Engineering and Mitigation
919 7th International Conference on Debris-Flow Hazards Mitigation Predicting debris-flow scour depth downstream from a check dam
Hua-Yong Chena,b,c*, Xiao-Qing Chena,b,c, Jian-Gang Chena,b,c, and Jin-Bo Tanga,b
a Key Laboratory of Mountain Hazards and Earth Surface Process/Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China b University of Chinese Academy of Sciences, Beijing 100049, China c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences (CAS), Beijing, 100101, China
Abstract
Debris flows often pose great threats to people’s lives and property in mountainous regions. For example on August 8, 2010, a debris flow with a volume of 220×104 m3 occurred in ZhouQu county, Gansu province, China. The debris flow resulted in not only significant loss of farmlands, but also in 1,248 deaths and 496 people missing. The construction of check dams in debris- flow valleys is a useful way to mitigate deaths and damages. Herein, we investigate scour downstream from check dam spillway structures. We propose that the main parameters which determine scour depth of a scour hole downstream from check dams, include flow density, flow depth, flow discharge per unit width, and acceleration due to gravity. Physical experiments were also carried out to investigate debris flow movement and scour characteristics. In addition, a theoretical expression was deduced to predict the maximum scour depth downstream from check dams. There was a good correlation between experimental data and results predicted from theory. The results obtained in this paper can provide constraints for the design of check dams in mountainous areas.
Keywords:Check dam, Debris flow; Energy dissipation; Scour depth prediction, Discharge per unit width
1. Introduction
Debris flows are poorly sorted, fast-moving mixtures of sediment and water that mobilize in upstream region of a valley, surge down slopes in response to gravity (Iverson, 1997). Excessive rainfall (Chen et al., 2014; Zhou et al., 2015; Melo et al., 2018), snowmelt (Decaulne et al., 2008; Decaulne et al., 2010), or glacier lake outburst floods (GLOF) (Breien et al., 2008) can all generate debris flows. When abundant loose soil particles are available along a valley, they can be eroded and entrained by flood waters and transformed into large debris flows. In China, debris flows are considered one of the most serious natural hazards in mountainous areas, especially after the occurrence of the May 12, 2008 Wenchuan Earthquake. Many large scale debris flows were triggered by intensive rainfall following the earthquake, such as September 24, 2008 Wenchuan debris flow (Huang, 2011), and the August 13, 2010 Qingping debris flow (Xu et al., 2012; Ma et al., 2013). Additionally, on 8 August, 2010, a large debris flow occurred in the Sanyanyu and Luojiayu gullies, north of ZhouQu county, Gansu province. This debris flow blocked the Bailongjiang River, resulting in the formation of a lake that inundated over half of ZhouQu (Cui et al., 2013). Many countermeasures have been applied to mitigate debris-flow hazards and reduce the risk to people and structures. Counter structures include check dams, discharge drainages or debris basins. A closed-type check dam with a rectangular or trapezoid spillway is one of the most effective structural countermeasures for debris-flow control. These dams are commonly installed to stabilize channel side-slopes, to reduce scour, to capture sediments, and to reduce debris-flow velocities (Shrestha et al., 2008; Shrestha et al., 2009; Zeng et al., 2009; Hsieh et al., 2013). Experimental and theoretical studies have been carried out to investigate local scour under different conditions. Sheppard and Miller (2006) conducted pier scour experiments and compared their results with local scour equations. Ballio et al. (2010) studied the temporal scales for scour at abutments. Local scour experiments were performed with four different uniform cohesionless sediment diameters, then a design equation is proposed based experimental data
______* Corresponding author e-mail address: [email protected]
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and existing live-bed scour data (Barbhuiya and Mazumder, 1997). Manes and Brocchini (2015) derived a new predictive formula to calculate the scour depth at piers, which merges the theory of turbulence with empirical observations. The validity of the proposed approach for local scour around bridge piers was tested for vertical-wall bridge abutments (Coscarella et al., 2018). Much previous attention was paid to the clear-water scour and sediment- laden water scour. However, relatively little attention has been focused on debris-flow scour, especially, the local scour downstream from check dams. Lenzi and Comiti (2003) examined local scouring characteristics downstream from 29 drop structures in a steep mountain river. Their results indicated that drop height, flow depth, and step spacing affect scouring dynamics in a complex way. Pan et al. (2013a) conducted flume experiments to investigate the laws governing a scour hole’s shape downstream of a debris-flow check dam under different circumstances. Results demonstrated that the position of the maximum depth point moved downstream with an increase in flume- slope angle. Pan et al. (2013b) gave a theoretical formula to calculate the depth of a scour hole downstream of a debris-flow check dam in 2D physical experiments. Results indicated that the scour hole mainly depended on channel slope, flow density, flow depth, mud and sand characteristic coefficients, and a scour-hole formation coefficient. Chen et al. (2016) investigated four different lateral contraction ratios of spillways and proposed an empirical model to predict the scour depth downstream of a check dam. In this paper, experiments were conducted to investigate the characteristics of a debris-flow nappes downstream a check dam. For each experimental test, video cameras were employed to record the trajectory of debris-flow nappes from the spillway to the debris-flow surface downstream. The characteristics of scour holes downstream from different spillway structures were analyzed in relation to nappe characteristics. Additionally, we employed momentum conservation theory to predict the depth of scour holes formed by debris flows. The results presented in this paper can provide information for the structure design of check dams in mountainous areas. Our experimental flume consisted of a hopper, a sluice gate, a rectangular channel, a check dam with a spillway, and a downstream erodible bed composed of the loose soil material with the maximum diameter no more than 20.0 mm (Fig.1a). The gate was used to control the discharge of debris flows passing through the spillway. In each experimental run, a volume of debris flow was held in the hopper. The sudden remove of the sluice gate was used to simulate a debris-flow event. The rectangular channel was about 4.0 m long, 0.4 m wide and 0.4 m high, with a slope of i=8° (Fig.1b). The mean velocity of the flow front in the channel was estimated by video after it ran out from the hopper. The spillway shapes of the check dam in our experiments are rectangular and Y-type, respectively. The check dam was made of steel material was set at the end of the rectangular channel (Fig.1).
(a) Photograph of the experimental setup
(b) Schematic diagram (unit: cm) (c) The rectangular and Y-type spillway of check dams Fig.1. The experimental setup
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The hopper of the check dam was filled with the sediment from Jiangjia ravine, with a slope of s=3° (Fig.1b). Particle size distribution of sediment may affect the debris-flow density and flow motion along the channel. In the prototype, the particle sizes of the debris flows change from clay to boulder. In the experimental model, the particle sizes of the debris flows should be relative small due to the limited model dimensions. The sediment used in this experiment was equivalent to a sample of typical debris flows and excluded particles larger than 20.0mm. The diameter of the sediments in the erodible bed was also smaller than 20.0 mm. In addition, the clay and fine particles (smaller than 1.0 mm) were excluded to avoid the effects of matric suction on the development of the scour hole. The particle size distribution of sediments for both debris flows and erodible bed is shown in Fig.2. In our experiments, the flow rate can be obtained by monitoring the flow velocities and flow depths at the outlet of spillway. Debris flow density can also be measured by monitoring the weight and volume of debris-flow samples taken at the end of the spillway. Although the experimental setup and sediments used in experiments were not strictly follow the similarity theory, dimensions of our physical model, debris flow volume, and particle sizes were roughly set based on the natural debris-flow events. Therefore, the results obtained in our experiments can reflect the dynamic evolution process and basic mechanism in prototype.
100 Particles of erodilbe bed 90 Particles of debris flow materials 80 70 60 50
40 Finer/% 30 20 10 0 0.001 0.01 0.1 1 10 100 Particle size/mm Fig. 2. The particle size distribution of the sediments
2. Experimental results and analysis
2.1. Typical flow pattern downstream form the check dam
When debris flows pass a check dam a lot of debris flow materials are retained in the channel, forming an obvious elevation difference downstream and upstream from check dam site. In our experiments, we saw that debris flows overflowed the spillway of check dams and then plunged into the dissipation pool with erodible bed (Fig.1). A debris flow nappe, which represented the debris flows from spillway outlet to downstream water level, was observed downstream from the check dam under different spillway structures. However, for different spillway structures, the profiles of the debris-flow nappe can be different. For example, when debris flows overflow the spillway of the rectangular cross-section, the flow nappe is relatively thin (no more than 60.0 mm) and the flow turbulence is relatively weak. But when debris flows overflow the spillway of the lateral contraction (Y-type spillway), the flow nappe is relatively thick and the flow turbulence is relatively strong, as shown in Fig.3.
(a) Debris flows passing through spillway with rectangular cross-section
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(b) Debris flows passing through spillway with "Y-type" cross-section Fig. 3. Typical debris flow patterns downstream from the check dam with different types of spillway
2.2. Plunging velocity analysis
During the plunging process of the debris-flow nappe, the debris-flow velocity increases when it plunges to the downstream flow surface due to gravity. Actually, the spillway velocity, v1, is easily estimated by a float method (testing the average velocity covering certain distance by floating a marker on flow surface) but this method cannot be used to estimate the plunging velocity due to strong turbulence at the end of the flow nappe. Fortunately, we can predict the plunging velocity based on the projectile motion theory as shown in Fig.4 (Hayen, 2003). If we suppose debris flows overflow the spillway at velocity,v1 (v1 is composed of vx1 and v1z components, Fig.4) and debris flows plunge into the dissipation pool at v2 (v2 is composed of v2x and v2z components), where x means the movement distance of the debris flow mass in the horizontal direction and z means the movement distance in the vertical direction, then we have
x v1 cos t (1)
1 2 z v1 sin t gt (2) 2 Combined (1) with (2), then 1 x2 z xtg g 22 (3) 2 v1 cos
So x can be expressed as v2 2gz x 1 sin cos cos sin2 2 (4) g v1
According Equation. (1), then x v 2gz t 1 sin sin2 (5) 2 vg1 cos v1
where x is distance in the x direction, z is distance in the z direction, v1 is flow velocity at spillway outlet,vx1 is flow velocity in the x direction, vz1 is flow velocity in the z direction, α is flow direction at the spillway outlet, t is time, and g is the acceleration gravity. Plunging velocity v2 of debris flows can be expressed as 2 2 2 v2 v 1 g t 2 v1 sin gt (6)
θ , shown in Fig.4 can be expressed as vsin gt arctg()1 (7) v1 cos
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Fig. 4. Diagram of debris flow trajectory
2.3. Debris-flow scour downstream from a check dam
When debris flows plunge into the dissipation pool at relatively high speeds, the debris flows can mobilize sediments in the erodible bed and transport them downstream. A scour hole can been found in the erodible bed as shown in Fig.5. When the impact stress of the debris flows which flow over the erodible bed is greater than the resistance of the erodible bed, the scour hole will develop further. The scour hole will reach an equilibrium status when the debris flows can’t initialize and transport the particles in the erodible bed. When the scour hole reaches its equilibrium status the slope of the scour hole is equal to the angle of repose because the erodible bed is formed by the non-cohesive particles. Debris flows can mobilize and transport much more sediments from the erodible bed with increasing flow velocity at spillway. Meanwhile, according to Equation (4) and Equation (6), the movement distance of the debris-flow mass in the horizontal direction and the plunging velocity will also increase. Therefore, with increasing the debris-flow volume, not only the maximum scour depth, but also the distance between the check dam site and deepest scour location will increase as shown in Fig.6.
Fig. 5. The shapes of scour hole downstream from the check dam
Fig. 6. The profiles of scour hole at different debris flow volumes: V1=0.16 m3; V2=0.10 m3; V3=0.06 m3; V4=0.03 m3.
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3. Prediction of debris-flow scour depth
Definitely, besides the debris flow volumes, the scour depth can been determined by some other factors such as the sediment concentration or debris-flow density, debris-flow velocity, particle-size distribution, and the maximum diameter of sediments. Obviously if all the factors are considered in our study, it will become very complex. So we fix the particle-size distribution and maximum diameter of sediments, and pay more attention to the dynamic parameters of debris flows such as flow density, velocity, and volumes. The diagram of scour dynamic processes in the erodible bed is shown in Fig.7. Herein, we considered a spillway with rectangular cross-section and a "Y"-type spillway. For each spillway structure, four different discharges were considered to measure the key parameters of debris flows and predict the maximum scour depth in the erodible bed. The values of key parameters measured are shown in Table 1. The study area is chosen as shown in Fig.7. The momentum equation between section 1 and 2 is expressed as follows
P1 P 2 GLsin q 0 (Vmm2 V 1 ) (8)
where P1 and P2 are the hydrostatic forces at section 1 and section 2 respectively, G is the gravity force of the debris flow, θ is downstream slope angle of the scour hole, τ0 is the shear stress, ρ is debris flow density, q is unit flow discharge, V1 and V2 are the average velocity at section1 and section 2 (Fig.7), respectively.
P1 can be expressed as
hhct / cos 1 2 P1 ghcos dh g hct h / cos (9) 0 2 P2 can be expressed as h / cos t 1 2 P2 ghcos dh ght / cos (10) 0 2 G can be expressed as g G( h 2 h ) / cos h / cos ctg (11) 2 c t c where φ, h are the angle of repose and the water depth downstream, respectively. When a debris-flow nappe reaches the bottom of the scour hole, the flow velocity changes its direction roughly parallel with the erodible bed. We assumed that the flow velocity V1 was the average velocity and also considered it as the boundary velocity which caused the shear stress 0 . Drag force F can be expressed as 2 d sin F L gV2 C 2 L g 0 C2 h/ cos ctg (12) 01mc hhct
where, C is the Chezy parameter (C=60.0), L is the downstream slope length of the scour hole, is constant ( =2.28), d0 is the thickness of the debris flow nappe, θ is the recipient angle of the debris flow nappe. From Equation (7) to Equation (11), then we have 2 ddsinsin g v 00C2 h/ sin q ( v V ) (13) 2 cm2 2 hc h t hc h t
In Equation (13), every parameter can be obtained based on the experiments except the erosion depth hc. Therefore, we can calculate the erosion depth by trial and error method. The value of each parameter and the results of the erosion depth hc are shown in Table 1. Fig.8 showed the results of the predicted scour depth and the experimental data. It indicated that the scour depth calculated by Equation (13) was in good agreement with the experimentally calculated scour depth.
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Fig. 7. Diagram of scour dynamic process Table 1. The values of key parameters under different spillway operating conditions
Parameters Spillway types Error ρ q φ V0 d0 θ ht hce hc V1 V2 (kg/m3) (m3/s) (°) (m /s) (m) (°) (m) (m) (m) (m /s) (m /s) 1500 0.0893 40 3.30 0.025 71.54 0.035 0.164 0.142 1.02 1.80 -13.41% Spillway with Rectangular 1500 0.0735 40 3.27 0.022 72.60 0.030 0.145 0.164 0.98 1.73 4.88% cross-section 1500 0.0592 40 3.23 0.017 73.56 0.025 0.123 0.129 0.92 1.55 -2.78% 1500 0.0484 40 3.00 0.015 74.50 0.020 0.110 0.138 0.85 1.46 18.52% 1500 0.0893 40 3.30 0.05 51.85 0.035 0.145 0.164 1.50 1.90 13.10% 1500 0.0735 40 3.27 0.042 53.00 0.030 0.123 0.129 1.48 1.80 25.45% "Y"-type Spillway 1500 0.0592 40 3.23 0.035 53.40 0.025 0.110 0.138 1.43 1.74 27.96% 1500 0.0484 40 3.00 0.030 52.30 0.020 0.108 0.105 1.30 1.53 -1.08%
Fig. 8. Comparison of predicted data and experimental scour depths
4. Conclusion
In this paper, we described experiments that we conducted to investigate the characteristics of debris-flow nappe downstream from a check dam. A theoretical analysis for predicting the maximum scour depth is also presented. The following conclusions were drawn from this analysis: 1) The geometry of debris-flow nappes are mainly determined by the type of spillway. For rectangular spillways, debris flows overflow with shallow flow depths, and the nappe is wide and thin. However, for Y-type spillways with lateral contraction, the nappe is relatively narrow and thick. 2) Scour holes formed when the debris-flow nappe plunged into the erodible bed and the deepest point of the scour hole occurred at the tip of the debris-flow nappe. With increasing debris-flow volume, the maximum scour depth increased and the deepest position of the scour hole extended further downstream. 3) Based on the momentum conservation law, a theoretical expression was proposed to predict the maximum scour depth downstream from the check dam. The predicted results exhibited good agreement with the experimental results. The maximum relative error of the predicted results was always smaller than 27.96%.
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Acknowledgements
The study presented in this paper was supported by the 135 Strategic Program of the Institute of Mountain Hazards and Environment, CAS,NO. SDS-135-1701, the National Natural Science Foundation of China (Grant No. 41661144028; 41372331), the Science and technology research project of China railway corporation (2017G008-F), and Foundation of Youth Innovation Promotion Association, Chinese Academy of Sciences (Grant No. 2017425).
References
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927 7th International Conference on Debris-Flow Hazards Mitigation Debris-flow mitigation measures and an application case in a small- scale watershed in China
Jiangang Chen, Xiaoqing Chen*, Wanyu Zhao, Yong You
Key Laborator y of Mountain Hazards and Earth Surface Process, Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (CAS), Chengdu, 610041, China
Abstract
The Wenchuan earthquake in China (May 12, 2008) triggered numerous debris flows, some of which caused serious secondary disasters, such as in Wenjia gully (Sichuan Province) and Sanyanyu gully (Gansu Province). To solve this problem, a planning method for debris-flow mitigation engineering measures was proposed based on watershed and river sediment transport characteristics. This method aids in the selection of engineering measures to effectively prevent secondary disasters and control unconsolidated soil and debris-flow movement from upstream to downstream. Several measures are considered in the proposed method, including check dams and transverse structures built across gullies, which are important engineering measures in debris- flow hazard mitigation. Based on the developed planning method, new types of check dams were proposed, the regulation effect of sediment particle size in slit check dams was qualitatively analyzed, and the design parameters (e.g., deposition slope, deposition length, and dam height) of the check dams were deduced. Moreover, a series of drainage channels was proposed to cross highways and discharge debris flows into the river or debris-flow deposition basin below. Finally, an engineering application case in Xiaogangjian gully, Sichuan Province, China, was examined using the proposed watershed planning method, in which a series of check dams with different orifices sizes was constructed. The engineering application results provide useful data for developing check dams as a restoration tool and hazard mitigation technology in small watersheds.
Keywords: Debris flow; mitigation measures; small-scale watershed; check dam; drainage channel
1. Introduction
Debris flows are sediment movements that can travel several kilometers as a series of surges; they are common on steep terrain and in deep gullies within mountainous areas (Iverson, 1997; Hungr et al., 2001; VanDine and Bovis, 2002; Godt and Coe, 2007; Cui et al., 2013). They can scour channel banks and gully beds, enhancing the debris- flow discharge due to the effects of the blockage and subsequent outbreak process in watersheds (Cui et al., 2013). The Wenchuan earthquake in China (May 12, 2008, Sichuan Province) triggered numerous debris flows, some of which caused serious secondary disasters, such as in Wenjia gully (Sichuan Province) and Sanyanyu gully (Gansu Province) (Zhou et al., 2013; Zhang and Matsushima, 2018). Subsequently, numerous large-scale debris flows occurred during the rainy season from 2008 to 2018, when portions of hillslope deposits were transferred to channel deposits, and the source materials in the channel were carried to the outlet of the watershed. Based on a study of the debris-flow activities along a highway in China, the debris flow volume, runout distance, and deposition width decrease over time due to the decreasing source material volume and a possible change in debris-flow type (Zhang and Zhang, 2017). Whereas 665 debris-flow events per year occurred from 2001 to 2008, the average number of debris-flow events per year reached 1153 from 2009 to 2016 in China (Fig. 1). This trend suggests that debris-flow hazard prevention and mitigation will remain an urgent issue in the coming decades, as stated by Stoffel et al. (2014). Debris-flow mitigation measures can be classified as structural measures (e.g., check dams, drainage systems, flexible barriers, and debris-flow basins) or nonstructural measures (e.g., warning and evacuation systems, appropriate land use, and building improvements) (Armanini and Larcher, 2001; Takahisa, 2008; Hassanli et al., ______
* Corresponding author e-mail address: [email protected]
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2009; You et al., 2011; Canelli et al., 2012; Chen et al., 2014; Chen et al., 2015; Liu et al., 2017; Wang et al., 2017; Chen et al., 2018; Wang et al., 2018). Although debris-flow mitigation measures have been constructed in a few watersheds in the Wenchuan earthquake area, China, large-scale debris flows have still occurred, proving that existing mitigation measures are inadequate (Wang et al., 2012; Wang et al., 2013). In addition, the dual impacts of the Lushan earthquake (April 20, 2013, Sichuan Province, China) and the Jiuzhaigou earthquake (August 8, 2017, Sichuan Province, China) are expected to further increase the incidence of debris-flow disasters (Cui et al., 2014; Chen et al., 2018). Therefore, developing new planning methods and techniques based on debris-flow characteristics in this earthquake-stricken region is a critical issue. This paper presents a new planning method for debris-flow hazard prevention and mitigation measures, and describes several corresponding engineering technologies. The proposed debris-flow hazard prevention and mitigation method was tested based on several medium-sized debris flows, and a related case study of a small, steep watershed is described. The results indicate that this method is effective in regulating debris-flow hazards; therefore, this planning method and related engineering technologies may provide a new process for debris-flow hazard mitigation.
2000
1600
1200
800
Number of debris flowsdebris of Number 400
0 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 Year
Fig. 1 Debris-flow events in China from 2001 to 2016
2. Debris-flow hazard mitigation engineering planning method and new technologies
2.1. Planning method
The early planning method for debris-flow hazard mitigation engineering, in 1980s in China, is primarily based on the target objects intended to be protected, which is effective for small debris flows, but not large debris flows. For instance, several large debris flows have exited the gully in which they originated and blocked the river below, such as in Wenjia gully (Sichuan Province) and Sanyanyu gully (Gansu Province). Therefore, the planning method requires reconsideration for large debris flows. Based on the calculation method of the debris flow block the main river (He, 2003) and to make full use of the sediment transport capacity of rivers, Chen et al. (2015) proposed the use of cascade check dams and deposition basins to regulate the scale of debris flows in gullies. Figure 2 shows the specific implementation process of the new planning method for debris-flow mitigation engineering measures, which includes consideration of the debris-flow peak discharge with a design standard (QTotal), the debris-flow peak discharge through the drainage channel (QDrainage), the debris-flow peak discharge requiring blockage by check dams (QBlock), and the debris-flow peak discharge that must be accommodated by the deposition basin (QDeposition). The primary steps in designing a mitigation system include the determination of QTotal based on the design standard and rainfall data, as well as the transport capacity of nearby rivers based on river discharge data, followed by the determination of QDrainage. Why we consider the drainage channel discharge capacity and the sediment transport capacity of the river, the reason is that the river can be blocked when the debris flow discharge into the river is large than the sediment transport capacity of the river. Based on the block condition of the river, the better process can be selected according to the flow chart in Figure 2. The relation between the debris-flow peak discharge with a design standard and the total volume Q of a debris flow is Q = 19TQP/72, T stands for the debris flow duration time and QP stands for the debris-flow peak discharge.
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Start
QTotal, QDrainage
Yes Drainage channel QTotalİQDrainage? End Q*Drainage=QTotal
No
Yes Drainage channel + Check dams QTotalİQDrainage+QBlock? Q*Drainage=QDrainage End Q*Block = QTotal-QDrainage No
Drainage channel + Check dams + Deposition basin
Q*Drainage = QDrainage, Q*Block= QTotal-QDrainage-QDeposition
End
Fig. 2 The implementation process of the proposed planning method
2.2. New debris-flow hazard-mitigation technologies
To better apply the proposed planning method, a large number of check dams and drainage channels were researched and developed. Several types of structures are described in the following sections. 2.2.1 Cascade check dams with orifices of various sizes A series of open check dams with rectangular orifices with various sizes can be used to block and separation debris particles and the interval spaces of the orifices was calculated by the debris flow impact force. When debris flows enter the circulation area, water and stones are separated.The mean particle size gradually becomes smaller further downstream, thereby decreasing the impact forces of large stones. Such structures can achieve a uniform distribution of debris flow materials along the length of the debris flow. As such, this method makes use of both the siltation and hazard mitigation functions of check dams. Cascade check dams have completely prevented debris flows (Liu et al., 2017); therefore, they have an important hazard mitigation function. Since cascade check dams have the advantage of reducing debris flow particles with sequentially decreassing orifice sizes, regulating the debris- flow peak discharge, this method is expected to have major impacts on the mitigation and treatment of large-scale debris flows.
Fig. 3 Cascade check dams with orifices of various sizes in Shaofang gully, China
2.2.2 Debris-flow blocking structure with piles Blocking structures can be constructed from both rectangular and T-shaped piles arranged in a staggered formation (shown in Fig. 4). In these structures, the horizontal coupling beam is made of reinforced concrete with a rectangular cross-section. Such structures can block stones, while having minimal effects on flow efficiency. The rectangular pile structure in the first row is relatively strong, and mainly blocks large stones; the T-shaped piles in
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the second row are used to form a soil arch effect. Overall, this enables the pile structure to block debris and form a dam. Debris-flow blocking structures with two rows of piles at equal intervals can block large stones while discharging smaller particles, protecting downstream buildings from the impact forces of large debris. Meanwhile, the T-shaped piles improve the stability of the structure. Compared with gravity retaining check dams, the pile spacing can be easily adjusted, reducing the project budget without affecting the function. Finally, the structure is relatively convenient, economical, and practical to construct in the poverty-stricken area.
Fig. 4 Debris-flow blocking sstructure with piles
2.2.3 Drainage channel with prefabricated reinforced concrete boxes The drainage channel with prefabricated reinforced concrete boxes described herein represents a new type of debris flow drainage channel, which is shown in Fig. 5. In this design, the prefabricated reinforced concrete boxes can be used to cconstruct the sidewall or as an energy dissipation section by filling with stones. The side walls, which can be either vertical or inclined, can be constructed from masonry, concrete, reinforced concrete, or prefabricated, reinforced concrete boxes. The ground sill was constructed by reinforced concrete. The energy dissipation section is located between the upstream and downstream sections of the channel slab, and stones are used to fill the structure. Different structural forms, such as a steel cable network can be used to avoid scouring damage, can be used as the slab of the energy dissipation section, and a concrete lining layer can be used to protect the bottom from scouring. Debris flows can flow through the top open surface as they pass through the structure. Some of the energy is dissipated via strong interactions between the debris flow and stones. In addition, the impact energy of the debris flow can be absorbed by the soft foundation, and exchange between the debris flow and bottom soil can be inhibited, controlling the erosion of the channel slab by the debris flow. Ultimately, this structure can achieve a balance between the reduction in the potential energy and the dissipated energy.
Fig. 5 Schematic diagram of a drainage channel with prefabricated reinforced concrete boxes
2.2.4 Drainage channel with a step-pool configuration Drainage channels with a step-pool configuration are composed of sidewalls, a channel slab (i.e., step section), upstream and downstream ground sills, and a pool section (Fig. 6). The step section includes the upstream ground-sill, the downstream ground-sill, and a concrete slab that connects the upsstream and downstream ground-sills. The pool section is located between two adjacent ground-sills and is filled with stones. Debris-flow energy can be dissipated via interactions between the debris flow and stones. In addition, debris-flow impact energy can be absorbed by the soft foundation, and exchange between the debris flow and bottom soil can be prevented to control erosion of the channel slab. Figure 6 shows the structural
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characteristics of a drrainage channel with a step-pool configuration. The sidewalls can be vertical or inclined, and different structural forms can be used in the pool sections. Additionally, steel cable networks, soft foundations, or concrete lining layers can be used in the pool bottom to protect against bottom scouring.
Fig. 6 Schematic diagram of a drainage channel with a step-pool
3. Engineering application case
3.1. Research area
The small-scale watershed of Xiaogangjian gully is located in the central Longmen Mountains area (31°02ƍN, 104°07ƍE). The area of this gully is 1.36 km2, the main channel of the gully is 2,590 m long, and the longitudinal gradient is 412‰. The altitude of the catchment ranges from 810 to 1,987 m, representing an elevation difference of 1,177 m. The area with slope gradients equal to or greater than 25° measures approximately 1.16 km2. The slopes in this catchment are mostly steep with large longitudinal gradients, favoring thhe concentratiion of stormwater runoff, making the area prone to debris flows.
Fig. 7 Location of Xiaogangjian gully (source: China Earthquake Administration; MS stands for the Richter scale)
The Wenchuan earthquake induced numerous landslides and avalanches in this gully, with total deposits of 80 × 104 m3. Ten debris floows occurred between May 12, 2008, and September 5, 2011, and caused a radical change to the landscape, particularly the outlet of the gully. The total volume of solid materials that exited the gully was estimated to be 50 × 104 m3, including materials from the old accumulation platform (40 × 104 m3) and those from
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the collapse upstream (10 × 104 m3). An area of 3 × 104 m2 was directly affected by the debris flow in the gully outlet, including the road below the gully. Major hazards included the blockage and siltation of the main road and river channel, the formation of a dammed lake, the inundation of the upstream road, and the rise in the downstream river bed due to high-intensity sedimentation, which resulted in the burial of the road downstream. For example, the road was buried by a debris flow that occurred on July 26, 2010, interrupting traffic for 18 days. Another larger- scale debris flow occurred on August 13, 2010, and blocked the road at the gully outlet to form a dammed lake that flooded the upstream road, interrupting traffic until August 20, 2010.
3.2. Debris-flow mitigation engineering measures in Xiaogangjian gully
To protect lives, properties, and the road, as well as create favorable conditions for a healthy economy and environment and for construction and development in the nearby town, a debris-flow mitigation project in Xiaogangjian gully was carried out in 2011. The design standard of the debris flow mitigation measure was to resist rainstorm-induced debris flows with a 20-year return period. Geological, geomorphological, hydrological, and meteorological conditions were considered in the project design. 3.2.1 Systematic planning of the debris flow mitigation project Based on the planning method for debris-flow hazard mitigation engineering, the debris-flow discharge into the river through the drainage channel is large than the sediment transport capacity of the main river, then the river can be blocked by the debris flow. Thus, the maximum debris-flow discharge into the river through the drainage channel 3 should not exceed the threshold of QDrainage > 17.5 m /s. Based on the hydrology of the small basin, the debris-flow 3 peak discharge with a 20-year return period, QTotal, was 90.3 m /s. Because QTotal > QDrainage, a drainage channel alone would be insufficient in this gully. A series of check dams with orifices of various sizes was used to raise the erosion base level and stabilize the inner bank slope. Based on an analysis of the geological and geomorphological conditions, it was determined that three check dams should be built in the gully. Based on a slope stability analysis, these dams could mitigate a peak 3 discharge of QBlock = 31.3 m /s. Because QTotal > QDrainage + QBlock, a debris flow basin was added to accommodate additional debris flow peak discharge. To avoid blocking the river, the debris flow basin should be capable of 3 accepting a peak discharge of QDeposit = QTotal í QDrainage í QBlock = 90.3 í 17.5 í 31.1 = 41.7 m /s. In summary, the design of the engineering project based on the debris flow peak discharge and the total volume of a single debris flow event is as follows: the peak discharge, QDrainage, of the debris flow that can drain into the 3 3 main river is 17.5 m /s; the reduction in peak discharge, QBlock, that can be produced by check dams is 31.1 m /s; the 3 peak discharge, QDeposit, that can be accepted by the debris flow basin is 41.7 m /s, with a minimum volume of 3 26,414 m , which can be calculated by the empirical formula Q = 19TQP/72, T stands for the debris flow duration time and QP stands for the debris-flow peak discharge. 3.2.2 Application of the drainage channel Three check dams with orifices of various sizes were constructed in Xiaogangjian gully to mitigate debris flows (Fig. 8). In downstream succession, the sequence of dams is as follows: the first dam is 12.5 m high with 7 orifices, the sizes of the orifices are 0.8 × 5.0 m and 0.8 × 3.5 m (width × height); the second dam is 11.5 m high with 7 orifices, the sizes of the orifices are 0.8 × 4.5 m and 0.8 × 3.0 m (width × height); the third dam is 12.5 m high with 10 orifices, the sizes of the orifices are 0.8 × 3.0 m, 0.8 × 2.5 m and 0.8 × 1.0 m (width × height). To protect the highway at the gully outlet, a drainage channel with prefabricated reinforced concrete boxes was constructed to enable discharge of the debris flow into a debris flow basin (Chen et al., 2015). The steps in designing the drainage channel are as follows. (1) Based on field measurements, the drainage channel slope is approximately i = 35%, and the length of the channel is 105 m. The cross-section is rectangular, the width of the channel is B = 6 m, and the height of the sidewall is 3.5 m. According to the construction material, the roughness coefficient is approximately n0 = 0.02. Based on the velocity characteristics of the scour resistance of the reinforced concrete materials, the admissible velocity in the drainage channel ranges from 2.7 to 8 m/s. (2) Assuming that the width and length of the energy dissipation section are b = 3.0 m and L = 15 m, respectively, the mean diameter of the filled stones is D = 0.4 m. The roughness coefficient of the energy dissipation structure section can be calculated using the equation proposed by Chen et al. (2018). (3) The debris-flow velocity can be obtained by substituting the parameters into the Manning equation. During
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the design process, thee relationship between the debris flow depth and the mean diameter of the filled stones is h = 4D. (4) The calculated debris-flow velocity and the admissible velocity in the first step are compared to determine whether the parameters are reasonable. If not, the design parameters can be adjusted, and the computational process repeated. Ultimately, for design parameters of i = 0.35, b = B = 6.0 m, L = 25.0 m, and D = 0.3 m, the roughness coefficient of the energy dissipation structure section is n = 0.109. The calculated debris flow velocity is 4.9 m/s, which meets the permissible velocity limit for the drainage channel.
3.3. Effect of the debris flow mitigation project in Xiaogangjian gully
The debris-flow mitigation project was completed in May 2012. From August 13 to 18, 2012, seven debris flows were triggered in the gully with a total flow volume to 23.2 × 104 m3. The mitigation project was designed by the rainstorm with a 20-year return period in this steep and rich source materials gully, thus, the highway was effectively protected by the mitigation engineering and the debris flows did not cause a disaster. This outcome indicates that the debris flow mitigation strategy used in this gully is effective and that the construction of the mitigation system is feasible. In general, strategies for controlling debris flows using a combination of check dams, drainage channels, and debris flow deposition basins are effectivve (Chen et al., 2015). However, the design standard of the mitigation measures should be improved if the areas to be protected are particularly important. To ensure the longevity of the debriss flow mitigation system, the following measures must be taken. During the construction, the construction quality should be inspected. After the debris flow mitigation measures are installed, their operation and maintenance, such as dredging to maintain the capacity of the check dams and repairing abrasion of the drainage channel, must be performed in a timely fashion.
Fig. 8 Application of the debris-flow mitigation structures in Xiaogangjian gully, Sichuan Province, China
4. Conclusion
Avalanches and landslides caused by the Wenchuan earthquake in the Longmen Mountains region provided an abundant source of loose slope material for debris flows. Based on the large-scale debris-flow characteristics of the Wenchuan earthquake, a new planning method for debris-flow mitigation measures in small-scale wateersheds was proposed, and new techniques for the planning and designing were suggested. A drainage channel constructed of prefabricated reinforced concrete boxes was developed and applied in a practical engineering case. In the case application in Xiaogangjian gully, a series of check dams with orifices with various sizes was constructed to promote the settling of debris-flow particles of various sizes and separate water from stone along the length of the gully channel. The impact forces of debris flows can be effectively reduced using this method. Moreover, this method can uniformly distribute debris flow materials among the group of check dams, more fully enabling the siltation and hazard mitigation functions of check dams. The results support the application of the new debris-flow
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mitigation measure and new planning method to small-scale watersheds, which can not only effectively protect roadways and minimize losses by debris flows in China, but also it can apply to debris-flow-prone regions worldwide.
Acknowledgements
This study was supported by the National Science Foundation of China (Grant Nos. 41661144028, 41790434, and 51709259), and the CAS “Light of West China” Program and Youth Innovation Promotion Association of the CAS (2017426).
References
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935 7th International Conference on Debris-Flow Hazards Mitigation Roles of barrier location for effective debris-flow mitigation: assessment using DAN3D
Shin-Kyu Choia, Tae-Hyuk Kwon a,*, Seung-Rae Leea , and Joon-Young Parkb aDept. of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Daejeon 305701, KOREA bGeo-Environmental Hazards Research Center, Korea Institute of Geoscience and Mineral Resources, 124 Gwahak-ro, Daejeon 34132, KOREA
Abstract
Debris flows can travel at rapid velocities and can cause economic and societal damages. Accordingly, barriers that can dissipate the energy of debris flows are frequently installed as a mitigation measure. However, the effect of barriers on debris-flow behavior is not fully understood. In this study, we used DAN3D to investigate the interactions between a debris flow and barriers, and evaluate the effect of barrier location on debris-flow velocity and volume. We chose a study site in Seoul, Korea, where a debris- flow event occurred in 2011. At the site, we numerically installed a closed-type barrier at four different locations along the flow path in the watershed. We then simulated the debris flow while monitoring debris-flow velocity and volume. The barriers decreased the velocity and volume of the debris flow compared to a simulation with no barrier. In particular, installation of the barrier at the upstream portions of watersheds resulted in the greatest reduction in velocity. Installation of the barrier at downstream portions of watersheds resulted in the greatest deposition of volume. These results contribute to a better understanding of debris-flow behavior associated with the installed barriers as a mitigation measure, and can be used for optimum and efficient design of the debris-flow barriers.
