京都大学防災研究所年報 第 55 号 B 平成 24 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 55 B, 2012

Upstream Channel Responses due to Partial Removal of a Weir
Hao ZHANG, Yasunori MUTO(1), Hajime NAKAGAWA and Shiro NAKANISHI(2)(1)

Department of Civil and Environmental Engineering, University of Tokushima, Tokushima, Japan(2)
Osaka Prefectural Government, Osaka, Japan
Synopsis
This paper presents an experimental study on the hydraulic and morphologicalresponses of a channel due to partial removal of a weir. In particular, the impacts of theremoval shape of the weir and the properties of the bed sediment are focused on. It isfound that weir removal promotes local scour and sandbar development in the upstream
channel. The removal shape significantly affects the three dimensional flow structure
and the local scour around the weir and poses influences on the propagation process of
upstream sandbars at the early stage. Nevertheless, the characteristics of the sandbar
system at the quasi-equilibrium stage are governed by the removal area rather than theremoval shape. The mean grain size and the geometric standard deviation of the bed
sediment mixtures are two practical parameters in characterizing the local scour and thesandbar structure in non-uniform sediment beds. Sand ribbons are observed in
non-uniform sediment beds due to the lateral sediment sorting and the ribbons also
influence the bed morphologies.
Keywords: dam/weir removal, channel response, sandbar, local scour, sand ribbon

1.

Introduction

The aging of dams and weirs, coupled withincreasing awareness of their environmental costs,
has brought dam/weir removal to the attention ofscientific communities, management agencies andthe general public (Doyle et al., 2003). Over the
past several decades, there have been a significant
amount of low head dam or weir removal examples
around the world, either successful or failed, e.g.
Wildman and MacBroom (2005), De Leaniz (2008),
Walter and Tullos (2010). In general, there areseveral alternatives during the modification of adam or weir, for example, a full removal, a partial
removal, a gradual removal, relocation of thestructure or construction of appendage structures.
Up to now, most of the research related to dam/weirremoval are based on limited field survey and/or

post-removal monitoring and the results obtained
are generally site-specific and problem dependent,
e.g. Doyle et al. (2005), Rumschlag and Peck(2007), Kibler et al. (2010). Research withnumerical methods is also carried out by someengineers and scholars (Cui et al., 2006, Cantelli etal., 2007, Zhang et al., 2009), but the applicability
and reliability of those models remain questionable
due to the hypotheses and simplifications
introduced during the model formulations.
Consequently, the understanding on the associatedhydro-morphological processes of post-removal
rivers is still fairly poor and the existing knowledge
is quite insufficient to guide considerations of
dam/weir removal in practices. In order to reduce
the risks and uncertainties in the strategies
development for dam/weir removal, there is a great
demand for expanded research (Doyle et al., 2003).

― 437 ―

A weir to be removed generally persists foryears, hence the river channel typically has adjusted
to the altered hydrologic, hydraulic and sedimenttransport regimes that the weir imposes and thechannel is usually in a quasi-equilibrium state(Doyle et al., 2003). Since the weir blocks the riverand causes river water to back up, the upstream andthe downstream of the weir usually exhibit
completely different sceneries and characteristics,
typically in terms of a water pool and sedimentstorage upstream and associated sedimentcoarsening and bed degradation downstream.
Removal of the weir will trigger hydraulic andsediment transport processes that propagate both
upstream and downstream. The changes in the flow
and geographic parameters furthermore exert
impacts on the biogeochemistry and stream habitat.
Therefore, removal of a weir, like the presence of aweir, represents a disturbance to the river channel.
Moreover, the fluvial process provocated by theweir construction and weir removal may not always
be reversible (Doyle et al., 2005). It is hence
necessary to characterize and quantify the
post-removal channel dynamics and associatedunderlying processes.
In the past several years, the authors’ researchgroup has conducted a series of studies toinvestigate the upstream channel responses to themodification of a weir, particularly in case of apartial removal (Zhang et al., 2008, Zhang et al.,2009, Muto et al., 2009, Muto et al., 2010). A lot ofknowledge has been accumulated on several
governing parameters such as the initial weir poolcondition, the flow discharge, the discharge
hysteresis and the modification scheme of the weir
section. It is found that the weir removal exerts
impacts both near the weir and far from the weir,
typically in terms of local scour development andsand bar propagation. Soon after the removal, thelocal flow and/or bed conditions in the proximity of
the weir change correspondingly. This change
triggers sediment movement and bed variation in
the upstream channel and the change in the
upstream channel then gradually affects theneighborhood of the weir. In general, the influencesof the local scour propagate towards upstream,
while those of the sand bars propagate towards
downstream. As a result, the channel adjusts itself

