自然災害科学 J. JSNDS 35 特別号 117 -129（2016 ）LOCAL SCOUR AND SEDIMENTSORTING AROUND A SERIESOF GROYNES
Hao ZHANG1・Hajime NAKAGAWA2LOCAL SCOUR AND SEDIMENT SORTING
AROUND A SERIES OF GROYNES
Hao ZHANG1 and Hajime NAKAGAWA2
Abstract
Groynes are widely constructed along alluvial rivers for disaster mitigation andenvironment restoration in the world. Understanding the local scour and the sediment sortingis important to maximize the benefits and to mitigate potential disasters associated withgroynes. In this paper, an experimental study has been presented on the properties of the bed
deformation around a series of groynes in non-uniform sediment beds with both bedload andsuspended load transport. The experiments are conducted for a group of impermeable groynes
under both emerged and submerged conditions. The results indicate that groynes signiﬁcantly
enhance the bed diversity in terms of promoting scour-deposition morphology and sedimentsorting in their neighborhood. The sediment sorting results in different grain size distributions
in longitudinal, transverse and vertical directions. Longitudinal coarsening of sedimentparticles are observed in the mainstream narrowed by the groynes. In the transverse directionin the groyne reach and its downstream, an evidently coarsened zone forms near the centerlineof the ﬂume. Groyne bays and groyne wakes are capable of trapping ﬁne sediment particles,
but the patterns of ﬁne deposition depend on their locations and submergence conditions. Local
scour areas are coarsened and the bottom sediment is coarser than that at the top within thescour hole.

Key Words： Groyne, emerged, submerged, local scour, grain size variation

1

2

Dr. Eng., Associate Prof., Center for Disaster Prevention
Promotion, Kochi University

117

Dr. Eng., Prof., Disaster Prevention Research Institute,
Kyoto University

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LOCAL SCOUR AND SEDIMENT SORTING AROUND A SERIES OF GROYNES

1. INTRODUCTION

deformation proper ties around a single groyne

Groynes as an effective solution for bank pro-have been obtained. Many common features weretection, navigation enhancement and ecologicalobserved between a homogeneous sediment bed

rehabilitation, are widely constructed in alluvial

and a heterogeneous sediment bed in terms of lo-

rivers on our planet. Protruding of groynes into

cal ﬂow structures and scour-deposition morphol-a river course results in changes in the local bed

ogies around a single groyne. On the other hand,configurations and the substrate types (Zhang

properties unique to a heterogeneous sediment

et al., 2011). The former, mainly in terms of local

bed were also noted mainly as a consequence ofscour, is a crucial concern for the channel sta-

sediment sorting. For more reliable design andbility and the structure safety. While the latter,management of groynes, it is of great meaning tolargely due to sediment sorting, exerts impacts on

explore research taking into account the impacts

the bio-richness and bio-diversity in the riverineof the heterogeneity of bed sediment. Moreover,aquatic system. Therefore, an insight into the local

as groynes are usually arranged in groups rather

scour and sediment sorting is important to know

than in the form of an individual structure in actual

the potential beneﬁts and disasters associated withrivers, it is necessary to investigate the local scour

groynes.

and sediment sorting around a series of groynes.

Over the past several decades, there have

As a first step, the authorsʼ group has inves-been a huge amount of literature contributing totigated the flow structure and bedload transport

the hydraulics, morphodynamics and local scour

around a series of groynes in non-uniform sedi-

around groynes. Zhang and Nakagawa (2008) con-ment beds (Mizutani et al., 2014). The differences

ducted a systematic review which covered most of

in terms of flow structures and bed variations

the early and influential works. Recent advances

between a single groyne and a group of groynes

are available from publications by research groups

with various arrangements have been clariﬁed. On

such as Yossef and de Vriend (2010), Koken andthe other hand, fine depositions were commonly

Constantinescu (2011), Cai and Tominaga (2012),

repor ted in field sur veys around river groynes

Henning and Hentschel (2013) and Sukhodolov(Zhang et al., 2011), indicating the impor tance(2014). Unfortunately, the sediment heterogeneityof suspended sediment transport. In this study,

that is a distinctive feature of alluvial riverbeds has

the bed variation characteristics in a non-uniform

been seldom considered in the existing studies. In

sediment bed around a series of emerged andthe past several years, the authorsʼ group has con-submerged groynes were investigated under bothducted a lot of fundamental research to clarify the

bedload and suspended load transport conditions.

