京都大学防災研究所年報

第 54 号 B 平 成 23 年 6 月

Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No.54B, 2011

Characteristics of Grain Size Distribution in Groin Fields and Their
Environmental Implications

Hao ZHANG, Xinhua ZHANG*, Hajime NAKAGAWA, Weilin XU*,
Pengzhi LIN* and Hideaki MIZUTANI*** State Key Laboratory of Hydraulics and Mountain River Engineering, Sichuan University, China** Graduate School of Civil and Earth Resources Engineering, Kyoto University, Japan
Synopsis
This paper presents the characteristics and environmental implications of thegrain size distribution around two typical groin sites in the Kizu River (the Yodo Riverbasin, Japan) and the Fuhe River (the Min River basin, China). The results show thatgroins enhance sediment sorting processes longitudinally and laterally. Longitudinalsediment coarsening is observed in the mainstream narrowed by groins. The relatively
closed bay area between two consecutive groins is capable of trapping fine sedimentmaterials such as silt and clay. Moreover, the area behind a permeable groin exhibits
more diversity in terms of grain sizes compared with that behind an impermeable one.
Due to lateral sediment sorting, sand ribbons form at the downstream of groins. The
results also indicate that the sediment size distributions are closely related to the local
channel morphology and the complex flow structure around the groins.
Keywords: groin, sediment heterogeneity, grain size distribution, sand ribbon, field survey

1.

Introduction

Alluvial rivers adjust their morphologicalfeatures and substrate compositions in response to
the water and sediment conditions in their drainage
basins. Although large variation might exist among
different river basins, the physical characteristics
such as the stream width, water depth, bed slope,
sediment property and channel pattern of a specificriver, vary consistently from the upstream origin to
the estuary. These characteristics exert significant
impacts on the biogeochemistry and stream habitat.
As a result, riverine species adapt themselves toaccommodate accordingly and an orderly transfer offlora and fauna is commonplace in a river.
Consequently, the continuum of the fluvial forms
and processes provides the basics of essential
concepts advanced in both fluvial geomorphology

and aquatic ecology (Pitlick and Wilcock, 2001).
Unfortunately, most of the alluvial rivers wanderingon our planet have been significantly altered andtheir continuum has been generally interrupted due
to various human interventions over the past several
decades. One of the most commonly introduced
human interventions is the construction of groins.
Groins protrude into the watercourse and makedisturbances to the river flow, sediment system andriverine environment. The discontinuity associatedwith groins has been widely utilized in alluvial
rivers for bank protection and navigability
improvement (e.g. Zhang, 2005). On the other hand,groins greatly enhance the flow and morphologicaldiversity in their proximities and they have beensuccessfully adopted in practices to restore extinct
or damaged aquatic habitats, e.g. Shields et al.
(1995), Poizat and Pont (1996), Hartman and Titus- 511 -

(2010) and Radspinner et al. (2010). In general, thegroins work at a relatively local scale, but may havebroader-scale implications. Understanding themechanisms and quantifying the characteristics ofthe discontinuity caused by groins are important for
the effective management of rivers and ecosystems.
The hydraulic and morphological consequencesof the interruption due to groins in rivers have been
documented by many researchers and engineers, e.g.
the early research conducted by Amhad (1953),
Garde et al. (1961), Gill (1972), Michiue et al.
(1984), Kuhnle et al. (1999, 2002) and others as
systematically reviewed by Zhang and Nakagawa(2008), and recent contributions such as Uijttewaal(2005), Biron et al. (2005), Thompson (2006),
McCoy et al. (2007), Koken and Constantinescu(2008a, 2008b), Zhang and Nakagawa (2009) andYossef and de Vriend (2010). According to thosestudies, it has been well understood that the flow
around a groin is quite complex and may begenerally separated into several components
including the downward flows, bow waves,
horse-shoe vortices and wake vortices. The riverbed
morphology is characterized by local scouring at
the groin toe, sediment deposition in the groin wakeand bed degradation in the mainstream.
Nevertheless, the discontinuity associated with thesubstrate composition around groins is still poorly
understood despite its crucial significance. It is
well-known that the motion of the bed sediment asconstitute of the bed itself exerts direct impact on
the bed morphology and hence the flow field (e.g.
Wu et al., 2000 and Zhang, 2005). On the other
hand, the bed sediment is used directly by benthic
species, fishes and plants and serves as an indirectindictor for aquatic habitat (e.g. Milhous, 1998 andShields and Milhous, 1992). In this paper, thecharacteristics of the grain size distribution in two
typical groin fields in Japanese and Chinese rivers
are investigated based on field surveys and theirenvironmental implications are addressed as well.

