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Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1051
DOI: 10.4408/IJEGE.2011-03.B-114
FUNDAMENTAL HYDRAULIC FLUME TESTS FOCUSED ON SEDIMENT
CONTROL FUNCTION USING A GRID-TYPE HIGH DAM
t
akaHiRo
ITOH
(*)
, s
HiGeo
HORIUCHI
(**)
, J
un
-i
CHi
AKANUMA
(***)
, k
azuHiko
KAITSUKA
(****)
,
s
enRo
KURAOKA
(****)
, t
akenoRi
MORITA
(****)
, m
inoRu
SUGIYAMA
(****)
& t
akaHisa
MIZUYAMA
(*****)
(*)
Takahiro ITOH El Koei Co. Ltd. (Now in Nippon Koei Co. Ltd), 2304 Inari-hara, Tsukuba, Ibaraki, 300-1259 Japan
(**)
Fujikawa Sabo Office, Ministry of Land, Infrastructure and Transport (MLIT) (Now in Sabo Frontier Foundation), Japan
(***)
Fujikawa Sabo Office, Ministry of Land, Infrastr. and Transp. (Now in Shimonoseki-city municipal office, Yamaguchi Pref.), Japan
(****)
Nippon Koei Co. Ltd, Japan
(*****)
Graduate School of Agriculture, Kyoto University, Japan
tionally, preliminary flume tests for sediment control
using a kind of “steel wire-nets” near the vertically
upper parts of the grid were conducted focusing on
the simple and the effective countermeasures for the
sediment storage in a sabo dam, in order to control the
sediment runoff passing through the sabo dam after a
lot of sediment is deposited in the sabo dam.
K
ey
words
: grid-type sabo dam, high dam, debris flow, flume
tests, sediment control with steel nets
INTRODUCTION
There are two kinds of sabo dams (check dams),
which are closed and open-type. Recently, in Japan,
open-type check dams are constructed taking into ac-
count the sediment continuity from upstream to down-
stream reach in a basin, though there are some matters
to be newly solved for sediment control function by the
closed type sabo dam. There have been a lot of experi-
mental and numerical researches for sediment control
with open-type check dams in all over the world (e.g.,
a
sHida
et alii, 1980, 1987; m
izuyama
et alii, 1997
& 2000, a
Rmanini
et alii, 2001; C
Hen
et alii, 1997;
H
eumadeR
, 2000; l
in
et alii, 1997; w
u
et alii, 2003;
s
atofuka
et alii, 2005 and t
akaHaRa
et alii, 2007), the
minimum grid size of grid-type check dam, which is a
kind of open-type check dam, is practically set to be
less than twice of 95 % size of sediment distribution of
bed material, d
95
. Based on previous experimental data,
in the technical standards establishing for sabo mas-
ABSTRACT
In Japan, the recent check dam (sabo dam) con-
struction has been the open-type of the continuity of
sediment routing from upstream to downstream a reach
in a river basin. Not only in Japan but also in other coun-
tries, a lot of experimental and numerical research has
been conducted on sediment control using open-type
sabo dams. Plans for size and location have been drawn
up for grid-type sabo dams with heights of around 20
meters, which is the highest grid-type sabo dam in Ja-
pan, in the Amahata river in Yamahashi Pref., where a
lot of sediment are yielded and deposited in numerous
torrents due to heavy rainfall in 1982. Flume tests are
conducted using a straight channel to determine the grid
size of a grid-type sabo dam and to confirm its sediment
control function. Subsequently, several runs of flume
test were conducted supposing several types of debris
flow regimes such as steady and uniform debris flows
and quasi-steady debris flows, and taking into account
sediment transport mode corresponding to bed slope
and longitudinal bed profiles in the basin.
In the present study, the sediment control func-
tions of the “grid-type high dam (GHD)”, which is de-
fined as a grid-type sabo dam with a height over 15 m,
were examined using several experimental data sets
such as the dimensionless sediment runoff volume
which is defined as the ratio of runoff sediment vol-
ume to inflow sediment volume, temporal changes of
mean diameter, sediment concentration and sediment
discharge rate passing through the check dam. Addi-
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t. ITOH, S. HORIUCHI, J. AkANUMA, k. kAITSUkA, S. kURAOkA, T. MORITA, M. SUGIYAMA & T. MIZUYAMA
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
ment concentration, bed profiles and mean diameter.