Debris flows; Debris-flow barrier; Location; Velocity; Volume; Entrainment
1. Introduction
Recently, damage caused by landslides has increased due to heavy rainfall. Debris flows, a flow-like type of landslides can travel at extremely rapid velocities and entrain basal channel materials with scouring. Installing debris- flow barriers that can dissipate the energy of debris flows is one of the frequently used methods for preventing damages. Many researches have conducted small-scale experiments to verify the effect of debris-flow barriers (Wenbing and Guoqiang, 2006; Lim et al., 2008; Takahara and Matsumura, 2008; Canelli et al., 2012; Kim et al., 2013; Xie et al., 2014; Ng et al., 2015; Choi et al. 2018). Also, research on flow patterns of debris flows using numerical analysis has been carried out (Remaître et al., 2008; Kwan et al., 2014), but only a little research has been done on analyzing the influence debris-flow barriers on debris-flow characteristics (Remaître et al., 2008; Jeong et al., 2015). Therefore, there is a need for research on this topic. In this study, we explored the influence of a debris-flow barrier on characteristics of a debris flow that occurred during heavy rainfall in Woomyeon Mountain, Korea in 2011. A closed-type barrier that traps all of the debris-flow sediment and water was numerically installed separately at four locations along the debris-flow channel, and 2011 debris flow was then simulated. The effect of the closed-type barrier on characteristics of the debris flow was evaluated with respect to velocity and volume. In all cases, the closed-type barrier significantly reduced velocity and volume compared to the 2011 debris flow without the barrier. Our results contribute to a better understanding of debris-flow behavior associated with installed barriers as a mitigation measure, and can be used to determine an optimum and efficient design for debris-flow barriers.
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* Corresponding author e-mail address: [email protected]
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a b
Both sides of the barrier were blocked
Flow direction
(m) 222-265 178-222 135-178 10
92-135 meters Elevation
Fig. 1. (a) Locations of the numerical barriers and debris-flow initiation area; (b) Schematic description of barrier installation
2. Numerical code
2.1. Governing equation
DAN3D (Dynamic Analysis of Landslides in Three Dimensions; McDougall and Hungr, 2004, 2005) is a commercially available code that simulates the flow dynamics of viscous, liquid-like debris. This numerical code is based on a smoothed particle hydrodynamics (SPH) method that was first developed by Hungr (1995). Changes in a complex 3D terrain cause non-hydrostatic, anisotropic internal stresses, which strongly affect landslide dynamics. The governing equations of DAN3D are composed of mass conservation equation and momentum conservation equations.
hbv vy hx (1) txyt
vx h h b hhgk xx z kyx z zxx v (2) t x y t
v h h b y (3) hhgk yy z kxy z zyy v t y x t
Where, h is the bed-normal flow depth, is the material bulk density, t is time, v is flow velocity, b is the bed- normal erosion-entrainment depth, g is the acceleration due to gravity, k is the stress coefficients. DAN3D simulates the local divergence (or convergence) of landslides flowing over complex 3D topography by using Rankine’s earth-pressure theory (Rankine, 1857). The DAN3D code can incorporate the increase in debris-flow volume based on the effect of momentum transfer between the main flow and the bed materials, assuming that the exponential growth in volume is correlated with the displacement of the debris flow. This code can utilize any of five different rheology models: Newtonian, Plastic, Bingham, Frictional, or Voellmy.
2.2. Rheological model
The volume increases due to entrainment phenomenon at the bottom of the channel can be considered with erosion rate and the flow characteristics (velocity and deposition) can be controlled by the rheological models. Among the rheology models, many researchers have used the Voellmy model for analysis of the debris flows. The voellmy model was originally developed to analysis snow avalanches (Voellmy, 1955; McDougall, 2017). However, the model began
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to be used for landslide analyses because ranges in the velocity and thickness of snow avalanches are similar to landslides (Korner, 1976; McDougall, 2017). The Voellmy model requires frictional and turbulence coefficient values as input parameters. The frictional coefficient is related to the deposition characteristics of debris flows, and the turbulence parameter is related to the velocity of debris flows (McDougall, 2017). For debris flows, the frictional coefficient previously used was in the range of 0 to 0.3 and the turbulence parameter was in the range of 0 to 1000 m/s2. Most debris flows in Korea have extremely rapid velocity (measured maximum velocity was 28 m/s) due to high water content from heavy rainfall. In order to satisfy these high-velocity characteristics, a low frictional coefficient and a high turbulence parameter were used in this study.
3. Case study
3.1. Research area
The study area was Mt. Woomyeon, located in Seoul, South Korea. Mt. Woomyeon has a maximum elevation of 293 m above sea level and is surrounded by buildings and roads within an area of 5,104,162 m2 (Park, 2014). Korean Society of Civil Engineers (KSCE) reported that 33 debris flows occurred from ~150 landslides on July 26–27, 2011. The estimated financial loss was approximately US$15 million, with sixteen lives lost (KSCE, 2012). One of the debris-flow events occurred in the Sindonga watershed, Mt. Woomyeon was chosen for this study. This event has been previously investigated by KSCE (2012). For the Sindonga debris flow case, the watershed area, runout length, average slope angle of the channel were 214,400 m2, 633.6 m, and 17.5°, respectively. The event consisted of three debris flow that coalesced in the main watershed channel and had a total combined volume of 45000 m3. All input parameters used for the back-analysis were based on the field investigation (KSCE, 2012). The debris flow reproduced by the back-analysis had the frictional coefficient of 0.03, the turbulence parameter of 800 m/s2 and the erosion rate of 0.0078 m-1. A debris flow that flowed out to the Sindonga apartment complex was selected as a reference case (Case REF) for numerical modeling in this study (Fig. 1a).
3.2. Condition of debris flow barriers
The effect of location of barrier installation was examined numerically. The distance (L) between the debris source and the roadway near to the Sindonga apartment was 596 m (Fig. 1a). Four locations along the debris-flow channel were determined; thereby, a barrier was placed at 0.3L (i.e., 179 m far from the debris source; Case 1), 0.5L (i.e., 298 m far from the debris source; Case 2), 0.7L (i.e., 417 m far from the debris source; Case 3), or 0.9L (i.e., 537 m far from the debris source; Case 4). The barriers were created by numerically increasing elevation values in the topography file to achieve the desired shape and size using ArcGIS 10.5 (Fig. 1b). The barrier was installed to be perpendicular to the debris flow direction. The barrier width was determined to be two times wider than the width of the front part of the reference debris flow occurred in 2011; it resulted in 48 m wide (Case 1), 50 m wide (Case 2), 60 m wide (Case 3), and 110 m wide (Case 4). The height and thickness of all barriers were set to be 7 m and 3 m, respectively (Table 1). Surface erosion near the barrier was prevented by setting the no erosion zone which was 50 m long and 50 m wide in the upstream and downstream sides of the barrier. Total five simulations, one without the barrier (or reference case; REF) and four with the barrier (Cases 1-to-4), were conducted, as shown in Table 1.
4. Results
Figure 2a shows temporal changes in velocities of the modeled debris flow with respect to the barrier locations, respectively. Herein, as the SPH method was used, the velocity of moving debris flows was determined from the average velocity of all moving particles. When any overflow was observed in some cases, the velocity represented the average velocity of the overflowed particles. The results indicate that debris-flow velocity gradually increased to approximately 7 m/s due to the steep slope angle in the upper part of the flow path, then gradually decreased as the debris flowed downslope. Each time that the debris collided with a barrier, the velocity decreased over time compared to the actual debris flow. Particularly, debris flows were not transferred to downstream in Case 1. In Case 4, debris flows were almost entirely deposited and only about 131 m3 of material was transferred to downstream locations. Immediately after the collision of barriers, velocities decreased to zero in the front part, but debris flows continuously come in. When volume of debris flows exceeded allowable deposition of the barrier, overflow was generated and velocities increased as it passed downstream due to the steep slope. This tendency occurred frequently in the
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downstream locations because volume of the debris flow grow further by entrainment to the downstream. In Cases 2 and 3, the overflowed debris slightly gained their velocities, mainly due to the steep slope after the barriers, and their volumes also increased due to the entrainment as those flowed downslope along the flow path. If there was no entrainment effect, the overflowed volume is expected to stay constant while the velocity is primarily determined by the slope of the channel.
Table 1. Barrier conditions
Width of the Width of the Height of the Thickness of Number of Distance from Case name reference debris barrier barrier the barrier barriers the source (m) flow (m) (m) (m) (m)
1 24 48 179
2 25 50 298 1 5 3 3 30 60 417
4 55 110 537
REF 0 -
Figure 2b shows the volume of the moving segments (or particles). The debris reached a volume of 4100 m3 as it approached the downstream roadway at 85 s in Case REF. Upon the barrier installation, the debris volume was significantly reduced to less than 1000 m3 in all cases with the barrier. As the debris overflowed the barrier in Cases 2, 3, and 4, the volume slightly increased. Toward the downstream area, the barrier became huge because of the increased debris volume and the widened channel width. The volume of the deposited debris (e.g., 200 m3 for Case 1, 600 m3 for Case 2, 1200 m3 for Case 3, and 2800 m3 for Case 4) progressively increased as the barrier location became more distant from the source area. These results implies that it is better to install a barrier near the source location, if it is predictable, before the debris grow further by entrainment. However, it is a daunting task to predict the source location, and thus, either multiple barriers along a debris flow channel or one gigantic barrier at the downstream is expectedly required, when the large- scale debris flow is predicted.
Fig. 2. (a) Changes of the average velocity; (b) Distribution of the volume of moving debris flows. Dots indicate the time of access to the village
5. Conclusion
The effects that barrier locations along a channel have on debris-flow behavior have been explored using the DAN3D numerical code. Temporal changes in debris-flow velocity and volume were observed for scenarios with and without barriers. The velocity of the debris flow significantly decreased at barriers and small volume (< 600 m3 in Case 4) of the debris flow was transferred to downstream locations. After collision with the barrier, the velocity of the overflowed debris flow increased again due to the steep slope after the barrier. Our work shows that it is possible to
939 Choi / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
prevent debris flows from being transferred downstream by installing one barrier in a proper location when the scale of debris flow is small (< 200 m3 in Case 1). Because volume of debris flows around the source and the width of the channel are small, the debris flow can be adequately blocked by small-sized barrier. However, it is difficult to install due to problems with the access road. Large-sized barrier should be installed because the volume increases by entrainment and the width of the channel grows as progressed downstream. For this reason, optimum location is required. These results can be used for optimum and efficient design of the debris-flow barriers.
Acknowledgements
This research was supported by the (BRL, Basic Research Laboratory) Program through the National Research Foundation of Korea (NRF) funded by the MSIT (NRF-2018R1A4A1025765).
References
Canelli, L., Ferrero, A. M., Migliazza, M., and Segalini, A., 2012, Debris flow risk mitigation by the means of rigid and flexible barriers- experimental tests and impact analysis. Natural Hazards and Earth System Sciences, v. 12, n.5, p. 1693. Hungr, O., 1995, A model for the runout analysis of rapid flow slides, debris flows, and avalanches. Canadian Geotechnical Journal, v. 32, n. 4, p. 610–623. Jeong, S. S., Lee, K. W., and Ko, J. Y., 2015, A study on the 3D analysis of debris flow based on large deformation technique (Coupled Eulerian- Lagrangian). Journal of the Korean Geotechnical Society, v. 31, n. 12, p. 45–57. Kim, Y., Nakagawa, H., Kawaike, K., and Zhang, H., 2013, Study on Characteristic Analysis of Closed-type Sabo Dam with a Flap due to Dynamic Force of Debris Flow. Annuals of the Disaster Prevention Research Institute, Kyoto University, n. 56. Korean Society of Civil Engineers, 2012, Research contract report: causes survey and restoration work of Mt. Woomyeon landslide. Korean Society of Civil Engineers. Körner, H.J. 1976, Reichweite und Geschwindigkeit von Bergstürzen und Fließschneelawinen. Rock Mechanics, v. 8, p. 225–256. Kwan, J. S. H., Chan, S. L., Cheuk, J. C. Y., and Koo, R. C. H., 2014, A case study on an open hillside landslide impacting on a flexible rockfall barrier at Jordan Valley, Hong Kong. Landslides, v. 11, n. 6, p. 1037–1050. Lim, Y. H., Jeon, G. U., Kim, M. S., Yeom G. J., Lee, J. H., 2008, Capture effect of slit dam for debris flow and woody debris with hydraulic model experiment -Focusing on A and D type. In 2008 Summer conference of Korean Forest Society, Korean Forest Society, p. 343–344 McDougall, S., and Hungr, O., 2004, A model for the analysis of rapid landslide motion across three-dimensional terrain. Canadian Geotechnical Journal, v. 41, n. 6, p. 1084–1097. McDougall, S., and Hungr, O., 2005, Dynamic modelling of entrainment in rapid landslides. Canadian Geotechnical Journal, v. 42, n. 5, p. 1437– 1448. McDougall, S., 2016, 2014 Canadian Geotechnical Colloquium: Landslide runout analysis—current practice and challenges. Canadian Geotechnical Journal, v. 54, n. 5, p. 605–620. Ng, C. W., Choi, C. E., Song, D., Kwan, J. H. S., Koo, R. C. H., Shiu, H. Y. K., and Ho, K. K., 2015, Physical modeling of baffles influence on landslide debris mobility. Landslides, v. 12, n. 1, p. 1–18. Park, D. W., 2014, Simulation of landslides and debris flows at regional scale using coupled model. Master’s Thesis, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea. Rankine, W. M., 1857, On the stability of loose earth. Philosophical Transactions of the Royal Society of London, v. 147, p. 9–27. Remaître, A., Van Asch, T. W., Malet, J. P., and Maquaire, O., 2008, Influence of check dams on debris-flow run-out intensity. Natural Hazards and Earth System Sciences, v. 8, n. 6, p. 1403–1416. Takahara, T., and Matsumura, K., 2008, Experimental study of the sediment trap effect of steel grid-type sabo dams, International Journal of Erosion Control Engineering, v. 1, n. 2, p. 73–78 Voellmy, A., 1955, Über die Zerstörungskraft von Lawinen. Schweizerische Bauzeitung, v. 73, p. 212–285. Wenbing, H., and Guoqiang, O., 2006, Efficiency of slit dam prevention against non-viscous debris flow, Wuhan University Journal of Natural Sciences, v. 11, n. 4, p. 865–869. Xie, T., Yang, H., Wei, F., Gardner, J. S., Dai, Z., and Xie, X., 2014, A new water–sediment separation structure for debris flow defense and its model test. Bulletin of Engineering Geology and the Environment, v. 73, n. 4, p. 947–958.
940 7th International Conference on Debris-Flow Hazards Mitigation Scour and erosion experience with flexible debris-flow nets
Nadine Feigera,*, Corinna Wendelerb
aDepartment of Civil, Environmental and Geomatic Engineering ETH Zurich, Stefano-Franscini-Platz 5, Zürich 8093, Switzerland bGeobrugg AG, Achstrasse 11, Romanshorn 8590, Switzerland
Abstract
Flexible debris-flow nets were developed in the frame of a three-year PhD thesis by Wendeler in 2008. Since then, they have been used all over the world for debris-flow protection or slope/riverbed stabilization. Some of these installed flexible debris-flow nets have already been filled a couple of times and verified the developed load model. Nevertheless, depending on soil properties, most problems with debris-flow nets appear to be related to construction, such as channel flank erosion, which exposes anchors, or undermines supporting foundations. Both cases lead to instability of the entire system and to increased maintenance costs and should therefore be avoided. In this contribution, we present a service ability method for scour and erosion issues on flexible debris- flow nets. In this context, “service ability method” means a design tool that should help to suggest construction possibilities to avoid erosion problems along the barrier in order to guarantee a lifetime of more than 25 years. The results are based on an analysis of existing barriers to determine occurring scour and erosion problems. Hence, an approach to calculate scour and erosion length and depth respectively around the barrier construction is developed. To validate this approach, a debris-flow simulation with the software RAMMS, or equivalent software, is used. In terms of economic efficiency, different construction measures for riverbed and flank stabilization are analyzed and implemented into the service ability. This tool will help designers and planning engineers to design and calculate their debris-flow protection system in a more economic and safe way. Further, maintenance costs will be minimized and a longer lifetime of the entire barrier system, including the anchors as the most cost-intensive parts, can be provided. Since the project is still ongoing, the final design tool with its equations cannot yet be discussed in detail in the following paper.
"Keywords: Flexible ring net, Debris flow, Service ability"
1. Introduction
Floods, debris flows, and slope failures are the most common natural hazards in Switzerland. In inhabited areas, material and personal damages occur from time to time. Therefore, investigations about debris-flow dynamics and protective structures have been a research topic for many years (e.g. Böll, 1997; Bergmeister, 2009). To increase knowledge about debris-flow behavior, several small- to large-scale flume experiments were recently conducted (e.g. Weber, 2003; de Haas et al., 2015; Major, 1997) and described (e.g. Iverson, 1997; Kowalski, 2008) worldwide. Nowadays, various mitigation structures are available against natural hazards, but erosion around them is a notable problem. One possible mitigation structure against debris flows is flexible ring nets. Until 2005, flexible nets were mainly used for rockfall barriers. Retained slides in these nets were observed, but there was no existing dimensioning concept which proved that these nets are capable of retaining large slides. As a result, real-scale experiments with flexible ring nets were performed at Illgraben test site in Switzerland between 2005 and 2008 (Wendeler, 2008). At least once per year a middle (10’000 – 20’000m3) to large (> 50’000 m3) debris flow naturally occurs in the Illgraben and is suitable to test and improve flexile ring nets. Tests showed that a single barrier can retain debris-flow material until its retaining capacity is reached, depending on the channel geometry. Afterwards the material can overflow the barrier without damaging the system. Therefore, a so-called multi-level system, with several barriers in a row, can be planned and constructed. Such a multi-level system was tested at Hasliberg and Merdenson (Switzerland) (Wendeler et al., 2008).
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* Corresponding author e-mail address: [email protected]
941 Feiger/ 7th International Conference on Debris-Flow Hazards Mitigation (2019)
Based on these tests, a dimensioning concept was developed by Wendeler (2008) and led with load distribution and simulation with the finite-element software FARO (Volkwein, 2004) to standardized flexible ring net barriers. Since then, flexible ring net barriers have been installed worldwide. Some of them have already been filled a couple of times. Experiences over the past ten years showed that most problems with ring nets appear to be related to construction, such as channel flank erosion, which exposes anchors or undermines supporting foundations. We present an initial progress on developing a service ability method based on an analysis of existing barriers, 1:1 field test, an intermediate-scale flume experiment and a debris flow simulation with Rapid Mass Movement Simulation (RAMMS) (WSL Institute for Snow and Avalanche Research SLF) to help designers and planning engineers to design and calculate their debris flow protection systems in a more economic and safe way. Furthermore, the lifetime of these flexible net systems can be optimized by protecting them against erosion and scour processes.
2. Study site and data
To develop an appropriate service ability method, scour and erosion problems on existing flexible ring nets have been analyzed. The main database provides 80 installed flexible ring net barriers in Switzerland since 2005 by Geobrugg AG. Few debris-flow events occurred in Switzerland since then, and therefore, data from filled barriers in Italy, Spain and Peru were additionally used. All installed flexible ring nets in Switzerland will be found on Geobrugg’s web page with information about installation year, barrier type, occurred events and problems beginning in May 2019. Calculation of possible erosion scenarios are based on commonly used equations in river and hydraulic engineering. Developed erosion scenarios are validated by new erosion data collected in an intermediate-scale flume experiment in 2018/2019. Within the same data set, behavior of a slope stabilization mesh (TECCO®) as erosion protection was tested. In addition, 1:1 field data with slope stabilization meshes are collected at Illgraben, Switzerland from December 2018 until June 2019. Experimental small-scale data from (Speerli, 2009), large-scale data from (Bugnion et al., 2012) and observations at Illgraben by WSL were used for comparison. Further data were required to simulate and verify the developed service ability with the Rapid Mass Movement Simulation (RAMMS) debris-flow module. These included (a) a digital elevation model (DEM), (b) a hydrograph or (c) a release area.
3. Methods
To develop a service ability concept, major problems have to be identified and their corresponding solutions designed. First, erosion in a mountain torrent can be triggered by a debris-flow event or normal water flow, for example due to rain. The design should cover both cases. Second, important parameters such as channel geometry, barrier type, soil properties and soil mechanical properties must be detected. Hence, possible scour areas related to flexile ring net barriers were identified upstream and downstream of a barrier construction (Fig. 1). In practice, a barrier overflow can lead to erosion and scours downstream of the barrier (Fig. 1c), whereas a construction post behaves as a pile scour and can lead to undermining of the foundation (Fig. 1d). In case of the barrier filling process or depending on the flow depth, flank erosion occurs at the anchors (Fig. 1a). To quantify these erosion and scours, different calculation formulas can be found in the literature. However, none of them are directly suitable for mountain torrents. To verify an appropriate approach, 1:1 field test (Section 3.1) and an intermediate-scale flume experiment was conducted (Section 3.2). Comparison with experimental data by Speerli (2009), Bugnion (2012), natural Illgraben events and RAMMS simulation led to the final design.
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Fig. 1. Examples of erosion/scour problems on flexible ring net and schematic overview. (a) Eroded anchors; (b) Filled barrier with an overflow; (c) schematic overview of erosion problem in case of an overflow; (d) undermined foundations due to riverbed erosion; (e) Schematic overview upstream erosion problem of the barrier (anchors due to flank erosion, and foundation due to riverbed erosion).
3.1. 1:1 field installation
In rivers, riprap or concrete bolts are commonly used for erosion protection; however, a slope stabilization mesh with or without a geotextile layer is another possible option. There are no data or experiences of such a mesh in a mountain torrent. To evaluate the effectiveness of stabilization mesh on erosion control, a Geobrugg AG slope stabilization mesh was installed at Illgraben (Switzerland) from December 2018 until June 2019 (Fig. 2). Two different mesh types G65/3 and G45/2 were chosen and installed. One only the mesh and one with the corresponding geotextile. In total, four meshes with a 3.9 m width were anchored with ramming nails (Fig. 2a and 2b). Based on the torrent topography, it was not possible to install the mesh in the middle of the torrent bed as well as to cover the mesh edges by a rope (Fig. 2a and 2b). In general, the mesh is anchored by a raster system and on all edges by a top, bottom and lateral boundary rope to fix the mesh as tightly as possible onto the slope (Fig. 2c). Erosion and mesh behavior were observed by a time series of pictures, which were mostly taken after rain events. After a significant event, estimation of acting shear force (휏) was carried out by a RAMMS simulation and calculation according to Equation 1:
휏 = 휌 ∙ 𝑔 ∙ ℎ ∙ 퐽 (1)
where ρ = density, g = gravitational acceleration, h = flow depth, and J = channel inclination.
943 Feiger/ 7th International Conference on Debris-Flow Hazards Mitigation (2019)
Fig. 2. (a) TECCO G45/2 with TECMAT (brown) and without (grey); (b) TECCO G65/3 with TECMAT (brown) and without (grey), Arrow indicates flow direction; (c) Schematic TECCO erosion protection (d) Study site overview.
Fig. 3. Schematic flume experiment view. (a) Channel cross section; (b) Side view; (c) Plan view; (d) Flume with force plate (yellow).
3.2. Flume experiment
To obtain data on overflow length - distance from the net to the impact of the discharge jet - (Fig. 1c), shear force, and erosion behavior of a slope stabilization mesh, an intermediate-scale flume was constructed as shown in Fig. 3. Flume geometry was determined by the available space and to be on a scale of around 1:10 to 1:15 (Fig. 3a-c). No laser sensors were available, therefore, flow velocity (u) and flow depth (h) were estimated by a high-speed camera. Shear (τ) and pressure force (N) were measured with a force plate (Fig. 3d), which was already used by Bugnion
944 Feiger/ 7th International Conference on Debris-Flow Hazards Mitigation (2019)
(2012). The shear and force plates were calibrated and sampled at a rate of 1 kHz. One flume flank is covered with the slope stabilization mesh (G45/2). The used slope material was taken from the Illgraben. The original Illgraben material was used for the debris-flow mud mixture. The release of smaller mud mixture amount (0.05-0.2 m3) was done by an excavator shovel, and larger amounts by a concrete mixer. In the first experiment series, different material volumes were released to identify the relation between measured shear force and Froude numbers (Fr). During that, erosion underneath the slope stabilization mesh was observed with images. In a second experiment series, overflow length of flows past a ring net barrier that was installed in the flume 2.5 meters downstream of the release point were evaluated. In these experiments, a single debris-flow release filled the net. Additional flows were released and the length of overflow past the net was measured. Collected shear force data were transformed from kg into kN/m2 and for each experiment its Froude number (Fr) was calculated according to Equation 2:
푢 퐹푟 = (2) 퐴 √𝑔∙ 푏 where u = flow velocity, g = gravitational acceleration, A = cross-sectional area, and b = water level width. To be able to compare our measured shear forces with the measurements made at different scales by Bugnion (2012) and measured at Illgraben (McArdell, 2016), the shear stress is converted to be a dimensionless number alpha (α) as shown in Equation 3:
푢 휏푐ℎ푎푟푎푐푡푒푟𝑖푠푡𝑖푐 ℎ 훼 = 휏푚푒푎푠푢푟푒푑 = 휏푚푒푎푠푢푟푒푑 (3) 휇 휇 where τchar = characteristic shear force defined as u/h = flow velocity over flow depth (shear rate), τmeasured = measured shear force, and µ = viscosity. Overflow length (Fig. 1c) calculation was done according to Equations 4-6 for each overflow event, as well as for existing data from Speerli (2009) and video recorded Illgraben events. Calculated lengths were then compared with the measured ones in the experiment. The most fulfilling equations were used for the design tool to determine the overflow length. Equation 4 is used for dimensioning stilling basin length (Bergmeister, 2009):
퐻 퐿 = (푢 + √2 ∙ 𝑔 ∙ ℎ) ∙ √ + ℎ (4) 푇 𝑔 where 퐿푇 = stilling basin length, u = flow velocity, g = gravitational acceleration, h = flow depth, and H = construction height. Equation 5 determines the overflow length based on the trajectory parabola with inclination:
2(퐻+ℎ) 퐿 = (퐻 + ℎ) ∙ 푡푎푛(훼) + 푢√ (5) 푃 𝑔∙푐표푠(훼) where 퐿푃 = trajectory length, α = slope inclination, g = gravitational acceleration, h = flow depth, and H = construction height. Equation 6 is used to calculate hydraulic jump length for inclined channels (Jirka et al., 2009):
퐿푤 = (6.1 + 4.0퐽) ∙ ℎ2 (6) where 퐿푤 = hydraulic jump length, J = channel inclination, h2 = flow depth downstream the barrier and ℎ2 = h1 2 (√1 + 8Fr − 1) with h1= flow depth upstream the barrier, and Fr = Froude number. 2 Further, other equations can be found in literature such as from Smetana, Rouse, and others (Bollrich, 2013) that are used for stilling basin length dimensioning. All of them are based on an empirical number and the h2 term. Therefore, an empirical number was determined for the taken flume experiment data to improve the equation.
945 Feiger/ 7th International Conference on Debris-Flow Hazards Mitigation (2019)
3.3. Service ability design
The service ability design works as a decision-making tool (Fig. 4). First, parameters, such as channel geometry, barrier type, soil properties and soil mechanical properties are added. Based on this input information and the most suitable equation identified in Section 3.2, a recommendation according to erosion can be made. Based on that a possible mitigation system (slope stabilization mesh, concrete bolts, riprap or a combination of them) can be suggested to ensure a long lifetime of the barrier.
Fig. 4. Schematic overview of service ability tool.
4. Results and Discussion
Since the project is still ongoing, some first results will be presented here. Note that they are not completed yet.
4.1. Field observation
Two weeks after slope stabilization mesh installation (3 December 2018) either a debris flow or flood event with significant sediment transport occurred at Illgraben. Unfortunately, all installed observation measurement devices (geophones, laser sensor etc.) from WSL (Swiss Federal Institute for Forest, Snow and Landscape Research) were not operating in December 2018. Figure 5 shows the slope stabilization meshes after the event. Flow depth is clearly visible, as well as erosion on the upstream and downstream side of the mesh. Upstream side mesh erosion could be expected due to the lack of a lateral boundary rope and the fact that the mesh is not as tight to the slope as it normally is when it gets anchored. However, ramming nails were not displaced by the event.
Fig. 5. Installed slope stabilization mesh (rectangular) after debris flow event of 3 December 2018. Red dashed line shows flow depth, yellow ellipse indicates erosion up- and downstream the mesh. Flow direction is indicated by the arrow.
Table 1 shows calculated shear force for the event on 3 December 2018 based on Equation 1, for granular flow as well as mud flow at two Illgraben locations. One at the mesh location with J=0.05, h=3 m and the other at the measurement device location with J=0.09, h=1.5 m. Comparison with 2006 recorded data, one can assume that the event was approximately the size of 10’000-18’000m3. A comparison with a RAMMS simulation is ongoing.
946 Feiger/ 7th International Conference on Debris-Flow Hazards Mitigation (2019)
Table 1. Shear force results at Illgraben.
Shear force (kN/m2) Shear force (kN/m2) mesh location measurement device location Granular flow 3.2 2.8 (ρ=2150 kg/m3) Mud flow 2.8 2.5 (ρ=1900 kg/m3)
4.2. Flume experiment
Reliable overflow length determination based on actual overflow events at Illgraben is almost impossible. Some overflow events are larger than the recorded video section. Therefore, one can only approximately say the overflow is larger than ten meters or smaller than ten meters. However, a first overflow length estimation based on recorded videos from Speerli (2009) experiments can be done as well as from our first flume experiments. Besides, overflow length according to Equation 4-6 can be calculated for this data as well as for Smetana. Percentage deviation for each trial from the calculated value (trajectory parabola with inclination, stilling basin, hydraulic jump, and Smetana) are shown in Figure 6. Trial 1-5 are data form Speerli (2009) and 6-15 from realized flume experiments. As expected, the length results for the stilling basin (orange dots) underestimate the measured results due to the lack of a slope correction factor within the equation. The trajectory parabola (blue dots) fluctuates around the measured values and has the lowest deviation from measured lengths. The expectation was that the hydraulic jump (green dots) 2 would deviate most. The equation according to Smetana (퐿푆 = 3 ∙ (√1 + 8퐹푟1 − 3)) (yellow dots) is basically the hydraulic jump with additional empirical values and one can see that it already deviates less. However, all of these equations have their limitations and are designed for water. Since all of them depend on the flow depth, flow velocity and hence, the Froude number, a sensitivity analysis was conducted (not shown in this paper). So far one can state that the flow depth uncertainty with respect to measurement error does not account as much as the velocity for the deviation from the measured value. To identify the optimal domain to use the equation, the results for each equation will be correlated with the Froude number. Consequently, the equations to use for the service ability tool will result out of that analysis. These results will be presented at the presentation.
Fig. 6. Percentage deviation from measured Trial (zero line) to calculated trajectory parabola with inclination (blue), stilling basin (orange), hydraulic jump (green), and Smetana (yellow).
947 Feiger/ 7th International Conference on Debris-Flow Hazards Mitigation (2019)
Some first results of dimensionless numbers (α) and Froude number (Fr) were calculated from available Illgraben data. The regression indicates a quadratic behavior, but a final statement cannot be made, since the results from the flume experiments are not fully analyzed.
5. Conclusion and Outlook
So far, no finalized conclusion can be made. The 1:1 field installation is under observation and during the winter months no significant event is expected. RAMMS simulation of the observed event is running. Flume experiments to determine overflow length are completed and shear force experiments are ongoing. Afterwards, interpretation of the data will allow the determination of equations to describe a lower and upper bound for the construction length. Measured shear forces will be analyzed and compared with the measured Illgraben data. Finally, the service ability design can be programmed and tested. The tool will be presented at the presentation. The eventual creation of the tool describes an overall start for guidelines and help to design a sustainable flexible ring net barrier. Further tests, experiments and especially experience are necessary to improve service ability on flexible ring net barriers.
Acknowledgements
This project was done within a master thesis frame at the Swiss Federal Institute of Technology in Zurich, Switzerland (ETHZ) in collaboration with Geobrugg AG, Switzerland. A grateful acknowledgement to Geobrugg AG for the opportunity to realize such a project and for all the assistance and helpful discussions. Especially, A. Lanter, S. Karrer and R. Wyss for helping in the field. Thanks to B. McArdel for sharing Illgraben knowledge and data from Swiss Federal Institute for Forest, Snow and Landscape Research (WSL).
References
Bergmeister, K., 2009, Schutzbauwerke gegen Wildbachgefahren: Grundlagen, Entwurf und Bemessung, Beispiele: John Wiley and Sons, 121 p. Bollrich, G., 2013, Technische Hydromechanik 1: Grundlagen, Berlin: Beuth Verlag GmbH, 289. Böll, A.,1997, Wildbach- und Hangverbau: Berichte der Eidgenössischen Forschungsanstalt für Wald, Schnee und Landschaft, Nr. 343. Bugnion, L., McArdell, B. W., Bartelt, P., and Wendeler, C., 2012, Measurements of hillslope debris flow impact pressure on obstacles: Landslides, v 9(2), p. 179-187, doi: 10.1007/s10346-011-0294-4. Haas, T., Braat, L., Leuven, J. R., Lokhorst, I. R., and Kleinhans, M. G., 2015, Effects of debris flow composition on runout, depositional mechanisms, and deposit morphology in laboratory experiments. Journal of Geophysical Research: Earth Surface, p. 1949-1972, doi: 10.1002/2015JF003525. Iverson, R. M., 1997, The physics of debris flows. Reviews of geophysics, p. 245-296. Jirka, G. H., and Lang, C., 2009, Einführung in die Gerinnehydraulik. Karlsruhe: Universitätsverlag Karlsruhe, 69 p. Kowalski, J., 2008, Two-phase modeling of debris flows [Ph.D. thesis]: Zurich, ETH Zurich. Major, J. J., 1997, Depositional Processes in Large‐Scale Debris‐Flow Experiments: The Journal of Geology, p. 345-366, doi: 10.1086/515930. McArdell, B. W., 2016, Field Measurements of Forces in Debris Flows at the Illgraben: Implications for Channel-Bed Erosion: International Journal of Erosion Control Engineering, p. 194-198, doi: 10.13101/ijece.9.194. Speerli, J., 2009, Murgang Modellierung. Rapperswil: Institut für Bau und Umwelt (unpublished). Volkwein, A., 2004, Flexible Murgangbarrieren: Bemessung und Verwendung. Birmensdorf: Berichte der Eidgenössischen Forschungsanstalt für Wald, Schnee und Landschaft, v. 8. Weber, D., 2003, Untersuchungen zum Fliess-und Erosionsverhalten granularer Murgänge [Ph.D. thesis]: Zurich, ETH Zurich. Wendeler, C. S., 2008, Murgangrückhalt in Wildbächen: Grundlagen zu Planung und Berechnung von flexiblen Barrieren [Ph.D. thesis]: Zurich, ETH Zurich. Wendeler, C., Volkwein, A., Roth, A., Herzog, B., Hahlen, N., and Wenger, M., 2008, Hazard prevention using flexible multi-level debris flow barrier, on Proceedings, Interpraevent,11th, Dornbirn:Voralberg, Austria, p. 547-554.
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956 7th International Conference on Debris-Flow Hazards Mitigation Debris-flow mitigation – research and practice in Hong
Kong Ken K.S. Hoa,*, Raymond C.H. Kooa, Julian S.H. Kwana
a Geotechnical Engineering Office, Civil Engineering and Development Department, Hong Kong SAR Government, China
Abstract
Dense urban development on a hilly terrain, coupled with intense seasonal rainfall and heterogeneous weathered profiles, gives rise to acute debris flow problems in Hong Kong. The Geotechnical Engineering Office (GEO) of the Hong Kong SAR Government has launched a holistic R&D programme and collaborated with various tertiary institutes and professional bodies to support the development of a comprehensive technical framework for managing landslide risk and designing debris flow mitigation measures. The scope of the technical development work includes compilation of landslide inventories, field studies of debris flows, development and calibration of tools for landslide runout modelling, back analysis of notable debris flows, physical and numerical modelling of the interaction of debris flow and mitigation measures, formulation of a technical framework for evaluating debris flow hazards, and development of pragmatic mitigation strategies and design methodologies for debris flow countermeasures. The work has advanced the technical understanding of debris flow hazards and transformed the natural terrain landslide risk management practice in Hong Kong. New analytical tools and improved design methodologies are being applied in routine geotechnical engineering practice.
