Field Study on Bank Erosion due to a Crank Flowin a River Channel which has
Divergence Convergence Meandering System
Tomokazu MISHINA* and Nyosen SUGA***Chief-Eng. Chuohteck. Co. Ltd, 310-0902, Watari3082, Mito city, Ibaraki, JAPAN** Prof. Emeritus, Univ. of Utsunomiya, 276-0023, Katsutadai4-2-4, Yachiyo city, Chiba, JAPANI.

INTRODUCTION

This paper deals with a new concept of bank erosion in ariver caused by bank attacking concentrated flow at sharp angle
due to crank flow as was introduced in Ref. [1] and [7].
Definition and characteristics of crank flow are shown below.
Crank flow is a flow pattern which has plain shape of crank.
And after development, crank flow becomes to a bank attacking
concentrated flow at sharp angle to the bank, and brings large
scale bank erosion. Development process of crank flow isdivided into two categories; the one by natural condition and theother by artificial condition.
This paper treats the case of crank flow developed by the
deformation of morphological meandering shape divergence
convergence channel system. This kind of crank flow occurs
frequently according to the increase of artificial actions recently.
Divergence-convergence meandering system which was also
introduced in Ref. [5] and [6] develops under the condition of
bed material sifting during flood flow. And the phenomena aremainly characterized to rivers which have graded large bed
materials. In this case, formation of jam-up deposit of largestones promote to make flat and wide channel up-stream, andalso steep and narrow channel down stream.
Practically in many rivers in Japan, the developments ofvarious types of crank flow are recently becoming remarkable.
As for the damages, the suffering cases of bank erosion causedby crank flow increase quantitatively and some of then arerecognized beyond sufferance qualitatively. For example, lengthof damage of bank erosion exceeds 100m (about one-third ofchannel wide) laterally in a flood time (one day) occurred in
Kinu-river 2001.
In this paper, main causes of development and theclassification of crank flow are discussed based on field
investigation in such rivers as Kinu and Watarase River. In thoserivers, typical strong bank attacking concentrated flows at sharp

angle to the bank due to the development of crank flow areappearing, according to the degree of deformation ofmeandering system mainly by artificial action. In this case, themeandering system in the fan shape land of Kinu and Watarase
river is characterized by the divergence convergencemeandering channel system.
Thus, the recent remarkable bank erosion caused by crank
flow in a river which has divergence-convergence system is
elucidated based on field study. Results of this study are
expected to contribute to practical application and offer
suggestions to farther development of the study. And also as
countermeasures, mitigated relieved method and restoration of
the channel meandering system are proposed besides the hither
to direct protection as revetment method.
II.

ACTUAL CONDITION OF CRANK FLOW

A General feature of Crank flow
Three items are mentioned as main characteristics of crank
flow as follows. a) plane shape of thal-weg is crank-like. b)bank attacking concentrated flow at sharp angle to the bank. c)crank flow brings large-scale bank erosion. Typical example ofcrank flow is shown in Photo. 1. And also, Ref. [1] and [7]shows that characteristics of crank flow and introduction ofseveral types of crank flow.
Development process of crank flow is divided into twoclassified categories based on the natural condition and artificial
condition. As natural conditions, meandering river at mountainalluvial land is mentioned as an example. In this case, theimportance of existence of many large stones is indicated as thatthe channel meandering is easily occurred where jam-up depositof large stones and the local scour around it develop, or in thecase of gravel bed without large stones, crank flow is formed asstraight flow from edge to edge of rock bank in the width offorced meandering alluvial land. Above-mentioned results were
Aerial photo, 1974

6k
8k

500m

Flow

Photo 1. A typical example of crank flow (Down stream alluvial reaches of Naka river )Velocity near the river bank
(Before and after the development)

Velocity V

Velocity at the center of the channel(Before and after the development)

Discharge Q or Water surface level H

Figure 1. Change of velocity pattern(Before and after the development of deformation of meandering system)
4,000

Observatory: Sanuki (107km)
3,000

④

③

⑤

2,000

1,000

0
1961
1963
1965
1967
1969
1971
1973
1975
1977
1979
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001

