River management viewed from long-term changes of the main
channel in the middle reach of Tama River
Katsuhiro Gotoh1, Takahisa Gotoh1, Shoji Fukuoka1
1

5

Research and Development Initiative, Chuo University, Tokyo, 112-8551, Japan
Correspondence to: Katsuhiro Gotoh (gotou-k@tamacc.chuo-u.ac.jp)

Abstract. In the middle reach of Tama River, past massive gravel-excavations and construction of fixed weirs
have caused lack of sediment transport and degradation of the river bed, resulting in the reduction of channel
reaches with characteristic sandbars. We investigated the long-term change in the river channel by the observed
data from 1947 to 2019 and evaluated the Tama River management up to now. Analysis showed that effects of10

river improvement works and foundation height of cross-structures were seen in the response of the main channel
to floods. The height of river bed has been maintained by the cross-structures such as bed protection works,
reconstructed weirs and the groundsills installed at proper longitudinal intervals. This brought sediment supply toriver bed by the increase in the main channel width, and resulted in stabilization of the main channel, recovery ofchannel with characteristic sandbars. Considering the main channel bed stability based on long-term river channel

15change leads to effective river management in harmony with flood control and river environment.

1 Introduction

30 formed, but sediment transport decreased due to massive
gravel-excavations and construction of fixed weirs during

In Japan, floods have become more severe and more

the period of rapid economic growth (1950s to 1960s). The

frequent due to climate change in recent years, resulting inshortage of sediment transport rate caused river bed

serious flood damage and sediment disaster. In order todegradation, narrowing and deeping the main channel,

20 create safe and manageable rivers against large floods, it isimportant to analyse the long-term change of river channel

35 resulting in the reduction in channel reaches withcharacteristic sandbars. In recent years, river improvement

in response to past floods and river improvement works,

works (Shimojo et.al., 2011, Ozawa, et.al., 2013 andand to obtain technical information for the proper river

Ohnami et.al., 2021) has been implemented in the harmony

management.
25 Figure 1 shows the annual changes of the river channel inthe middle reach of Tama River. The middle reach of Tamaof flood control and environment against problems such as
40 formation of unstable channel bed, damage to river
structures and forestation on sandbars.

River has a bed gradient of approximately 1/200 to 1/400,
and is active in sediment transport. In the 1940s, a vast
sandbar channel with a rich natural environment was1

40.0k

41.0k

1947 flood

44.0k

43.0k

43.0k

40.0k

41.0k

River channel excavationfrom 1967-1968
Installationofbed
protection works in 1970(b) Aerial photo in 1974(after the 1974 flood)
42.0k

40.0k

41.0k

39.0k

43.0k
Chuo
expressway
BridgeHino Bridge

Hino Weir

Installationofbed
protection works in 1986

JR
Chuo
Line Bridge

Yaji River

46.0k

37.0k

Yotsuyahonjyuku
WeirTamaohashi
BridgeJR HachikoLine Bridge

45.0k

Yotsuyahonjyuku
Weir(constructed
in 1967)

Chuo expressway
BridgeInstallation of bed protection
works before 1967

44.0k

39.0k

38.0k

Yaji River
Installationofbed
protection works in 1973

3

Asa River

Hino Bridge

JR HachikoLine Bridge

44.0k

42.0k

JR
Chuo
Line Bridge

Tamaohashi
Bridge31982 flood (3,345m /s)

37.0k(a) Aerial photo in 1947 (after the 1947 flood)

1974 flood (3,486m /s)

45.0k

Hino Bridge

JR HachikoLine Bridge

Yaji River

38.0k

JR
Chuo
Line Bridge

45.0k
46.0k

Hino Weir(constructed
in 1959)
46.0k

39.0k

42.0k

Asa River

38.0k
37.0k

(c) Aerial photo in 1999

3

3

3

Asa River3

1999 flood (2,353m /s) 2001 flood (2,165m /s) 2007 flood (3,440m /s) 2019 flood (4,600m /s)
42.0k