roles of the sediment heterogeneity plays in the

In particular, the properties of the local scour andbed deformation process around a single groyne

the grain size distributions were emphasized.

installed along a straight channel. The research
has revealed the importance of several governingparameters such as the bed sediment non-unifor-

2. LABORATORY EXPERIMENTS
2. 1 Experiment setup

mity (Zhang et al., 2012), the flow over topping

Two cases of experiments were performed inratio (Mizutani et al., 2012), the type of the groyne

a glass-sided tilting ﬂume in the Ujigawa Open Lab-

(Zhang et al., 2013) and the protruding angle oforatory of Kyoto University (Japan). The groynes

the groyne (Mizutani et al., 2013). In addition,

were kept emerged from the water throughout thethe detailed three-dimensional flow field and bed

experiment in Case1 and were overtopped from119

自然災害科学 J. JSNDS 35 特別号（2016 ）the beginning by the water in Case2. The experi-

types of sediment were 2.43 mm and 0.09 mm,

ment ﬂume was 8 m-long, 40 cm-wide and 30 cm-

respectively. The sieve analysis result of the initial

deep as shown in Fig. 1. A 20 cm-thick wooden

sediment bed materials was shown in Fig. 2. Ac-

deck was put on the bottom of the ﬂume to make acording to the figure, the mean grain size of theﬁxed ﬂatbed and a 2.5 m-long sediment recess was

initial sediment bed was 1.03 mm.

situated at 4 m downstream from the inlet bound-The hydraulic conditions applied in the exper-

ary. The recess was ﬁlled with silica sands to form

iments were tabulated in Table 1. It was evident

a movable bed. Four impermeable groynes werethat the friction velocity in the approach ﬂow area

installed on the movable bed along the right side of

was much smaller than the critical friction velocity

the ﬂume. Each of the groyne was 1 cm-thick andof the coarse fraction and much larger than that of

10 cm long. The groynes were perpendicular to thethe ﬁne fraction. Moreover, the ratio of the friction

side of the flume and were labelled as groyne A,

velocity in the approach ﬂow to the settling velocity

B, C and D from the upstream to the downstream.

of the fine fraction was calculated as 2.93, which

The length of the groyne bay between two consec-

was larger than the critical values for sedimentutive groynes was a constant of 20 cm. The flow
discharge was the same for both experiments,
while the height of the groynes differed from caseto case. In the emerged case, i.e. Case1, the heightof the groynes was much larger than that of thewater level. In the submerged case, i.e. Case2, theheight of the groynes was 2.5 cm above the initial
sediment bed (Fig. 1c).
The movable bed was prepared by mixing
two types of uniform silica sands with a ratio of
2:3 (coarse: ﬁne). The mean diameters of the two

Fig. 2

Sediment sieve analysis results

Table 1

Details of the hydraulic conditions

Flow discharge Q (l/s)

5.70

Channel slope I

1/1,280

Approach ﬂow depth h (cm)4.43

Approach ﬂow velocity U (cm/s)

32.17

Sediment density σ (g/cm3)

2.65

Sediment mean diameter D (mm)1.03

Friction velocity u＊(cm/s)

1.67

Critical friction velocity (coarse fraction) u＊c (cm/s) 5.85

Fig. 1

Experiment setup (unit: cm, not to scale)Critical friction velocity (ﬁne fraction) u＊c (cm/s)

1.29

Settling velocity (coarse fraction) wf (cm/s)

15.88

Settling velocity (ﬁne fraction) wf (cm/s)

0.57

Reynolds number Re

10,894Froude number Fr

0.49

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LOCAL SCOUR AND SEDIMENT SORTING AROUND A SERIES OF GROYNES

suspension proposed by most of the researchers

the final bed and the initial flatbed are plotted in(Chanson, 2004). Therefore, both bedload andFig. 3 and several characterizing parameters are

suspended load would be observed in the experi-

extracted from the ﬁgure as listed in Table 2. For

ments.

