Morphometric Analysis of Debris Flows and Their Source Areas Using

Geomorphology 129 (2011) 387–397

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Geomorphology

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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. Yu et al. (2006) Shanbu Typhoon Herb (1996); Toraji (2001) 17 residents dead, 26 houses collapsed, and 29 houses buried by debris masses Yu et al. (2006) during Typhoon Toraji. Songhe No. 1 Torrential rains (2000); Typhoon Mindulle One resident dead, one injured, and 30 houses buried by debris masses during Chen et al. (2007c) (2004) Typhoon Mindulle.

a Source: SWCB, http://www.swcb.gov.tw. large TWI value indicates a lower slope gradient and/or large m grid intervals were available. Eight basins (Nanpingkeng, Junkeng, catchment area in the valley floor area. Further, a large TWI value Erbu, Shanbu, Chushui, Fengqiu, Hoshe No. 1, and Hoshe No. 3) belong may also feature high slope gradient and very small catchment area to tributaries of the Chenyoulan River in Nantou County in central near the drainage divide (Conoscenti et al., 2008). Taiwan (Fig. 1), and their geomorphological characteristics and past The relief ratio (R), which has been used to describe debris flow debris flows have been well documented (Lin P.S. et al., 2002, 2006; travel distance and event magnitude (Corominas, 1996), is defined as: Chen et al., 2005; Lin et al., 2005; Lin G.F. et al., 2006; Yu et al., 2006; Wan et al., 2008). The basins of the Houtong and Chonghe creeks in R = H = D ð9Þ Taipei County in northern Taiwan and the Songhe creek in Taichung County were also investigated. These basins have high potentials of where H is the elevation difference between the starting point and the debris-flow hazards. Most of the debris-flow source areas of the basins lowest point of deposition of debris flow, while D is the corresponding were associated with shallow landsliding. Field investigation verified horizontal distance. The parameter has been used to describe the that both headwall and sidewall landslides provided large debris debris flow mobility from a landslide head scarp to a debris deposition masses. front (Bathurst et al., 1997; Iverson et al., 1998; Rickenmann, 1999; The basins have experienced landslide-induced debris flows, and Toyos et al., 2007). in the Houtong and Chonghe basins, failure of landslide dams took

The effective basin area (A15;%)isdefined as the area of slope over place. Most landslides were triggered during typhoons Herb (with a 15° in a drainage basin divided by the basin area. The parameter rainfall of 1890 mm, 31 July to 1 August 1996), Xiangsane (964 mm, defines topographic steepness of a basin (Lin G.F. et al., 2006; Chen et 31 October to 1 November 2000), Toraji (757 mm, 30 to 31 July 2001), al., 2007a). and Mindulle (2200 mm, 30 June to 4 July 2004) (Table 1). The average annual rainfall for the whole of Taiwan between 1949 3. The study area and 2007 was 2493 mm, with 2932 mm in northern Taiwan and 2144 mm in central Taiwan (Water Resources Agency, 2009). The River basins in Taiwan with documented past debris flow events basic geomorphological and geological properties of the 11 basins are were selected for the study. For all basins DEMs with both 10- and 40- listed in Table 2, and Fig. 2 shows their field photographs. The major

Table 2

Terrain indices and geology of the studied basins in relation to debris ﬂows. A15: effective basin area.

Basin Length Basin area A15 Geology Relief (m) (km2) (%) Mean (m) Max. (m) Mean slope (%)

Chonghe 1950 1.24 54.8 Andesite/andesitic pyroclastics 463 1045 26.6 Chushui 5755 8.63 60.7 Sandstone, shale 2109 2779 37.2 Erbu 1871 1.58 60.8 Quartzite, slate, coaly shale 912 1530 31.7 Fengqiu 1929 1.62 100 Quartzite, slate, coaly shale 1477 2078 39.1 Hoshe No. 1 3770 3.11 51.8 Sandstone, shale 1321 1840 33.5 Hoshe No. 3 1509 1.63 100 Sandstone, shale 1518 2123 40.8 Houtong 1541 1.91 90.6 Sandstone, shale/andesite 397 659 31.2 Junkeng 1572 0.51 100 Quartzite, slate, coaly shale 764 1184 29.7 Nanpingkeng 1684 1.48 73.0 Quartzite, slate, coaly shale 780 1204 28.8 Shanbu 2442 3.34 44.0 Argillite, slate, phyllite/quartzite, 951 1553 28.2 slate, coaly shale Songhe No. 1 1339 3.71 91.4 Quartzite, slate, coaly shale 1425 2385 40.4 390 C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397