Maximum dischargeof one year (m3/s)
年最大流量（m3/s）

obtained by the field investigation of natural meandering pattern
at mountain alluvial land in Yosasa-river, as stated in Ref. [2]and [3].
As artificial conditions for the development process of crank
flow deformation of the meandering features the construction
by channel works in the divergence-convergence meanderingriver is mentioned. The river bed degradation is an alternate bar
channel is also one of the important additional causes as shownin photo. 1.
In this paper, the cases of divergence-convergence channels
where the development of crank flows are remarkable areinvestigated. The objected rivers are Kinu-river and
Watarase-river, tributaries of the Tone River. Basic structure ofdivergence convergence channel system is described later.
Concerning the power of crank flow for bank erosion,
relationship between discharge/water depth and velocity was
shown in Fig.1.
Velocity increased near the bank occurs even during small
flood when the water level is below the surface of sand bars,
under the condition of development of deformation ofmeandering system. Those phenomena occur, corresponding to
middle scale flood and also decreasing term of discharge in big
flood , in accordance with the development of bed degradationof the thal-weg. But, the velocity near the bank is moderate, andthe maximum velocity during large scale flood appears at thecenter of the channel laterally, before the deformation ofmeandering system occurs, maintaining the original two lane
meandering channel pattern. As stated in Ref. [4], two lane
meandering system is stable under the natural condition, andeven big flood flows do not exert influence on river bank due tothe moderate velocity near the bank.
But recently, meandering pattern is gradually deforming.
Among various way of deformation, primary item is thedevelopment of flow velocity near the bank and the extension ofthal-weg to the down system side along and in front of the
newly constructed straight works. This brings the developmentof extension of distinct straight channel along and in front of thebank which is protected by rigid revetment. Though there aremany causes for the change of channel pattern such as dredging,decrease of sediment run off, change of bed material, change offlood scale and flood duration, or vegitation, construction ofstraight revetment, among them, is considered to be the directcause by artificial action. Concerning extension of the straightthal-weg, the meandering shape deforms, abrupt change of flow
direction occurs in the thal-weg to the other side of the bankacross the river, and the bank attacking concentrated flow at

Figure 3. The rate of maximum discharge in Kinu-river(No data shows non-observation value.)

sharp angle to the bank develops.
B Relation between the Revetment and Crank flow
Fig. 2 shows relationship between extension of revetment
and existent of crank flow in Kinu-river. The number of existentcrank flow is in increasing tendency. 14 crank flows existed as
maximum in 2002. After the war, large-scale disaster restoration
works were enforced 5 times as, 1) 1950. 2) 1958-1960. 3)
1966-1967. 4) 1977-1982. 5) 1990-1995. And the maximum
rate of flood discharge in those case are shown in Fig. 3. Resultof decipherment, shows the increasing of the existence of crank
flow after restoration works, and number of crank flows⑤

Right bank
Left bank
Number of Crank flow

20
15

20

④

15
③

10

10

②

5

5

①

Figure 2. Cumulative extension of revetment works ofand Existent number of crank flow in Kinu-river(Section: 83-101.5k (L=18.5k), Existent number of crank flow is judged from Aerial photo.)

2001

1998

1995

1992

1989

1986

1983

1980

1977

1974

1971

1968

1965

1962

1959

1956

1953

0
1950

0

Existent number of crank flow

25

1947

Cumulative extension of
construction of revetment (km)25

gradually decreases afterwards in each case. Furthermore
number of existent crank flow increases again after thelarge-scale disaster restoration works.
C Suffering case by Crank flow
Examples the suffering cases in Kinu-river due to crank flow
are picked up. The location belong to the reach where bed slope
abruptly changes from 1/500 to 1/1,500, longituanaly. The sizeof bed material also changes from stones to the sand and gravel.
Since the channel stability is not enough in this reach, lateralfluctuation of the thal-weg is remarkable compared to the
upstream reaches. Photo.2 shows the development of the crank
flow between 1994 and 2002, and two pictures of bank erosionof 30m lalerally, 250 m longitudinally, caused by bank attacking
Local photo, 2003