40.0k

41.0k

39.0k

43.0k
Chuo expressway
BridgeInstallation
Installationofofbed
bed
protection works in
2005

River channel
excavation in 2013

JR HachikoLine Bridge

Hino Weir

River channel
excavation in 2006

Yaji River
River channel excavationfrom 2015-2016

JR Chuo Line
BridgeRiver channel
excavation in 2006

45.0k

Hino Bridge

44.0k

Installation ofgroundsill in 2015

46.0k

Tamaohashi
BridgeBackfilling of the riverbed
andinstallationofgroundsills from 2014-2016

Reconstruction of
Yotsuyahonjyuku
Weir in 2005
River
channel
38.0k improvementfrom 2002-2004
37.0k

Asa River(d) Aerial photo in 2019(after the 2019 flood)
Figure 1: Annual changes of the river channel in the middle reach of Tama River.

5 In this study, we investigated the long-term change in themain channel response to large floods and continuous river
improvement works in the middle reach of Tama River
from the Hino weir (45.2km) to the confluence point of the
Asa River (37.0km). We focussed on the change in bed
10 height and width of the main channel for the proper rivermanagement. The analysis revealed that the main channel
Figure 2: History of flood events in the past 73 years of the

bed level has been controlled by river improvement works

middle reach of Tama River(maximum observed flow

and foundation height of structures installed across thedischarge in each year).

channel.

2

1954

1974flood
1982flood

1974

1999flood
2001flood

1983

1999

2007flood 2019flood

2002

2008

2019

rock

Foundation
height構造物高of cross-structure
Confluence of Asa River

A series foundation heightof cross-structures

Height of soft

sedimentary
土丹高
Yotsuyahonjyuku Weir

Chuo expressway Bridge

Hino Bridge

JR ChuoLine Bridge

Groundsill

Tamaohashi Bridge

Groundsills
installed
continuously

JR Hachiko Line Bridge

Bed elevation(A.P.m)

gravelexcavations
Hino Weir

84
82
80
78
76
74
72
70
68
66
64
62
60
58
56
54

Before reconstruction
After reconstruction

Exposed section of soft sedimentary rock

45

44.5

44

43.5

43

42.5

42

41.5

41

40.5

40

39.5

39

38.5

38

37.5

37

Longitudinal distance(km)1954

1974flood
1982flood

1974

1999flood
2001flood

1983

1999

2007flood 2019flood

2002

2008

2019

Height of soft
土丹高
sedimentaryrockFoundation
height構造物高of cross-structure
Confluence of Asa River

Yotsuyahonjyuku Weir

Chuo expressway Bridge

Hino Bridge

JR ChuoLine Bridge

Groundsill

Tamaohashi Bridge

Groundsills
installed
continuously

JR Hachiko Line Bridge

84
82
80
78
76
74
72
70
68
66
64
62
60
58
56
54
52

gravelexcavations
Hino Weir

Bed elevation（A.P.m)

Figure 3: Changes in longitudinal distributions of the annual mean bed level of the main channel.

Before reconstruction
After reconstruction

Exposed section of soft sedimentary rock

45

44.5

44

43.5

43

42.5

42
41.5
41
40.5
Longitudinal distance(km)

40

39.5

39

38.5

38

37.5

37

Figure 4: Changes in longitudinal distributions of annual the deepest bed level.

