HYDRAULIC MODEL TESTS FOR DEBRIS FLOW DUE TO BREAK OF A SMALL NATURAL LANDSLIDE DAM
Shigeo Horiuchi1, Jun-ichi Akanuma2, Kazuhiko Ogawa3, Senro Kuraoka3, Minoru Sugiyama3, Takenori Morita3, Takahiro Itoh4*, Takahisa Mizuyama5
ABSTRACT
In mountainous region, it is reported that there are a lot of formations and failures of landslide dams due to heavy rainfall or earthquake in Japan and in abroad. When natural landslide dams are formed in torrents, flash floods such as debris flow and sediment laden flow can be produced by dam’s break. A new plan for constructing grid-type check dam with around 20 m, which was the highest for grid-type check dam in Japan, was proposed in Okusawa creek for mitigation of massive sedimentation. The basin is located in a branch of Fujikawa-River in Yamanashi Pref. In present study, hydraulic model tests were conducted focusing on erosion by overflow of landslide dam, and several hydraulic parameters, such as increase of peak discharge rate for inflowing discharge rate, temporal changes of sediment-water mixture discharge rate, sediment grain size and spread of flow width and the control of flash floods by grid-type check dam, were analyzed using experimental data. The calculations obtained with several empirical formulas and preliminary numerical prediction models for peak discharge rate of debris flow due to natural landslide break are compared to experimental data obtained by hydraulic model tests, and those flow characteristics are comprehensively discussed.
Key Words: Natural landslide dam, Hydraulic model tests, Debris flows, Grid-type check dam
INTRODUCTION
Check dams (Sabo dams) are effective structures in order to control sediment transported by debris flows and flush floods in mountainous torrents. Generally, there are two kinds of check dams, which are closed and open-type. Recently, in Japan, open-type check dams are constructed taking into account the continuity of sediment routing from upstream to downstream reach in a basin, though there are also some topics concerning to sediment control function to be solved for the closed type check dam. There has been a lot of experiments and research which has been carried out sediment control on open type check dams. As known from the studies carried out in Japan and aboard (e.g., Ashida et al.
1987; Ikeya et al. 1980; Mizuyama et al. 1997 & 2000 in Japan, Armanini et al. 2001; Chen
1 Fujikawa Sabo Office, Ministry of Land, Infrastructure and Transport (MLIT), Japan (The present post: Sabo Frontier Foundation, Japan)
2 Fujikawa Sabo Office, MLIT, Japan (The present post: Kanto Regional Development Bureau, MLIT, Japan) 3 NIPPON KOEI Co., Ltd., Japan
4 El KOEI Co., Ltd, Address: 2304 Inarihara, Tsukuba, Ibaraki 300-1259, Japan (*Corresponding Author; Tel.:
+81-29-871-2092; Fax: +81-29-871-2022; Email: a6556@n-koei.co.jp) 5 Professor, Graduate School of Agriculture, Kyoto University, Japan
et al. 1997; DeNatale et al. 1997; Heumader 2000; Lin et al. 1997 and Wu et al. 2003), the minimum grid size of grid-type check dam, which is a kind of open-type check dam, is practically set to be equal to 95 % size of sediment distribution of bed material, d95. Based on previous experimental data, in the technical standards establishing for Sabo Master plan for debris flow and driftwood (2007) by the National Institute for Land and Infrastructure Management in Japan, it is recommended that the minimum grid size of grid-type check dam is 1.0×d95.
On the other hand, it is well known that the countermeasures for huge sediment movements such as landslide and debris flow due to heavy rainfall and earthquake or so are needed, and that the flow due to break of a natural landslide dam formed by mass movements is also controlled by countermeasures. There are a lot of studies for numerical simulations and filed investigations focusing on the formations and breaks for landslide dams in Japan and aboard (e.g., Tabata et al., 2002; Takahashi 2007; Satofuka et al., 2007; Awal et al., 2009 in Japan, Costa 1988; Fleming et al., 1988; Wahl 1988; Fread 1991; Schuster et al., 2001; Ermini at al., 2003 in abroad). However, it seems that there are few researches about countermeasures for debris flow with grid-type check dams.