corresponding to these two dominant processes andtheir interactions. The dominant discharge of theriver is mainly responsible for the longitudinal bed
features and meso-scale sandbar systems in the
upstream channel. After the weir removal, lowflows have a potential to trigger minor channels
while high flows likely to promote bed forms ofdunes/anti-dunes. Irrespective of the initial bed
conditions resulted from different flow discharges,
the channel beds at the quasi-equilibrium states
exhibit similar geographic features after acontinuous running of the dominant discharge. If aweir pool exists before the removal, the remaining
size of the pool is almost inversely proportional to
the removal area of the weir section and local scour
develops near the weir depending on the removal
shape to some extent. If sandbar fronts alreadyapproach the weir section before the removal, thesandbars will enlarge in wavelength and graduallyshift from a single system to multiple ones with anincrease of the removal area. The local changes
near the weir are closely related to the removal
shape of the structure. However, the exact role thatthe removal shape plays is still poorly understood.
Moreover, all the aforementioned conclusions are
drawn based on relatively uniform sediment beds
which are rarely encountered in actual rivers. In fact,as constitute of the bed itself, bed sediment exerts
impact on both the fluvial geomorphology and theaquatic ecology since bed materials are useddirectly by periphyton, benthic species, fishes andplants and serve as an indirect indictor for aquatic
habitat (Shields and Milhous, 1992, Milhous, 1998).
Therefore, the sediment heterogeneity should also
be accounted for prior to any removal action. In
view of those arguments, the responses of the
upstream channel of the weir due to various
removal shapes of the weir section and different
types of sediment beds are focused on in this study.
2.

Experiments

2.1 Experiment setup
A series of experiments were performed in astraight tilting flume at the Ujigawa Open
Laboratory, Disaster Prevention Research Institute,
Kyoto University (Japan). The flume has arectangular cross-section and is 21m in length,― 438 ―

B1

50cm in width and 30cm in depth. An inlet tank anda small reservoir locate at the upstream and thedownstream of the flume, respectively, and thewater in which is re-circulated with a pump-pipe
system. The slope of the flume is kept as 1/200
during the experiments. A model weir of 11.5cm inheight, 50cm in width and 2cm in thickness is set at
13m from the inlet tank. The upstream reach of theweir is covered with 8.5cm-thick silica sedimentwith a mean diameter of 0.163cm, forming a
movable bed area (see Fig.1). The experiment setup
is sketched in Fig.1 and the photo of the experiment
flume is shown in Photo1.
Weir

B2
B
Relative removal width B* and height H*
B*=B0/B,H*=H0/H
Fig.3 Definition of governing parameters

Case1
A

50

y
x
Instrument carriage

Movable bed

Inlet tank(a) Plan view

8.5

zy

8.5 1.5 1.5

Photo 1 Experiment flume17

Case1,7,8

9

32

Case8

Case8
Case7

Flow meter(b) Section A-A
Fig.1 Sketch of experiment setup (Unit: cm)

17 16

Case7

Case1

x

Pump

B2

1300

A

Fixed bed

B0

H

Percentage finer (%)

800

H0

Bs

9

9

32

9

13

24
8

13

Case2

Case3

11 8 12 8 11

85 9 6 9 58

Case4
Case5
Case6
Fig.2 Weir section post- removal (Unit: cm)

Sieve size D (mm)Fig.4 Sieve analysis curves of sediment samples
2.2 Experiment conditions
In general, two kinds of experiments are
performed consisting of 8 cases in total. In all thesecases, the weir is partially removed with the section
being lowered both across the channel and in thevertical plane. The first kind (Case1 to Case6,
hereafter referred to as the U-series) tests theimpacts of the removal shape on the upstream
channel processes. In these experiments, themovable bed is composed of uniform sedimentmaterials with a mean diameter of 0.163cm. The
removal area is fixed, while the removal shapevaries from case to case. The post-removal weir
sections are sketched in Fig.2. In general, they canbe categorized into three types. Type1 tests
rectangular removal section with different removal
heights (Case1 and Case2). Type2 uses trapezoidal
removal section with bases of different lengths(Case1, Case3 and Case4). Type3 adopts various
slit-type removal sections (Case1, Case5 andCase6). For clarity, several governing parameterscharacterizing the removal shape are sketched in― 439 ―

Fig.3. The second kind (Case1, Case7 and Case8,
hereafter referred to as the M-series) tests theimpacts of the bed composition on the upstream
channel processes. The removal shape and the mean
diameter of the movable bed is kept the same, while
the bed composition differs in each case. Threetypes of sediment beds are prepared for theexperiments by mixing uniform silica sands ofdifferent sizes. The sieve analysis curves of thesediment samples taken from the initial flatbeds areshown in Fig.4 and several characterizing
parameters are listed in Table1. It is evident that the
sediment mixtures may be categorized as uniformly
graded (Case1), well graded (Case7) and gap
graded (Case8), respectively.
Based on the knowledge and experiences fromprevious studies of the authors, the water discharge
in the flume is kept constant as 8.16l/s, representing
a shear stress ratio of τ*/τ*c=2.0. Here, τ* and τ*c are
dimensionless shear stress and dimensionlesscritical shear stress, respectively. This flow
condition, applying on the sediment beds in thecurrent experiments, corresponds to a dominantdischarge scenario in typical Japanese rivers
according to Yamamoto (1997). The hydraulicconditions are detailed in Table2.
Table 1 Sediment properties
Parameters

