River Sedimentation – Wieprecht et al. (Eds)© 2017 Taylor & Francis Group, London, ISBN 978-1-138-02945-3

Study for restoring bank protection functions of longitudinal dikes existingin the river with alternate bars
S. KatoGraduate School of Science and Engineering, Chuo University, Tokyo, Japan

T. Gotoh & S. Fukuoka
Research and Development Initiative, Chuo University, Tokyo, Japan

ABSTRACT: Alternate bars are formed in the main channel of the Kurobe River on the alluvial fan. Patternsof the alternate bars are controlled by the Aimoto ground sill which is directed slightly toward the right bank at
the apex of the alluvial fan. At flow attacking points, longitudinal dikes have been installed for bank protection.
However, the extension of revetments at the downstream of the ground sill brought bed scouring along the newrevetments, resulting in changes in the pattern and the flow attacking points. Therefore, the longitudinal dikes
are not able to work effectively against flood flows. In this study, we propose river improvement works forrestoring pattern of alternate bars and maintaining functions of the existing longitudinal dikes by using sandbars
with boulders and improving main channel. Stability of the proposed channel was investigated by applying thetwo-dimensional flood flow and bed variation analysis.

1

INTRODUCTION

The Kurobe River, Japan, is a steep river and forms
alternate bars in the reach between 13.4 km and 6.0 km
on the alluvial fan. The channel slope in this reachresults in change of flow attacking points at the downstream of the Aimoto ground sill. The ground sill
located at the apex of the alluvial fan is directedslightly toward the right bank for protections of theleft banks (see Fig. 1). The free meander experimentsof Friedkin (1945) and Kinoshita (1957) showed thatthe direction of flow at upstream ends controlled patterns, widths and lengths of alternate bars. Flood flow
and meander pattern in the Kurobe River were controlled by the obliquely installed Aimoto ground sill.
At the flow attacking points, a series of longitudinal
dikes have been constructed for bank protection since1991 (see Fig. 2).The longitudinal dikes were designed
based on pattern of the alternate bars by the large-scale
hydraulic model experiments (Public Works ResearchInstitute 1993). However, since flow attacking points
have changed after 1995 flood which was the largest
flood in the past 40 years, most of the longitudinal
dikes have not been effective for bank protection (seeFig. 2). This flood caused severe bank erosions andbed scouring along the outer bank at the downstreamof the Aimoto ground sill. After the flood, the revetment at downstream of the ground sill was extended
to prevent the outer bank from erosions. The extended
revetments accelerated scouring of river bed along newrevetments. Therefore, developments of the bed scoring along the revetments have brought changes in thepattern of alternate bars.

The process of bed scouring by flood flows along
the revetments and changes in longitudinal patternof alternate bars were clarified in the Joganji River(Osada et al. 2007). The improvement work of the
Joganji River was executed by installing sandbars withboulders for regulating flood flows along the revetments (see Fig. 11) (Osada et al. 2012; Koikeda et al.
2012). The Sandbar with boulders could navigate flood
flow accelerated along revetments and restore original
pattern of the alternate bars in the Joganji River.
In this study, we consider that installing the sandbars
with boulders is effective method to improve accelerated flows along the revetments in the Kurobe River.
Therefore, we propose a river improvement work for

Figure 1. Alluvial fan of the Kurobe Rover (Aimoto observation station) (October 2003).

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Figure 2. Temporal changes in alternate bars of the Kurobe River.

Figure 3. Longitudinal dikes for bank protection.

restoring pattern of the alternate bars and maintaining
functions of the existing longitudinal dikes by using
the sandbars with boulders.

2

FLOOD IMPROVEMENT WORKS OF THE
KUROBE RIVER

2.1 Temporal changes in the alternate bars

Figure 4. Maximum annual discharges (Aimoto observation station).

Figure 1 shows locations of the Aimoto ground sill
and the Aimoto weir. The Aimoto ground sill is slightly
directed toward the right bank for protection of the leftbank. Figure 2 represents temporal changes in patternsof the alternate bars in the Kurobe River from 1989 to
2013. The locations of the revetments and longitudinal dikes are shown in these Figures. The longitudinal
dikes with 8m in height and 50 m in length of concrete
blocks have been constructed at 100 m longitudinalintervals in 1991 (see Fig. 3). A series of longitudinal dikes were designed according to patterns of thealternate bars in 1989 channel. Then, geometric characteristics of the alternate bars almost agreed withthose of natural rivers. The amplitude of the alternatebars were almost river width of the main channel andthe wave lengths were about 1 km as shown in Figure 2 (a). The effects of the designed longitudinal dikes
against flood flows were confirmed by the large scale
hydraulic model experiments (Public Works ResearchInstitute 1993).
The July 1995 flood of the Kurobe River was thelargest flood in the past 40 years. Figure 4 and Figure 5
show the observed maximum annual discharge and theobserved discharge hydrographs of July 1995 floodat Aimoto observation station, respectively. The peak633