Amahata River basin
●Tokyo Kyoto ●
the Sea of Japan
the Pacific Ocean Amahata River basin
●Tokyo Kyoto ●
the Sea of Japan
the Pacific Ocean
< Okusawa River basin >
0 500 1000 1500 2000 2500
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
4.4 deg. 12.1 9.3 21.9 19.6 32.7
Bed elevation (m)
Main torrent A= 17.7 km2
A reach for hydraulic model tests
Distance (m) Torrent 4: A= 1.72 km2
Torrent 3:
A= 3.47 km2 Torrent 2:
A= 2.1 km2 Torrent 1:
A= 1.68 km2
Section for check dam Averaged bed slope
0 500 1000 1500 2000 2500
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
4.4 deg. 12.1 9.3 21.9 19.6 32.7
Bed elevation (m)
Main torrent A= 17.7 km2
A reach for hydraulic model tests
Distance (m) Torrent 4: A= 1.72 km2
Torrent 3:
A= 3.47 km2 Torrent 2:
A= 2.1 km2 Torrent 1:
A= 1.68 km2
Section for check dam Averaged bed slope
< Longitudinal bed profiles along each torrent>
Figs. 1 Okusawa River basin and longitudinal bed profiles in its basin
Figure 1 shows Okusawa River basin and longitudinal bed profiles in the basin. The basin is located in Fuji River basin in Yamanashi Prefecture, and 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, which is a branch of Fuji River. It is reported that about 0.8 to 1.0 million m3 of sediment is deposited in the parts of torrents and that sedimentation is quite active in the basin. A plan to construct a grid-type check dam with around 20 meters high in Okusawa creek in the basin, in which the watershed area is 16.6 km2, mean bed slope near the section is 1/13.5 (= 4.2 deg.), is proposed for sediment which is around 400 thousand m3. Generally, a check dam with more than 15 m high is called “high dam” in Japan and some high dams are constructed gradually in the basin with active sedimentation.
In present study, several runs of hydraulic model tests are conducted for countermeasure using grid-type high dam to evaluate the plan for constructing the check dam in a view of sediment transportation. Herein, the decision for the position and the grid size of grid-type high dam has already done using flume tests by authors (Horiuchi et al., 2008). The sediment control function of the check dam is discussed using several experimental data such as temporal changes of discharge rate of debris flow, sediment concentration, bed distribution and mean diameter. The experimental runs with/without a check dam and the prediction for peak discharge rate of debris flow due to overflowing natural landslide dam using a preliminary numerical simulation were conducted supposing that the peak discharge rate of debris flow could be larger than a design discharge rate of a check dam.
FLUME TESTS
Experimental flume and check dam
Figure 2 shows the experimental flume, which is 4.0 m wide, 1.0 m high and 14.3 m long, and the model scale is set to 1/50 referring to geometrical similarity, and there are a lot of curved parts in the flume. Figure 3 shows the schematics of a natural landslide dam and bed profiles in experimental flume. A 40.0 cm high check dam (20 m in proto type scale) with grids is set at the downstream reach in the flume. Based on the flume data obtained using prismatic straight channel and previous experimental data, the scale of the grid is specified as follows: the clearance of the grid is 1.0×d95 and the clearance between bed surface and horizontal bar is set to 1.5×d95 by authors’ flume tests (e.g., Horiuchi et al., 2008). As shown in Fig. 3, at the section of natural landslide dam, a fully saturated debris flow is produced after flowing over the top of dam. The produced debris flow is a flow over rigid bed until sediment reach check dam, and then the flow changes into a flow over erodible bed due to sediment deposition on the bed. The bed slope along the channel is 4.2 deg. (=
1/13.5) (See Figs. 1 and 3). The position for natural landslide dam is specified as the section, 1690 m, as shown in Fig. 2, referring to the locations of previous landslide occurrences in the area for hydraulic model tests.