Case

Maximum grain size D90(cm)
Geometric
standard
deviation σg
Uniformity coefficient Cu
Curvature coefficient Cc

Case1

Case7

Case8

0.204

0.250

0.252

1.200

2.151

2.953

1.453

5.932

8.815

1.286

1.968

0.168

Table 2 Hydraulic parameters
Discharge Q (l/s)

8.16

Bed

1/200

slope I

Flow depth h (cm)Mean velocity U (cm/s)

4.18
39.0

Sediment size D (cm)0.163

u*/u*c1.40

Reynolds number

12,651

Froude number

0.61

In Table1 and Table2, σg is the geometricstandard deviation and is obtained from (d84/d16)1/2,
Cu is the coefficient of uniformity and is defined as
d60/d10, Cc is the coefficient of curvature and isexpressed by d302/(d60d10) , u* is the friction velocity
and u*c is the critical friction velocity.
Near the inlet of the flume, the bed elevation ismade a little higher than the immediately
downstream area as sediment reserves. Whileduring the experiments, additional sediment supply
is not made from the upstream end.
2.3 Experiment procedures
Each experiment follows such a procedure: 1)
The sediment bed surface is levelled to a flat initial
one with a scraper blade; 2) When the desired water
depth is achieved upstream of the weir by slowly
filling water in the flume, the pump is set to thedesired discharge; 3) The propagation process of
sandbars in the channel is memorized if necessary
until a quasi-equilibrium condition is reached; 4)
The water level along the centerline of the flume isrecorded with a point gauge and the velocity near
the weir is measured with an electromagnetic
velocimetry (Model ACM250-A, JFE Alec Co.,Ltd); 5) The pump is stopped and the flume isdrained out; 6) The bed level is measured with alaser displacement meter (Model LK-500, Keyence,
Co., Ltd) and the setup of the pre-removal
condition is completed; 7) The flume is again filledwith water until the desired water depth is achieved
and the desired discharge is then set properly andthe weir is partially removed at the same time; 8)
The propagation processes of sandbars as well as
the water level, the velocity near the weir and thebed deformation at the quasi-equilibrium state areinvestigated following the same procedure as thatof the setup of the pre-removal condition. The
quasi-equilibrium state is mainly examined basedon the propagation characteristics of sandbars. In
Case7 and Case8, sediment samples are taken fromthe bed surface at several representative points witha sampling spoon soon after the bed elevations are
surveyed. The sampling depth is about 2.5mm,
corresponding to the maximum grain sizes of thesediment materials used in the experiments. The
samples are analyzed with a nested column ofsieves after being completely dried, together with a― 440 ―

3.

Results

3.1 Temporal variation of bar front(1) Pre-removal stage
Soon after the flow passes the initial flatbed,
sediment particles move. But the movement isgenerally confined in the area a little far from theweir at the beginning of each experiment. Sandbar
develops quickly and the bar front is obviously
distinguishable within several minutes. The
movement of the bar front is depicted in Fig.5.
In the U-series experiment cases, the bar front
propagates towards downstream with a decreasing
propagation velocity and becomes almost stagnant
with a propagation velocity less than 0.5cm/min at alocation about 2m away from the weir after 160min.
A stable weir pool forms and a quasi-equilibrium
condition is assumed.
In the well graded case (i.e. Case7), Bar front
mainly consisting of fine particles appears soonafter the start of the experiment and propagatesdownstream with a decreasing propagation velocity.
During the propagation of the bar front, new bar
front consisting of coarse sediment forms in the
upstream area. The new bar front moves faster thanthe old one. It overlies on the old one andpropagates ever downstream with a decreasing
propagation velocity. The new bar front is not the
only competitor, a newer bar front composed ofeven coarser particles was born before the first front
is colonized by the second one. The newer one
propagates downstream and soon replaces the
position of the second one. The process is notfinished yet, the fourth and the fifth bar fronts
emerge one by one, consisting of coarser andcoarser sediment particles. The fifth bar front
becomes the final winner, it becomes almost
stagnant at a place around 1.5m away from the weir
after 210min. It has to be mentioned that five kindsof relatively uniform sediment particles are used toprepare the mixed sediment in this case, with amean size of 0.31, 0.48mm, 1.03mm, 1.61mm and2.38mm, respectively. The development of the bar
fronts gives evidence on the sediment sorting
process of these different size fractions.

Distance from the weir(cm)high-resolution balance scale (UW220H, Shimadzu
Co., Ltd).

