Preliminary Assessment of Debris Flow Hazard in
a Catchment under Extreme Condition
L. GAO, L.M. ZHANG and H.X. CHEN
Department of Civil and Environmental Engineering, the Hong Kong University of Science and Technology, Hong Kong
Abstract. Due to hilly terrain and frequent heavy rainstorms, debris flow is one of the most common and catastrophic hazards in
Hong Kong. To mitigate the debris flow risk, prediction of the magnitude and consequence of debris flow is essential. The objective of this paper is to simulate debris flow hazards in a study area in Hong Kong. Assessment of the surface geology is firstly conducted to ascertain the source materials. The physical parameters of the loose materials such as size, location, and soil type are investigated. The volume, sediment concentration and resistance parameters for debris flows are subsequently evaluated and adopted as the input for the debris flow simulation. Numerical debris flow simulation is then conducted according to the established parameters. The maximum flow depth and maximum flow velocity are calculated by solving the continuity equation, the momentum equation and a friction slope equation. Based on the simulation results, a debris flow hazard map as a function of both the maximum flow depth and the maximum flow velocity is finally produced.
Keywords. Debris flow, hazard assessment, engineering geology, deposition
1. Introduction
Hong Kong has a sub-tropical climate characterized by distinguished dry and wet seasons. The terrain in Hong Kong is hilly, with 30% of the land steeper than 30q (Au, 1998). Inevitably, debris flow is one of the most common and catastrophic hazards in Hong Kong. Therefore debris flow hazard assessment plays a central role in its sustainable development.
A debris flow is defined as a fast or extremely fast flow of sediment and water mixtures in terms of continuous fluid driven by gravity (Takahashi, 2007). Quantitative methods are needed to describe a debris flow. Chen and Lee (2000) developed a 3D dynamic model for simulating debris flows based on finite element analysis, and applied it to the Shum Wan Road landslide and the Fei Tsui Road landslide. Chau and Lo (2004) assessed the debris flow hazard for Leung King Estate of Hong Kong by incorporating GIS with numerical simulations and proposed a hazard map based on computer simulations. Kwan and Sun (2007) developed a 3D debris mobility model and simulated a number of benchmarking cases.
Generally, researchers have focused on simulating the debris flows under normal conditions; the studies on debris flow hazard under extreme conditions are rather limited. This paper aims to evaluate, through numerical simulation, the debris flow hazard under extreme condition in an urban area.
2. Study Area
The study area is located on western Hong Kong Island as shown in Figure 1. Three channelized debris flows occurred in the study area in 1967, with travel distances of 235.7 m, 172.7 m, and 119.1 m, respectively.
The study site has an area of 3.8 km2. The ground surface elevation and the building distribution on the GIS platform are shown in Figure 2. The elevation of the catchment ranges between 0 to 551 m. The slopes in the catchment are rather steep and many buildings are located in front of the buildings. The terrain conditions are discretized using a 10 m×10 m grid. The solid geology in the study area is composed of fill, granite and tuff, as shown in Figure 3.
© 2015 The authors and IOS Press. This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License. doi:10.3233/978-1-61499-580-7-540
The superficial geology is divided into volcanic colluvium, granite colluvium and vegetation-covered bedrock corresponding to the solid geology conditions (Fyfe et al., 2000), as shown in Figure 4. The bedrock in the study area
Figure 1. Location of the study area.
Figure 2. Elevations and buildings in the study area.
Figure 3. Geology of the study area.
Figure 4. Superficial geology of the study area.
has been subjected to weathering. The typical depth of the deposit is from 2 m to 5 m (Lumb, 1975). A large amount of loose materials in the study catchment is recognized as potential sources for debris flows.
3. Numerical Method 3.1. Governing Equations
The FLO-2D model (FLO-2D Software Inc., 2009, Chen et al., 2013) describes well unsteady surface flows and debris flows and is used in this study. The governing equations include the continuity equation (1) and the momentum equation (2): i x hV t h w w w w (1) t V g x V g V x h S Sf o w w w w w w 1 (2)
where h is the flow depth; t is time; V is the depth-averaged velocity in eight flow directions, including four compass directions and four diagonal directions; i is the excess rainfall intensity on the flow surface; Sf is the flow resistance slope.
Based on the study of O’Brien et al. (1993), the flow resistance slope, Sf, can be expressed as:
3 / 4 2 2 2 8 h V n h V K h S td m m y f J K J W (3)
where Wy is the yield stress; K is the dynamic viscosity; Jm is the unit weight; K is the laminar flow parameter and set to 2,500, which has been used effectively for urban studies (O’Brien et al., 1993, FLO-2D Software Inc., 2009); ntd is the equivalent Manning coefficient and calculated as (FLO-2D Software Inc., 2009):
V C td ne n 6.0896 0538 . 0 (4) where n is the Manning coefficient and Cv is the volumetric sediment concentration, defined as the ratio of the volume of the solid material to the whole mixture. In the study carried out by Chau and Lo (2004), the value of Cv is set according to field measurements at Tsing Shan, and the initiation value is around 0.5. For simplicity, Cv is taken as 0.5 in this study.