2.2 River improvement works for restoring thealternate bars and functions of the existing
longitudinal dikes

Figure 5. Observed discharge hydrographs in July 1995
flood (Aimoto observation station).

discharge of the flood was about 2,400 m3 /s and theduration time was a few days. The existing longitudinal
dikes worked effectively for the river bank protection
at the flow attacking points against this flood (Kamada
et al. 2001). However, 1995 flood caused the bankerosion about 40m and the river beds scoring about
3m of the right bank at 12.6 km. Figure 6 indicatesobserved cross-sectional river bed profiles at 12.6 km
from 1990 to 2012. After the 1995 flood, the revetmentof the right bank was elongated to about 300m downstream at downstream of the Aimoto ground sill asshown in Figure 2 (b). The extension of the right bankrevetment brought river bed scoring along the newbank revetments as shown in Figure 6. The bed scoring have gradually extended from 12.6 km to 12.0 km
after 1995 flood (see Fig. 2 (b) (c)). Therefore, patterns of the alternate bars and flow attacking points
have been also changed by successive floods as shownin Figure 2 (b) and (c). Figure 7 designates the sectionof the bank erosions around the longitudinal dikes at
10.0 km after 2005 flood. In 2005 flood, changes inpatterns between 12.0 km and 10.0 km caused sharp
flow attacks to the longitudinal dikes (see Fig. 2 (c)).
Consequently, the longitudinal dikes have not become
effective for the bank protection due to changes in flow
attacking points.
Figure 8 shows longitudinal and temporal changes
in main channel widths which are defined as widthsof water surfaces in the maximum annual discharge(about 1,000 m3 /s). The main channel width has
been decreasing between 12.0 km and 10.0 km due tochanges in the pattern of the alternate bars after 1995
flood.

We executed river improvement for restoring original
pattern of the alternate bars and maintaining functionsof the existing longitudinal dikes by several improvement works. Figure 9 presents locations of proposed
two sandbars with boulders to control flood flows flowing from the Aimoto ground sill. The contour diagrams
indicate differences from averaged bed elevations in
1993 and 2012, respectively. Installing two sandbars
with boulders at the downstream of the Aimoto groundsill were executed in order to reproduce longitudinal arrangement of the alternate bars in 1993 which
had an original pattern. Situations of the sandbars
with boulders at 12.6 km are shown in Figure 10 (a).
The height of the sandbars with boulders was determined with heights of surrounding natural sandbars.
Boulders about 1,000 mm in size are put in front of
the sandbars (see Fig. 11 (a)). Figure 11 shows photographs of the sandbars with boulders constructed in
the Jyoganji River. In stony Jyoganji River, Koikeda
et al. (2012) proved that sandbars with boulders wereable to direct flood flows concentrated along the riverbanks toward the main channels and were effective forcontrolling flood flows (see Fig. 11 (b)).
Sandbars in the sections between 12.0 km and9.0 km were excavated for widening the main channel. The main channel widths were widened similar tothese of 1989 and 1995 channel (see Fig. 8 and 10 (b)).

3

STABILITY OF PROPOSED CHANNEL

3.1 Calculation method
For investigating stability of the proposed channels,
we conducted the two-dimensional flood flow and bed
variation analysis in stony bed rivers (Osada et al.
2013). The Kurobe River has a wide range of particlesize distributions including large stones. Osada &Fukuoka (2013) developed the numerical model of
two-dimensional river bed variations in stony-bed

Figure 6. Cross-section at 12.6 km point.

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where Fξ , Fη = ξ- and η- components of form resistance of d90 ; ND90 = number of d90 on the bed surface
2
defined as ND90 = 0.2/α2 d90; εD90 = shielding coefficient of d90 (=0.48); ρ = water density; CD = dragcoefficient (=1.0); AD90 = projected area of d90 ;uD90 = flow velocity acting on d90 which is determinedby logarithmic velocity distributions.
3.1.2 Average height and particle size fraction on
the bed surface
Average height of each particle size ZPk is related topick-up rate VPk , deposit rate VDk and each particle
size fraction on the bed surface Pk by Equation (2).

Figure 7. Bank erosion around the longitudinal dikes after2005 flood (10.0 km point).

where subscript i, j indicates computational grid number and k indicates particle numbers; α2 , α3 = twodimensional and three-dimensional shape factors of
the particles (= π/4, π/6), respectively. Each particle
size fraction on the bed surface Pk is estimated bypick-up rate VPk and deposit rate VDk . Average elevation of riverbed ZB is calculated by subtracting average
particle radius from average height of particles.