After deciding the grid size, preliminary flume tests
focusing on effect of flow discharge size on sediment
storage are evaluated with/without steel nets on the
grid, which is supposed as one method for controlling
easily sediment and for easy maintenance.
SEDIMENT TRANSPORTATION MODE
OkUSAwA RIVER BASIN
Figure 1 shows longitudinal bed profiles of the
Okusawa River. The basin is located in Amahata River
basin in the Fuji River in Yamanashi Prefecture. The
geologic characteristic is classified into sedimentary
rock in the Paleozoic and the Mesozoic. Huge volume
of sediment was yielded by landslides and debris flows
due to heavy rainfall, whose rainfall intensity was 49.5
mm/h and its accumulated rainfall depth in one day
was 386 mm, in 1982 in Okusawa creek in Amahata
River basin. It is reported that about 0.8 to 1.0 million
m
3
of sediment is deposited in the parts of torrents, and
that huge sediment transportation still now takes place
in the basin. A plan to construct a GHD with around 20
meters high in Okusawa creek in the basin, in which
the watershed area is 16.6 km
2
and mean bed slope near
the section is 1/13.5 (= 4.2 deg.), is proposed for 400
thousand m
3
of sediment volume including pore. In
this paper, we will call define sediment volume includ-
ing pore as “apparent sediment volume”.
wATER DISCHARGE AND SEDIMENT CHRAC-
TERISTICS IN THE BASIN
Figure 2 shows the schematics of sediment trans-
port mode (e.g., e
GasHiRa
, 2006) from mountainous
region to downstream reach on a fan. Sediment trans-
ter plan for debris flow and driftwood (2007) by the
National Institute for Land and Infrastructure Manage-
ment in Japan, it is recommended that the minimum
grid size of grid-type check dam is 1.0×d
95
.
On the other hand, it is well known that the coun-
termeasures for huge sediment movements such as
debris flows and flash floods due to heavy rainfall are
needed using an effective check dam with height over
15m taking into account a plan size of storage sedi-
ment volume in a dam, costs of check dam’s construc-
tion in comparison to sediment control with many
small size of check dams. Herein, supposing that we
define a grid-type check dam with height over 15 m
as “grid-type high dam”, which is shortly called as
“GHD” in present study, there are no researches for
sediment control function using such kind of high dam.
There are a lot of experimental researches focusing
on debris flows and saturated soil mass movements in
steep slope channel, in which bed slope is over 12 de-
grees and those mass movements have high speed in a
flume. It was reported that large boulders could make
“arch structure” at the grids in a slit dam in case of de-
bris flows with non-uniform sediment. However, the
movements of those boulders are not active in case of
debris flow over mild slope bed considering sediment
transport modes. There are few researches focusing
on sediment control function with a GHD consider-
ing sediment transport modes over a bed in storage
area of the dam. In present study, several runs of flume
tests are conducted for countermeasure using GHD to
evaluate the plan for constructing the check dam in a
view of sediment transport modes and the appropriate
grid size of GHD is discussed using flume data such
as temporal changes of debris flow discharge, sedi-
Fig. 1 - Longitudinal bed profiles
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FUNDAMENTAL HYDRAULIC FLUME TESTS FOCUSED ON SEDIMENT CONTROL FUNCTION USING A GRID-TYPE HIGH DAM
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1053
grain size distribution, d
95
, is 1800 mm. In the fig-
ure, the sediment grain size distribution used in
flume tests (model) is also shown.
The clear water discharge is 185 m
3
/s in the
peak value (design discharge of clear water), which
is estimated using rainfall intensity (49 mm/h), wa-
tershed area and peak runoff coefficient (0.8) based
on previous maximum rain fall observed in 1982. A
steady clear water supply is set taking into account
large boulders movements in critical condition for
uniform and non-uniform sediment particles in clear
water flow (e.g., JSCE 1999).
On the other hand, critical erosion capacity of
bed large boulders by debris flows (i
toH
et alii,
2007) is calculated using calculated data obtained by
one dimensional numerical simulation introducing
erosion/deposition rate formula data for debris flow
(i
toH
et alii, 2003) along a main channel as shown
in Fig. 1. Minimum clear water discharge for large
boulders’ movement was estimated as 60 m
3
/s (proto
type) and the minimum discharge for every compo-
nent of sediment particles was estimated as 160 m
3
/s
(proto type) in flume tests.
port mode is debris flow (in other words, sediment
laden flow which has two layers of both clear water
and hyper-concentrated sediment mixture flow) refer-
ring to Fig, 2, when bed slope is 1/13.5 (= 4.2 deg.).