Upstream Natural landslide dam
Downstream Grid of the grid-type check dam Check dam
Digital gauge check dam (height: 20 m in proto scale)
1 m 4 m
pool water outlet
channel for water supply ( length: 5 m,
width: 30cm)
Section for a natural landslide dam
・surface: smooth bed made by mortar (thickness: 3～
4cm）
・section: plywood
・Inside: aggregate (sediment) 14.3 m
water supply
1.0×d95 1.0×d95
Upstream Natural landslide dam
Downstream Grid of the grid-type check dam Check dam
Digital gauge check dam (height: 20 m in proto scale)
1 m 4 m
pool water outlet
channel for water supply ( length: 5 m,
width: 30cm)
Section for a natural landslide dam
・surface: smooth bed made by mortar (thickness: 3～
4cm）
・section: plywood
・Inside: aggregate (sediment) 14.3 m
water supply
check dam (height: 20 m in proto scale)
1 m 4 m
pool water outlet
channel for water supply ( length: 5 m,
width: 30cm)
Section for a natural landslide dam
・surface: smooth bed made by mortar (thickness: 3～
4cm）
・section: plywood
・Inside: aggregate (sediment) 14.3 m
water supply
1.0×d95 1.0×d95
Fig. 2 Experimental flume for hydraulic model tests
460 480 500 520 540 560 580 600
1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500
0 5 10 15 20 25 30 35 45
40 35
30 No.28+40 m
No.29+5 m
No. No. No.
Distance (m) Elevation
(m) No.27
No.29+5 m (1455 m)
No.33+40 m (1690 m)
No.38+30 m (1930 m) No.32+40.2 m
(1640.2 m)
No.34+22.5 m (1722.5 m) Position of grid-
type dam
Natural landslide dam
Volume in a reservoir
・42,623 (m3 )：Proto-type
・341 (l)：Model Natural landslide dam
・Height：16 m （Model: 32 cm）
・Apparent sediment volume：
16,250 m3(Model：130 l )
235 m
1 15.1
1 10.9
3.78 deg. 5.24 deg.
Gauge for free surface elevation
No.34+22.5 m (1722.5 m) Gauge for free surface elevation
Width (m)
Flow width of low water Flow width
460 480 500 520 540 560 580 600
1300 1400 1500 1600 1700 1800 1900 2000 2100 2200 2300 2400 2500
0 5 10 15 20 25 30 35 45
40 35
30 No.28+40 m
No.29+5 m
No. No. No.
Distance (m) Elevation
(m) No.27
No.29+5 m (1455 m)
No.33+40 m (1690 m)
No.38+30 m (1930 m) No.32+40.2 m
(1640.2 m)
No.34+22.5 m (1722.5 m) Position of grid-
type dam
Natural landslide dam
Volume in a reservoir
・42,623 (m3 )：Proto-type
・341 (l)：Model Natural landslide dam
・Height：16 m （Model: 32 cm）
・Apparent sediment volume：
16,250 m3(Model：130 l )
235 m
1 15.1
1 15.1
1 10.9
1 10.9
3.78 deg. 5.24 deg.
Gauge for free surface elevation
No.34+22.5 m (1722.5 m) Gauge for free surface elevation
Width (m)
Flow width of low water Flow width
Fig. 3 Schematics of natural landslide dam and bed profiles in experimental flume
Experimental condition
The grain size distributions of sediment shown in Fig. 4 used in the hydraulic model tests (model) are obtained using data sampled in the Okusawa creek (proto type). The physical parameters characteristic of the non-uniform sediment, which are illustrated in Fig. 4, used in the experiment is shown in Table 1. in which σ/ρ is the specific weight of sediment, c* is the volumetric sediment concentration in non-flowing layer, dm is the mean diameter of sediment, dmax 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 φs is the interparticle friction angle of sediment particles.
Table 1 Physical parameters of the non-uniform sediment in flume tests σ/ρ c* dm
(mm)
dmax
(mm) d95
(mm) d84
(mm) d20
(mm) d16
(mm) d10
(mm) k
(cm/s) φs (deg.) Model 2.64 0.634 5.08 53 36 15 0.537 0.494 0.437 0.0922 38 to 40 Proto type 2.64 0.634 254 2,650 1,800 750 26.9 24.7 21.9 0.652 38 to 40
0 10 20 30 40 50 60 70 80 90 100
0.01 0.1 1 10 100 1000 10000
proto type (%)
sediment grain size (mm) d95 (4 m) (1.8 m)
dmax model (1/ 50 )
sediment for flume tests (model scale: 1/ 50) 95
Fine sediment in the region are omitted taking into account Froude similarity.