reference, the photos of the bed surface taken at
the ﬁnal stage are presented in Fig. 4. It is evident

2. 2 Experiment measurements
Both experiments started from a flatbed andlasted 3 hours when a quasi-equilibrium condition
was achieved. The quasi-equilibrium condition wasdeﬁned as a condition that the bed conﬁgurationsin the proximity of the groynes exhibited insigniﬁcant changes. In the experiments, no sediment was
supplied from the upstream of the ﬂume. The ﬁnal
bed levels were measured with a laser displacement meter (LK-500, Keyence co., Ltd) after thebeds were completely drained out. The bed materials of the surface layer at several representative lo-

Fig. 3

cations were taken with a special sampling spoon.

Bed deformation from the initial flatbed(unit: cm, Top: Case1; Bottom: Case2)

The sampling depth was approximately 2.86 mm,corresponding to D90 of the coarse fraction of thesediment. Finally, the dry sediment samples wereanalyzed with a nested column of sieves, together
with a high-resolution balance scale (UW220H,
Shimadzu co., Ltd).

3. RESULTS AND DISCUSSIONS
3. 1 Local scour and bed deposition

Table 2 Local scour depth and bed deposition
depth (unit: cm)
Parameter/Case

Case1

Case2

Max. scour depth

7.96

3.65

Max. deposition depth in bay AB

1.39

0.89

Max. deposition depth in bay BC

0.62

0.70

Max. deposition depth in bay CD

0.80

0.51

Max. deposition depth behind groyne D

1.17

0.76

Local scour is one of the most important concerns in the design and implementation of groynes.
Excessive scour may result in the undermining of
the groyne structures, which should be avoided
from the perspective of disaster mitigation. On
the other hand, the pool habitat created by local
scour has a potential to improve the river ecology,
which should be promoted from the viewpoint ofenvironment restoration. Moreover, local scour isgenerally followed by immediately downstream
sediment deposition. Therefore, an insight into thescour-deposition morphology is a key to understand the performance of groynes.
The dif ferences of the elevation between

Fig. 4

Bed surface conditions at the final stage(Top: Case1; Bottom: Case2)

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自然災害科学 J. JSNDS 35 特別号（2016 ）that the bed morphologies in Case1 and Case2 arepatterns in the groyne bays are different in dif-similar to some extent but differ a lot in terms offerent cases. The first groyne bay, i.e. bay AB isdetailed scour-deposition patterns.

inﬂuenced a lot by the local scour around groyne

In the approach flow zone, the bed shows

A. Due to the large scour hole, sediment carried

insignificant changes due to the formation of anby the horse-shoe vortex is transported to the fore-

armor layer which prevents the bed from con-side of groyne B and deposits there in Case1. On

tinuous degradation. On the other hand, ripples

the other hand, the deposition in bay AB of Case2

consisting of ﬁne sediment are observed in somelocates along the leeside of groyne A due to theplaces especially in the proximity of the rightsmall scour and the ﬂow overtopping the groyne.

side of the flume as shown in Fig. 4. The main-

The deposition patterns in bays BC and CD are

stream area is slightly degraded due to the flow

somewhat similar. In either of the groyne bay,

acceleration caused by the local narrowing of thethere are two ridges in the deposition area. One ischannel width. Compared with that in Case2, thelocated along the leeside of the upstream groynes,

bed degradation in Case1 is more drastic, indicat-

and another is near the center of the bays in Case1

ing the different constriction effect of groynes inand near the foreside of the downstream groynes

different cases. In the wake zone behind the mostin Case2. The maximum deposition depth and thedownstream groyne, i.e. groyne D, a relatively longdeposition area in Case 1 are generally larger than

distance of deposition with ripples of ﬁne sediment

those in Case2 as shown in Fig. 3 and Table 2,

is observed in both cases although the deposition

corresponding to the differences in the local scour

pattern is slightly different. The bed morphology

volume and the degree of the mainstream bed deg-in the proximity of the groynes is characterizedradation.

by local scour and corresponding deposition inboth cases. In Case1, severe local scour up to 7.96

3. 2 Grain size variation

cm takes place at the most upstream groyne, i.e.