River basin) near the Kyoto City, Japan (naming as
Kizu site hereafter) and the other is located in the
Fuhe River (the Min River basin) near the Chengdu
City, China (terming as Fuhe site hereafter). The
Min River is one of the largest tributaries of the
Yangtze River as shown in Fig.2. The Kizu site
consists of two groins to restore a beach landscapefor recreation and to enhance riverine bio-diversity
nearby. One is impermeable and the other one ispermeable. The impermeable one is made ofgeo-bags and the permeable one is composed offour rows of timbers as shown in Photo1. The mean
grain size of the bed materials in this reach isaround 5.6mm.The Fuhe site consists of threeimpermeable groins mainly for the purpose ofnavigability improvement. For the time being, only
the groin bay between the most downstream two
groins is selected for investigation as shown in
Photo2. The bed in this reach mainly consists ofgravels and the mean grain size is about 50mm.
Lake Biwa

N

Shiga Pref.

Kyoto Pref.
Kyoto

Katsura R.

Uji R.
Kizu site

Kizu R.

Yodo R.
Osaka Bay

Mie Pref.

Osaka

Osaka Pref.
Nara Pref.

Fig.1 The Yodo River basin and the Kizu site
Gansu Prov.

Shannxi Prov.
Han R.

Hubei Prov.
Min R.
ChengduDadu R.
Fuhe site

Research methodology

Yichang
Jialing R.

Tuo R.

Sichuan Prov.

2.

Seta R.

Chongqing

Yibin

This research is mainly based on field surveys
along two rivers in Japan and China. The locationof the two study sites are sketched in Fig.1 andFig.2. One is located in the Kizu River (the Yodo

Yangtze R.
Yalong R.
Panzhihua

Qing R.

Hunan Prov.
Wu R.
GuiyangN

Guizhou Prov.

Fig.2 Location of the Min River and the Fuhe site

- 512 -

Flow
Permeable groin
Impermeable groin

Photo 1 Groins along the Kizu River(positioning around 34o51’N and 135o44’E)

Flow

Impermeable groin
Impermeable groin

Photo 2 Groins along the Fuhe River(positioning around 30o28’N and 104o03’E)

8.4 8.8 9.2 9.6(m)

10 10.4 10.8 11.2 11.6 12 12.4

1120

T1

L1

T2

Vegetated sandbar
T3
1080

L2

L1Main channel

1040

L3
1000

L2

Deposition
T1

960

L3
1680

T2

Minor channel
1720

1760

T3

1800

1840

1880

(m)

Fig.3 Sediment sampling points superimposed upon
the reference bed bathymetries and GPS tracks
at the Kizu site

460.1 460.3 460.5 460.7 460.9 461.1 461.3 461.5(m)
0Main channel

-20

-40

Deposition
Minor channel

-60

-80
-60

-40

-20

0

20

40

60

(m)

Fig.4 Sediment sampling points superimposed upon
the reference bed bathymetries and GPS tracks
at the Fuhe site

