THEME C: River Management647

EVALUATION OF THE EFFICACY
OF WATER CHANNEL PLANNING
FOR THE NAGAOKA CITY AREA OF THE SHINANO RIVER
Tsuchiya S.1 and Shoji Fukuoka2
Foundation for River front Improvement and Restoration, Tokyo, Japan2
Department of Civil and Environmental Engineering, Hiroshima University,
1-4-1 Kagamiyama, Higashi-Hiroshima City, 739-8527, Japan,
Telephone 081-824-24-7821, fax number 0824-24-7821
1Abstract: An effective flood-control strategy for mitigating flow attack points in ameandering river with a steep bed gradient and alternate bars is to preserve the floodchanneland stabilize the water course. In the Nagaoka City Area River Plan, main channel alignment
was determined according to the results of hydraulic model tests, and training levees werebuilt as the primary means of stabilizing the main channel. To assess the channelimprovement efficacy of these works over the 25 years that have passed since the project
began, the authors used the results of field observations, model tests based on the adopted
plan, and computational analysis. This paper compares and considers the actual observed
effectiveness of the flood control works in the river and the results of model tests andnumerical analysis with regard to main channel planning, bed evolution, flow attack point
mitigation, and other factors.
Keywords: flood control work, training levee, field observation, hydraulic model test,
numerical computation, flow attack point mitigation, bed evolution

1. INTRODUCTION
The Shinano River has a drainage basin area of 11,900 km2 and is 367 km long, making it oneof Japan’s largest rivers. The river’s Nagaoka section, defined as that between the 15.5 km
and 22.5 km upstream points of the Okozu Diversion Channel, runs through urban Nagaoka
City and so is one of the most important sections with regard to flood control. This is also
where the waters of the Shinano River enter the Niigata Plain after descending throughmountainous terrain. Consequently, the bed gradient is steep and sharply changes in this area,with extensive meandering and longitudinal variation in main channel width and inter-levee
distance, accompanied by alternate bars and deep bed scouring. This section is alsocharacterized by a long period of snowmelt runoff, typical of snow-accumulation regions.
Consequently, river courses have been poorly managed, with numerous flow attack points andextensive damage to revetments and other river structures. Before implementing riverplanning to reduce this flood danger through bed stabilization, hydraulic model tests wereconducted to assess main channel alignment, and methods and procedures for channel
improvement works.1), 2) Based on these results, full-scale work was begun around 1975.
Roughly 10 years later, changes in actual river conditions brought about by the flood control
works were compared to the results of hydraulic model tests3). Another thirteen years later,
further investigation showed that the flood control works had indeed been effective.

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This paper compares and considers the actual observed effectiveness of the flood control
works and the results of model tests and numerical analysis with regard to main channel
planning, bed evolution, flow attack point mitigation, and other factors. In particular, we
compare the experimentally predicted and observed effectiveness of the training levees thatcharacterize this river plan and attempt an assessment using numerical analysis of the flows.

2. RIVER PLANNING FOR THE NAGAOKA CITY AREA
2.1. RIVER HYDRAULIC PROPERTIESThe Nagaoka City Area of the Shinano River is situated in the alluvial fan region of theNiigata Plain, and its alignment is characterized by extensive meandering, particularly at the18.25 km point, near the Chosei Bridge. The bed profile undergoes a sharp change in gradientat the 17.5 km point, from 1:650 upstream to 1:1,200 downstream.
Temporal change in mean bed level exhibited a degrading tendency in the years 1965–1975
but has been fairly stable recently. Inter-levee distance ranges from a maximum of 1,000 m toa minimum of 800 m; the main channel width is 250–450 m.
Upstream, bed material comprises gravel with a mean particle diameter of 40–50 mm andtransitions to sand 0.1–0.5 mm in diameter beginning at the 12.0 km point. The section iscategorized as a Segment 2-1 gravel waterway, intermediate between alternate sandbars anddouble-row sandbars. Flow attack point positions (Figure 1) are nearly stable, and theparticularly severe ones are at Mizunashi (21.5 km, right bank), Shibumi River confluence(19.5 km, left bank), Kusozu (18.0 km, right bank), and Zao (15.0 km, right bank).
The average annual maximum discharge recorded at the Nagaoka Observation Station isapproximately 4,000m3/s (for the years 1978–1998). The average maximum discharge for thesnowmelt season can be as great as 2,400m3/s for as long as 1 month. Snowmelt discharge
accounts for 30–50% of total annual discharge.

