changes in river bed around the fukawa...

The 10th Int. Conf. on Hydroscience and Engineering (ICHE-2012), Nov. 4 – Nov. 7, Orlando, USA 1

CHANGES IN RIVER BED AROUND THE FUKAWA CONTRACTION AREA

BY FLOODS AND CHANNEL IMPROVEMENT WORKS

IN THE LOWER TONE RIVER

Naoki IWAYA1 and Shoji FUKUOKA

2

ABSTRACT

In the lower Tone River, channel dredging and widening have been conducted for the river

improvement to respond to the flood discharge increase. Especially, the flood discharge capacity in

the Fukawa contraction area has been an important problem of flood control in the lower Tone

River. In the Fukawa contraction area, the river bed elevation lowered below the design bed

elevation, and therefore channel dredging stopped in 1966. Nevertheless, the river bed degradation

has continued until around 1998. In this study, we investigated the causes of the river bed

degradation in the Fukawa contraction area in relation to flood flows, the channel dredging and

widening by both observed data and analysis of flood flow and bed variation considering channel

widening and dredging. Observed data demonstrated that the river bed degradation in the Fukawa

contraction area has occurred due to floods, channel dredging and widening in the upstream and

downstream of the Fukawa contraction area. Numerical analysis provided a good explanation for

mechanics of river bed degradation and scouring during floods.

1. INTRODUCTION

In the lower Tone River, river improvement works including channel dredging and widening have

been conducted to respond to the flood discharge increase. The Fukawa contraction area is located at

76km from the river mouth, and the flood discharge capacity in the Fukawa contraction area has

been an important issue of flood control in the lower Tone River. Because there were many houses

along the Fukawa contraction area, channel dredging had been main countermeasures for

improvement works. In the Fukawa contraction area, the river bed elevation lowered below the

design bed elevation, and therefore channel dredging stopped in 1966. Nevertheless, the river bed

degradation has continued until around 1998.

The objective of this study is to clarify the causes of the river bed degradation and scouring in the

Fukawa contraction area in relation to the channel dredging, widening and flood flows. First, we

investigated the causes from the observed data of the channel dredging, widening and flood flows.

Secondly, we conducted the unsteady quasi 3-D flow analysis and bed variation analysis using

1 Graduate Student of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo, Japan,

Email: [email protected] 2 Professor, Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo, Japan, FL

Email: [email protected]


observed temporal changes in water surface profiles in order to understand the bed variation in and

around the Fukawa contraction area from 1981 to 1983.

2. CHANGES IN RIVER BED IN AND AROUND THE FUKAWA CONTRACTION

AREA IN THE LOWER TONE RIVER

Figure 1 shows study area and location of observation points in and around the Fukawa

contraction area in the Lower Tone River. Figure 2 shows the maximum annual discharge at Toride

(85.3km) and Fukawa (76.5km) observation point from 1962 to 2007, and the broken lines indicate

the design high water discharge at the observation points. The dredging volume in the upstream and

downstream of the Fukawa contraction area from 1972 to 2007 is shown in Figure 3. Since 1999, the

dredging has not conducted in the upstream of the Fukawa contraction area and the dredging volume

has been decreasing year by year in the downstream.

Figure 4 shows the changes in main channel width in the study area. Main channel has been

widened longitudinally from 1961 to 1983 in the upstream and from 1961 to 1998 in the

Figure 1 Study area and location of observation points in and around the Fukawa contraction area

in the Lower Tone River.

.

Figure 2 Maximum annual discharge.

Figure 3 Dredging volumes in the upstream and downstream of the Fukawa contraction area.

0

2,000

4,000

6,000

8,000

10,000

12,000

1962 1967 1972 1977 1982 1987 1992 1997 2002 2007

the design high water discharge (Toride)

the design high water discharge (Fukawa)

0 2,000 4,000 6,000 8,000

10,000 12,000

1962 1967 1972 1977 1982 1987 1992 1997 2002 2007

Toride(85.3km) Fukawa(76.5km)

0

10

20

30

40

50

1972 1977 1982 1987 1992 1997 2002 200701020304050

1972 1977 1982 1987 1992 1997 2002 2007

The downstream of the Fukawa contraction area (76km～61km)

The upstream of the Fukawa contraction area (85.5km～78km)

