposition was obser ved near thethis case. In addition, no deposition was observed

inlet boundary, and the sediment was transported

in the vicinity of the open outlets. This outcome

and more uniformly distributed in the downstream

is attributed to the high velocity and turbulencearea owing to the high shear stress. This phenom-

in this area. In case 5, the main flow path clearlyenon is more common in practical ﬂood disasters.

deﬂected to the right side because the two outlets

Some field sur veys show that the soil near theon the right boundary were open. Similar to casebreach point is usually eroded and forms a pond

4, most of the sediment was deposited in the down-(Fujimoto et al., 2019). The ﬁgure also shows thatstream area. In the earlier part of the main flowsome sediment was retained in the upstream area

path, the deposition pattern showed the bed-loadby the paddy ridges.

movement, but, in the latter part, the deposition
was almost ﬂat, which means that suspended depo-

3. 2

Results of group 2

sition was the main reason for the sedimentation in

Fig. 5 shows the experimental results andthis area. In case 6, because all the channel outlets

deposition thickness distribution of each case of

were closed during the experiments, water could

group 2. In case 4, channel outlets 1, 2 and 3 were

only over flow from the downstream boundar y.

opened during the experiments. The water depth

Therefore, the channel network did not have any

was lower than the height of the wall at the down-

influence on the sediment deposition on the land

stream boundary, and all the water could only ﬂow

surface, and the results presented a similar patternout from the open channel outlets. Because of theto that of case 1. The maximum deposition thick-high bed shear stress near the inlet boundary, no

ness was found in the vicinity of the breach point

sediment deposition was found, and most of the(approximately 6.5 mm), and deposition occurred

sedimentation occurred on the four pieces of woodalong the centerline of the inundation platform.

along the downstream boundar y and presented

The experimental data also show that the operationan almost symmetrical distribution. Moreover, theof channel outlets can significantly influence thesedimentation in this case was accompanied bytotal amount of deposited sediment. The total vol-a strong sand-wave migration, which means thatumes of deposited sediment in cases 4, 5 and 6 are

Fig. 4

Vertical velocity distribution along the centerline of case 2

118

Experimental investigation of suspended load deposition on paddy fields during fluvial inundation

Fig. 5Experimental results and deposition thickness distribution on land surface of group 2554.9 cm3, 1301.5 cm3 and 1530.8 cm3, respectively.

shows the reach number and deposition thickness

In case 4, the outlets on the downstream boundary

along the channel network for the three cases. As

were open, and the water with a higher sedimentshown in the ﬁgure, the deposition thickness dis-concentration could be drained out more effective-

tribution in the channel network is closely related

ly. Therefore, the bed shear stress was higher andto the main ﬂow path on the land surface. In case 4,

the sediment quantity was less in this case, so thecontinuous sedimentation was formatted from thetotal deposition thickness was obviously lower thanpoint 70 cm from the inlet boundary in channel 2,that of the other two cases.

which is much closer than that of the land surface

Five channels, which were divided into 17(over 100 cm). By contrast, in channel 4, unlike thereaches, were set on the platform, and sedimentcontinuous sedimentation in channel 2, the depo-

deposition was found in some reaches. Fig. 6

sition was only symmetrically distributed in a part

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自然災害科学 J. JSNDS 40 特別号（2021 ）Fig. 6

Sedimentation along the channel network of cases 4, 5 and 6of reaches 4-2 and 4-3. In addition, the deposition

channel 2 (approximately 6 mm) and on the land

here presented a slope along the cross section

surface (approximately 2.7 mm), but the deposi-of the channel. At the upstream side, the highest

tion thickness at the downstream side was only

thickness was over 1 cm, much higher than that inless than 5 mm. The bed load was the main reason

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Experimental investigation of suspended load deposition on paddy fields during fluvial inundation

for the blockage. A similar phenomenon was also

relatively larger bed shear stress. Because of theobser ved in case 5, but because the main flowhigh shear stress, once the suspended load waspath deﬂected to the right side, sedimentation diddeposited, it moved from the bed to the channels

not occur in reach 4-3, but in reaches 4-2 and 3-1.

in the form of bed load and was then drained out,

Moreover, because the outlets of channel 2 were

but the existence of ridges could decrease the bed

closed, the sedimentation thickness in this channelload transport rate and retain more sediment on

was much higher than that in case 4. In case 6, thethe land surface.

location of deposition in the channels agrees betterIn this group of experiments, only the channel

with the land surface than cases 4 and 5 because

perpendicular to the main flow direction showedsuspended load was the main reason for sedimen-

signiﬁcant sedimentation. In the ﬂuvial inundation

tation in this case. However, owing to the higher

in Kinu River in 2015, serious sedimentation waswater depth, the deposition thickness in the chan-also found in the channels that were on the mainnels was obviously higher than that on the adjacentflow path. Fig. 9 shows the deposition thickness

surface area.

along the channel. The ﬁgure shows that the setting ridges at the upstream side could effectively

3. 3

Results of group 3

alleviate sedimentation in the channel network.