Wreckage of revetment

concentrated flow during a small flood in 2003. Photograph lot
and direction are shown in the serial photograph below right.
In this case, main cause of development of crank flow is
considered to be the influence of construction of straight
revetment at the left bank. Flow angle at the right bank changed
a little and bank materials are also loose composed of sands,
gravels and stones. Disaster outbreak first by destruction ofexisted old revetment, and then erosion of loose bank. The
wreckage of old revetment is seen in the picture above left.
D Velocity of Crank flow
Concerning velocity of crank flow at bank attacking
concentrated flow does not comprehend sufficiently whichsome case rivers of really existence of crank flow. Therefore,
250m

Aerial photo, 1994

30ｍ

Flow
↑Nakazima bridge①①

Aerial photo, 2002

250m

Local photo, 2003

Relative height:5m
Sand and gravel

1994
②
100°
Flow
↑Nakazima bridge

②

250m

Extension of thal-weg

Photo 2. Development of crank flow between 1994 and 2002,
and river bank erosion after suffering (2003) in Kinu-river
Kinu river

Watarase river[ Aerial photo, 2003 ][ Aerial photo, 2003 ]Kinou Bridge

Ｂ

Bank protection whichＡof a crowd groin

Kobayashi bridge

Bank protection of concrete block

標高（Y.P.m）
Altitude(Y.P.m)Kinu river [A]235

Bed slope : 1/150
Bed slope along the crank flow : 1/60
Manning’s roughness coefficient : 0.035
Water depth at middle scale flood : 2 m
Velocity of bank attacking
concentrated flow : 5.9 m/s

⊿Z=4m

1

230

Bed slope : 1/150
Bed slope along the crank flow : 1/40
Manning’s roughness coefficient : 0.035
Water depth at middle scale flood : 2 m
Velocity of bank attacking
concentrated flow : 7.2 m/s

105
標 高（Y.P.m）
Altitude(Y.P.m)240

100

Watarase river [B]

95

⊿Z=4m

60

1
40

225

90
0

50

100

150

200

250

Distance横断距離（m）
from the river bank (m)

300

350

-350

-300

-250

-200

-150

-100

-50

Distance横断距離（m）
from the river bank (m)Figure 4. River bed topography along the crank flow [ Cases of the Kinu-river and Watarase-river ]0Plain shapefield investigation of crank flow is enforced in Kinu-river and
Watarase river. Fig. 4 shows the topography of the crank flow in
Kinu-river and Watarase-river.
The channels of crank flow in Kinu-river was investigated byfield measurement. The results show that the difference ofelevation of the bed of the thal-weg at right and left bank is 4m,
bed slope of crank flow channel is 1/60, speed of bank erosion
between 1999 and 2002 is approximately 17m/year. Afterdisaster suffering caused by the bank attacking concentrated
flow due to this crank flow, concrete block revetment isrecognized to be constructed as shown in Flg. 4.
On the other hand, many parks and playgrounds wereconstructed alternatively in the channel of Watarase-river.
Though, the original channel of Watarase had also divergence–convergence meandering system, sub lanes of two lane
meandering channel were buried to make new land including
central bar area, and the two-lane meandering system thuschanged into alternate bar meandering system in order to use
the land in the channel artificially. And the thal-weg in thealternate bar system has gradually developed to become stablecrank flow system. The locations of bank attacking concentrated
flow in stable and were found to coincide with the historicallocation of lateral deposition of jam-up large stones.
Results of topographical field measurement about the crank
flow in Watarase-river show that the bed elevation difference ofthe channel of bank attacking concentrated flow at the positionsof left and right bank is 4m, and the slope is 1/40. Large-scale
groins is now constructed along the bank as direct protection
countermeasure.

Bank erosion by crank flow
Deposition of Sand

River bankLongitudinal
Deposition of large stones

Flow
Crank flow

Lateral Jam-up
Straight revetment
deposit ofOriginal former thal-weg
large stones
Extension to the down stream by construction revetment

Longitudinal shape
1/150〜1/300

Lateral shape⊿Z=3-4m

Figure 6. A schematic figure basic structure
based on jam-up deposit of large stones
and development of crank flow
In this connection, velocity was assumed by calculation as
its maximum, based on such assumptions as uniform flow,
Manning’s formula and water depth 2m, which corresponds tothe height of crank flow channel below the top of thesurrounding sand bars. According to trial simple 2D calculation
in Ref. [9] and [10], this water depth is considered to be suitable