We suppose three kinds of sediment transport modes
in present flume tests, and those are as follows: a)
Debris flow due to natural landslide dam break (Type
NL), b) Steady and longitudinally uniform debris flow
(Type U) and c) Quasi-steady debris flow with a hy-
per-concentrated front part (Type QS).
Type NL can be rarely formed in the mild slope
torrent but the flow scale and magnitude can be large
because of huge size of floods due to break and out-
burst of natural landslide dam. On the other hand,
the flow patterns of Type U and Type QS can model
equilibrium debris flow for the bed slope. In order to
evaluate sediment control with GHD, it is better to
suppose several kinds of debris flow modes.
Figure 3 shows the sediment grain size distri-
bution sampled in the torrent bed. The distribution
is wide; i.e., maximum size of sediment is about
4000 mm and minimum size is 0.1 mm, and mean
diameter, d
m
, is 258 mm and 95 % size of sediment
Fig. 2 - Sediment transport mode
(e.g., e
GAShirA
et alii, 2006)
Fig. 3 - Sediment grain size distribution
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t. ITOH, S. HORIUCHI, J. AkANUMA, k. kAITSUkA, S. kURAOkA, T. MORITA, M. SUGIYAMA & T. MIZUYAMA
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
a flow body. In present study, relationship between cf
and bed slope as shown in Fig. 4 can be applied for the
setting of sediment supply. Initial bed is set as rigid
bed considering sediment deposition in the dam.
FLUME TESTS
HYDRAULIC CONDITION
Figure 5 shows an experimental flume, which is
30 cm wide, 80 m high and 20 m long, and the model
scale is set to 1/50 referring to geometrical similarity.
The flume is two parts, in which one is 12m in length
with 4.2 degrees (= 1/13.5) in downstream reach and
the other is 8 m length with 16 degrees (= 1/3.5) in
upstream reach as shown in Fig. 5. A 40.0 cm high
sabo dam (20 m in proto type) with grids is set at the
downstream reach in the flume.
Herein, hydraulic parameters and geometrical val-
ues in present study are mainly shown in model values.
Sediment supplying volume is set as 0.6 m
3
supposing
that a storage area can be trap its volume. Fully saturated
debris flow is formed in a channel for three kinds of flow
regime (Type NL, QS and U) as shown in Fig. 5. Sedi-
ment supply and flow condition for three kinds of sedi-
ment transport mode is as follows:
a) Type NL: Saturated sediment with height from
61.8 to 129 cm is quickly supplied with water in
the upstream reach, where bed slope is 16 degrees.
Channel B was used for the flume test as shown in
Fig. 5, and bed was erodible bed in the reach 2.0
to 7.5 m from downstream end to prevent friction
between sediment particles and bed surface from
changing very much.
SEDIMENT CONCENTRATION
There are two kinds of definition of volumetric
mean sediment concentration such as cross sectional
mean sediment concentration and flux sediment con-
centration (e
GasHiRa
et alii, 1997).
in which cf is the flux sediment concentration, c is
the depth averaged (cross sectional) mean sediment
concentration, h is the flow depth, c is the volumetric
sediment concentration, u is the local mean flow ve-
locity and z is the axis normal to flow direction.
Figure 4 shows experimental relationships between
equilibrium bed slope and flux sediment concentration
in debris flow with uniform sediment over an erodible
bed. In the figure, several lines of estimation for cf (e.g.,
Egashira et alii, 1997; JSCE 1999) are drawn to compare
to flume data. The differences between c and cf are re-
markable in wide flow regime from debris flow to flow
with bed loads, though these can take similar values over
around 14 degrees in bed slope, and the setting for sedi-
ment discharge rate using cf is important in debris flows
in case that bed slope is mild such as 1/13.5 (= 4.2 deg.).