Keywords: Debris flow mitigation; landslide risk management
1. Introduction
Starting in 2010, systematic study and mitigation of natural terrain landslide risk has become a core component of the Hong Kong Government′s Landslip Prevention and Mitigation (LPMit) Programme, which is managed by the Geotechnical Engineering Office (GEO). In order to tackle natural terrain landslide hazards, technical development work has been in progress by GEO since the late-1990s. Through systematic mapping and studies of notable landslides, advances have been made in the understanding of the mechanisms and classification of natural terrain landslides and debris movement, together with the formulation of risk management and hazard mitigation strategies. Based on the state-of-the-art knowledge, GEO developed a technical framework for evaluating landslide hazards (Ho et al., 2015), and implemented R&D studies to advance the strategy and design of mitigation measures in order to reduce landslide risk to an as low as reasonably practicable (ALARP) level. This paper presents the progressive development of the natural terrain risk mitigation practice in Hong Kong, and the advances made by the R&D work. The practical challenges in relation to the design, construction and maintenance of landslide mitigation measures are discussed.
2. Nature of Natural Terrain Landslides
Hong Kong has a population of over 7 million and a small land area of 1,100 km2, only 15% of which is developed land. The terrain is hilly, with 75% of the land being steeper than 15° and 30% steeper than 30°. Rainfall intensities exceeding 70 mm/hour and 300 mm/day are not uncommon. The dense urban development on a hilly terrain, together with intense seasonal rainfall and variable weathered profiles, gives rises to acute slope safety problems in Hong Kong. This is reflected by a death toll of over 470 fatalities due to landslides since the 1940s.
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* Corresponding author e-mail address: [email protected]
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Hong Kong comprises a hilly terrain with dense urban development close to steep hillsides. The natural terrain is typically mantled by weak and heterogeneous saprolite or colluvium, which is susceptible to shallow, small to medium- scale landslides (see Figure 1), usually several hundreds cubic metres, or occasionally more sizeable, due to loss of suction or build up of local perched water pressure as a result of intense rainstorms. This can be further complicated by ongoing progressive deterioration of the condition of the natural hillside due to successive heavy rainstorms. These landslides can develop into debris flows where debris reaches drainage lines with surface water flow resulting in increased mobility (i.e. larger velocity and greater runout distance). Based on the landslide inventory, on average about one landslide occurs each year for every 2 km2 of natural hillside in Hong Kong. Occasionally, larger scale debris flows (see Figure 2) can occur given adverse site setting and intense rainfall. The inventory, compiled using aerial photographs, contains records of more than 100,000 past failures on the natural hillsides in Hong Kong. Apart from structural, geological and hydrogeological factors, unfavourable topographical factors can also contribute to increased susceptibility to landslide initiation, such as breaks in slope, topographic depression, head of drainage line, and presence of regolith downslope of a rock outcrop. Channelised debris flows along incised drainage lines or pronounced topographic depressions with concentrated surface water flow tend to be more mobile (as compared to landslides on a planar hillslope) with notable velocities (in the order of 10 m/s or more). Due cognizance needs to be taken of the nature of channelised debris flows in the design of mitigation measures. Debris flows can occur in pulses and may entrain loose materials due to erosion along the flowpath. They can also engulf large boulders, which can be isolated or in clusters occurring as a bouldery front, typically with an inverse grading due to reverse segregation (see Figure 3). The complex and transient nature of such surge two-phase flows can be further complicated by the presence of large broken tree trunks. Additionally, there is the possibility of dam break pulses occurring along the drainage line due to build-up of a temporary debris dam.
Fig. 1. Landslide-prone natural terrain of Hong Kong Fig. 2. The 1990 channelised debris flow at Tsing Shan g
Shek Mun Kap Yi O Village Nam Chung Tsuen Fig. 3. Bouldery front of channelised debris flows observed in June 2008 in Hong Kong
3. Natural Terrain Landslide Risk Management
The Geotechnical Engineering Office (GEO) has launched a holistic R&D programme and collaborated with various tertiary institutes and professional bodies to support the development of a comprehensive technical framework for managing landslide risk and designing debris flow mitigation measures with more scientific rigour. The scope of the technical development work includes compilation of landslide inventories, field studies of debris flows, development and calibration of tools for landslide mobility modelling, back analysis of notable debris flows, physical and numerical modelling of the interaction of debris flow and mitigation measures, formulation of a technical framework for evaluating debris flow hazards, and development of pragmatic mitigation strategies and design methodologies for debris flow countermeasures. The work, which spans the last two decades, has advanced the technical understanding of debris flow hazards and transformed the natural terrain landslide risk management practice
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in Hong Kong. New analytical tools and improved design methodologies are being applied in routine geotechnical engineering practice by local practitioners including geotechnical engineers and engineering geologists. One of these new tools is an emphasis on risk-based management. Landslide risk can be quantified as follows:
Risk = Pi × Ci (1)
where Pi is probability of occurrence of landslide hazard and Ci is landslide consequence.
The risk posed to a given facility can be managed by reducing Pi by means of stabilisation works or by reducing Ci through mitigation measures, or by doing both. For existing facilities such as buildings or roads subjected to natural terrain hazards, slope stabilisation on the steep hillside is often neither practically nor economically and environmentally justifiable. Instead, an active mitigation strategy involving the implementation of mitigation measures (such as debris-resisting rigid barriers, steel flexible barriers, or boulder fences) is more practicable (Ho, et al., 2015). In view of the complexities and uncertainties associated with debris flows, emphasis has been given by GEO in developing and adopting pragmatic and suitably simplified barrier design methods. An overview of the advances in geotechnology for slope stabilisation and landslide mitigation was given by Ho (2005).
4. Evolution of Barrier Design Practice
4.1. Phase 1 – Development of Barrier Design Guidelines
Traditionally, the assessment of natural terrain landslide hazards was undertaken by engineering geologists through an engineering geological approach, with a qualitative risk assessment and the necessary risk mitigation measures determined largely by experience and judgement. The process was typically not particularly transparent. Starting in the late 1990s, significant advances have been made by GEO in developing practical numerical tools for debris mobility assessment and calibrating the rheological models and input parameters through systematic back analysis of local case histories of the more mobile landslides (Kwan & Sun, 2007). GEO also promulgated guidance on the assessment of debris discharge, flow velocity and thickness, debris run-up, retention capacity of barriers, and surface drainage provisions (GEO, 2014). The technical guidance on mitigation measures promulgated by GEO at that time covers primarily the design of rigid barriers against debris and boulder impact. In developing the guidance, a holistic approach was adopted including benchmarking against international practice and reviewing relevant laboratory and field studies, back analysis of instrumented field data, performance review of barriers upon impact by landslides, etc. In essence, the basis of the guidance promulgated at this early stage was largely empirical, supported by literature review and limited field studies.
4.2. Phase 2 – Rationalisation and Enhancement of Barrier Design Guidelines
From about 2010 onwards, GEO initiated further R&D work focusing on the use of flexible and rigid barriers to arrest natural terrain landslides. The advances have led to an improved understanding which enables the guidance on barrier design to be rationalised and expanded. The basis of the enhanced design approaches is multi-pronged, including back analysis of field observations, use of physical models (laboratory flume), numerical techniques, analytical solutions, etc. A key consideration is to build in sufficient robustness to cater for the uncertainties in the field associated with the complex characteristics and variable composition of debris flows. The work culminated in the promulgation of new or improved design guidance covering the following areas: (a) a design methodology for the impact of debris and boulders on rigid and flexible barriers using a force approach (Kwan & Cheung, 2012); (b) a design methodology for debris impact on flexible barriers using the energy approach based on insight from Discrete Element Model (DEM) analysis and a simplified analytical framework (Sun & Law, 2012); (c) a design methodology for debris impact on rigid or flexible barriers using the force approach, including a multiple-phase debris impact model which accounts for dynamic impact pressure and static earth pressure of the deposited debris , with due allowance made for the variation in debris velocities at different phases of debris impact as computed from debris mobility analysis (GEO, 2015), together with allowance for the additional drag force in the event the debris overtops the barrier;
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(d) an analytical framework for the design of multiple barriers (with the upstream barriers acting as check dams) based on a newly developed staged mobility analysis (Kwan et al., 2015); and (e) a design framework for the use of prescribed flexible barriers in mitigating open hillslope landslides in order to streamline the design process (GEO, 2014). The current design approaches adopted in Hong Kong are summarised in Figures 4 and 5.
Step 1
(i) Calculate energy loading for pile-up mechanism (Ep) (ii) Calculate energy loading for run-up mechanism (Er) ( ) ( ) cos( + ) sin( + ) = = 4( cos sin3 ) 48 5 ( cos sin ) 훼휌푄0 푈0 휌푄0 푈0 휃 훾 휃 훾 where α is dynamic퐸푝 coefficient (taken to be 2.0 for flexible barrier); ρ is debris density; Q is discharge푟 rate; U is debris2 impact velocity; 2µ is basal friction 0 퐸 0 0 coefficient (i.e. tan φ); φ is휇 debris휃 friction− 휃 angle;푔 θ is inclination of channel base; γ is inclination of ramp formedℎ by푔 debrisµ behind휃 − the barrier;휃 and g is gravity Step 2 (i) Calculate kinetic energy of landslide debris when the (ii) Calculate kinetic energy of landslide debris that debris front reaches the design location of flexible barrier E( k1) pass through the design location of flexible barrier (Ek2) Step 3
Design energy loading E = min {max(Ep, Er), max(Ek1, Ek2)} Step 4 Check if design energy loading≤ 0.75 × energy rating of flexible barrier certified by ETA full-scale rockfall test. Notes: (1) For other design checks, see (ii), (iii) & (iv) of Figure 5. (2) If Step 4 cannot be satisfied, then use force approach for design.
Fig. 4. Summary of the energy approach for design of flexible barriers
Fig. 5. Summary of the force approach and key design checks for flexible and rigid barriers
The concepts of a composite structure comprising a rigid barrier with baffles to dissipate the energy of landslide debris and arrest some of the boulders, together with a cushioning layer on the rigid barrier front face to help reduce boulder impact load, are promoted to enhance robustness. Apart from the promulgation of technical design guidelines, GEO has also published guidance on other related design and construction issues as follows: (a) suitable detailing of rigid and flexible barriers (e.g. avoiding damage of posts in flexible barrier due to boulder impact, improving drainage provisions, and enhancing resilience against scouring of the substrate of the barrier foundation, detailing of a deflector at the crest of a rigid barrier to avoid spillage of debris due to debris run-up upon impact, etc.); (b) improvement of contract specification for new flexible barriers to enhance durability based on a performance review of about 100 local barriers, together with retrofitting of deteriorated steel components of existing barriers; and
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(c) guidance on slope landscaping and use of bioengineering techniques to improve the aesthetics and biodiversity of the plants on or close to the barriers (GEO, 2011). GEO is currently also using Building Information Modelling (BIM) model to examine buildability issues and construction sequencing in order to optimise the design layout of barriers and minimise cut and fill operations.
4.3. Phase 3 – Optimisation of Barrier Design
To validate or calibrate the various design approaches and improve the understanding of barrier behaviour with a view to optimising barrier design, GEO has continued to undertake in-house development work and collaborate with practitioners and with local tertiary institutes and overseas experts in pursuing various R&D initiatives. These include the use of state-of-the-art physical modelling (centrifuge as well as laboratory and field flume tests) to study mechanisms, application of advanced numerical modelling, and development of new analytical approaches.
i. Displacement-based approach for assessing geotechnical stability and flexural response of rigid barriers Conventional force-based design approaches often result in over-design of rigid barriers subject to debris impact which is transient in nature. The newly proposed displacement-based approach could provide a more realistic evaluation of the performance of rigid barriers subject to boulder impact. Based on fundamental principles of dynamic analysis, Lam & Kwan (2016) developed closed-form formulae for estimating the translational and rotational movements, as well as the flexural deflection and tensile reinforcement strain of rigid barriers, due to boulder impact. A series of small scale impact tests were carried out to verify the predictions using this displacement-based approach (Lam et al., 2017) and good agreement was obtained. A comparison was made between the displacement-based approach and the conventional limit equilibrium analysis. Based on the impact scenarios that are typically encountered in routine design (i.e. a 1 m diameter boulder with a velocity of 10 m/s impacting onto a typical 6 m high, 10 m long rigid barrier), the predicted translational and rotational movements of the barrier were found to be insignificant based on the displacement-based approach. Large-scale tests were also carried out to investigate the structural response of a rigid barrier subject to impact by a solid steel impactor, which successfully validated the enhanced flexural stiffness method. The above have demonstrated that substantial cost savings could potentially be achieved in barrier designs by accounting for the inertia effect of a rigid barrier.
ii. Field testing of cushioning materials for reducing boulder impact load on rigid barriers Field monitoring and observations together with recent centrifuge tests indicate that impacts due to hard inclusions (i.e. boulder front) of a debris flow can result in high magnitude and transient loads on a rigid barrier. With a view to damping out these force spikes, a systematic study on the use of different cushioning materials to shield the barrier was initiated by the GEO. In general, the cushioning materials are deformable and thus capable of prolonging the impact process and attenuating the impulsive forces due to the hard inclusions. A large-scale instrumented pendulum impact test facility involving a 1.16 m diameter concrete ball (2,000 kg in weight) with a maximum impact velocity of 8.4 m/s and a kinetic energy of up to 70 kJ was set up. Four types of cushioning materials, namely rock-filled gabions, recycled glass cullet, ethylene-vinyl acetate (EVA) foam, and cellular glass, were tested. The results show that the cushion layer could effectively reduce the maximum impact forces although it would become less effective after successive impacts (Ng et al., 2018). The test data were also used to calibrate numerical models for further parametric studies. Recent large-scale impact tests have also shown the effectiveness of a gabion cushioning layer in preventing localized structural damage (such as cracking, penetration, perforation and scabbing) in a reinforced concrete barrier, and in substantially reducing the flexural deflection at barrier crest (by 67% to 90%).
iii. Study on use of baffles to dissipate energy of debris flow Baffles are flow-impeding structures installed along the flow path to dissipate the energy of debris flows and screen out large boulders. A series of instrumented flume tests and back analyses were carried out to investigate dry sand flow impact on an array of baffles (Choi et al., 2014). The influence of baffle height, number of rows, and transverse and longitudinal spacing of baffles was systematically examined. These small-scale tests with dry sand indicate that increasing the baffle height from 0.75 to 1.5 times the approaching flow depth would lead to a more effective development of subcritical flow condition which promotes energy dissipation of the debris. Increasing the number of rows from a single row to a staggered three-row array results in about 70% additional energy loss. Energy loss is attributed to the deflection of granular jets and backwater effects.
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iv. Advanced coupled analysis of debris-barrier interaction Advanced numerical modelling has been adopted to simulate debris-barrier interaction using the computer program LS-DYNA. Various researchers (e.g. Kwan et al., 2015, Koo et al., 2018) have demonstrated that the use of Arbitrary Lagrangian-Eulerian method in LS-DYNA appears to be a promising tool for modelling debris flow and debris-barrier interaction. Such modelling has been benchmarked against laboratory flume tests and actual landslide cases in terms of debris runout characteristics. In the conventional approach, landslide mobility analyses and structural analyses of the barrier are carried out separately. The landslide mobility is first simulated under a free-field condition to obtain design parameters such as flow velocity and depth (e.g. 3d-DMM by Kwan & Sun, 2007), which are then converted into a pseudo-static impact force as input to a separate structural model (e.g. computer program NIDA-MNN by Sze et al., 2018). This latter approach however neglects the dynamics of debris- barrier interaction. Coupled analyses can be carried out using LS-DYNA, with the landslide mass modelled as a continuum in a finite element formulation (see Figure 6). The results successfully reproduced the deformation and forces in various structural components as observed in instrumented case studies (Cheung et al., 2018). The coupled analyses also provided insight on the energy dissipation of landslide debris in the debris-barrier interaction process. The preliminary findings are that the overall strain energy absorbed by the flexible barrier upon debris impact only amounted to a fairly small portion (generally less than 35% based on parametric studies) of the total debris impact energy, as due to internal distortion of the debris and changes in momentum flux direction under a debris run-up mechanism upon impact. It is noteworthy that the continuum model adopted in LS-DYNA has certain limitations as it may not fully simulate particle-fluid interaction and the presence of hard inclusions at the debris front. Other research tools such as coupled analysis using discrete element models and computational fluid dynamics models are being used to examine the potential effects of particle-fluid interaction (Li & Zhao, 2018).
v. Parametric study of varied debris composition and different barrier configurations using physical tests Centrifuge and/or flume tests were conducted for various types of mitigation structures (e.g. flexible barrier, curved rigid barrier, slit barrier, etc.) to examine the effects of impact mechanisms and influence of different debris composition under controlled conditions (Choi et al., 2016; Ng et al., 2016; Song et al., 2017). During the frontal impact of a two-phase debris flow without hard inclusions, the measured dynamic pressure coefficient in the hydrodynamic approach is close to unity (which confirms the principle of conservation of momentum), for both rigid and flexible barriers that are upright (note that the coefficient is less than unity for a curved rigid barrier subject to impact by coarse granular flow). Increasing the solid fraction of a debris flow was found to promote transition from run-up to pile-up mechanisms. Furthermore, test results indicate that the presence of large hard inclusions (boulders) in the debris flow are liable to induce transient force spikes reflecting significant impulse loading on a rigid barrier.
Fig. 6. LS-DYNA simulation of debris Fig. 7. Smart rigid barrier system impact on flexible barrier
5. Way Forward
The above studies have provided useful yardsticks for calibrating or bracketing existing design approaches. They further highlight that there is potential scope for further rationalising and optimising the design, e.g. the numerical coupled analyses suggest that the impact energy transmitted to a flexible barrier could be much lower than that assessed by the current design approach because of internal distortion of the debris and change in momentum flux direction. Similarly, the displacement approach, corroborated by laboratory model tests, suggests that the dynamic
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force exerted on a rigid barrier would be much lower than conventional elastic theory taking due account of the inertia effect. Notwithstanding the above, it should be borne in mind that the physical models are constrained by the use of idealised materials as compared to real life debris flows, and potentially by uncertainties involved in scaling up the observed behaviour. As a basis for validation of the observed insights from the latest R&D work with a view to optimising the design methods, large-scale field tests are planned with a failure volume up to about 500 m3. Class A predictions could be made, which would be calibrated by large-scale field tests using a material composition that resembles real debris flows as much as possible.
6. Ongoing Challenges
Some ongoing challenges and pertinent issues faced by the practitioners are highlighted below: (a) Behaviour of energy dissipation (or brake) elements – brake elements are an essential component of a flexible barrier in dissipating the impact energy. However, there is as yet no internationally recognised testing standard to check their stress strain characteristics at an appropriate strain rate and assess their degree of variability. Based on limited site observations following debris impact (e.g. Kwan et al., 2014), there is an element of uncertainty regarding the actual behaviour of different types of brake elements, particularly when they become buried by landslide debris (as some of them apparently were not activated following debris impact and barrier deformation). (b) Potential for under-estimation of landslide hazards due to climate change – recent local experience with extreme rainfall events has shown that the response of natural hillsides in Hong Kong is highly sensitive to more severe rainfall in that the number, scale and mobility of landslides are much elevated. The assessment of the landslide hazard to be designed for during the design life of the mitigation measure is fraught with considerable uncertainty and difficulty, given that the relatively short time window available for compiling the landslide inventory may not have captured the extreme rainfall events. This is exacerbated by the increased likelihood of occurrence of more frequent and intense weather events associated with potential climate change. The possibility of barriers being under-designed and overwhelmed by more sizeable and/or more mobile landslide hazards than those anticipated by designers based on prior knowledge and experience calls for a paradigm shift in the strategy for managing the associated landslide risk. A recent initiative by the GEO is the development of smart barriers incorporating the use of real time wireless sensors and Internet of Things (IoT) and cloud computing technology to provide early warning of landslide impact and facilitate timely emergency response (see Figure 7). Other recent advances in the management of landslide risk associated with extreme weather events in Hong Kong entailed refinement of the landslide warning system (Ho et al., 2017), development of rainfall-based landslide susceptibility zoning (Ko & Lo, 2018), and innovative approaches in enhanced public education. The above are some of the non-structural measures of landslide risk management under a systems approach in addressing landslide risk in a holistic manner. (c) Durability and long-term maintenance of flexible barriers – a cost effective long term strategy for maintenance of flexible barriers is needed, given that the steel components are subject to progressive deterioration in hot and humid climates like Hong Kong. It is also necessary to have improved knowledge on the rate of corrosion of steel components and various forms of treatment in corrosive environments.
7. Concluding Remarks
The design of landslide risk mitigation measures for debris flows and other flow-like landslides is highly challenging in light of the many uncertainties involved. A holistic and progressive approach has been adopted in Hong Kong to improve our fundamental knowledge of debris flows and to provide scientific insight into the behaviour of debris-resisting landslide barriers as a result of debris-structure interaction (e.g. effect of Froude number of debris flows on impact behaviour, influence of debris impact mechanisms, presence of a dead zone associated with debris deposition upon initial impact, effect of varied debris composition including solid fraction, postulated effect of suction on debris mobility and impact behaviour, influence of compressibility of debris flow on impact behaviour, etc.). The systematic technical development work carried out on landslide mitigation measures has led to an improved understanding of the related mechanisms and the controlling parameters. Nevertheless, it is important to remain pragmatic and to strike a suitable balance in translating research findings into practice with due account taken of the simplifications made in the model testing and computational analyses as opposed to the complex and random nature of real debris flows in the field. Due allowance should also be made in the design for enhanced robustness and redundancy in managing the uncertainties.
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Apart from the consideration of appropriate technical standards and improved design methodologies, it should be borne in mind that there are other pertinent issues that are of relevance to practitioners, including guidance on proper detailing of the works, consideration of buildability, the structural form to be adopted (e.g. post-supported flexible barrier versus side-anchored flexible barriers), an appropriate acceptance system for flexible barrier products for quality assurance and quality control, landscaping works, etc.
8. Acknowledgements
This paper is published with the permission of the Head of Geotechnical Engineering Office and Director of Civil Engineering and Development, Hong Kong SAR Government, China.
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Kwan, J.S.H. & Sun, H.W., 2007, Benchmarking exercise on landslide mobility modelling - runout analysis using 3dDMM. Proceedings of the 2007 International Forum on Landslide Disaster Management, v. 2, p. 945–966. Li, X.Y. and Zhao, J.D., 2018, Dam-break of mixtures consisting of non-Newtonian liquids and granular particles, Power Technology, v. 338, p. 493–505. Lam, N.T.K., Yong, A.C.Y., Lam, C., Kwan, J.S.H., Perera, J.S., Disfani, M.M., and Gad, E., 2017, Displacement-based approach for the assessment of overturning stability of rectangular rigid barriers subjected to point impact, Journal of Engineering Mechanics, in press. Lam, C. & Kwan, J.S.H., 2016, Displacement-based Assessment of Boulder Impacts on Rigid Debris-resisting Barriers - A Pilot Study, GEO Technical Note No. TN 5/2016. Geotechnical Engineering Office, Civil Engineering and Development Department, Hong Kong SAR Government, China, 67 p. Ng, C.W.W., Song, D., Choi, C.E., Kwan, J.S.H., Shiu, H.Y.K., and Koo, R.C.H., 2016, Impact mechanisms of granular and viscous flows on rigid and flexible barriers, Canadian Geotechnical Journal, v. 54(2), p. 188–206. Ng, C.W.W., Su, A.Y., Choi, C. E., Lam, C., Kwan, J.S.H., Chen, R., and Liu, H., 2018, Comparison of cushion mechanisms between cellular glass and gabions subjected to successive boulder impacts. Journal of Geotechnical and Geoenvironmental Engineering, (accepted). Song, D., Ng, C. W. W., and Choi, C.E., Kwan, J.S.H., and Koo, R.C.H., 2017, Influence of debris flow solid fraction on rigid barrier impact, Canadian Geotechnical Journal, v. 54(10), p. 1421–1434. Sun, H.W. and Law, R.P.H., 2012, A Preliminary Study on Impact of Landslide Debris on Flexible Barriers, GEO Technical Note No. TN 1/2012. Geotechnical Engineering Office, Civil Engineering and Development Department, Hong Kong SAR Government, China, 43 p. 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964 7th International Conference on Debris-Flow Hazards Mitigation Flume investigation of the interaction mechanisms between debris flows and slit dams
H. S. Hu a,b*, Gordon G. D. Zhou a,b, D. Songa,b
a Key Laboratory of Mountain Hazards and Earth Surface Process/Institute of Mountain Hazards and Environment, Chinese Academy of Sciences (CAS), #.9, Block 4 , Renminnanlu Road, Chengdu, 610041,China b University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan District, Beijing, 100049, China
Abstract
Slit dams are designed to mitigate debris-flow hazards. However, according to field surveys and past experimental studies, slit dams constructed using currently prescribed design methods usually become blocked, which then leads to the loss of capacities of the slit dam’s capability to mitigate debris-flow hazards. In this study, a series of flume tests were conducted to investigate the interaction mechanisms between debris flows and slit dams. This work aims to contribute to the design of slit dams more reliable. The influence of debris-flow water content (w) and the slit-dam relative post spacing b/dmax (b: post spacing; dmax: maximum particle diameter) were examined. Experimental results reveal that when w<22%, dead zones and pile-ups occur during the interaction processes. When w≥22% and b/dmax≤2.3, run-up, overtopping, and backwater effects can be observed, and with no apparent formation of dead zones. Moreover, when w≥26% and b/dmax>2.3, majority of the granular-water mixtures pass through the slit dam in the form of jet flows with no obvious overtopping.
Keywords: Debris flow; slit dam; interaction mechanisms; flume tests;
1. Introduction
Slit dams, as one type of open-type dams, designed with one or several vertical opening(s) (Chanson, 2004), are initially designed to retain large particles and weaken the peak discharge (Lien et al., 2000; Choi et al., 2018). The relative post spacing (b/dmax, b: post spacing, dmax: maximum particle diameter) is the key parameter (Johnson and McCuen, 1989; Lien et al., 2003), which directly affects the trapping or regulation function of a slit dam. Mizuyama et al. (1988) and MLR (2004) recommended that b/dmax should be between 1.5 and 2.0 for design of slit dams. However, experimental results from Lin et al. (1988) revealed that slit dams have notable effect on trapping debris materials when b/dmax≤1.7. Furthermore, Han and Ou (2006) reported that when b/dmax<1.5, the slit dams become prone to blockage. In addition, field investigations (Shima et al., 2016) showed that slit dams are more likely to be filled up by granular materials contained in debris flows when the relative post spacing is narrower (b/dmax≈1.5). This effectively diminishes the trapping capacity of a slit dam (Fig.1a and Fig.1b). The results from both engineering practice and past experimental studies have shown that slit dams will be blocked with condition of b/dmax≤1.5~2.0, and it will trap granular materials contained in debris flows until the trapping capacity is lost. Ideal behavior of slit dams is to weak the peak discharge of debris flow while is not to be blocked rapidly. Accordingly, the interaction mechanisms between debris flows (with different water contents) and slit dams (with different post spacings) are investigated, which contributes to improving the reliability of slit dams designing.
Fig. 1 Slit dam filled up by granular materials (the pictures from Shima et al., taken in Rishiri island, Japan, 2016) ______* Corresponding author e-mail address: [email protected]
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2. Flume model tests
2.1. Scaling
Small-scale flume modelling is widely adopted to investigate the complex flow interaction between mass movement and structures (Choi et al., 2014; Ng et al., 2015), since it can provide a controlled and systematic manner to study mechanisms of flow-structure interaction (Choi et al., 2015). The Froude number (Fr), which macroscopically quantifies the ratio of the inertial to gravitational forces, is widely adopted to characterize the dynamic similarity between channelized granular flows (Chehata et al., 2003; Hauksson et al., 2007) and geophysical flows (Hübl et al., 2009; Choi et al., 2015). It can be expressed as (Choi et al., 2015): v Fr (1) gh cosT where v is the frontal velocity, g is the gravitational acceleration, h is the maximum approaching flow depth (because the damage of structures usually appeared when debris flows approach with maximum flow depth), and θ is the inclination of the channel.
2.2. Model setup and instrumentation
The experiments were carried out using a flume model with an overall length of 7.0 m, a channel width of 0.3 m, and depth of 0.35 m. Figure 2a shows the flume, which consists of a storage tank, a channel with two different inclinations, and a deposition section. The upper part of the channel with has a steeper angle and is usually regarded as the transportation zone. The lower part is the deposition zone. A model of the slit dam was installed 2.8 m upstream of the outflow plain (Fig. 2b). The slit dam consists of three posts. The post spacing of the modelled slit dam varies from 27 mm to 72 mm by decreasing thickness of the posts. To measure the flow depth of the debris flows, three laser sensors (Leuze, ODSL 30/V-30M-S12, named Lasers A to C) with a resolution of 1 mm were used at the monitoring sections A, B, and C. Meanwhile, three cameras (SONY FDR-AX40, named camera A to C) with a resolution of 1440×1080 pixels and a frame rate of 25 frames per second (fps) were fixed to capture the kinematics of the tests. Three grid lines, with intervals of 0.01 m, were drawn at the base of the channel at sections A, B, and C to approximate the frontal velocity of the flow using the high-speed cameras. In addition, a fourth camera (Nikon D 610, named camera D), with a resolution of 1280×720 pixels and a frame rate of 60 fps, was positioned at the side of the flume to capture the interaction process between debris flows and slit dams. One differential strain-gauge pore pressure transducer (PPT, model KPSI 735, 0 ~ 18 kPa) was used to record the variation in the pore water pressure of debris flows.
Fig. 2 Setup of flume model tests. (a) side view of flume model; inset on the left: model slit dam; inset on the right: a natural debris flow channel in Kangding county, Sichuan, China; (b) plan view of flume model; (all dimensions in m)
2.3. Experimental materials and program
The granular materials used in the tests were obtained from the debris-flow deposition fan of the Jiangjia Ravine, in the Dongchuan District of Yunnan Province, China. The granular material with diameters larger than 20 mm were removed to make sure that all particles flow smoothly in the flume (Cui et al., 2015). Figure 3 shows the topographic
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map of Jiangjia Ravine, the sample collecting site, and the grain-size distribution of the granular material used for the tests. The bulk density of the granular materials was measured to be 2680 kg/m³ in the laboratory. Sixty-five kg dry granular material was used in each test.
Fig. 3 (a) The topographic map of Jiangjia Ravine in Yunnan province and the sample collecting site; (b) particle size distribution of the granular material The water content of the debris flows and the relative post spacing were varied to discern their influence on the flow-dam interaction. The range of water contents that were used in the experiments was selected based on trial and error. When the water content of debris flow is less than 18% (15% adopted), the granular-water mixture is not yet considered to be saturated. The velocity of the flow is very low and the debris stops upslope of the slit dam. On the other hand, when the water content of debris flow is greater than 38% (40% adopted), the Fr of the approaching flow becomes higher than 8.5, exceeding the common Fr range of natural debris flows. Therefore, the water content is varied from 18% to 38% with an interval of 4%. Narrow relative post spacings were set to be b/dmax < 2.0 (i.e., b/dmax = 1.4 and 1.8) while wide relative post spacings varying from 2.3 to 3.6. Details of the modelling tests were summarized in Table 1. Table 1. Test program of debris flow-slit dam interaction
Relative post Water Bulk Solid Degree of Approach Flow Froude Test ID spacing content density concentration liquefaction velocity depth number b/dmax W (%) (kg/m³) (Cs) VwVt 漏m/s漐 (m) Fr w18-1.4 1.4 1.56 0.056 2.11 w18-1.8 1.8 1.62 0.056 2.19 w18-2.3 2.3 1.42 0.070 1.72 w18-2.7 2.7 18 2160 0.69 0.17 1.39 0.073 1.65 w18-3.1 3.1 1.70 0.061 2.21 w18-3.6 3.6 1.65 0.070 2.00 w22-1.4 1.4 3.00 0.045 4.53 w22-1.8 1.8 3.25 0.035 5.57 w22-2.3 2.3 3.25 0.035 5.57 w22-2.7 2.7 22 2010 0.63 0.45 3.00 0.039 4.87 w22-3.1 3.1 3.00 0.040 4.81 w22-3.6 3.6 3.08 0.043 4.76 w26-1.4 1.4 3.25 0.043 5.02 w26-1.8 1.8 3.38 0.040 5.42 w26-2.3 2.3 3.50 0.036 5.92 w26-2.7 2.7 26 1970 0.59 0.55 3.38 0.036 5.71 w26-3.1 3.1 3.38 0.040 5.42 w26-3.6 3.6 3.33 0.040 5.34 w30-1.4 1.4 3.50 0.049 5.07 w30-1.8 1.8 3.62 0.050 5.19 w30-2.3 2.3 3.88 0.044 5.93 w30-2.7 2.7 30 1920 0.56 0.59 3.75 0.041 5.94 w30-3.1 3.1 3.88 0.040 6.22 w30-3.6 3.6 3.50 0.046 5.23 w34-1.4 1.4 4.00 0.040 6.41 w34-1.8 1.8 3.88 0.046 5.80 w34-2.3 2.3 4.00 0.043 6.18 w34-2.7 2.7 34 1880 0.53 0.65 4.00 0.044 6.11 w34-3.1 3.1 4.12 0.036 6.96 w34-3.6 3.6 4.00 0.046 5.98 w38-1.4 1.4 4.12 0.049 5.97 w38-1.8 1.8 4.25 0.045 6.42 w38-2.3 2.3 4.12 0.036 6.96 w38-2.7 2.7 38 1830 0.50 0.83 4.25 0.039 6.90 w38-3.1 3.1 4.25 0.042 6.65 w38-3.6 3.6 4.00 0.043 6.18
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3. Observed interaction process between debris flow and slit dam
In this study, two typical water contents (i.e., w=18%, w=30%) and two typical relative post spacings (i.e., b/dmax =1.4, b/dmax=3.1) were chosen to illustrate the interaction processes between debris flows and slit dams, which contributes to the understanding of the interaction mechanisms between slit dams and debris flows.
3.1. Interaction process between slit dam and debris flows with low water content
Debris flows with low water contents (low Fr condition), such as those in w18-1.4, approach the slit dam with narrow relative post spacing. The debris-flow front is thin and wedge-shaped. It approaches slit dam at t =0 s (Fig. 4a) and impacts the slit dam at t = 0.22 s. The measured frontal velocity is 1.56 m/s (Fig. 4b). When the front of the debris flow impacts the slit dam, few debris are observed to pass through while the majority of the debris are retained. Sediments depositing behind the slit dam form a dead zone (a region in which the debris are static). At t = 0.35 s, the trajectory of the flow started to change as a thin layer of run-up (the debris layer keep running while its height increasing) develops before the slit dam (Figs. 4c&d). As the interaction proceeds, more debris pile up (debris accumulation) on top of the dead zone (Fig. 4e). The pile-up stops when the sediments reach the highest point of the flow (Fig. 4f). Afterwards, the deposited mass begins to propagate upstream along the surface of dead zone (Fig. 4g). The deposits eventually reach a static state at t = 1.33 s (Fig. 4h). Figure 5 shows the interaction between debris flows with a low water content w=18% (low Fr condition) and a slit dam with wide relative post spacing b/dmax = 3.1. The interaction process observed in this test was similar to that observed in test w18-1.4 (Fig. 4) as previously discussed, including the tapered approaching debris flow with a measured frontal velocity of 1.70 m/s, impacting on the slit dam (Fig. 5a&b), runs-up (Fig. 5c&d), and piles-up on top of the dead zone (Fig. 5e), backflows (Fig. 5f), and eventually assumes a static state (Fig. 5h). When the debris flow impacts on the slit dam, much more of granular material-water mixture pass through the posts of slit dam. In addition, a weaker backflow was observed in this test as compared to that of w18-1.4.