Case1
Case7
Case8

Time (min)Fig.5 Movement of bar front before removal
In the gap graded case (i.e. Case8), sedimentconsists of obviously distinguished two size
fractions: a fine fraction with a mean size of 0.31mm
and a coarse fraction with a mean size of 2.38mm.
Ripples of fine sediment particles immediately
appear after the start of the experiment. After that,bar front mainly consisting of fine sediment isobserved, but the front expands towards downstream
and soon becomes invisible to the naked eye.
Therefore, the obviously distinguished front
consisting of coarse fraction sediment is recorded. It
is very clear that zigzags are observed on the path
down to the weir as shown in Fig.5, especially at theinitial stage. It indicates that the bar front propagatesdownstream following such a pattern as fast, slowly,
fast again and slowly again and this pattern isrepeated until the front approaches the weir. In thisprocess, the accumulation of fine sediment plays acrucial role. The existence of thin fine sedimentlayers reduces the local bed resistance and results inthe movement of coarse sediment. In fact, the rapid
movement of coarse particles on fine sediment
stripes is evidently observed during the experiments.
The overlay of fine sediment stripes on the coarse
sediment texture is termed sand ribbons in this paper
and is discussed later in the context. When the bar
front propagates close enough to the weir after
230min, particles on the upper part of the front istransported over the weir to the downstream by the
local flow and the height of the front is lowered.
After that, such process is repeated that the sedimentfrom upstream is transported to the front and is
carried by the local flow to the downstream of theweir. It has to be mentioned that the bar front
maintains a shape like a chin although it expands to― 441 ―

Distance from the weir(cm)(2) Post-removal stage
As soon as the weir is removed, the bar front
propagates immediately and the weir pool shrinks
rapidly as shown in Fig.6. The bar front moves
downstream with an ever decreasing velocity,
approaches the proximity of the weir section andloses its identity finally. The propagation velocity ofthe bar front, and hence the time to approach theweir, differs from case to case. After thedisappearance of the dominant bar front, bars in the
more upstream area develop towards downstream
and repeat the life cycle of the dominant one.
In the M-series experiment cases, the bar front
approaches the weir section earlier if σg is larger.
Moreover, sand ribbons also gradually change withtime. The variations of the sandbars and the sandribbons are not two independent processes, but
exhibitlinkagestosomeextent.
The
quasi-equilibrium conditions are assumed after a
180min-running (Case7) and a 90min-running(Case8) based on examinations of the propagation
characteristics of the bars and the sand ribbons.
In the U-series cases, the bar front generally
arrives at the weir earlier if the removal shapeexhibiting a smaller value of H*/B* (Type1), B2/B1(Type2) or Bs/B0 (Type3). As it will be discussed
later, those values show strong relations with the
maximum local scour depth around the weir. On theother hand, the local scour around the weir has apotential to alter the downstream boundary
conditions governing the propagation of sand bars
such as the water stage, channel thalweg and bed
slope. However, there is also an exception, i.e.
Case3 and Cas4. Although the value of B2/B1 in
Case3 is larger than that in Case4, the propagation
velocity in Case3 is faster than that in Case4. It
suggests that the removal shape should influence thesandbar properties in a rather complex way and the
maximum scour depth along would be not enough to
explain the development of the bar fronts. In theuniform bed cases, the quasi-equilibrium conditionis assumed 180min after the bar front arrives at theweir section based on the examinations of thetransport properties of the bar systems.

Case1

Case7

Time (min)(a) Non-uniform bed cases
Distance from the weir(cm)the weir section. The experiment finally lasts
300min when the sand bar structures and sandribbons in the flume show insignificant changes.

Case3
Case2
Case6

Case1
Case5
Case4

Time (min)(b) Uniform bed cases
Fig.6 Movement of bar front after removal
3.2 Bed deformation
The changes of the bed level from the initial
flatbeds to the pre-removal and final stages areshown in Fig.7 and Fig.8.
(1) Influence of removal shapeComparing the post-removal cases with the
pre-removal one in Fig.7, it is evident that the weirremoval triggers sediment movement both near andfar from the weir. It is very clear that sandbars
occupy almost the whole domain of the movable
bed area and the local scour develops at theproximity of the weir section after the partial
removal of the weir. The weir pool formed at the
pre-removal stage is significantly aggraded due tothe invasion of the sandbars. Alternating bars areevident in all the experiment cases. The propertiesof the alternating bar after the removal are very
similar in each case irrespective of the differencesin the removal shape. It provides further evidenceson the authors’ previous findings, i.e. it is theremoval area, rather than the removal shape, thatplays the dominant role in the determination of theproperties of the meso-scale sandbar system.

― 442 ―

Fig.8 Bed deformation from the initial flatbed preand post- weir removal (Case7-8)

Fig.7 Bed deformation from the initial flatbed preand post- weir removal (Case1-6)(2) Influence of sediment heterogeneity
Comparing the pre- and post-removal cases in
Fig.7 (a), (b) and Fig.8 (a)-(d), one may recognize
significant differences in the bed morphologies
among them. The differences are observed in termsof not only the size of the weir pool but also theproperties of the sandbar system.