Fig. 2. Photos of the studied debris ﬂow channels/gullies. A) Songhe No. 1 creek. B) Chushui creek. C) Chonghe creek. D) Junkeng creek. E) Erbu creek. F) Houtong creek. G) Hoshe No. 1 creek. H) Nanpingkeng creek. I) Fengqiu creek. J) Shanbu creek. K) Hoshe No. 3 creek. geology types are sandstone, shale, quartzite, slate, and coaly shale in 4. Methods the eight basins in the Chenyoulan catchment; andesite and andesitic pyroclastics in the Chonghe catchment; and quartzite, slate, and coaly Topographic indices are often evaluated using GIS (e.g. De Roo, shale in the Songhe basin. In the Chenyoulan basin 92% of the land is 1998) and the increasing DEM availability and computational capacity forest, 4% farms, and only 0.37% residential areas. Similarly, over 90% of PCs allow the rapid topographical analysis for large catchments of the land in the Chonghe, Houtong, and Songhe basins is used for (Claessens et al., 2005). One important factor for DEM analysis is the forests and farms. The Chushui basin has the highest relief of 2779 m, inﬂuence of DEM resolution (or grid interval) on the result. In general, while the Houtong has the lowest of 659 m. the use of coarser DEMs leads to higher TWI values (Saulnier et al., C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397 391

Fig. 3. DEMs before debris ﬂow events and erosion/deposition areas due to debris ﬂows in six basins.

1997; Wolock and McCabe, 2000; Wu et al., 2008). The choice of DEM The major input data for our topographic analysis are 10-m resolution depends on data availability and the type of analysis resolution grid DEMs before and after a debris ﬂow event. The DEMs (Claessens et al., 2005). for the Chenyoulan catchment (Fig. 3) were provided by the Council of

Fig. 4. Interpretations of the landslide head scarp, debris flow source area, flow path, deposition area, and initiation point based on DEMs before and after the debris flow due to Typhoon Herb in the Junkeng basin. 392 C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397

Table 3

Basin topographic features analyzed. LSRUSLE: sediment transport capacity index, TWI: topographic wetness index, SPI: stream power index, E: elevation–relief ratio, F: form factor, HI: hypsometric integral, R: relief ratio, V: volume of landslide source area, VC: debris volume delivered to the channel.

3 3 Basin LSRUSLE TWI (source/basin) (%) SPI E F Hi (%) RV(m ) VC (m ) Chonghe 14.7 5.29/5.4 3.86 0.37 0.33 35.8 0.21 57,481 57,481 Chushui 19.0 5.4/5.36 4.17 0.52 0.26 52.6 0.29 1,505,612 1,351,741 Erbu 18.0 4.9/5.17 4.08 0.40 0.45 39.7 0.27 121,400 108,400 Fengqiu 20.4 4.75/4.86 4.40 0.58 0.44 56.6 0.44 73,711 65,494 Hoshe No. 1 18.5 4.9/5.08 4.18 0.46 0.22 45.0 0.33 47,703 39,590 Hoshe No. 3 19.7 4.8/4.8 4.46 0.47 0.72 47.2 0.26 606,354 464,258 Houtong 17.2 5.12/5.18 4.08 0.54 0.80 53.3 0.26 218,000 201,700 Junkeng 17.5 5.21/5.26 4.01 0.48 0.21 49.1 0.35 6947 4526 Nanpingkeng 17.1 5.1/5.29 3.97 0.50 0.52 47.4 0.32 104,100 102,900 Shanbu 15.6 5.0/5.35 3.91 0.41 0.56 40.8 0.20 311,165 292,706 Songhe No. 1 17.6 4.75/4.8 4.46 0.45 2.07 45.1 0.25 522,000 408,000