Fig. 4 Interaction process between debris flow with Fig. 5 Interaction process between debris flow with low water content (w=18%) and slit low water content (w=18%) and slit dam with dam with wide relative post spacing (b/dmax=3.1): test w18-3.1. DZ represents “dead narrow relative post spacing (b/dmax=1.4): test w18- zone”. 1.4. DZ represents “dead zone”. 3.2. Interaction process between slit dam and debris flows with high water content
Debris flows with high water contents (high Fr condition) were set to impact the slit dams with narrow relative post spacings. Taking the test w30-1.4 as an example, a thinner debris-flow front with a larger velocity of 3.5 m/s approaches the slit dam at t = 0 s (Fig. 6a). Upon impacting the slit dam, part of the debris flow, the main of the slurry, passes through the slit dam and develops a distinct run-up along the face of the slit dam (Figs. 6b&c). Overtopping is observed at t = 0.26 s (Fig. 6c) and t = 0.43 s (Fig. 6d). Run-up continues to overtop the slit dam and the run-up region becomes thicker (Fig. 6d). Meanwhile, backward rolling occurs in the run-up region, where part of
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debris flow hits the posts of the slit dam and is bounced backward (Fig. 6d). The vertical jet begins to fall down towards the channel base (Fig. 6e). At t = 0.93 s, more distinct falling towards the channel base is observed, as well as a bouncing phenomenon which happens when the granular-water mixtures splatter against the channel base (Fig. 6f). Then the granular material-water mixtures upstream of the slit dam start to flow back, increasing the flow’s depth (Fig. 6g). Finally, the slit dam retains the sediments, and the slurry contained in the granular material-water mixtures flows through the slit dam (Fig. 6h). Debris flows with high water contents (high Fr condition) were also set to impact on the slit dam with wide relative post spacings. Taking the test w30-3.1 for example, the measured frontal velocity of a thin debris flow was 3.9 m/s (Fig. 7a). The debris flow makes impact on the slit dam and more of the granular material-water mixture flow through the post spacing. Meanwhile, run-up is observed to develop (Fig. 7b), but was not distinctly thicker than that observed in test w30-1.4 (Figs. 7c&d). No apparent overtopping was observed. At t = 0.67 s, backward rolling occurs (Fig. 7e), which leads to a bouncing phenomenon upstream of the slit dam and is accompanied by the falling of the debris flow down into the channel base downstream of the slit dam (Fig. 7f). Then, the granular material-water mixture upstream of the slit dam start to flow back (Fig. 7g). Finally, majority of the sediment flow through the post spacing, leaving behind a relatively low fraction of its volume behind the slit dam (Fig. 7h).
Fig. 6 Interaction process between debris flow with high Fig. 7 Interaction process between debris flow with high water content
water content (w=30%) and slit dam with narrow relative (w=30%) and slit dam with wide relative post spacing (b/dmax=3.1): test post spacing (b/dmax=1.4): test w30-1.4. w30-3.1. 4. Interaction process influenced by water content and relative post spacing
4.1. Influence of water content on interaction process
Comparing Fig. 4 with Fig. 6, with the same b/dmax=1.4, the difference in the interaction process is obvious. With a water content of 18%, debris flow-slit dam interaction phenomena such as run-up, dead zone, pipe-up, and backflow are observed. However, when the water content is increased to 30%, additional interaction processes such as overtopping, backwater effect, and bouncing (happens in the form of sediments splash down to the base of the flume) are observed. It is also noted that there is no formation of dead zones. Similarly, set-ups with water contents of 18% (Figs. 5) and 30% (Figs. 7) with relative post spacing of 3.1 are compared. The main interaction processes observed in Fig. 5 are very much similar to those in Fig. 4. The case in Fig. 7 however, shows a much more violent interaction process as debris flows impact the slit dam. Most of its content fly through the slit dam with only a small portion of the debris flow running up along the posts of slit dam and then falling back down to the base of the flume, causing a bouncing phenomenon (Fig. 7f).
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Figure 8 show the different interaction processes caused by different water contents that are varied from 18% to 38% and the relative post spacing is 1.8. Here it is observed, that overtopping or run-up are at their highest points. It is noted that when the water content of debris flow is 18% (Fig. 8a), no overtopping phenomenon is observed. However, when w≥26%, there is distinct overtopping. Moreover, the height of overtopping increases with the increasing water content.
4.2. Influence of relative post spacing on interaction process
Cases where the debris-flow water contents are both kept at 18%, while the b/dmax is increased from 1.4 (Figs. 4) to 3.1 (Figs. 5) are compared to isolate the influence of the relative post spacing. The interaction processes are almost identical, except that more debris passes through the slit dam when b/dmax is wider. Similarly, comparing Fig. 6 with Fig. 7, the water contents of debris flow are kept at 30% and the b/dmax also increases from 1.4 to 3.1. The differences in these two tests are obvious. In test w30-1.4, apparent run-up, overtopping, backwater effect, and bouncing after the debris flow falls down to the base of the flume are observed. In contrast, in test w30-3.1, due to the wider b/dmax, more debris pass through the slit dam in a jet flow manner, and no overtopping phenomenon is observed. The run-up and bouncing phenomena are still observed but are not as obvious as compared with those in test w30-1.4. The different interaction processes caused by different b/dmax are also shown in Fig. 9, where the water contents are kept at 30% and the b/dmax is increased from 1.4 to 3.1. In these cases, overtopping or jet flow are at their highest. When b/dmax≤2.3, overtopping after the debris flow makes impact on the slit dam dominates. Increasing b/dmax, decreases the height of the run-up (he). However, when b/dmax>2.3, majority of the granular material-water mixture pass through the slit dam as a jet flow and no obvious overtopping is observed.
Fig. 8 Snapshots of each test when the overtopping or run-up is at Fig. 9 Influence of the relative post spacing on the interaction its highest point. Relative post spacing is kept at 1.8 while the process between debris flow and slit dam. Water content is kept at water content is varied from 18% to 38% at an interval of 4%. 30% and the relative post spacing ranges from 1.4 to 3.6.
5. Influence of water content and relative post spacing on the height of run-up
Overtopping phenomenon occurs after the interaction between debris flows and slit dams. This potentially causes the foundation of dams to be eroded and result in structural instability (Pan et al., 2013). The degree of overtopping is determined by the height of run-up. Accordingly, it is imperative to ascertain the height of run-up after the interaction between debris flow and slit dam. In this study, the influence of water content and relative post spacing on the height of run-up is investigated. The height of the run-up (he) is normalized by the depth of approaching flow (h). This value is named the relative run-up height (he/h). Figure 10 shows the relationship between the relative run-up height (varying water contents) and the relative post spacing. When the relative post spacing is constant, the relative height of run-up increases with the water content. When w≥22%, the relative run-up height decreases with the increasing relative post spacing. Indeed, the relative run-
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up height reaches its maximum value (about 30) when w=38% and b/dmax=1.4. For debris flow with w=18%, the relative run-up height is obviously lower than those with w≥22%, and the influence of the relative post spacing on the relative run-up height is negligible. It is also noticed that the overtopping phenomenon is more likely to occur when w≥22% than when w=18%.
Fig. 10 The relationships between relative run-up height (he/h) and relative post spacing (b/dmax) for different water contents; he: height of run-up.
6. Interaction mechanisms between debris flows and slit dams
Based on the analysis of the interaction processes between debris flows and slit dams, it is evident that water content w and relative post spacing b/dmax are two key variables influencing the interaction mechanisms. Hürlimann et al. (2015) demonstrated, through laboratory experiments, that water content strongly influences the run-out distance of debris flows. In fact, water content essentially reflects the degree of liquefaction of debris flows. The degree of liquefaction, which is defined as the ratio of pore water pressure (Vw) to the total normal stress of debris flow (Vt), is used to represent the normalized influence of basal pore pressure on Coulomb resistance (Iverson et al., The total normal stress (ߪݐ) was estimated by the bulk density and approaching flow depth, that is ߪݐ ൎ .(2010 ߩ݄݃ܿݏߠ, where g is the gravitational acceleration; and θ is the inclination of the channel (Iverson et al., 2010). Both flume experiments (Iverson, 1997; 2010) and field observation (McArdell et al., 2007) suggested that the basal fluid water pressure (proportional to the degree of liquefaction) contributes to the mobility of debris flows. With lower degree of liquefaction, the grain-contact effective stress dominates. Force chains form much easier and the internal shearing of solid grains is enhanced. From the energy point of view, energy dissipation efficiency of the grain-contact effective stress is much higher than the viscous stress of the liquid phase. Accordingly, debris flows approach the slit dam with a lower velocity. This explains why, when w<26%, the debris flow with a lower velocity impacts on the slit dam, no distinct overtopping is observed, and the trapping capacity of slit dam is obviously influenced by b/dmax. On the contrary, with high degree of liquefaction, the effective stress of debris flows decreases, and debris flows are more fluid-like. Thus the basal resistance becomes minor, leading to higher mobility of debris flows. In addition, solid inertial force dominate during the movement, resulting in highly energetic debris flows. Accordingly, when w≥26%, debris flows with higher velocities impact the slit dam. When b/dmax is narrow, the granular-water mixtures can run up and overtop the slit dam. This further explains why when water content w≥26%, the influence of relative post spacing b/dmax on the trapping capacity is less obvious.
7. Conclusions
A set of flume experiments were carried out to study the interaction mechanisms between slit dams and debris flows. The key findings that can be drawn from this study are: (1) The relative post spacing and water content govern the interaction mechanisms of slit dams and debris flows. When water content w<22%, pile-up occurs and no distinct overtopping is observed, showing that the
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trapping capacity of the slit dam is obviously influenced by b/dmax. When w≥22% and the relative post spacing b/dmax≤2.3, run-up, overtopping, and backwater effects are apparent, and dead zones do not form. The trapping capacity of slit dam is obviously influenced by water content. Moreover, when w≥26% and b/dmax>2.3, majority of the granular material-water mixtures pass through the slit dam in the form of a jet flow and no obvious overtopping phenomenon is observed. (2) The run-up height reaches its maximum value (about 30) when w=38% and b/dmax=1.4. The relative run-up height is obviously lower for debris flows with w=18%, than those with w≥22%, and the influence of the relative post spacing on the relative height of run-up is negligible. (3) The degree of liquefaction (Vw/Vt) dominate the interaction mechanisms between debris flows and slit dams are when the relative post spacing is kept constant. With a lower degree of liquefaction, the grain-contact effective stress dominates. Force chains are much easier to be formed and the internal shearing of solid grains is enhanced. On the contrary, with a high degree of liquefaction, the effective stress of debris flows decreases. Thus the basal resistance becomes minor, leading to highly mobile of debris flow.
Acknowledgements
The authors acknowledge the financial support from the National Natural Science Foundation of China (grant No. 11672318), the Youth Innovation Promotion Association, CAS, and the Chinese Academy of Sciences (CAS) Pioneer Hundred Talents Program. The financial support from research grant T22-603/15-N provided by the Research Grants Council of the Government of Hong Kong SAR, China is also greatly appreciated.
References
Chanson, H., 2004, Sabo check dams‐mountain protection systems in Japan: Internal Journal of River Basin Management, v. 2, no. 4, p. 301-307, doi: 10.1080/15715124.2004.9635240. Chehata, D., Zenit, R., and Wassgren, C.R., 2003, Dense granular flow around an immersed cylinder: Physics of Fluids (1994-present) v. 15, no. 6, p. 1622-1631, doi: 10.1063/1.1571826. Choi, C.E., Ng, C.W.W., Song, D., Kwan, J.H.S., Shiu, H.Y.K., Ho, K.K.S., and Koo, R.C.H., 2014, Flume investigation of landslide debris– resisting baffles: Canadian Geotechnical Journal, v. 51, no. 5, p. 540-553, doi: 10.1139/cgj-2013-0115. Choi, C.E., Ng, C.W.W., Au-Yeung, S.C.H., and Goodwin, G., 2015, Froude characteristics of both dense granular and water flows in flume modelling: Landslides, v. 12, no. 6, p. 1197-1206. Choi, S.K., Lee, J.M., and Kwon, T.H., 2018, Effect of slit-type barrier on characteristics of water-dominant debris flows: small-scale physical modelling: Landslides, v. 12, no. 6, p. 111-122. Cui, P., Zeng, C., and Lei, Y., 2015, Experimental analysis on the impact force of viscous debris flow: Earth Surface Processes and Landforms, v. 40, no. 12, p. 1644-1655, doi: 10.1002/esp.3744. Han, W., and Ou, G., 2006, Efficiency of slit dam prevention against non-viscous debris flow: Wuhan University Journal of Natural Sciences, v. 11, no. 4, p. 865-869. Hauksson, S., Pagliardi, M., Barbolini, M., and Jóhannesson, T., 2007, Laboratory measurements of impact forces of supercritical granular flow against mast-like obstacles: Cold Regions Science and Technology, v. 49, no. 1, p. 54-63, doi: 10.1016/j.coldregions.2007.01.007. Hübl, J., Suda, J., and Proske, D., 2009, Debris flow impact estimation, in Proceedings, The 11th international symposium on water management and hydraulic engineering, Ohrid, Macedonia, September 2009, Volume WMHE2009, p. 137-148 . Hürlimann, M., McArdell, B.W., and Rickli, C., 2015, Field and laboratory analysis of the runout characteristics of hillslope debris flows in Switzerland: Geomorphology, v. 232, p. 20-32, 10.1016/j.geomorph.2014.11.030. Iverson, R.M., 1997, The physics of debris flows: Reviews of geophysics, v. 34, no. 3, p. 244–296, doi: 10.1029/97RG00426. Iverson, R.M., Logan, M., LaHusen, R.G., and Berti, M., 2010 The perfect debris flow? Aggregated results from 28 large‐scale experiments: Journal of Geophysical Research: Earth Surface, v. 115, no. F03005, doi: 10.1029/2009JF001514. Johnson, P.A., and McCuen, R.H., 1989, Slit dam design for debris flow mitigation: Journal of Hydraulic Engineering, v. 115, no. 9, p. 1293- 1296, doi: 10.1061/(ASCE)0733-9429(1989)115:9(1293). Lien, H.P., and Tsai, F.W., 2000, Debris flow control by using slit dams: International Journal of Sediment Research, v. 15, no. 4, p. 391-409. Lien, H.P., 2003, Design of slit dams for controlling stony debris flows: International Journal of Sediment Research, v. 18, no. 1, p. 74-87. Lin, Y.Y., and Jiang, Y.Z., 1988, Experimental study on the effectiveness of slit dam on debris flow: Journal of Chinese soil and water conservation, v. 19, no. 1, p. 40-57 (in Chinese). Mizuyama, T., Suzuki, H., Oikaka, Y., and Morita, A., 1988, Experimental study on permeable sabo dam: Journal of Japan erosion control engineering society. v. 41, no. 2, p. 21-25 (in Japanese). MLR (Ministry of Land and Resources), 2004, Design standards for debris flow hazard mitigation measures (DZ/T0239-2004), Beijing, China: Chinese Geological Survey, Ministry of Land and Resources (in Chinese). McArdell, B.W., Bartelt, P., and Kowalski, J.,2007, Field observations of basal forces and fluid pore pressure in a debris flow: Geophysical Research Letters, v. 34, no. 7, doi: 10.1029/2006GL029183. Ng, C.W.W., Choi, C.E., Song, D., Kwan, J.H.S., Koo, R.C.H., Shiu, H.Y.K., and Ho, K.K.S., 2015, Physical modelling of baffles influence on landslide debris mobility: Landslides, v. 12, no. 1, p. 1-18. Pan, H., Wang, R., Huang, J.,2013, Study on the ultimate depth of scour pit downstream of debris flow sabo dam based on the energy method: Engineering Geology, v. 160 p. 103-109, doi: 10.1016/j.enggeo.2013.03.026. Shima, J., Moriyama, H., Kokuryo, H., Ishikawa, N., and Mizuyama, T., 2016, Prevention and Mitigation of Debris Flow Hazards by Using Steel Open-Type Sabo Dams: International Journal of Erosion Control Engineering, v. 9, no. 3, p. 135-144, doi: 10.13101/ijece.9.135.
972 7th International Conference on Debris-Flow Hazards Mitigation Empirical model for assessing dynamic susceptibility of post-earthquake debris flows
Kaiheng Hua,b*, Zhang Wanga,b,c, Cheng Chena,b,c, Xiuzhen Lia,b
aKey Laboratory of Mountain Hazards and Earth Surface Processes, Chinese Academy of Sciences, Chengdu, Sichuan 610041,China bInstitute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, Sichuan 610041, China cUniversity of Chinese Academy of Sciences, Beijing 100049, China
Abstract:
Debris-flow susceptibility is controlled not only by static effective factors such as topography and lithology, but also by dynamic effective factors such as earthquake, rainfall and human activity. In this paper, a simple model of calculating the dynamic susceptibility is developed based on the assumption of linear relationship between the static and dynamic susceptibilities. The influence of earthquake and rainfall events is represent by two coefficients. The earthquake coefficient is considered as an exponential function of intensity, and a negative power function of elapsed time. The rainfall coefficient is proportional to the occurrence days of heavy rainfall. This model is applied to assess the debris-flow susceptibility of Hengduan mountainous area from 2000 to 2015. Four static effective factors including relative relief, slope, lithology and fault density are used to calculate the static susceptibility by ARCGIS grid toolbox. There are six earthquake events since 1995 whose intensity zones of >= VI are intersected with the Hengduan area. The earthquake coefficient is calculated with the intensity zoning data of each of the six events and then is accumulated to get the final earthquake coefficient in each year. TRMM satellite rainfall data from 2000 to 2015 are collected to extract the occurrence days of heavy rainfall which is used to calculate the rainfall coefficient. The dynamic susceptibilities from 2000 to 2015 are obtained by multiplying the static susceptibility with the earthquake and rainfall coefficients in respective year. The 2015 susceptibility map shows a qualitative agreement with the distribution map of disasters in 2015.
Keywords: Debris flow; Susceptibility;earthquake;rainfall;dynamic assessment;
1. Introduction
Risk assessment is one of the most important non-engineering countermeasures against debris-flow hazards. The risk assessment models have evolved from qualitative evaluation into quantitative evaluation, and from geological statistical methods into physical models and numerical simulation methods since the 19th century (Wei et al., 2003). There are three different levels of risk assessment: susceptibility, hazard and risk assessment (Hu et al., 2013). The susceptibility can be considered as a kind of occurrence probability of hazards at a place. Most of susceptibility assessing methods relate it to effective environmental factors such as rainfall, slope, slope direction, lithology,etc. by geological and statistical models (Carrara, 1991; Jade,1993; Dai, 2002; Carrara, 2008; Tang, 1994; Wei, 2000; Liu , 2004; Hu, 2012; Cheng, 2015). Debris-flow susceptibility is controlled not only by static effective factors such as topography and lithology, but also by dynamic effective factors such as earthquake, rainfall and human activity. So the susceptibility varies with dynamic factors. For example, the magnitude and frequency of debris-flow events increased largely in Longmeng mountain after the 2008 Wenchuan earthquake (Cui et al, 2011). However, the present susceptibility assessing models are based on static effective factors such as slope, relative elevation, lithology etc., and cannot reflect such a variation. Due to lack of awareness of such susceptibility changes, reconstructions such as residential buildings and tourist facilities were built in the areas with highly increasing susceptibility, and suffered a great deal of damages. In order to assessing varying susceptibility resulted from earthquakes and rainfalls, a simple model is developed based on the assumption of linear relationship between the static and dynamic susceptibilities. The influence of earthquake and rainfall events represent by two coefficients. The
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earthquake coefficient is considered as an exponential function of intensity, and a negative power function of elapsed time. Hengduan mountainous area in the southwestern China is chosen as the study case (Fig 1).
2. Dynamic model and method
2.1. Dynamic model
The susceptibility is determined by two factors: static and dynamic. Static factors are determined by background conditions, such as relative relief, slope, lithology, fault density, etc., which vary very little over hundreds of years and can be considered as unchanged. Other factors such as earthquakes, rainfalls and human activities can result in the rapid change of water and soil supplies and are considered as dynamic factors (Fig 2).
Fig.2 Static and dynamic effective factors with geo-hazard susceptibility
Fig.1 The location map of Hengduan mountainous area
Due to the complexity of human activities, we only consider the effects of earthquake and rainfall. It is assumed that the static factors, the effects of earthquake and rainfall are independent statistically. The time unit of assessing is simplified to one year, and the annual susceptibility is assumed to be proportional to the static susceptibility under the influence of earthquakes and rainfalls. Then the dynamic susceptibility can be expressed as the product of static susceptibility, earthquake and rainfall influence coefficients:
(ሺݐሻ ൈܴሺݐሻ (1ܧሺݐሻ ൌܵൈܪ where, H is the annual susceptibility of debris flows; S is the susceptibility caused by static factors (i.e. background value of susceptibility); Ek and Rk are annual influence coefficients of earthquake and rainfall.
2.2. Static evaluation method
The static susceptibility assessment is evaluated by factors that remain unchanged or change slowlyˈsuch as topography, geology, vegetation, soil and other underlying surface factors. The following conditional probability model is adopted: ୬ (ൌσ୧ୀଵ ߱ܲሺݔሻ (2ܵ
where,߱ is the weight coefficient of the i factor, ܲሺݔሻ is the probability of debris-flow occurrence when the i factor is equal to x, i.e. the conditional probability of the i factor. Usually x takes some interval or some category. If a square grid cell is used as evaluation unit, then
ሺ௫ሻ (ሺݔሻ ൌ (3ܲ ேሺ௫ሻ
where, ܰሺݔሻis the total number of grid cells at which the value of the i factor = x, and ݊ሺݔሻ is the total number of grid cells where the value of the i factor = x and debris flows occurred. The weights of static factors
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are determined by statistical methods, such as Grey Relational Degree or Analytic Hierarchy Process (Li et al., 2010). Based on the debris-flow inventory data in Hengduan area, the principal component analysis method found that relative relief, slope, lithology and fault density had the closest relation with disasters (Wei et al., 2008). The relative relief is defined as the difference between the maximum and the minimum elevation within a certain radius. It represents the energy condition of soil and water migration and reflects the energy of local terrain. The slope is the height difference of unit length, reflecting the energy gradient. The lithology is divided into quaternary deposits, soft rock, intercalated soft and hard rock, and hard rock. The fault density is defined as the total length (km) of faults per unit area (km2). The conditional probability of the four factors is calculated respectively in ARCGIS according to Eq.(3) (Fig 3).
Fig.3 Occurrence probabilities of the hazard in different background factors in Hengduan mountainous region (a. relative elevation, b. slope, c. lithology, d. fault density)
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The Grey Relational Degree and GIS are used to determine the weight values of relative relief, slope, lithology and fault density in the susceptibility, respectively (Li et al., 2010): relative relief=0.2048ˈ slope=0.1969ˈlithology = 0.1836ˈfault density = 0.2196. The static susceptibility of the Hengduan area is obtained by summing the weighted conditional probability of each factor (Fig 4).
Fig.5 Decaying curve of effective coefficient of earthquake Ek for different seismic intensities according to Eq(4) Fig.4 Static susceptibility of the hazard in in Hengduan mountainous region
3. Seismic effect coefficient
Strong earthquakes often trigger a large number of landslides. Lots of loose material caused by seismic landslides, avalanches and rockfalls accumulates at hillslopes or upstream of catchments, providing abundant material for debris flow, which can easily occur eroded by rainstorms. Large earthquakes in mountainous area at home and abroad (such as the Great Kanto Earthquake in Japan, the Chi-Chi earthquake in Taiwan, the Chayu Earthquake in Tibet, the Wenchuan Earthquake in Sichuan, etc.) indicate that the activity of debris-flow disaster is obviously enhanced, namely the number, scale and frequency are increased after the earthquake (Nakamura, 2000; Lin, 2004; Lin, 2006; Cui, 2008; Bao, 2004). Relevant studies have shown that the number of landslide initiated by earthquake in intensity region of V and below is very small (Xin and Wang, 1999). The number of landslide below VI degree in Wenchuan Earthquake is only 0.18% of the total (Cui et al., 2011). Moreover, the numbers of the landslides show an exponentially decreasing trend with the distances to epicenters or seismogenic faults (Prestininzi and Romeo, 2000; Chen and Hu, 2017). The seismic intensity has a close positive relationship with the epicentral or fault distances. Therefore, it is assumed that the influence coefficient of earthquake is a power law function of seismic intensity. On the other hand, the debris flow triggered by earthquakes is a type of limited loose material supply (Bovis and Jakob, 1999). As time goes by, the loose materials are transported to the downstream channel or the deposition fan, or consolidate gradually and strengthen, which reduces their volume and erodibility. The activity of debris flows after the earthquake will decrease along with the reducing loose materials. As a result, the susceptibility decreases over time. According to the observation data of debris flows at Guxiang catchment after the Chayu Earthquake in Tibet in 1950, an empirical power function describes well the decay process of the debris-flow activity. The power function exponent is about -0.63 by linear regression method (Hu et al., 2011). Based on the above analysis, the susceptibility is equal to the static one if the seismic intensity is below VI, and the susceptibility doubles when the intensity is one level higher. Meanwhile, the susceptibility will decrease exponentially along with time. Therefore, it can be expressed by the following formula for a single earthquake event: ͳǤͲǡ ݔ ൏ ܧ ൌቊ (4) ͳǤͲ ʹ௫ିܰିǤଷǡݔ
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where, x is the intensity value (=6.0~11.0 from VI to XI), and N is the total number of years after the earthquake. Ek is set to 1.0 when x < VI, indicating that the earthquake has no effect on the susceptibility. According to Eq.4, the influence coefficients in seismic area of VII, VIII and IX approximate to 1.0 after about 30 years from the curve, where is still very high after 30 years of XI seismic area, about 4.8 (Fig 5). In order to analyze the impact of the earthquake on the debris-flow susceptibility, all of earthquake events whose the area of > VI intensity overlaps with the Hengduan region are collected since 1995 (Ms > 6.5) (Table 1). Then, according to the intensity range of each event, Eq.4 is used to calculate the total influence coefficient of seismic events from 1995 to 2015 in the Hengduan area (Hu et al., 2018).
Table 1. Historical earthquakes in Hengduan mountainous area since 1995
Time Latitude Longitude Maximum Date Depth(km) Magnitude Location (Beijing time) (Degree) (Degree) Intensity 1995.10.24 6:46:49 25.9 102.2 15 6.5 đ Wuding, Yunnan 1996.2.3 19:14:20 27.2 100.3 10 7.0 đ Ljiang, Yunnan 2000.1.15 7:31:02 25.5 101.1 30 6.5 Đ Yaoan, Yunnan 2008.5.12 14:28:01 31.0 103.4 14 8.0 ē Wenchuan, Sichuan 2013.4.20 8:02:46 30.3 103.0 13 7.0 đ Lushan, Sichuan 2014.8.3 16:30:10 27.1 103.3 12 6.5 đ Ludian, yunan
4. Rainfall event effect coefficient
Debris flows in Hengduan area is mainly caused by heavy rain and storm. In general, if topographic and geological conditions are same, then the more heavy rain events happen, the more disasters can occur. Therefore, the influence coefficient of rainfall can be represented by occurrence days of heavy rain:
ܴ ൌܾൈܫவଶହሺͷሻ where, I>25mm is days of heavy rain, b is the proportion coefficient. By using TRMM satellites data, we calculate occurrence days of heavy rain at each grid cell in the area from 2000 to 2015 (Table 2). The cell size is 0.25° by 0.25°. There are 84497.13 days of heavy rains with an average daily rainfall of more than 25 mm per year added up for all of cells in China. At the same time, the average annual number of disasters is 23980.6 according to the National Geological Disaster Bulletin from 2001 to 2015. Some of the disasters were related to earthquakes, and most of them were triggered by rainfalls. The ratio of the disaster number to the day•cell number is 0.284. That means a heavy rain event can cause about 0.284 disasters on average per year in China. The occurrence probability of disasters in Hengduan area should be higher than the national average. However, there is no historical data of disasters in the area. Therefore, the value of b is considered as 0.284.
Table 2. Occurrence days of heavy rainfall and storm from 2000 to 2015 in the Hengduan
Number of Day•Cell of rainfalls (> 25mm) Number of Day•Cell of rainfalls (> 25mm) Year Year In Hengduan In China In Hengduan In China 2000 5243 80913 2008 4975 88308 2001 6755 77337 2009 5344 78796 2002 5299 89154 2010 5198 94035 2003 5059 81621 2011 3737 72396 2004 5452 75345 2012 5501 95659 2005 4753 83898 2013 4733 89546 2006 4280 79684 2014 5505 85036 2007 5322 81314 2015 6123 95328
5. Dynamic evaluation result
Combining the static susceptibility, seismic influence coefficient and rainfall influence coefficient, the dynamic susceptibility is calculated by Eq.1 in Hengduan area from 2000 to 2015. The susceptibility is divided
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into four levels: very high, high, moderate and low (Fig 6). By comparing the distribution of disaster events with the susceptibility map in 2015, it is found that they are spatially consistent by and large (Fig 7). The disasters mainly occurred on the west side of Longmen mountain, the lower reaches of Jinsha river and the northwest of Yunnan where the susceptibility is relatively high.
Fig.6 Maps of the dynamic susceptibility in 2005 (a), 2008 (b), 2010 (c) and 2015 (d) in the Hengduan
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Fig.7 Maps of the hazard susceptibility and inventory in 2015 in the Hengduan
6. Conclusions
The susceptibility of debris-flow disaster is a dynamic process affected by earthquake, rainfall and human activities. The paper classifies the influencing factors of debris-flow susceptibility into static and dynamic factors, and analyzes the impact of earthquake intensity and its decay effect and heavy rain event on disasters. The method and model are used to calculate the seismic influence coefficient of debris-flow susceptibility and annual susceptibility from 2000 to 2015 in the Hengduan area. The results showed that the susceptibility in meizoseismal areas is still four times higher than that before the earthquake even though the quake happened 30 years ago. From 2000 to 2015, days of heavy rain in the Hengduan mountainous area varied greatly, and the maximum year was 1.81 times that of the minimum year. The actual distribution of disaster events is consistent with the results of susceptibility zoning in 2015. The dynamic assessment model and method of disaster susceptibility is acceptable qualitatively, but more disaster event data are necessary for verification and improvement. The lack of disaster event data, errors of rainfall, geology and other data may lead to deviation in statistical calculation. In addition, there is no widely accepted model for the quantitative relationship between earthquake and debris flows. The linear hypothesis of dynamic susceptibility and the decaying model of seismic influence coefficient need more tests with real monitoring and study data.
Acknowledgements
This work has been supported by the National Basic Research Program of China (973 Program) (Grant No. 2015CB452704), and the National Natural Science Foundation of China (Grant No. 41790434 and 41601011).
References
Bao, Y.J., 2004, Probability analysis and first-step predict of earthquake-induced landslides[Master thesis]: Institute of Geophysics, China Earthquake Administration. Bovis, M.J., and Jakob, M., 1999, The role of debris supply conditions in predicting debris flow activity: Earth Surf. Process. Landforms, v.24, p.1039-1054.
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Carrara, A., Cardinali, M., Detti R., Guzzetti, F., Pasqui, V., and Reichenbach, P., 1991, Gis techniques and statistical-models in evaluating landslide hazard: Earth Surface Processes and Landforms, v.16, p.427-445. Carrara, A., Crosta, G., and Frattini, P., 2008, Comparing models of debris-flow susceptibility in the alpine environment: Geomorphology, v.94, p.353-378. Chen, C., and Hu, K.H., 2017, Comparison of distribution of landslides triggered by Wenchuan, Lushan and Ludian earthquakes: Journal of Engineering Geology, v.25, p.806-814. Cui, P., He, S.M., Yao, L.K., Wang Z.Y. et al., 2011, Formation Mechanism and Risk Control in Wenchuan earthquake of Mountain Disasters: Science Press, Beijing. Cui, P., Chen, X.Q., Zhu, Y.Y. et al., 2011, The Wenchuan earthquake (May 12, 2008), Sichuan province, China, and resulting geohazards: Nat Hazards, v.56, p.19-36. Cui, P., Wei, F.Q., Chen, X.Q., and He, S.M., 2008, Geo-hazards in Wenchuan earthquake area and countermeasures for disaster reduction: Bulletin of the Chinese Academy of Sciences, v.23, p.317-323. Dai, F.C., and Lee, C.F., 2002, Landslide characteristics and, slope instability modeling using GIS, Lantau Island, Hong Kong: Geomorphology, v.42, p.213-228. Hu, K.H., Cui, P., Han, Y.S., and You, Y., 2012, Susceptibility mapping of landslides and debris flows in 2008 Wenchuan earthquake by using cluster analysis and maximum likelihood classification methods: Science of Soil and Water Conservation, v.10, p. 12-18. Hu, K.H., Cui, P., You, Y., and Chen, X.Q., 2011, Influence of debris supply on the activity of post-quake debris flows: The Chinese Journal of Geological Hazard and Control, v.22, p.1-6. Hu, K.H., and Ding, M.T., 2013, Discussion on framework of landslide and debris-flow risk assessments: The Chinese Journal of Geological Hazard and Control, v.24, p.26-30. Hu, K.H., Chen, C., Li, X.Z, and Li, P., 2018, Dynamic assessment of debris-flow susceptibility under the influence of earthquake and rainfall events: The Chinese Journal of Geological Hazard and Control, v.29, no.2, p.1-8. Jade, S., and Sarkar, S., 1993, Statistical-models for slope instability classification: Eng Geol, v.36, p.91-98. Li, X.Z., Wang, C.H., and Deng, H.Y., 2010, Application of grey relation analysis and distance discrimination analysis methods in discriminating potential landslides of Xiluodu Reservoir area: The Chinese Journal of Geological Hazard and Control, v.21, p.77-81. Lin, C.W., Shieh, C.L., Yuan, B.D., et al., 2004, Impact of Chi-Chi earthquake on the occurrence of landslides and debris flows: example from the Chenyulan River watershed, Nantou, Taiwan: Engineering Geology, v.71, p.49-61. Lin, C.W., Liu, S.H., Lee, S.Y., et al., 2006, Impacts on the Chi-Chi earthquake on subsequent rainfall-induced landslides in central Taiwan: Engineering Geology, v.86, p.87-101. Liu, X.L., Wang, Q.C., Kong, J.M., et al., 2004, Hazard assessment of debris flows and their developing trend along Dujiangyan-Wenchuan highway: Journal of Disaster Prevention and Mitigation Engineering, v.24, p.41-46. Nakamura, H., Tsuchiya, S., Inoue, K., et al., 2000, Sabo against Earthquakes: Kokon Shoin, Tokyo, Japan, p.190-220. Prestininzi, A., and Romeo, R., 2000, Earthquake-induced ground failures in Italy: Engineering Geology, v.58, p.3-4. Tang, C., and Liu, Q.Z., 1994, A study on hazardous intensity and risk regionalization in china: The Chinese Journal of Geological Hazard and Control, v.5, p.30 -36. Wei, F.Q., Gao, K., Hu, K., Li, Y., and Gardner, J. S., 2008, Relationships between debris flows and earth surface factors in Southwest China: Environmental Geology, v.55, p.619-627. Wei, F.Q., Hu, K.H., Lopez, J. L., et al., 2003, Method and its application of the momentum model for debris flow risk zoning: Chinese Science Bulletin, v.48, p.298-301. Wei, F.Q., Xie, H., and Zhong, D.L., 2000, Risk factors zoning of debris flow in Sichuan Province: Journal of Soil and Water Conservation, v.14, p.59-63. Xin, H.B., and Wang, Y.Q., 1999, Criteria for earthquake-induced landslide and avalanche: Chinese Journal of Geotechnical Engineering, v.21, no.5, p.591-594.
980 7th International Conference on Debris-Flow Hazards Mitigation From practical experience to national guidelines for debris-flow mitigation measures in Austria
Johannes Huebl a*, Georg Nagl a
aUniversity of Natural Resources and Life Sciences, Peter Jordan Straße 82, Vienna 1190, Austria
Abstract
In the second half of the 19th century a lot of torrential disasters occurred in the Alps causing substantial damage. These catastrophes may be traced back to non-sustainable land-use management with inadequate alpin pasture farming, leading to forests in a poor condition, large denuded areas and steep incised channels. Starting with an analysis of these disasters, political decisions were made to implement mitigations measures, including forestry, agricultural and structural measures. Additional financial regulations were established to facilitate mitigations measures even for poor rural communities. The technological background for the design of structural measures was founded in forestry, because of the experience with construction of log driving. Basic hydraulic models were used for designs, sometimes empirical values were added to consider bedload or the impact of point loads. The structures served to stabilize the channel bed, to deposit bedload upstream of settlements, to restrict sedimentation or inundation on the fan or to redirect the flow to areas of low interest. In the early years of torrent control in the Alps, structures were usually built as dry masonry walls, later cement mortar masonry walls and concrete gravity walls were favoured. New static concepts and the use of reinforced concrete formed the basis of a colloquium in Vienna (1973), where load models and static concepts were discussed. In practice, the multiplier of the static load of the Lichtenhahn-model was used within a broad range, leading to different structural design of check-dams. In the beginning of the 21th century, based on the design concept of EUROCODE, new technical guidelines for barrier design (ONR-Series 248xx) were developed. The load cases include flood and debris-flow and combination of events for all kind of torrential structures. The experience in the application of these guidelines is currently being evaluated and will lead to a new national standard.
