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The initiation of debris flow at high slopes: experimental results Carlo Gregoretti
a
a
Dipartimento Territorio e Sistemi Agro-Forestali, Agripolis, Via Romea, 35020, Padova, Italy E-mail: Version of record first published: 07 Jan 2010.
To cite this article: Carlo Gregoretti (2000): The initiation of debris flow at high slopes: experimental results, Journal of Hydraulic Research, 38:2, 83-88 To link to this article: http://dx.doi.org/10.1080/00221680009498343
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The initiation of debris flow at high slopes: experimental results La mise en mouvement de lave torrentielle sur fortes pentes: resultats experimentaux CARLO GREGORETTI, Assistant Professor - Dipartimento Padova - Italy e-mail:
[email protected]
Territorio e Sistemi Agro-Forestali
- Agripolis - Via Romea - 35020
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ABSTRACT Debris deposit laying over an impermeable high slope surface (e.g. in steep streams and gullies) could develop a water sediment mixture flow (usually known as debris flow) when a high enough water flow is flowing over the sediment bed. In particular the rainfall first saturates the debris deposit and then mobilizes it by an overland flow. The mechanisms which make unstable the granular material layer in such a case were studied by experimental tests performed in a tilting flume filled with uniformly distributed granular material. The experimental results are compared with theoretical relationships on the occurrence of debris flow and with experimental findings pertinent to sediment transport phenomena. The material employed in the experiments was nearly uniform gravel of three different sizes with mean diameters respectively equal to d = 0.023, 0.029 and 0.034 m. RÉSUMÉ Un dépöt de débris sur une surfavce impermeable a forte pente (par exemple dans des torrents ou des ruisseaux torrentiels) peut donner lieu a une lave torrentielle lorsqu'un écoulement liquide suffisamment important se développe sur le lit de sediments. Ainsi, la pluie commence par saturer le depot de sediments et ensuite le met en mouvement par submersion. Les mécanismes qui rendent instables la couche de matériau granulaire dans un tel cas ont été étudiés a l'aide d'essais experimentaux dans un canal inclinable rempli avec du matériau granulaire de granulométie uniforme. Les résultats experimentaux sont compares avec des relations théoriques d'occurrence des laves torrentielles et avec des résultats experimentaux relatifs au transport solide. Pour les essais, on a utilise du gravier plus ou moins uniforme de trois différents types, avec un diamètre moyen des 0.023,0.029 et 0.034 m respectivement.
1 Introduction Debris flow usually denotes the motion of a water-sediment mixture driven by gravity, which forms wherever the simultane ous availability of water, debris material and an adequate slope, steeper than 10° are satisfied. Various mechanisms are likely to trigger debris flow, namely, the mobilization of debris deposits laying on steep impermeable surface, landslides, the collapse of natural or artificial dam. In the present paper the attention is focused on the first mecha nism. The state of art on the second and third mechanism is rewied, respectively by Iverson et al. (1997) and Takahashi (1991). Let then consider a debris flow which takes places in a gully or in a riverbed, when a large gravel layer accumulated by previ ous erosive events (surface erosion, landslide and volcanic eruptions) is saturated and moved by the water flow following an intense rainfall event. Takahashi (1978, 1991) was the first who investigated the occurrence of debris flow in such a configuration. He studied the equilibrium of an infinitely long saturated layer of uniform cohesionless sediments laying over an impermeable surface whose slope angle is , when a stream of water of depth h flows over the sediment surface (fig. 1). In particular, Takahashi (1978, 1991) compared the shear stress x and the resisting stress xs, at depth y from the sediment surface, namely: T = g [ v * ( p , - p)y + p(y + h)]sm&
(1)
t s = gv ( p s - p)ycosf>tan
(2)
where p, ps are respectively the water and sediment densities, v* is the dry volumetric sediment concentration (assumed to be uniform within the sediment bed), 0 is the friction angle of the sediments and g is the gravitational acceleration (9.81 m/s 2 ). Figure 1 shows that, when dxldy < dxs/dy, a sediment layer of thickness 6 becomes unstable and is susceptible of slowing down. Equating the shear and the resisting stress at depth 6 leads to: t a n d = tan
(P'~P)V*
(3)
( p , - p ) v » + p ( l + 5) which gives the minum slope above which a debris flow can develop under the flow conditions of figure 1. Takahashi (1991) suggested that b-nd,d being a characteristic grain size of the sediment layer and n a numerical constant superior or equal to one. Also, he argued that ratio hlh should be next to unity in order to ensure that also the larger sediments can spread over the whole flow depth, as long as the erosion process enhanced by the instability of the surface layer proceeds, thus giving rise to a so called mature debris flow. By fitting equation (3) to field data obtained from debris flow occurred on gullies, Takahashi (1978, 1991) found that approx imately, blh > 0.7, recommending, however, to estimate experi mentally the value of this ratio.
