Advances in River Sediment Research – Fukuoka et al. (eds)© 2013 Taylor & Francis Group, London, ISBN 978-1-138-00062-9

How the river improvement works have been conductedin the Lower Tone River and how effective they are
N. IwayaCTI Engineering, Co., Ltd., Saitama, Japan

S. Fukuoka

Research and Development Initiative, Chuo University, Tokyo, Japan

ABSTRACT: In the Lower Tone River, channel dredging and widening have been conducted to accommodate large flood discharge. However, the current river channel has still insufficient capacity for thedesign flood. It is therefore required to implement the appropriate river improvement works. However,effectiveness of the past channel dredging and widening has not been assessed so far. In the present paper,
we carry out the assessment of the past improvement works in the Lower Tone River and clarify byinvestigating changes of channel shape and longitudinal distributions of hydraulic quantities in the river
how the river improvement works have played important roles. The results of the past river improvement
works demonstrated that the sediment deposition tended to vanish away at the downstream from 18.5 km
and bed scouring near the outer bank in the river bends nearly disappear from 40 km to 18.5 km.
1

InTroduction

In the Lower Tone River, channel dredging andwidening have been conducted as one of flood
control countermeasures. As a result, the discharge capacity of the river has increased year by
year. However, the current river channel capacity
has still insufficient for the present design flood
discharge. The sediment deposition near the innerbank of river bends and the large bed scouring atouter banks tend to be marked. It is importantto understand effects on changes of the river bed
elevation in the past river improvement works
and to make it the proper use for the present rivermanagement.
Moro et al. (2011) investigated the effects ofthe channel dredging from Sawara (40.0 km) toriver mouth (0.0 km) in relation to changes inmain channel widths and cross section areas.
Iwaya & Fukuoka (2012) estimated influences of
the channel dredging, widening and flood eventson the changes in the river channel between 1981
and 1983 around Fukawa contraction area (from85.5 km to 66.5 km) from the result of analyses offlood flows and bed variations. However, effects of
the river improvement works have not been investigated from long-term viewpoints and also in many
river sections except at Fukawa contraction area.
In this study, we investigate the past Lower Tone
River improvement works and clarify how theirimprovement works have had effects on channel

cross-sectional shape and longitudinal distributions of hydraulic quantities, such as shear velocity and relative depth (flood channel depth/main
channel depth).
2

The CHRONICLE of the River
improvement works in theLower Tone River

Figure 1 shows study area in the Lower Tone River.
The river improvement works in this area have been
conducted continuously from Tone River Improvement Plan made in 1900 to the basic policy forriver improvement established in 2006. Figure 2
shows changes in the design flood discharge, theperiod, sections and sorts of the river improvement
works. Figure 3 shows an aerial photo displaying
river condition from 16 km to 8 km in 1948.
After the enactment of the river law in 1896, thechannel dredging and widening have been conductedin the Tone River, and the embankment has been
constructed in primary (1900) Tone River Improvement Plan from 40 km to 20 km and ­secondary(1907) from 85.5 km to 40 km (See Fig. 2b). This
construction work played an essential role to lay afoundation for the current river channel.
Tone River Expansion Plan made in 1939 wasreviewed by the disaster of Typhoon Kathleen in
1947. In Tone River Revised Improvement Plan,
the design flood discharge was increased as shown

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Figure 1. Study area in the Lower Tone River.

Figure 2. Changes in the flood control plan and the river improvement works.

Figure 3. Aerial photo from 16 km to 8 km in 1948.

in Figure 2a. But, it was difficult to expand the
Fukawa contraction area (from 78 km to 76 km)
and its downstream. Accordingly, the constructionof the Tone River floodway and capacity expansion of the Sugo, Inadoi and Tanaka retarding
basins in the upstream of the Fukawa contraction
area have been planned from Tone River Revised
Improvement Plan established in 1949 to thebasic plan for the implementation of construction
work established in 1980. In Tone River Revised520