Keywords: Mitigation measures; Debris-flow; Design Standards; Torrent control service; Austria;
1. Introduction
There is a long tradition of natural disaster mitigation in Alpine regions. In the 18th century, Joseph Walcher (1719- 1830), an Austrian Jesuit, physicist and mathematician, worked on the topic of hydraulic and glacier lakes. In his work ”Nachrichten von den Eisbergen in Tyrol” (Walcher, 1773), he investigated the Vernagt glacier, Gugler glacier and especially the Rofner glacier lake, which threatened the Ötz valley by repeated glacial lake outburst floods (Fig 1). These outburst floods had catastrophic consequences for the province of Tyrol in the years 1600, 1678, 1680 and at 1845. In the year 1788, an edict promulgated by Wenzel Graf von Sauer, incited working-groups to establish mitigation measures (Graf von Sauer, 1788). Franz Seraphin von Zallinger zum Thurn (1743-1828), priest and physicist, worked on this edict and addressed the issue of inundation in Tyrol in his work “Abhandlung von den Überschwemmungen in Tyrol” in the year 1779 (Zallinger zum Thurn, 1778;1779).
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* Corresponding author e-mail address: [email protected]
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Fig. 1. Glacier lakes illustration of (Walcher, 1773); (a) Rofner glacier lake; (b) Passyer lake
One of the first technical drawings can be found in this publication. Also in France, Jean Antoine Fabre (1748- 1834) was involved in the topic of mitigation measures in Alpine regions with his book “Essai sur la théorie des torrens et des rivéres” (Fabre, 1797). In the next decades, the topic gained interest, but most work remained theoretical (Aretin, 1808; Streffleur, 1852; Müller, 1857). It was the work of Josef Duile (1776-1863), that started practical implementation of mitigation measures (Duile, 1826). He was an engineer, primarily in the field of hydraulics and road construction with a keen interest in torrent control. Due to his leading role in this field, he was called to Switzerland and applied his knowledge there. He can be considered as father of the European torrent control (Fig 2). In this time, two technical domains characterized the progression of the torrent control: one part was hydraulic engineering and the other part was forest engineering.
Fig. 2. Technical illustration of mitigation measures; (Duile, 1826)
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Hydraulic engineering was mainly applied to the lower part of the torrent while forest engineering addressed the headwaters. The political question who should further be responsible for torrent control works led to a controversy between Arthur Freiherrn von Seckendorff (forest engineering) and Weber von Ebenhof (hydraulic engineering) (Länger, 2003). During this time, Seckendorff started to study the methods developed in France, especially the works of Demontzey, (1880).
Fig. 3. These drawing originate from a report of an eyewitness (forester) of a debris-flow near Faucon (Barcelonette, France) in (Demontzey, 1880). The longitudinal section shows the accumulation of coarse boulders at the debris-flow front and the right picture displays the debris-flow cross section during the passage of the front.
Owing to the major catastrophes in the years 1846 and 1856 along the rivers Loire and Rhone in France (Fig 3), increasing knowledge developed in this field and public support started and led to a legal foundation of mitigation works in the years 1860 and 1864, especially related to reforestation. The severe floods in Tyrol and Carinthia in the year 1882 initiated, as in France, a rethinking. Julius Graf Falkenhayn travelled to France in the year 1883 to learn of the methods used, and to implement them in Austria (Wang, 1901). The emperor of Austrian-Hungarian monarchy passed the torrent control act in 1884, stating that torrent control works have to be executed on a national level, with public funding and with a systematic approach. Seckendorff introduced the forest engineering background from France successfully. The government implemented a legal basis for financing and organizing the torrent control service to ensure further development. Educational courses were established at the University of Natural Resources and Life Sciences, Vienna (BOKU) for students and the staff of the new established service.
Fig. 4. Example of technical structure of (Wang, 1901)
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2. Empirical design approaches
The construction materials used at this period consisted of the locally available construction materials near the torrents, see Fig 4 (Wang, 1901). Wood and dry-stone masonry were used for mitigation measures. Progress was achieved through dissemination of experience and not by research because no research facilities existed at the time. Nonetheless, first innovative experimental structures were raised, for example: • Barrier with a self-cleaning function (dosing) was installed in the Fischbach, Tyrol (1924-1928) (Fig 5) • A cement mortar arch dam was built in the years 1951/52 in the Finsingbach, Tyrol, to study the failing conditions of this construction type. • Prefabricated construction of a barrier of reinforced concrete in the Winklergraben, Upper Austria (1929- 1931).
Fig. 5. Barrier with a self-cleaning function (dosing); “Strele-Sperre”, Fischbach, Längenfeld, Tyrol (IAN-Archive)
Due to the improvement of materials and mechanization on building sites in the second half of the 20th century, new methods were established in the torrent control service. Increasing usage of heavy machinery in the steep regions enhanced construction efficiency. Depending on the two main types of structures used, the arch dam and the gravity wall, different formulas were used for design. The focus of the gravity wall was external stability (safety against tilting, sliding and bearing capacity failure) and the dimensions of the bottom width of the structure. Arch dams reduce the construction volumes by using the arching effect for stability. The arch dam designed by Hampel (1960) in the Finsingbach set a trend for the dimension of arch dam with a simple formula based on the ring tension. The formula reads ( + ) (1) = ℎ 𝑢𝑢 ∙ 𝑟𝑟 ∙ 𝛾𝛾 𝑑𝑑 with the thickness of the dam d, the height h, flow depth𝜎𝜎𝑑𝑑 u, radius of the arch dam r, allowable pressure loading of the material and the density of water with 1 metric ton/m³ (Leys, 1968). The Austrian Centre for Forest Research (BFW) held an international colloquium on check-dams in Vienna in the 𝑑𝑑 year 1972. Lichtenhahn𝜎𝜎 presented at this𝛾𝛾 colloquium the often-used load model for design (Lichtenhahn, 1972). He proposed an empirical multiplier k to increase the hydrostatic pressure to include load of debris-flow pressure. In practice, this multiplier ranges significantly and led to different structural designs of check-dams. 𝑝𝑝𝑠𝑠𝑠𝑠
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= (2)
𝑝𝑝𝑠𝑠𝑠𝑠 𝑘𝑘 ∙ 𝜌𝜌 ∙ 𝑔𝑔 ∙ ℎ 3. Adaptation to EUROCODE concept
Due to this long tradition in torrent control works in the Austrian Alps, numerous different types of protection works were designed in different regions in Austria. The design of technical structures is now based on the EUROCODE concept, that specifies how structural design should be conducted within the European Union (EU). Based on this, the ONR Series 248xx was established, encompassing torrential processes, snow avalanches and rock fall. An interdisciplinary working group (ON-K-256) developed the new standards for the load models, design, construction and life cycle assessment of torrent control works (technical standard series ONR 24800). The following national documents have been developed:
• ONR 24800: 2009 02 15 (Austrian Standards, 2009), Protection works for torrent control - Terms, definitions and classification; Contains the terminology and classifications of torrent control including the terms concerning the design and function of torrential barriers. An important classification is the definition of functional barrier types. • ONR 24801: 2013 08 15 (Austrian Standards, 2013), Protection works for torrent control - Impacts on structures; stresses on torrential barriers result from water (hydrostatic, dynamic), earth and debris-flow impacts. In special cases effects from avalanches, falling rocks and earthquakes must also be considered. • ONR 24802: 2011 01 01 (Austrian Standards, 2011), Protection works for torrent control - Design of structures; for the design of torrential barriers, the Ultimate Limit States (ULS) and the Serviceability Limit States (SLS) must be considered. The concept gives specific design rules (e.g. stress combinations) for torrential barriers. • ONR 24803, 2008 02 01 (Austrian Standards, 2008), Protection works for torrent control - Operation, monitoring, maintenance; a fundamental requirement to guarantee a minimum safety level of the protection works is periodic monitoring of their condition and effectiveness. The monitoring concept, in the ONR 24803, is divided in two parts, the inspection and the measurement or intervention part.
By these technical standards, the “traditional” assessment and construction concepts for torrent control structures were adapted to the EUROCODE standards. The documents are based on and interact with EN 1990 (basic of structural design), EN 1992-1-1 (design of concrete structures), EN 1997-7 (geotechnical design) and the related documents for the Austrian national specifications. Torrential barriers with the functions energy-dissipating, dosing, filtering or deflecting (Hübl, 2018) are subject to extreme dynamic stress that presupposes the application of high safety standards for design, construction and maintenance. The Austrian Standard ONR 248xx provides a standardized model for the design of torrent barriers under debris-flow impact, which was has been developed from comparative calculation of common debris-flow models from engineering practice in torrent control and calibrated by impact measurements of debris-flow events. The proposed method should enable practitioners to properly design debris-flow countermeasures with the restriction that usually only little data are available. Naturally, simplifications and assumptions are necessary. Therefore, the process “debris-flow” and the interaction with the structure itself are separated. At the interface of the barrier and the debris-flow process the process parameters are transferred to impact parameters that act on a specified load area. The design basis is the load distribution and the force of the stress model (Fig 6).
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Fig. 6. Schematic illustration of the distinction between load and process model. The standard distinguishes between the process part (right section) the moment of impact (middle section) and the load model for the design (left section), where v is the velocity, h the flow depth, the density, Hk the barrier height, pst the static pressure, pdyn the dynamic pressure, pa the self-weight of the overtopping material and the impact force 𝜌𝜌 of a single element FE.
For the assessment of impact by channel processes on structures, a process model and a stress model are combined. The process model represents the behavior of a debris-flow process according to its physical properties. At the interface characteristic parameters of the debris-flow process (e.g. energy, density, flow height, flow velocity) are transferred to the impact model, which simulates the interaction of the process with the structure and comprises the representative stress (areal or single load) and the related load distribution. For the design of torrent barriers for engineering purposes, simplifications concerning the model parameters, the stress model and the load distribution are required. A revised version, based on the gained experience with the ONR-Series within the last years, will be published in 2019 as ÖNORM B4800.
4. Conclusion
The challenge in developing national guidelines was to connect the experience of the practitioners with scientific research results and to derive a state of the art, starting with terminology, definitions, construction rules and design procedures. One the one hand the guidelines should provide a standardized method for the design of torrent control structures, on the other hand a degree of freedom must be left to adjust the design according to the peculiarity of the torrent. In Fig.7 is shown a recently completed debris-flow barrier, designed according to the ONR National Guidelines.
Fig. 7. Recently completed debris-flow barrier with the sediment retention basin (Schallerbach, Tyrol), designed according to the ONR National Guidelines.
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References Aretin, G., 1808, Über Bergfälle und die Mittel, denselben vorzubeugen oder wenigstens ihre Schädlichkeit zu vermindern: Innsbruck. Austrian Standards, 2008, Protection works for torrent control - Operation, monitoring, maintenance, (ONR 24803: 2008 02 01) Austrian Standards, 2009, Protection works for torrent control - Terms and their definitions as well as classification, (ONR 24800: 2009 02 15) Austrian Standards, 2011, Protection works for torrent control - Design of structures, (ONR 24802: 2011 01 01) Austrian Standards, 2013, Protection works for torrent control - Static and dynamic actions on structures, (ONR 24801: 2013 08 15) Demontzey, P., 1880, Studien über die Arbeiten der Wiederbewaldung und Berasung der Gebirge. [translated by Seckendorff-Gudent, A.], Vienna: C. Gerold's sohn. Duile, J. 1826, Ueber Verbauung der Wildbäche in Gebirgs-Ländern, Zum Gebrauche für Bau- und Forstbeamte, Obrigkeiten, Seelsorger und Gemeinds-Vorstände: Innsbruck, Wagner, p. 179, http://doi.org/10.3931/e-rara-19671 Fabre, J.A.,1797, Essai sur la théorie des torrens et des rivières: chez Bidault, Libraire, An VI 1797, p. 284, http://dx.doi.org/10.3931/e-rara- 44675 Graf von Sauer, W., 1788, Gubernialverordnung Hampel, R., 1960, Bruchversuch an einer Bogensperre der Wildbachverbauung. In Österreichische Wasserwirtschaft (12 Heft 8/9), p. 187. Hübl, J., 2018, Conceptual Framework for Sediment Management in Torrents: Water, 10(12), 1718, p.13, https://doi.org/10.3390/w10121718 Länger, E., 2003, Der forsttechnische Dienst für Wildbach- und Lawinenverbauung in Österreich und seine Tätigkeit seit der Gründung im Jahre 1884. Die Grundlagen für die Aufgabenerfüllung der Dienststellen der Wildbach- und Lawinenverbauung im Bereich der heutigen Republik Österreich, sowie Die Grundlagen für die Aufgabenerfüllung der Dienststellen der Wildbach- und Lawinenverbauung im Bereich der heutigen Republik Österreich, sowiedie allgemeine Entwicklung der Tätigkeiten der Dienststellen, insbesondere im Bundesland Kärnten. [Ph.D. thesis]:Vienna, Universität für Bodenkultur Leys, E., 1968, Zum Bau von Gewölbesperren in der Wildbachverbauung: In J. Kar (Ed.): Oesterreichische Wasserwirtschaft. 20. Jahrgang. Wien, New York: Springer, p. 243–249. Lichtenhahn, C., 1972, Berechnung von Sperren in Beton und Eisenbeton: In Forstlichen Bundesversuchsanstalt in Wien (Ed.): Kolloquium über Wildbachsperren. 1 Volume. Wien: Österreichischer Agrarverlag, p. 91–118. Müller, F., 1857, Die Gebirgs-Bäche und ihre Verheerungen wie die Mittel zur Abwendung der Letzteren: Krüll'sche Universitäts-Buchhandlung, p. 44 Streffleur, V. R. v., 1852, Über die Natur und die Wirkung der Wildbäche: In Sitzungsberichte der Akademie der Wissenschaften mathematisch- naturwissenschaftliche Klasse 08 1852, p. 248–261. Walcher, J., 1773, Nachrichten von den Eisbergen in Tyrol: Kurzböck. Wang, F., 1901, Grundriss der Wildbachverbauung. Leipzig: S. Hirzel. Zallinger zum Thurn, F. S., 1778, De causis et remediis inundationum in Tyroli. Innsbruck. Zallinger zum Thurn, F. S., 1779, Abhandlung von den Ueberschwemmungen in Tyrol. Innsbruck.
987 7th International Conference on Debris-Flow Hazards Mitigation Flexible debris-flow nets for post-wildfire debris mitigation in the western United States
William F. Kanea*, Mallory A. Jonesb
a,bKANE GeoTech, Inc., 7400 Shoreline Drive, Suite 6, Stockton, California 95219 USA
Abstract
Wildfires are a continual threat in the western United States. Post-fire debris flows annually cause millions of dollars in damage and often result in loss of life. Rapid post-fire response is essential to prevent additional hazards in terms of debris-flow damages. Flexible systems utilizing high-strength steel ring nets have proven to be reliable and cost effective. These systems can be installed rapidly to minimize or eliminate the dangers caused by post-fire debris flows. Wildfires in the western United States generally occur during the dry season in late summer and fall. Although monsoonal storms can cause debris flows in the summer months, seasonal rain storm events occur in fall and winter, often resulting in devastating debris flows. In the Rocky Mountain states debris flows occur during the summer monsoon season. In both areas, storm cells can remain stationary over mountain peaks for hours dropping large amounts of rain in a very short time. The resulting runoff and erosion can cause damaging debris flows miles away from the rain event. Debris impacts differ significantly from rockfall impacts. The debris nets must withstand both a surge in pressure on impact, and a static load once the flow has dissipated. Tested and engineered, flexible nets can be rapidly deployed in strategic locations to lessen or eliminate the threat. Compared to large, rigid structures and debris basins, these nets are cost-effective, rapidly constructed, environmentally friendly, and approval by regulatory agencies can relatively quick. This study focuses on current mitigation and protection practices using flexible debris-flow nets as developed in Switzerland and used in the United States. Case studies of projects in Colorado, New Mexico, and California detailing site investigations, engineering, and construction of flexible debris-flow nets are described. Of special interest are the steps and protocols taken as a result of debris flows following the Thomas Fire in California.
Keywords: debris flow; ring nets; protection; wild fire
1. Introduction
Each year, across the globe debris flows cause substantial damage and loss of life. Torrential rains from typhoons and hurricanes, or post-wildfire rain events can trigger large masses of vegetation, soil, and rock to flow catastrophically from mountain valleys and canyons out into inhabited areas. To date, the majority developmental work on debris-flow nets has been conducted by the Swiss government in conjunction with the Swiss company Geobrugg, AG, Romanshorn, Switzerland. This paper describes design and projects using the Swiss/Geobrugg debris mitigation products. Other manufacturers also provide debris nets. These nets were not used in the projects described. The principle behind debris nets is to catch debris flows close to the source, usually in mountain canyons, stop the massive flow, and then, if desired, allow the material to be placed back in the channel to allow natural process to return to normal sediment transport conditions. Flexible debris nets have been installed in hundreds of locations around the world to protect people and infrastructure in a low-impact, environmentally sound way. The basic debris-flow protection system consists of a custom ring net engineered to resist the velocities and dynamic and static pressures unique to debris flows. Support ropes are installed into channel banks and transfer debris impact and pressure loads from ring nets to the ground. Excessive energy is absorbed by net braking elements in the support ropes. In addition, the rings in the system allow the passage of water and fine sediment beneath and through the net.
*Corresponding author e-mail address: [email protected]
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1.1 Development from rockfall barriers
European countries pioneered the development of rockfall protection barriers in the mid 20th Century. Rockfall barriers were the logical extension of snownets already installed in mountainous regions. Early snownets were composed of wire rope nets which held snowfall until spring melting thereby preventing avalanche formation. Post-thaw inspection of the nets showed them to have caught boulders which had fallen from above. This led to research and development of rockfall barriers on a full scale by Brugg Cable (now Geobrugg), Maccaferri, and other wire and wire rope manufacturers. The California Department of Transportation (Caltrans), an early American adopter of rockfall barriers, observed that the barriers were effective in stopping small debris flows. This led to increased interest and research in the use of flexible nets to stop debris events.
2. Research
2.1 Theory
Existing methods for determining debris-flow protection were meant for large watersheds and large-scale structures such as basins and check dams (Bradley, et al., 2005). Early research on debris nets, including the use of anti-submarine ring nets, was conducted by the California Department of Transportation (Caltrans) and the United States Geological Survey (USGS). They installed a flexible ring net at the base a small flume (De Natale, et al., 1996). Other researchers were also conducting research on flexible nets in Japan, Europe and other countries. Conventional debris-flow net design is based on field observations and full-scale testing in controlled situations (Muraishi and Sano, 1997). Other publications related to the design of debris-flow protection systems includes Mitzuyama, et al. (1992), Rickenmann (1999, 2001), and PWRI (1988).
2.2 Illgraben research
After catastrophic debris flows in Switzerland in 2005, the Swiss government partnered with Geobrugg to conduct a major research program to determine if the nets could be used as lightweight, low-cost, environmentally sound replacements for concrete check dams and debris basins. The goal was to develop a standardized approach to debris-flow mitigation using flexible high-strength steel ring nets (Wendeler, 2017) The main focus of the Swiss research was a full-scale test site on the Illgraben near Leuk, Switzerland. The Illgraben usually produces five to six debris-flow events per year. A fully instrumented debris net was installed, Fig 1. Geophones installed upstream signaled the instrumentation at the net site to begin collecting data. They also turned on floodlights for nighttime events. Instruments monitored flow height, net rope forces, flow velocity, and weight. The channel dimensions were well known so volume and density could be determined.
2.3 Mechanics of debris flow impacting flexible barrier
Geobrugg examined the forces involved in the impact of debris material into a flexible net. They found that flows occur in pulses or waves. The first pulse was stopped at the base of the net. Subsequent pulses flow up and over the previous pulses. Nets were designed using this information with the maximum forces at the base of the net. In addition, the flexing and deformation of the net and net ropes will also absorb the dynamic impact pressures. Once the debris material is stopped the net must then support the static load of the material.
It should be noted that debris flows tend to be sequential events so that after an initial dynamic impact, additional surges add only a quasi-static load to the net, instead of a fully dynamic load. In addition, the debris material already impacted and de-watered on the net serves to absorb some of the energy of the subsequent surges. The result is that much of the debris-flow material is not against the net, resulting in decreased energy absorption and height requirements, Fig 2.
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2.4 Design concept
As a result of its research, Geobrugg (2003) developed a methodology suitable for the design of its debris-flow net systems. Existing research results allow a peak discharge to be calculated and the flow velocity estimated. Once the mass and velocity are known, the design pressures can be determined. Finally, the design height is calculated.
2.4.1 DEBFLOW software Fig 1. Instrumented test net in the Illbraben, Leuk, Switzerland.
This methodology is incorporated into the software program, DEBFLOW, which determines the appropriate Geobrugg debris-flow system as a function of the characteristics of a given debris-flow basin and channel. The DEBFLOW program is based on the full scale testing in controlled situations at Illgraben, solutions of the rope equation, and finite element modeling.
Fig 2. Schematic showing successive impact pressures from a debris flow being applied to a net. The net and its anchorages must be designed to withstand dynamic and static (Rankine) pressures. Note that successive debris impacts after the first flow lose energy by having to go up the previous flow and also stop debris material back up in the channel.
Given input parameters such as debris material, channel dimensions, number of pulses, etc. DEBFLOW provides the user with recommended Geobrugg nets. The type of net specified depends on the width of the channel and the calculated dynamic and static pressures. There are two basic versions of the Geobrugg debris net systems. The VX net is intended for relatively narrow channels up to 40-ft (15-m) wide, Fig 3. The UX net is installed in wider channels up to 90-ft (25-m) wide and has posts to keep the top net support rope from sagging, Fig 4.
2.4.2 Debris-flow Volumes
In the United States, initial volumes can be estimated following debris flows using WERT and BAER Reports. However, these estimated total debris-flow volumes will frequently exceed the one-event capacity of the available flexible net designs. Therefore, for design purposes, nets can be assumed to fill completely. Debris-flow volume storage area can be based on field observations, previous flow volumes, and measurements of channel geometry (Gartner, et al., 2008, and others,). For DEBFLOW analyses, the calculated volume of sediment Fig 3. Post-fire VX net installed above running stream in Nambé detained by each net is based primarily on a uniform Pueblo, New Mexico. Note basal opening allowing water and fish passage beneath. Animals can pass either underneath or through rings.
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geometry of each net and channel gradient. This assumes the storage area is a trapezoidal prism extending upstream from the net. This volume estimate does not take into account changes in channel shape upstream from each net location. However, sites are chosen to maximize storage area, so the volume estimates should be considered minimum values of sediment retained. Optimum locations are where channel geometry is constricted and upstream geometry widens to provide maximum storage capacity.
3. Design and Construction
In order to produce installation plans for the nets, it is necessary to consider strength of the anchoring rock and, if Fig 4. UX net with posts for wider channels Camarillo, California. required, the design of foundations for the posts. Design loads are supplied to the engineer by the manufacturer as a result of their testing and modeling. Rock and soil properties are determined during the field investigation at each installation site. Flexible debris nets can be constructed rapidly with minimal environmental impact and can be combined with the existing debris basins to maximize material storage in the canyons. They have a small construction footprint and do not change channel flow unless a debris-flow event occurs.
3.1 Anchor Design and Testing
Anchor design for UX and VX nets consists of determining the depth required to support the loads on the wire ropes. Previous work by the Post Tension Institute (PTI) (2014) gives a methodology for anchor design that is used for soil walls, tie-back walls, slope post-tensioning, slope stabilization system design, and rockfall and debris net anchor design. The PTI provides design charts with a recommended shear, or bond, strength for a particular rock/grout combination as determined by the geologist. The data comes from thousands of actual installations. For example, from PTI tabulated data, a weathered and fractured sandstone will have a bond strength of 100-psi to 120-psi. The maximum test load, as provided by Geobrugg, for a debris net anchor is about 80,000-lbs. Using the PTI criteria and assuming a 4-in drill hole and minimum bond strength of 100-psi, the necessary depth to hold the anchor in the fractured sandstone is 10.6-ft. This is well within the capability of a small rock drill. Rather than using estimates of bond strength material type, it is preferable when possible to perform actual field pull- out tests on anchors to determine the site-specific bond-strength characteristics. Verification anchors are sacrificial anchors installed in typical sections of colluvium or rock. The anchors are drilled to various depths and tested. The load at pullout can then be back-calculated to determine the actual bond strength for the particular rock in the field. Tabulated data is often very conservative and time and money can be saved by performing verification tests prior to net installation.
3.2 Foundation Design
UX nets require the construction of post foundations. Early practice involved using a large block of reinforced concrete about 1-m x 1-m. These blocks were not engineered and consisted of reinforced threaded bars inserted in the concrete to anchor the post base plate. Although easy and inexpensive to construct, they were prone to large foundation displacements and cracking on impact. Subsequently, engineered shallow foundations were used. These foundations are designed using concrete and building codes for steel reinforcement. In general they are designed to use the passive pressure of the soil as the soil resistance. Since lateral loading on post foundations can exceed 80,000-lbs, foundation dimensions can become quite large and introduce additional complications during construction. Because of the high loads and large foundation blocks, cast-in-drilled-hole (CDIH) foundations are considered economical alternatives to large concrete blocks. These foundations resist the loads by deformation and changing soil
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resistance. The design approach was developed by Reese & O’Neill (1988). Developed for laterally loaded pile foundations, the method utilizes the finite difference method and p-y curves (Reese & O’Neill, 1988). These curves model the soil or rock as systems of springs which push back on the foundation as it deforms from the bending of the foundation. Design consists of comparing the maximum moment developed in foundation with foundation strength. Displacement of various depths on the foundation can be calculated. Controlling factors in design are foundation size and displacement. The approach involves significant computational effort. Generally, lateral foundation loads are too great to use shallow foundations.
4. Construction
Fig 5. Changes in risk in the town of Brienz, Switzerland before (A), after the catastrophic debris flows of 2005 (B), and after the installation of a system of Geobrugg debris nets in the Alpine drainages above the town (Geobrugg, 2017). Debris net construction initially consists of drilling anchor holes and grouting wire rope anchors. Drilling equipment varies but in general downhole hammers are used. Sometimes these are hand operated but can be equipment mounted. Specialized equipment that can negotiate narrow canyons like the Kaiser SL2 are versatile and can drill quickly. For UX nets and SLBs, foundations are constructed and posts erected. Because debris channels often contain loose sand and rock, foundation usually require an excavation. A reinforcing bar cage is fabricated and placed in the hole, concrete is then poured into the excavation. If the excavation is large, a sonotube or form is necessary. Backfill around the form must be compacted to perform as the material used in the design. Alternatively, a controlled density fill (CDF) or soilcrete can be used. Support ropes are installed between the anchors. Then the ring nets are hung and secured with shackles. If a backing mesh is used, it is installed at this time. Finally, the overtopping plates are installed to the top rope.
5. Risk Reduction
After the flooding of August 2005 in Switzerland, the Swiss government and Geobrugg worked to reduce the debris risk to residents living in high risk zones by using environmentally sound debris nets. Fig 5 shows the changes in risk in the town of Brienz, Switzerland along the Trachtbach River after two catastrophic debris flows in summer 2005.
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6. Case Studies
6.1 Glen Eyrie Conference Center, Colorado Springs, Colorado In the summer of 2012, the Waldo Canyon Fire destroyed 11-mi2 of mountainous land above the city. Debris flows following the fire created substantial damage. Glen Eyrie Conference Center at the base of Queen’s Canyon installed three flexible debris nets. To protect the structures, a VX net was installed in the channel near the Center with a UX net directly above it. These nets were meant to be cleaned out. Further up the Canyon, a UX net was installed to protect a small water supply dam. It was not intended for clean out. The lower UX net was a unique design. The channel was too wide to install a stock UX net. In addition, the Center required access to the back of the net. An engineered fill embankment was designed and constructed to allow the support ropes to pass through the embankment and be anchored directly in the bedrock, Fig 6. This design allowed the net to deform as intended since none of the actual debris net was embedded in the embankment.
6.2 Santa Clara Pueblo, New Mexico
The Jemez Mountains portion of the New Mexico lands of the Santa Clara Band of Pueblo Indians was devastated by the Las Conchas Fire in the summer of 2011. Initially nine sites were selected for flexible debris nets. Funding delays led to further degradation of the channels resulting in the construction of only five nets in the fall of 2014. Equipment to construct the CIDH foundations for the UX nets could not be obtained so the contractor excavated the foundations and formed the shafts using sonotube tubes. A mixture of colluvium and cement was used to backfill around the tubes, resulting in higher strength than the native material originally there. The following summer the nets experienced an impact. The resulting cost savings to the Pueblo from not having the material impact their road was significant, Fig 7.
6.3 Nambé Pueblo, New Mexico
The Pacheco Fire of 2011 burned Sangre de Cristo mountains above the Nambé Pueblo. Subsequent debris flows impacted the Nambé Rio and Nambé Reservoir. The Pueblo installed three debris nets along the water to prevent further degradation of the reservoir. The nets were designed with a relatively large basal opening to allow wildlife and small debris to pass beneath, Fig 3.
6.4 Camarillo Springs, California
The Springs Fire of May 2013 burned a large swath of the Santa Monica Mountains. A debris-flow event consisting of mostly ash occurred in October 2014. During cleanup, a second debris flow occurred in December 2014 doing significant damage to a number of homes in the community, Fig 8. Several types of debris mitigation structures were constructed including shallow landslide barriers, UX and VX debris nets and berms to conduct surface runoff into the channels. The short time frame in which to construct the mitigation led to the use of soilcrete as backfill around excavations. Instead of a sonotube, a corrugated metal pipe was used for the foundation form. This added stiffness also enabled the shafts to be shortened, which in turn, allowed the construction time to be reduced. Two days after construction was completed, the first major rainfall of the season occurred filling several of the nets, Fig 9. Fig 6. VX net (below) and UX net (above) with armored access abutment, Glen Eyrie Conference Center, Colorado Springs, Colorado.
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6.5 Montecito, California
In December and January, 2018, the Thomas Fire became the largest fire in California history. In early January 2018, torrential rains pounded the Santa Ynez Mountains above the coastal community of Montecito. The ensuing debris flows killed 23 people, destroyed 10% of the housing in the community, blocked the U.S. Highway 101 Freeway interrupting commerce, Fig 10. A preliminary risk assessment of the mountain canyons and the community was made to determine the need for debris-flow mitigation prior to the upcoming rainy season (BGC, 2018). BGC concluded that a large supply of fine-grained sediment, boulders, tree-trunks, and branches remain in the canyons and is readily available for future debris-flow events in the coming rainy season. They also pointed out that the existing sediment basins in Montecito are inadequate to catch and store the volume of debris likely to be mobilized during a debris-flow event similar to the January 9, 2018 event. BGC recommended that immediate mitigation action be taken and that an instrumentation and warning system be installed. They recommended that flexible debris nets be placed in the canyons to help protect against large-scale debris-flow events. Fig 7. Filled Geobrugg UX debris net in Santa Clara Pueblo, Seventy-one sites in the five canyons that drain into New Mexico. Montecito were identified as potential flexible debris net sites. Of those, 16 were to be constructed the first summer. However, delays caused by environmental permitting issues resulted in construction being scaled back to 12 debris nets. Another outcome of the environmental community concern was that no foundation construction could occur in the channels. This eliminated the consideration of UX nets. A “Super VX” net has been engineered to span the larger sites that conventional VX nets cannot span.
6.6 Conclusions
Flexible debris nets have been shown to be effective measures in the protection of people and property in post-fire debris events. Among the advantages are:
• Lightweight - easily deployed • Rapid construction • Economical • Significant risk reduction • Environmentally sound
With the size and frequency of wildfires in the American southwest increasing, the use of these nets has demonstrated that it is a proven technology for protecting people and infrastructure.
Fig 8. Aftermath of post-fire debris flow in Camarillo Springs, California.
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Fig 9. Filled VX debris net in Camarillo, California. Fig 10. Aerial view of debris-flow damage in Montecito, California, January 13, 2018.
References
BGC Engineering, 2018, Letter to Suzanne Elledge: “Montecito Debris-Flow Risk Management – Urgent Action Needed.” BGC Project No. 1890- 001, August 31, 2018. Bradley, J. B., Bahner, C. D., Richards, D. L., Bahner, C. D., 2005, “Debris Control Structures – Evaluation and Countermeasures.” Hydraulic Engineering Circular 9, 3rd Edition, Federal Highway Administration, FHWA-IF-04-016 HEC-9. Cannon, S. H., Gartner, J. E., Santi, P. M., and Dewolfe, V. G., 2008, Empirical models to predict the volumes of debris flows generated by recently burned basins in the western U.S.: Geomorphology, v. 96, p. 339-354. De Natale, J. S. et al. 1996, “Response of the Geobrugg Cable Net System to Debris Flow Loading.” Report, California Polytechnic State University, San Luis Obispo, California. Denk, M., Roth, A., Volkwein, A., Wartmann, S., & Wendeler, C., 2007, Field measurements and numerical modeling of flexible debris flow nets. Debris-Flow Hazards Mitig. Mech. Predict. Assess. Millpress, Rotterdam, 681-687. Denk, M., Gröner, E., Wendeler, C., 2017, “Zehn Jahre Erfahrung mit flexiblen Murgangbarrieren.” (“Ten years of experience with flexible debris barriers.”) Austrian Journal of Engineers and Architects, v. 162, p. 209-214. (In German). Geobrugg, 2003, “Design Concept, VX/UX Protection System Against Debris Flow.” Fatzer AG, Geobrugg Protections Systems, Switzerland. Geobrugg, 2013, “Product Manual for VX/UX Debris Flow Barriers.” Geobrugg North America, 22 Centro Algodones, Algodones, New Mexico. Geobrugg, 2017, “DEBFLOW for advanced users.” Design Academy HYDRO, August 29-31, 2017, Romanshorn, Switzerland. O’Neill, M.W., Reese, L.C., 1988, “Drilled Shafts: Construction and Design”. FHWA Publication No. HI-88-042. Post-Tensioning Institute (PTI), 2014, Recommendations for Prestressed Rock and Soil Anchors. Post-Tensioning Institute. PWRI ,1988, “Technical Standard for Measures Against Debris Flows (Draft).” Ministry of Construction, Japan. Rickenmann, D., 1999, “Empirical Relationships for Debris Flows.” Natural Hazards, 19(1), 47-77. Rickenmann, D., 2001, “Estimation of Debris Flow Impact on Flexible Wire Rope Barriers.” Birmensdorf, interner Bericht, unver ffentlicht.
995 7th International Conference on Debris-Flow Hazards Mitigation Application of an innovative, low-maintenance weir to protect against debris flows and floods in Ottone, Italy
Chiara Morstabilinia,*, Ilaria Boschinib, Federica Zambrinib, Giovanni Mendunib, Marco Luigi Deanac, Nadia Zorzia
aMaccaferri Innovation Center, Via Volta 13/a, 39100 Bolzano, Italy b Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy cOfficine Maccaferri S.p.A., via Kennedy 10, 40069 Zola Predosa (BO), Italy
Abstract
The need of a low-maintenance and easy-applicable apparatus against debris flow led Maccaferri Innovation Center and the Politecnico of Milan to a new hydraulic-based approach that was focused on the application of a special weir called Mini Skirt Check Dam (MSCD). After a three-years research on applicable equations, a construction site in Ottone (Italy) was identified to have been affected by destructive debris flow in the past years. The site is characterized by the presence of an underground pipe that collects the stream flow rate flowing under the village square. The purpose of this work is to design a MSCD, able to prevent a pressure driven flow in the underground pipe and to avoid the related risk for the inhabitant of the village. MSCD is a special weir, which consists in large wings to slow the flow and a ring net to block boulders and logs, as to become a sifting filter of the debris. As to design the best performing apparatus, materials and type of anchoring are crucial; for this reason, an analysis of the impact pressure was performed. The case study has considered several different aspects: hydrology, size of the material and its characteristics and previous events to have a complete analysis of what could happen in the next events. The result of this collaboration is the complete design of a MSCD, ready to be installed.
Keywords: debris flow; management of the hydraulic risk;
1. Introduction
Ottone is a town in the province of Piacenza, in the Emilia-Romagna region of northern Italy, with an important extension (98.96 km2) which is compared to a small resident population of only 495 inhabitants (ISTAT, 2017). The town is capital of the municipality and it is located at an elevation of 510 m above sea level, while the hamlets are located at higher altitudes and they are a major tourist destination especially in the summer. The village of Ottone is located in the Alta Val Trebbia, on the Ligurian Apennines. Some of the numerous hamlets that make up the territory are in the Val Boreca. The territory includes the Trebbia and Aveto basins, the main tributary of the Trebbia. The main course of the Trebbia river develops in SW-NE direction for 116 km, from the source on Mount Prelà on the Ligurian Apennines (1406 m), up to the outlet in the river Po near Piacenza. The portion of the basin close to Ottone drains approximately 207 km2. The municipal territory develops from a minimum elevation of 344 m to a maximum of 1667 m above sea level, and the average slope of the area is 53%. The inhabited nucleus of the Capital is set on an alluvial fan that has stabilized over time and overlooks the river bed of the Trebbia on the hydrographic right. The hydrology of Ottone town is characterized by the waters of Rio del Montone and Rio della Ghiossa flow underneath the old town (see Fig. 1), in particular they join below Piazza Vittoria and then continue flowing towards
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* Corresponding author e-mail address: [email protected]
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the river Trebbia, draining a total area of about 40 hectares. This peculiarity has caused several problems to the population and the purpose of this project is to permanently solve this problem with a lower maintenance weir.