In case of debris flow with non-uniform sediment,
there is little knowledge for equilibrium sediment
concentration because the effect of vertical and lon-
gitudinal sediment sorting and sieving on the flow are
remarkable, and these are few researches for the flow
conditions. However, it is expected that mean diam-
eter of debris flow body except frontal part does not
change very much, while sediment sorting is active in
(1)
(2)
Fig. 4 - Relationship between equilib-
rium bed slope and sediment
concentration
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FUNDAMENTAL HYDRAULIC FLUME TESTS FOCUSED ON SEDIMENT CONTROL FUNCTION USING A GRID-TYPE HIGH DAM
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1055
mined using Froude similarity. Grain size in model is
affected both by geometrical similarity and by Froude
similarity. Herein, let us examine the influence of Rey-
nolds shear stress on flow field using shear velocity
Reynolds number defined as R
e*
= u
τ
d/v, in which v is
the kinematic viscosity of clear water. In case of R
e*
>
20 to 30, ripples can not be formed on the bed surface,
and fine sediment particles less than 0.7 mm are omit-
ted to avoid occurrence and formation of ripples and to
avoid suspended flow in flume tests (See Fig. 3).
Table 1 is a part of experimental conditions. Table
2 shows the hydraulic conditions for the flows of Type
and UQ. Table 3 shows physical characteristics of sed-
iment particles. In those table, n is the Manning’ bed
roughness [m
-1/3
s], Q
w
is the clear water discharge, c
f
is
the flux sediment concentration, c is the cross section-
al mean sediment concentration, h
0
is the uniform flow
depth, d
m
is the mean diameter of sediment, h
0
/d
m
is
the relative flow depth, τ* is the nondimensional bed
shear stress defined as u
τ
2
/{(ρ-1)gd}, u
τ
is the shear
velocity defined as
, θ is the bed slope, Fr
is Froude number, σ/ ρ is the specific weight of sedi-
ment, c* is the sediment concentration in non-flowing
layer, d
max
is the maximum diameter. The subscript of
d respectively correspond to the percentage (%) size
of sediment distribution, while k is the permeable co-
efficient of sediment and f
s
is the interparticle friction
angle of sediment particles. Flux sediment concentra-
tions (e.g., c
f
<0.0264) are specified using Fig. 4 to
form the steady debris flow over the rigid bed.
The clearance of the grids of a dam is selected so as
to obtain several combination of distance for horizontal
bar, L
v
, and vertical bar, Lh. Figure 6 shows schematics
b) Type U: Sediment and water is supplied steadily
in upstream end of channel A
c) Type QS: Sediment, which is 0.1 m in height, 3 m
in length and 0.09 m
3
in apparent volume, is set
on the bed in upstream reach of channel A to form
a frontal part with hyper-concentrated flow at the
beginning of Type U flow. Herein, the sediment
volume corresponds to sediment deposition for 10
m in flow width, 1.6 m in erosion depth and 700 m
in length in a torrent. In the experiment b) and c),
crosspieces on the flume bed were set for rough-
ness, and flow resistance of clear water flow and
sediment-water mixture flow are confirmed so as
to satisfy hydraulic conditions.
Pictures of flow regime are taken by a digital camera
form side wall to obtain temporal changes of flow sur-
face and bed surface. Flow rate of sediment-water mix-
ture is measured at the downstream end of the flume and
experimental data such as sediment concentration and
grain size distributions are obtained. Cross sectional bed
profiles are measured at several sections just after stop-
ping debris flow to obtain longitudinal mean bed profiles,
and temporal changes of bed surface were obtained.
In flume tests for steady debris flow, sediment and
water were stopped twice within one run as follows:
380, 780, 1180 (Final stage) sec. (Type U) and 260,
660, 1060 (Final stage) sec (Type QS) in model scale.
In Type NL, temporal changes of hydraulic parameters
except longitudinal bed elevation and sediment runoff
volume from downstream end were not measured be-
cause of high speed and large amount of volume.
As shown in Fig. 3, sediment grain size distribu-
tion has wide profile. Hydraulic parameters are deter-
Fig. 5 - Debris flow regime and schematics
of experimental flume
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t. ITOH, S. HORIUCHI, J. AkANUMA, k. kAITSUkA, S. kURAOkA, T. MORITA, M. SUGIYAMA & T. MIZUYAMA
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
sabo dam to supplied sediment volume.
Figure 7 shows temporal changes of debris flow
discharge in unit width and flux sediment concen-
tration, which are measured at downstream end of
the flume. Figure 8 shows temporal changes of mean
diameter for runoff sediment passing through a dam.
