E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

River improvement techniques for mitigating
river bed degradation and channel width
reduction in the sandy Hii River where sedimenttransport occurs at normal times
Takahisa Gotoh1,*, and Shoji Fukuoka1
1

Research and Development Initiative, Chuo University, Japan
Abstract. In the sandy Hii River, a large amount of sediment yield fromupper river basin had brought developments of braided channels covered
with sand waves. In the braided channels, sediment materials on the river
beds are capable to move in normal discharge conditions. In recent years,
however, the sediment yield decreases due to constructions of check dams
and ground sills in the upper river basin. Thus, the river beds downstreamof the ground sill have gradually degraded and the main channel widths
have been narrowed with the progressing bed degradation. Firstly, weclarified that the effects of non-equilibrium sediment transports around theground sill during normal discharge conditions on the bed degradation andthe channel width reduction by using annual observed data and numericalsimulations for bed variations. In addition, we provided the river
improvement techniques for mitigating bed degradation and channel width
reduction by improving state of non-equilibrium sediment transports
passing through the ground sill.

1. Introduction
The Hii River is located at Shimane prefecture in Japan and flows into the Sea of Japanthrough the Lake Sinji and the Lake Naka-umi (see Figure 1). The upper basin of the Hii
River had yielded a large amount of sediment composed of fine sand materials. It had
caused great aggradation of the river bed in the lower Hii River until 1960s. Due to the bed
aggradation, the bed elevations of the main channel in the lower Hii River have raised than
the ground level of the urban area along the river. And it has brought flood inundation
disasters around this area in the past years.
Meanwhile, a large amount of fine sediment yield has formed braided channels which
are covered with well developed sand waves. The braided channels in the lower Hii River(around 11.0km) at the normal discharge conditions are shown in Figure 1. In the braided
channels, sediment materials on the river bed are capable to move due to normal discharge
flows since the river bed materials are almost uniform sizes of fine sands which areapproximately 1mm to 2mm in diameter. Figure 2 shows ratio of shear stresses to thecritical shear stress in the braided river section (12.4km) during the normal discharge* Corresponding author: goto510@tamacc.chuo-u.ac.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons
Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

condition. Here, the normal discharge in this section is approximately 50m3/s by the annual
observation records. The ratio of shear stresses to the critical shear stress indicates to beabout 3 to 5 times at the cross-section.

14

Ratio of shear stress ofcritical shear stressagainst mean diameter

7

Water level in normal
discharge flow

5

Survey data in 20126

Elevation（T.P.m)

13.5
13

4
3

12.5

2
12

1

Shear stress / Critical shear stressFig. 1. Location of the Hii River and the braided channels in the river.

0

11.5
0

50

100

150

200

250

300

350

Lateral distance(m)Fig. 2. Ratio of shear stress to critical shear stress in the braded river section.

However, in recent years, the sediment yield from the upper river basin has decreased
due to constructing a series of check dams and ground sills since 1960s. It has caused
rapidly degradation of the river bed from the downstream of the ground sill in the lower Hii
River. The bed degradation has brought to reduce the main channel width in thedownstream of the ground sill. Moreover, the narrowed main channel led to advance thefurther river bed degradation.
In this study, we investigate mechanism of the bed degradation and the channel width
narrowing in the sandy Hii River. In addition, we attempt to provide the idea of river
improvement techniques for mitigating bed degradation and channel width narrowing insandy braided rivers where sediment supplies from upper river basins are almost zero atnormal discharge conditions.

2. Mechanism of river bed degradation and channel width
narrowing
In this section, we investigated mechanism of the bed degradation and the main channel
width reduction in the lower Hii River by using annual survey data, hydraulic records andaerial photographs.
Figure 3 shows the aerial photographs and the changes in the main channel widths since1975 until 2011. Here, the main channel widths shown in Figure 3 were determined byusing the cross sectional shapes (see Figure 4). Figure 4 shows cross sectional shapes at
2

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

17.0km and 18.6km in the periods. In 1975, the braided channels were formed across thefull river width defined by widths between the levees. However, since 1992, the reductionof the main channel widths has begun and gradually progressed toward the downstream
from the Igaya ground sill which was installed at 23.3km. Figure 5 indicates the annualchanges in the observed mean bed elevations of the main channel. Due to the reductions of
the main channel width, the mean bed elevations in the main channel have graduallydegraded from the Igaya ground sill toward the downstream. The bed degradation hasreached around 14.4km point until 2011.