1.1. Problems occurred in Ottone
The main risk for the municipality of Ottone is given by the hydraulic security of Piazza Vittoria. Originally, in the urban plan, the inhabited nucleus was placed outside the path of the two streams Rio della Ghiossa and Rio del Montone. Around the second half of the 1800’s, the two streams started to be covered because in the urban planning the area was intended as a market square. The covering of the last segments of the two streams was completed between 1873 and 2000. The sewer is in masonry with sections of 1.30 × 1.40 m or 2.00 × 1.50 m and with vaulted roof. The drainage disposal capacity, assessed with a maximum filling level of the sections equal to 2/3 of the available height and considering clear water, is 8 m3/s, equivalent to a multisecular flow rate. However, the flood events regarding the two streams are characterized by a strong presence of solid transport that can plug the sewer system. This scenario of occlusion made by massive solid transport has occurred more than once in recent history and the events of considerable importance are: • 19 September 1953 • 13-16 October 2000 • 13-14 September 2015 The most recent event had a particularly high intensity that led to the complete obstruction of the eastern channel of the sewer of Rio del Ghiossa and the current situation of inadequacy of the network that puts at risk the appurtenances of the square. In the period immediately following the September 2015 event, works were carried out to try to restore the flow of Rio del Ghiossa by bypassing the blocked section to canalize the waters up to Trebbia through the western channel. The image in Fig. 1 makes it possible to appreciate the situation following the first interventions for restoring the transport capacity: the black section is out of service due to the clogging caused by the debris carried by the flood, while the green section is the bypass.
Fig. 1 Representation of the channel system of Rio della Ghiossa and Rio del Montone below Piazza Vittoria after the first restoration: the black section is out of service due to the clogging made by the September 2015 event, while the green section is the bypass made to allow the flow of the waters of Rio della Ghiossa up to Trebbia, following the West channel.
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The slopes of the square exceeding 5% allow a good drainage capacity to the underlying sewer. The system is not specifically affected by a possible regurgitation of the Trebbia. To protect the inhabited area, there are four weirs installed upstream of the sewer entrance, at a distance of about 30 meters one to the other (Fig. 2). The structural reliability of these structures should be verified, together with the state of consistency and maintenance.
Fig. 2 Photos of the last two weirs.
1.2. How to reduce the risk
From the past events and from the geomorphological and hydraulic studies of the basin it was noticed that the biggest problem in the case of debris flow comes from Rio del Ghiossa. The measures to reduce the risk of debris flow on Piazza Vittoria are divided into two categories: structural and non-structural. For the non-structural part, the installation of a monitoring and alerting system designed by CAE S.p.A. in collaboration with the Politecnico di Milano, financed by the Emilia-Romagna region, is about to be completed. Thanks to the study of the precursors to the calamitous event, it is possible to alert the population in advance in order to follow predefined emergency procedures that increase security and reduce material damage. The system logic is based on the correlation between precipitation duration and intensity and the occurrence of debris flow (Guzzetti et al., 2007; Pizziolo et al., 2005). The structural interventions in the project can be distinguished in interventions for the changing layout of the square and mitigation measures for “debris-flow hazard”. After the September 2015 event, concrete barriers with a jersey profile were laid along the western side of the square in order to preserve the entrance to the shops and create a safe walkway for pedestrians who unfortunately had to be in the area at the time of the manifestation of the landslide event. This floodproofing intervention should be developed in the square renewal project. The hydraulic restoration works in the square were entrusted to a local engineering company, which defined a technological solution that provides the adjustment of the West channel and of the sewage system. The choice to refer to this setting was made on the basis of operational and economic evaluations. On the other side, the installation of an apparatus called Mini Skirt Check Dam upstream of the entrance of the sewer is planned. It is a slit weir with a central net that is raised with respect to the riverbed, see Fig. 3. This product allows the lowering of the debris discharge, reducing substantially the speed and the highest discharge. The advantage over the traditional weirs is that, with the addition of the net, larger debris and timber that are often the most destructive part of the debris flow can be retained more effectively. Moreover, the presence of the net guarantees the need for a wider slot, which entails a lower expenditure of materials and, in particular, a lower impact on the structure that can be designed with smaller anchorages and greater durability. This weir does not need maintenance at the end of each event and for this reason it is optimal in all contexts where the accessibility of the site is problematic and linked to weather conditions.
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Fig. 3 General hydraulic scheme of a MSCD (University of Trento, 2016). The wings could be made in gabions or concrete and are designed with a specific R=B/Bf for each Froude number and volume of debris. The net in the middle is designed with a specific hm/a parameter, where hm is the flow level in the upper part of the weir.
2. Hydraulic and hydrological modelling
In order to properly design the MSCD, a hydraulic model and hydrological study was implemented. While creating the models, the reference setting has been the “project scenario”, with the new configuration of the sewer that will be completed soon.
2.1. Hydraulic modelling
1D hydraulic modelling has been carried out applying the HEC-RAS model (US Department of Defense, Army Corps of Engineers, 2016), to a series of cross-section, with the aim of identifying the critical section, which is obviously the one with worst debris accumulation. The definition of the geometry has been simple to implement since 88% of the channel is artificial and was constructed using a precast concrete box of defined size. For the remaining part, the reference was to a topographic evaluation received from the technical service of the municipality and to an in situ evaluation carried out with a geologist. The technical service department of Ottone Municipality had also provided information regarding the elevation of the submerged channel in correspondence to manholes for inspections so that an accurate reconstruction of the submerged part was possible. Since no data was available regarding the elevation of the riverbed upstream of the covered part, an assumption of constant slope (5%) was made. Considering all the obtained data, the available sections were 41; due to the model structure and function, some of the sections need to be doubled, so that a total of 64 sections were introduced into the model. The junctions among the channels composing the sewer were solved using the momentum equation. The roughness factor has been kept constant along the whole transverse section, adopting 3 reference values for the main characteristics of sections (precast concrete box, natural open sky section, and open sky section with concrete).
The adopted values of discharge were taken by the previous work from Politecnico di Milano (Menduni, 2016), imposing as an input value that the whole Rio della Ghiossa contribution is channeled in the East segment, as planned in the restoration project. The selected upstream boundary condition was due to the hydraulic jump on the weir. From the output, it has been possible to compute the transport capacity, whose profile is shown in Fig. 4, and identify the critical section.
Critical section
Fig. 4 Transport capacity along the sewer channels based on Hec Ras.
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2.2. Hydrological study
Since no hydrometer and rain gauges are present in the basin, calibrating and predicting the peak of flow rate for each return period chosen in the analysis was not a simple task. The total surface of the hazardous basin is less than 0.5 km2, so it it was extremely inaccurate to use the measuring apparatus installed on Trebbia river to calibrate the model. A simulation based on a GIS tool was preferred to find the water discharge for each return period. A calibration was possible with the data from the previous study (Menduni, 2016), that was calibrated on the amount of rain near the basin and the amount of material accumulated in the village. The procedure followed consists in different steps: • GIS analysis to identify the basin and its feature • Hydraulic manipulation using the Peak Flow model (Rigon et al., 2011) to obtain hydrograph • Amplification of the obtained curve according to the previous studies (Menduni, 2016) • Calculation of the solid flow rate It was fundamental to perform a complete GIS hydrological analysis to identify the basin and some hydraulic characteristic supplied as input data to the Stage tool implementing Peak Flow model and obtain the complete shape of the liquid discharge in time (as it is fully explained in Rigon et al., 2011) an example of the GIS analysis could be found in Fig. 5.
Elevation [m]
Fig. 5 DTM applied on the reference basin and direction of the main flow carried out with QGis, each colour correspond to an elevation (in m) above the sea level.
Starting from the output of the GIS study, it was possible to perform a further step of analysis using the software Peak flow. The calibration took into account the peak of the flow rate found in the previous studies (Menduni, 2016), so all the curve was multiplied for a factor that gives the same peak. The resulting discharge was amplified to accomplish the values found in previous studies, as shown in Fig. 6.
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12 Return period 100 years 10
Q Q [m3/s] amplified liquid flow rate 8
6 liquid flow rate from model
4
2
0 t [hh:mm] 00:00 01:12 02:24 03:36 04:48
Fig. 6 Example of the amplification of the resulting discharge from the application of the Peak Flow model. For the return period of 100 years the factor of amplification is 4.12, similar factor has been used for the other return periods.
A series of four curves was obtained for four reference return periods. Starting from these curves, the Takahashi criterion (Takahashi, 1978; Lanzoni, 1993) was chosen to obtain the complete curve of the solid and liquid flow rate. An example of the application on the obtained curves is provided in Fig. 7. 16 Return period 100 years 14
12 Q [m3/s] amplified liquid flow rate 10 8 total flow rate 6 4 2
0 t [hh:mm] 00:00 01:12 02:24 03:36 04:48
Fig. 7 Example of the amplification of the resulting return period of 100 years event, using the Takahashi (1978) criterion.
Errore. L'autoriferimento non è valido per un segnalibro. shows c and n parameters defining the depth-duration- frequency curves for each return period that were fixed from the local administration and which play an important role in the Peak Flow model (Rigon et al., 2012); QL max is the maximum flow rate for each return period and was fixed by Menduni (2016). We call “scale parameter” the coefficient applied to adapt the discharge from the Peak Flow elaboration, to the Peak imposed by the previous studies. VL represents the total volume of water during the flood event, calculated as an integral of the amplified curve. Vtot represents the calculation of the total volume of both sediment and water that flows during the event and is calculated with the Takahashi criterion. Table 1 Final results of the hydrological study
Tr 20 yrs Tr 100 yrs Tr 200 yrs Tr 500 yrs c mm/h 35.99 70.68 77.81 87.2 n - 0.348 0.335 0.332 0.328
3 QL max m /s 7.55 10.12 11.2 12.64 Scale parameter - 3.88 4.12 4.14 4.21
3 3 VL 10 m 17 23 24 27 3 3 Vtot 10 m 23 32 33 37
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3. Design of MSCD
The characteristics of the basin and the presence of a sewer portion of the stream, forced the design procedure to install an apparatus like the MSCD, that could dilute the amount of water and sediment but also requiring less maintenance. A section for the installation that is near to the road and easy to access was chosen and at the same time far enough from the bridge to avoid problems related to the pressure in that section. This job site is a part of the whole works in the County to improve safety against debris flow that will occur in the future. This installation, in particular, will ensure a constant amount of flow rate in the channel and thus, avoiding the pressure inside the sewer. To avoid the sediment to overflow the hydraulic obstacle, all the volume of the debris moved by the 500 years return period flood must be contained in the chosen section in the space upstream, so a check on the available storage capacity was necessary while choosing the position. With reference to Fig. 3, the design parameters to be determined were: • a, how much the net is uplifted from the ground: this parameter could be found in the abacus of the design (Morstabilini & Deana, 2018) • R what is the ratio between B (the width of the undisturbed section) and Bf (the distance between the wings of the MSCD), see Fig. 3. The considered base section is 4.7 m wide, so two possible Bf: Bf = 3 m and Bf = 3.5 m were chosen leading to two design options: R = 1.57 and R = 1.34. one of the resulting graphics for these options is reported in Fig. 8. 12 mini skirt R=1.57; hm/a=0.6 10 mini skirt R=1.57; hm/a=0.7 mini skirt R=1.57; hm/a=0.8 8 ] mini skirt R=1.34; hm/a=0.8 - 6 mini skirt R=1.34; hm/a=0.9 4 Δz/hm Δz/hm [ 2 0 0 0.5 1 1.5 2 2.5 -2 Fr [-]
Fig. 8 Example of a resulting design abacus for R=1.57 and R=1.34 and different hm/a parameters
The ability of the MSCD to dilute the debris flow is determined by the ratio Δz/hm which is a non-dimensional parameter that expressed the maximum amount of material that is temporarily blocked by the apparatus. Considering that the maximum height of the weir should be 2 m, the options for the design are in Table 2 the chosen solution is the third, with final drawing of Fig. 9. Table 2 Final results of the hydraulic model
R a hm / a hm dz / hm dz dz + hm Fr U Q [-] [m] [-] [m] [-] [m] [m] [-] [m/s] [m3/s] 1.57 0.35 0.70 0.25 6.72 1.65 1.89 2.20 3.41 3.42 1.57 0.30 0.80 0.24 7.04 1.69 1.93 1.90 2.92 2.50 1.57 0.70 0.60 0.42 3.61 1.52 1.94 2.00 4.06 8.14 1.34 0.35 0.80 0.28 6.14 1.72 2.00 2.20 3.65 3.66 1.34 0.30 0.90 0.27 6.22 1.68 1.95 1.90 3.09 2.66
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Fig. 9 Final design of the MSCD: in order to be easy to apply, the particular asset of the wire ensure the hole to stay open during the event.
4. Conclusions and future development
The Ottone research was an interesting case study to apply the results of the research on MSCD. In addition, the creation of a work team that involved Maccaferri Innovation Center and the Politecnico of Milano was fundamental in finding the best design solution for this site. During 2019 data load cells will be installed in the clamps which will communicate the tensile strength on anchoring. Combing this information with data coming from the sensor already installed for the early warning system, it will be possible to find a correlation between the rain and the pressure on the barrier, enlarging the knowledge on the phenomena occurring in the area.
Acknowledgements
This research was possible thanks to a collaboration between Maccaferri Innovation Center and the Hydraulic department of the Università degli Studi di Trento, in the person of Prof. Aronne Armanini, as part of a research project to study innovative apparatus for the control of debris flow phenomenon.
References
Brunner, G. W., 2016, HEC-RAS: River Analysis System: Hydraulic Reference Manual (5.0 ed.): US Department of Defense, Army Corps of Engineers. Guzzetti, F., Peruccacci, S., Rossi, M., and Stark, C., 2007, Rainfall thresholds for the initiation of landslides in central and southern Europe: Meteorology and Atmospheric Physics, v. 98, p. 239–267, doi: 10.1007/s00703-007-0262-7. Rigon, R., d'Odorico, P. and Bertoldi, G., 2011, The geomorphology structure of the runoff peak: Hydrogeology and Earth System Science, doi:10.5194/hess-15-1853-2011. Takahashi, T., 1978, Mechanical characteristics of debris flow: Journal of the Hydraulics Division 104 HY8, 1153 - 1169. Arpa, 2005, Evaluation of the risk of landslides in Appennines in Emalia Romagna [translated from Valutazione del rischio da frana nell'Appennino Emiliano-Romagnolo]. https://ambiente.regione.emiliaromagna.it/it/geologia/temi/dissestoidrogeologico/pdf/soglie_pluviometriche.pdf/@@download/file/soglie_pluvio metriche.pdf Menduni, G., 2016, Analysis of the hydrogeologic risk in Ottone related to the events of 13-14 september 2015 and residual risk [translated from Analisi del rischio idrogeologico nel Capoluogo del Comune di Ottone con particolare riguardo agli effetti calamitosi del 13-14 settembre 2015, ai conseguenti scenari di intervento e alla gestione del rischio residuo], Milan. Morstabilini, C. and Deana, M. L., 2018, Debirs Flow: a new design approach: Geomechanics and Geodynamics of Rock Masses. European Rock Mechanics Symposium, vol 2. Lanzoni, S., 1993, Meccanica di miscugli solido-liquido in regime granulo inerziale. [ph. D thesis] Padova, University of Padova.
1003 7th International Conference on Debris-Flow Hazards Mitigation Laboratory tests of an innovative check dam Chiara Morstabilinia,*, Marco Luigi Deanab
aMaccaferri Innovation Center, Via Volta 13/a 39100 Bolzano, Italy bOfficine Maccaferri S.p.A., via Kennedy 10 40069 Zola Predosa (BO), Italy
Abstract
The flow of both solid and liquid particles, especially in mountain basins, is impulsive, not easily predictable and for this reason extremely dangerous. As to avoid the damages caused by this phenomenon, barriers are usually used. They are easy to install but are high maintenance and, in addition, they are aimed at blocking all the sediment, running the risk of under-designing the volume to be collected. Starting from this assumption, the need of a maintenance-friendly approach was investigated by Maccaferri Innovation Center with an innovative apparatus called Mini Skirt Check Dam (MSCD). This is a special dam which is made up of two parts: solid wings that have a vertical fissure in which a ring net is applied. The net is uplifted from the ground as to leave a part of the debris flowing and blocking only the top part of it. The research analyzed the design procedure of a standard weir and several laboratory tests were performed in cooperation with the University of Trento (Italy) that ended with the creation of a new approach against debris flow. The MSCD permits the cutting of the peak of the flow rate and, at the same time, blocking tree trunks and boulders. Laboratory tests conducted showed that the combination of weir and net slows down the debris and collects only the most dangerous part of the flow with a lower maintenance requirement and a good hydraulic performance.
Keywords: debris flow; on scale laboratory tests on weirs; hydraulic design procedure
1. Introduction
According to Takahashi (1991), debris flow is a phenomenon of transport of both liquid and solid particles that occurs in mountain areas characterized by a severe slope where the motion of the solid phase is driven by gravity. The shape of the sieve curve of the solid phase is usually various, from a few centimeters to diameters of over one meter and it is transported by a water and mud matrix (Armanini et al, 2005). One of the more relevant factors of the phenomenon is that boulders and big stones usually float over the debris. The triggering factors of debris could be related to the presence of solid material and a fixed range of slope, between 15°9’ and 23°5’, as indicated by Takahashi (1978). One other possible cause is the rain, generally the event that causes the debris comes after a continuous light rain that is sufficiently able to saturate the soil.
This phenomenon is not constant in time and space and, in addition, is extremely violent and impulsive; this last characteristic makes forecasting and design extremely difficult because the rheology of the flow and its motion has not been fully explained yet.
1.1. Solutions in use for debris flow and their problems
The solution normally applied for debris flows are barriers. The principle application of this product is to block all the material that is flowing up to a certain prefixed retain volume and to leave all the other material overpassing the barrier. The threshold for the determination of the typology of barrier is a function of the recharge basin and its shape (so by the total amount of the material that could potentially be moved) and by the return period of design (in this case the amount of the material must be estimated for the design return period). ______
* Corresponding author e-mail address: [email protected]
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As shown in Fig.1. there are two typical barrier applications: Fig. 1. (a) shows a typical barrier application on an open hill, this application is commonly used for both debris flow and landslides; Fig. 1. (b) shows the debris flow barrier application in a channel, in this case the barrier is a hydraulic structure that represents an obstacle for the material flow of the whole basin up to the install section.
b a
Fig. 1. (a) typical barrier for open hills; (b) typical barrier for channel (Courtesy of Maccaferri)
Based on existing knowledge of the phenomenon, the principle of blocking all the sediment that is flowing seemed to be the only solution against the violence and the destructive power of the flow. The cost of these types of nets is reduced because they are light and easy to install. However, the cost is extremely increased when taking the maintenance costs into account. As it is shown in Fig. 2. Barriers in general get clogged and the material in the upper section that has been successfully blocked, needs to be cleaned. In most cases, apart from the cost of the work and the related risk to this type of job the cleaning of the objects must also be considered as it is not possible in every site of installation due to the slope or the presence of unstable boulders.
Fig. 2. Barrier clogged by rocks installed in the Alps (Courtesy of Maccaferri)
2. New approach
Starting from the hydraulic approach normally in use for weirs, a new apparatus called Mini Skirt Check Dam (MSCD), designed for debris flow, was studied. The main idea, as shown in Fig. 3 is not to block all the sediment as done in regular barriers but to dilute the debris, to cut the maximum amount of discharge and to release it in a second
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part of the event. This approach is environmentally-friendly because the discharge is reduced and could be designed according to the return period of the reference event and, in addition, effectively reduces the damage of the debris, ensuring a constant flow rate during the event that could be acceptable for the basin.
1.2 a) debris flow b ) 1 MSCD
0.8
0.6 Q [m3/s] 0.4
0.2
0 0 0.5 1 1.5 time [h] c
Fig. 3. (a) Performance of MSCD in comparison to the natural event in term of flow rate in the section after the apparatus; (b) the picture shows the control volume to which all the balance of energy and flow rate are referred to. The balance could be written between two generic sections 1 and 2, taking into account the direction of the flow rate. The picture on the right shows the different concentration in flow depth and the α angle.
The main goal of the research was to study and identify a new approach, based on the performance of the apparatus, that could be suitable for both weirs and MSCD, applicable for rivers and small basins (<10 km2) with intense sediment transport.
The general procedure studied for the design started from the application of the Navier-Stokes set equation in the direction of the flow, 푥, for both liquid and solid phase. The application of this approach on two general sections, 1 and 2, allows a system of three equations to be written: the conservation of the solid mass, the conservation of the liquid mass and the momentum, as done in Eq.1.
(퐶휌푠퐵푈ℎ)1 = (퐶휌푠퐵푈ℎ)2 { ((1 − 퐶)휌푤퐵푈ℎ)1 = ((1 − 퐶)휌푤퐵푈ℎ)2 (1) 휕 퐴 ∫ (푝 + 휌 𝑔푧 + 휌 푢2/2) (푢푑퐴) = −𝑖 휌 𝑔푄 휕푥 퐴 푤 푤 퐸 푤
Where 퐶 is the concentration of solid material in the control volume, 휌푠 is the density of the solid phase, 퐵 is the width of the channel, 푈 is the medium velocity, ℎ is the flow depth, 휌푤 is the density of the water, 푝 is the pressure, 𝑔 is gravity, 푢 is the local velocity, 퐴 is area, 𝑖퐸 is the loss of energy, 푄 is the flow rate, 푥 is the coordinate of the flow (supposed mono directional) and 푧 is a vertical coordinate, defined in the opposite direction from the application of the vector 𝑔.
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tubulalight r
Fig. 4. General scheme of a MSCD: wings could be made by concrete or gabion, the nets, located downstream from the wall, is made by two layer of nets; the ring net for the structural function of transferring the force of the debris to the ropes and the anchoring, the double torsion net ensures a thin filter for the lower part of the sieve curve of the material.
The main goal of the project was to apply Eq. (1) on a new apparatus called Mini Skirt Check Dam. A general hydraulic scheme can be found in Fig. 4. The apparatus is composed of wings that are designed according to the hydraulic criteria used for weirs and a ring net (diameter 35 cm) with an optional double torsion net (Morstabilini et al., 2018). The advantage of the net is to block boulders and logs and at the same time to reduce the quantity of concrete needed for the wings. This reduction of material, if compared to a regular weir, allows for an easier installation procedure, so less time and cost. The presence of the concrete, that could be substituted by gabions, is necessary to force the accumulation of the material before the apparatus. The lower part of the net is void, this means that the net is uplifted from the ground, so that a certain amount of flow discharge is being left free to flow. The design of a parameter (with reference to Fig. 4) is a function of the regular flow rate of the channel, the maximum space available before the MSCD and the threshold of the return period in which the MSCD should be active.
3. Experiments on weirs
The simplification of this system was fully explained in Morstabilini et al. (2018) and resulted in a final design equation expressed in Eq. (2):
2 푈2 푈 (푈 −푈 )2 훥푧 + ℎ 푐표푠 훼 + 훼 푚 = ℎ 푐표푠 훽 + 훼 푓 + 푚 표′ (2) 푚 3 2푔 푓 푓 3 2푔 2푔 where: 훼3 is a hydraulic coefficient that should be calibrated on the characteristics of the flow, 푈𝑖 are velocities in each section and the other variables are expressed in Fig. 5. The coefficient is defined below and the quantity indicated with the capital letter is the average on the section volume and the lowercase letter is the punctual quantity:
ℎ ∫ 푐푢푑푦 훼 = ℎ (3) 3 퐶푈ℎ
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Fig. 5. Hydraulic scheme of a weir.
The application of Eq. (2) on weirs give the possibility of writing a design procedure focused on the 휟풛 parameter (see Fig. 5), the weir considered for the application is reported in Fig. 5. The hypothesis of the method is that inside the weir the condition of Fr=1 is reached and that the weir is completely impermeable, so no filtration motion is allowed. This procedure could be extremely effective in defining the capacity of the weir to temporary block the flowing material, as a function of the Froude number and was fully explained in Morstabilini et al. (2018). In the case of a normal weir, Eq. (2) is applied between sections m and f and needs to define two different 휶ퟑ coefficients because the local velocity 푼풊 is different in the two sections. The design parameter is represented by the R number, which is the ratio between B and Bf (see Fig. 5). In this case the design equation is:
2 2 2 2 훥푧 2/3 1/3 퐹푟푚 푅 𝑖 퐹푟푚 2 −2/3 𝑖 = 푅 퐹푟푚 푐표푠 훽푓 + [( 2 ) 훼3푓 − 훼3푚] − 푐표푠 훼 + [1 − (퐹푟푚푅) ] (4) ℎ푚 2 퐹푟푚 𝑖 2 3 𝑖
Eq. (4) was calibrated by a series of on scale experiments, conducted in the CUDAM laboratory of the Università degli Studi di Trento (Armanini et al., 2017). These experiments were conducted on a steady flow channel 0.3 m width for several categories of R parameters. Eq. (4) was simplified to be no-dimensional, as to have a reasonable comparison between the on-scale results and the real parameters on a full scale. In Fig. 6 and 7 the calibration for R=6.3 in two experiment settings is reported. In Fig. 6 the theoretical curve is compared to simulated debris flow with different d50 of the material. In Fig. 7 the theoretical curve is calibrated with different types of mixture by variating the concentration of the solid particles. Both the approaches of calibration show that there is a good correspondence between data and curve; moreover, it seems that the approach is independent to the sieve curve of the material.
h/d50=7-10 h/d50=10-13 h/d50=13-16
Fig. 6. Calibration of Eq. (4) for different sieve curve, as a function of d50 of the material and the flow depth.
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c=0.1-0.2
c=0.2-0.3
Fig. 7. Calibration of Eq. (4) for different concentrations of the mixture.
4. Experiments on MSCD
The application of Eq. (2) on MSCD required defining a new coefficient related to the occlusion of the mesh, Cf. This is a dimensionless coefficient that accounts for the degree of occlusion of the net, in case of total occlusion (safer hypothesis) Cf =1, otherwise Cf >1. In this case, the application of Eq. (2) could be expressed as:
2 2 2 훥푧⁄ 2 훥푧 푎퐶푐푣퐶푓 퐹푟푚 2 ℎ푚 𝑖 퐹푟푚 ℎ푚 𝑖 = 푐표푠 훽푓 − 푐표푠 훼 + [푅 ( ) − 훼3푚] + ( ) (5) ℎ ℎ 2 푎퐶 퐶 퐶 𝑖 2 1+훥푧⁄ 𝑖 푚 푚 푐푣 푐표 푓 ℎ푚
Where 퐶푐𝑖 are coefficients related to the reduction of the horizontal and vertical flow depth, normally considered constant and equal to 0.61. Eq. (5) was calibrated with a series of on scale experiments conducted in the CUDAM laboratory of the Università degli Studi di Trento (Armanini et al., 2017). These experiments were conducted on a steady flow channel 0.3 m width for several categories of R and hm/a parameters.
Fig. 8. Hydraulic scheme of a MSCD.
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hm/a=0.2-0.3 hm/a=0.3-0.4 hm/a=0.4-0.5 hm/a=0.5-0.6 hm/a=0.6-0.7
Fig. 9. Calibration of Eq. (5) for R=6.3, Cco = 1, Ccv = 0,61 and Cf = 1,1, different types of hm/a curves were investigated.
hm/a=0.45 hm/a=0.4-0.5 hm/a=0.55 hm/a=0.5-0.6 hm/a=0.65 hm/a=0.6-0.7 Normal weir
Fig. 10. Calibration of Eq. (5) for R=6.3, Cco = 1, Ccv = 0,61 and Cf = 1,4, different types of hm/a curves were investigated.
Fig. 9 shows the calibrating result for R=6.3 and for several values of hm/a parameters. In Fig. 9 this calibration involved a complete occluded mesh, with Cf=1.1. On the other hand, Fig. 10 represent the calibration for a partially occluded mesh, with Cf=1.4. The data shows a good correspondence between Eq. (5) and the on-scale experiments. It seems that this correspondence is better for Cf=1.1, which suggests that the design equation is more effective with a completely occluded mesh. This fact is related to the issue that, the partial occlusion of the mesh does not block all the material on the net but leaves some part of this material flowing through it. This suggests that another equation set should be studied in order to have a specific shape of the net for a filtering use and to lower the pressure of impact on anchoring. The calibrating process was successfully done and showed that, if Eq. (5) is applied for the design, Cf should be considered equal to 1 for safety.
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5. Conclusions and future development
A new approach for the characterization of the hydraulic apparatus has been defined by this research. This project investigated a new apparatus and its performances in diluting the discharge of a debris flow. Data used for the calibration of Eq. (3) suggests a no-correlation to the sieve curve of the material, which should be investigated by later experiments. Data used for the calibration of Eq. (5) suggests the need of investigating the filtering properties of the net to apply a more realistic coefficient that will express these properties and will be useful in reducing the pressure on the net.
The whole research investigated the innovative hydraulic design criteria for MSCD. The full-scale design of this apparatus requires further investigation of the impact pressure on a partially open obstacle. In addition, the filtering properties of the net should be correlated to the potential reduction of the pressure, as to have a more realistic design procedure for anchoring.
Acknowledgements
This research was possible thanks to a collaboration between Maccaferri Innovation Center and the Hydraulic department of the Università degli Studi di Trento, in the person of Prof. Aronne Armanini, as part of a research project to study innovative apparatus for the control of debris-flow phenomenon.
References
Armanini, A., Fraccarollo L., and Larcher M., 2005, Debris Flow: Encyclopedia of Hydrological Sciences, v. 4(12), p. 2173-2186. Takahashi, T., 1991, Debris Flow: International Association for Hydraulic Research. Takahashi, T., 1978, Mechanical characteristics of debris flow: Journal of the Hydraulics Division v. 104, no. HY8, p. 1153–1169. Morstabilini, C., and Deana, M. L., 2018, Debris Flow: a new design approach: Geomechanics and Geodynamics of Rock Masses. European Rock Mechanics Symposium, v. 2. Morstabilini, C., Ferraiolo, F., and Armanini, A., 2018, Mini Skirt Check Dam: an innovative apparatus against debris flow: IAHR, doi: 10.3850/978-981-11-2731-1_251-cd.
1011 7th International Conference on Debris-Flow Hazards Mitigation Numerical study of debris flows in presence of obstacles and retaining structures: A case study in the Italian Alps
a a,b b a Marina Pirulli *, Mario Manassero , Carmine Terrioti , Alessandro Leonardi , Giulia La Portaa
aPolitecnico di Torino, Corso Duca degli Abruzzi 24, Torino 10129, Italy bGeotechnical Engineering, Corso Duca degli Abruzzi 42, Torino 10129, Italy
Abstract
Debris flows are one of the most frequent mass movement processes and occur in all regions with steep relief and at least occasional rainfall. Their high flow velocity, impact forces, and long runout, combined with poor temporal predictability, cause debris flows to be one of the most hazardous landslide types. An essential aspect of debris-flow risk management is the design of mitigation measures, which reduce the existing risk to an accepted level of residual risk, by reducing the potential damage that the moving mass can produce in terms of loss of human life and destruction of structures and infrastructures. Among these mitigation measures, transverse retention structures are used to delimit storage basins and prevent dangerous debris flows from reaching high- consequence areas. Due to the enormous impact forces that debris flows can exert on obstacles in their path, a reasonable planning requires that dynamic stresses are taken into account during the structural designing process, regardless of the complete (solid body barrier) or partial (open barrier) retention function that the type of selected structure can exert on the flowing mass. Since the village of Cancia, close to Cortina d’Ampezzo (Italian Dolomites), is hit by destructive debris flows for a long time, a storage basin delimited by natural and gabion barriers was built in 2000. In 2009 a severe event caused the partial collapse of the gabions and the overflow of the flowing mass. The present paper analyses through numerical modelling the dynamics of the flow and the influence of an abandoned building, existing inside the storage basin, on the occurred event.
Keywords: Debris flow; Retention barriers; Numerical modelling; Risk mitigation
1. Introduction
Debris flows are fast-flowing mass movements composed of a mixture of water, mud and debris, discharging through steep and confined channels (Iverson, 1997). This natural process represents a widespread threat to villages and infrastructures in mountain areas. Therefore, countermeasures have to be adopted by the local governments to mitigate the risk related to these phenomena. In order to protect elements at risk and to reduce expected losses, different passive (e.g. land-use management, hazard delimitation) as well as active (e.g. structural measurement, protection forest) mitigation strategies are available (Holub and Fuchs, 2008). In particular active structural measures, such as retention basins, check dams and channelization are established in the management of mountain hazards. But, knowledge of debris-flow dynamics, impact forces and loads is needed to design these engineered structures strong enough to withstand the forces of the impacting mass. A contribution to this knowledge can to a certain extent be obtained through numerical modelling (e.g., Iverson and Denlinger 2001; Pitman and Le 2005; Pudasaini et al. 2005; Pirulli 2005), if measures and observations for the model validation and calibration are available. Data from real events would allow one to completely bypass any possible scale effects affecting laboratory-scale results, but available data are usually limited. ______
* Corresponding author e-mail address: [email protected]
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In the present paper, the numerical code RASH3D (Pirulli, 2005) has been used to back-analyse the debris-flow event that affected the village of Cancia (Italian Dolomites) in July 2009. Since destructive debris flows have long impacted this area, a storage basin delimited by a compacted soil embankment surrounded by gabions was built in 2000 as temporary mitigation structure. This barrier was partially destroyed by the July 18th, 2009 event. The collapse allowed the flowing mass to impact a house located downstream of the retention structure, where two people died. Numerical modelling carried out with RASH3D was aimed to investigate the dynamics of the flow and the influence on the event on an abandoned building located inside the storage basin.
2. Study area
The investigated area is the sector of the Boite river valley that is located at the foot of the Antelao Mountain, where the Cancia hamlet is (Fig. 1a). This hamlet is part of the Borca di Cadore municipality, which is located a few kilometers from Cortina d’Ampezzo (North-Eastern Italian Alps).
Fig. 1. (a) The Cancia study area (modified after Boreggio 2014); (b) the low deposition area where the partially destroyed storage basin is located. The red circle indicates the house impacted by the July 18th 2009 debris flow (image modified after Boreggio, 2014).
From a geological and structural point of view, the Antelao mountain range is between two important south-verging thrust faults: the Antelao line to the North and the Pieve di Cadore line to the South. The segmentation of the regional structure is further enhanced by movements along the faults and some existing paleotectonic fractures with NW-SE and NNW-SSW orientation. Cataclastic processes due to the above dislocations have originated the thick debris layer that characterizes the area (Turconi and Tuberga, 2010). From a geomorphological point of view, the western side of the Antelao mountains presents an extremely irregular profile, because of the existing geological structure but also due to the intense modifications that occurred during the Würm glaciation. The retreat of glaciers made numerous mountainsides unstable and prone to collapse (Turconi and Tuberga, 2010). Over the last few decades, the Antelao slope overhanging the Cancia hamlet has been object of periodic and intense instability phenomena in the form of debris flows. The source area of these events extends from 1005 m a.s.l. at the terminus to 3264 m a.s.l. (Antelao Mountain), comprising a drainage area of about 1.8 km2. Its main channel, namely Ravina di Cancia, has a length of about 2400m with a mean slope of about 20°. It originates at the feet of the Salvella Fork (2500 m a.s.l.) and ends at 1005m a.s.l in a storage basin (low deposition area), which was built in 2000 to protect the Cancia village. At the confluence with the Bus del Diau torrent (1335 m a.s.l.), the Ravina di Cancia pattern intersects a flat area (high deposition area) that was specifically built to divert and slow down flow events (Manassero et al., 2018). The debris flow occurred on July 18th, 2009 was triggered by heavy but not exceptional rainfall; nevertheless, it was the most catastrophic of the historically occurred events, causing the loss of two human lives. The event, which
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triggered from the upper part of the Ravina di Cancia channel and the Bus del Diau (Fig. 1a), mobilized about 30.000 m3 of material. This volume estimation is based on on-site surveys and comparison between pre- and post- event digital topography data and is defined as a function of the material eroded and deposited from the high deposition area up to the low deposition area and the Cancia hamlet (Fig. 1a). During its propagation, it impacted and caused the partial collapse of some gabion barriers located upstream (1013m a.s.l.) and at the end (1001m a.s.l.) of the above mentioned storage basin (Fig. 1b). The partial collapse of the 1001m a.s.l. gabions allowed the mass to continue its running downstream and impact against a house located along its main flow trajectory, where two people were killed by the flowing mass (Fig. 1b). The flowing mass entered the impacted house through windows and doors facing upstream and splashed up to the ceiling of the rooms. Immediately after, the mass fell through the floor into the downstairs garage and then exited the house from openings and pointed downstream. After the event it was observed that gabions were not joined together, thus preventing the structure to act as a monolithic mass against the flow. Furthermore, a large quantity of fine size aggregates filled the inner part of the gabions, thus preventing the water to flow through the wall and not minimizing the build-up of pressure behind. The collapsed gabions were found emptied by confirming the washing away of fines by the moving mass.