In the figure, time averaged mean sediment grain
size in sediment within every water supply is also
shown. The effects of distance between steel bars
in the grid on sediment trapping are confirmed by
Figs. 7 and 8. The sediment is trapped well in case
of L
v
1.0 ×L
h
1.0 (Run 9 and 10) in comparison to L
v
1.0 ×L
h
1.5 (Run 7 and 8), and the mean diameter of
runoff sediment passing through the grid gradually
of grids and definitions for several parts of grids. In Ta-
ble 1, for example, “L
v
1.0 ×L
h
1.5” means that, for the
grid, the ratio of the horizontal bar to bar distance, L
v
, to
d
95
is 1.0 and the ratio of the vertical bar to bar distance,
L
h
, to d
95
is 1.5, while distance between bed and a near-
est horizontal bar to the bed is set as L
v
/d
95
= 1.5.
SEDIMENT AND PEAk DISCHARGE CONTROL
wITH A DAM
Sediment trapping rate obtained by flume tests are
shown in Table 1. Sediment control with the grids is
discussed focusing on sediment runoff rate for debris
flow modes. Herein, sediment runoff rate is defined as
the ratio of sediment runoff volume passing through a
Tab. 1 - A part of experimental cases for flume tests (in model scale)
Fig. 6 - Schematics of grids and definitions of
several parts of grids
Tab. 2 - Hydraulic conditions of flume tests (Type U, QS)
Tab. 3 - Physical characteristics of sediment particles for flume tests
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FUNDAMENTAL HYDRAULIC FLUME TESTS FOCUSED ON SEDIMENT CONTROL FUNCTION USING A GRID-TYPE HIGH DAM
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1057
in which V
sin
is the sediment supplying volume, V
sout
decrease due to time development in
Run 9 and 10, though the deviations
for time averaged of d
m
is large in Run
9. If we focus on sediment transport
mode, the sediment runoff rate is still
large just after a debris flow reaches a
dam and then sediment particles are
gradually deposited near the grids
in Type U. There is some range of
temporal change of mean diameter
passing through a dam, because large
boulders reach intermittently grids in
Type U. In case of Type QS, the fron-
tal part of debris flow has some large
boulders, and the grids seem to be
blocked easily by them.
Figure 9 shows relationship between L
min
/ d
95
and
sediment runoff rate passing through grids of a dam. The
value of L
min
/ d
95
shows the ratio of minimum clearance
of a grid to d
95
. The trapping rate of sediment in a dam
and the sediment runoff rate from a dam are defined re-
spectively as follows (e.g., H
oRiuCHi
et alii, 2009):
Fig. 7 - Temporary changes of flow rate of sediment-water mixture and flux sediment concentration at downstream end
Fig. 8 - Temporary changes of mean diameter passing through a check dam at downstream end
(3)
(4)
Fig. 9 - Relationship between dimensionless grid-scale and dimen-
sionless sediment runoff rate passing through sabo dam
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5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
is the sediment runoff volume passing through the
grids, Q
sin
is the sediment discharge rate, Q
sout
is the
sediment runoff rate passing through the grids, t is
the time, t
in
is the sediment and water supplying time
and t
is the time when the sediment runoff from a
dam continues. In the figure, previous experimental
data (a
sHida
et alii, 1987; t
akaHaRa
et alii, 2007) is
indicated focusing on bed slope of channel.
The sediment runoff rate depends slightly on bed
slope as shown in previous research (a
sHida
et alii,
1987). The non-dimensional sediment volume such
as supplying rate and runoff rate of sediment can be
usually affected by initial bed slope and volume of
storage area in a sabo dam. It seems that influences of
those parameters on dimensionless sediment runoff
volume are a little small because of small model scale
of flume tests. Dimensionless sediment runoff rate
changes suddenly in a range that L
min
/ d
95
is 2.0 to 1.5
and takes about 0.05 to 0.1 in a range that L
min
/ d
95
is
1.5 to 0.5. The value of dimensionless sediment run-
off rate of Type U and QS takes lower value in enve-
lope lines which can be estimated in Fig. 9. Sediment
runoff rate of Type NL takes upper limit value on the
envelope. Those differences in sediment control can
be reasons why activities of large boulders in debris
flows depend on sediment transport mode, referring
to temporal changes of flux sediment concentration
and mean grain size of sediment passing through the
grids. Frontal part and flow body of a debris flow can
transport large boulders in case of Type QS and Type
NL, but the movements of sediment particles includ-
ing large boulders are inactive in case of Type U. It is
reported that the arch structure due to large boulders
can be formed near the grids in steep slope channel
(t
akaHasHi
, 2000; 2007). The arch structures can be
rarely seen in experimental run, though the arches
were found in a few runs in case of Type NL.