5.4 Chushui Chonghe are two major types of two-dimensional algorithms: single (e.g. ArcGIS 9) and multiple (e.g. Tarboton, 1997; Qin et al., 2007) flow directions. The Geta GIS implemented the commonly used single flow Junkeng direction (D8) algorithm. The sedimentary transport capacity index 5.2 LSRUSLE was calculated by SATEEC (Sediment Assessment Tool for Houtong Effective Erosion Control; Lim et al., 2005). TWI The horizontal runout distance of each debris flow on a fan was Nanpingkeng fl 2 measured in ArcGIS along the debris ow path, from the curvilinear Y = -0.04X + 6.34, r = 0.95 fl 5 Sanpu feature at the debris ow initiation point to the deposition front identified by field surveys, aerial photo interpretation, and changes in Erpu DEM elevation before and after the debris flow. Fig. 4 shows the debris Hoshe No.1 flow source area, flow path, and depositional area along the Junkeng

Mean source-area Hoshe No.3 creek. The landslides were mapped based on the elevation change 4.8 Fengqiu Songhe No.1 computed from the DEMs. The volumes of erosion and deposition were also estimated from the DEMs. The terrain indices are used for the debris ﬂow source area, depositional area, and the whole drainage basin. We obtained the

following terrain indices: TWI, mean slope (SR) and volume (V) of the 4.6 landslide source area; slope (S ), A, A , R, E, F, LS , SPI, and TWI for 28 32 36 40 44 48 B 15 RUSLE the drainage basin; slope (SD), area (AD), volume (VD) and runout Mean source-area slope, SR (%) distance on the fan (LF) for the depositional area. We analyzed their relationships by regression analysis. The effect of DEM resolution was Fig. 5. Mean slope and mean topographic wetness index TWI for debris ﬂow source areas. also discussed using parameter values from both the 10- and 40-m DEMs.

Agriculture (http://www.afasi.gov.tw/) using analytical photogram- 5. Results metry applied to 1/5000 aerial photos taken in 1993 and 1996, before fl and after debris ows induced by Typhoon Herb. The DEMs have an Table 3 lists the obtained values of LSRUSLE, TWI, SPI, E, F, Hi, R, V, RMSE (Root Mean Square Error) of about 1.0 m (COA, 2005). The and debris volume delivered to the channel for each basin (VC). V DEMs for the other basins were also made using analytical and VC were far beyond the minimum criteria of debris flow photogrammetry. The aerial photos of the Houtong and Chonghe initiation proposed by Millard (1999) for a coastal environment. The basins were taken in 1994 and 2002, while those of the Songhe basin upslope contributing areas are all above 3000 m2 and thus were taken in 2003 and 2004. Flow direction and accumulation were preferentially susceptible to debris flows according to Godt and analyzed, and catchments and stream networks were extracted using Coe (2007). In the following, the topographical features of the the ArcGIS ArcHydro 1.2 extension. The threshold for determining the debris-flow source area, drainage basin, and depositional area are location of a channel head was assumed to be 1000 cells. Then the described separately. basin area above the debris flow initiation point was computed. Slope angles were also computed from the DEMs using ArcGIS 9. Debris 5.1. Topographic features of source areas flows were categorized into three types: those from landslides on headwalls, those from landslides on channel sidewalls, and those from The slope of the source area is between 30° and 42°, slightly higher both headwalls and sidewalls. than typical values for debris-flow source areas (27° to 38°; Takahashi, TWI and SPI were calculated using Geta (Grid-based program for 1981; Hungr et al., 1984; Rickenmann and Zimmermann, 1993). The Estimating Terrain Attributes) GIS (SINICA, 2003). The values of TWI debris flows were initiated from landslides having a steeper slope or and SPI are influenced by the algorithms for calculating As and tanβ, large TWI, especially for the Chushui basin (Fig. 5). The potential slope fl and computation of As depends on the ow direction algorithm. There instability area can be estimated using the relationship between SR