0 10 20 30 40 50 60 70 80 90 100
0.01 0.1 1 10 100 1000 10000
proto type (%)
sediment grain size (mm) d95 (4 m) (1.8 m)
dmax model (1/ 50 )
sediment for flume tests (model scale: 1/ 50) 95
Fine sediment in the region are omitted taking into account Froude similarity.
Fig. 4 Sediment grain size distribution
The clear water discharge rate is specified as 60 m3/s of minimum magnitude in a plan size of hydrograph, in which the hydrograph with peak discharge rate is 185 m3/s (design discharge rate of clear water), minimum discharge rate is 60 m3/s and continuing flood duration is 29 hours. The clear water discharge rate and the capacity of sediment movements, especially
for large sediment particles are confirmed using Shields parameters proposed for uniform and non-uniform sediment (e.g., JSCE 1999). Figure 5 shows the relationships between height and apparent sediment volume of natural dam formed by several mass movements such as landslide, debris flow and rock-fall (e.g., Costa et al., 1988; Tabata et al., 2002; MLIT in Japan, 2008). The minimum value of the sediment volume is specified to make small size landslide dam in flume tests. The possibility of occourence of landslide is specified based on several scenarios and previous reports and the height of landslid dam, the area and sediment volume of landslide are as follows: the height of dam is 16 m, the area of a landslide is 4,200 m2 and the apparent sediment volume is 16,250 m3. In addition, the shape of triangle of dam form is set supposing that all mass of sediment is deposited on the river bed.
The angle of a triangle landslide dam, θ* in Figs. 5, is specified as around 22 degrees, which corresponds to the angle of repose for fully saturated sediment (Egashira et al., 1997), to obtain huge peak discharge rate.
0.001 0.01 0.1 1 10 100 1000 10000 100000
1 10 100 1000
Height of dam (m) Apparent sediment volume Vs×10-4 (m3)
△Landslide and debris flow (Tabata et al., 2002)
■Debris flow (Tabata et al., 2002)
◇Rock fall (Tabata et al., 2002)
●Landslide (Tabata et al., 2002)
○Landslide (Costa, 1988)
◇
◆
Landslide (Earthquake in Niigata Pref. in 2004/ temporary estimations by MLIT, Japan ) Landslide (Earthquake in Iwate and Miyagi Pref. in 2008/
temporary estimations by MLIT, Japan )
16,250
16
Sediment volume for flume tests
< Definition of parameters for natural landslide dam>
<Schematics of natural landslide dam and reservoir >
θ*= 22 deg., θ1= 17.8 deg., θ2= 26.2 deg., Hd= 16 m, L1= 49.8 m, L2= 32.5 m
L1
L2
θ Hd
θ θ1
θ2
Qin
Debris flow Gauge
(Measurements for free surface)
Gauge (Measurements for free surface) No.29+5 m (1455 m) No.33+40 m (1690 m)
No.34+22.5 m (1722.5 m)
x= 267.5m (Model: 5.35m) x= 235m (Model: 4.7m) x/ Hd= 14.7
L1
L2
θ Hd
θ θ1
θ2
0.001 0.01 0.1 1 10 100 1000 10000 100000
1 10 100 1000
Height of dam (m) Apparent sediment volume Vs×10-4 (m3)
Qin
No.34+22.5 m (1722.5 m) No.34+22.5 m (1722.5 m) No.34+22.5 m (1722.5 m)
Debris flow Gauge
(Measurements for free surface)
Gauge (Measurements for free surface) No.29+5 m (1455 m) No.33+40 m (1690 m)
x= 267.5m (Model: 5.35m) x= 235m (Model: 4.7m) x/ Hd= 14.7
L1
L2
Hd
θ
θ1
θ2
*
2 θ θ θ θ
θ = + u= + θ*
θ θ θ θ
θ1= d− = *− θ*
, θ*
d=
= θ θu
△Landslide and debris flow (Tabata et al., 2002)
■Debris flow (Tabata et al., 2002)
◇Rock fall (Tabata et al., 2002)