3. RASH3D model
The back analysis of the aforementioned event was carried out using the RASH3D numerical code (Pirulli, 2005; Pirulli et al., 2007). RASH3D is based on a single-phase continuum mechanics approach and on depth-averaged St. Venant equations. This implies that both the depth and length of analysed flowing masses are assumed large, if compared to the characteristic dimension of the particles involved in the movement. The real moving mixture of the solid and fluid phases can therefore be replaced with a homogeneous continuum, whose rheological properties are intended to approximate the bulk behaviour of the real mixture, and the motion can be described using a model that consists of the balances of mass and momentum (Pirulli and Marco, 2010). Furthermore, assuming that the vertical structure of the flow (i.e depth) is much smaller than its characteristic length allows one to integrate the balance equations in depth and to obtain the so-called depth-averaged continuum flow model (Savage and Hutter, 1989):
h hv yhv x 0 t x y 2 x hv hv yxhvv hσ (1) ρ x xx ρg h zx x t x y x 2 yhv yhv xyhvv yy hσ ρ ρg h zy y t x y y where 푣̅̅푥̅ , 푣̅̅푦̅ denote the depth-averaged flow velocities in the x and y directions (z is normal to the topography), h is the fluid depth, τzx, τzy the shear resistance stress (transverse shear stresses τxy are neglected), ̅̅̅푥푥̅̅ , ̅̅̅푦푦̅̅ the depth- averaged normal stress and gx, gy the projection of the gravity vector. Different rheologies exist to describe the basal shear that develops at the interface between the flowing material and the rough surface and are implemented in RASH3D. As far as the Voellmy rheology is concerned, Rickenmann and Koch (1997) and Revellino et al. (2004) showed that this model, originally developed for snow avalanches (Voellmy, 1955), offers a good simulation of velocities for debris flows and debris avalanches. According to these authors and our experience, the Voellmy rheology was selected for the numerical back analysis of the July 18th event. The Voellmy model combines a frictional term, which includes the friction coefficient μ (= tan , where is the friction angle) and a turbulent term, which includes the turbulence coefficient ξ:
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2 v g = gh+ i sgnv (where i=x,y) (2) z i
4. Dynamic analysis of the July 18th debris flow
The analysis of the July 18th debris flow is here presented. An attempt was made to investigate the influence of an abandoned building (Mi.No.Ter. in Fig.1b), located inside the storage basin, on the flow trajectory and dynamics. A set of numerical analyses in presence and absence of the Mi.No.Ter. building have been then carried out. Both a Digital Surface Model (DSM) and a Digital Terrain Model (DTM) generated from LiDAR, with a 1m grid spacing of the pre-event topography, were provided by the Ministry for the Environment and Protection of the Territory and the Sea. The starting position of the 30,000 m3 mass was the confluence of Ravina di Cancia with Bus del Diau torrent at 1335m a.s.l (Fig. 2). The Voellmy resistance model shown in Eq. (2) was used. The two Voellmy parameters were systematically adjusted until simulations approximately reproduced the observed distribution of deposits in the storage basin.
Fig. 2. RASH3D numerical simulation: starting position of the released mass on the topography.
The model provided a good match of the general extent and distribution of the storage basin deposit using a friction coefficient () equal to 0.1 and a turbulence coefficient () equal to 300 m/s2. A comparison between numerical simulations with the above calibrated parameters (Fig. 3b) and survey data collection (Fig. 3a) shows that the model reasonably simulates the event in terms of distribution of the final deposit.
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Fig. 3. Comparison between deposit depth as obtained processing pre- and post-event available digital topographic data (a) and RASH3D numerical results (b).
Regarding the flow velocity (Fig. 4), it is observed that the flow reaches high velocity (>10 m/s) along the channel and in the upper part of the retention basin. The velocity decreases downstream of the gabion barrier to 5-6m/s.
Fig. 4 RASH3D numerical results: maximum computed flow velocity.
To highlight the role of the Mi.No.Ter building (Fig.1b) on flow dynamics, we modeled the flow with (Fig. 5a) and without (Fig. 5b) the building in the sediment basin. The results show that without the Mi.No.Ter. building, the flow would still have overflowed the storage basin (Fig. 5b, Scenario B). Nevertheless, a smaller quantity of material would have escaped from the storage basin and affected the downstream area. In particular, a lower height flow would have impacted the house below the debris basin where two people died. Based on the height of the windows, a lower flow height would have reduced the amount of flow into the house (highlighted with red squares in the legend of Fig. 5a and 5b). In particular, numerical results indicate a decrease of the flow thickness from a maximum value of 2 m (Fig. 5a, Scenario A in the presence of Mi.No.Ter.) to a maximum value of 1.4 m (Fig. 5b, Scenario B in the absence of Mi.No.Ter.). Results are justified by the fact that with the building, a larger quantity of material overflow the gabion barrier. While, without the house, the storage capacity of the basin increases and a minor quantity of material overflow the gabion barrier.
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Fig. 5. RASH3D numerical simulation: deposit depth distribution with (a) and without (b) the Mi.No.Ter. building. Red arrow indicate the position of the mainly impacted house during the July 18th event.
5. Conclusions
The design of countermeasures and the hazard zoning in debris-flow prone basins need an estimation of the debris- flow magnitude and require the understanding of the flow dynamic characteristics. With this in mind, the continuum mechanics based numerical code RASH3D was used to investigate the role of a storage basin, built to protect the village of Cancia from regularly occurring debris flows, and of an abandoned building, located inside the above storage basin, on the dynamics and trajectory of the event here occurred on July 18th 2009. First, the calibration of the model parameters was made on the basis of the deposit shape and depth distribution surveyed on site. Simulations in presence and absence of the abandoned building were carried out and results compared. We found that removal of the building would not have prevented the mass from exiting the storage basin. But, without the abandoned building, the impact depth of the flowing mass would have decreased from about 2 m to 1.4 m. This would have reduced the flow of material into the house. Our results show that the RASH3D model can adequately simulate the complex flow dynamics of this event and, therefore, can be used quantify potential debris-flow impacts to infrastructure and inform the design of mitigation structures. Specific numerical analyses to investigate the partial collapse of the gabion barrier are foreseen in future.
References
Boreggio, M., 2014, Simulazione degli eventi di colata detritica avvenuti nei bacini del Rio Lazer (TN), Fiames (BL) e Cancìa (BL) mediante i codici di calcolo “D.F.R.M.” e “T.R.E.N.T.2D-df” [Master thesis]: Università degli Studi di Padova, 284 p. Denlinger, R.P., and Iverson, R.M., 2001, Flow of variably fluidized granular masses across three‐dimensional terrain: 2. Numerical predictions and experimental tests: J. Geophys. Res., no. 106(B1), p. 553–566. Holub, M., and Fuchs, S., 2008, Benefits of local structural protection to mitigate torrent-related hazards: WIT Transactions on Information and Communication Technologies, no. 39, p. 401–411. Iverson, R.M., 1997, The physics of debris flows: Rev. Geophys., no. 35, p. 245–296. Manassero, M., Pirulli, M., and Terrioti, C., 2018, The influence of obstacles in debris-flow dynamics: the case study of Cancia (Italian Dolomites), in Proceedings, Congresso Nacional de Geotecnia, 16th, Acores: Portugal, 27-30 May 2018, p. 1-9. Pitman, E., and Le, L., 2005, A two-fluid model for avalanche and debris flows: Royal Society of London Transactions Series A - Mathematical, Physical and Engineering Sciences, no. 363 (1832), p. 1573–1601. Pirulli, M., 2005, Numerical modelling of landslide runout, a continuum mechanics approach [Ph.D. thesis]: Italy, Politecnico di Torino, 204 p. Pirulli, M., Bristeau, M.O., Mangeney, A., and Scavia, C., 2007, The effect of the earth pressure coefficients on the runout of granular material: Environmental Modelling and Software, no. 22(10), p. 1437-1454. Pirulli, M., Marco, F., 2010, Description and numerical modelling of the October 2000 Nora debris flow, Nothwestern Italian Alps: Canadian Geotechnical Journal, no. 47, p. 135-146. Pudasaini, S., Wang, Y., and Hutter, K., 2005, Modelling debris flows down general channels: Natural Hazards and Earth System Sciences, no. 5(6), p. 799–819. Revellino, P., Hungr, O., Guadagno, F.M., and Evans, S.G., 2004, Velocity and runout simulation of destructive debris flows and debris avalanches in pyroclastic deposits, Campania region, Italy: Environmental Geology, no. 45, p. 295-311.
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Rickenmann, D., and Koch, T., 1997, Comparison of debris flow modelling approaches, in Proceedings, Int. Conf. on Debris Flow Hazards Mitigation: Mechanics, Prediction, and Assessment, 1st, Chen, C.L., ASCE, Reston, Va., p. 576-585. Savage, S.B., and Hutter, K., 1989, The motion of a finite mass of granular material down a rough incline: Journal of Fluid Mechanics, no. 199, p. 177-215. Turconi, L., and Tuberga, S., 2010, Relazione tecnica su incarico per consulenza evento 18.07.2009, Cancia (Borca di Cadore). Report, 90 p. Voellmy, A., 1955, Uber die Zerstorungskraft von Lawinen, Schweiz: Bauzeitung, no. 73, p. 212-285.
1018 7th International Conference on Debris-Flow Hazards Mitigation Design of a debris retention basin enabling sediment continuity for small events: the Combe de Lancey case study (France)
Guillaume Pitona,*, Vincent Manob, Didier Richarda, Guillaume Evina, Dominique Laiglea, Jean-Marc Tacneta, Pierre-Alain Riellandb
a Univ. Grenoble Alpes, IRSTEA, ETNA, Grenoble, 38402, France bARTELIA Eaux Et Environnement, Echirolles, 38130, France
Abstract
The Combe de Lancey stream is a relatively calm tributary of the Isère River flowing through the city of Villard-Bonnot near Grenoble (France). In 2005, a long-lasting extreme rainfall event triggered dramatic erosion processes in this 18 km² granitic catchment. A volume of 20,000 m3 of sediment and logs deposited in the paper factory located near the fan apex. This paper focuses on the definition of a new protection system, namely a debris retention structure made of an excavated basin and an open check dam. The design is based on expert knowledge and tested byphysical small scale modelling. The particularity of this case study relies on two points: (i) its design scenarios and (ii) the structure capacity to transfer small events. Attention was paid to define several 100-year return period events used as “design events” for which the structure must have its best effectiveness. Extreme events with higher return periods called “safety check events”, for which the structure must still withstand the event without failure but with acceptable marginal damages, were also modeled. The definition of the scenarios is described in the paper. Secondly, the debris retention basin should have the capacity to transfer small debris floods without trapping sediment in order to prevent downstream incision and heavy maintenance costs. It must however trap nearly totally the sediment and large woods that are erratically supplied by the catchment during extreme events. Classical debris retention basins are usually not able to achieve such a dual objective. Here, two concepts developed in past works, namely a guiding channel and a hybrid open check dam with mechanical – hydraulic control were successfully tested. The paper presents the design and testing procedure of this case study exemplifying the next generation of debris retention structures.
Keywords: debris flood, hazard scenario, open check dam, guiding channel
1. Introduction
Steep mountain streams erratically experience debris-flows and debris-flood events releasing massive amounts of sediment and large woods on fans. The municipality of Villard-Bonnot is partially built on the Ruisseau de la Combe de Lancey alluvial fan and aims at removing here a closed paper factory to create new residential areas. Before doing so, the French state requested a torrential hazard mitigation plan to protect the whole fan. Past studies demonstrated that a debris retention basin was part of the best option (SOGREAH 2010). The best location to build it is at the slope break between the upstream gorges and the alluvial fan (Fig. 1). Indeed, massive deposition was observed in the last extreme event at this location. Designing a debris retention basin in a city center deserved a detailed analysis which has been performed using expert assessment along with numerical and small scale physical modelling. This case study had two particular challenges so far poorly addressed in the literature and worthy of publication: (i) determination of the debris-flood multiple scenarios and (ii) solutions to enable sediment transport in normal conditions while trapping all gravels and large woods for events overloading the quite low channel capacity. This paper synthetizes these points after a short presentation of the catchment and before to briefly describe the structure behavior in physical small scale model subjected to various scenarios. All details may be found in the technical report by ARTELIA and IRSTEA (2018).
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* Corresponding author e-mail address: [email protected]
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2. Catchment short presentation
The Ruisseau de la Combe de Lancey is a steep stream flowing from a 18 km² catchment in the Belledonne mountain range, north-east to Grenoble. Its confluence with the Isère River is located on the Villard-Bonnot municipality that occupies its alluvial fan. The torrent passes through four geomorphic units along its path (Fig. 1a & b). The steep headwaters experience erratic debris-flow activity. Along the 6 km of 12%-steep mid-mountain range, the torrent has a very stable, about 4 m wide steep-pool pattern with a wider paleo-bed. A very steep gorge with a bedrock and cascade bed connects the mid-range with the alluvial fan. The fan is almost entirely occupied by a huge 19th century paper factory now closed, the village center, a regional road and the railway to Italy (Fig. 1c). The final torrent section is 1.5% steep before its confluence with the Isère River. It is worth being stressed that where the river channel crosses a deep ditch dug to drain the Isère floodplain a long time ago (Fig. 1c), the torrent has only the capacity to carry 7 m3/s, so that any additional discharge supplied by the catchment is lost in the drainage ditch.
c) Overflowing in the drainage ditch
Large wood jam likely to jam the bridge
Massive deposit in 2005
Open check dam Torrent axis Bridge & culverts Drainage ditch
Fig. 1: a) full longitudinal profile of the Combe de Lancey torrent, split in four sections (i) headwater, debris-flow reaches, (ii) mid-range 12%- steep section, (iii) 28%-steep final gorges and (iv) alluvial fan; b) zoom on the fan section with location of the debris deposition basin, drainage ditch and confluence with the Isère River; c) map of final gorge and alluvial fan areas, location of bottleneck sections with deposition, jamming and overflowing issues (©IGN BD Alti 25m and Scan25).
The granitic geology of the catchment makes primary sediment production quite low; consequently, bedload sediment transport is supply-limited under normal conditions. Large sediment supplied to the fan is in any case limited by the mid-mountain range 12%-slope, except in the case of landslide in the final gorges which has been considered possible but very improbable and consequently not studied further in detail. On Aug. 21st and 22nd, 2005, a long lasting extreme rainfall event hit the Belledonne mountain range (IRMA 2006). Most torrents with sources located close to the summits experienced flood events with high intensity lasting at least 24 h, triggering massive geomorphic adjustments and debris releases on fans. The Combe de Lancey experienced debris flows in the headwaters and debris floods further downstream. Bridge clogging and bed widening in the mid-range occurred and 20,000 m3 of gravels deposited under and around the paper factory at the slope break between fan and gorges. Water peak discharges were not extreme (Q~22 m3/s, return period ~ 30 yrs, SOGREAH 2010) but the high flows lasted about 24 h, much longer than under usual thunderstorm conditions. The multivariate nature of this event, i.e., peak flows, duration, sediment transport, large wood recruitment or resulting damages, makes a direct estimation of a return period difficult but it can certainly be considered as exceptional. If considering simply damages, the historical records let us think that the return period of such events is closer to centuries than to decades. Not all details could be provided here for the sake of conciseness. The paper rather aims at conceptually explaining the coupled historical – geomorphic – physical appraisal used to fine-tune the structure to the torrent.
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3. Design scenarios for debris-flood events
3.1. Event status in design scenarios
Debris retention basins fundamentally aim at partially changing the sediment cascade processes but should optimally (i) only influence events likely to create hazardous problems, (ii) fully cope with a certain range and variability of events and (iii) not aggravate hazards when overloaded beyond their capacities. Several event magnitudes were consequently studied. (Fig. 2 and Table 1): As mentioned above, the torrent has the capacity to convey down to the Isère River only discharges up to 7 m3/s, i.e., about the annual peak flows. Trapping the bedload transport occurring below those conditions would increase cleanout costs for the structure, trigger sediment starvation downstream and should therefore be prevented with a suitable design. Several short runs on the physical small scale model with low sediment supply were tested to verify this point and are hereafter referred to as “routine events”. The structure should be able to cope with a similar event than the one experienced in 2005, which is assumed to have a return period of damages of about 100 years, these damage being mostly related to the volume of sediment transport. Assuming that sediment transport is mostly related to the hydrograph (see below for a discussion on this point), one must consider that a 1:100 years return period event could also have other features, e.g., a shorter duration but higher peak discharge. Two events with exceedance probability of about 1:100 years were tested and are hereafter referred to as “project design events”. Under these various harsh conditions, the structure must fully protect the area, i.e., with a certain safety factor taken as a 1 m-high freeboard on the flow level. A protection structure should not fail and aggravate the hazards even under a certain range of events with magnitudes higher than these project design events or more rare events with extraordinary features, e.g., massive armor breaking in the stable step-pool systems or cascading hazards of landslide and strong thunderstorm triggering an abnormally sediment-laden flood. Such events were tested as “safety check events” to control the structure robustness and reliability. For these events, a null freeboard is considered acceptable but full structure failures are not. They aim at understanding possible failure modes and at raising stakeholder awareness that structures protect up to a certain limit.
Table 1: Synthesis of event scenarios Name Event Sediment main source Return Peak Event* Solid Structure objective for this status period discharge duration volume event [yr] [m3/s] [h] [m3] Small events Routine Armored torrent bed <<10 3-7 3-7 <<1,000 Sediment transport transfer events downstream, no trapping
Type 2005 Project Debris flows in the ~100 22 30 20,000 Maximum effectiveness with event headwaters 1m freeboard and without large Long Project Debris flows in the ~100 35 18 20,000 woods releases to the thunderstorm event headwaters downstream channel Armor breaking Safety Large scale armor >100 35 18 20,000 Observation of potential failure check breaking modes for safety check: the Short & Hyper- Safety Landslides or massive >100 22 14 20,000 structure should not aggravate concentrated check bank erosion in the the hazards but may be catchment inter range overloaded and not able to cope with such events. Freeboard Volume Safety Larges debris flows in the >>100 35 36 40,000 overloading check headwaters may be null Debris flows Extreme Landslide in the steep >>100 ? ? ? Considered too unlikely to be gorges just above the fan studied. * Event duration: full hydrograph duration, assumed to be four times longer than the rainfall and two times longer than the flood high stages as displayed in Fig. 2.
3.2. Computation of event hydrographs
The first step was to reconstruct the 2005 event. Analyzing historical data, crisis management reports, ex-post technical reports and rainfall data, a hydrograph has been proposed to model the event with the following features:
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3 peak discharge Qp=22 m /s, high flow stage duration, i.e., with Q>Qp/2, t=10 h, a full duration of 36 h and a water volume of 760,000 m3. 3 The other ~ 1:100 years flood event was considered to have a hydrograph with maximum value Qp=35 m /s 3 (SOGREAH 2010), a high flow stage shorter duration t1=5 h, a full duration of 18 h and a volume of 560,000 m of water as a response to a 5 h rainfall event. To the best of authors’ knowledge, recommendations based on the determination of discharges – durations, of given exceedance probability, in the context of debris flood-prone ungauged mountain streams – have never been tried or tested. To estimate longer hydrologic events of similar exceedance probabilities, a straightforward technique has been used: assuming that peak discharges are proportional to the rainfall intensities - the so-called “rational method” in discharge estimation - one can express peak discharges as a power equation on time:
1−푏 1−푏 퐶(푇).퐴.퐼(푇,푡) 퐶(푇).퐴.푎푇.푡 푇 푄푝(푇,푡1) 푡1 푇 푄푝(푇, 푡) = = ⇔ = ( ) (1) 3.6 3.6 푄푝(푇,푡2) 푡2
3 with event time return T [years]; peak discharge Qp [m /s]; runoff coefficient C [-]; rainfall intensity I [mm/h], catchment area A [km²], rainfall intensity-duration coefficients aT [mm/h] and bT [-], and rainfall durations t, t1 and t2 [h]. Fig. 2 displays the peak discharge reduction against rainfall duration using bT=0.37, mean value of the frequency-intensity-duration curves measured at the rain gauges located on or near the Belledonne massif (Allevard, Fond de France, Saint Martin d’Hères, according to Djerboua 2011). The longer project design event, corresponding to the 2005 disaster, falls in the ±20% uncertainty range around this 1:100 year exceedance probability domain (Fig. 2, light blue area). The same method has been used to estimate other exceedance probability domains based on discharge knowledge for 1:10 years and extrapolating using daily rainfall extreme values and discharge-rainfall relationships according to Carré and Fretti (2010) for 1:1,000 years.
Fig. 2: Theoretical peak discharge – flood high stage duration for both routine, project and safety check events; exceedance probabilities of event magnitudes are based on rainfall intensity and peak discharge – rainfall relationships; sediment yields are computed with a bedload transport formula.
3.3. Computation of sediment yield and geomorphic scenarios
Bedload transport has been computed in a second step according to the “travelling bedload” framework specifically suitable for heavily armored bed experiencing colluvial sediment inputs, e.g., debris flows and landslides (Piton and Recking 2017). All the fan being urbanized and mostly covered, no alluvial material could be found near the structure location to assess the fan apex grain size distribution representative of debris-flood events. Measurements were thus realized upstream in the mid-mountain range (D84=332 mm) and downstream on the fan distal part close to the Isère confluence (D84=98 mm). Based on evidences of 2005’s disaster pictures, debris-flood events were assumed to supply grain sizes finer than the first sample but coarser than the latter. Project design events were tested with a mixture of 40% of the coarse gorge sample and 60% of the fine fan one. It has additionally been
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observed that cobbles of approximately 400 mm of diameter tended to deposit in the fan channel and hardly to be transported until the Isère confluence. Using this grain sizes with the aforementioned 2005’s hydrograph and the mid-mountain range 12% slope resulted in a sediment yield estimation of 58,000 m3, nearly three times the 20,000 m3 observed in 2005. This is consistent with the supply-limited state observed in normal condition: only rare triggering of massive debris flows in the headwaters can supply the material to generate debris floods. As often done in practice, the slope has been artificially lowered to 6.8% in order to model this supply-limitation and obtain the 20,000 m3 transport under the reconstructed hydrology. Several hypotheses had to be done on hydrology and sediment sizes and availability as described above. Scenarios testing the occurrence of events different than expected were used as safety check events (Fig. 2, Table 1): A safety check test was performed using coarser grain sizes corresponding to 60% of the gorge sample and only 40% of the fan sample. This event is assumed to correspond to a large scale armor breaking event. This test aimed at verifying the structure capacity to cope with the supply of sediment coarser than usual. Another possible geomorphic scenario is the occurrence of landslides and mass wasting processes into the torrent bed and the occurrence of a flood event of moderate magnitude, e.g., exceedance probability 30 years, before that channel cleaning with earth moving machinery could be performed. Such an event would trigger sediment laden flows loaded at full transport capacity, i.e., with the geometrical slope of 12%. This test aimed at verifying the structure capacity to cope with the supply of sediment with a solid concentration much higher than usual. Performing hydrology studies of ungauged high mountain catchments is highly uncertain. In addition to the two project design events which vary mostly in term of hydrograph, another safety check event aimed at testing how the structure respond to both a long (38 h) and intense event (Qp=35 m3/s) supplying 40,000 m3 of sediment, thus strongly overloading the debris basin volume capacity. Its hydrological exceedance probability is assumed to be about 1:1,000 years (Fig. 2).
3.4. Large wood recruitment
In addition to sediment and water, extreme debris-flood events recruit large woods, i.e., logs longer than 1 m and diameter higher than 0.1 m, on banks and from mass wasting processes. During the 2005 disaster, a cumulated volume of 600 m3 of large wood jams was measured in the catchment (ONF-RTM 2005). This value is close to the lower envelope of values estimated from empirical formulas (400 m3-3,000 m3) as reviewed in Piton and Recking (2016a). Large wood jams in retention basins increase obstruction ratio and trapping performances but are also likely to be released aggravating downstream hazards. They should consequently be considered cautiously during the design procedure (Bezzola et al. 2004). Based on historical evidences and observations of large wood pieces in the final gorges, a total absence of large wood pieces was considered really unlikely. However, in order to prevent an overestimation of trapping capacity, a relatively small volume of 100 m3 of large wood pieces were introduced during the raising limb of the hydrograph during all events tested in the laboratory. Close attention was paid to observe large wood accumulation and abrupt releases by overflowing possibly generating jamming problems further downstream (Fig. 1).
4. Debris retention basin design: basic and optimized
The basin has a diamond shape adjusted to the area available in the current and future urban fabric. It is roughly 100 m wide and long. The design is inspired by the state-of-the-art reviewed by Piton and Recking (2016a, 2016b) and by Schwindt et al. (2017, 2018) for the adaptations to enable sediment transport continuity during routine events while fully trapping sediment transport for larger events. The basin bottom slope has been chosen at precisely 1.5%, i.e., value of the bottleneck downstream reach, from the drainage ditch to the Isère River (Fig. 1). In essence, the structure should be able to transfer precisely the amount of sediment that the most constrained part of the whole downstream torrent system is able to export. This bottleneck reach has a hydraulic capacity of 7 m3/s. A guiding of channel of precisely the same capacity was dug in the basin to enable the continuity: the slope is not sufficient to transfer the sediment load if flows spread in the basin, flows must also be laterally constrained to keep their transport capacity (Piton et al. 2018a). Designing an open check dam able to transfer all sediment transport for routine events and trapping nearly all higher sediment supplies was an unresolved challenge until recently. The recent works by Schwindt et al. (2017) and Roth et al. (2018) provided clear recommendations to do so with so-called “hybrid” open check dams. Such
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structures have a bottom outlet that takes advantage of (i) a orifice hydraulic functioning to rapidly increase head losses and trapping capacity when discharges overpass the guiding channel capacity, along with (ii) a preceding grill with bottom clearance that prevent self-flushing during flow recession. The grill bottom clearance was 0.4 m high, the space between bars was 0.2 m and the bottom orifice was 4 m wide and 0.5 m high in the first option, adjusted to 0.8 m after preliminary tests (Fig. 3).
a)
b) Fig. 3: a) Basic and b) optimized versions of the open check dam: upstream and left side views at prototype scale and pictures of the 1:40 scaled physical model. Blue arrows display the flow direction. The bottom slot was equipped with an inclined grill according to the hybrid mechanical- hydraulic controlled described by Schwindt et al. (2017); optimization consisted in increasing the bottom slot height of 0.3 m, lowering the spillway level by 0.5 m and adding a rack on the spillway to retain large woods in the basin
Above this outlet dedicated to the routine event management, eleven vertical slits were added to enable flows from routine events up to project design events to pass through the dam. They were 1.0 m high, adjusted to 0.35 m in a second round and 0.4 m wide. The slits’ width was narrow enough to rely on their clogging by the cobbles of the same size observed in the channel. Additionally empirical evidences from other catchments let us think that mixtures of large wood pieces and even finer gravels will very likely lead to the clogging of the whole dam for heavy solid supply. Finally a typical 6 m-wide trapezoidal spillway was added at a height of 2.35 m above the river bed, decreased to 1.85 m after optimization. This crest feature should prevent structure by-passing and guide extreme event flows or design event flows when outlets’ clogging occurs. After a few tests, the following observation made clear some slight optimization needs as illustrated by difference between Fig. 3a & b: The 4 m-wide, 0.5 m-high bottom slot and inclined grill triggered a head loss a bit higher than expected likely to trap sediment for discharge in the range 4-7 m3/s. The bottom slot height was consequently increased to 0.8 m. The structure was able to trap all of the project design events’ sediment supplies; some room even remained in the basin distal part due to the quite steep deposits (see later). Lowering the water level would enable the deposit front to prograde faster in the basin and optimize its filling. The spillway level strongly controlling the water level, the structure crest had consequently been lowered by 0.5 m, thus lowering too the vertical slit height. Large wood accumulated against the open check dam, clogged the bottom slot and some slits, thus increased head losses and water levels. No abrupt and massive release of large woods was observed for discharges up to 28 m3/s, while nearly full large wood overtopping was observed for discharge approaching 34 m3/s. A rack dedicated to retain large woods in the basin was added on the spillway crest to prevent it.
5. Short description of structure behavior on the physical small scale under various event scenarios
The optimization proved to have satisfactory results: (i) flows up to 7 m3/s were only marginally influenced by the basin and new dam and (ii) large woods were mostly retained upstream of the rack, only a few pieces passed in uncongested transport. The prevention of wood release maintained however higher head losses and the basin filling did not significantly change between the basic and the optimized designs. Safety check events were then tested to check failure modes and structure functioning robustness.
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In addition to those empirical observations, classic measurements methods were used on the small scale model: photogrammetric analysis and surface velocity measurements by image analysis (Piton et al. 2018b). All events followed similar geomorphic trajectories although with various celerity and slight random variations. In essence: some bedload transport occurred in the guiding channel, however most of the supplies massively deposited near the basin inlet as soon as water discharge overpassed the 7 m3/s threshold. A small fan-like pattern grew and progressively prograded in the basin, the active channels wandered, split and merged on the depositional form. Since basin bottom slope was low and head losses rapidly grew, most of the basin was flooded and deposition actually occurred under a delta shape rather than a pure alluvial fan shape. The delta eventually reached the open check dam and large wood accumulation only occurred near the end of the events. The basin thus trapped all of the sediment supply, except for the volume overloading event. During this particular event, the bedload transport continuity was 3 3 recovered near half of the run and 13,000 m out of the 40,000 m supplied were transferred in the channel downstream. The basin was thus capable to store 27,000 m3 at most, i.e., with null freeboard. Maximum deposit elevations were measured to design the lateral embankments (Fig. 4). The project design event deposits plus the 1 m freeboard, as well as the safety check events (volume overloading and hyperconcentrated) with null freeboard, were measured above the ground level on a length of about 50 m from the basin inlet. Embankments suitably protected from erosion are required in this section. Conversely the simple excavation of the current ground is sufficient to contain the flows and depositions further downstream. Depositions slopes are relatively constant between events, around 5-6%, except for the hyperconcentrated and armor breaking events, which naturally resulted in steeper deposits with slopes around 7-8%.
Fig. 4: Longitudinal synthetic profiles of ground level, i.e., current terrain in the area, basin bottom level, i.e., excavation to be performed to dig the basin, and deposit maximum elevations for the various scenarios: either project design events (PDE) or safety check events (SCE). Both project design events have very similar deposit maxima despite their different dynamics; both the hyperconcentrated and the volume overloading safety check events were necessary to determine the deposit maxima for extreme events enabling to check the embankment length.
6. Discussion and conclusion
Debris retention basins are complex structures whose filling and responses to the natural variety of processes have been seldom directly observed (Piton et al. 2018a). The deposition process complexity (Piton and Recking 2016a) and interactions between structures and large woods (Piton and Recking 2016b) are consequently not fully understood. To the best of the authors’ knowledge, numerical models are not yet considered to be fully reliable tools to model debris-flood events, i.e., gravel, water and large wood mixtures flowing on slope of 5-15%. Small scale physical modelling was and is still a powerful tool to perform comprehensive analysis of such processes. But
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performing each run is costly in labor force and time, typical case-studies can consequently afford only a handful of runs. Ensuring the necessary representativeness of those runs and not missing a likely surprising behavior resulting in the structure failure is thus of utmost importance. Testing the great variability of torrential flows observed in Nature by clearly highlighting the hydrologic and geomorphic scenarios likely to occur is thus a key preliminary study of any expensive small-scale physical model campaign. Based on a recent case study of the Ruisseau de la Combe de Lancey in France, this paper shortly describes how the authors chose to deal with historical, geomorphic and physical data and methods to fine tune a debris retention basin. Three types of events were modeled: Routine events enabled to test and optimize the design so that small magnitude events, not threatening the village, could be transferred with marginal trapping to prevent maintenance effort and environmental degradation. Project design events, of various peak discharges and durations but globally having a ~ 1:100 years return period, enabled to verify the structure capacity to cope with such events with a reasonable freeboard. Safety check events, less probable though without specified exceedance probability due to the lack of data, enabled to test a greater variability of hydrological and geomorphic events and to make sure that no failure mode would appear for those magnitudes that remain possible although quite rare.
We hope that this example of applied research will help practitioners in other case studies or researchers in connecting scientific challenges in hydrology, sediment transport and extreme value theory with the challenges posed by the application data-scarce context.
Acknowledgements
The work of G.P. was funded by the H2020 project NAIAD [grant no. 730497] from the European Union’s Horizon 2020 research and innovation program. The authors would like to thanks the Villard-Bonnot municipality for allowing re-using the study results in this paper. The authors gratefully thank Kevin McCoy and an anonymous reviewer for numerous suggestions that helped us to improve the paper content.
References
ARTELIA and IRSTEA. 2018. Etude de l’Aménagement d’une Plage de Dépôt sur le Torrent de la Combe De Lancey - Site des Anciennes Papèteries de Villard Bonnot - Rapport de Modélisation Physique. Ville de Villard-Bonnot. 69p. Bezzola, G. R., H. Sigg, and D. Lange. 2004. Driftwood retention works in Switzerland [translated from Schwemmholzrückhalt in der Schweiz]. INTERPRAEVENT Conference Proceedings. VII:29-40. Carré, J., and B. Fretti. 2010. Analyse et critique de la méthode Speed (Système probabiliste d’étude par évènements discrets). SOGREAH. Technical report. 107p. Djerboua, A. 2001. Prédétermination des pluies et crues extrêmes dans les Alpes franco-italiennes: prévision quantitative des pluies journalières par la méthode des analogues. [PhD thesis] Grenoble, INPG, 214p. IRMA. 2006. On n’a jamais vu ça ! ou l’incorrigible nature... Institut des Risques Majeurs. Grenoble. 24p. ONF-RTM. 2005. Evènements des 22 et 23 août 2005 dans le massif de Belledonne. Préfécture de l’Isère. Technical report. 10p. Piton, G., F. Fontaine, H. Bellot, F. Liébault, C. Bel, A. Recking, and T. Hugerot. 2018a. Direct field observations of massive bedload and debris flow depositions in open check dams. Pages 1–8 Riverflow conf. Proc. E3S Web of Conferences. doi: 10.1051/e3sconf/20184003003 Piton, G., and A. Recking. 2016a. Design of sediment traps with open check dams. II: woody debris. Journal of Hydraulic Engineering 142:1–17, doi:10.1061/(ASCE)HY.1943-7900.0001049 Piton, G., and A. Recking. 2016b. Design of sediment traps with open check dams. I: hydraulic and deposition processes. Journal of Hydraulic Engineering 142:1–23. doi:10.1061/(ASCE)HY.1943-7900.0001048 Piton, G., and A. Recking. 2017. The concept of travelling bedload and its consequences for bedload computation of mountain streams. Earth Surface Processes and Landforms, 42(10)1505-1019. doi:10.1002/esp.4105 Piton, G., A. Recking, J. Le Coz, H. Bellot, A. Hauet, and M. Jodeau. 2018b. Reconstructing Depth-Averaged Open-Channel Flows Using Image Velocimetry and Photogrammetry. Water Resources Research 54:4164–4179. doi:10.1029/2017WR021314 Roth, A., M. Jafarnejad, S. Schwindt, and A. Schleiss. 2018. Design optimization of permeable sediment traps for fluvial bed load transport. E3S Web of Conferences 40:03009. doi:10.1051/e3sconf/20184003009 Schwindt, S., M. Franca, G. De Cesare, and A. Schleiss. 2017. Analysis of mechanical-hydraulic bedload deposition control measures. Geomorphology. 295:467-479, doi:10.1016/j.geomorph.2017.07.020 Schwindt, S., M. J. Franca, A. Reffo, and A. J. Schleiss. 2018. Sediment traps with guiding channel and hybrid check dams improve controlled sediment retention. Natural Hazards and Earth System Science 18(2):647–668. doi:10.5194/nhess-18-647-2018 SOGREAH. 2010. Plage De Dépôt Sur Le Torrent De La Combe De Lancey Sur Le Site Des Papéteries - Etude De Faisabilité. Commune De Villard Bonnot. Technical report. 26p.