Longitudinal shape
Step

Divergence part
Small bed material
Mild bed slope
Wide river channel

Lateral jam-up deposit of large stones

Influenced width by jam-up deposit,
gradations wide divergence area

Longitudinal jam-up deposit of large stones

Plain shape

Double lanes flow

Front line of Lateral

Convergence part
Large bed material
Bed slope: sharp (downstream)
Narrow river channel

jam-up deposit large stones

Open levee
Tributary

Figure 5. A schematic figure of jam-up deposit large stones and divergence convergence channel
Convergence position

Divergence position
Aerial photo, 1947

102k

Villages

Front line of lateral
jam-up deposit of large stones

Longitudinal jam-up deposit of large stones
106k

100k
104k
Kobayashi bright

Photo 3. Divergence Convergence channel in Kinu river [ Aerial photo, 1947 before construction of levee ]Extension of thal-weg with straight revetment

Lateral jam-up depositof large stonese)d)Aerial photo, 2002
500m

c)
b)
Two lane flow

Two lane flow a)
Crank flow
Disaster revetment works by rank flow
(Direct measure construction method)

Longitudinal jam-up deposit of large stones

Photo. 4 Two lane meandering channel with the development of longitudinal jam-up deposit of large stones

III.

DIVERGENCE CONVERGENCE
MEANDERING SYSTEM

Among various meandering patterns, typical divergence
convergence meandering system in Kinu-river and
Watarase-river was selected for field investigation. The basic
structure of divergence-convergence meandering system basedon stable condition of jam-up deposit of large stones, isdiscussed in Ref. [5] and [6].
Morphological concept of river channel structure has been
formed based on bed material sifting during flush flood flows.
Those phenomena are mainly characterized to a river which has
graded large bed materials. Fig. 5 shows schematic figure ofdivergence convergence meandering system. Formation of
jam-up deposit of large stones causes flat and wide surface
up-stream divergence reach, and also steep and narrow
convergence reach down stream. Photo. 3 shows that plane
shape of divergence convergence channel is watched as inexample of aerial photograph at Kinu-river in 1947.
Especially, at the downstream side of jam-up deposit of largestones, open levee and confluence of small tributary are usuallyexist, and is judged to be the stable deposition.
In this paper, the positions of highly stable lateral jam-up
deposit as shown in Fig. 5, are defined according to such items
as longitudinal size of bed material, change of bed slope, changeof channel width, existence of open levee and confluence of255

small tributary, existence of step in the protected land, and also
direct investigation of bed material by dredging around the
proposed position. Fluctuations of the shift and change ofthal-weg were investigated too in Ref. [6].
Position of flow channel at divergence area locates to thecenter of the channel where supercritical flow occasionallyoccurs at near the peak of big flood, when the big stones aretransported and deposited just down stream of the steplongitudinally. This deposit forms longitudinal jam up deposit,
makes the lateral bed profile convex and becomes the maincauses to stabilize the two lane meandering system atconvergence reach, as is shown Fig. 6.
Photo. 4 shows that typical two lane meandering channel in
220
G-10

Flow

G-11

G-9

Altitude (Y.P.m)

to the condition that the strength of the crank flow becomes to
its maximum during a flood. Calculated results of velocity are
6m/s in the bank attacking concentrated flow at sharp angle atKinu River, and 7m/s in that of Watarase-river respectively.

218

G-8

216
101.8

101.9
102.0
102.1
102.2
Longitudinal distance (km)Figure 8. Longitudinal shape of longitudinal
jam-up deposit of large stones
Sand and Gravel (D<10cm)

Cobbles (10cm<d<20cm)

Boulders (20cm<d<50cm)

Big stones (D>50cm)

a)

100%

245
b)

240

c)

235

d)

230

e)

The rate of bet material (%)

Altitude (Y.P.m)

250

80%
60%
40%
20%
0%

225
0

100
200
300
Lateral distance from left river bank (m)Figure 7. Lateral shape at divergence reachin Kinu river

G-8

G-9

G-10

G-11

Figure 9. The rate of bed materialon the surface of river-bed material rateof longitudinal jam-up deposit of large stones

102.3

86k

D

150

50

94
95

92
93

90
91

88
89

86
87

84
85

83

0

Figure 11. Standard deviation of lateral position of deepest
river bed at 83k-101.5k in Kinu-riverThis numerical value is the standard deviation of distance to the maximum
river bed from the left bank annually. The measurements were done 13 times
during 1963-2001.

