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Morphometric Analysis of Debris Flows and Their Source Areas Using

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Morphometric Analysis of Debris Flows and Their Source Areas Using Geomorphology 129 (2011) 387–397 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph Morphometric analysis of debris flows and their source areas using GIS Chien-Yuan Chen a,⁎, Fan-Chieh Yu b a Department of Civil and Water Resources Eng., National Chiayi University, Chiayi City 600, Taiwan b Department of Soil and Water Conservation, National Chung Hsing University, Taichung 402, Taiwan article info abstract Article history: Important factors for the initiation of debris flows include available loose sediment, torrential rainfall, and Received 21 February 2010 topographic conditions. The objective of this study is to identify topographic features of debris flows and Received in revised form 24 February 2011 conditions favorable for debris-flow initiation based on geomorphological analyses of 11 river basins in Accepted 6 March 2011 northern and central Taiwan. Morphometric indices were derived from 10-m grid digital terrain models Available online 11 March 2011 before and after debris flow events using GIS. The indices include the stream power index (SPI), topographic wetness index (TWI), sediment transport capacity index, elevation–relief ratio, form factor, effective basin Keywords: fl Debris flow area, and slope gradient. The results show that debris ows tend to initiate from steep slopes or landslides Terrain analysis with higher TWI values. Debris flows are expected in basins with higher SPI and TWI. Basins with lower slope Digital terrain data gradients and SPI but higher TWI may also have a high potential for debris flow. SPI changes most significantly GIS due to a debris flow event particularly in steep basins. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Front Range, Colorado. Some other studies have also related the initiation of debris flows to the slope of source areas, with typical Taiwan is characterized by frequent rainfall-induced mass move- values between 27° and 38° (Takahashi, 1981; Hungr et al., 1984; ments. There are 1420 debris flow prone creeks in Taiwan (COA, Rickenmann and Zimmermann, 1993). A channel gradient greater 2005), categorized into three groups with high, medium, and low than 25° is also necessary for debris flow initiation and it decreases potential for debris flows based on topographic and geologic with an increasing catchment area (Van Dine, 1996). Millard (1999) conditions (Lin P.S. et al., 2002, 2006). In the catchments of these indicates that debris flows from channel sidewalls tend to be larger creeks, 685 landslide-induced debris flows occurred between 2001 and occur on steeper slopes than those from headwalls. Although this and 2004. Recent climate change with increased stormy precipitation inference agrees with the concept of sediment transport limit has increased the frequency of massive debris flows and landslides in (Marshall et al., 1996), it was based on data for a coastal environment Taiwanese mountains (Chen et al., 2008). and may have only limited applicability to mountains. Debris flows were divided into three categories by the type of This study analyzes Digital Elevation Models (DEMs) before and initiation: shallow landsliding, rilling, and the “firehose effect” in after debris flow events for 11 mountainous river basins in Taiwan, to alpine landscapes (Godt and Coe, 2007). The firehose effect is caused discuss topographic changes, debris-flow magnitudes (volume and by debris masses washed away by a concentrated flow of water in an runout) and controlling variables of debris flows. The results may help alpine landscape (Johnson and Rodine, 1984; Godt and Coe, 2007). to evaluate debris-flow potentials for disaster prevention. Numerous studies investigated relationships between drainage-basin topography and debris flows. Wichmann et al. (2007) modeled debris-flow initiation locations in relation to channel gradient, 2. Topographic indices related to debris flow susceptibility discharge and sediment contributing area using GIS. Topographic form controls the location of the head of a debris channel Numerous topographic indices have been proposed to represent (Montgomery and Dietrich, 1994a; Vandaele et al., 1996) and a the geomorphological characteristics of a river basin. Of these, we threshold relation exists between slope angle and the contributing used those relevant to debris flow susceptibility. The sediment area (Dietrich et al., 1992; Montgomery and Dietrich, 1994b). Godt transport capacity index (LSRUSLE; Moore and Burch, 1986) is based and Coe (2007) show that slope angles N32° and upslope contributing on the unit stream power theory (Moore and Wilson, 1992) and is areas b3000 m2 are favorable for debris flow initiation for the central equivalent to the length-slope factor of the RUSLE in certain circumstances. It is a function of local slope and contributing area: ⁎ Corresponding author at: Room A05B-401, No. 300, Syuefu Rd., Chiayi City 60004, Taiwan. Tel.: +886 5 2717686; fax: +886 5 2717693. ðÞðÞ= : mðÞβ= : n ð Þ E-mail address: [email protected] (C.