Revision received July, 1999. Open for discussion till October 31, 2000.
JOURNAL OF HYDRAULIC RESEARCH. VOL. 38. 2000. NO. 2
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In the present contribution, the results of a series of laboratory experiments suitably designed to investigate the triggering mechanisms of debris flow occurring on a steep impermeable surface, are presented. Experimental data are compared with the theory of Takahashi (1978, 1991) and with the particle incipient motion condition (Shields, 1936), typical of usual sediment transport phenomena.
sedime imper
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Fig. 1. Stress distribution within the layer. 2 Experimental set-up The experiments were performed in the sloping duct facility at Hydraulic Research Wallingford Ltd laboratories. The sloping duct facility consists of a 6.75 m long, 0.6 m wide and 0.25 m high, partially transparent, closed channel which can pivot on a central support. The downstream end of the duct has a sediment trap. In the present research the upper wall was removed and the duct could act as a tilting flume. Figure 2 shows a schematic drawing of the tilting flume apparatus. The slope of the tilting flume was measured by an inclinometer whose maximum error is 0.2°. The water discharge was supplied by a centrifugal pump and was measured by an Altoflux K380 electromagnetic flowmeter (with a measuring error of 0.00008 mVs), situated in a straight portion of 0.20 m diameter feeder pipework. A gate valve between the pump and the flowmeter (0.5 m before the latter) allowed to set the appropriated water discharge. At the downstream end of the flume a chute was placed below the trap to carry water and sediments to an underlying tank (fig. 2). A pipe connected to the tank allowed water to flow to the main collecting channel of the laboratory. A cohesionless sediment bed 0.15 m thick and 1.53 m long was placed in the flume between two grids (0.15 m high), the down stream one placed just before the sediment trap. In order to localise the hydraulic jump arising because of the supercritical character of the incoming water current and to obtain uniform flow conditions, at the upstream edge of the sediment bed an additional layer of coarser material (with an average diameter of 0.05 m) was placed just before the upstream grid. This addi tional layer was 0.15 m high, 2 m long and was protected at the upstream edge by a triangular permeable wood ground. During each experiment the free surface level of the water flow ing over the sediment bed was measured by using three piezo meters placed close to the first half of the right side glass wall, 0.01 m from the flume bottom (fig. 2). In particular, a grid (0.0256 m square spacing) installed inside the flume against the wall allowed to estimate the level of water within the piezome
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ters. The flow pattern was recorded with a videocamera (1/1000 shutter). A Broadcasting Umatic system at a speed of 25 frames per second allowed to analyse the images and to measure water level in the piezometer with the help of the grid. The value of the piezometer water level was assumed as the average over six consecutive measurements taken at intervals of 0.04 sec. The maximum standard deviation was estimated to be 0.001 m, the resolution of the level measurements being equal to 0.0005 m. The material employed in the experiments was fracturated gravel irregularly shaped (fig. 3) of three different sizes: A) 0.020 < d < 0.026 m; B) 0.026 < d < 0.032 m; C) 0.032 < d < 0.036 m. In the following we will refer to the different types of sediment through their median sieve-diameter: A) d = 0.023 m; B) d = 0.029 m; C) d = 0.034 m. The characteristics of adopted sediments are shown in table I. The values of the dry volumetric concentration v* and of the friction angle O are the averages among 15 measurements. In particular, the friction angle O was measured with the help of a table whose surface was made rough by gluing grains. The table then was covered with a layer of grains and was tilted until a slide took place. Further details of the experimental apparatus and of the measur ing tecniques are given in Gaspardo et al. (1997). Table I. Characteristics of material Material gravel A grave] B gravelC
d(m) 0.023 0.029 0.034
1 (deg) 50.2 51.3 47.7
v* 0.512 0.541 0.568
St. dev. of $ 2.76 2.29 3.77
St. dev. of V* 1.05I0"2 1.31 10 2 1.23102
Fig. 2. Experimental apparatus.
§?5p|| HS v* p ps T T,
numerical constant diameter of the gravel gravity acceleration water stream depth average depth of the scour water depth corresponding to the piezometric head numerical constant numerical constant unit width surface water discharge unit width seepage discharge unit width total discharge depth averaged velocity incipient motion flow velocity fall terminal velocity of a particle co-ordinate of the depth of the gravel layer value of the thickness of gravel layer at a fixed depth slope angle dry volumetric concentration of the gravel density of the water density of the gravel shear stress resisting stress material friction angle
JOURNAL DE RECHERCHES HYDRAULIQUES. VOL. 38. 2000. NO. 2