1026 7th International Conference on Debris-Flow Hazards Mitigation Review of the mechanisms of debris-flow impact against barriers
S. Poudyala,*, C.E. Choia,b, D. Songc,d, G.G.D. Zhoud, C.Y. Yunee, Y. Cui a, A. Leonardif, M. Busslingerg, C. Wendelerh, G. Pitoni, E. Moasej & A. Strouthj
a Department of Civil and Environmental Engineering, The Hong Kong University of Science and Technology, Hong Kong bHKUST Jockey Club Institute for Advanced Study, The Hong Kong University of Science and Technology, Hong Kong c University of Chinese Academy of Sciences, Beijing, China dInstitute of Mountain Hazards and Environment, Chinese Academy of Sciences & Ministry of Water Conservancy, Chengdu, China eGangneung-Wonju National University, South Korea f Politechnico di Torino, Turin, Italy gTetra Tech Canada Inc, Vancouver Geotechnical Engineering, Vancouver, Canada h Geobrugg AG, Switzerland iInstitut national de recherche en sciences et technologies pour l'environnement et l'agriculture (IRSTEA), France j BGC Engineering, Vancouver
Abstract
Our limited understanding of the mechanisms pertaining to the force exerted by debris flows on barriers makes it difficult to ascertain whether a design is inadequate, adequate, or over-designed. The main scientific challenge is because flow-type landslides impacting a rigid barrier is rarely captured in the field, and no systematic, physical experimental data is available to reveal the impact mechanisms. An important consideration in flow-structure interaction is that the impact dynamics can differ radically depending on the composition of the flow. Currently, no framework exists that can characterize the impact behavior for a wide range of flow compositions. This review paper examines recent works on debris-flow structure interactions and the limitations of commonly used approaches to estimate the impact load for the design of barriers. Key challenges faced in this area and outlook for further research are discussed.
Keywords: Debris flows, barriers, impact mechanisms, flow compositions,
1. Background
The Association of Geohazard Professionals (AGHP) is an industry association that was created in 2013 to support the development of standards, specifications, and best practices for the design and implementation of geohazard- related technologies and products; and to provide education to the geohazard community. The AGHP Debris Flow and Steep Creek Hazards Mitigation Committee (Committee) was formed in 2017 and currently includes members from North America, Asia, and Europe. The first committee workshop was held on June 3, 2018 in Canmore Alberta, Canada and included a discussion of the wide range of design guidelines that are available. The Committee recognized that design practices for debris-flow mitigation structures vary between different world regions, and some aspects of practice are not well described in the existing guidelines. This paper is a collective effort by AGHP members and focuses specifically on current understanding of debris-flow impact mechanism against barriers. Steep creek flows made of mixtures of soil, rock and water, surge downslope at high velocities. These flows include floods, hyper-concentrated flows, debris flows, and rock avalanches (Hungr et al., 2014). To mitigate these hazardous flows, rigid barriers (e.g., Takahashi, 2014) and flexible barriers (e.g., DeNatale et al., 1999; Wendeler, 2008; Bugnion et al., 2012) are commonly installed in the predicted flow paths. Correspondingly, a reliable estimate of impact load is required to design these barriers. However, current design approaches for estimating impact loads relies heavily on empiricism and do not explicitly consider the composition of the flow, including the particle size and the ratio of solids
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* Corresponding author e-mail address: [email protected]
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to fluids. Such approaches make it difficult to ascertain whether a barrier design is robust, inadequate, or over- designed. Our present knowledge in this area is deficient for three main reasons. First, debris flows impacting structures are rarely captured in the field. Second, debris flows are scale-dependent phenomena (Iverson, 1997; Zhou and Ng, 2010). More specifically, small-scale physical experiments (e.g., Canelli et al., 2012; Scheidl et al., 2013) cannot holistically model the absolute stress state in a granular assembly, the timescale for pore pressure dissipation, and the degree of viscous shearing observed in prototype flows (Iverson, 2015). To capture the granular and fluid stresses in real flows more holistically, centrifuge model tests (Bowman et al., 2010) or large-scale physical experiments are necessary (Iverson, 2015). Third, depending on flow composition, specifically the ratio of solids to fluids (Iverson, 1997; Iverson and George, 2014) and particle size (Faug, 2015; Song, 2016; Song et al., 2017a and 2017b), the impact dynamics of debris flows can vary drastically. A framework that characterizes the impact mechanism of debris flows by considering a wide range of flow compositions—solid-fluid interaction and particle size effects—is necessary to make reliable estimations of the impact load.
2. Impact Models
Current opportunities for advancing our understanding of the impact mechanism of debris flows are reflected in international guidelines (VanDine, 1996; MLR, 2006; NILIM, 2007; Kwan, 2012). An estimate of impact force exerted by debris flows, assuming continuum-like behavior, is based on force equilibrium in hydrostatic models and momentum conservation in hydrodynamic models. Another type of loading that needs to be considered is discrete loading, which is created by short duration impulses from large particles (Ng et al., 2018). Existing approaches for estimating loading are discussed below.
2.1. Continuum loading
Continuum-based approaches adopt empirical coefficients to account for various uncertainties, including unknown impact mechanisms and flow composition. For example, the momentum-based equation for estimating impact (Hungr et al., 1984; Kwan, 2012; Volkwein et al., 2014) is given as follows: 퐹 = 훼휌푣2ℎ푤 (1) where 훼 is the empirical pressure coefficient, 휌 is the bulk density, 푣 is the impact velocity, ℎ is the flow thickness and 푤 is the channel width. Clearly, flow composition is not explicitly considered in equation 1, and 훼 accounts for the complexity of variables involved in natural geological material and natural settings. To highlight the empiricism of equation 1, a literature review shows that 훼 values are not consistent (Table 1). For example, 훼 of 3.5 is recommended for less viscous flows and 1.0 to 5.3 for more viscous flows (Scotton and Deganutti, 1997). Thurber Consultants Ltd. (1984) recommended 훼 value of 3 to 5 for flow compositions in Austria and Switzerland. Kwan (2012) recommended 훼 values from 2.0 to 2.5, depending on the type of structural countermeasure. Sovilla et al. (2016) demonstrates that the dimensions of the structure also fundamentally influence the impact pressure. Clearly, a scientifically based approach is urgently required to characterize the impact behavior for a wide range of debris flows.
Table 1. Summary of hydrodynamic models for estimating debris flow impact on a rigid barrier
Pressure coefficient (α) Reference α = 1.0 VanDine (1996) α = 3.0 to 5.0 Zhang (1993) α = 1.0 for circular structure α = 1.3 for rectangular structure MLR (2004) α = 1.5 for square structure α = 2.5 to 3.0 Lo (2000), Kwan (2012) α = 2.0 Vagnon and Segalini (2016) α = 1.5 to 5.5 Canelli et al., (2012) α = 2.0 to 4.0 Hübl and Holzinger (2003) α = 1.0 NILIM (2007) α = 1.0 SWCB (2005)
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Ancey and Bain (2015), Faug (2015), Ashwood and Hungr (2016), and Song (2016) all suggest that to more appropriately characterize impact, both static and dynamic loading must be explicitly considered as follows: 퐹 = 0.5푘휌𝑔(훽ℎ)2푤 + 훼′휌푣2ℎ푤 (2) where 훼′ is the coefficient for dynamic effect only, 푘 is the coefficient for static effect only, and 훽 is the ratio between the height of the static deposit and flow thickness before impact. Song et al. (2017b) further characterized the pressure coefficient to represent both dynamic and static loading with clearer physical meaning as portrayed by the following relationship: 휅′훽2 1 ′ (3) 훼 = 2 + 훼 2 퐹푟 where 퐹푟 is the Froude number (푣⁄√𝑔ℎ). The 퐹푟 is characterised by the ratio of inertial to gravitational forces of flow- type landslides in an open channel flow (Hübl et al., 2009; Choi et al., 2015a).
2.2. Discrete loading
Discrete loading is generated from large particles entrained in debris flows. These particles exert a concentrated impulse that can destroy structures in the flow path. To capture discrete loads exerted by these large particles, the Hertz equation is often used in design guidelines (Lo, 2000; NILIM, 2007; Swiss Federal Road Authority, 2008). The impact force calculated based on the Hertz contact theory (Johnson, 1985) assumes an elastic impact scenario which is given as follows: 4퐸 1 3 퐹 = 푅2(푥)5 (4) 3 2 2 where 퐸 is the effective modulus of elasticity, which is given as 1⁄퐸 = (1 − 휈1 )⁄퐸1 + (1 − 휈2 )⁄퐸2 (subscripts 1 and 2 denote parameters relating to the barrier and boulder, respectively) and 휈 is the Poisson’s ratio. 푅 is the equivalent radius, which is given as 1⁄푅 = 1⁄푅1 + 1⁄푅2, (푅1, 푅2 are the radius of curvature of contacting bodies) and x is the deformation, which is given as follows: 15푚푣2 푥 = 1 (5) 16퐸푅2 where, m is the mass of boulder and 푣 is the impact velocity. Kwan (2012) introduced a modified version of Eqn. 4 for design to estimate the impact force between a granite boulder and a reinforced concrete rigid barrier. The equation is given as follows:
1.2 2 퐹 = 퐾푐4000푣 푅 (6) where, 퐾푐 is a load-reduction factor, 푣 is the velocity of the boulder and 푅 is the radius of the boulder.
A fully elastic solution is generally believed to be over conservative (Hungr et al., 1984; Lo, 2000; Sun et al., 2005). Therefore, a load-reduction factor 퐾푐 was introduced. This factor is empirical and recommended values vary in the literature (Table 2). Equation 6 is for a single boulder and an equation that can capture the mechanics of a cluster of boulders impacting a surface simultaneously remains a crucial scientific challenge that needs to be addressed.
Table 2. Summary of Hertz equations for estimating boulder impact load on rigid barrier
Load reduction factor Kc Reference
퐾푐 = 0.1 Lo (2000) 퐾푐 < 0.1 NILIM (2007)
0.2 < 퐾푐 < 0.5 SWCB (2005)
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3. Impact Mechanisms
Equation 2 is convenient for engineering design, but fails to explicitly capture the key mechanisms of impact observed in physical experiments. These key mechanisms include the accumulation of static deposits called ‘dead zones’ (Chanut et al., 2010; Faug et al., 2009, 2011, 2012 and 2015; Choi et al., 2017), the pile-up of highly frictional flows (Koo et al., 2016) or the vertical-jet-like behavior of viscous flows (Choi et al., 2015a). Physical experiments have demonstrated that the impact mechanism strongly influences the impact load, and consequently the load distribution along a structure (Song et al., 2017a). As such, more details pertaining to impact mechanisms in two most extreme types of geophysical flows: dry granular flow and water flow are discussed below.
3.1. Pileup and vertical-jet mechanisms
To illustrate how the impact mechanism is governed by flow composition, let us consider two of the most extreme types of geophysical flows, specifically dry granular flow and water, impacting a rigid barrier. A dry granular flow is highly frictional with air as the interstitial fluid, which has a low viscosity and plays a relatively insignificant role in regulating the flow dynamics. Instead, frictional and inertial grain stresses dominate. Choi et al., (2015a) demonstrated that when a dry granular flow composed of Leighton Buzzard (LB) fraction C sand with uniform grain diameters of about 0.6 mm, impacts a rigid barrier, a pileup mechanism develops (Fig. 1a).
(a) = 0s (i) t = 0 s t
Flow direction Flow direction
(ii) t = 0.30 s (b) t = 0.2s
(iii) t = 0.60 s Vertical-jet
(c) t = 0.4s (iv) t = 0.90 s Rolling-back
(a) (b) Fig. 1 (a) Observed pileup impact mechanism for supercritical dry sand impacting an orthogonal barrier (b) Observed vertical-jet mechanism for supercritical water flow impacting a vertical barrier installed along a channel inclined at 5⁰ (redrawn from Choi et al., 2015a)
This mechanism exhibits a rapid attenuation of flow kinetic energy from the high degree of enduring frictional contacts between grains and their boundaries. Furthermore, a granular material with angular grains, such as sand, exhibits a high degree of bulk compressibility, assuming fragmentation does not occur. This feature is controlled by the changes in void ratio from elastic shear distortions of angular grain contacts (Iverson, 2015). High compressibility leads to bulk deformation during impact through shearing between grains, which is a very effective dissipater of flow kinetic energy compared to viscous shearing contributed by the interstitial fluid (Choi et al., 2015b). The properties of dry sand therefore inherently limit accretion along the free surface upon impacting a barrier. Instead, bulk deformation, for 퐹푟 within the transitional range (Faug, 2015), leads to the development of a granular bore that propagates or piles- up along the upstream direction in the channel. Compared to dry granular flow, water exhibits a vertical-jet mechanism upon impact if the initial 퐹푟 conditions are supercritical (Armanini, 1997; Choi et al., 2015a). This impact mechanism is characterized by the redirection of flow vertically along the barrier (Fig. 1b). A vertical-jet mechanism develops because the inertia of the flow is significantly
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larger than the restoring gravitational field, which is responsible for ‘pulling’ the flow towards the channel. The obvious transfer of flow momentum in the vertical direction for water, compared to dry sand, is because less flow kinetic energy is lost during the impact process. The energy loss is only limited to viscous shearing of the fluid and shearing along its boundaries (Choi et al., 2015b). The effects of viscous shearing in the dissipation of flow kinetic energy is less significant compared to enduring frictional-grain stresses in dry sand. Furthermore, water, has a relatively low bulk compressibility compared to that of dry sand. This lower bulk compressibility promotes run-up upon impact in the only unconfined boundary within a channel, and that is the free surface. By contrast, dry granular flow can compress along the slope-parallel direction during impact and can also pileup towards the free surface of the channel. Aside from the flow composition, the dynamics of channelized flow, specifically the 퐹푟 before impact also strongly influences the resulting impact mechanism (c.f., equation 3). Physical model tests have already demonstrated that water in supercritical flows exhibit a vertical-jet mechanism. By contrast, water in subcritical flow exhibits a reflective- wave mechanism upon impacting a rigid barrier. This mechanism can be characterized by limited transfer of momentum along the vertical direction or free-surface of the channel, the flow impacts the barriers and is allowed to reflect back upstream. Any disturbance to the flow in the channel, such as the barrier, will transfer energy as a wave. The flow inertia for subcritical flows is less than the restoring gravitational field. Therefore, a reflective-wave mechanism is exhibited for subcritical flows. For granular flows, the 퐹푟 of the flow before impact strongly influences whether gravitational or inertial effects are dominant (Faug, 2015; Sovilla et al., 2016).
4. Flow composition effects on dynamic response
4.1. Solid-fluid interaction
The complex flow dynamics of debris flows are governed by the interaction between the solid and the fluid phases. Solid-fluid interactions control the changes in the pore fluid pressure, which in turn regulates the Coulomb friction within and at the boundaries of a landslide (McArdell et al., 2007; Iverson and George, 2014; George and Iverson, 2014). The degree of interaction between the solid and fluid phases in the flow can be represented by the solid fraction, or the proportion of solids to fluids by volume (Cui et al., 2015). Flows with a higher solid fraction more readily dissipate flow energy by shearing between grains (Choi et al., 2015b). Although a great foundation has been established for the structural response of different types of barriers (DeNatale et al., 1999; Wendeler et al., 2006; 2007; Kwan et al., 2014), there remains a knowledge gap on how different flow types can result in very different impact loads. To remedy this gap in the literature, Ng et al., (2016a; 2016b) and Song et al., (2017a; 2017b) carried out a set of centrifuge tests to model the impact mechanisms of debris flows, dry sand and viscous flows, with varying flow composition, on rigid and flexible barriers. Depending on the flow composition, the impact behavior differed drastically. For dry granular flows, the dissipation of the flow kinetic energy was significantly enhanced via stress-dependent friction, unlike viscous flows, which dissipated the flow kinetic energy less readily. As discussed, a dead zone is useful for attenuating the impact load on an obstacle or barrier, but it can also contribute to the overall load acting on a structure. Song et al., (2017b) carried out a series of centrifuge experiments modelling the impact of two-phase flows on a rigid barrier. In these experiments, the solid fraction was progressively increased from 0 to 0.5. As expected, as the solid fraction increased, particle image velocimetry analysis (White et al., 2003) showed large dead zones. The larger the dead zone observed, the higher the resulting peak impact load measured on the barrier. These findings confirmed that the impact process for two-phase flows is as much a dynamic process as it is a static process. The higher the solid-fraction in the flow, the more pronounced the dead zones. These deposits in turn augment the overall load acting on the orthogonally-configured barrier (Fig. 2a).
4.2. Influence of particle size
Another important feature that adds to the complexity of investigating the impact mechanisms of debris flows is the effects of particle size. Song (2016), Song et al., (2017b), and Song et al., (2018) demonstrated that as the particle size increases, more discrete loads with higher magnitudes are generated. The impact dynamics resulting from large glass spheres differ significantly from dry sand or a two-phase mixture, with the same equivalent volume and 퐹푟 conditions before impact. Dry sand exhibits a progressive loading pattern to its static state without an obvious dynamic peak or sharp impulses. The two-phase mixture, however, exhibits a continuous loading behavior, which reaches its
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peak load before softening towards a static state. The two-phase mixture is fluidized and takes a longer time to reach a static state because of a lack of shear resistance in the flow. Clearly, a comparison of the different flow types highlight the distinct loading pattern exerted by a cluster of large particles compared to dry sand and two-phase flows.
6 6 Viscous liquid 20% solid fraction
h 5 5
/ 40% solid fraction α H 50% solid fraction 4 4
3 3 = 2.5 (Kwan 2012)
2 2 Pressure coefficient Pressure coefficient
Flow depth h 1 1 Normarlised height barrier Normarlised
0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Normalised peak pressure P/ρv2 Normalised particle diameter δ / h (a) (b) Fig. 2 (a) Influence of solid fraction on dynamic response of rigid barrier (redrawn from Song et al., 2017b) (b) Effects of particle size on dynamic response of rigid barrier (redrawn from Song et al., 2018) A comparison of the loading time-histories with existing impact models show that both the dry sand and two-phase mixture are bounded by the superposition of both equations 2 and 3. However, the cluster of glass sphere, resembling a bouldery flow, generates sharp impulses that exceed the superposition of equations 2 and 3. These results indicate that the entrainment of large and hard inclusions in debris flows warrants consideration in the design of structural countermeasures, to safeguard against local damage. To further investigate the effects of particle size, the performance of the hydrodynamic approach, based on different normalized particle sizes was investigated (Song, 2016). The peak loads were compared (Fig. 2b), and results showed that continuum-based mechanics (equation 3) fail to capture sharp impulse loads at a normalized particle size of 22 mm, based on the recommended dynamic coefficient of 2.5 (Kwan, 2012). Although a solution for capturing the impulse loads for a cluster of large particles was not provided, test results help to evaluate the current impact models for discerning the effects of particle size. A crucial challenge remains to account for impulse loads from a cluster of large particles and to distinguish what particle size is generating impulses that cannot be captured using continuum mechanics (equation 3). Sharp impulses can be attenuated by increasing the contact time between a particle and a surface. Depending on overall stiffness of a structure, the effects of particle size can diminish. For instance, flexible barriers were originally adopted for capturing rock fall and have been adopted for resisting debris flows in the past decade (Kwan et al., 2014). Another approach for attenuating sharp impulses is to install cushioning materials in front of rigid barriers to diminish these loads (Ng et al., 2017).
5. Summary
Examination of the current state of research on the impact mechanisms of debris flows is presented in this paper. This study highlights the importance of considering the composition of a debris flow to assess the resulting dynamic response and impact mechanisms induced on a rigid barrier. Some key aspects from this review paper is summarized as follows: 1) The effects of the particle size are manifested in the inertial grain stresses of the flow during impact. As the particle size increases, the debris-flow transitions from contact-dominated (continuum) to inertial-dominated (discrete) grain stresses. The larger the particle size, the higher the magnitude and number of sharp impulses that are induced on a barrier. 2) The effects of solid-fluid interaction, specifically the ratio of solids to fluids dictates the force exerted on a barrier.
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The higher the solid fraction, the more predominant grain-contact stresses are, thereby inducing higher static loads on the barrier during impact. 3) The flow type governs the mechanism of impact on rigid barriers. Granular flow, which consist of angular grains, readily dissipate flow kinetic energy through enduring shear contacts between grains and deformation from its high bulk compressibility. By contrast, inviscid flow does not readily dissipate flow kinetic energy from internal viscous shearing and viscous shearing along its boundaries. The ratio of inertial to gravitational forces before impact dictate the impact mechanism. These mechanisms are critical for discerning the design load and distribution on barriers.
Acknowledgements
Members of the Association of Geohazard Professionals provided valuable input to this paper during and after the June 2018 workshop organized by the Debris Flow and Steep Creek Hazards Mitigation Committee. The authors are grateful for the financial sponsorship from the National Natural Science Foundation of China (51709052), the theme- based research grant T22-603/15-N and the General Research Fund 16209717 provided by the Research Grants Council of the Government of Hong Kong Special Administrative Region, China, by the H2020 project NAIAD [grant no. 730497] from the European Union’s Horizon 2020 research and innovation program. The authors would like to gratefully acknowledge the support of the HKUST Jockey Club Institute for Advanced Study and the Hong Kong Club Charities Trust and the Hong Kong Jockey Club Disaster Preparedness and Response Institute (HKJCDPRI18EG01). The first author gratefully acknowledges generous support from Hong Kong PhD Fellowship for his PhD study. Support from Tetra Tech Canada Inc. for preparation of this manuscript is gratefully acknowledged.
References
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1034 7th International Conference on Debris-Flow Hazards Mitigation Small scale impact on rigid barrier using transparent debris-flow models
Nicoletta Sanvitalea,*, Elisabeth Bowmana, Miguel Angel Cabrerab
aUniversity of Sheffield, Sir Frederick Mappin Building, Mappin St., Sheffield, S1 3JD, UK bUniversidad de los Andes, Carrera 1 Este No. 19ª-40, Bogotá, 111711, Colombia
Abstract
Fast landslides, such as debris flows, involve high speed downslope motion of rocks, soil and water. Their high flow velocity, high degree of runout and potential for impact make them one of the most hazardous gravitational mass flows. While the estimation of the pressure generated by the impact of debris flows on civil engineering structures has been widely investigated, the state of the knowledge is still insufficient to accurately describe the dynamics and load evolution of the impact process. Both fluid and solid forces influence the dynamics of debris flows but existing design approaches for barrier or mitigation structures tend to treat these geophysical flows as a single continuum, neglecting the solid fluid-interactions. Hence in the literature, impact models are yet largely semi-empirical. This paper presents the first results of experiments using transparent debris flows in a small-scale flume aiming at investigating the mechanism of impact on rigid barriers. The use of a transparent debris-flow model allows the movements of particles and fluid within the medium to be probed. We examine flows consisting of uniform and well graded particle size gradings at two different fluid contents. The evolution of the impact load, bed normal pressure and fluid pore pressure for the different flows are measured and analysed in order to gain a quantitative comparison of their behaviour before and after impact.
Keywords: Debris flows; Barriers; Physical modelling; Transparent soil
1. Introduction
A debris flow is a rapid surging flow of saturated-debris and soil in a steep channel that may present high impact load and long runout (Iverson, 1997; Takahashi, 2007; Hungr et al., 2013). A common method to prevent these flows from reaching vulnerable areas is by obstructing their channelized paths with barriers, dampening the overall flow inertia, trapping most of the transported debris, and, therefore, decreasing their expected runout. These barriers can be rigid walls or flexible nets, with a main goal of withstanding the impact forces from the transported debris and suspended material. Rigid barriers, also called catch dams, are the most common mitigation structure against debris flows, due to the minimal technical skills required in their construction and easiness in the supply of building materials for reinforced concrete. These barriers sustain the lateral impact forces by self-weight, consisting of a debris-flow breaking structure, a retention basin and a pre-structure (Hübl et al., 2009). While the estimation of the pressure generated by the impact of debris flows on civil engineering structures has been widely investigated (Moriguci et al., 2009, Armanini et al., 2011; Bugnion et al., 2011; Hu et al., 2011; Scheidl et al, 2012; Cui et al., 2015; Zhou et al., 2018).), the state of knowledge is still insufficient to accurately describe the dynamics and load evolution of the impact process. The current paper presents the first results of experiments using transparent analogue debris flows in a small-scale flume, aiming at investigating the bulk impact forces on rigid barriers. The experimental variables are the initial particle size distribution and fluid content while measuring the impact forces against the obstacle, the basal total- and fluid-pressures, flow height, and the cross-sectional flow dynamics at impact observed via Planar Laser Induced Fluorescence, PLIF (Sanvitale & Bowman, 2012). Section 2 presents details on the experimental setup and its
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* Corresponding author e-mail address: [email protected]
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instrumentation, describing the materials employed and experimental protocol. Section 3 focuses on the direct measurements and discusses the impact mechanisms. Finally, Section 4 presents the main conclusions of the current work and provides insights into ongoing research.
2. Methods
2.1. Experimental set up
The material is stored in a rectangular tank, at the top of the channel. A slice gate, fitted between acrylic seals, releases the material by hand pull. The material flows down a rectangular flume 2.57 m long and 0.15 m wide, at an adjustable angle (see Fig. 1). Experiments reported in this paper are performed for a flume inclination of 20º. The lateral walls are made of borosilicate glass and the flume’s bottom is roughened with 3D-printed PLA plates with a hexagonal packing of 3 mm semi-spheres. The roughened bed is instrumented along its base with three pore pressure transducers, PPT2, 3 and 4 (PDCR 810 Druck) placed at 175 mm centres along the flume at 30 mm from the centerline with PPT2 placed closest to the barrier at 8.5cm distance and PPT3 and PPT4 placed 175mm and 350mm further upslope. One load cell (LUX-B-ID Kyowa) with a circular sensing plate of 23mm diameter is mounted flush with the flume across from PPT2 (at 30 mm on the other side of the centerline). All basal sensors have 3D printed disk headings, equivalent to the roughness of the rest of the base. The barrier model is made of a PMMA plate 10 mm thick, 150 mm wide and 190 mm tall (Fig. 1). The plate is connected to an aluminum support, connected at one end, on its mid-width, to an axial load cell (U9C, HBM) and fixed on its base to a linear bearing (LZMHS12-37T2P1, SKF). The barrier model is fixed to the flume bed at 2.25 m from the gate release, and is orientated normal to the flow direction (see inset in Fig. 1).
Fig. 1. Experimental setup after test. (Inset) Barrier model front view.
A 0.5 mm thick 532 nm laser light sheet is allowed to pass through the roughened bed and barrier model base, illuminating the flowing material at the flume’s mid-width. The illuminated plane is located about in front of the barrier lighting the material from the flume’s bottom (Fig. 1). The longitudinal motion is recorded with a Phantom high speed camera at a frame rate of 2000 fps. For more details on the PLIF technique please refer to Sanvitale and Bowman (2012).
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2.2. Materials
In order for the PLIF technique to work under optimum conditions, the fluid and solid should present a match of their refractive indices. The current experiments are performed with hydrocarbon oil (Cargille laboratories) dyed with a fluorescent powder, and mixed with borosilicate glass beads (Sigmund Lindner GmbH). The fluid has a viscosity that is higher than water and a density that is lower, such that particle / fluid consolidation behaviour is equivalent to that using quartz particles that are four times smaller in water (Sanvitale & Bowman, 2012). Three particle size distributions (PSD) are employed in experiments; two uniform samples with glass beads of 3 mm and 7.5 mm, and a well graded sample (coefficient of uniformity CU=6) with mean particle size of 7.5 mm (see Fig. 2). These samples are intended to provide an insight on the potential effects of particle size distribution on the impact mechanism.
Fig. 2. Glass beads samples particle size distribution.
2.3. Test procedure
Experiments explore the effects of 28% and 32% fluid content fc, defined as massfluid/masssolid, impacting a rigid barrier model. Prior to experiment, the flume is cleaned, avoiding the presence of oil films on the roughened bed and lateral walls. For each experiment 10 kg of glass beads are used. Oil is gently poured into the container and mixed with the glass beads, paying special attention to the removal of air bubbles inside the mixture. Once the desired amount of fluid is added into the container, the laser beam is set-on, the high-speed camera is activated, and the slice gate is opened. At release, a triggering shutter connected to the gate activates the sensor recording at a sampling rate of 36 kHz, for a duration of 9 s.
3. Results and discussion
Qualitative features of flow impact can be revealed by the analysis of the video images. Figure 3 presents a sequence of images for each PSD at the fluid content, fc 28% (images for fc 32% are not shown for brevity). Fig. 4 shows the runup height at the barrier measured on the video footage for all the tests except the well graded flow with 32% of fluid content, for which the movie was not available. The images show different instants during the impact of the mixture against the barrier with respect to the time t=0 at which the slice gate was opened. Images on the left column show the initial phase of the impact, the middle image is taken when the runup of the material occurs and the image on the right shows the final phase of each test. All the tests displayed similar dynamics during the impact, with preliminary collisions of dry bouncing particles against the barrier prior the arrival of the flow front. Only in the test with 3mm beads and fc 28% could we observe the arrival of a first small frontal surge before the impact of the main flow surge. The impact of the leading surge led to a runup that exceeded for a few centimeters the height of the barrier for all the experiments except for the 3mm mixture at fc 28% (Figure 4). However during runup only a small quantity of mixture was able to override the barrier because the vertical wave produced rapidly broke backward, resulting in an upstreaming propagating shock wave. The same behavior was observed by Iverson et al. (2016) in large scale flume tests using soil and water mixtures.
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Fig. 3. Illuminated section at centerline of flume during impact (a) 3mm uniform, (b) 7.5mm uniform, (c) well-graded with D50 = 7.5mm, CU = 6
The velocity of the tests is generally larger at higher fluid content. The front velocity of the tests at fc 28%, estimated using the instant at which the flow front arrive at the barrier, is approximately 1.1.m/s , 1.9 m/s and 1.8 m/s ± 0.2 m/s for the 3mm, 7.5mm and well graded PSDs, respectively. When the fluid content is 32% the estimated front velocity is 1.6m/s, 2.1m/s, 1.9 m/s ± 0.2 m/s for the 3mm, 7.5mm and well graded PSDs, respectively. The estimate of the instant in which the flow arrives at the barrier is not an easy task due to the fact that the tests are run in a dark environment to avoid any light except that produced by the fluorescent dye excited by the laser being recorded by the high speed camera. As the flow front is usually unsaturated, we can only track the position of the flow front from the reflected light coming from the reflections of the laser with the surface of the beads. Further work is ongoing to obtain the field velocity during impact using particle image velocimetry analyses on the flow images.
3.1. Basal pressure development
Figs 5 and 6 shows the responses of the pore pressure transducers using a running filtering window of 300 Hz. The PPT responses are dominated by the increase in the height of the fluid-saturated debris behind the barrier (which is effectively impermeable) after impact. Therefore, although flows initially pass over PPT4, then PPT3 and then PPT2 in succession on their descent (resulting in relatively small recorded pressures of the order of 0.2 kPa), PPT2 (closest to the barrier) then produces the largest and earliest response to this impact with recorded pressures ranging between 1 and 2 kPa. There is a considerable difference in pore pressure behaviour after impact between flows at fc 28% and 32%, particularly for the 3mm flows which, considering the ratio of pore pressure to total stress, appears to be close to hydrostatic for fc 28% but above this immediately post impact for fc 32%. For the well graded flows, it is clear that the
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pore pressures are much greater than hydrostatic upon and post impact at both fluid contents. For the 7.5mm flows, the picture is mixed – appearing slightly above hydrostatic for the 28% and considerably above for fc 32%. These results point to several things: excess pore pressures do not necessarily generate within uniform flows of spheres, except where sufficient fluid is present and sufficient agitation is generated (e.g. during an impact event). The 7.5 mm flows generate large forces at impact, higher velocities and greater agitation than the 3mm flows, although otherwise it would be expected from consolidation theory the finer material should both generate and maintain higher pore pressure. This only occurs for the 3mm flow at fc 32%, showing that the fluid content plays a role in developing excess pore pressure which is necessary for enhanced mobility. Conversely, for well-graded flows (at least for the chosen grading and same fluid contents) excess pore pressures are both generated and maintained at impact. This is likely due to there being both larger particles to agitate the flow upon impact and fines to reduce permeability and hence maintain the excess pore pressure for longer.
Fig. 4. Measured run-up height at the barrier (time t=0 is the time of the flow front arrival)
3.2. Impact load
The recorded raw impact signals present high frequency spikes that can be due to random effects depending on the resonance frequency of the load cell and on single instantaneous impact of large particles. In order to filter the data we followed the procedure proposed by Scheidl et al. (2012), applying a low pass filter with a maximum high frequency estimated considering the average maximum front velocity vf and the maximum particle diameter, as follows fi=vf/dmax. For the uniform tests with 3 mm and 7.5 mm particles this produced a low pass frequency of 450 Hz and 270 Hz respectively. For the well graded material we decided to assume as dmax the value of d90=20mm and obtained a low pass frequency of 100 Hz. Figures 5 and 6 present the impact load measured during tests after filtering. The influence of the particle size on the response of the barrier is clear. The uniform flows with the 7.5 mm particles generate the greatest initial impact load, while that of the well graded flow (with d50 of 7.5 mm and d90 of 20 mm) is intermediate to that of the 3 mm and 7.5 mm for both fluid content series. The well graded tests such as those with 7.5 mm show a number of spikes in the impact signal recorded from the load cell, representing collisions of large particles against the barrier, however, both the peak force during and after the impact reach values similar to those of 3 mm mixtures. This indicates the dampening influence of the fine particles within this flow material, considering that it has larger particles than the uniform 7.5 mm flows. Comparing the videos with the development of the impact load it is possible to recognize common dynamics for all the tests. We observe an initial dynamic impact characterized by the rapid increase of the load due to the front surge arrival and the subsequent material runup hitting the barrier. The impact force peak is followed by a quick drop when the runup wave breaks backward down, suppressing the action of the incoming flow for a subsequent runup. From this point on, the increase of the impact force can be considered as a result of the progressive deposition of the mixture behind the material already settled in front of the barrier. The peak load due to the initial dynamic impact of the flows appears to be enhanced by the presence of a larger quantity of fluid for the 7.5 mm and well graded tests. The 3 mm tests do not show a greater initial impact at larger fluid content but instead it is possible to observe a slight increase in the load exerted from the flow after the impact,
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probably due to the fact that the more fluidised mixture results in a faster flow and hence most of the particles within the mixture are able to travel till the end of the channel, increasing the weight of the material behind the barrier (see Fig. 4).
Fig. 5. Fluid content 28% (Left) Load on barrier (Right) Basal pressures (a) 3mm uniform, (b) 7.5mm uniform, (c) well-graded with D50 =
7.5mm, CU = 6
Closer examination shows that the flow at fc 28% for the 3 mm particles behaves similarly to that of the 7.5 mm flows with a near constant pressure response after impact, however at fc 32%, it is more similar to that of the well graded flows where the impact pressure initially rises then falls then rises again. Coupled with the pore pressure response for 3 mm, this suggests that the behaviour of this uniform flow strongly depends on the fluid content. Only the 7.5mm tests exhibited a peak load generated by the initial dynamic impact on the barrier that is higher than that exerted from the material deposited behind. The reason for that is that such fast mixtures, once they hit the
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barrier, can produce a high and large runup wave with large particles that, despite their size, can be easily mobilized and pushed upward and against the barrier exerting high pressure. In contrast, it is clearly visible from the high speed camera video, that for the fc 28% well graded test, most of the top part of the runup wave is comprised of fluid as the large particles at the flow front are too heavy to be pushed any higher than approximately the middle height of the barrier (Fig. 3). Furthermore, the presence of finer material can also have a damping effect on the large particle collisions. The combination of these factors can explain the similar value of the impact load between the 3 mm and the well graded tests.
Fig. 6. Fluid content 32% - (Left) Load on barrier (Right); Basal pressures (a) 3mm uniform, (b) 7.5mm uniform, (c) well-graded with D50
7.5mm, CU = 6
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4. Conclusions
The paper presents the results of tests performed using a small-scale flume in which the impact of a transparent debris-flow model on rigid barrier is investigated. The roughened base of the channel is instrumented along its base with three pore pressure transducers and a load cell for the measure of the total normal stress. The barrier is fixed to the flume bed at 2.25 m from the gate release and is orientated normal to the flow direction. The evolution of the impact load, bed normal pressure and fluid pore pressure for flows consisting of uniform and well graded particle size gradings at two different fluid contents, 28% and 32%, is measured and analyzed before and after impact. It has been found that excess of pore pressures do not necessarily generate within uniform flows of spheres, except where sufficient fluid is present and sufficient agitation is generated (e.g. during an impact event). The particle size of the material has an effect on impact. The uniform flows with the larger particles generate the greatest initial impact load while for the well graded mixture the presence of fine particles within the flow can provide a dampening influence. Further work is ongoing to obtain the field velocity during impact using particle image velocimetry analysis on the flow images in order to investigate the impact kinematics in detail.
Acknowledgements
This research was supported through the Engineering and Physical Sciences Research Council (EPSRC), UK project no. EP/M017427/1 “High speed granular debris flows: new paradigms and interactions in geomechanics”. The authors would like to acknowledge the assistance of technicians at the University of Sheffield in the construction of the apparatus. MAC was partly funded by the Early-stage Researcher fund (FAPA) from the Universidad de los Andes, under the Grant agreement No. PR.3.2016.3667.
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