Fig. 6. Relationships of debris volume delivered to the channel (VC) versus topographic indices: A) effective basin area A15, B) elevation–relief ratio E, C) relief ratio R, D) sediment transport capacity index LSRUSLE, and E) stream power index SPI. C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397 393

10000000 10000000

) A ) B 3 3 (m (m

C Chushui C Chushui V V 1000000 1000000 Hoshe No.3 Songhe No.1 Songhe No.1 Hoshe No.3 Sanpu Sanpu Houtong Houtong Erpu Erpu Nanpingkeng Nanpingkeng 100000 100000 Chonghe Fengqiu Chonghe Fengqiu Hoshe No.1 Hoshe No.1

10000 10000 Junkeng Junkeng Debris volume delivered to the channel, Debris volume delivered to the channel, 1000 1000 100000 1000000 10000000 0.35 0.4 0.45 0.5 0.55 0.6 2 Effective basin area, A15 (m ) Elevation-reliefratio, E 10000000 10000000 ) ) 3

3 C D (m (m C C Chushui

V Chushui V 1000000 1000000 Songhe No.1 Hoshe No.3 Hoshe No.3 Songhe No. 1 Sanpu Sanpu Houtong Houtong

Erpu Nanpingkeng NanpingkengErpu 100000 100000 Chonghe Chonghe Fengqiu Hoshe No.1 Fengqiu Hoshe No.1

10000 10000 Junkeng Junkeng Debris volume delivered to the channel, Debris volume delivered to the channel,

1000 1000 0.15 0.2 0.25 0.3 0.35 0.4 0.45 14 16 18 20 22 Relief ratio, R Sediment transport capacity index, LSRUSLE 10000000 )

3 E (m

C Chushui V 1000000 Songhe No.1 Sanpu Houtong Hoshe No.3 Nanpingkeng 100000 Erpu

Chonghe Fengqiu Hoshe No.1

10000

Junkeng Debris volume delivered to the channel,

1000 3.8 4 4.2 4.4 4.6 Stream power index, SPI 394 C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397

Table 4 2000 ﬂ Debris ow volume VD, deposition area AD, runout distance on the fan LF and total Sanpu Chushui runout distance L. A Partly after Yu et al., 2006. Songhe No.1 3 2 Basin VD (m ) AD (m ) LF (m) L (m)

(m) 1000

Chonghe 5944 7876 256 1280 F 900 Chushui 376,200 115,800 1487 4741 L 800 Fengqiu Erpu 31,500 17,722 313 2929 700 Hoshe No.3 Fengqiu 310,000 91,200 690 1606 600 Hoshe No.1 42,000 10,800 330 2102 500 Houtong Hoshe No.3 54,700 16,500 699 4507 Houtong Parts eroded out by trunk channel 34,934 457 1456 400 Hoshe No.1 Junkeng 46,600 21,900 148 1459 Erpu Nanpingken 16,410 12,536 197 1661 300 Sanpu 61,500 36,000 1544 4164 Chonghe Songhe No. 1 198,570 149,615 1164 2931 200 Nanpingken Runout distance on the fan, Junkeng 0.45 2 and mean TWI of the source area except for the data of the Chushui Y =2.26X , r = 0.69 basin:

100 − : : ; 2 : ð Þ TWI = 0 04SR +634 r =095 10 1000 10000 100000 1000000 10000000 Landslide volume,V (m3) 5.2. Topographic features of basins 160000 B Songhe No.1 The topographic features of the basins, i.e., SB, A, A15, R, E, F, LSRUSLE, TWI and SPI before the debris ﬂow were determined. A15, R, E, LSRUSLE, – and SPI were plotted against VC (Fig. 6). The basins have E of 0.35 0.6, Chushui

) 120000

R of 0.15–0.45, LSRUSLE of 14–21, and SPI of 3.8–4.5. Higher values of 2

A15, E, LSRUSLE,orSPI correspond to larger VC. Higher values of R (m ﬂ D correspond to lower VC that may re ect more depositional volume on A 2 Fengqiu the fan. The minimum A15 is 0.5 km (Junkeng basin), and VC clearly increases with increasing A15. 80000