●Landslide (Tabata et al., 2002)
○Landslide (Costa, 1988) Landslide (Earthquake in Niigata Pref. in 2004/ temporary estimations by MLIT, Japan ) Landslide (Earthquake in Iwate and Miyagi Pref. in 2008/
temporary estimations by MLIT, Japan )
16,250
◇
◆
16
Sediment volume for flume tests
< Definition of parameters for natural landslide dam>
<Schematics of natural landslide dam and reservoir >
θ*= 22 deg., θ1= 17.8 deg., θ2= 26.2 deg., Hd= 16 m, L1= 49.8 m, L2= 32.5 m
L1
L2
θ Hd
θ θ1
θ2
Qin
Debris flow Gauge
(Measurements for free surface)
Gauge (Measurements for free surface) No.29+5 m (1455 m) No.33+40 m (1690 m)
x= 267.5m (Model: 5.35m) x= 235m (Model: 4.7m) x/ Hd= 14.7
L1
L2
θ Hd
θ θ1
θ2
Qin
Debris flow Gauge
(Measurements for free surface)
Gauge (Measurements for free surface) No.29+5 m (1455 m) No.33+40 m (1690 m)
x= 267.5m (Model: 5.35m) x= 235m (Model: 4.7m) x/ Hd= 14.7
L1
L2
Hd
θ
θ1
θ2
*
2 θ θ θ θ
θ = + u= + θ*
θ θ θ θ
θ1= d− = *− θ*
, θ*
d=
= θ θu
L1
L2
Hd
θ1
θ2
θ
*
2 θ θ θ θ
θ = + u= + θ*
θ θ θ θ
θ1= d− = *− θ*
θ* u= θ
,
d= θ
Figs. 5 Relationships between height and apparent sediment volume of natural dam, and schematics of natural landslide in flume tests
Table 2 Experimental conditions in ca se of peak discharge without check dam (Sabo dam)
Table 2 shows the experimental conditions in case of peak discharge rate. In Table 2, subscripts of “p” and “m” for flow width, water discharge rate and uniform flow depth mean
“proto type” and “model” respectively. In the table, B is the flow width at the slit part of
B Qm cf h0 h0/dm w0/uτ τ* Fr
Bm= 34 cm Bp= 17 m
Qmm=10.0 l/s Qmp=177 m3/s
0.00687 h0m= 3.32 cm h0p= 1.66 m
6.54 1.49 0.293 1.55
check dam, Qw is the discharge rate of clear water, cf is the flux sediment concentration defined as cf =
∫
0hcudz h, in which c is the volumetric sediment concentration, u is the local mean velocity, h is the flow depth, h0 is the uniform flow depth of debris flow, h0/d is the relative flow depth, d is the mean diameter of sediment particles, uτ is the shear velocity defined as uτ = ghtsinθ , τ* is the non-dimensional bed shear stress defined as( )
{
gd}
u2 1
*= σ ρ−
τ τ , w0 is the settling velocity of sediment particle in the clear water and Fr is the Froude number. As seen in Table 2, sediment suspension cannot take place actively for every case because the ratio, w0/uτ, is greater than unity (w0/uτ >1) based on the researches on suspended flows. In this flume tests, quantities of flow characteristics are set to satisfy Froude similarity and each class diameter of sediment particles shown in Fig. 4 and Table 1 satisfy also its similarity. Sediment movements and modes depend on bed shear stress, and the movements of fine sediment particles could not usually satisfy Froude similarity because those are affected by viscosity of clear water in flume tests. Herein, let us examine the influence of Reynolds shear stress on flow field using shear velocity Reynolds number defined as Re* = uτ d/ν , in which ν is the kinematic viscosity of clear water. Fine sediment particles less than 0.7 mm in model scale are omitted, and the modified sediment particles, as shown in Fig. 4, are used in flume tests, supposing that the sediment size is estimated at 0.7 to 0.8 mm in model scale (0.1 mm in proto type scale) in case of Re*= 100.