If simply quantifying the non-uniformity of the
bed sediment with σg, there are several interestingfindings. At the pre-removal stage, the weir poolarea decreases with an increase of σg. Moreover, thehead of the sandbar protrudes into the weir pool andthe protruding length increases with an increase ofσg. In addition, the elevation of the bar head ishigher if the value of σg is larger. Due to the
protruding of the bar head, the weir pool exists only
along the two sides of the channel in Case8. As tothe sandbar system in the upstream channel,
alternating bar is clear in the uniform case, i.e.
Case1, while the bar system in the non-uniform
cases is a bit difficult to be characterized. On theother hand, longitudinal streaks are visible from thefigures, especially in Case8. These streaks arealmost parallel to the side of the flume channel andshow certain relationship with sand ribbons which
will be discussed later. It may also indicate thatsand ribbons influence both the bed composition
and the bed morphology.
After the partial removal of the weir, the local― 443 ―

3.3 Water level and longitudinal bed slope
The water level and the bed elevation along thecenterline of the flume are shown in Figs.9-11. For
clarity, the impacts of the removal shape (U-series
experiments) and the bed sediment heterogeneity(M-series experiments) are discussed separately.
Case3

z (cm)

Pre-removal

Case1

Case2

Case4

Case5

Case6

x (cm)
Fig.9 Water level along the centerline of the flume(Cases 1-6)

z (cm)

Case4

Case5
Case1

Case2
Case3

Case6

Pre-removalx (cm)
Fig.10 Bed level along the centerline of the flume(Cases 1-6)

Pre-removal: Case1
Water stage Post-removal: Case1

Case2 × Case3
Case2 ж Case3z (cm)

scour develops at the proximity of the weir and thesandbar front approaches the local scour area ineach case. In the uniform case, i.e. Case1, thewavelength of the alternating bar increases. In the
non-uniform cases, i.e. Cases7 and 8, the bed levelof the head of the sandbar is significantly lowered
and the weir pool area is aggraded. As a result, weirpools are not distinguishable at the post-removal
stage. The sandbar system in the non-uniform cases
is not readily recognizable from the figures.
However, the similarities of the contours suggestthat there seem no inherent changes in the bar
structures except certain changes in the
wavelength and the wave height.

Pre-removal: Case1
Bed level Post-removal: Case1

Case2
Case2

Case3
Case3

x (cm)
Fig.11 Water level and bed elevation along thecenterline of the flume (Cases 1, 7 and 8)
Before the weir removal, the pool area isobviously distinguishable according to either thewater surface profile or the bed level as shown in
Figs.9 and 10. The pool area is characterized by arather big water depth. After the removal, waterlevel decreases, particularly in the previous weirpool area. On the other hand, the bed in theprevious weir pool area is greatly aggraded. The
reduction in water depth is expectable. Both theslope of the water surface and the longitudinal bed
slope become much steeper than the pre-removal
condition. It is known that all these changes
promote sediment movement and the propagation ofsandbars. Moreover, the slopes of the water surface
and the longitudinal bed are very similar in the area
a little far from the weir in all the post-removal
cases, while discrepancies are recognized in theproximity of the weir. The observations andfindings here well coincide with those in Fig.7.
Concerning the M-series experiments, the bed
elevations vary a lot from case to case before theweir removal. A bigger σg generally corresponds toa higher bed elevation. The dominant bar front at
the pre-removal stage is evident in each case.
Immediately upstream of the bar front, the local bed
slope becomes negative. Compared with the
non-uniform cases, this slope is much steeper in theuniform case. The bar system is recognizable in theuniform case, but is not evident in the non-uniform
cases. After the partial removal of the weir, the bar
front propagates downstream and the negative slope
area is enlarged in the uniform case. In the― 444 ―

non-uniform cases, bed slopes become muchsteeper with the disappearance of dominant sandbar
fronts and severe bed degradations. Bed level andbed slope are similar in the area far from the weir in
the two non-uniform sediment cases. But near theweir, bed level is higher and bed slope is steeper in
Case8 if compared with those in Case7.
Despite the great differences in the bed
elevations, the water levels near the weir are very
similar in all the M-series experiment cases,
indicating that the weir structure plays an importantrole in controlling the water stage in its
neighborhood. In the far upstream area, the impactof the weir becomes weak and the water levelprofile follows that of the longitudinal bed
elevation in each case. Before the weir removal, theslope of the water surface is almost 0 or evennegative, corresponding to the weir pool or thenegative bed slope area following the dominant bar
front. After the removal of the weir, water levels
decrease. The decreasing amount is particularly
large near the weir due to the significant change inthe weir elevation there. Since the decreasing
amount gradually increases from the upstream tothe downstream, the slope of the water surface
becomes steeper than that at the pre-removal stage
either near the weir or far from the weir.

Taking a look at the U-series experiments, it isfound that the scour depth increases with anincrease of either H*/B*, B2/B1 or Bs/B0. In otherwords, deeper scour holes may be intendedly
created by increasing the removal height or theremoval length at the lower part of the weir or bysimply decreasing the number of openings in caseof a slit type removal. This may be explained byconsidering the mechanism of the formation of
local scour holes, especially the strength of thevortex systems in the proximity of the weir section.
It is understandable that stronger vortices are easily
induced by the removal schemes with larger valuesof H*/B*, B2/B1 or Bs/B0 as will also be seen in theplots of the velocity profiles later. The local slopesof the scour holes also deserve special attention. In
each case, the slope in the longitudinal direction is
generally steeper than that in other directions andthe slope towards the centerline of the flume isgenerally steeper than that towards the side of the
flume. The slopes of the scour holes are closely
related to the local flow structure, particularly thestrong circulating flows in the scour area.