It is confirmed that effective grids for sediment
trap in a sabo dam are specified as follows: L
v
1.0 ×L
h
1.0, which means that the distance of horizontal and
vertical bars to
d
95
is unity respectively except 1st
rung of grid from bed surface.
SEDIMENT CONCENTRATION wITH STEEL NETS
In the flume tests, appropriate distance between
steel bars in the grids is specified to control well sedi-
ment runoff due to debris flow. However, when the
sediment is deposited in a dam, bed slope decreases
due to sediment deposition, and the sediment trans-
port modes change from debris flow to clear water
with bedloads. Spatial changes of bed and channel
shifting due to sand bars are active (See. Fig. 10).
Those movements cause sediment runoff to be large
from vertically upper parts of the grids.
m
izuno
et alii (1996) proposed that additional steel
bars needed to be vertically installed in order to reduce
the sediment runoff volume from vertically upper parts
of the grids by making steel bar to bar distance of the
grids be small. However, influences of channel shifting
and sand bars in the storage area of the dam on the sedi-
ment runoff passing through the dam are significant.
It is quite difficult to estimate their movements, and it
might not be always better way to reduce the sediment
runoff from the grids using additional steel bars.
Herein, it seems better to use easy structure and to
apply new idea for the countermeasure to control sedi-
ment runoff from the grid; using steel nets, setting up
large boulders upstream reach of the grids and so on.
We can consider the easy countermeasure for sediment
control by easy installation and removing steel nets
from the surface of the bars of the grids, while it is usu-
al to fasten steel nets to the surface of bars in the grids.
There are preliminary tests to capture debris flows with
steel nets in field (e.g., t
abata
et alii, 2004; J
aPanese
f
lexible
b
aRRieR
a
ssoCiation
, 1996; w
endeleR
et alii,
2008) and in preliminary flume tests (e.g., d
e
n
atale
et alii, 1997) for hyperconcentrated debris flow.
Flume tests were conducted to examine how bed-
loads prevent from running out heavily from the grids of
sabo dam under the conditions that the clearance of the
grids was set as the minimum size such as L
min
/d
95
=1.0.
The model scale (λ
L
= 1/50) and the setting of flume
channel are set as same to flume experiment described
above. Figure 11 shows schematics of a flume used in
present tests and the flume is a part of the flume shown
in Fig. 5. In order to simplify discussions and flume
tests, sediment control function with/without steel nets
Fig. 10 - One example for channel shifting and formation of
sand bars on the bed in a storage area of sabo dam
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FUNDAMENTAL HYDRAULIC FLUME TESTS FOCUSED ON SEDIMENT CONTROL FUNCTION USING A GRID-TYPE HIGH DAM
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1059
supplied sediment. Sediment runoff rate becomes
about 0.07 to 0.08 with nets, and decreases about 0.12
to 0.13 in comparison to flume data without nets (0.2).
Supposing that sediment control in a storage area
of a sabo dam is additionally needed using a slit dam
with steel grids, it is better to apply several easy meth-
ods such as ‘double nets structures’, ‘rearrangement of
large boulders’ behind the grids on the bed surface and
so on (H
oRiuCHi
et alii, 2009). Sediment can be control-
led well with nets structure in the upper part of grids
in open-type sabo dam, when sand bars and channel
is evaluated for sediment transportation by steady
small size of flood, after a half part of storage area
in a dam is approximately filled with sediment
due to steady and large size of debris flow.
Next, let us discuss modelled steel nets.
Figure 12 shows the setting of steel nets on the
grids. Steel nets are set at upper part of grids and
those nets are approximated using square nets
instead of circle nets referring to a standardized
article for steel nets, which is 300mm (e.g., Ja-
pan flexible barrier association). Effective diam-
eter of circle nets is estimated as 240 mm con-
sidering overlaps of the nets, and the diameter of
a net is approximated as 225 mm using square
nets. Additionally, the errors for the shapes of
the nets are 6.25% in length and 10.7 % in area.
Hydrograph for flume tests is specified as
two kinds of steady flow discharge, which are
10.5 l/s and 3.39 l/s, respectively. Flow rate,
10.5 l/s and 3.39 l/s, corresponds to peak dis-
charge, 185 m
3
/s, and minimum discharge, 60
m
3
/s, in design hydrograph, respectively. After
sediment and water is steadily supplied within
14.8 min. by peak discharge until 6th rung of
grids from a bed, the grids between 7th and 8th
rung of grids are filled by small size of floods
(3.39 l/s) within 35.4 min in case of with/with-
out nets. Total time for sediment and water sup-
ply is 50.2 min.. Temporal changes of longitu-
dinal bed profiles and sediment runoff volume were
measured to confirm sediment control with steel nets.