Measurements
Flow rate of sediment-water mixture 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 points at a section just after stopping debris flow to obtain longitudinal mean bed profiles. Temporal changes of free surface are measured at the reservoir and the section of check dam by digital gauge (See Fig. 2), and pictures of flow regime are taken by digital camera and temporal change of spreading flow width on the natural landslide dam is measured using digital images. Reproduction of flume tests is confirmed by several preliminary tests.
SEDIMENT AND PEAK DISCHARGE RARE CONTROL FUNCTION BY GRID-TYPE CHECK DAM
Figures 6 to 8 shows temporal changes of free surface elevation elevation in a reservoir and a channel, temporal changes of debris flow discharge rate and flux sediment concentration in the downstream reach of check dam, longitudinal profiles of cross sectional mean bed elevation before and after occurrence of debris flow due to break of landslide dam, temporal change of flow width of landslide dam due to overflowing on a landslide dam and temporal change of mean grain size at the downstream end. Sediment deposition and peak discharge reduction of debris flow by check dam is active because the large components of sediment particles reach the check dam and because the closeness of the grid progresses. The longitudinal bed surface profile seems to be linear due to sediment transportation by natural landslide dam break. Finally, the bed slope in upstream reach is adjusted to the supplied sediment concentration. Except for large components, the sediment particles are passing through check dam and then are transported in the downstream reach of check dam, and the size of mean diameter becomes almost constant due to the grid of check dam. Spreading of flow width is shown in Fig. 8. Mean flow is estimated as 15 to 17m and is similar to original
flow width although it depends on the flow regime over the top of natural landslide dam.
Sediment is controlled well by check dam because the increase of free surface in the check dam in comparison to data without a check dam. However, it seems that peak discharge rate and temporal change of debris flow is affected and reduced by curved channel because there are three curved reach in the flume as shown in Fig. 2, and those need to confirm using flume tests, numerical calculation and so on.
0 5 10 15 20
-600 0 600 1200 1800 2400 3000
Time (sec.) Depth (m)
Time when debris flow reach the section of dam Start of overflow on
the dam
Without grid-type check dam
Elevation of water level in a reservoir
0 5 10 15 20
-600 0 600 1200 1800 2400 3000
Depth (m)
Time when debris flow reach the section of dam Start of overflow on
the dam
With grid-type check dam
Elevation of water level in a reservoir
Time (sec.) Increase of free surface
elevation due to grid-type check dam
Without check dam With check dam Figs. 6 Temporal changes of free surface elevation in a reservoir and a channel
177 104
0 50 100 150 200 250 300 350 400 450 500
0 600 1200 1800 2400
Qm (m3/s)
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045
Time at downstream end (sec.) Without grid-type check dam
0.00687 0.00265
With grid-type check dam
Inflowing clear water discharge rate
Equilibrium flux sediment concentration for initial bed slope
cf
330 375
1 D. debris flow simulation without a check dam and a flow width spreading
177 104
0 50 100 150 200 250 300 350 400 450 500
0 600 1200 1800 2400
Qm (m3/s)
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045
Time at downstream end (sec.) Without grid-type check dam
0.00687 0.00265
With grid-type check dam
Inflowing clear water discharge rate
Equilibrium flux sediment concentration for initial bed slope
cf
330 375
1 D. debris flow simulation without a check dam and a flow width spreading
No. 28+40m No.29+5m No.29+20m No.29+35m No.30 No.30+15m No.31+10m No.32+5m No.33 No.34 No. 35 No.36
No.28 No.28+10m No.28+25m No.32+20m No.32+35m No.33+15m No.33+30m
470 480 490 500 510 520 530
1350 1400 1450 1500 1550 1600 1650 1700 1750 1800
Initial bed
Without grid-type check dam With grid-type check dam Bed elevation (m)
Distance (m) Cross section of grid-type dam (1455 m)
Without grid-type check dam (1 D. simulation)
No. 28+40m No.29+5m No.29+20m No.29+35m No.30 No.30+15m No.31+10m No.32+5m No.33 No.34 No. 35 No.36
No.28 No.28+10m No.28+25m No.32+20m No.32+35m No.33+15m No.33+30m
470 480 490 500 510 520 530
1350 1400 1450 1500 1550 1600 1650 1700 1750 1800
Initial bed
Without grid-type check dam With grid-type check dam Bed elevation (m)
Distance (m) Cross section of grid-type dam (1455 m)
Without grid-type check dam (1 D. simulation)
Figs. 7 Temporal changes of debris flow discharge rate and flux sediment concentration in the section of check dam (Time of “0 sec. means the time when a flow reach at the downstream end.), and longitudinal profiles of cross sectional mean bed elevation before and after occurrence of debris flow
0 10 20 30 40 50
0 600 1200 1800 2400 3000
Width on the top of a dam (m)
Time from time when a overflow takes place on the dam (sec.)