Improvement Plan (1949), the channel widening
has also been conducted from 40 km to 18.5 km so
as to increase the discharge capacity. On the other
hand, the main channel was about 1200 m wide atthe downstream from 18.5 km, where the sedimentdeposited considerably as shown in Figure 3. Thus,
the channel dredging was conducted and flood
plains were created by using dredged sediments.
However, the channel dredging has not been conducted very so by the saltwater intrusion due to theunusual drought occurred in 1958 and 1959. To
prevent saltwater intrusion, the river mouth weir
was constructed at 18.5 km in 1971. Afterward,
the channel dredging was resumed in the downstream of the river mouth weir. In the upstream
from 40 km, the channel dredging has been continuously conducted for the river improvement andfor need of the construction material. However, theriver bed elevation was lowered below the design
bed elevation in the Fukawa contraction area,
and therefore channel dredging there stopped in1966 (the area enclosed by purple line in Fig. 2b).
In the basic plan for the implementation of construction work (1980), the design flood discharge
was increased because of the progress of improvement works in the Upper Tone River as shown in­Figure 2a. The channel widening and dredging

have been conducted at the upstream from 40 km
until around 1998, while the channel widening was
not conducted at the downstream from 40 km forassuring the safety of levees (the area enclosed by
red line in Fig. 2b).
The basic policy for river improvement established in 2006 determined the shift of construction
location of the Tone River floodway to the downstream of the Fukawa contraction area because
the floodway planned in the upstream of the
Fukawa contraction area brought a large impact
on the regional communities. As a result, thedesign flood discharge was determined 10,500 m3/s
in the upstream and 9,500 m3/s in the downstreamof the branched section of the Tone River floodway (See in Fig. 2b).
3

The relationship between
river improvement works andchanges in river bed elevation

On this section, we investigate the chronologicalchanges in the river bed elevations and river crosssections by the river improvement works.
Figures 4 and 5 show water surface profiles
at the peak discharge of the floods occurred in

Figure 4. Water surface profiles at the peak of each flood and changes in average river bed elevation.

Figure 5.

Changes in main channel width and total channel width.

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Aug. 1959, Sep. 1972, Sep. 1983, Sep. 1998 and
Sep. 2007 as well as changes in the average river
bed elevation and changes in main channel width
and total channel width, respectively. Figures 6
and 7 show river bed elevation contour in 1961 and2007 before and after the river improvement works
and changes in the lowest bed elevation, respectively. Large bed scourings are seen at the placesindicated by red color in Figure 6 and the area
shaded by green color in Figure 7. Here, we define“large bed scouring” as the bed scouring near thebank and places of relatively low elevation of river
bed seen in the longitudinal profiles of the lowest
bed elevation in Figure 7. Plan shape of the Lower
Tone River has changed markedly between 1961
and 2007 as shown in Figure 6.
Okada et al. (2002) showed the characteristic
diagram of plan shape of the meandering compound channel in Figure 8, in which the meandering channel is assumed to be described by sine
generated curves in Figure 9. Here, bmc, Amp and L
are main channel width, meander amplitude andFigure 6. River bed elevation contour in 1961 and 2007 in the Lower Tone River (from 80 km to 31 km).

Figure 7. Changes in the lowest bed elevation.

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wave length, respectively. Figure 8 shows that threeparameters 2Amp/L, bmc/(2Amp + bmc) and bmc/L
are important to express plan shape of meandering channels. The higher the horizontal axis bmc/
(2Amp + bmc) is, the more features of flood flow
approach features in the straight channel. Thus,
both abscissa values bmc/(2Amp + bmc) and curvedlines bmc/L vary with the main channel width bmc.
Table 1 shows physical quantities of plan shapes
in 1961 and 2007. The values of bmc/L in 1961 and2007 shown in Table 1 are found to correspondwith the values of a family of curved lines bmc/Lshown in Figure 8. We can evaluate changes in plan
shape of the Lower Tone River by using Figure 8.
Figure 10 shows maximum annual discharges at
Toride (85.3 km) and Fukawa (76.5 km) observation points from 1959 to 2007, and the broken lines
indicate the design flood discharge at the observation points. In this area, large floods occurred inAug. 1959, Sep. 1972, Sep. 1983, Sep. 1998 and
Sep. 2007. In order to clarify the interrelationshipbetween the river improvement works and changes
in river bed elevation, we investigate longitudinal
distributions of cross section area, shear velocity
and relative depth Dr (flood channel depth/main
channel depth) in the peak flood level of large
floods.
From 1961 to 1980, the channel dredging has
been conducted at the upstream from 40 km.
The amount of sediment transported from the
upstream area has reduced due to the erosion andsediment control and the construction of dams in
the Upper Tone River in the 1960s. Therefore, river
bed degradation occurred as shown in ­Figure 4.
The channel widening was conducted from 40 km
to 18.5 km in Figure 5. As a result, the peak waterlevel in the Sep. 1972 flood (peak discharge:
6300 m3/s) was lower than that in the Aug. 1959