2 Long-term river channel change and mechanism of
bed stabilization of the main channel

2.1 Change in channel bed due to gravel-excavation
and bed foundations of cross-structures

We investigated the long-term river channel change using

20 The 1947 river channel as in Figure 1(a) was a simple vast

floods and channel data in the middle reach of Tama River.

channel with characteristic sandbars because there were no

5 Figure 2 shows the flood history of the Tama River. In the

cross-structures. During the 1950s and 1960s, massive

Tama River, floods exceeding 2,000m3/s have occurred six

gravel-excavations were conducted, and the Hino weirtimes in the past 46 years from 1974 to 2019. No large(45.2km) and the Yotsuyahonjyuku weir (38.2km) were

floods occurred between 1983 and 1999, and the frequency

25 constructed for the water use across the river. Theseof large floods has increased afetr 1999. Figure 3 andchannel improvement works reduced the rate of sediment

10 Figure 4 show changes in longitudinal distributions of thetransport, resulting in the river bed degradation. The riverannual mean bed level and annual the deepest bed level ofbed degradation progressed as flood flows become

the main channel, respectively. Figure 5 shows the annual

concentrated in the main channel formed by gravel-changes of the cross-sectional profiles. Figure 6 shows the

30 excavation, and the river channel became monotonous, as

longitudinal distribution of sediment deposition andshown in Figure 1(b). In the 1960s and 1970s, bed

15 erosion in the bed and bank of the main channel and flood

protection works around the bridge were conducted tochannel bed due to 1974 flood and 2019 flood obtained

protect against scouring around bridge piers.

from cross-sectional survey before and after the floods.

3◆ 1999 flood mark ◆ 2007 flood mark ◆ 2019 flood mark3
3
3
(2,353m /s)(3,440m /s)(4,600m /s)
1999flood
1974flood
2001flood 2007flood
2019flood
1982flood1974

Bed elevation(A.P.m)

84

1983

1999

2002

2008

2017

250
Amount of sediment deposition(m2)85■main channel bed ■main channel bank ■flood channel bed

2019River channel
excavation in 2015

83
82
81
80
79
78
77

Height of soft sedimentary rock76

Longitudinal distance(km)Amount of sediment deposition(m2)

2019

73
72
71

Foundation height of bed protection
works of JR Chuo Line bridge
400

450

Bed elevation(A.P.m)

1983

1999

150
100
50
0
-50
-100
-150
-200
0
-250

Exposed section ofsoft sedimentary rock45 44.5 44 43.5 43 42.5 42 41.5 41 40.5 40 39.5 39 38.5 38 37.5 37(b) 2019 Flood
Figure 6: Longitudinal distribution of sedimentdeposition and erosion due to floods.

500(b) 41.4km(directly upstream of the JR Chuo Line bridge)
1974

200

Longitudinal distance(km)Height of soft sedimentary rock
0
50
100
150
200
250
300
350
Cross sectional distance(m)

74

250

(a) 1974 Flood(No data)

Bed elevation(A.P.m)

2017

45 44.5 44 43.5 43 42.5 42 41.5 41 40.5 40 39.5 39 38.5 38 37.5 37
Confluence of Asa River

2008

River channel
excavation in 2006

75

0
-250

Exposed section ofsoft sedimentary rock

Yotsuyahonjyuku Weir

2002

74

68

-200

Chuo expressway Bridge

1999

75

69

-150

Hino Bridge

1983

76

70

-100

JR ChuoLine Bridge

77

0
-50

Tamaohashi Bridge

1974

50

Foundation height of bed protection
works of Tamaohashi bridge
200
250
300
350
400
450
500
Cross sectional distance(m)(a) 43.8km(directly upstream of the Tamaohashi bridge)
78

100

Hino Weir

0150

JR Hachiko Line Bridge

Backfilling ofriverbed in 2016
50
100
150

75

200

2002

2008

2017

2019

the main channel bed and deposition of sediment on

73
72

sandbars between 1974 and 1983, and the elevation

71
70

15 difference between bed height of the main channel and69
68

sandbar increased. After 1982 flood, there were no floods

67

Height of soft sedimentary rock66

exceeding 2,000m3/s until 1999, so the sandbars were

65
0

50

100

150
200
250
300
Cross sectional distance(m)

350

400

450

hardly flooded. As a result, flood flows were concentrated(c) 40.6km(upstream of Hino bridge)
Figure 5: Annual changes in cross-sectional profiles.

in the main channel, causing further bed degradation of the
20 main channel. The 1999 river channel shown in Figure 1(c)has lost characteristic morphological structures of the riverwith sandbars due to narrowing and deeping the main

2.2 Loss of characteristic sandbars in the channel

channel and brought forestation on immobilized sandbars.