Dis

charg

e (m

3/s

)D

redgin

g v

olu

me

(×10

4m

3)

85.5km

66.5km

5.0km

Tone River

76.0km

78.0km

Water level observation point

Discharge observation point

Toride

SugaFukawa(76.5km)

Oshitsuke(78.5km)

Todai(0.5km)

Toyota weir(1.0km)

Nakago

Takasu(3.5km)

Yahoo Japan map

Koka River

Levee alignment


downstream for increasing the capacity of the river channel. In this area, large floods occurred at

Sep.1983, Sep.1998 and Sep.2008. Figure 5 shows flood marks and observed maximum water level

of each flood. Figure 6 and Figure 7 are the changes in average and deepest elevation of river bed in

the study area. In the Fukawa contraction area, although the dredging has not conducted since 1966,

the average and deepest river bed has greatly degraded from 1961 to 1983. On the other hand, from

1983 to 1998, the degradation of the river bed has caused mainly near the exit of the Fukawa

contraction area (from 77.0km to 76.0km). And, the river bed elevation has not much degraded since

1998.

From 1961 to 1983, the dredging has been conducted actively in the upstream and downstream of

the Fukawa contraction area. And, the amount of sediment from the upstream has reduced due to the

erosion and sediment control management and the construction of dams in the upper Tone River in

the 1960s. Therefore, the average and deepest elevation of river bed has degraded severely in the

upstream and downstream of the Fukawa contraction area (See Figure 6 and Figure 7). And, the

-5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

1961 1983 1998 2008

-10.0 -9.0 -8.0 -7.0 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

1961 1983 1998 2008

6.0

7.0

8.0

9.0

10.0

11.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

Figure 5 Flood marks and observed maximum water level of each flood.

Figure 6 Changes in average elevation of river bed.

Figure 7 Changes in deepest elevation of river bed.

6.0

7.0

8.0

9.0

10.0

11.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

Sep.1983 flood marks Sep.1983 observed water level



6.0

7.0

8.0

9.0

10.0

11.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0




Longitudinal distance (km)



Bed e

levati

on (Y

.P.m

)B

ed e

levati

on (Y

.P.m

)W

ate

r ele

vati

on (Y

.P.m

)

0

100

200

300

400

500

66.5 67.5 68.5 69.5 70.5 71.5 72.5 73.5 74.5 75.5 76.5 77.5 78.5 79.5 80.5 81.5 82.5 83.5 84.5 85.5

1961 1983 1998 2008


wid

th (

m)

The Fukawacontraction area

Figure 4 Changes in main channel width.

Dredging stopped in 1966 Dredging stopped in 1999


river bed elevation in the Fukawa contraction area has also lowered due to flood flows and the river

bed degradation in the upstream and downstream of the Fukawa contraction area from 1961 to 1983.

From 1983 to 1998, the dredging has continued in the upstream and downstream as shown in

Figure 3. And, the amount of the river bed degradation in the downstream is greater than in the

upstream (See Figure 6 and Figure 7). Figure 8 shows cross sectional form at 76.5km after Sep.1983

and Sep.1998 flood, and broken lines indicate the peak water levels of the floods. Table 1 indicates

the cross section area at the peak water levels. As shown in Figure 8 and Table 1, the water level in

Sep.1998 flood is lower than in Sep.1983 flood. However, the cross section area in Sep.1998 flood

is almost as large as in Sep.1983 flood. In the downstream of the Fukawa contraction area, because

the river bed elevation decreased more greatly than in the upstream and the main channel has been

widened, the water level has degraded. It is supposed that the scouring near the exit of the Fukawa

contraction area progressed to get the almost same cross section area of Sep.1983 flood. From the

above discussions, the dredging and widening in the downstream of the Fukawa contraction area are

closely related to the river bed degradation near the exit of the Fukawa contraction area from 1983

to 1998.

After 1998, although the floods comparable to the design high water discharge have occurred (See

Figure 2), the river bed elevation has not much degraded as shown by Figure 6 and Figure 7. There

are two reasons for this. One is the decrease of dredging volume and the other is the geological

factor in the Fukawa contraction area. Figure 9 shows the geologic cross section at 76.5km. The

Figure 8 Peak water levels and cross sectional form at 76.5km.

Table 1 Cross section area at peak water levels.