Flow

Section A
Maximum scour

3.4 Local scour near the weir
It has been found in Figs. 7 and 8 that partial
removal of the weir will promote local scour near
the weir. The typical scour hole is sketched in
Fig.12. Although the local scour covers a very
limited area, it exhibits great differences from caseto case according to Figs. 7 and 8. It is also foundfrom the figures that the number of scour holes
generally equals to the number of openings plus one
in case of a partial removal. The scour holes may
also change their geometries periodically according
to the shifting of sand bars immediately upstream ofthem. In this study, the mean values of the scour
hole depth of each case are selected to characterize
the local scour phenomena and the maximum andminimum values are also listed for reference. These
values are denoted as d, d1 and d2 correspondingly
as shown in Table3. Moreover, representative
angles showing the surface slope of the left-side
scour hole are also listed.

― 445 ―

Movable bed

Section B
Fig.12 Sketch of scour dimensions
Table 3 Local scour dimensions
Case1

Case2

Case3

Case4

H*/B*
B2/B1

3.125
1

0.781
-

0.333

0

Bs/B0

1

-

-

-

d1(cm)

8.50

2.86

7.70

4.13

d2(cm)

6.01

1.63

6.58

1.15

d(cm)

7.26

2.25

7.14

2.64

o

()
(o)

32.0
30.0

32.0
33.0

32.0
21.0

33.0
12.0

(o)

24.0

7.7

7.5

10.0

Table 3 Local scour dimensions (continued)
Case5

Case6

Case7

Case8

H*/B*

-

-

-

-

B2/B1

-

-

-

-

Bs/B0

0.5

0.333

-

-

d1(cm)d2(cm)

5.72
5.19

4.92
4.08

7.14
6.49

5.72
5.61

d(cm)

5.47

4.45

6.82

5.67

o

()

31.0

28.0

27.0

30.0

o

()

19.0

21.0

29.0

22.0

o

16.0

20.0

16.0

11.0

()

Upstream

Photo 2 Sand ribbons in Case8 (pre-removal)
Upstream

In the M-series experiments, the scour depth
decreases and the local slopes increases with anincrease of σg. The findings are similar to those ofscour holes around a spur dyke as observed byZhang et al. (2012) based on a series of experimentswith non-uniform sediment. The sediment sorting
process in the proximity of the weir is mainly
responsible for this phenomenon. Similar to theresults of the U-series experiments, the local slopeof the scour surface in each case generally follows
the sequence of α>β>γ , in which α, β and γ are
local slopes of the scour hole in three representative
directions as depicted in Fig.12.
3.5 Bed composition and sand ribbons
In the M-series experiments, the sedimenttransport not only promotes changes in the bed
topography, but also results in changes of the bed
composition. The appearance of sand ribbons asmentioned before is a phenomenon that requires
special attention. The typical sand ribbons areshown in Photos 2 and 3 for Case8. During theexperiments, the fine sediment stripes which are
distinguishable with a naked eye are memorized
and are sketched in Fig. 13, together with the bed
contour maps. It has to be mentioned that the finesediment here is not readily defined by a specificgrain size. If the particles are obviously finer than
the surrounding ones, they are termed fine sediment.
Moreover, sediment particles in sand ribbons are
not necessarily uniform. The dimensionless grain
sizes, i.e., the ratios of the mean grain sizes of thequasi-equilibrium bed to those of the flatbed, atrepresentative locations are also plotted in Fig.13.

Photo 3 Sand ribbons in Case8 (post-removal)

Fig.13 Dimensionless grain size distribution(scatters), sand ribbons (belts) and bed
deformation (background maps)― 446 ―

3.6 Velocity profiles near the weir

The flow field around the weir is an important
key to understand the sediment transport and bed
variation processes. When local scour occurs, theflow generally becomes three-dimensional and iscomposed of complex vortex system. In this study,
the velocity profiles in a typical transverse section
A and a longitudinal section B (sketched in Fig.12)
in the U-series experiments are measured. The
experimental data is plotted in Figs.14 and 15,
together with the bed profiles and weir sections.
An investigation on both the transverse and thelongitudinal velocity profiles at the pre-removal
stage demonstrates that a strong upward flow withalmost no transverse velocity component forms
when the flow approaches the weir section. Near
the bed, the flow velocity is quite small. As a result,almost no sediment movement could be observed in
the proximity of the weir at this stage.
After the partial removal of the weir, the flow
velocity and the local bed dramatically change andinteract with each other. The plots showing thetransverse cross section (Fig.14) suggest that thetransverse velocity component has been intensified
and that the directions of the velocity are strongly
dependent on the geometry of the local scour holes.
The intensified transverse velocity plays a dominantrole in the lateral expanding process of the scourhole. In addition, weak vortices occur in Cases 5
and 6, corresponding to the scour geometries.
10 cm/s

6z (cm)

4
2
0
-2
-4
-6

Pre-removal
0

4

8

12

16

20

y (cm)