Figure 8. Changes in non-dimensional plan shapeparameters before and after river improvement works.

Figure 9. Definition of plan shape parameters.

Table 1. Physical quantities of plan shapes in 1961 and 2007.
1961

2007

L
(m)

2Amp
(m)

bmc
(m)

bmc/L

L
(m)

2Amp
(m)

bmc
(m)

bmc/L

80.0 km∼69.0 km
69.0 km∼62.0 km
66.0 km∼58.0 km
62.0 km∼52.5 km

8830
6500
6750
8500

3040
1330
1580
2350

260
260
280
290

0.029
0.040
0.041
0.034

8830
6500
6750
8500

2850
1200
1460
2350

290
360
350
360

0.033
0.055
0.052
0.042

47.5 km∼38.5 km

7670

1460

300

0.039

7670

1420

370

0.048

43.0 km∼33.5 km

8200

1200

330

0.040

8200

1080

440

0.054

38.5 km∼26.0 km

12000

2340

320

0.027

12000

2130

450

0.038

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Location of large bed
scouring
Sannbannwari (72.5 km)
Fujikura (67.0 km)
Yako (63.5 km)
Ryusaku (58.5 km),
Namekawa (56.5 km)
Kokuno (42.0 km),
Sawara (40.0 km)
Mukozu (38.0 km),
Tsunomiya (37.0 km)
Kusabayashi (33.0 km)

Figure 10. Maximum annual discharges.

Figure 11. Changes in cross section area.

Figure 12. Changes in shear velocity.

flood (peak ­discharge: 5800 m3/s) at the upstream
from 20 km as shown in Figure 4.
Figure 8 shows that characteristic values vary
greatly by the channel widening from 1961 to 2007
in each meandering section of Mukozu (38 km),
Tsunomiya (37 km) and Kusabayashi (33 km).
Especially, bmc/(2Amp + bmc) vary more greatly than
2Amp/L. In the downstream of 40 km, differenceof the river bed elevation between 1961 and 1972
is small. Local bed scouring is mostly dependent
on plan shape. In the meandering section whichbmc/(2Amp + bmc) vary greatly, the plan shape ofmain channel changed as indicated in Figure 6,
and the flood flows have behaved like flows in thestraight channel. From this reason, it is consideredthat bed scouring near the outer bank in the river
bend nearly disappeared in the downstream from40 km because of the decrease of the secondary

flow intensity. Figures 11 and 12 show longitudinal distributions of cross section area and shear
velocities at peak water level of the each flood,
respectively. The cross section area of the Aug.
1959 flood was different between upstream anddownstream of 20 km. At the downstream from
18.5 km before 1961, the sediment deposition
was considerable as shown in Figure 3. Becauseof channel widening from 18.5 km to 40 km afterAug. 1959 flood, the difference of cross section
area between upstream and downstream of 20 km
has reduced in Sep. 1972. The distribution ofshear velocities has become uniform at the downstream from 40 km. Therefore, it is considered thatthe sediment deposition tended to reduce at thedownstream from 18.5 km after 1961 (See Fig. 4).
On the other hand, the difference of main channel width between upstream and downstream of524

Figure 13.

Changes in relative depth (flood channel depth/main channel depth).