Sediment sorting is a common phenomenon

groyne A, and a large area of sediment deposition

in alluvial rivers. Sediment sorting affects both theforms behind it. The scour-deposition pattern in

bed topography and the bed materials composition.

general is similar to that around a single groyne

Hereafter, the distributions of the dimensionless

which is mainly controlled by the local vortex sys-

mean grain size along typical longitudinal andtem (Zhang et al., 2012 and Zhang et al., 2015). The

transverse cross-sections are investigated. The di-local scour at groyne A exerts great impacts on themensionless mean grain size is deﬁned as the ratio

bed deformation around the downstream groynes.

between the mean grain size at the final stage D

Although local scour holes and deposition areas

to that on the initial bed D0. The bed is coarsened

are also obser ved around groynes downstream,

if the value is larger than 1.0. On the other hand, athey are basically superimposed on the wake depo-

value less than 1.0 indicates an increase of the ﬁnesition induced by groyne A. In Case2, the maxi-

fraction.

mum scour is found at the toe of groyne A which
is similar to that of Case1, but the scour depth is
only 46% of that in Case1. Moreover, the inﬂuence

3. 2. 1 Grain size variation along longitudinalcross-sectionsof the wake deposition due to groyne A on the bed

The changes of the grain size along typicalmorphology around downstream groynes is muchlongitudinal cross-sections are plotted in Fig. 5.

less compared with that in Case1. The deposition

In the figure, Sections y=20 cm and y=30 cm are

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LOCAL SCOUR AND SEDIMENT SORTING AROUND A SERIES OF GROYNES

Fig. 5Grain size variation along typical longitudinal cross-sections (Left: Case1; Right: Case2)

123

自然災害科学 J. JSNDS 35 特別号（2016 ）sections in the mainstream, Section y=10 cm is thethe groyne bays in Case1. In the approach flow

section connecting the heads of all groynes andzone, sediment becomes finer than the original

Section y=5 cm passes the geometric center of the

one due to the formation of a thin layer of ﬁne sed-

groyne bays. For reference, the bed level change

iment ripples. In the groyne reach, bed sediment iswith respect to the initial flatbed of each section,

greatly coarsened. The sediment at the bottoms ofdenoted by zb, is also plotted in the ﬁgure with thescour holes is extremely coarsened and is aroundsecondary vertical axis.

2.2 times of the original one, which indicates that

Fig. 5(a) and Fig. 5(b) draw a clear image on

the bottoms almost completely consist of coarse

the longitudinal bed variation in the mainstream

sands. In the downstream of the groyne reach,in Case1. In the approach ﬂow zone (x<0 cm), thesediment is coarser than that on the initial bed.

bed level almost maintains and the bed materials

Moreover, the sediment becomes coarser andare slightly coarsened. This phenomenon is caused

coarser with the elevation on the foreside and ﬁnerby the transport of ﬁne sediment and the formationand finer on the leeside of the deposition hill. Inof an armor layer. In the reach narrowed by theCase2 as shown in Fig. 5(g), surface armoring andgroynes (0 cm<x<60 cm), bed degradation is obvi-

ripples are distinguishable in the approach flowous due to the ﬂow acceleration, especially in thezone. Sediment coarsening is also evident in thearea near to the groynes (Section y=20 cm). In thisgroyne reach, but the coarsest grains are not al-reach, the bed materials are significantly coarser

ways found at the bottoms of the local scour holes.

than those on the initial bed. The coarsest grains

This phenomenon is closely related to the local

are found in the proximity of the end of the groyne

flow field. According to previous research of thereach (x=60 cm-70 cm), where the grain size isauthors group, the horse-shoe vortex within the

more than twice of that on the initial bed. In thescour hole which is the main engine for the local

downstream of the groyne reach (x>60 cm), coarse

scour development contributes dominantly to thesediment is still dominant for a long distance. Un-bottom coarsening in emerged conditions (Zhang

fortunately, the experimental data availablenesset al., 2012). On the other hand, the overtopping

does not allow a quantitative estimation of the in-ﬂow plays an important role in case of submerged

ﬂuence range. Similar trends are observed on thegroynes (Mizutani et al., 2012). The mixing of the

bed variation in Case2 according to Fig. 5(e) andtwo ﬂow patterns in the transition zone is responsi-

