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 FUKUOKA2

ABSTRACTIn 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 andwidening 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 anddownstream of the Fukawa contraction area. Numerical analysis provided a good explanation formechanics of river bed degradation and scouring during floods.

1.

INTRODUCTION

In the lower Tone River, river improvement works including channel dredging and widening havebeen 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 thedesign 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, weinvestigated 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: naoki-iwaya@civil.chuo-u.ac.jp
2
Professor, Research and Development Initiative, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo, Japan, FL
Email: sfuku@tamacc.chuo-u.ac.jp

The 10th Int. Conf. on Hydroscience and Engineering (ICHE-2012), Nov. 4 – Nov. 7, Orlando, USA2

observed temporal changes in water surface profiles in order to understand the bed variation in andaround 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
Nakago5.0km
Takasu(3.5km)
85.5kmWater level observation point
Discharge observation point
Levee alignment

Koka River
Toyota weir(1.0km)
TorideTodai(0.5km)

Tone River

Oshitsuke(78.5km)
66.5km
78.0kmFukawa(76.5km)

Suga

76.0kmYahoo Japan map

Discharge (m3/s)

Figure 1 Study area and location of observation points in and around the Fukawa contraction areain the Lower Tone River.
.
12,00010,000Toride(85.3km)

Fukawa(76.5km)

the design high water discharge (Toride)

8,000
6,000

the design high water discharge (Fukawa)

4,000
12,00010,000
2,000
8,000
6,000
4,00002,000 1962
1967
1972
1977
0
1962 1967 1972 1977 1982 1987 1992 1997 2002 2007

1982

1987

1992

1997

2002

2007

Dredging volume(×104m3)

Figure 2 Maximum annual discharge.

50

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

40

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

30
20
10

0
1972

50
1977 40
30
20
10
0

1982

1987

1992

1997

2002

2007

Figure 3 Dredging volumes in the upstream and downstream of the Fukawa contraction area.2002
1972

1977

1982

1987

1992

1997

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

2007The 10th Int. Conf. on Hydroscience and Engineering (ICHE-2012), Nov. 4 – Nov. 7, Orlando, USA

3

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 levelof 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, from1983 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 since1998.
width (m)500

The Fukawa
contraction area

400

1961

1983

1998

2008

300

200
100
0
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

Longitudinal distance (km)Water elevation (Y.P.m)

Figure 4 Changes in main channel width.

11.0
Sep.1983 flood
marksflood marks
Sep.1983
Sep.1983
observed
Sep.1983 observed
waterlevel water level
10.0Sep.1998 flood
marksflood marks
Sep.1998
Sep.1998
observed
Sep.1998 observed
waterlevel water levelmarksflood marks
Sep.2008
Sep.2008
observed
Sep.2008 observed
waterlevel water level
9.0Sep.2008 flood
8.0
7.0
078.0
74.080.0
76.082.0
78.084.0
80.086.0
82.0 84.0 86.06.0
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 elevation (Y.P.m)

Figure 5 Flood marks and observed maximum water level of each flood.
Dredging stopped in 1966

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

Dredging stopped in 1999

1961
66.0

68.0

70.0

72.0

74.0
76.0
78.0
Longitudinal distance (km)

80.0

1983

1998

82.0

2008
84.0

86.0

Bed elevation (Y.P.m)

Figure 6 Changes in average elevation of river bed.
2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0
-6.0
-7.0
-8.0
-9.0
-10.0

1961
66.0

68.0

70.0

72.0

74.0

76.0

78.0

80.0

1983
82.0

1998

2008
84.0

86.0

Longitudinal distance (km)Figure 7 Changes in deepest elevation of river bed.
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 theerosion and sediment control management and the construction of dams in the upper Tone River inthe 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The 10th Int. Conf. on Hydroscience and Engineering (ICHE-2012), Nov. 4 – Nov. 7, Orlando, USA

4river 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 indicatesthe 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 floodis 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 theabove discussions, the dredging and widening in the downstream of the Fukawa contraction area areclosely related to the river bed degradation near the exit of the Fukawa contraction area from 1983
to 1998.
Bed elevation (Y.P.m)