Figure 4 shows that after gravel-excavation in 1974, theriver bed in the main channel degraded and soft
sedimentary rock shown by the brown dotted line was
5 exposed in the main channel. The soft sedimentary rock is

2.3 Channel bed stabilization and recovery of channel
25 with sandbars due to occurrence of large floods andproper river improvement works

easily worn away by gravel movement on it and makes it

After 1999, many large floods such as 1999 flood

difficult for sands and gravels to be deposited (Tadatsu(2,353m3/s),et.al., 2009). Therefore, Figure 3 and Figure 6(a) show thatthe river bed degradation due to 1974 flood (3,486m3/s)

3

2001

flood

(2,165m3/s),

2007

flood

3

(3,440m /s) and 2019 flood (4,600m /s) occurred, and the
30 effects of large floods and river improvement works

10 and 1982 flood (3,345m3/s) was significantly larger in the

became apparent, resulting in bed stabilization of the main

exposed soft rock section than in the unexposed section.
Figure 5 shows that those floods caused the degradation of4channel. The mechanism of bed stabilization of the main
channel is described below.

river morphological structure has become visible in the
40 middle reach of Tama River.

In the section between the upper reaches of the JR Chuo

On the other hand, from the JR Hachiko line bridge toLine bridge and the Yotsuyahonjyuku weir (42.2 toTamaohashi bridge section (44.8 to 43.8km), river bed in

5 38.2km), the river bed degradation in the main channel has

the main channel continued gradually to decline after 2007

been controlled since 1999, as shown in Figure 3. This isflood, as shown in Figure 3. The bed protection works ofbecause bed protection works were installed around the

45 the Tamaohashi bridge (installed in 1986) was installed

Hino bridge (39.9km) and the JR Chuo Line bridgelater than that of the JR Hachiko Line bridge (installed(41.35km), and mean bed level of the main channel

before 1967), and the elevation difference between the two

10 upstream of these bridges has been almost maintained at

bed protection works has been increasing, so the river bed

the foundation height of the bed protection works installed

has continued to decline, as shown by the black dotted line

by a certain interval. In addition, the Yotsuyahonjyuku

50 in Figure 3. As shown in Figure 5(a), the immobilized

weir (38.2km) was damaged by 2001 flood, so thesandbar in this reach was hardly flooded even in the 1999

groundsill was constructed for lowering the foundation

flood exceeding 2000m3/s, indicating that flood flows were

15 height of the weir to improve flow capacity and sedimentconcentrated in the main channel, and the lack of sedimenttransport rate in the main channel (Shimojo et.al., 2009).

supplied from the sandbar banks affected to the river bed

Figure 3 shows that the longitudinal river bed profile of the

55 degradation.

main channel has been controlled by the foundation height

Therefore,of the cross-structures and maintained mainly by a series

implemented in the section upstream and downstream of

20 of foundation heights shown by the black dotted line. The

the Tamaohashi bridge (44.8 to 43.4km) from 2014 to 2016,

bed protection work of the JR Chuo Line bridge (installed

including backfilling of gravels to the degradation reach,in 1973) and that of the Hino bridge (installed in 1970)

60 excavation of immobilized sandbars, and installation ofwere installed before 1974 flood which caused the rivergroundsills at the certain interval to maintain the river bed

bed degradation significantly. The section between thoselevel (Ozawa et.al., 2013 and Ohnami et.al., 2021). In

25 (41.2 to 40.0km) is stable now with a gentle riverbed slope

2019 large flood, the concentration of flood flows to thearound the 1970s.