According to available bed level data, aerial
photos and site accessibility, sediment samples at
representative points of the two sites (sketched in
Fig.3 and Fig.4) are taken from the surfaces of theriverbeds during the winter season of year 2010.
Relatively coarse sediment is analyzed with acolumn of nested stainless sieves and an electronicbalance after completely dried out. Fine particles
with a maximum size obviously less than 1mm isanalyzed with two laser diffraction particle size
analyzers: SALD-3000 (Shimadzu co., Ltd., Japan)
and Mastersizer 2000 (Malvern Instruments Ltd.,UK).
Prior to this study, the authors’ research group
has investigated the morphological processes in theproximity of the Kizu site with large-scale
experiments and numerical simulations undervarious flow conditions (e.g. Zhang et al., 2006 andZhang et al., 2007). It was found that local scour
developed at the toe of the impermeable groin at
low flow conditions. During high flow conditions,the scour hole had a potential to be filled withsediment transported from the upstream reach andthe area along the channel bank might suffer from
erosion due to return currents in the groin wake.
Furthermore, the flow overtopping the crest of thegroin generally led to severe bed degradation along
the lee side of the groin body. The erosion at the leeside developed towards downstream and a minor
channel would appear. On the other hand, thepermeable groin reduced the flow velocity in its
wake and generally resulted in bed aggradation
downstream of it. Compared with that behind theimpermeable groin, the depositional area behind thepermeable groin was smaller. Moreover, numericalsimulation results suggest that the morphologicalconsequences of the groins were closely related tothe complex local flow field and the findings werealso applicable to groins in other rivers.
In this study, the grain size distributions arefocused on, while the bed bathymetries are notmeasured in details. The typical topographic
features, in particular the extents of the depositionalarea, are recorded with the aid of a mobile GPS(Global positioning system) unit GPSmap 60CSx(Garmin Cooperation). The sketches of thedepositional area are depicted in Fig.3 and Fig.4(dotted lines), together with available historic bed- 513 -

level daata of the Kizu site (year(2006, 5m
resolutionn) and the Fuuhe site (yeaar 2000, scattteredpoints) ffor reference. It is found that the sketccheddepositioonal area in thet Kizu site agrees well withthat plottted from the 2006 bed baathymetrical ddata.
In fact, s imilarities in other topograaphic featuress areiandalso recoognized durinng the field investigation
photograaphy. These suggest that thhe year 20066 bed
bathymettry could exppress the riveerbed topograaphic
features in year 20100 with a reassonable accurracy.
Hence, thhe bed level information of year 20006 is
used direectly in the analyses of the Kizu sitte if
necessaryy. In the Fuuhe site, the bed geometrry ismuch sopphisticated inn the groin baay, indicatingg thecomplex flow condittions there. There are ssome
similaritiies between the GPS traccks and the year
2000 bedd contours suchs
as the locations off thedepositioonal area and the minor chhannel. Howeever,discrepanncies are alsoo evident, e.gg. the incisionns ofseveral sm
mall channelss into the dep
positional areaa are
distinguishable in booth years but the locationns of
those ch annels are a bit differentt. These finddingsbmorphological propeerties
demonstrrate that the basicaround thhe groins havve almost maiintained althoough
there havve been channges in the bed level deetails
over the past decade. Keeping thiss in mind, wee are
not too particular abbout the dettails of the bed
elevationns for the anaalyses of the Fuhe site inn thefollowingg contexts. Innstead, the innherent propeerties
associateed with the beed morpholog
gies are extraacted
and emphhasized if neccessary.
3.

No.1
N

No.8

No..5

No.9

No..6

No..10

No.77

No.3
N

No.88

No.4
N

No.11

Fig.5
F
Sketch ofo representattive samplingg points

No.8

No.5

No
o.1

No.9
N

No.6

No
o.2

No.10
N

No.7

No
o.3

No.11
N

No.8

No
o.4

ults
Resu

ons, it has bbeen
Durring the fieldd investigatio
found thaat the reachess equipped with groins exhhibit
obvious diversity inn bed sediiment propeerties
comparedd with other reaches of th
he channels. The
sedimentt particles vaary in space not only inn thelongitudiinal direction but also in the latteral
directionn. Moreover, thet sediment properties inn thebay areaa between twotconsecuutive groins areevidentlyy distinguishhable from those in themainstreaam even withh the naked ey
ye. In this secction,
one of the most im
mportant parrameters for thequantification of thee substrate compositioncandn size distribuution,
habitat h eterogeneity, i.e. the grain
is selecteed for analysees.