Maximum Minimum
1 10 100 1000
0 600 1200 1800 2400
Time (sec.) Mean diameter (mm)
Initial condition (Sediment of dam) Without check dam
(Grid-type dam)
With check dam
1 10 100 1000
0 600 1200 1800 2400
Time (sec.) Mean diameter (mm)
Initial condition (Sediment of dam) Without check dam
(Grid-type dam)
With check dam
Figs. 8 Temporal change of flow width over a landslide dam due to break of landslide dam and temporal changes of mean grain size in the downstream end (Time of “0 sec. means the time when water flows over top of dam.)
Figures 9 show the temporal change of flow depth and flow rate at the downstream. Those calculations are conducted focusing that effects of shape of natural landslide dam on peak flow rate, and the sediment volume of each natural landslide dam takes same value (16,250
m3). Herein, the calculation for debris flow due to erosion for flow over a landslide dam was conducted using one dimensional governing equations set for one-phase debris model without flow width spreading over a natural landslide dam (e.g., Itoh et al., 2003; Egashira et al., 1997). It has been reported that one-phase debris model can explain flow characteristics in a wide flow regime from debris flow to flow with bed-loads (Egashira et al., 1997) in case that the differences between solid and fluid phase in velocity are not significant. In addition, calculation is divided into two parts: the reservoir and the torrent including a natural landslide dam respectively, to avoid critical flow condition (Fr= 1) and the related numerical instability due to the overflow on the top of the dam in the calculation. The leap-frog scheme and the values of Δx= 5m and Δt= 0.00125-0.005 sec. were used in the calculations. Physical parameters of water and sediment are set as the same values in flume tests and the flow width was specified as 17 m, which is mean flow width as shown in Fig. 3. It is found that trapezoidal natural landslide dam can form small magnitude of debris flow in comparison to that of triangle dam. Additionally, in Figs. 7, calculated data are compare to flume data in case of triangle shape of natural landslide dam without a check dam. There is a discrepancy between flume data and predictions for peak discharge rate. It could be matters to be solved precisely because the peak discharge rate depends on inflowing discharge rate, geographical features and shape parameters of dam such as the height and the slope of dam in downstream reach etc..
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0 600 1200
0 100 200 300 400 500
Time at downstream end (sec.) Inflowing clear water discharge rate Flow depth (m)
Debris flow discharge rate,
Qm(m3/s)
495 500 505 510 515 520
0 50 100
Distance from a dam (m) Bed elevation (m)
Shape A Shape B
Qm(Shape B)
Qm(Shape A) Flow depth
(Shape B) Flow depth (Shape A)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0 600 1200
0 100 200 300 400 500
Time at downstream end (sec.) Inflowing clear water discharge rate Flow depth (m)
Debris flow discharge rate,
Qm(m3/s)
495 500 505 510 515 520
0 50 100
Distance from a dam (m) Bed elevation (m)
Shape A Shape B
495 500 505 510 515 520
0 50 100
Distance from a dam (m) Bed elevation (m)
Shape A Shape B
Qm(Shape B)
Qm(Shape A) Flow depth
(Shape B) Flow depth (Shape A)
{ }
out
in q
dt q t LH
dα () = −
< Schematics of calculation methods for dam break and reservoir >
Hd θ θ1
θ2
Qin
Debris flow Zone of continuity equation for clear water
( )t H
Zone of governing equation’s set for debris flow
# Connection between reservoir and dam
* Fr=1 on the top of dam
* Run-off discharge rate of clear water is evaluated using a formula for run-out discharge rate from a weir.