14.0

14.0

10.0

10.0
6.0

6.0

2.0

1983
1998
the peak water level in Sep.1983 flood
the peak water level in Sep.1998 flood

2.0

-2.0
-6.0

-2.0

-10.0
-100

-6.0
0

100

200

300

400

Lateral distance (m)

-10.0
-100

0

100

200

300

400

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)
Sep.1982 flood
7700
Sep.1998 flood
7600

Cross section area (m2)
2500
2550

Bed elevation (Y.P.m)

14.0
Sheet pile

10.0

Embankment

6.0

Alluvial cohesive soil

2.0

S36

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

-2.0

-6.0
-10.0

1961
S58
1983
H10
1998
2008
H19

Alluvial sandy soil
Diluvial cohesive soil
Diluvial sandy soil

66.0
68.0
70.0
72.0
74.0
76.0
78.0
80.0
82.0
84.0
86.0

-100

0

100

200

300

400

Lateral distance (m)Figure 9 Geologic cross section at 76.5km.
After 1998, although the floods comparable to the design high water discharge have occurred (SeeFigure 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 geologicalfactor in the Fukawa contraction area. Figure 9 shows the geologic cross section at 76.5km. The

The 10th Int. Conf. on Hydroscience and Engineering (ICHE-2012), Nov. 4 – Nov. 7, Orlando, USA5geologic 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 consideredthat 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

Volume percentage finer than (%)

The foregoing section showed that flood flows, channel dredging and widening had an effect onchange 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 ideathat 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.
100
90
80
70
60
50
40
30
20
10
0

100

69.0km
71.0km72.0km73.0km75.0km
76.0km
78.0km80.0km81.0km82.0km
83.0km
84.0km

90
80

70
60
50

40
30
20

10
0

0.01

0.1

0.01

10.1

Grain size (mm)1

10The downstream

The Fukawa
contraction area

The upstream

10

Figure 10 Grain size distribution in 1996.
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 thegrain 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 andamount 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.

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

6

Table 2 Dredging range, volume and implementation year.
1981

1982

dredging location
81.25k right bank78.75k left bank
74.5k right & left bank
74.0k left bank
68.5k right & left bank
68.0k right bank（ｍ３）
30,465
6,850
21,84418,80035,06035,83075.0km

74.5km1983（ｍ３）
32,535
37,11912,321
17,10024,56035,229dredging location
81.5k right bank
74.5k left bank73.75k left bank
71.5k right bank
68.0k right bank
68.75k right bankdredging location
81.0k right bank
75.0k left bank
73.5k left bank69.5k left bank
69.0k right bank
68.0k right bank

75.0km（ｍ３）
41,46043,44019,446
5,020
49,74040,55074.5kmThe dredging area

The dredging area

Elevation (Y.P.m)b) Calculated river bed after the floodsa) Observed river bed in 1983

Figure 11 Observed river bed contour in 1983 and calculated river bed contour after the floods.
Table 2 shows dredging range, dredging volume and implementation year. But, there was no dataof dredging depth and area of each dredging location. We determined the dredging depth and area sothat 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 thefloods 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 thecalculated 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.
Table 3 Widening place name, kilometer post and implementation year.
1982

1983

TorideInba

TorideInba
79.0km72.0km79.0kmBed elevation (Y.P.m)

14.0
12.0
14.0 10.0
12.0 8.0
10.0 6.0
8.0 4.0
6.0 2.0
4.0 0.0
2.0 -2.0
0.0 -4.0-2.0 -6.0
-4.0
-200
-6.0
-200

1981

1980 (before widening)

1983 (after widening)
0

0

200

200

400

600

Lateral distance (m)
400

600

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

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

14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
-2.0
-4.0
-6.0

7

Widening area
1980 (before widening)

1983 (after widening)

-200
200
400
600
Figure 13 Planarchange0in main channel
widthfrom 1981
to 1983.