Fig. 5(f). However, the changes of both the bed

ble for the differences in the bed variations. Down-

level and the bed composition in Case2 are muchstream of the groyne reach, the bed level is slightly

milder than those in Case1. It is also interesting toaggraded and the bed materials gradually turns ﬁn-note that the grain size in the groyne reach does

er with the reduction of the ﬂow velocity. Different

not monotonically increase in the groyne reach.

from that in Case1, the deposited sediment is ﬁner

The grain size at x=40 cm is relatively smaller com-than that on the initial bed after a certain distancepared with other areas in both cases. It is probably

from groyne D.

due to the boundary on the left side of the ﬂume

In Fig. 5(d) and Fig.5(h), the bed properties

which exerts inﬂuences on the ﬂow separated fromin the approach flow zone are similar to those in

the heads of the groynes, particularly the most up-

Fig. 5(c) and Fig. 5(g). Nevertheless, the char-stream groynes.

acteristics of the grain size distributions in otherFig. 5(c) shows the grain size distribution inareas are quite different. The grain size in bay AB

the transition zone between the mainstream andis a little complex and is largely inﬂuenced by the124

LOCAL SCOUR AND SEDIMENT SORTING AROUND A SERIES OF GROYNES

local ﬂow around groyne A. The bed materials arethat the grain size of the former is larger thanmostly coarser than those on the initial bed exceptthat of the latter in all sections. In both cases, thea small area along the leeside of groyne A in Case2.

grain size distributions in the approach ﬂow zone,

In bays BC and CD, the grain size distributions

the groyne reach and the downstream area of theshow a close relation with the local bed topogra-

groyne reach exhibit quite different characteristics

phy. In general, ﬁne sediment accumulates at highin the transverse direction.

elevations and coarse sediment is dominant at low

In the approach ﬂow zone (x<0 cm), the bed iselevations. In either bay BC or bay CD, two deposi-

generally coarsened, but the locations of ﬁne sed-tion ridges as mentioned before are evidently con-iment ripples and the geometry of the local scour

firmed in the figures. However, as shown in Fig.

hole inﬂuence the mean grain size of the samples.

4, the thickness of the ﬁne deposition in Case1 isAlong Section x=-30 cm, sediment near the rightmuch larger than that in Case2. In both cases, theside of the flume is finer than the initial bed ma-

grain size on the foreside of the groyne is muchterials and gradually turns coarse towards thelarger than that on the leeside. In the wake zone ofleft side in Case1 as shown in Fig. 6(a), while ingroyne D in either of the case, two regions of ﬁne

Case2, the bed is coarsened everywhere especiallysediment are obser ved. The two fine deposition

near the sides of the ﬂume according to Fig. 7(a).

areas are closely related to the scale of the horse-

Along Section x=-10 cm, the distribution patterns

shoe vortex and the wake vortex around groyne D.

change a lot. In Case1, the local scour extends to

The area of the ﬁne deposition immediately behind

this cross-section and the sediment materials at

groyne D in Case1 is larger than that in Case2. But

the bottom of the scour are coarser than those on

the situation changes when the ﬁne deposition far

the upper layer. Outside of the scour, the grain size

from groyne D is concerned. Compared with thoseis slightly coarser than that on the initial bed asin Case1, the area is larger and the mean grain size

shown in Fig. 6(b). On the other hand, sedimentis smaller in Case2.

near the right side of the flume almost consistsof only fine sediment due to the development of

3. 2. 2 Grain size variation along transverse
cross-sections
The changes of the grain size along typicalripples and the mean grain size gradually turns
coarse towards the left side of the ﬂume in Case2
as shown in Fig. 7(b).

transverse cross-sections are plotted in Fig. 6

The distribution of the grain size in the groyne

and Fig. 7 for Case1 and Case2, respectively. In

reach (0 cm<x<60 cm) is signiﬁcantly inﬂuenced by

the figure, Sections x=-30 cm and x=-10 cm are

the existence of the groynes. In the extent of thesections in the approach flow zone, Sections x=0

scour hole, for example, Figs.6(b)-(e) in Case1 andcm, x=20 cm, x=40 cm and x=60 cm are sections