river

improvement

works

has

been

main channel was alleviated and the main channel bed

The maintenance of the main channel bed level brought

65 level between groundsills is mostly free from theriverain erosion consisting of the sandbar banks due to thedegradation as in Figure 3. These works created the stableoccurrence of frequent large floods since 1999, as shown

main channel and contributed to recover the channel with

30 in Figure 5(b) and (c). This enlarged the main channel

sandbars around 1970s as in Figure 1(d) and Figure 5(a),width and stabilized the main channel bed level bywhich was the main target of river improvement works of

sediment supplied from the sandbar banks. In 2019 large

70 the Tama River.

flood, which was the largest flood in the past, the main
channel banks formed by sandbars were significantly

3. River management considering long-term change ofthe main channel

35 scoured in the section upstream and downstream of the JR
Chuo Line bridge (42.2 to 40.4km), as shown in Figure

In the middle reach of Tama River, long-term data of bed

6(b). Most of the scoured sandbar banks are composed of
sediments deposited after 1974 flood, and recovery of thechanges showed that river improvement works in the past
75 were implemented towards the bed stabilization. The bed
protection works of river cross-structures are functioning5

to stabilize the longitudinal bed profile of the main channel,

resulted in sediment supply from banks to river bed,

and the desirable channel with characteristic sandbars is

stabilization of the main channel and recovery ofrecovering as the main channel bed is stabilized and the40channel with characteristic sandbars.

main channel width extends. In river channel with a stable

3. Considering the main channel bed stability based on

5 bed level, the elevation difference between bed height oflong-term river channel change leads to effective riverthe main channel and sandbars is maintained, which

management in harmony with flood control and river

restricts the forestation on sandbars as in Figure 1(d).

environment.

Thus, information viewed from long-term river channelchange provides important technical knowledge for the
10 rational river management in the future.

45 References

In general, the check of sediment transport rate caused by

Shimojo, Y., Ishikawa, T., and Fukuoka, S. : Towards

cross-structures such as weirs is serious common problems

preventive maintenance of river crossing structure andforchannel in the Tama River system, Advances in Riverflood

control

and

the

river

environment.

Engineering, JSCE, 17, 329-334, 2011.

Reconstructing aging weirs into movable weirs or
15 groundsills, and improving the river channel so that

50 Ozawa, T., Fukushima, Y., Kaizu, Y., Gotoh, T., andexisting bed protection works and groundsills are placed at

Fukuoka, S. : River management in consideration of both

appropriate foundation height and longitudinal intervals, isflood control and river enviroment at the upper reach of thean effective river management technique to improve theTama River, Advances in River Engineering, JSCE, 19,

flow capacity and sediment transport rate in the upstream

471-476, 2013.

20 and downstream reaches and to stabilize the longitudinal

55 Ohnami, H., Shimojo, Y., Gotoh, K., and Fukuoka, S. :bed profile of the main channel as well as conservation of

Response of river crossing structures against scouring of

the river environment.

exposed shales bed during the 2019 october flood,
Advances in River Engineering, JSCE, 27, 129-134, 2021.
Tadatsu, T., Suzuki, K., Uchida, T., and Fukuoka, S. :4 Conclusion

60 Temporal elevation changes in the shale bed and increase

We investigated the long-term change of the river channelin scour depth with sandbar deformation around the weir

25 in response to large floods and river improvement works

during floods, Advances in River Engineering, JSCE, 15,in the middle reach of Tama River and evaluated the Tama249-254, 2009.

River management up to now.
1. Long-term data analysis showed that effects of river
improvement works and foundation height of the cross30

structures were evident in the bed stabilization of themain channel to floods.
2. The river bed height has been controlled by the crossstructures such as bed protection works, reconstructed
weirs and groundsills installed at proper foundation

35

heights and longitudinal intervals, which has caused
waterside erosion of the immobilized sandbars. Thisbrought the increase in the main channel width, and6