- 514 -

No.2
N

Fig.6 Samples at representaative samplingg points

3.1 A general image of grain sizes around groins
In order to obtain a general idea on the grain
size distribution around groins, the photos and thesediment sieve analysis curves of samples at several
representative points taken at the Kizu site arepresented first. The sketch of the groins as well as
the locations of the points is depicted in Fig.5. For
clarity, the size of the circle in the figure gives
qualitative information on the mean grain size ateach point, while the details on the grain size
distribution are shown in Fig.6.
The photos provide visible evidences on thesediment heterogeneity in the proximity of thegroins. The sediment covers a broad range of grain
sizes, including silt, sand and gravel obviously. The
distribution of the grain sizes does not follow therule of thumb any more that alluvial rivers exhibit
gradual fining of bed materials in the down-valley
direction. The continuum of the river in terms of
substrate composition is clearly interrupted by thegroins. Moreover, different from the effect of theinterruptions caused by dams or weirs which is
generally simplified as upstream fine sedimentdeposition and downstream bed coarsening, thediscontinuity due to groins is much more complex.
The mean grain sizes and their distributions show
dramatic variations in a very confined local area inthe proximity of the groins.
3.2 Longitudinal distribution of grain sizes
The mean grain sizes D and the geometricstandard deviations σg of the samples at typicallongitudinal cross-sections in the Kizu site are
depicted in Fig.7 and Fig.8. For clarity, the bed
level, the projected area of the groin bay and themean sediment size within the reach are plotted in
the figures as well. The locations of thecross-sections are sketched in Fig.3. Amongst them,Section-L1 lies in the mainstream, Section-L2 isacross the groin bay and Section-L3 is near thebank and across the groin bay.
Due to the existence of a large vegetatedsandbar as sketched in Fig.3, the bed topographyaround the groins is a bit complex. However, thetypical longitudinal morphological features are still
well distinguishable in the three sections as shownin Fig.7. The bed profile in the mainstream isrelatively simple, highlighted by severe local

scouring. In the groin field, the bed levels vary a lot
in the longitudinal direction. The immediately
upstream of the permeable groin is eroded and thesediment deposits in the groin bay. Both the
upstream and the downstream of the impermeable
groin suffered from erosion and deposition takes
place in the wake zone.
As is known, bed materials in alluvial rivers
generally turn finer and finer if one goes down anddown the river. However, it is noted in all
longitudinal sections of Fig.7 that the sediment isfine in the upstream of the groin bay and is coarse
in the downstream of it. In the upstream area, themean sizes of all sediment samples are less than themean sediment size within the reach. In other areas,
differences are recognized between the section in
the mainstream and sections in the groin bay. The
groin bay (Section-L2 and Section-L3) is found to
be covered with very fine sediment particles, while
the part of the mainstream just in front of the groinbay (Section-L1) shows obvious bed sedimentcoarsening indicating a possible existence of anarmor layer. The findings here are not surprisingconsidering the flow velocity reduction in the groin
field and the flow velocity acceleration in themainstream caused by groins as have been well
documented by previous researchers such as Koken
and Constantinescu (2008b), Zhang and Nakagawa(2008), Zhang et al. (2009) and Yossef and deVriend (2010).
As to the value of σg, it is found that sediment
samples having a mean size less than 0.5mm are
generally characterized by a relatively small σg andhence are relatively uniform according to Fig.7 andFig.8. It implies that the relatively coarse sedimentis possibly more non-uniform since there are morevoids in the mixture allowing various finer particles
to fill in. The value of σg in the mainstream(Section-L1) seems not show any obvious relation
with the location or the mean size of the sediment
samples. But in the groin field, there are someinteresting findings. Section-L2 and Section-L3
suggest that the most uniform sediment locates in
the groin bay, followed by the upstream of the groinbay and the downstream area of the groin bay. It
indicates that the existence of the groin bay plays
an important role in the longitudinal sedimentsorting process.

- 515 -

D=5.6mm

Doownstream

D (mm) Bed leevel (m)B level (m)D (mm) Bed

Bed level (m)Upstream
Groin
ns

Distance (m
m)

D (mm
m)

D=5.6
6mm
Upstrream

Downstreeam

Downstreaam

Groinsm
UpstreamGroinsDisstance (m)Distancce (m)Fig.77 Bed level (liines) and meaan sediment ssizes (points) along longitu
udinal sectionns L1, L2 and L3(Left: Secttion-L1; Midddle: Section L2;
L Right: Secction L3)

D ownstream

Upstreamσg

σg Bed leevel (m)σg BedB level (m)Bed level ((m)

Downstrream

UpstreamU

GroinsDisstance (m)Distance (m))

Upstreaamm
Downstream

GroinsGroin
nsDistancce (m)Fig.8 Beed level (liness) and geomeetric standard deviation of sediment (points) along loongitudinal seections
L1, L2
L and L3 (Leeft: Section-L
L1; Middle: Section L2; Right: Section L
L3)
Bed level (m)G
Groin field