Flow

Flow

Flow

Flow

Igaya ground sill
Main channel width

17k

23
22
21
20
19
18
17
16
15

Channel width until 1985
Channel width since 1998

050

100 150 200 250
Lateral distance(m)

2012
2010
2006
1998
1985
1975
1966
300

Elevation（T.P.m)

Elevation（T.P.m)

Fig. 3. Annual changes in main channel width in the Hii River.
2012
1985

25
24
23
22
21
20
19
18
17

18.6k
2006
1975

1998
1966

1992

Channel width until 1985
Channel width since 1992

0

50

100
150
200
Lateral distance(m)250Fig. 4. Annual changes in cross sectional bed profiles and definition of main channel widths.

Figure 6 indicates the annual changes in observed water levels in the normal discharge
at Kamishima observation station (18.6km) which is located about 5km downstream fromthe Igaya ground sill. The observed data shows that the water levels in the normal discharge
have gradually declined with the river bed degradation. The declining of water levels per
year since 1993 until 2011 is approximately -7.2cm. It seems to be almost independent
from frequency and scale of the past flood events shown in Table 1. Table 1 indicates thehistory of main flood events in the past about 50 years. Figure 7 shows observed crosssectional shapes at upstream and downstream of the Igaya ground sill (23.4km, 23.0km). At
the upstream section of the ground sill, the bed elevations of the main channel in recent
years became lower than the elevation of the ground sill which was 29.05 T.P.m, T.P. :3

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

Tokyo peil, Japanese datum of leveling. Therefore, the ground sill has prevented sediments
transported from the upstream section during the normal discharge conditions and small
scale floods. At the cross section of 23.0km downstream of the ground sill, the river bed in
the main channel was scoured during the period from 2006 to 2010 although large scale
floods did not occur in this period (see Table 1). It means that the river bed degradation has
progressed due to effects of non-equilibrium sediment transports passing across the groundsill during the normal discharge conditions and small scale floods.
On the other hand, in large scale flood of 2006 flood, flood flows were able to transport
sediment materials across the ground sill and the sediment materials were deposited
downstream of the ground sill (see Figure 7). Therefore, we found that the sediment passing
through the ground sill during large scale floods mitigated the bed degradation and the
reduction of main channel widths in the downstream of the ground sill. It was found that
improving the state of non-equilibrium sediment transports passing across the ground sill
was important to mitigate the bed degradation and the channel width narrowing.

Kamishima

Fig. 5. Annual changes in longitudinal distributions of observed mean bed elevations in the main
channel.
30

Water level（T.P.m)

28
26
24
22

Kamishima(18.6k)
Ootsu(12.4k)
Shinigaya(24.1k)

20-7.2cm/year

18
16
14
12
1990/5

1995/5

2000/5

2005/5

2010/5 (year)

Fig.6. Annual changes in observed water levels in normal flow condition.
Table 1. History of flood events in the past 50 years of the Hii River.

Year/Month1971/7
1972/7
1979/101983/91993/93

Peak discharge(m /s)
1,500
2,300
1,600
1,500
1,600

Year/Month1998/10
2006/7
2011/5
2011/94

3

Peak discharge(m /s)
1,700
2,400
1,500
1,500

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018
2010
2006
2005
1998
1992
1966

Elevation（T.P.m)

33
32
31
30

35

Upstream of theground sill (23.4km)

29

Elevation of
Igaya groundsill（29.05 T.P.m)

28
27
0

50

100

150

200

Lateral distance(m)250

Elevation（T.P.m)34

https://doi.org/10.1051/e3sconf/20184002033

30

2010
2006
2005

1975
1966

1998
1992
1985

25

Bed scouring since2006 until 201020

Downstream of theground sill (23.0km)

15Deposition
0 by 50
2006 flood

100

150

200

Lateral distance(m)

250

300

Fig. 7. Annual changes in observed cross sectional shapes of upstream and downstream of the Igaya
ground sill (23.4km and 23km).