Figure 8. Longitudinal and temporal changes in the main
channel width.

rivers. Figure 12 shows the numerical analysis procedure of the two-dimensional riverbed variations.
The numerical analysis model was validated by theSatsunai River (Fukuoka 2013) and large-scale field
experiments in the Jyoganji River (Osada et al. 2013).

3.1.3 Estimation of sediment transport rate
Sediment transport rate of each particle size is evaluated by products of the volume of sediment transport
Vm and particle velocity upk .

3.1.1 Unsteady 2-D flood flow analysis
General coordinate systems are employed for flood
flow analysis. Since boulders bring dominant flood
flow resistances in stony-bed rivers, the flow resistances are evaluated by a boulder of diameter d90 as
follows:

The volume of sediment transport per unit of area
Vm is calculated by differences of sediment transport
rate, pick-up rate and deposit rate in each control
volume.

Figure 9. Contour diagrams of bed elevations differences from average bed elevations.

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Figure 10. Cross-sections of the current channel and the proposed channel.

Figure 11. Sandbars with boulders (Jyoganji River, 2012).

Figure 12. Procedure of the 2-D riverbed variation analysis.

Particle velocities of each particle size uPξk , uPηk
are calculated by saltation analysis using 3D equationof motions of each particle size. The saltation analysis is computed on the bed surface of mean diameter.

Longitudinal and cross-sectional bed gradient are considered in the saltation analysis. Restitution coefficient(=0.65) is used for calculating collisions with bed
materials.

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Figure 13. Observed discharge hydrographs in July 1995
flood for the upstream boundary condition.

Pick-up rate VPk is calculated as follows:

where εPk = rate of pick up calculated by particle
height distribution; εwsk = rate of pick up controlled
by shielding effect of large materials; NPk = numberof particles of each particle size on bed surfaces(= Pk /α2 dk2 ); TPk = time required for pick up from bed
surfaces.
Deposit rate VDk is calculated as follows:

where PCk is the rest ratio of each particle estimated by the saltation analysis considering particle
size distributions and particle height distributions.
3.2

Calculation conditions

Meander pattern and bed variations of the proposed
channel are compared to those of the current channel. Flood condition for the calculation was July 1995
floods. The boundary condition of the upstream end
was given by observed discharge hydrograph of the
Aimoto observation station (13.4 km) as shown in
Figure 13. Sediment supplies at the upstream end
were given by the equilibrium sediment discharge
conditions. The downstream boundary condition wasgiven as longitudinal gradients of velocities and water
depth being zero. Sediment size distributions in thecalculation conditions were given in Figure 14.
3.3

Figure 14. Particle size distributions.

controlling flood flows between 13.4 km and 11.0 km.
These meander patterns almost coincide with thoseof 1993 channel and the existing longitudinal dikes
work for bank protection against flood flows. Figure 16 represents contour diagrams of the calculated
bed variations after the flood. In the proposed channel,
bed scouring along the revetments of the right banks
between 12.8 km and 12.4 km section are mitigated byinstalling the sandbars with boulders.
Figure 17 presents calculated water surface profiles
and river bed profiles in the current channel and the
proposed channel at downstream of theAimoto groundsill.At the peak discharge (about 2,400 m3 /s), the sandbar with boulders in 12.6 km rise water level between
12.8 km and 12.7 km. Therefore, the water surface profiles of the proposed channel are milder than those in
the current channel. Figure 18 shows calculated crosssectional bed profiles, velocity distributions and water
surface profiles in the peak discharge at 12.8 km. The
calculation results of the proposed channel mitigate
flood flows concentrated along the right bank. Therefore, two sandbars with boulders at downstream of the
Aimoto ground sill are found to be effective for restoring patterns of the alternate bars and functions of thelongitudinal dikes.
Fukuoka (2010, 2012) proposed Fukuoka’s equations which clarify the relations between dimensionless width, dimensionless depth and dimensionlesschannel-forming discharge in natural alluvial rivers as
Equation (7) and (8),Calculation results of the proposed channel

Figure 15 shows contour diagrams of calculated discharges per unit width at the peak discharge (about
2,400 m3 /s) in the current channel and the proposed
channel. The calculation result in the current channel
indicates that channel straightening along the revetment between 13.0 km and 12.4 km causes the changes
in the pattern of the alternate bars. And, the longitudinal dikes are not effective for bank protection due tothe changes in flow attacking points (see Fig. 15 (a)).
On the other hand, installing two sandbars with boulders improve meander pattern of the main channel by

where Q = channel-forming discharge; B = riverwidth; h = water depth; g = gravitational acceleration;
I = energy gradient; dr = representative grain diameter (d60 ). Figure 19 shows the relations between
Fukuoka’s equation and calculated results of the current and the proposed channel to different discharges
in the flood rising period. The dimensionless widthof the current channel varies abruptly with increasingof flood discharges. This issue means that hydraulic637

Figure 15. Contour diagrams of discharges per unit width.