Figs.7(c), (e), (g) and (i) in Case2, the sedimentpassing the normal centers of groynes, Sections

becomes coarser and coarser with the decreasing

x=10 cm, x=30 cm, x=50 cm are sections passingof the channel bed elevation. It demonstrates thatthe geometric centers of groyne bays, and Sections

the local scour not only alters the local bed con-x=70 cm, x=80 cm and x=120 cm are representative

ﬁguration but also leads to the vertical coarsening

sections in the downstream of the groyne reach.

of bed materials within the scour hole. In general,

Along the sections passing the normal centers of

the change of the grain size in the groyne reachthe groynes, samples are taken from both the fore-

follows such a pattern of ﬁne-coarse-ﬁne from theside and the leeside of each groyne, and it is foundgroyne bay toward the mainstream. Here, the自然災害科学 J. JSNDS 35 特別号（2016 ）Fig. 6

Grain size variation along typical transverse cross-sections (Case1)

125

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LOCAL SCOUR AND SEDIMENT SORTING AROUND A SERIES OF GROYNES

Fig. 7

Grain size variation along typical transverse cross-sections (Case2)

127

自然災害科学 J. JSNDS 35 特別号（2016 ）grain size of the fine area is not necessarily finermaterials. As the approach ﬂow area shows insig-

than that on the initial bed. The coarsest sedimentniﬁcant changes in bed elevations and there is no

is commonly found in the range of y=10 cm-20 cm,

sediment supply from the upstream inlet boundary,which is more than 2.2 times of that of the sedi-

it may be considered that the sediment particles onment on the initial ﬂatbed. An evidently coarsened

the initial ﬂatbed are re-distributed by the groynes.

area is formed in the proximity of Section y=20 cm.

To understand the functionality of groynes, it is of

In Case1, the grain size at most of the represen-meaning to know how the sediment moves after

tative points in the bay areas is ﬁner than that on

the installation of the groynes.

the initial bed, especially in bays BC and CD. On

During the running of the experiments, inter-the other hand, only the sample on the leeside ofval timer shooting on a digital single-lens reflex

each groyne is ﬁner than the initial bed sedimentcamera (Model: Nikon D5000) was conducted.

in Case2. These observations are also visually con-

Based on the records of the camera and obser-

ﬁrmable from the experiment photos in Fig. 4.

vations during the running of the experiments,

In the downstream of the groyne reach, i.e.

sketches are made in Fig. 4 to draw a picture on

Sections x=70 cm, 80 cm and 120 cm, the grain

the primary sediment transport near the bed in thesize also follows a trend of fine-coarse-fine fromneighborhood of the groynes. The sketch also pro-the right side to the left side of the ﬂume in bothvides information on the near-bed mass exchanges

cases. The coarsened region extends to this reach

between the mainstream and the groyne bays/

and is located in the proximity of Section y=20 cm

wakes. As the mainstream is the major source ofas well. Although the percentage of ﬁne sedimentsediment supply for the groyne bays/wakes, thevaries with locations, the mean grain size in theexchange process plays an important role in theproximity of the left side of the ﬂume is generally

bed deformation properties. The sketch demon-larger, and that in the proximity of the right sidestrates why the sediment bed is scoured and coars-of the ﬂume is smaller than the mean grain size ofened as well as how and where the fine particles

the initial bed materials. The existence of an obvi-

accumulate to form the deposition area. Common

ously coarsened region is also observed in case offeatures and differences between different casesa single groyne according to the authorsʼ previous

and among different groyne bays are distinguish-

research, e.g. Zhang et al. (2012) and Zhang etable from the sketches, which provide importantal. (2015). In those research, two fine sediment

information for the understanding of bed variation

ribbons have been obser ved in the groyne wakemechanisms.

and near the coarsened region in different non-uni-

It is no doubt that the movement of sedimentform sediment beds due to sediment sorting inparticles strongly depends on the ﬂow ﬁeld and thethe transverse direction. However, the latter is not

local bed topography. In particular, the transport ofevidently distinguishable as the grain size of thesuspended load is directly inﬂuenced by the struc-ﬁne fraction adopted in this study is too ﬁne and isture of the local flow. Although detailed flow ve-

mainly transported in suspended load.

locities are not measured in this study, the sketch
is found to coincide with the findings obtained

3. 3 Discussions

from previous studies of the authors group on the

The results above indicate that groynes havethree-dimensional flow structure under bedload

significant influences on both the bed configura-transport conditions, e.g. Zhang et al. (2012) andtions and the distribution characteristics of the bed

Mizutani et al. (2014).