-Y. Chen). LSRUSLE =m+1 A 22 13 sin 0 0896 1 0169-555X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2011.03.002 388 C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397 where A is the upslope contributing area (m2), β is the slope gradient SPI is used to describe potential channel erosion and landscape (in degrees), and m and n are constants (=0.4 and 1.3, respectively). processes (Moore et al., 1991). The elevation–relief ratio (E)isdefined as (Pike and Wilson, Another topographic index related to As and for estimating 1971): transportation capacity is the terrain characterization index (TCI; Park et al., 2001): E=ðÞ mean elevation–min elevation =ðÞmax elevation–min elevation κ ð Þ ð2Þ TCI = ln As 6 where κ is the total topographic curvature (in m m− 1). E is equivalent to the hypsometric integral, HI (Willgoose and The form factor (or shape factor) is a measure of relative width and Hancock, 1998; Awasthi et al., 2002; Bishop et al., 2002), and can be area of the basin. It is defined as: easily calculated within the GIS environment (Singh et al., 2008). Stream power per unit length of channel (Ω; Bagnold, 1966)is = = 2 ð Þ defined as: F = W Lo = A Lo 7 Ω = γQS ð3Þ where Lo is the length of the river (in m), W is the average width of the basin (in m; W=A/Lo), and A is the area of the basin (in m2). F is where γ is the specificweightofwater,Q isthewaterdischarge(m3 s−1), related to the peak flow rate (Wohl and Pearthree, 1991) and debris and S is the slope of water surface (m m−1), which can be approximated flow occurrence (Wan et al., 2008). by the slope of the channel bed, tanβ. It expresses river flow strength The topographic wetness index (TWI), which has been used to (Worthy, 2005) or converted potential energy (Knighton, 1999). describe the spatial soil moisture patterns (Kirkby, 1975; Beven and If Q is proportional to specific catchment area As (the upslope Kirkby, 1979; Wilson and Gallant, 2000), is defined as: contributing area per unit contour length; Moore et al., 1991), the ðÞ= β ðÞ relative stream power (RSP) is calculated as (Lindsay, 2005): TWI =lnAs tan 8 RSP = Astanβ ð4Þ TWI is useful for landslide susceptibility studies (Conoscenti et al., 2008; Gorum et al., 2008, Nefeslioglu et al., 2008). TWI can be used to The logarithm of RSP is called the stream power index (SPI; Wilson assess the spatial pattern of potential soil moisture and changes in soil and Gallant, 2000): texture due to erosion (Schmidt and Persson, 2003; Grabs et al., 2007). It is commonly used to quantify topographic control on hydrological processes (Sørensen et al., 2006), and higher TWI values SPI =lnðÞAstanβ ð5Þ are commonly found in landslide bodies (Nefeslioglu et al., 2008). A Chonghe Creek Houtong Creek Nanpingken Creek Junkeng Creek N Erpu Creek Sanpu Creek W E S 0 10 Kilometers Fengqiu Creek Taipei County No. 21 Expressway Nantou County Hoshe No.1 Creek Hoshe No.3 Creek N W E Songhe Creek Taiwan N S W E 0 10 Kilometers S 0 10 Kilometers Chushui Creek Chenyoulan Basin Taichung County Fig. 1. Locations of the 11 study basins/creeks in the Chenyoulan catchment (Nantou County), Taipei County, and Taichung County. C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397 389 Table 1 Debris flow database. After COA, 2005. Basin Initiation events and years Descriptiona References Chonghe Typhoon Lynn (1987); Xiangsane (2000) Buildings downstream and three bridges destroyed by debris flow Chen et al. (2006) during Typhoon Xiangsane. Chushui Typhoon Herb (1996); torrential rains (1998, Bridges destroyed during torrential rains and Typhoon Toraji. Yu et al. (2006), Chen et al. 1999); Toraji (2001) (2007b) Erbu Typhoon Herb (1996); Toraji (2001) Eight residents dead, 14 houses collapsed, and three houses affected by Yu et al. (2006) debris masses during Typhoon Herb. One resident missing, 14 houses collapsed, and seven houses buried by debris masses during Typhoon Toraji. Fengqiu Typhoon Nilson (1985); Herb (1996); Otto Ten houses collapsed, 11 houses affected by debris masses, and two residents Yu et al. (2006) (1998); Toraji (2001) dead during Typhoon Herb. Hoshe No. 1 Typhoon Herb (1996); Toraji (2001) Bridge destroyed and parts of a primary school in downstream inundated. Chen and Su (2001); Yu et al. (2006) Hoshe No. 3 Typhoon Herb (1996); Toraji (2001) Bridge destroyed. Chen and Su (2001); Yu et al. (2006) Houtong Typhoon Xiangsane (2000) Over 20 houses buried by debris masses, seven residents buried, one missing, Chen et al. (2004) and bridge destroyed. Junkeng Typhoon Herb (1996); Toraji (2001) Debris masses impacted residents and killed 5 people during Typhoon Herb. Yu et al. (2006) Nanpingkeng Typhoon Herb (1996); Toraji (2001) A house destroyed during Typhoon Herb.
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