In the study area, the widening place name, kilometer post and implementation year have beenrecorded as shown in Table 3. But, widening area has not been recorded. In order to determine thewidening 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 ofchannel widening is determined as indicated in Figure 13.
Conclusively, we set the planar area of channel dredging and widening of each year as shown infigure 14.
TorideDredging area
After 1981 flood
After 1982 floods
After 1983 floods

Widening area
After 1981 flood
After 1982 floods

Inba

After 1983 floods

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

Water elevation (Y.P.m)

3.2

11.0
10.0
9.0
11
10
9
8.0
8
7
6
5
7.0
4
3
2
6.0
0
5.0
4.0
3.0
2.0

Toride(85.3km) Obs.
Toride(85.3km) Cal.

Oshitsuke(78.5km) Obs.
Oshitsuke(78.5km) Cal.

Fukawa(76.5km) Obs.
Fukawa(76.5km) Cal.

Suga(66.5km) Obs.
Suga(66.5km) 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.1983
Sep.1983
Aug.1982
Sep.1982
Aug.1981

Figure 15 Comparison between observed and calculated water level hydrographs of the floodsin the Tone River.

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

8

9000
8000

7000

Toride(85.3km) Obs
Toride(85.3km) Cal.

6000

Discharge (m3/s)

5000
9000

Fukawa(76.5km) Obs.
Fukawa(76.5km) Cal.

Takasu(3.5km) Obs.
Takasu(3.5km) Cal.

4000
8000
3000
7000
2000
Tone River
6000
1000
5000 0
-1000
4000
0
3000
2000
Kokai River
1000
0
-1000
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.1982
Sep.1982
Aug.1981
Aug.1983
Sep.1983

Figure 16 Comparison between observed and calculated discharge hydrographs of the floodsin the Tone River and the Kokai River.

Water elevation (Y.P.m)

Figure 1511.0shows the comparison between observed and calculated water level hydrographs of the
10.0
9.0
floods in Tone
River. Figure 16 shows the comparison between observed and calculated discharge
8.0
7.0 of the floods in the Tone River and the Kokai River. The calculated water level andhydrographs
6.0
5.0
discharge hydrographs
correspond
well
with
the observed
water leveland82.0dischargehydrographs.
66.0
68.0
70.0
72.0
74.0
76.0
78.0
80.0
84.0
86.0
Aug.1981 flood marks (left bank)Aug.1982 flood marks (left bank)
Sep.1982 flood marks (left bank)

11.0

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

AUg.1981 flood peak Cal.
Aug.1982 flood peak Cal.
Sep.1982 flood peak Cal.

The Fukawa
contraction area

10.0
9.0
8.0

7.0
6.0

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 elevation (Y.P.m)

Figure 17 Comparison between observed and calculated water surface profiles
at the peak of each flood.
average bed elevation
before floods Obs.
after floods Obs.
after floods Cal.

2.0
1.0
0.0
-1.0
-2.0
-3.0
-4.0
-5.0
-6.0
-7.0
-8.0
66.0

68.0

70.0

72.0

74.0

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

76.066.0
78.0
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.

80.0

82.0

84.0

86.0

Longitudinal distance (km)Figure 18 Comparison between observed and calculated river bed elevation
before and after the floods.
Figure 17 is the comparison between observed and calculated water surface profiles at the peak ofeach flood. The calculation coincides with observed water surface profiles. Figure 18 shows thecomparison 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.

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

Before 5 floods(contour of bed elevation)

9

After dredging and widening in 1982
Bed elevation(Y.P.m)

After dredging and widening
in 1981(contour of bed variation)

After Aug. and Sep. 1983
floodsAfter Aug. and Sep. 1982
floodsAfter dredging and widening
in 1983

Dredging area
Widening area

1981

1982

1983

1982

1983

Scouring

Bed variation (m)Deposition

Figure 19 Calculated bed variation contour in and around the Fukawa contraction areafrom 1981 to 1983.
As a result, we reproduced the bed variation in and around the Fukawa contraction area from1981 to 1983 by using numerical analysis as shown in Figure 19. The more the floods occurred, themore 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 thelocation 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 inriver channel morphology can be analyzed by this method.
CONCLUSION

1. Observed data shows that river bed degradation and scouring in the Fukawa contraction area haveoccurred 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 andbed 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 theanalysis result provided a good explanation for the observed results of bed variation in and aroundthe 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.

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

10

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