5.3. Topographic features of depositional areas Y = 26.3X - 6367, r2 = 0.76

Table 4 lists VD, AD, LF and total runout distance (L) extracted from Depositional area, 40000 Sanpu DEMs and aerial photos. LF (m) is approximately proportional to V 3 Nanpingken (m )(Fig. 7A), Junkeng Erpu Hoshe No.3 : 0:45; 2 : ð Þ LF =226V r =069 11 Hoshe No.1 Chonghe 2/3 2 0 The index VD provides a good approximation of AD (m )(Fig. 7B), 0 1000 2000 3000 4000 5000 6000 2/3 3 VD (m ) : 2 = 3– ; ; 2 : ð Þ AD =263VD 6 367 r =076 12 10000 C The Houtong basin was excluded from the following analysis because a significant portion of the debris was eroded away by the trunk stream (Table 4). The Songhe No. 1 basin shows a farther deviation from the regression line that may reflect higher SPI and F values (Table 3). (m) L Chushui Hoshe No.3 5.4. Topographic indices changes due to debris flows Sanpu

The topographic indices of A15, R, E, F, and LSRUSLE did not change significantly in response to debris flows. TWI is inversely proportional Erpu Songhe No. 1 to SB before debris flows except for the Chushui basin (Fig. 8A):

2 Hoshe No.1

− : : ; : ð Þ Total runout distance, TWI = 0 037SB +637 r =070 13 Nanpingkeng ﬂ Fengqiu The results indicate that debris ows can be initiated under both a Junkeng lower S with a higher TWI and a higher S with a lower TWI. Fig. 8A B B Chonghe shows conditions both before and after the debris ﬂows. There was no 1000 1000 10000 100000 1000000

ﬂ 3 Fig. 7. Characteristics of debris ow deposition areas: A) runout distance on the fan LF Debris flow volume,VD (m ) and landslide volume V in the basin. B) Debris ﬂow volume VD and depositional area AD.

C) Debris ﬂow volume VD and total runout distance L. C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397 395

5.5 10 A Lower gradient, higher TWI, A Chonghe but lower SPI in Fig.8B High potential Medium potential 5.4 Chushui A025 Sanpu Low potential low gradient, high TWI, Non-debris flow Nanpingkeng 5.3 but low SPI in Fig. 9B TWI TWI Nanpingkeng Junkeng A021 9 Junkeng A016 5.2 Houtong A024 A023 A001 Houtong Erpu

Erpu Heshe No.3 5.1 001 Heshe No.1 005 2 5 Y = -0.037X + 6.37, r = 0.70 8 Chushui A012 4.9 Topographic wetness index, Topographic wetness index, before debris flow Fengqiu 007 Songhe No.1 high gradient, low TWI 4.8 after debris flow Songhe No.1 Heshe No.3 004 4.7 7 20 25 30 35 40 45 10 20 30 40 Mean basin gradient (%) Mean basin gradient (%)

6 10 B Songhe No.1 B before debris flow High potential after debris flow Fengqiu Medium potential 5.5 Low potential high gradient, high SPI Chushui A001 Non-debris flow 005 A016 001 2 2 SPI 8 SPI Y = -1.34 + 0.27X -0.0025X , r = 0.75 5 increased SPI after debris flow Heshe No.1 Erpu Heshe No.3 Houtong Songhe No.1 A023 4.5 Nanpingkeng Fengqiu Sanpu Heshe No.3 6

Heshe No.1 Stream power index, A024

Stream power index, Houtong A021 Chonghe Junkeng Chushui high gradient, low gradient, low SPI 4 Erpu Junkeng high SPI Nanpingkeng A025 Sanpu 2 Chonghe Y = 0.04X + 2.79, r = 0.95 4 3.5 0 10203040 20 25 30 35 40 45 Mean basin gradient (%) Mean basin gradient (%) Fig. 9. Topographic indices for 43 debris flow basins and nine non-debris flow basins in Fig. 8. Changes in the topographic indices due to debris flows. A) Mean basin gradient Chiayi County, Taiwan. A) Mean basin gradient versus the topographic wetness index versus the topographic wetness index TWI. B) Mean basin gradient versus the stream TWI. B) Mean basin gradient versus the stream power index SPI. power index SPI.