Peak dischrge (m3/s) Cross section area (m2)Sep.1982 flood 7700 2500Sep.1998 flood 7600 2550

-10.0

-6.0

-2.0

2.0

6.0

10.0

14.0

-100 0 100 200 300 400

S36

S58

H10

H19-5.0 -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

1961

1983

1998

2008

Lateral distance (m)

Alluvial cohesive soil

Alluvial sandy soil

Diluvial cohesive soil

Diluvial sandy soil

Embankment

Figure 9 Geologic cross section at 76.5km.

Bed e

levati

on (Y

.P.m

)

Sheet pile

-10.0

-6.0

-2.0

2.0

6.0

10.0

14.0

-100 0 100 200 300 400

Lateral distance (m)

Bed e

levati

on (Y

.P.m

)

-10.0

-6.0

-2.0

2.0

6.0

10.0

14.0

-100 0 100 200 300 400

1983

1998

the peak water level in Sep.1983 flood

the peak water level in Sep.1998 flood


geologic cross section indicates that the river bed surface has been covered with the diluvial sandy

soil since 1998, which is more difficult to erosion than alluvium. For this reasons, it is considered

that the river bed degradation near the exit of the Fukawa contraction area almost stopped.

3. BED VARIATION ANALYSIS CONSIDERING THE CHANNEL DREDGING AND

WIDENING DURING THE FLOODS

3.1 Analysis Method

The foregoing section showed that flood flows, channel dredging and widening had an effect on

change in the river bed in and around the Fukawa contraction area. But, flood flows and bed

variation in the study area during the floods is not discussed. Okamura, Fukuoka et al. (2010)

clarified bed variation and flood flows in the Tone River mouth during the flood by analysis method

using time series of the observed water surface profiles. The analysis is conducted based on the idea

that the effects of river conditions and bed variation during flood appear in time series of the water

surface profiles. We conducted the numerical analysis of flood flow and bed variation using

observed temporal changes in the water surface profiles for the five large floods in Aug. 1981, Aug.

and Sep. 1982 and Aug. and Sep. 1983. In this period, the river bed elevation in the Fukawa

contraction area lowered severely.

The analysis consists the unsteady quasi 3-D flow (Uchida and Fukuoka, 2011) and bed variation

analysis. The bed variation analysis was considered both bed load and suspended load because the

grain diameter is relatively so small as shown in Figure 10 that the amount of suspended load is not

negligible. The rate of bed load transport was calculated by Ashida and Michiue formula (Ashida

and Michiue, 1972). The vertical distribution of suspended load concentration, fall velocity and

amount of suspended load were calculated by using Lane-Kalinske formula (Lane and Kalinske,

1941), Rubey formula (Rubey, 1933) and Kishi-Itakura formula (Itakura and Kishi, 1980),

respectively. The suspended load concentration was calculated by depth averaged planar 2-D

convective diffusion equation.

The initial bed forms were measured in 1980. The upstream and downstream boundary conditions

were given by the time series data of observed water level at Toride (85.3km), Suga (66.5km)

observation point in the lower Tone River and Nakago (5.0km) observation point in the Kokai river

(See Figure 1). The mean grain diameter is 0.25mm in the study area. In the Fukawa contraction

area, we assumed the river bed degradation does not occur at the place of diluvium in the analysis.

Manning's roughness coefficients were set to 0.020 for the main channel and 0.038 for the flood

channel.

In this calculation, flood flows and bed variation were calculated during the 5 floods continuously.

The channel dredging and widening have been conducted in the study area after Aug. 1981 flood,

Sep. 1982 flood and Sep. 1983 flood, respectively. So, the bed variation analysis considered channel

dredging and widening after each flood in the following manner. In this study, we defined dredging

as excavation of river bed and widening as extension of main channel width.

Grain size (mm)

Volu

me p

erc

enta

ge fin

er

than (

%)

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10

0

10

20

30

40

50

60

70

80

90

100

0.01 0.1 1 10

69.0km71.0km72.0km73.0km75.0km76.0km78.0km80.0km81.0km82.0km83.0km84.0km

The downstream


The upstream

Figure 10 Grain size distribution in 1996.