IV.

DEVELOPMENT PROCES OF CRANK FLOW

The crank flow treated here develops at divergence reach in
the divergence-convergence channel. This area is rather flat andwide, and the stability of the channels on it depends mainly on
the scale, number, and their stability of longitudinal jam-up
deposits of large stones. Artificial actions such as constructionof straight revetment, are another causes to decrease stability as
stated in Ref. [7].
Basic structure of divergence area is constituted with several
longitudinal jam-up deposits of large stones. And lateral shape isconvex. Fig. 6 shows the outline of the structure of deposition ofsifted materials, lateral and longitudinal shape, and low water
thal-weg in the divergence reach. The occurrence point of crank
flow is near the peak of longitudinal bed shape at center. It is
illustrated in this case that the construction of straight revetment
causes extension of thal-weg along river bank. This extension
promotes development of crank flow and also bank attacking

Front line of lateral jam-up deposit large stones
91k

92k

89k

B section [down stream section: 86k-92k]
93k

98k

94k
95k

F

J

100

E

92k

I

E

B

90k

D

F

101

A

99
100

deepest river bed. (m)Standard deviation of lateral position of

G

88k

C

H

C
200

87k

1947
1960
1966
1973

A section

B section
250

96
97
98

Kinu-river, and lateral shape in this area shows Fig. 7 at somecase of divergence reach. Fig. 8. and Fig. 9. shows that
longitudinal shape and bed material at longitudinal jam-up
deposit of large stones. Bed material rate appears on the surface
river bed by field investigation.
In this area, longitudinal jam-up deposits clearly at thedivergence reach, and thal-weg of two lane develops along theright and left bank in channel. In this case, divergence reachwith flat and wide is formed by stabled lateral jam-up deposit oflarge stones. Basic lateral shape of longitudinal jam-up depositof large stones becomes convex shape, and size of bed material
differenced each position as longitudinally. Large-size bed
material (cobbles, boulders, big stones) deposit at upper reach,and Small-size bed material (sand or gravel) deposit at down
stream, as relatively.
Concerning of formation of longitudinal jam-up deposit oflarge stones promote to make convex shape laterally at thedivergence reach. Point a) exist down stream reach of lateral
jam-up deposit of large stones, as a lateral shape is concave.
Formation of concave shape is stabled which basic of lateral
jam-up deposit of large stones stability. The flow patterngradually changes from mono lane flow to double lane flow isshown point b) and c). Especially, point c) is formed convex
clearly with condition below, degradation of thal-weg at near
the both bank developed, and stability of longitudinal jam-up
deposit of large stones is high. Also point d) is convex same as
point c), presently flowing at the right bank is flowed higher
than flowing left bank. Crank flow at this point e) is caused byflow condition of point d), it is described below. Main cause ofdevelopment of crank flow at the point e) is considered to be theinfluence of straight revetment at the right bank. In this case,extension of thal-weg to the down stream side is become bystraight revetment. Relative height of the thal-weg between rightand left banks is 2m at point d) of Fig. 6. Relative height of river
bed in crank flow is 4m at point e) of Fig. 7. Damage of bankerosion by crank flow of point e), left bank was eroded 17m in
2001.

96k

G

97k

H

A section [up stream section: 92k-98k]
Figure 10. Change of thal-weg at up stream section and down stream section in Kinu-river.
(C-H in the figure shows decrease section to minimum value from the maximum value)concentrated flow at sharp angle at the other side bank.
There is some difference in stability of thal-weg fluctuation
between upstream section and down stream section of
Kinu-river. Fig. 10. shows that change of thal-weg up stream
section and down stream section in Kinu-river.
The up stream section (A section : 92k-98k) has been formedon the original fan shape land and the stable lateral jam-up
A section
各範囲別の頻度 各範囲別の頻度 各範囲別の頻度 各範囲別の頻度