Figure 16. Calculated bed variations after the flood.

Figure 17. Calculated water surface and river bed profiles in the peak discharge in the current channel and proposed channel.

Figure 18. Calculated cross sectional bed profiles before and after the flood, velocity distributions and water level in thepeak discharge.

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Figure 19. Relationships between dimensionless width, dimensionless depth and dimensionless channel-forming discharge(Fukuoka’s equation).

phenomena during a flood such as velocity would
not change smoothly with the increase in flood discharges. In contrast, the dimensionless width anddimensionless depth of the proposed channel increase
almost parallel to the Fukuoka’s equations. Therefore,
it is confirmed that the proposed channel has geometric characteristics of natural stable rivers by the
improvement works.
4

CONCLUSIONS

The following conclusions were derived in this study.
1. The 1995 flood caused severe bank erosions andbed scouring along the bank revetment at the downstream of the Aimoto ground sill. The extensions of
the bank revetments after 1995 flood brought bed
scouring along the revetments and caused changes
in the pattern and the flow attacking points. As aresult, a series of the existing longitudinal dikes
have not become effective for the bank protection.
2. We proposed flood improvement works for restoring patterns of the alternate bars and maintaining
functions of the existing longitudinal dikes. Two
set of sandbars with boulders reproduced longitudinal arrangement of the original pattern in thealternate bars, together with the widening of themain channel width.
3. The two-dimensional flood flow and bed variation
analysis was conducted for investigating effects ofthe proposed improvement works on flood flows.
We demonstrated that two sandbars with boulders
were able to divert concentrated flows along therevetments toward the center of the channel. The
pattern of the alternate bars approached to theoriginal alternate bar pattern in the Kurobe River.
The existing longitudinal dikes recover functions
for bank protection by the proposed improvement
works.
4. We confirmed that the relations between dimensionless width, dimensionless depth and dimensionless channel-forming discharge in the proposed
channel almost corresponded to those Fukuoka’s
equation found in stable alluvial natural rivers.

REFERENCES
Friedkin, J. F. 1945. A laboratory study of the meandering of alluvial rivers, U.S. Army Engineers Waterways
Experiment Station, Vicksburg, Mississippi.
Fukuoka, S. 2010. River engineering adaptations against
the global warming¯towards generalization of close tonaturerivers, Journal of Japan Society of Civil Engineers,
Ser. F, 66 (4), 471–489, in Japanese.
Fukuoka, S. 2012. Dimensionless width and depth and sediment transport rate in stable rivers, 3rd International
Symposium on Shallow Flows (ISSF), CD-R.
Fukuoka, S. 2013. Study of stable cross-sectional forms
harmonizing the flood control and river environment in
the Satsunai River, Annual Report of River Center of
Hokkaido, in Japanese
Kamada, T., Tsuchiya, S., Nakahira, Y. & Takashima, K.
2001. The longitudinal facility plan of the Kurobe riverand bank erosion prevention effect of longitudinal facility
for medium floods, The Journal of Japan Society of Civil
Engineers, Ser. 45, 787–792, in Japanese
Kinoshita, R. 1957. Formation of dunes on river bed, Transactions of Japan Society of Civil Engineers No.42, pp. 1–21,
in Japanese
Koikeda, S., Ishii, A., Iwai, H., Ishikawa, T., & Fukuoka, S.
2012. Nature friendly bank protection works using sandbars with boulders in the Jyoganji River,Advances in River
Engineering, JSCE 18: 233–238, in Japanese
The Ministry of Construction, Public Works Research Institute, 1993. The Kawamichi special quality of Kurobegawa and channel planning, Public Works Research Institute material; 3139th number, in Japanese
Osada, K., Abe, T., & Fukuoka, S. 2007. Formation mechanism of channels developing along the low water revetment in gravel bed rivers, Advances in River Engineering,
JSCE, 13, 321–326, in Japanese
Osada, K., Fukuoka, S., & Ujiie, K. 2012. Comprehensive
channel planning by the use of sandbars in stony bed rivers,
Advances in River Engineering, JSCE 18: 227–232, inJapanese
Osada, K., & Fukuoka, S. 2013. Two-dimensional riverbed
variation analysis method focused on bed surface unevenness and mechanism of sediment transport in stony-bed
rivers, THESIS 2013, CD-R
Osada, K., Fukuoka, S., & Ohgushi, H. 2013. Study of flood
flow and gravel riverbed variation analysis in the Satsunai
River, Proceeding of 12th ISRS, pp. 589–598.

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