D (mm) Bed
B level (m)M
Mainstream

D (mm) Bed level (m)Grooin field

Mainsstream

D (mm
m)

Groin fi
field

Mainstreeam

D=5.6mmm
D=5.6mm

Distance (mm)

Distancce (m)Diistance (m)Fig.9 Bed level (lines) and meean sediment sizes (points)) along transv
verse sectionss T1, T2 and T3
T(Left: Section-T1; Midddle: Section-T
T2; Right: Secction-T3)
σg BedB level (m)Bed level (m)G
Groin fieldM
Mainstream

Distance (mm)σgGroiin field

nstreamMain

Distance (m)Dσg

Bed level (m)Groin fiieldeam
Mainstrence (m)Distanc

Fig.10 B
Bed level (linnes) and geom
metric standarrd deviation ofo sediment (ppoints) along ttransverse sections
T1, T2
T and T3 (Leeft: Section-T
T1; Middle: Seection-T2; Right: Section- T3)- 516 -

3.3 Lateral distribution of grain sizes
In Fig.9 and Fig.10, the mean diameter and thegeometric standard deviation of sediment samples
taken from the Kizu site are plotted along severaltransverse cross-sections, together with the bed
level, the projected area of the groins and the mean
sediment size within the reach. The locations of thecross-sections are sketched in Fig.3. In details,
Section-T1 situates in the upstream of the groin bay,
Section-T2 is across the groin bay and Section-T3
locates at the downstream of the groin bay.
It is noted in Fig.9 that the bed level in the groin
field is lower than that in the mainstream along
Section-T1, coinciding with the fact that the bed iseroded immediately upstream of the permeable
groin. In Section-T2, bed degradation in themainstream and the complex bed topography in thegroin bay are evidently visible. The bed level in
Section-T3 gives a clear image on the main channel,
the minor channel and the deposition area between
them.
The changing trend of either D or σg along
Section-L1 and Section-L2 is rather similar
according to Fig.9 and Fig.10. From the left bank of
the channel towards the mainstream, the sedimentgradually increases in size and the σg also shows
general increase within the groin field except
several points. In the mainstream area, both D andσg are much larger than those in the groin field. The
sediment mean sizes in the mainstream of
Section-T2 are particularly large, around twice ofthe mean sediment size of the reach. It is
understandable as the width of this area isdramatically narrowed by the groin bay andsignificant flow acceleration takes place there.
In the downstream of the groin bay, i.e.
Section-L3, the situations are completely different.
The alternated distribution of coarse and finesediment is observed along this section, indicating
the existence of sand ribbons. Sand ribbons here
refer to the overlay of longitudinal fine sediment
strips on the coarse sediment texture and there areconfirmed with naked eye during the field
investigation. The locations of the fine sedimentpoints indicate that the sand ribbons are situated on
the slopes of the main channel and the minor
channel. The mean sediment sizes of the two sandribbons are very similar but the values of σg are a

bit different. The sediment in the sand ribbon near
the main channel is found to be more uniform thanthat near the minor channel. In fact, the sand ribbon
near the main channel is also much thicker andwider than that near the minor channel according to
the field investigation. These differences are causedby the differences in the bed topography (seeFig.10) and the secondary flow between the main
channel and the minor channel. Previous studies
such as Colombini (1993) suggest that theformation of sand ribbons is closely related tosecondary flows. Although the secondary flows
have a lot of origins such as those due to theinhomogeneity of turbulence, the gradient inchannel geometry and the difference in bottomroughness, the change of the bed topography due tothe existence of groins is considered to play themost important role.
3.4 Grain size distribution in the groin bay
Both longitudinal and lateral grain size
distributions indicate that the sediment particles in
the groin bay are finer and more uniform comparedwith those outside of the bay. Due to the complexityof the bed topography in the groin bay, thedistribution patterns of sediment particles are also
complex. Therefore, it is necessary to draw aclearer image by focusing on the groin bay area. In
Fig.11 and Fig.12, The grain size distributions inboth the Kizu site and the Fuhe site are plotted,
together with the bed level information. It has to bementioned that the upstream groin in the Kizu site
is a permeable groin but that in the Fuhe site is animpermeable one. The permeable groin allows
water and sediment passing through, while theimpermeable one blocks the flow and sediment
until it is submerged.
The sediment samples in the groin bay of the
Kizu site cover a much wider range of sedimentsizes compared with those of the Fuhe site although
the bay area of the former one is rather smaller thanthat of the latter one. Moreover, most of thesediment in the Fuhe site falls in the range of silt
and clay and the size of which is much finer thanthat of the Kizu site. There are two probablereasons. One is the approach flow condition, inparticular the flow direction. Due to the existenceof the sandbar, the approach flow direction in the- 517 -