than the general trend. Underground water flow combined with the obvious change in TWI due to a debris flow except for the Hoshe No.3 steep streambed slope was probably responsible for the debris flow and Chushui basins. (Chen et al., 2007b). The Songhe No. 1 basin experienced a marked SPI increased markedly due to debris flows and is proportional to topographic change due to the debris flow, and its SPI value was the mean basin gradient (Fig. 8B): higher than that of the other basins; whereas, the Junkeng basin showed smaller changes. Fig. 8B shows that basins with higher : : ; 2 : ðÞðÞ gradients tend to have more increases in SPI in response to debris SPI =004SB +279 r =095 before debris flow 14 flow. This indicates that flow in steeper basins tends to have higher erosive power to rapidly change landforms. − : : – : 2; 2 : ðÞðÞ SPI = 1 34 + 0 27SB 0 0025SB r =075 after debris flow 15 In the Songhe No. 1 basin the depositional area was wider than that of the other debris flows with a similar volume (Fig. 7), probably due to the greater amount of rainfall from Typhoon Mindulle (2200 mm) 6. Discussion than Typhoons Herb (1890 mm) and Toraji (757 mm). Therefore, rainfall conditions also affect topographic changes due to debris flows.

TWI decreases but SPI increase with increasing SB (Figs. 8 and 9). TWI and SPI show a higher statistics correlation to the potential of This result is consistent with the original deﬁnition of these debris ﬂows than the other morphometric indicators (A15, E, R,andLSRUSLE parameters in Eqs. (5) and (8). An exception is the Chushui basin in Fig. 6). In review of literatures, SPI is an indicator of sedimentary

(Fig. 8A), where the debris ﬂow was initiated under higher TWI and SB transport capacity and TWI is an alternative indicator for landslide 396 C.-Y. Chen, F.-C. Yu / Geomorphology 129 (2011) 387–397

Table 5 basins and nine non-debris flow basins in Chiayi County, Taiwan List of 43 debris flow basins in Chiayi County, Taiwan. SPI: stream power index. H: high, (Table 5, partly sourced from Chen et al., 2007c) validates the inferred M: medium, L: low. effects of DEM resolution. The indices were derived from a 40-m DEM, Creek Damage SPI Mean basin slope (%) and the correlation holds for this coarse DEM resolution (Fig. 9), 001 M 8.25 32.54 showing that lower basin gradient and SPI as well as higher TWI has high 002 M 7.09 32.18 debris flow potential (No. A021, A023, A024, and A025 in Fig. 9). 003 M 7.16 32.45 004 M 7.20 36.28 005 M 8.36 34.87 7. Conclusions 007 M 7.23 38.12 008 L 7.12 31.07 Topographic features of debris flow deposits and source areas 010 M 7.65 21.03 including basin area, runout distance, and depositional area were studied 011 M 7.32 33.01 012 M 7.24 27.78 for 11 river basins in Taiwan. The features were derived from 10-m DEMs 013 H 6.67 27.45 before and after debris flow events using GIS. The debris flows initiated 014 M 6.36 21.98 from steep source areas or landslides with higher TWI values. Among the 015 H 7.02 25.59 selected topographic indices, SPI most clearly changed due to a debris A001 M 8.37 30.58 flow. The SPI and TWI indices can be used for identifying the topographic A002 L 7.76 28.51 A003 M 7.16 28.3 characteristics of debris flows. Steep basins with high SPI and TWI are A004 L 7.84 31.38 expected to have high debris flow potential. Lower basin gradients, lower A005 M 7.73 23.3 SPI and higher TWI may also have high potential to debris flow. Coarser A006 L 7.26 24.62 DEMs may yield the same results. A007 H 7.24 27.76 A008 M 7.16 30.4 A009 H 7.08 27.78 Acknowledgments A010 M 7.26 30.82 A011 M 6.93 32.97 A012 M 7.17 35.81 The authors would like to thank the editor and anonymous A014 H 7.03 22.86 reviewers for constructive review. 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