3. River improvement techniques for mitigating river bed
degradation and channel width narrowing in the sandy Hii
River
3.1. The idea of river improvement techniqueIn this section, we proposed river improvement techniques for mitigating bed degradationand channel width narrowing downstream of the Igaya ground sill on the basis of themechanism clarified in the section 2. The idea of the river improvement technique wascomposed of lowering the elevation of the ground sill for improving the state of nonequilibrium sediment transports and widening the main channel widths in downstream of
the ground sill. Effects of the proposed river improvement technique were examined byconducting numerical simulations of river bed variations under conditions of a series offlood flows and normal discharge flows.
Elevation(T.P.m)(c) Improvement of the ground sill
Installed elevation of the ground sill
Lowered part of theground sill

30
29.5
29
28.5
28
27.5

0.3m
case1
0

50

case2

23.4k

100
150
200
Lateral distance(m)

250

300

Fig. 8. River improvement techniques.

Figure 8(a) and (b) show the proposed main channel widths and the longitudinal mean
bed elevations of the main channel. The main channel widths and the mean bed elevations
were determined so as to agree with those in 1992 when the reduction of the main channel
was not noticeable problems for the river managements. In addition, for increasing
sediment replenishments from the upstream of the Igaya ground sill, we attempted to lower
the elevation in the central part of the ground sill as shown in Figure 8(c). The width of the5

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

lowered part of the ground sill was determined based on the width of sand bars in 1975
when the river bed profiles were relatively stable (see Figure 8(d)). The height of thelowered part of the ground sill was 0.3m. It was determined by taking into accountelevations of sand bars at the upstream sections of the ground sill and elevations of water
intake facilities upstream of it.
3.2. Calculation method and conditions
The BVC Method, the Bottom Velocity Computational Method[1], which was able toestimate vertical velocity distributions of flows based on the depth integrated model were
employed for simulating flows and bed variations. The vertical velocity distributions in the
numerical model were calculated by using depth averaged horizontal vorticity anddifference between water surface velocities and bottom velocities. The bed variation was
estimated by calculating sediment transport of bed load and suspended load. The bed loadtransport was calculated by using bed load formula of Ashida and Michiue. The suspended
sediment transport was calculated by three dimensional convection–diffusion equation ofsuspended load concentrations. The entrainment rate of suspended load from river bed was
calculated by Kishi & Itakura formula[2].
A series of discharge hydrographs based on the past observation records were given as
the boundary conditions of upstream ends (see Figure 9). The normal discharge of rainyseasons and snowmelt seasons were considered in this simulation. The observed water level
hydrograph in the Lake Shinji was given as boundary conditions of downstream end.
Manning's roughness coefficient in the main channel was 0.027(m-1/3s) which was
determined by using temporal changes in observed water surface profiles during 2013 floodin our previous studies[3] in the Hii River.
The simulation cases were as follows. In Case1, channel widths and bed elevations
downstream of the ground sill were improved as described in Figure 8(a), (b) and thecentral part of the ground sill was lowered as shown in Figure 8(c). In Case2, channel
shapes were same as Case1. But the elevation of the ground sill remained as the current
conditions.

Discharge(m3/s)

10000
1000

Flood

Flood

Flood Flood

Flood

Flood

Flood

Shin Igaya Observation station
Flood Flood Flood

Snowmelt season Rainy season

100
10
1

1st year

2nd year 3rd year

0500

4th year 5th year1000

6th year1500

7th year2000

8th year2500

9th year

10th year

3000

3500 (day)Fig.9. A series of discharge hydrographs given at the upstream end.

3.3. Calculation results
Figure 10 shows the calculated contour of bed profiles downstream of the ground sill ineach case. In Case2, the numerical results show that the bed profile downstream of theground sill was obviously channelized due to the repetitions of flood flows and normal
discharge flows. At the upstream section of the ground sill, the sand bar graduallydeveloped and it gave rise to meandering pattern of the main channel in the downstream of
the ground sill. On the other hand, by lowering elevation of the ground sill shown in Case1,
6

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

the normal discharge flows were able to transport sediment materials smoothly across theground sill without sand bar developments at the upstream section. Hence, the channelized
bed morphology did not form clearly in downstream section of the ground sill. Figure 11
shows calculated cross-sectional shape at 20.6km and 22.9km after the repetition of flood
flows and normal discharge flows for 10 years period. In the calculation results of Case2
which formed channelized river bed morphology, the differences of bed elevations between
main channel and sand bars became larger than those of Case1. And the maximum heightof the differences was more than 2m.
Case1(5th year)