Previous experiments found that most of the sedi-

The maximum deposition thicknesses of cases 7, 8,

ment was deposited only on the speciﬁc area thatand 9 were 15.96, 7.56, and 12.19 mm, respectively.

the main flow passed. Therefore, to reduce themeasurement time, the width of the inundation

4. Conclusions

platform was decreased from 300 to 160 cm. By

A series of physical experiments were con-changing the location of the ridges, three cases

ducted to investigate the suspended load depo-

were performed to investigate the sedimentation

sition on the paddy field caused by embankment

on paddy fields. Fig. 7 shows the experimental

failures by considering the influence of paddy

results and the deposition thickness distribution.

ridges and the channel network. For the high-Unlike cases 1 and 2, the influence of ridges was

er-water-depth cases, the sediment was mainly

only limited to the adjacent area less than 10 cm,

deposited along the main ﬂow path, and the maxi-

and the sedimentation results of the three cases

mum deposition thickness was found in the vicinity

were quite different. In case 7, two rectangularof the inlet boundary. Under this scenario, settingareas upstream of the ridges retained most of

paddy ridges did not change the total amount of

the sediment (shown in Fig. 7(a)), and most of

sedimentation on the bed, and its influence was

the deposition in case 8 was formatted along thelimited to the surrounding area because of theridges. In addition, paddy ridges significantly in-three-dimensional flow characteristics. However,ﬂuenced the total amount of sedimentation. Fig. 8

for the shallow-water-depth cases, the sediment

depicts the quantity of sediment deposited on thewas carried by water and more uniformly deposit-

land surface. Compared with cases 7 and 8, lessed in the downstream area. The ridges could effec-

sediment was retained on the bed by paddy ridges

tively decrease the bed-load transport rate, whichin case 9. The different layout of ridges could only

resulted in a large amount of sediment depositedchange the deposition distribution, but not thein the upstream area and along the ridge, and thetotal amount of sediment. This ﬁnding is attributed

maximum deposition thickness was found at theto the very small water depth (less than 1 cm) andfront side of the ridge. In the experiments, the op-

自然災害科学 J. JSNDS 40 特別号（2021 ）Fig. 7

Experimental results and deposition thickness distribution of group 3

121

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Experimental investigation of suspended load deposition on paddy fields during fluvial inundation

References
Foster, M., Fell, R., and Spannagle, M (2000): The statistics of embankment dam failures and accidents,
Canadian Geotechnical Journal, Vol. 37, No. 5, pp.

Fig. 8Fig. 9The quantity of deposition on the bed ofcases 7, 8, and 9

Deposition thickness along the channel

eration of the channel outlets signiﬁcantly changed
the water depth and flow velocity, and the main
ﬂow direction clearly deﬂected to the open outlets.
The results show that opening the channel outlets
could drain out the sediment more effectively andalleviate sedimentation on the land surface. However, the channel can also be blocked by sediment.
The maximum deposition thickness in the channel
occurred at the point where the main ﬂow passed,
which was mainly caused by the bed load from theland surface. By setting ridges on the bed, the ridges retained more sediment on the bed. Conversely,
setting ridges at the channelʼs upstream side would
be helpful in decreasing the sedimentation in thechannel network. However, some factors were notconsidered in this study. For example, the vegetation in paddy ﬁelds may have signiﬁcant inﬂuenceon the ﬂow propagation and sediment deposition.
Further physical experiments and numerical simulations considering different conditions are necessary to better understand the mechanism.

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Frank, P. J. (2016): Hydraulics of spatial dike breaches, PhD thesis, Swiss Federal Institute of Technology Zurich, Switzerland.
Fujimoto S., Koyama T., and Yamada M. (2019): Preliminary report on ﬂooding and building damage
surveys on the Naka River due to Typhoon No.
19 in 2019 (in Japanese).
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failure, Bulletin of the Disaster Prevention Research Institute, Kyoto University, Vol. 44, Part 2,
No. 380, pp. 69-92.
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Numerical simulation of ﬂood inundation and sedimentation considering the effects of paddy ﬁeld,
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No. 4, pp. 381-402.
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要

旨

本研究では，水田における畦畔と灌漑水路が洪水氾濫時の土砂堆積に及ぼす影響を評価する
ため，室内実験を行った。堤防決壊条件を模擬するため，流入境界において一定の流量と土砂を供給した。下流端境界と水路の流出条件を変更することで，大まかに 3 種類の実験を行った。
その結果，氾濫水位が高い条件では，主に氾濫流の主流方向沿いに土砂が堆積し，畦畔の影響
はその周辺に限定された。さらに，水路の流出条件が変われば水深と氾濫流の主流方向が大きく変更し，それに応じて氾濫原での土砂堆積分布も変化した。氾濫水位が低い条件では，掃流砂量が減少するためより多くの土砂が畦畔の周辺に堆積した。水路やその合流点は氾濫原からの掃流砂が堆積するが，氾濫流の上流側に畦畔を設置することで水路への土砂の堆積を防止することができる。