24

10 cm/s

42z (cm)The spatial variation of the mean grain size isevident according to Fig.13, which is a direct resultof the sediment sorting process. The sedimentsorting process occurs both longitudinally andlaterally, triggering bed changes in terms of bothconfiguration and composition. The sorting process
is furthermore affected by the sandbar systems andsand ribbons. Consequently, the distribution of themean grain sizes exhibits a very complex pattern. In
general, sediment is coarser in the bed surface witha relatively higher elevation where the water isshallow and the flow is rapid. Due to the
propagation of sandbars, the bed is generally
degraded in the upstream part and is aggraded in
the downstream part from the pre-removal stage tothe post-removal stage. Correspondingly, the grain
sizes generally become smaller in the upstream partand turn larger in the downstream part if one
compares the post-removal stage with the
pre-removal stage. However, the occurrence of sandribbons makes the situation rather complex. It isfound in Fig.13 that relatively fine sediment also
appear in the bed with a very high elevation due tothe existence of sand ribbons.
Sand ribbons are caused by lateral sedimentsorting and are documented to exhibit a strong
relation with secondary currents in channels(McLean, 1981, Tsujimoto, 1989, Colombini, 1993,
Zhang et al., 2012). Since the formation of sandbar
systems in the channel triggers much stronger
secondary currents compared with a flatbed, theformation of sand ribbons in both pre-removal andpost-removal stages are understandable. The sandribbons are almost parallel to the side of the flume
but show certain changes where bed level changes
significantly, demonstrating that the properties ofsand ribbons are directly affected by the sandbar
structure. On the other hand, the characteristics ofthe sandbar system in the non-uniform beds are notunambiguous determinable and longitudinal steaks
are visible in the bed contour maps as have been
mentioned before. It suggests that the sand ribbons
exert influences on the properties of the sandbar
system. In other words, sandbars and sand ribbons
are strongly coupled phenomena in non-uniform
sediment beds.

0
-2
-4
-6

Case1

-8
0

4

8

12

16

20

24

y (cm)
Fig.14 Velocity vectors (v, w) in the transverse
cross-section A― 447 ―

10 cm/s
4

6

10 cm/s

4

4

2

2

0

0

10 cm/s

-2
-4
-6

Case2

-8
0

4

8

12

16

20

24

y (cm)

-2

-4

-6

-6

Pre-removal
0

10 cm/s

4

Case1

-8
0

8

4

8

x (cm)

2

12

10 cm/s

6

-4
-6

Case3

-8
0

4

8

12

16

20

y (cm)

24

10 cm/s

4
2

4

4

2

2

0

0

z (cm)-2-2

10 cm/s

-4

-6

-6

Case2
0

0

-2

-4

-8
4

16

x (cm)0z (cm)z (cm)

-2

-4

-8

4

z (cm)z (cm)0z (cm)z (cm)

2

Case3

-8
0

8

4

8

12

x (cm)x (cm)-2

10 cm/s

-4
4

-6

Case4
0

4

8

12

16

20

y (cm)

2

24

z (cm)-8

10 cm/sz (cm)

4

0
-2

2

-4

0

-6

-2

Case4

-8

-4

0

4

-6

Case5
0

4

8

12

16

20

y (cm)

10 cm/s4z (cm)z (cm)

2
0-2

12

10 cm/s24

10 cm/s

4

4

2

2

0

0

z (cm)-8x (cm)

8

-2

-2

-4

-4

-6

-6

-4
-6

Case6

-8
0

4

8

12

16

20

24

y (cm)
Fig.14 Velocity vectors (v, w) in the transverse
cross-section A (continued)

Case5

-8
0

4

8

12

Case6

-8
0

x (cm)
Fig.15 Velocity vectors (u, w)
longitudinal cross-section B― 448 ―

4x (cm)
in the8

Along the longitudinal section (Fig.15), an
evident circulating flow occupying the local scour
hole is observed in each case. This circulating flow
is an energy source for the development of thescour hole in the longitudinal direction.
It is also found that the bed level at the openingsof the weir section is generally higher than thesurrounding area. As a result, the flow is directed tothe openings with an upward vertical velocity
component. The observations suggest that thepartial removal of a weir triggers several 3D
vortices and these vortices are the major controllerof the scour process around the weir. The transverse
velocity components are sensitive to the removal
shape of the weir. Therefore, by carefully selecting
the removal shape, the transverse velocity and the
local bed profiles may be altered as desired.
4.