Kizu site varies according to upstream flow
discharges, which may exert certain impacts on the
direction of sediment transport and sediment size
distributions in the groin bay. But the second one,i.e. the upstream groin condition is mainly
responsible for the differences. The groin bay
closed with an upstream permeable groin exhibits
more diversity in terms of sediment sizes, andhence possibly more diversity from the viewpointof the environmental system. It is interesting tomention that the Kizu site is colonized by several
plant species, while the Fuhe site is dominated by asingle species during the field investigations.

(m)

120

320

520

720

(μm)

920 1120 1320

1040

10.8

10.4
1030

11

1020

10.8

10.6 10.8

High

11

1010
1000

10.6
10.8

990
980

10.4
10.4

10.8

1800

10.6

4.

10.8

1810

1820

1830

11

12

1840

1850

1860

(m)
Fig.11 Mean grain size distribution in the groin bayof the Kizu site(m)0
20 40 60 80 100 120 140 160 (μm)

-20

High

Low

High

-40

High

-60

Low
High

-80
-60

-40

-20

0

20

Discussions

10.8

Low
10.2

1790

10.8

11

Despite the significant differences in themagnitudes of sediment mean sizes, there are somecommon features between the two sites. In the area
close to the mainstream, sediment sizes are
generally larger compared with those in the area far
from the mainstream. It is due to the great
differences in the flow velocity. The sediment sizes
also show some relation with the bed topography. If
one characterizes the bed morphological features inthe two groin bays, the beds are simplified asfollows. In the Kizu site, the bottom left corner may
be considered as a low land area and the other area
is relatively high land with scattered pools. In the
Fuhe site, a large depositional area occupies most of
the groin bay. The depositional area is surroundedby low land area and is like an island. The island isin an ε-shape and has a potential to be separated
into three parts. In both sites, the sediment samples
taken at a high elevation are generally lager in sizes
than those taken at a low elevation although there
are several exceptional points. It indicates that the
low land area in the groin bay serves as sink tankfor fine sediment.

40

60

(m)
Fig.12 Mean grain size distribution in the groin bayof the Fuhe site

4.1 Flow structure
From the field survey results, it comes that thegrain size distributions show strong relations withthe local bed morphology. However, different fromlaboratory experiments under controlled flow andsediment conditions, an explicit relationshipbetween the grain size distribution and the bed
morphology is not readily derivable with limited
information. Besides the possible uncertainties in
the field survey data, the complexity of the bed
surface itself is also a great concern. As is known,
the surface of the alluvial riverbed is generally
active and experiences various flow discharges andcorresponding topographic changes. Each factor orevent has a possibility to leave its footprint more orless upon the bed and these footprints are not easily
isolated. However, these arguments do not meanthat the grain sizes are distributed randomly. On thecontrary, the results presented in the previous
contexts draw clear images of the typical patternsof grain size distributions in the proximity of groinsin actual rivers, corresponding to the typical bed- 518 -