8
94ｋ

6
4

+25≦ｘ<+75+75≦ｘ<+125+125≦ｘ<+225+225≦ｘ<+325+25≦ｘ<+75+75≦ｘ<+125+125≦ｘ<+225+225≦ｘ<+325+25≦ｘ<+75+75≦ｘ<+125+125≦ｘ<+225+225≦ｘ<+325+25≦ｘ<+75+75≦ｘ<+125+125≦ｘ<+225+225≦ｘ<+325+425≦ｘ<+525-25≦ｘ<+25-25≦ｘ<+25-25≦ｘ<+25-25≦ｘ<+25+325≦ｘ<+425-75≦ｘ<-25-75≦ｘ<-25-75≦ｘ<-25-75≦ｘ<-25+325≦ｘ<+425+425≦ｘ<+525+325≦ｘ<+425+425≦ｘ<+52595ｋ+425≦ｘ<+525-125≦ｘ<-75-125≦ｘ<-75-125≦ｘ<-75-125≦ｘ<-75

94.5ｋ+325≦ｘ<+425-225≦ｘ<-125-225≦ｘ<-125-225≦ｘ<-125-225≦ｘ<-12542-325≦ｘ<-2256-325≦ｘ<-22508-325≦ｘ<-2252-325≦ｘ<-2254-425≦ｘ<-3256-425≦ｘ<-3258-525≦ｘ<-425

0-425≦ｘ<-3252-425≦ｘ<-3254-525≦ｘ<-425

6-525≦ｘ<-42508-525≦ｘ<-425

2

95.5ｋ

0

B section
8

deposits of large stones are clarified to be stable. On the other
hand, down stream section (B section: 86k-92k) was considered
to be created about 7,000 years ago by channel shift and has
been developing by eroding ash accumulated lager mainly
laterally. Lateral jam-up deposits also developed at someinterval longitudinally. According to the field investigation,
most of them have enough stability but some are found to move100~200m down stream due to the large scale dredging of the
bed after the War.
Standard deviation of lateral position of deepest river bed
between 83k-101.5k using 13 data from 1963 to 2001 is shownin Fig. 11. And it is found from Fig. 11 that the lateral change ofthal-weg is little in up-stream section (A section) and large indown-stream section (B section). The figure of thal-weg in A
section divides cleanly focus positions and emission positions,and it can be recognized that there are divided reaches,
continuously connecting each reach longitudinally, where
maximum value of deviation appears at upstream point of one
reach and minimum value at down stream point of the same
reach. In fact these down-stream points correspond to theexisting positions of the stable lateral jam up deposits of largestone. In the down-stream section (B section), above tendency is
not clear than that in A section.
As stated above, these causes are related to the scale and thestability of longitudinal jam-up deposit of large stones.
Thal-weg in A section are formed under the condition of stable
longitudinal deposits composed of larger stones, than those in B
section. Fig. 12. shows frequency distribution of the position of
the deepest bed in Kinu River.
In A section, the positions of thal-weg at 94k are stable near at
center part and then approaches to right and left direction
according to shift down stream. This tendency means thedevelopments of thal-weg along right and left bank, andfluctuations of the flowing position also become large due to thechanges of stability of longitudinal jam-up deposit of large

88k

6

Aerial photo, 2002

4

100m

Bank erosion by crank flow is 100m laterally208←Crank flow

6

88.5k

4
2

250m

0

Northeast Shinkansen→

8
6
4
2
0
8
6

89k

Photo 5. Crank flow of Kinu-river[Northeast Shinkansen area, 94km]
River bank89.5 ｋ420+425 ≦ｘ<+525+325 ≦ｘ<+425+225 ≦ｘ<+325+125 ≦ｘ<+225+75 ≦ｘ<+125+25 ≦ｘ<+75-25 ≦ｘ<+25-75 ≦ｘ<-25-125 ≦ｘ<-75-225 ≦ｘ<-125-325 ≦ｘ<-225-425 ≦ｘ<-325-525 ≦ｘ<-425

Sharp angle of crank flow is relievedby construction of groin.

Figure 12. Frequency distribution of the positionof deepest river bed in Kinu-river.
Lateral axis is a difference from the mean distance from left bank to thedeepest riverbed. Vertical axis is frequency within each range. The
measurements were done 13 times during 1963-2001.

After executing
crank flow→←Before executing of
crank flow

The sand-bar forms by thegroin.