Initial bed profile
24

24

24

Flow

22.9km23Flow

22.9kmZB
Elevation(T.P.m)

21

21

20

20

30
29
28
27
26
25
24
23
22
21
20
19

22

21

21

20.6km20

20

24

sandbar24

22.9km 23

23

22

ZB
Elevation(T.P.m)
30
29
28
27
26
25
24
23
22
21
20
19

Flow

23

23

22

sandbar

Case2(10th year)
24

24

Flow
23

22

24

24

23

22

Case2(5th year)

Case1(10th year)

24

ZB
Elevation(T.P.m)
22

21

21

20.6km
20

20

30
29
28
27
26
25
24
23
22
21
20
19

22
22

21

21

20.6km20

22.9km 23
23

23

Elevation(T.P.m)
ZB

ZB
Elevation(T.P.m)

30
29
28
27
26
25
24
23
22
21
20
19

20

22
22

21

21

20.6km

20

30
29
28
27
26
25
24
23
22
21
20
19

20

Fig.10. Contour of the initial and calculated bed profiles of Case1 and Case2.

Fig.11. Calculated cross sectional bed profiles of Case1 and Case2.

Figure 12 presents the calculated longitudinal distributions of accumulated sediment
discharge during a year. The sediment discharge of Case2 rapidly decreased at thedownstream of the Igaya ground sill. However, in the results of Case1, the sediment
discharge at the downstream of the ground sill was increased by lowering elevation in thecentral part of the ground sill compared with Case2. The calculation results showed that theriver improvement techniques of Case1 were able to mitigate the state of non-equilibrium
sediment transports around the ground sill and channelization and channel incisions
downstream of the ground sill.
As a result, the numerical simulations demonstrated that the lowering elevation in thecentral part of the ground sill were effective for mitigating bed degradation with main
channel width reduction.

7

E3S Web of Conferences 40, 02033 (2018)
River Flow 2018https://doi.org/10.1051/e3sconf/20184002033

Snowmelt season
accumulated sediment
discharge (m3)

Dry season

Rainy season

Case2

Igaya ground sill

Case1
10000

10000

1000

1000

Flood

Total
Igaya ground sill

100

100
20

21
22
23
Longitudinal distance(km)

24

20

21
22
23
Longitudinal distance(km)24

Fig.12. Longitudinal distributions of accumulated sediment discharge for a year in 10th year.

4. Conclusions
In this paper, firstly, we investigated mechanism of the bed degradation and the main
channel width reduction in the sandy Hii River by using annual observed data. Secondary,
we proposed river improvement techniques for mitigating bed degradation and channel
width narrowing. The main conclusions in the study are as follows.
1. The normal discharge flows were able to transport sediment materials on river bed in the
braided channels in the sandy Hii River. Meanwhile, the ground sill prevented sedimenttransports from the upstream during the normal discharge flows and small scale floods.
These non-equilibrium conditions of sediment transports across the ground sill caused
the bed degradation and the main channel width reduction. The channel width reduction
induced further bed degradation in the main channels.
2. The proposed river improvement technique was composed of lowering elevation in thecentral part of the Igaya ground sill and widening of the main channel width based on
the past river conditions downstream of the ground sill. The numerical simulations
applying the BVC Method demonstrated that lowering elevation in the central part of
the ground sill improved the state of non-equilibrium sediment transports over the
ground sill. As a result, we were able to mitigate river bed degradation and channel
width narrowing in the sandy Hii River where sediment transports occurred at normal
discharge flows.
References
1.
2.
3.

T. Uchida, S. Fukuoka : Numerical calculation for bed variation in compoundmeandering channel using depth integrated model without assumption of shallow water
flow, Advances in Water Resources, Vol.72., pp.45-56 (2014)
T. Itakura, T. Kishi : Open channel flow with suspended sediments., J. Hydr. Div.,
106(8), pp.1325–1343 (1980)
T. Gotoh, S. Fukuoka, R. Shibata: Diversion of flood flow associated with a large
amount of sediment transport into the Hii River Floodway and measures reducing
inflow sediment discharge, Journal of Japan Society of Civil Engineers,
Ser.B1(Hydraulic Engineering), Vol.73, No.4, pp.I_895-I_900 (2017)8