Figure 1 Plan view of Nagaoka district and time change in the deepest riverbed locations
2.2. PLANNING, POLICIES AND HYDRAULIC MODEL TESTS
The primary objective of the Nagaoka City Area River Plan is to mitigate flow attack points;
other policies are as follows:a.) Levee alignment is to remain unchanged.
b.) To correct the main channel’s extensive meandering, a compound cross-section is to beutilized, mitigating dangerous attack points and permitting utilization of the floodchannel.
c.) Main channel width and depth should be 320 m and 4.5–5 m, respectively, to handle adischarge of up to 4,000 m3/s, which is expected to occur once every 2–3 years.

THEME C: River Management649d.) The first proposal for main channel alignment is to follow temporal change in the water
course and shift the attack points toward the channel’s center to the extent possible. The
best proposal shall be selected according to the results of hydraulic model test.
Main channel stability was examined by means of hydraulic model tests with a mobile bed
having a distortion scale of 1:100 on the horizontal and 1:70 on the vertical. Testing of thefirst and second proposals for main channel alignment suggested that these main channel
designs would not satisfy the requirements. The third proposed design was indeed found inexperiment to smooth the main channel flow and to greatly alleviate damage to the floodchannel. It was therefore decided to adopt the third proposal and construct a flood channel toprotect the banks from flow attack. The entire project involved constructing revetments anddeepening the main channel to protect the flood channel, and constructing training levees toform the flood channel and mitigate flow attack.
The decision was also made to perform model tests to assess the work sequence in which thevarious elements of the project were implemented, identify the position and range of the flow
attack points, determine the post-completion effects of flow attack mitigation, and assess theconstruction method.
Full-scale work began in 1975 and by 1992 was approximately 70% complete, with nosignificant change in either the work itself or its sequence relative to the initial plan.

3. POST-CONSTRUCTION CHANNEL CHANGES AND COMPARISONWITH THE RESULTS OF HYDRAULIC MODEL TEST
Figure 2 shows peak discharges associated with major floods since 1979. For threeconsecutive years (1981–1983), large floods ranging from 7,000 to 8,300m3/s occurred,compared with the planned flood discharge of 11,000m3/s. There followed a long period of no
significant flooding other than the annual periods of snowmelt runoff. In 1998, however, aflood of 6,700m3/s was recorded. Figure 3 shows changes in the main channel between 1977
(before full-scale improvement works commenced) and 1995 as determined from aerial
photographs. Before the improvement works, the channel bed had complex meanderingalignment and many large sandbars, and so there were flow attack points where the maincurrent directly struck the levees. After the improvement works, in contrast, the main channel
is more distinct, and the flow attack points (e.g., at Mizunashi and Kusozo) have been
mitigated. Figure 4 (a comparison of changes in the water channel and in the cross-sectional
form between 1975 and 1998) shows pronounced mitigation of flow attack, including a higher
bed level near the levee foot at a flow attack point and the movement of deepest bed levelfrom the levee toward the center of the channel.
Figure 5 compares the flow regime as determined from model tests with the actual water
course as it appeared in 1999, and permits the conclusion that the model test in generalaccurately predicted the 1999 channel flow. Differences exist at a detailed level, but these are
attributable to such factors as progress of the improvement works.

4. VALIDATING THE EFFICACY OF TRAINING LEVEES
4.1. TRAINING LEVEE EFFICACY AND SEDIMENTATION
Longitudinally oriented training levees placed in front of flow attack points utilize the forcesof nature to correct the watercourse, maintaining the main channel in a favorable condition
and reducing the current’s velocity behind it. In this section of the Shinano River, training
levees formed the heart of this plan to construct and maintain a flood channel by utilizing thesedimentation that occurs in the long snowmelt runoff period.

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3Discharge (m /s)

650

Figure 2 Discharge of previous floods

Figure 3 Changes in the main channel

Figure 4 Changes in the water course and channel’s cross-sectional shape

Water course in 1998
Flow pattern obtained
from model test

Figure 5 Comparison between hydraulic model test results and measurement data
The training levee structure comprises three layers of two-ton irregularly shaped blocks
placed in 12-m-wide rows on a 20-m-wide fascine mattress. The height was set at slightly lessthan the planned flood level in anticipation of sediment deposition at annual maximum
discharge.
4.1.1. Mizunashi Area (21.0–22.5 km, right bank)
After construction of main-channel revetments on the left bank and groins at the right-bankflow attack points, training levees were built in the Mizunashi Area beginning in 1977.
Sedimentation on the bank side of the training levees eventually formed the flood channel,
where vegetation now grows (Figure 6). This resulted in movement of deepest bed level fromnear the right-bank levee to the main channel’s center.