Figure 13. Jetty construction method forrelief against crank flow

stones, according to shift down-stream.
Because the scale of the phenomena are larger in up stream
section (A section), the scale of crank flow and also the strengthof bank attacking concentrated flow becomes larger in A section
than in B section.
For example in Kinu-river, at northeast shinkansen area of A
section, bank erosion of 100 m laterally in a flood of one time
(1day) occurred. Photo. 5 shows the crank flow at northeast
shinkansen in Kinu-river. Direct causes of this area areconsidered as follows; 1) Existence of large-scale lateral jam-up
deposit of large stones. 2) Development of thal-weg caused by
the construction of straight revetment at left bank. 3) Stability of
longitudinal jam-up deposit of large stones is high, and lateralshape is convex clearly. 4) Difference of elevation between left
and right bank is large.
In this case, stability of longitudinal jam-up deposit of largestones and two lane flow are recognized even in Fig. 12. As
result of description above, severe change of flow to the rightbank from left bank is occurred after the extension of thal-weg
down stream at left bank and the strong crank flow at developed
along and in front of the lateral jam-up deposit of large stones.

V.

MEASURE OF CRANK FLOW

Direct bank protection method was adopted in Watarase-river
because local erosion came lose to the levee. But the structure ofjetty was too heavy, and the straight of bank attacking
concentrated flow at sharp angle to the bank aid not to be
mitigated. Moreover, environmental problem becomes severe.
Countermeasure for local erosion and environment is urgent
to discuss, because the developments of crank flow has recently
increased quantitatively and also qualitatively, as discussed Ref.
[8]. Basically, then methods are considered as countermeasure
for crank flow as shown in Table I.
VI.

CONCLUSION

This paper dealt with a new type local erosion along a riverbank caused by crank flow which has powerful bank attacking
concentrated flow at sharp angle to the bank. Among various
patterns of crank flow, crank flow which develops by
deformation mainly due to artificial actions is investigated. As
original meandering channel system, divergence convergencemorphological system was adopted.

Plain shape
River bank

Lateral shape [A][A]
Arrangement of big stones

Low water

1-2m

[B]
Lateral shape is convex, and height of sand-bar must be pulled down
middle-scale flood level.
2-3mLongitudinal shape [B]Longitudinal jam-up deposit of large stones
Up-stream side arranges of large stones.

Figure 14. Restoration of double lanes meandering system
Table I. Countermeasures for crank flow
Countermeasures

Construction method

Point of construction method

Direct measure
for river bank

Groin, Jetty
Revetment,
Foot protection, et al1) These construction method is effective for bank erosion by crank flow.
2) The groins at the bank attacking concentrated points are effective. It is
possible to move position of from the front of river bank to the central part of
the river.

Relief of causeof crank flow

Relief of sharp angleof the flow

1) Change of shape of a little by using groin, is shown Fig. 13.
(Angle of crank flow from sharp angle to loose angle modification.)
2) Small-scale groin system to form sandbar, and loose angle.

Restoration ofriver channel system

Double lanes meanderingsystem(Longitudinal jam-up
deposit of large stones)

1) The restoration of longitudinal jam-up deposit of large stones is examined.
2) Change of flow pattern to two lane meandering from single lane. Velocity at
the river bank will be decreased.
3) The large stones are deposited at the upstream side of longitudinal jam-up
deposit in other to get stability, is shown Fig. 14.