morphologies. It suggests that there should be someunderlying processes and certain rules which thesediment particles follow during their movement.
In order to further the understanding of thegrain size distribution patterns, the flow structures
corresponding to the bed morphologies in theproximity of the Kizu site are investigated withnumerical methods. The flow directly exerts forceson sediment particles and is the most important
engine for sediment movement. The representative
flow fields are simulated with a 2D numericalmodel developed by the first author (Zhang et al.,2006) under two kinds of discharge scenarios:
500m3/s and 2000m3/s. The model is formulated on
unstructured mesh and allows the exact resolving of
the groin details and the irregular river boundaries.
The discharge of 500m3/s corresponds to thecondition that the vegetated sandbar just becomes
submerged. Under this flow discharge, the impactof groins is dominant. The discharge of 2000m3/s isequivalent to the bankfull discharge of the river andthe effects of meso-scale and large scale bed
deformations such as the migration of sandbars are
not negligible any more. The simulated velocity
vectors and their magnitudes are plotted in Fig.13
and Fig.14. Fig.13 shows evidently that the flow
direction is significantly controlled by the groins.
In the upstream reach of the groins, the flow is in a
direction intending to attack the left side of thechannel. But when it approaches the upstream
permeable groin, the flow becomes almost parallelto the bank of the channel. After passing thedownstream impermeable groin, part of the flow isdiverted towards the mainstream. As is known, thedirect benefit is that the bank is prevented from
erosion. In Fig.14, however, the controlling of theflow direction by groins is not evident. Since theflow depth is relatively large, the groins function as
roughness elements. As to the magnitudes of thevelocity, the absolute values are not paid too muchattention. It is observed that alternated distributionsof high and low velocity zones in the longitudinal
direction and significant flow acceleration in themainstream area near the groin bay are evident ineither case. The flow structure gives some hints on
the sediment sorting processes around the groins
and is able to partially explain some of the typical
distribution patterns observed in the field.

(m) 0.2 0.4 0.6 0.8

1

1.2

1.4

1.6

1.8

2

(m/s)

1m/s

1240
1200
1160
1120
1080
1040
1000
960
1680 1720 1760 1800 1840 1880 1920 1960 2000

(m)
Fig.13 Simulated 2D flow vectors and isovels in theproximity of the Kizu site (500m3/s)

(m)

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

(m/s)

1m/s

1240
1200
1160
1120
1080
1040
1000
960
1680 1720 1760 1800 1840 1880 1920 1960 2000

(m)
Fig.14 Simulated 2D flow vectors and isovels in theproximity of the Kizu site (2000m3/s)
4.2 Environmental implications
The results discussed above clearly demonstrate
the discontinuity of substrate compositions causedby groins. Since direct measurements of the aquatic
species are not made in this study for the time being,a quantitative assessment of the quality of thehabitat created by the groins is not readily available.
However, the environmental implications of thegrain size distributions are discussable with the aidof the reference information from previous research
conducted by morphologists and ecologists.
It is well-known that simple relations between
substrate grain size distributions and biological- 519 -

variables may be misleading but the grain sizes do
exhibit close linkage with the distribution andheterogeneity of aquatic species. The periphyton, as
an important indicator of water quality and foodsources for benthic organisms, relies strongly on
the properties of the substrate, e.g. Yamada andNakamura (2002) provide evidences from fieldsurveys that the increasing of fine sediment resultsin decreasing of chlorophyll a and increasing of
non-living periphyton. The bed sediment materials
are inhabited by benthic organisms that provide
food for fish. Hence, the substrate grain size plays
the most important role in the community
composition and density of benthic species. Field
studies on the distribution of macroinvertebrates
indicate that highest diversity is generally found incommunities composed of attachment types inrelatively coarse sediment beds, while cohesive soft
sediments are often densely populated by lessdiverse assemblages of burrowing types (Shields
and Milhous, 1992). The mobility of fishes,
especially adult fishes, complicates the linkages
between sediment properties and fish populations.
However, fishes use sediment directly for cover,feeding and spawning, indicating the importance of
substrate sediment in their life cycles. There isabundant research work related to various fishes
and substrate properties, e.g. Wood and Armitage,
1999, Osmundson et al., 2002 and Sternecker andGeist, 2010.
In general, there is a consensus that a cobble ora gravel bed exhibits more bio-diversity comparedwith a sand or a fine material one and there is awell-established review paper given by Shields andMilhous (1992). Moreover, the deposition of finesediment usually has negative biological effects inthe lotic environment and a recent review article isavailable from Kemp et al. (2011). As the groins
result in riverine areas with a mosaic of substrate
types in their proximities, a diverse distribution of
biological species around them is expectable. On
the other hand, it is also noted that the possible
appearance of immigration species is foreseeable
since new substrate types are created by the groins.
As different substrate types distribute in a verysmall reach around the groins, the tropic levels of
the pristine local species might be changed and thepositions of the immigration species on the foodchain may exert impacts on bio-richness andbio-diversity. From the viewpoint of habitat quality,
the groin bay enclosed by permeable groins ispreferred because generally less fine sediment willaccumulate in the bay area compared with that
enclosed by impermeable ones. A comprehensive
understanding on the habitat quality should bemade by including also other information such flow
fields and bed configurations.
5.