[Reservoir] [Dam and beds]
Erosion due to Clear water and debris flow
( ){1tanθ*+1tanθ}
=H t L
qin qout
2 /
2 3
3
2C gh
qout =
< Continuity equation for reservoir >
) (t h L
< Governing equations >
* Mass conversation equation (Sediment-water mixture, Sediment)
* Momentum conversation equation + Constitutive relationships
for debris flow (Sediment-water mixture)
* Equation for bed elevation + erosion rate of sediment
, C=0.577 (Fr=1)
{ }
out
in q
dt q t LH
dα () = −
< Schematics of calculation methods for dam break and reservoir >
Hd θ θ1
θ2
Qin
Debris flow Zone of continuity equation for clear water
( )t H
Zone of governing equation’s set for debris flow
# Connection between reservoir and dam
* Fr=1 on the top of dam
* Run-off discharge rate of clear water is evaluated using a formula for run-out discharge rate from a weir.
[Reservoir] [Dam and beds]
Erosion due to Clear water and debris flow
( ){1tanθ*+1tanθ}
=H t L
qin qout
2 /
2 3
3
2C gh
qout =
< Continuity equation for reservoir >
) (t h L
< Governing equations >
* Mass conversation equation (Sediment-water mixture, Sediment)
* Momentum conversation equation + Constitutive relationships
for debris flow (Sediment-water mixture)
* Equation for bed elevation + erosion rate of sediment
, C=0.577 (Fr=1)
Figs. 9 Effects of dam shape on temporal change of flow depth and flow rate of debris flow at the section of a check dam ( x/ Hd =14.7).
Tables 3 and 4 shows debris flow control for peak discharge rate and sediment volume using a grid-type check dam and the predicted peak discharge of debris flow, which is shown in Figs 7, at the section of a check dam respectively. In Table 3, the peak values for debris flow and flux sediment concentration at the section of grid-type check dam without and with a check dam are compared each other. Flume data and calculated data are compared to the values obtained using empirical relations (e.g., Costa 1988; Tabata et al. 2002) in Table 4.
Sediment volume and peak discharge rate are controlled well by a check dam although the check dam is grid-type check dam. On the other hand, in flume tests, peak discharge rate without a check dam is very close to design discharge rate (= 263 m3/s) in spite that the size of a natural landslide dam is small.
Table 3 Debris flow control for sediment and peak discharge rate using a grid-type check dam
Qp (m3/s) Qp / Qpwithout a dam cf p×103 cf p / cf p without a dam Qp / Qin
Without check dam 177 1 6.87 1 2.95
With check dam 104 0.587 2.65 0.386 1.73
Table 4 Estimation for peak discharge of debris flow at the section of a check dam
Methods Peak discharge rate (m3/s)
Costa (1988) 153
Tabata et al. (2002) (flow width, B, is 17 m) 27.8
Hydraulic model tests 177
Calculation using 1 dimensional one–phase debris flow model (B= 17 m) 330 to 375 Design flow rate (Clear water discharge rate obtained by rational equation: f= 0.8 ) 263 (185)
CONCLUSION
In present study, hydraulic model tests and numerical calculations were conducted to discuss the control function of sediment and flow rate of debris flow due to break of a natural landslide dam, and several results are obtained. Those are summarized as follows.
1. Sediment runoff and flow rate of debris flow is also controlled by grid-type check dam of open type’s check dam by capturing large sediment particles in the grids and by increase of free surface elevation in a check dam in case that movements of sediment particles are active.
2. Peak discharge rate of debris flow due to break of natural landslide dam can be close to design discharge rate of check dam, although the small size of natural landslide dam is formed in torrents
3. Numerical calculations for estimation of peak discharge rate of debris flow were preliminarily conducted using one-phase debris flow simulation model without flow width spreading and those data were compared to flume data. Peak discharge rate is overestimated in the calculations, and it need to be clarified taking into account effects of geographical features, shape parameters of dam etc..
Preliminary numerical simulations for debris flows due to natural landslide dam break were conducted using debris flow model. There were discrepancies between calculations and data obtained by hydraulic model tests in peak discharge and temporal changes of free surface in dam’s reservoir, and, therefore, more examinations will be executed taking into account effects of flow width and the shape of dam on debris flows.
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