The parameters of yield stress and dynamic viscosity require more information. According to laboratory studies (O’Brien and Julien, 1988), the range of dynamic viscosity for natural soils is from 0.05 to 15 paxs. The value of K is taken as 8.0 paxs in this study. A static limit-equilibrium equation is adopted to estimate the yield stress:
T U
Wy ghsin (5) where Wy is the yield stress; U is the density of the mixture, and is estimated to be 1900 kg/m3; h is the debris depth, set to be 2 m as an average; T is the average slope of the debris flow fan, assumed to be 7q. The yield stress is calculated to be 4.5 kPa.
3.2. Building Blockage
The buildings in the study area are densely populated and may block the debris flow or change its flow route. Hence the flows inside the grid elements at the building locations are blocked. Due to the use of a node spacing of 10 m, the accuracy of the building element is 10 m.
4. Assumed Extreme Conditions 4.1. Source Zones
A debris flow is commonly triggered by a landslide and develops along its runout path through entrainment. A Natural Terrain Landslide Inventory (NTLI) in Hong Kong was reported by King (1997). The corresponding landslide traces are shown in Figure 5. A ravine distributed with many failed slopes is selected to be the target study area. The west part of the ravine is covered by volcanic colluvium (Figure 4), which is considered as the potential debris source zone. The area of the landslips is about 4,000 m2. The thickness of the decomposed volcanic rock is considered as 5 m. Suppose all the loose deposits in the catchment turn into a debris flow. The material supply is therefore 20,000 m3. This is recognized as an extreme case for simulation since in reality the landslide size could be much smaller.
Figure 5. Landslide inventory in the study area. 4.2. Discharge Hydrograph
The shape of the hydrograph is assumed to be a triangle with a duration of 0.5 h. The curve is fitted to make the total discharge volume equal to 20,000 m3. The discharge hydrograph is shown in Figure 6.
Figure 6. Discharge hydrograph.
5. Debris Flow Simulation 5.1. Maximum Flow Depth
Figure 7 shows the maximum debris flow depth under the assumed extreme condition. The maximum flow depth is up to 3.9 m. At locations in front of the buildings that face the debris flow path, the debris deposition can be up to 3 m thick, equivalent to 1-story height. The travel distance is about 1.2 km.
Figure 7. Maximum flow depth. 5.2. Maximum Flow Velocity
The maximum flow velocity is shown in Figure 8. The maximum velocity is up to 7.3 m/s. The maximum velocity seems to be critical as a result of steep slopes distributed in the study area. The buildings in front of the slope are at risk if no prevention measures are taken.
Figure 8. Maximum flow velocity. 5.3. Hazard Map
To evaluate the effect of an individual debris flow, a hazard map as a function of both flow velocity and flow depth is needed. Based on the debris flow simulation results, the hazard level in different elements can be defined. Generally, the hazard level is considered to be high if persons are in danger both indoor and outdoor, and the buildings are in danger of being destroyed; and is recognized to be medium if persons are in danger outdoor (Jakob and Hungr, 2005). A hazard index is used in FLO-2D to describe the hazard level. The intensity is considered to be high if the maximum depth (h) is greater than 1 m, or the product of the maximum velocity (V) and the maximum depth (h) is greater than 1 m2/s; while
Figure 9. Hazard map. 0.00 5.00 10.00 15.00 20.00 25.00 0 0.1 0.2 0.3 0.4 0.5 Disch arg e (m 3/s) Time (h)
the intensity is medium if h is between 1 m and 0.2 m, or the value of Vxh is between 1 m2
/s and 0.2 m2/s. The hazard map is shown in Figure 9.
6. Conclusions
In this paper, a debris flow scenario under an extreme condition is simulated via FLO-2D, and the hazard intensity is characterized by both maximum flow depth and maximum flow velocity. A debris flow hazard map is obtained. The following conclusions can be drawn:
(1) The travel distance of the severe debris flow may reach 1.2 km. The buildings along the debris flow path may be affected if a severe debris flow occurs, especially for those that face the slopes or on the slopes.
(2) The maximum velocity of the ravine (7.3 m/s) seems to be critical as a result of steep slopes in the study area.
It should be noted that the volume of the debris flow in this study is assumed to represent an extreme value. The calculated impact area and hazard level, therefore, may be overestimated.
Acknowledgement
The terrain data provided by Geotechnical Engineering Office of the Civil Engineering and Development Department (CEDD) is gratefully acknowledged. The views expressed in this paper do not represent those of the CEDD.
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