Conclusions

In this study, spatial distribution characteristics
of grain sizes on the bed surfaces around typicalgroins in the Kizu River (the Yodo River basin,
Japan) and the Fuhe River (the Min River basin,
China) are investigated. Based on the research
results, the impact of groins on grain size
distributions in alluvial rivers and its environmental
implications are clarified.
The results indicate that the grain size
distribution exhibits some typical features andshows close relations with the local bed
morphology and the velocity field. The groins
enhance sediment sorting processes in theirneighborhood both longitudinally and laterally. In
the mainstream area, dramatic sediment coarsening
is found in front of and downstream of the groinbay. In the groin field, coarsest sediment appears in
the downstream of the groin bay, followed by the
upstream of the groin bay and the finest sedimentaccumulates in the groin bay. The grain size
distribution patterns in the groin bay is rather
complex. In general, the sediment at a low
elevation is finer than that at a high elevation
within the groin bay. The permeability of the
upstream groin of the groin bay also exerts impact
on the grain size distributions. The groin bay
behind a permeable groin is more diverse in termsof grain sizes. The formation of sand ribbons in thedownstream of groins deserves special attention,
which corresponds to the secondary flows triggered
mainly by changes in the local bed morphology
around groins.
The information presented in the paper provides
first-hand evidences on the grain size distributions
around groins in alluvial rivers. It has to bementioned that the results are consistent with those- 520 -

obtained from fundamental laboratory experiments
and numerical simulations associated with a single
impermeable groin under idealized flow conditions(Zhang et al., 2010). The conclusions drawn fromthe study is of reference for scientific management
and possible improvement of the river groins inboth sites. Moreover, it is also of reference for abetter understanding on the mechanism andconsequences of heterogeneous sediment transport
in local scales. Nevertheless, further field surveys
on the temporal changes and comparisons withexperiments as well as simulations are also
necessary, and which are crucial concerns in the
future study.
Acknowledgements
This research is financially supported by the
Grant-in-Aid for Young Scientist (B), MEXT,
Japan (PI: Dr. H. Zhang, Grant No. 22760369),
SKLH Open Research Fund, Sichuan University(PI: Dr. H. Zhang, Ref. No. SKLH-OF-0904) and
JSPS Asia-Africa Science Platform Program(Coordinator: Dr. H. Nakagawa). Mr. H. Teraguchi,
Mr. S.H. Zhao, Mr. J.Z. Luo and Miss Y. Chen are
sincerely acknowledged for their kind assistanceduring the field surveys. Thanks also go to the
Ujigawa Open Laboratory of Kyoto University andthe State Key Laboratory of Hydraulics andMountain River Engineering of Sichuan University,where the data analyses work were conducted.
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水制域における河床材料の粒度分布特性とそれらの環境関連性に関する研究

張浩・張新華*・中川一・許唯臨*・林鵬智*・水谷英朗***

中国四川大学水理学と山地河川工学国家重点実験室**

京都大学大学院工学研究科

要 旨
本稿は，京都府八幡市木津川（淀川流域）にある流れ橋周辺の環境再生用の水制２基，そして中国成都市近郊府河（長江の支川であるみん江流域）にある航路整備用水制群の２つの領域を対象に，水制周辺の地形及び河床材料の現地調査結果の基にした，河床表層材料の粒度分布特性とそれらの環境関連性に関する研究報告である。今回の調査により，水制周辺における流砂の縦断及び横断方向における分級現象の重要性を指摘した。水制により河床材料の連続性は失われ，
河川主流部の河床では粗粒化現象が見られ，水制と水制の間ではシルトや粘土などの細かい砂が堆積しやすい区間になる。さらに，透過型水制の背後では不透過型のそれよりも河床材料の粒度分布の粒径範囲が広く，ハビタットとしての質は良く，環境にも優しいと考えられる。水制直下流では二次流などの影響で顕在化する粗粒化領域，そしてその周囲に細粒化する領域（サンドリボン）の存在も確認した。また水制周りの地形及び流れに関する情報に配慮したところ，
粒度分布は河床形態と局所的流れ構造と密接な関連性があることも判明された。

キーワード: 水制，流砂の非均一性，粒度分布，サンドリボン，現地計測- 523 -