Figures 13 and 14 show temporal changes of lon-
gitudinal profiles in a dam without/with nets. Figure
15 shows sediment trapping rate and runoff rate pass-
ing through a dam. In small size of flow discharge,
the mobility of large components of sediment particles
decreases due to decrease of bed shear stress, because
of decrease of bed slope in a storage area of a dam.
Sediment runoff of fine sediment increases without
steel nets, and the grids with nets can fully capture
Fig. 14 - Temporal changes of longitudinal bed profiles with nets
Fig. 11 - Schematics of experimental flume
Fig. 12 - Schematics of modelled steel nets
and setting for steel nets on the grids
Fig. 13 - Temporal changes of longitudinal bed profiles without nets
background image
t. ITOH, S. HORIUCHI, J. AkANUMA, k. kAITSUkA, S. kURAOkA, T. MORITA, M. SUGIYAMA & T. MIZUYAMA
1060
5th International Conference on Debris-Flow Hazards Mitigation: Mechanics, Prediction and Assessment Padua, Italy - 14-17 June 2011
shifting are dominant on the bed surface. One prelimi-
nary method for easy sediment control was proposed in
present study using present flume tests.
In upper parts of the grids, it is necessary to con-
sider the logs jamming in a storage area of the dam.
This is a problem to be soled near future. Data col-
lections for behaviour of large components of sedi-
ment particles are needed focusing on debris flow
regime, bed condition, velocity of sediment particles
and sediment concentration.
CONCLUDING REMARKS
Several results were obtained by flume tests fo-
cusing on sediment control with the GHD. Those are
summarized as follows.
(1) Experimental conditions were specified suppo-
sing three kinds of debris flow regimes which
were debris flow due to natural landslide dam
break (Type NL), uniform debris flow (Type U)
and quasi-steady debris flow (Type QS). Sediment
concentration was set focusing on flux sediment
concentration for uniform sediment.
(2) Sediment trapping rate in a dam with grids was
experimentally confirmed using the relationship
between the ratio of minimum clearance to the
grain size of d95 and dimensionless sediment ru-
noff rate from a dam, Vsout/Vsin. Sediment runoff
rate depended on debris flow regimes, and the se-
diment runoff form a dam was active in Type NL
and the runoff rate was inactive in Type U and QS.
Those differences in sediment runoff depend on
mobility of large components of sediment parti-
cles. Data collections for behaviour of large com-
ponents of sediment including boulders in non-
uniform sediment particles are needed focusing on
flow regime of debris flow, velocity and sediment
concentration in debris flows.
(3) Experimental data supported that effective grids
in a sabo dam are specified as follows: Lv 1.0 ×Lh
1.0, which means that the distance of horizontal
and vertical steel bars to d95 is unity respectively
except 1st rung of grid from bed surface.
(4) Sediment runoff can be large from the upper part
of grids in open-type sabo dam when sand bars
and channel shifting are dominant on the storage
area of sediment in high dam. It is better to apply
several easy methods such as ‘double nets structu-
res’, ‘rearrangement of large boulders’ behind the
grids and so on, while the clearance of the grids
are set as minimum value.
One preliminary method, which is sediment con-
trol with flexible steel nets, is proposed to control sed-
iment runoff due to small size of floods and its control
function was confirmed using flume data.
ACKNOWLEDGEMENTS
Experimental data were obtained in flume tests
when authors participated in sabo project for plan-
ning a high dam, which were financially supported
by Fujikawa Sabo Office, Ministry of Land, Infra-
structure, Transport and Tourism (MLIT). We are
thankful to present staff of Fujikawa Sabo Office for
approval of using experimental data and submission
of research paper.
Fig. 15 - Temporal changes of accumulated sedi-
ment runoff rate passing through a dam
and trapping rate in a sabo dam
background image
FUNDAMENTAL HYDRAULIC FLUME TESTS FOCUSED ON SEDIMENT CONTROL FUNCTION USING A GRID-TYPE HIGH DAM
Italian Journal of Engineering Geology and Environment - Book www.ijege.uniroma1.it © 2011 Casa Editrice Università La Sapienza
1061
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