Discussions

The results obtained from the experiments
demonstrate that the partial removal of a weirstructure exerts great impacts on the upstream
channel dynamics. The following parameters andprocesses are considered to be of great importanceprior to a removal action.
According to the results of the current
experiments, it is found that σg is the mostdominant parameters to characterize the responsesof the upstream channel to a weir removal action in
non-uniform sediment beds if the mean grain size is
kept constant. Therefore, the mean diameter D andthe geometric standard deviation σ g may be used as
simple indicators in the analyses for practical uses.
The experimental observations and quantifiabledata indicate that the sediment sorting process is an
important phenomenon in non-uniform sedimentbeds. The dominant flow and the secondary flow
trigger sediment sorting both longitudinally andlaterally. The sediment sorting processes influence
the channel geometry, the bed resistance and the
bed composition. Consequently, bed morphology ina non-uniform bed exhibits quite different featuresfrom that of a uniform bed. The topographic
patterns of sandbars in a non-uniform bed are more
irregular than a uniform one and are not easily
determinable mainly due to the formation of sandribbons. Although more quantitative information isneeded to understand the weir removal impacts on
the sediment sorting process, it is evidently
confirmed that the weir removal will likely enhance
the lateral sediment sorting process.
The removal shape continuously controls the
local scour development and influences the
propagation velocity of the sandbar front before it
approaches the weir proximity. Nevertheless, thefinal sandbar properties at the quasi-equilibrium
condition are found to be somehow independent of
the removal shape. Based on the experiment results,
there are some probable reasons. Although the weir
section provides a downstream control for the
upstream hydraulic and morphological processes,
the influences of the removal shape are mostly
eliminated due to the self-adjustments of the local
bed geometry and the local flow field around theweir. As a result, the information on the removal
shape is almost lost when the influences of the weirremoval propagate towards the upstream of thechannel at the quasi-equilibrium stage. Since ittakes time for the self-adjustments of the flow andthe bed, the removal shape has a chance to makeinfluence but on limited area and within limited
time before the completion of the self-adjustments.
Three dimensionless parameters H*/B*, B2/B1
or Bs/B0 have been proposed to characterize theremoval shape. According to Figs.14 and 15, one
may conclude that those parameters make a wellcontrol on the local flow structures near the weiralthough the flow field is highly three dimensional.
With those parameters, one may obtain some
general idea on how many circulating cells may
occur, where the flow circulation may take placeand hence what kind of scour phenomenon may be
expected. In addition, it also sheds a light on
possible prediction of the local scour depth.
Unfortunately, the limited experiment data in thisresearch does not allow the derivation of a reliable
scour formula for the time being. However, theresults suggest that a removal shape of a shallow
rectangular or a triangle, or a removal byintroducing a lot of slits is recommended in orderto reduce the local scour around the weir.
5.

― 449 ―

Conclusions
A series of experiments on the upstream channelresponse to the partial removal of a weir structure
have been performed.
Weir removal promotes local scour near theremoved section and sandbar development in the
upstream channel. The two processes dominant thehydraulic and morphological processes in thechannel. Both the local scour and the sandbar
development are strongly related to sedimentnon-uniformity. Moreover, sand ribbons appear in
non-uniform sediment beds as a result of lateralsediment sorting, which furthermore promotecoarse sediment movement and the development ofsandbars and hence exert impacts on the flow
structure. The removal shape directly affects the
local flow structure and local scour depth aroundthe weir. The removal shape initiates and, to someextent, provides an important boundary for thedevelopment of the local flows. The local flow is
generally highly three dimensional. Moreover, thelocal flow and the local scour are strongly coupledparameters. They adjust themselves according to
the changes in each other. The removal shape also
exerts impact on the propagation characteristics of
upstream sandbars before the disappearance of theweir pool but does not exhibit significant influenceson the final properties of the sandbar system at thequasi-equilibrium stage. The removal shape can becharacterized with three dimensionless parameters,
i.e. H*/B*, B2/B1 or Bs/B0. It is found that the local
scour depth shows positive relationship, while thepropagation velocity of the sandbar front shows
negative relationship with the three parameters.
The findings in this research encourage thedevelopment of practicable scour prediction
formulae and possible methods to quantify sandbar
properties such as bar types, wavelength and wave
height. However, more experiment data is necessary
to further the understanding on and to characterize
the flow structure and the sediment movement.
Acknowledgements
The research is financially supported by the
Grant-in-Aid for Scientific Research (C), MEXT,
Japan (PI: Dr. Y. Muto, Grant No. 22560511) andthe JSPS Asia-Africa Science Platform Program:Creation of Research Hub for Sediment Disasters
and River Environment in Mountainous Area

(Coordinator: Dr. H. Nakagawa).
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堰の部分撤去による上流河道の応答特性

張浩・武藤裕則(1)・中川一・中西志郎(2)(1)

徳島大学大学院ソシオテクノサイエンス研究部
(2)

大阪府

要 旨
本稿は，堰の部分撤去による上流河道の水理学及び地形学的な応答特性に関する実験的研究である。特に，堰の改変
形状及び河床材料の粒径特性について注目した。堰を部分撤去した場合は，堰周辺には局所洗掘，河道上流域には砂州の移動が生じた。堰の改変形状が堰近傍の三次元流れ場及び局所洗掘を促進し，上流砂州の伝播特性にも影響していることが確認された。準平衡状態においては，砂州の特性は堰の改変形状よりも堰の改変面積との関連性は深いことが示された。混合砂での実験では，河床材料の平均粒径及び標準偏差による河道の応答特性に大きいな違いが見られた。また，河道横断方向の流砂分級現象による河道縦断方向にはサンドリボンと呼ばれる細粒径砂による帯が形成され，そのサンドリボンは河床地形に影響を及ぼすことも確認された。
キーワード:ダム・堰撤去，河道応答特性，砂州，局所洗掘，サンドリボン― 451 ―