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THEME C: River Management

4.1.2. Kusozu Area (18.0–18.5 km, right bank)
Work here includes revetments on the main channel’s left bank, underpinning for the piers of
the Chosei Bridge, and deepening of the main channel to alter the watercourse and the flow
direction. Next, training levees were built along the right bank starting in 1983. Work wascompleted in 1992, including aligning the edge of the leave to the bank.
Because of the river–tributary confluence as shown in Figure 7, the training levee was built tocreate a closed, V-shaped area where sediment has accumulated. The flow attack point has
moved farther downstream, and the main flow has remained concentrated in the channel
center, even during recent flooding. Furthermore, the position of deepest bed level has moved
from the right bank toward the center.
Training levee
Main channel revetment
75 778786 85 84 838292 91 90
817678757992787774
77

90

91
80

76

87
86 85 84 83 82
77
74

80 79
77

73
72

75

7376

74

81 80

79

78

79

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77 80

88
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90

71

1974
1988

1974
1988
1999

1999

Distance (m)Distance (m)Figure 6 Effects of the training leveeconstructed in Mizunashi
4.1.3 Zao Area (15.0–16.0 km, right bank)
In 1986–1988, a training levee was built,
and large point bar on the opposite bank was
dug out, altering the water course. Although
the position of deepest bed level remainednear the embankment toe, bed level ingeneral has risen over 2 m, indicating thatthe problem of bed scouring has been
eliminated. Overall, the main flow has
moved toward the channel center (Figure 8).
Dataontemporalchangeinsedimentation behind the training levees
built in the Mizunashi, Kusozu, and Zao
areas (Figure 9) show faster rates ofsedimentation in the years 1975–1985 and1995–1998 (although the training levees
were not all built in the same timeframe).
This is indicative of a significant

Figure 7 Effects of the training leveeconstructed in Kusozu

88 878688
86

92 919088

94868785 83 82 808419741988
1999
Distance (m)Figure 8 Effects of the training leveeconstructed in Zao

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relationship between sedimentation and flooding.

33

Volume of soil deposited (X 10 m )4.2. RESULTS OF NUMERICALANALYSIS
Training levee efficacy was also assessedcomputationally through flow analysis
based on plane two-dimensional unsteadyflow calculations. Figures 10–12 show
the planar distribution of discharge flux
resulting from a simulated discharge of
8,300m3/s (equivalent to the maximum
value recorded after 1945) in two
scenarios: 1974 (before the river
improvement works began) and 1992
(when the works were essentially
Figure 9 Changes with time in volume of soil
completed).
Discharge
flux
wasdeposited behind training levee
calculated as the product of velocity anddepth and so is equivalent to unit-width discharge.
Training levee
Main channel revetment

Figure 10 Numerical analysis results (Mizunashi)

Figure 11 Numerical analysis results (Kusouzu)

Computational results show that in the Mizunashi
area, discharge along the embankments in the1992 scenario was approximately 50% lower thanin the 1974 scenario, mitigating the flow’s attack
force on the embankments. In addition, thelocation of maximum discharge flux near the 21.0
km point had moved from the embankment to thechannel center. In the Kusozu area, discharge atthe embankments was 30–50% lower, indicating
a similar mitigation of flow attack points. In theZao area, discharge along the embankments Figure 12 Numerical analysis results (Zao)decreased roughly 50%, main-channel discharge
increased, and the flow was more rectilinear.
Regarding bed evolution in a period of frequent flooding (1980–1985), Figure 13 shows
actual scouring in the Mizunashi area and computationally obtained bed evolution. The
calculation results in general agree with the observed results, in which the site of extensive
scouring roughly corresponds to the location of maximum discharge flux shown in Figure 10.
In addition, the results of numerical simulation on bed variation were in generally goodagreement with measurement results.

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THEME C: River Management

Actual results

Scouring (m)Training levee
Main channel revetment

Computation results

Figure 13 Bed variations in Mizunashi

5. CONCLUSIONSThe principal conclusions of this paper are as follows.
(1) Field observations clearly showed that the training levees—intended to utilize sedimentdeposition during flooding—were effective in stabilizing the main channel and mitigating
flow attack points.
(2) General agreement with the results of field observations verified that model test is aneffective technique for use in river planning having the objective of main channelstabilization.
(3) Plane two-dimensional unsteady flow computations verified flow attack mitigation. Other
basic characteristics (e.g., flow concentration) revealed by the computations also agreed
with actual bed evolution.

REFERENCES[1] [2] Kyozo Suga and Takeharu Hamatani: “Studies on Main Channel Alignment” and“Riverbed Variation Drawings,” Report on the Results of Hydraulic Model Tests for theWater Channel Improvement Plan for the Nagaoka City Area of the Shinano River (PartI), Technical Memorandum of PWRI, No.786, 1967; (Part II) No.904, 1969
[3] Masakatsu Saito, Hiroyuki Kishida, and Katsuhiko Nagumo: ”The Planning of Main
Channel and Effects of Its Measures in the Nagaoka Area of Shinano River, Proceedingsof the 32nd Japanese conference on Hydraulics, pp.359-364, 1988

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AUTh, Thessaloniki, Greece