Table 2 shows dredging range, dredging volume and implementation year. But, there was no data

of dredging depth and area of each dredging location. We determined the dredging depth and area so

that calculated bed profiles after the floods reproduce the observed bed profiles. Concretely, Figure

11 shows the observed river bed contour in 1983 and the calculated river bed contour after the

floods and the area enclosed by black line indicates the determined channel dredging area. The

channel dredging depth and area is estimated to correspond with the dredging volume in the location

which the calculated river bed elevation is higher than the observed river bed elevation. If the

calculated bed profiles after the floods do not sufficiently reproduce the observed bed profiles, we

adjust the channel dredging depth and area by trial and error so that the calculated bed profiles

reproduce the observed bed profiles.

dredging location （ｍ３） dredging location （ｍ３） dredging location （ｍ３）

81.25k right bank 30,465 81.5k right bank 32,535 81.0k right bank 41,460

78.75k left bank 6,850 74.5k left bank 37,119 75.0k left bank 43,440

74.5k right & left bank 21,844 73.75k left bank 12,321 73.5k left bank 19,44674.0k left bank 18,800 71.5k right bank 17,100 69.5k left bank 5,020

68.5k right & left bank 35,060 68.0k right bank 24,560 69.0k right bank 49,74068.0k right bank 35,830 68.75k right bank 35,229 68.0k right bank 40,550

1981 1982 1983

Table 2 Dredging range, volume and implementation year.

Figure 11 Observed river bed contour in 1983 and calculated river bed contour after the floods.

Elevation (Y.P.m)

The dredging area

75.0km 74.5km

The dredging area

75.0km 74.5km

a) Observed river bed in 1983 b) Calculated river bed after the floods

1981 1982 1983

Toride Toride 72.0km

Inba Inba 79.0km

79.0km

Table 3 Widening place name, kilometer post and implementation year.

Figure 12 Cross sectional form before and after the river widening at 72.5km.

-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0

-200 0 200 400 600

Bed e

levati

on (Y

.P.m

)

Lateral distance (m)-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0

-200 0 200 400 600

1980 (before widening)

1983 (after widening)


In the study area, the widening place name, kilometer post and implementation year have been

recorded as shown in Table 3. But, widening area has not been recorded. In order to determine the

widening area, we used the observed cross sectional form before and after the floods and the data

shown in Table 3. First, we set the widening depth by comparing the cross sectional forms every

point which cross-sectional leveling was conducted (e.g., Figure 12). From this, the planar area of

channel widening is determined as indicated in Figure 13.

Conclusively, we set the planar area of channel dredging and widening of each year as shown in

figure 14.

3.2 Analysis Results

Toride

Inba

After 1981 flood

After 1982 floods

After 1983 floods

Dredging area

After 1981 flood

After 1982 floods

After 1983 floods

Widening area

Figure 14 Planar area of channel dredging and widening of each year.

Widening area

-6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 8.0

10.0 12.0 14.0

-200 0 200 400 600

1980 (before widening)

1983 (after widening)

Figure 13 Planar change in main channel width from 1981 to 1983.

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

0 20 40 60 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60

Aug.1981 Sep.1982 Aug.1983 Sep.1983

Wate

r ele

vati

on (Y

.P.m

)

23456789

1011

0

Toride(85.3km) Obs. Oshitsuke(78.5km) Obs. Fukawa(76.5km) Obs. Suga(66.5km) Obs.

Toride(85.3km) Cal. Oshitsuke(78.5km) Cal. Fukawa(76.5km) Cal. Suga(66.5km) Cal.

Figure 15 Comparison between observed and calculated water level hydrographs of the floods

in the Tone River.

Aug.1982


Figure 15 shows the comparison between observed and calculated water level hydrographs of the

floods in Tone River. Figure 16 shows the comparison between observed and calculated discharge

hydrographs of the floods in the Tone River and the Kokai River. The calculated water level and

discharge hydrographs correspond well with the observed water level and discharge hydrographs.

Figure 17 is the comparison between observed and calculated water surface profiles at the peak of

each flood. The calculation coincides with observed water surface profiles. Figure 18 shows the

comparison between observed and calculated river bed elevation before and after the floods. The

numerical computation provided a good explanation for the observed bed elevation after the floods.

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0

Toride(85.3km) Obs Fukawa(76.5km) Obs. Takasu(3.5km) Obs.Toride(85.3km) Cal. Fukawa(76.5km) Cal. Takasu(3.5km) Cal.