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LOCAL SCOUR AND SEDIMENT SORTING AROUND A SERIES OF GROYNES

4. CONCLUSIONSThe belt of the evidently coarsened zone in

The characteristics of the local scour andthe mainstream extends to the downstream of thegrain size distribution around a series of emerged

groyne reach. While in the wake of the most down-

and submerged groynes in a non-uniform sedimentstream groyne, two areas of fine deposition form

bed are presented. It is found that the groynes sig-

due to the horse-shoe vortex around and the wakeniﬁcantly alter the local bed morphologies and pro-vortex behind the groyne in each case. The scales

motes sediment sorting longitudinally, transversely

and the mean grain sizes of the two ﬁne depositionand vertically.

areas are different in emerged and submerged cas-

Under the current experimental settings, the
bed in the approach flow area generally showses corresponding to the differences in the vortex
system.

insignificant changes in elevation and is slightly

The characteristics of the scour-deposition andcoarsened due to the selective transport of finegrain size distribution above and their implications

particles and the formation of an armor layer inon potential disasters and beneﬁts should be pru-

the bed surface. However, ripples consisting ofdently considered for the design and assessment ofﬁne particles appear in the right half domain of thegroynes. Research with ﬁeld surveys is ongoing toﬂume due to the blockage effect of the groynes.

further the understanding in actual rivers.

In the groyne reach, the mainstream area isdegraded and local scour holes take place at the

Acknowledgement:

toes of groynes. The scour hole around the mostThis research is supported by the JST/JICA

upstream groyne is the most severe and exer t

SATREPS Program on Disaster Prevention/Miti-

impacts on the groynes downstream, especiallygation Measures against Floods and Storm Surges

in emerged case. The bed in the mainstream andin Bangladesh (PI: Dr. Hajime Nakagawa). Mr.

within the local scour holes are signiﬁcantly coars-

Kei Nishio and Mr. Masatoshi Ogura are acknowl-

ened due to flow accelerations and strong horse-

edged for their support in the laboratory experi-

shoe vor tices, respectively. A belt of evidently

ments.

coarsened zone is found in the proximity of thelongitudinal centerline of the ﬂume. On the other

References

hand, fine deposition is obvious in the second1 ）Cai, F. and Tominaga, A. (2012), Effect of longitudinal block on flow structures around box
groyne, Annual Journal of Hydraulic Engineering,
JSCE, 56, 55-60.

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要旨

河川災害の防止並びに多様な河川環境の創出を目的として，伝統的な河川工法である水制工は，世界中の沖積河川に広く導入されている。水制工が有する防災と環境機能を水工学的に評価するには，水制工周りの局所洗掘と流砂分級現象の解明が重要である。本論文では掃流砂と浮遊砂の卓越した状況での混合砂河床において，水制群周辺の砂礫の分級と河床変動特性に関する水理模型実験結果について考察を行っている。すなわち，不透過型水制群を用いて越流および非越流の場を対象とした実験を行い，水制群周辺の洗掘・堆積地形の形成および流砂の分級特性について検討した。その結果，水制群は河床の多様性を大幅に促進することが確認された。特に，実験水路の縦断，横断および鉛直方向における河床材料の粒度分布は流砂の分級により大きく異なること，水制工で水刎ねされた主流部では，流下方向に平均粒径は大きくなる
こと，水制工設置区間とその直下流域では，帯状の粗粒化領域が水路の中心線付近で見られること，一方，細粒土砂は水制間および水制背後域に堆積し，その堆積形態は場所や水制工の水没状況により異なること，洗掘孔は粗粒化領域で，河床材料は上部よりも底部のほうが粗いこと等が明らかになった。このような成果は水制工を用いて河川の多様な環境を創生するうえで重要な情報を提供するものといえる。