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. FukuokaResearch 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 atthe 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 dikesare 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 sandbarswith 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 formsalternate bars in the reach between 13.4 km and 6.0 kmon the alluvial fan. The channel slope in this reachresults in change of flow attacking points at the down-stream of the Aimoto ground sill. The ground silllocated 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 pat-terns, widths and lengths of alternate bars. Flood flowand meander pattern in the Kurobe River were con-trolled by the obliquely installed Aimoto ground sill.At the flow attacking points, a series of longitudinaldikes have been constructed for bank protection since1991 (see Fig. 2).The longitudinal dikes were designedbased on pattern of the alternate bars by the large-scalehydraulic model experiments (Public Works ResearchInstitute 1993). However, since flow attacking pointshave changed after 1995 flood which was the largestflood in the past 40 years, most of the longitudinaldikes 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 revet-ment at downstream of the ground sill was extendedto prevent the outer bank from erosions. The extendedrevetments accelerated scouring of river bed along newrevetments. Therefore, developments of the bed scor-ing along the revetments have brought changes in thepattern of alternate bars.

The process of bed scouring by flood flows alongthe revetments and changes in longitudinal patternof alternate bars were clarified in the Joganji River(Osada et al. 2007). The improvement work of theJoganji River was executed by installing sandbars withboulders for regulating flood flows along the revet-ments (see Fig. 11) (Osada et al. 2012; Koikeda et al.2012).The Sandbar with boulders could navigate floodflow accelerated along revetments and restore originalpattern of the alternate bars in the Joganji River.

In this study, we consider that installing the sandbarswith boulders is effective method to improve acceler-ated 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 obser-vation 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 maintainingfunctions of the existing longitudinal dikes by usingthe sandbars with boulders.

2 FLOOD IMPROVEMENT WORKS OF THEKUROBE RIVER

2.1 Temporal changes in the alternate bars

Figure 1 shows locations of the Aimoto ground silland theAimoto weir.TheAimoto ground sill is slightlydirected 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 to2013. The locations of the revetments and longitudi-nal dikes are shown in these Figures. The longitudinaldikes with 8m in height and 50 m in length of concreteblocks have been constructed at 100 m longitudinalintervals in 1991 (see Fig. 3). A series of longitudi-nal dikes were designed according to patterns of thealternate bars in 1989 channel. Then, geometric char-acteristics of the alternate bars almost agreed with

Figure 4. Maximum annual discharges (Aimoto observa-tion station).

those 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 Fig-ure 2 (a).The effects of the designed longitudinal dikesagainst flood flows were confirmed by the large scalehydraulic 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 5show the observed maximum annual discharge and theobserved discharge hydrographs of July 1995 floodat Aimoto observation station, respectively. The peak

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

discharge of the flood was about 2,400 m3/s and theduration time was a few days.The existing longitudinaldikes worked effectively for the river bank protectionat the flow attacking points against this flood (Kamadaet al. 2001). However, 1995 flood caused the bankerosion about 40m and the river beds scoring about3m of the right bank at 12.6 km. Figure 6 indicatesobserved cross-sectional river bed profiles at 12.6 kmfrom 1990 to 2012.After the 1995 flood, the revetmentof the right bank was elongated to about 300m down-stream 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 scor-ing have gradually extended from 12.6 km to 12.0 kmafter 1995 flood (see Fig. 2 (b) (c)). Therefore, pat-terns of the alternate bars and flow attacking pointshave 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 at10.0 km after 2005 flood. In 2005 flood, changes inpatterns between 12.0 km and 10.0 km caused sharpflow attacks to the longitudinal dikes (see Fig. 2 (c)).Consequently, the longitudinal dikes have not becomeeffective for the bank protection due to changes in flowattacking points.

Figure 8 shows longitudinal and temporal changesin main channel widths which are defined as widthsof water surfaces in the maximum annual discharge(about 1,000 m3/s). The main channel width hasbeen decreasing between 12.0 km and 10.0 km due tochanges in the pattern of the alternate bars after 1995flood.

Figure 6. Cross-section at 12.6 km point.

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

We executed river improvement for restoring originalpattern of the alternate bars and maintaining functionsof the existing longitudinal dikes by several improve-ment works. Figure 9 presents locations of proposedtwo sandbars with boulders to control flood flows flow-ing from theAimoto ground sill.The contour diagramsindicate differences from averaged bed elevations in1993 and 2012, respectively. Installing two sandbarswith boulders at the downstream of theAimoto groundsill were executed in order to reproduce longitudi-nal arrangement of the alternate bars in 1993 whichhad an original pattern. Situations of the sandbarswith boulders at 12.6 km are shown in Figure 10 (a).The height of the sandbars with boulders was deter-mined with heights of surrounding natural sandbars.Boulders about 1,000 mm in size are put in front ofthe sandbars (see Fig. 11 (a)). Figure 11 shows pho-tographs of the sandbars with boulders constructed inthe Jyoganji River. In stony Jyoganji River, Koikedaet 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 chan-nel. 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 bedvariation analysis in stony bed rivers (Osada et al.2013). The Kurobe River has a wide range of particle-size distributions including large stones. Osada &Fukuoka (2013) developed the numerical model oftwo-dimensional river bed variations in stony-bed

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

Figure 8. Longitudinal and temporal changes in the mainchannel width.