0 20 40 60 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60 80 100 0 20 40 60

Aug.1981 Sep.1982 Aug.1983 Sep.1983

Figure 16 Comparison between observed and calculated discharge hydrographs of the floods

in the Tone River and the Kokai River.

Tone River

Kokai RiverDis

charg

e (m

3/s

)

Aug.1982

Wate

r ele

vati

on (Y

.P.m

)


-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

average bed elevation

before floods Obs.

after floods Obs.

after floods Cal.

-8.0

-7.0

-6.0

-5.0

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

deepest bed elevation

before floods Obs.

after floods Obs.

after floods Cal.

6.0

7.0

8.0

9.0

10.0

11.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

66.0 68.0 70.0 72.0 74.0 76.0 78.0 80.0 82.0 84.0 86.0

Aug.1981 flood marks (left bank) Aug.1981 flood marks (right bank) Aug.1981 flood peak Obs. AUg.1981 flood peak Cal.Aug.1982 flood marks (left bank) Aug.1982 flood marks (right bank) Aug.1982 flood peak Obs. Aug.1982 flood peak Cal.Sep.1982 flood marks (left bank) Sep.1982 flood marks (right bank) Sep.1982 flood peak Obs. Sep.1982 flood peak Cal.


Figure 17 Comparison between observed and calculated water surface profiles

at the peak of each flood.


Figure 18 Comparison between observed and calculated river bed elevation

before and after the floods.

Bed e

levati

on (Y

.P.m

)


As a result, we reproduced the bed variation in and around the Fukawa contraction area from

1981 to 1983 by using numerical analysis as shown in Figure 19. The more the floods occurred, the

more bed scouring progressed at the Fukawa contraction area and sediment deposition became

noticeable at the exit of the Fukawa contraction area. Channel dredging was conducted around the

location in which sediment deposition was considerable at the end of the contraction area. However,

sediment deposition occurred again at the dredging location after the flood, in each year. In this way,

we think that respective influences of channel dredging, widening and flood flows on change in

river channel morphology can be analyzed by this method.

CONCLUSION

1. Observed data shows that river bed degradation and scouring in the Fukawa contraction area have

occurred due to flood flows, channel dredging and widening in the upstream and downstream of

Fukawa contraction area.

2. We estimated dredging area, depth and widening area so that calculated water surface profiles and

bed profiles reproduced the observed water surface profiles and bed profiles by using dredging

volume, implementation year and the cross sectional form before and after the floods, and the

analysis result provided a good explanation for the observed results of bed variation in and around

the Fukawa contraction area.

3. We evaluated the respective influences of channel dredging, widening and flood flows on change

in the river channel from the result of the analysis.

After Aug. and Sep. 1982

floods

After dredging and widening in 1982

Bed variation (m)Scouring Deposition

Before 5 floods(contour of bed elevation)

After Aug. and Sep. 1983

floods

After dredging and widening

in 1983

1981 1982 1983Dredging area

1982 1983Widening area

Bed elevation(Y.P.m)

After dredging and widening

in 1981(contour of bed variation)

Figure 19 Calculated bed variation contour in and around the Fukawa contraction area

from 1981 to 1983.


REFERENCES

Ministry of Construction, Kanto Regional Construction Bureau. 1987. 100 year’s history of the

Tone River.

Moro, Y., Kazama, S. and Fukuoka, S. 2011. Change of river improvement works of the Lower

Tone River and effectiveness of channel dredging, Advance in River Engineering, JSCE, vol.17,

pp101-106. Okamura,S., Fukuoka,S. and Takemoto,T. 2010. Bed Form and Bed Variation during Floods of the

Tone River Mouth, Annual Journal of Hydraulic Engineering, JSCE, Vol.54, pp.751-756. Shirai, K. and Fukuoka, S. 2011. Changes of Tonegawa River channel by river improvement since

the Meiji era and hydroscience and hydraulic engineering analysis on its factor, Advance in River

Engineering, JSCE, vol.12, pp217-222.

Uchida, T. and Fukuoka, S. 2011. A bottom velocity computation method for estimating bed

variation in a channel with submerged groins. Proceedings of JSCE, vol.67, No.1, pp.16-29.

changes in river bed around the fukawa...

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