40 km has increased in 1972 by the channel widening from 40 km to 18.5 km. Accordingly, shear
velocities at the upstream from 40 km have been
greater than at the downstream. For this reason,
it is considered that the river bed aggradation
occurred from 40 km to 18.5 km since 1972.
From 1980 to 1998, the river bed degradation
occurred and main channel width widened at
the upstream from 40 km by the river improvement works. Since 1998, the variations of river
bed elevation and main channel width are relatively small (See Figs. 4 and 5). Consequently,
the peak water level in the Sep. 1998 flood (peakdischarge: 8600 m3/s) was lower than that in the
Sep. 1983 flood (peak discharge: 7800 m3/s). On
the other hand, it was much the same as thepeak water level in the Sep. 2007 flood (peak discharge: 7200 m3/s) as shown in Figure 4. Due tochannel widening at the upstream from 40 km,
the difference of the main channel width between
upstream and downstream of 40 km disappearedin 1998. As a result, longitudinal distribution ofcross section area has become nearly uniform in
the Lower Tone River since the Sep. 1998 flood,and longitudinal variation of shear velocities has
become smaller.
The characteristic values at the upstream
from 40 km indicated in Figure 8 do not vary
so much except for the meandering section of
Fujikura (67 km) because the widening width at
the upstream from 40 km is smaller than that at
the downstream from 40 km. From this, it is supposed that change in plan shape hardly has aneffect on flows. But, the bed scouring progressed
greatly at the upstream from 40 km since 1980
as shown in Figure 7. This reason is discussed asfollows. F­ igure 13 indicates longitudinal distribution of relative depth Dr of each flood. Here, Dr
is defined as the ratio of flood channel depth tomain channel depth. As a result of channel dredging and widening at the upstream from 40 km,
relative depth Dr gradually decreased and is distributed around 0.3 since Sep. 1998 flood. Thismeans that the simple meandering channel flow
became dominant at the upstream from 40 km(Fukuoka, 2005). And, the ratio of main channel
width to total channel width increased as shown in­Figure 5. For this reasons, the sediment transport
rate increased as the flood discharge in the main
channel increased, and therefore it is consideredthat the large bed scouring occurred at the upstream
from 40 km (Okada et al., 2002). ­Additionally, thelarge bed scouring in the ­meandering section of
Fujikura (67 km) occurred near the inner bank of
the river bend because the feature of flood flow
has approached that in the straight channel as bmc/
(2Amp + bmc) increases (See Fig. 8).
4ConClusions

In this study, we investigated the chronologicalchanges of the river improvement works in theLower Tone River and clarified how the river
improvement works have given effects on changes
in the river bed elevation by investigating thechanges of channel shape and longitudinal distributions of hydraulic quantities.
Main conclusions were drawn as follows.
1. From 1961 to 1980, the channel widening has
been conducted between 18.5 km and 40 km.
As a result, distributions of cross section areaand shear velocities have become almost uniform at the downstream from 40 km. There is aclear distinction of cross section area and shear
velocities between upstream and downstream of
40 km. Therefore, the aggradation disappearedat the downstream from 18.5 km and occurredfrom 40 km to 18.5 km.
2. In the meandering section downstream from40 km, the flood flow features have approachedthose in the straight channel due to the channel
widening. Bed scouring near the outer bank in
the river bends nearly disappeared as the secondary flow intensity decreased.
3. From 1980 to 1998, the channel widening anddredging have been conducted at the upstream
from 40 km. The sediment transport rateincreased as the flood discharge in the main
channel increased because the relative depth

525decreased and the ratio of main channel width
to total channel width increased. For this reason,
the bed scouring progressed greatly upstream
from 40 km.
References
Fukuoka, S. 2005. Flood Hydraulics and Channel Design,
Tokyo: Morikita Publishing Company.
Iwaya, N. & Fukuoka, S. 2012. Changes in river bed
around the Fukawa contraction area by floods andchannel improvement works in the Lower Tone River,
Advances in Hydro-Science and Engineering, ICHE,
Vol. 10.

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Okada, S. & Fukuoka, S. 2002. Characteristics of flood
flow, sediment discharge and bed topography in compound rivers, Proceedings of JSCE, JSCE, No. 754,
pp. 19–31.
Shirai, K. & Fukuoka, S. 2011. Changes of Tonegawa
River channel by river improvement since the Meiji
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Vol. 12, pp. 217–222.

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