In Japan recently, channel characteristics has been changing
and new type disaster due to bank erosion is increasing,
especially by crank flow developed in divergence convergencemeandering channel. Therefore, revetment works as thecountermeasure for disaster are now constructed at several
rivers. Those direct works change the characteristics of the river
channel furthermore and promote to change again.
In order to contribute to improve above mentioned urgent
practical conditions, field investigation was considered to beimportant in the study and was made mainly in Kinu andWatarase River. Then, general consideration was made and get
following results. Suggestions were also added for the idea ofcountermeasure, including further problems for studies.
1) General concept of the development of crank flow wasclarified in the case of divergence convergencemorphological meandering channel system.
2) Local erosion along river bank caused by bank attacking
concentrated flow at sharp angle to the bank is a new typephenomenon. This phenomenon has lose relation with thedevelopment of crank flow. Especially crank flow caused bydeformation of meandering system in a stable divergence
convergence channel develops the power to the bankattacking concentrated flow at sharp angle to the bank, in theways to increase angle, velocity and rate of crank flow
discharge.
3) The causes of increase of power to crank flow are as follows;a) degradation by dredging, enlargement of channel anddecrease of sediment run off, b) development of thal-weg,
extension of thal-weg down stream along constructed straight
revetment, c) increase of relative height of bar and its
development, growth of grasses and trees on the bar, d)decrease of disturbance due to dam construction, e)
destruction of lateral and longitudinal jam up deposit,
decrease of big bed materials, decrease of meandering mode
from double lane meandering to mono meandering,
deformation of original meandering system.
4) Basic structure of divergence convergence channel
meandering system is formed based on bed material sifting
during flush flood flows. And main characteristics have
graded large bed material. Formation of jam-up deposit oflarge stones causes flat and wide surface up-stream
divergence reach, and also steep and narrow convergencereach down stream. Especially, divergence side is constituted
with several longitudinal jam-up deposit of large stones, andlateral shape is convex.
5) Crank flow develops mainly in the divergence reach. Scaleand power of crank flow depend on the stability of
longitudinal jam up deposit of large stones and the scale ofconvex bar of 3D (at three dimensional) curved surface, these
condition bring long extension of thal-weg, big bed elevationdifference, and abrupt change when deformation exceeds its
critical states.
6) Countermeasure of clank flow is needed to discuss both
direct construction method at bank attacking concentrated
position and relief of cause of sharp angle of the crank flow.

This paper proposed new construction method at relief ofcause of crank flow. The other measure is characterized
restoration of longitudinal jam-up deposit of large stones inorder to recover the double lanes from single lane. Thismethod is also evaluated in the sense of environment such asstone and gravel river beach simultaneously.
7) This study is expected to offer fundamental idea of measure
for crank flow.
REFERENCE[1] N. Suga, ”A study on the basis of crank flow which has
power to bring big bank erosion,” Annual Jour. Hy. Eng. JSCE.
Japan, Vol.49, pp955-960, February 2005. (In Japanese)[2] T. Mishina, N. Suga, et al, ”Changes of channel
characteristics of yosasa-river after disaster restoration,” Annual
Jour. Hy. Eng. JSCE. Japan, Vol.46, pp343-348, February 2002.
(In Japanese)[3] T. Mishina, N Suga, et al, ”Fielding investigation on bankprotection of natural meandering river at mountain allvial land,”Advances in River Eng. JSCE. Japan, Vol.9, pp131-136, June
2003. (In Japanese)[4] K.Suga, ”Stable condition of double track meanderingchannel,” Annua Jour. Hy. Eng. JSCE. Japan, Vol.33,
pp397-402, February 1989. (In Japanese)[5] N. Suga, ”Basic structure of river morphological channel
pattern in case of graded large bed materials,” Advances inRiver Eng. JSCE. Japan, Vol.10, pp95-100, June 2004. (InJapanese)[6] T. Mishina, N Suga, et al, ”Comprehensive field
investigation including protected land about jam-up deposit
composed of large stones formed in divergence-convergencechannel,” Advances in River Eng. JSCE. Japan, Vol.11,
pp375-380, June 2005. (In Japanese)[7] N. Suga, T. Mishina, et al, ”Crank flow and bank attacking
concentrated flow at sharp angle in a river channel ofdivergence-convergence meandering system,” Annual Jour. Hy.
Eng. JSCE. Japan, Vol.50, pp979-984, February 2006. (InJapanese)[8] N. Suga, ”Fundamental idea on individual river method inhigh regard of river characteristics and reflection of social andnatural environment based on new river law,” Advances inRiver Eng. JSCE. Japan, Vol.11, pp203-208, June 2005. (InJapanese)[9] Y. KOGAWA, N. SUGA, H. IKEDA, F.
IWASAKI, ”Structure of the open channel flow with hydraulicjump on sand bar front” Annua Jour. Hy. Eng. JSCE. Japan,
Vol.49, pp523-528, February 2005. (In Japanese)[10] Y. KOGAWA, H. IKEDA, F. IWASAKI, N.
SUGA, ”Plane angle of sand bar front and side wall roughness
exerted influence on structure of the open channel flow withhydraulic jump” Annua Jour. Hy. Eng. JSCE. Japan, Vol.50,
pp643-648, February 2006. CD-R. (In Japanese)