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

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

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

where Fξ , Fη = ξ- and η- components of form resis-tance of d90; ND90 = number of d90 on the bed surfacedefined as ND90 = 0.2/α2d2

90; εD90 = shielding coef-ficient 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 onthe bed surface

Average height of each particle size ZPk is related topick-up rate VPk , deposit rate VDk and each particlesize fraction on the bed surface Pk by Equation (2).

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

3.1.3 Estimation of sediment transport rateSediment transport rate of each particle size is evalu-ated by products of the volume of sediment transportVm and particle velocity upk .

The volume of sediment transport per unit of areaVm is calculated by differences of sediment transportrate, pick-up rate and deposit rate in each controlvolume.

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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 , uPηkare calculated by saltation analysis using 3D equationof motions of each particle size. The saltation analy-sis is computed on the bed surface of mean diameter.

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

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

Pick-up rate VPk is calculated as follows:

where ε′Pk = rate of pick up calculated by particle

height distribution; εwsk = rate of pick up controlledby shielding effect of large materials; NPk = numberof particles of each particle size on bed surfaces(= Pk/α2d2

k ); TPk = time required for pick up from bedsurfaces.

Deposit rate VDk is calculated as follows:

where PCk is the rest ratio of each particle esti-mated by the saltation analysis considering particlesize distributions and particle height distributions.

3.2 Calculation conditions

Meander pattern and bed variations of the proposedchannel are compared to those of the current chan-nel. Flood condition for the calculation was July 1995floods. The boundary condition of the upstream endwas given by observed discharge hydrograph of theAimoto observation station (13.4 km) as shown inFigure 13. Sediment supplies at the upstream endwere given by the equilibrium sediment dischargeconditions. The downstream boundary condition wasgiven as longitudinal gradients of velocities and waterdepth being zero. Sediment size distributions in thecalculation conditions were given in Figure 14.

3.3 Calculation results of the proposed channel

Figure 15 shows contour diagrams of calculated dis-charges per unit width at the peak discharge (about2,400 m3/s) in the current channel and the proposedchannel. The calculation result in the current channelindicates that channel straightening along the revet-ment between 13.0 km and 12.4 km causes the changesin the pattern of the alternate bars. And, the longitudi-nal 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 boul-ders improve meander pattern of the main channel by

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 dikeswork for bank protection against flood flows. Fig-ure 16 represents contour diagrams of the calculatedbed variations after the flood. In the proposed channel,bed scouring along the revetments of the right banksbetween 12.8 km and 12.4 km section are mitigated byinstalling the sandbars with boulders.

Figure 17 presents calculated water surface profilesand river bed profiles in the current channel and theproposed channel at downstream of theAimoto groundsill.At the peak discharge (about 2,400 m3/s), the sand-bar with boulders in 12.6 km rise water level between12.8 km and 12.7 km.Therefore, the water surface pro-files of the proposed channel are milder than those inthe current channel. Figure 18 shows calculated cross-sectional bed profiles, velocity distributions and watersurface profiles in the peak discharge at 12.8 km. Thecalculation results of the proposed channel mitigateflood flows concentrated along the right bank. There-fore, two sandbars with boulders at downstream of theAimoto ground sill are found to be effective for restor-ing patterns of the alternate bars and functions of thelongitudinal dikes.

Fukuoka (2010, 2012) proposed Fukuoka’s equa-tions which clarify the relations between dimension-less width, dimensionless depth and dimensionlesschannel-forming discharge in natural alluvial rivers asEquation (7) and (8),

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

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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 wouldnot change smoothly with the increase in flood dis-charges. In contrast, the dimensionless width anddimensionless depth of the proposed channel increasealmost parallel to the Fukuoka’s equations. Therefore,it is confirmed that the proposed channel has geo-metric characteristics of natural stable rivers by theimprovement 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 down-stream of the Aimoto ground sill. The extensions ofthe bank revetments after 1995 flood brought bedscouring along the revetments and caused changesin the pattern and the flow attacking points. As aresult, a series of the existing longitudinal dikeshave not become effective for the bank protection.

2. We proposed flood improvement works for restor-ing patterns of the alternate bars and maintainingfunctions of the existing longitudinal dikes. Twoset of sandbars with boulders reproduced longi-tudinal arrangement of the original pattern in thealternate bars, together with the widening of themain channel width.

3. The two-dimensional flood flow and bed variationanalysis was conducted for investigating effects ofthe proposed improvement works on flood flows.We demonstrated that two sandbars with boulderswere able to divert concentrated flows along therevetments toward the center of the channel. Thepattern of the alternate bars approached to theoriginal alternate bar pattern in the Kurobe River.The existing longitudinal dikes recover functionsfor bank protection by the proposed improvementworks.

4. We confirmed that the relations between dimen-sionless width, dimensionless depth and dimen-sionless channel-forming discharge in the proposedchannel almost corresponded to those Fukuoka’sequation found in stable alluvial natural rivers.

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