Sedimentary Impacts from Landslides in the Tachia River Basin, Taiwan

ARTICLE IN PRESS GEOMOR-02793; No of Pages 11 Geomorphology xxx (2008) xxx–xxx

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Geomorphology

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Sedimentary impacts from landslides in the Tachia River Basin, Taiwan

Chien-Yuan Chen ⁎

National Chiayi University, Dept. of Civil & Water Resources, No. 300, Syuefu Rd., Chiayi City 60004, Taiwan article info abstract

Article history: A case study of coseismic landslides and post-seismic sedimentary impacts of landslides due to rainfall Received 6 May 2008 events was conducted in the Tachia River basin, Taichung County, central Taiwan. About 3000 coseismic Received in revised form 9 October 2008 landslides occurred in the basin during the ML 7.3 Chi-Chi earthquake in 1999. The deposits from these Accepted 20 October 2008 landslides provided material for numerous debris flows induced by subsequent rainfall events. The estimated Available online xxxx 4.1×107 m3 of landslide debris produced in the upland area caused sediment deposition in riverbeds, and flash floods inundated downstream areas with sediment during torrential rains. The landslide frequency-size Keywords: distributions for the coseismic landslides and the subsequent rainfall-induced landslides were analyzed to Landslides Sedimentary deposits determine the sediment budgets of the post-seismic geomorphic response in the landslide-dominated basin. Frequency-size distribution Both the coseismic and the rainfall-induced landslides show a power–law frequency-size distribution with a Power–law rollover. It was found that the rainfall-induced landslide magnitude was smaller than the coseismic one, and that both have comparable negative scaling exponents in cumulative form, of about −2.0 for larger landslides (>10−2 km2). This may be attributed to ongoing movement or reactivation of old landslides, and a natural stabilisation of small landslides between 10− 4 and 10− 2 km2. It is proposed that the characteristics of geological formations and rainfall as well as changes in landslide area are reflected in the power–law distribution. © 2008 Elsevier B.V. All rights reserved.

1. Introduction of landslide debris is a function of the transport capacity of the stream at thesiteofblockageinducedbylargelandslides(e.g.Korup, 2005a,b).

Following the ML 7.3 Chi-Chi earthquake in 1999 in Taiwan, the Landslides can be modeled by a power–law distribution for frequency sudden occurrence of thousands of coseismic landslides became a versus magnitude (Stark and Hovius, 2001; Guzzetti et al., 2002; Chen et al., substantial concern to national and local authorities. The area studied 2007). Van den Eeckhaut et al. (2007) reviewed existing studies, and found in this research, the Tachia River basin in central Taiwan, suffered an average scaling exponent α of −2.3 for the non-cumulative landslide numerous coseismic landslides as well as debris flows and flash floods frequency-size distribution. Chen et al. (2007) demonstrated the existence due to subsequent rainfall events (Ku et al., 2006; Chiou et al., 2007). of a power–law in the landslide frequency-size distribution for Chushui The mass wasting by rainfall also induced new and further extension of Creek in Taiwan. They also speculated that the landslide frequency-size landslides, riverbank erosion, and massive floodplain and in-channel curve could be useful to reveal the critical state of the watershed. aggradation. The flash floods caused inundation in downstream areas In this study, the sedimentary impacts from landslides following the and blockage of connecting roads (NCDR, 2004). Major post-earth- rainfall events were examined to quantify the post-seismic sedimentary quake pulses include those resulting from the impact of Typhoon Toraji characteristics of the Tachia River basin. The frequency-size distribution in 2001 (Cheng et al., 2005), Typhoon Mindulle in 2004 (Chen and for the post-seismic landslides was investigated. Because landslides are Petley, 2005), and typhoons Aere and Haitang in 2004 and 2005 (Chen the major sediment supply mechanism in the study area, the power–law et al., 2008). Table 1 lists the sequence of major rainfall events that have characteristic of the frequency-size distribution was examined to better triggered landslides in the basin since the 1999 Chi-Chi earthquake. understand the potential control of rainfall characteristics and rock Hovius et al. (2000) studied the characteristics of landslide sediment types on sediment supply. production and delivery to the channel network in the Central Range of easternTaiwan. Long-term (>25 yr) monitoring has shown that the rivers 2. Site location do not transport significant amounts of sediment unless the sediment is provided by hillslope mass wasting in the catchments. Also, the removal The Tachia River is located in Taichung County in central Taiwan. It originates in the Central Mountain Range, and has a length of 140 km and a basin area of 1,336 km2. This river is one of the main water ⁎ Tel.: +886 5 2717686; fax: +886 5 2717693. resources in central Taiwan for power generation, water supply, and E-mail address: [email protected]. recreation. The five main dams for water resource usage along the river

Please cite this article as: Chen, C.-Y., Sedimentary impacts from landslides in the Tachia River Basin, Taiwan, Geomorphology (2008), doi:10.1016/j.geomorph.2008.10.009 ARTICLE IN PRESS

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Table 1 slumps and debris flows blocked the road during torrential rains. Landslide initiating events in the basin studied Fig. 3a depicts the coseismic landslides in the upper basin along the Event Rainfall characteristics (NCDR, 2005a) No. 8 expressway. Numerous regolith slides and debris flows blocked Time interval Max. cumulative and rainfall the expressway after the earthquake. Fig. 4a,b shows the debris that intensity in the basin buried a sentry post during Typhoon Toraji in 2001 and the rock Chi-Chi EQ (1999) 09/21 – avalanche that impacted a power plant during Typhoon Mindulle in Typhoon Toraji (2001) 07/28–07/31 482 mm, 95 mm h− 1 2004. Further, a bridge that connects the expressway to the down- – − 1 Typhoon Mindulle (2004) 07/01 07/05 1680 mm, 133 mm h town area was destroyed by a debris flow (Fig. 4c). Typhoon Aere (2004) 08/23–08/26 1383 mm, 76 mm h− 1 Typhoon Haitang (2005) 07/15–07/21 no information Typhoon Matsa (2005) 08/04–08/06 1243 mm, 90 mm h− 1 3.2. Techi Reservoir

The Techi Reservoir is located in the upper part of the basin, and are, from downstream to upstream, the Shigang, Maan, Tianlun, Guguan, has a catchment area of 592 km2. Operations at the reservoir ceased and Techi (Fig. 1). The Techi Reservoir, located deep in the mountain, is when Typhoon Mindulle-induced debris masses jammed the drainage the most important engineering project for water resources in the basin. tunnel (Fig. 3b,c). The elevation of the basin ranges from nearly 4000 m down to sea level. There are five townships in the basin, with Heping and Renai 3.3. Guguan area Townships located in the upper reaches of the mountainous areas. The No. 8 expressway is the main traffic artery to the upstream areas and The Guguan hot springs resort area, situated downstream of the the Techi Reservoir (Fig. 2a). Guguan Dam, was impacted by landslide debris from the upper basin, following torrential rains. As shown in Fig. 5a,b, the Guguan Power 3. Historic landsliding episodes and their impact Plant drainage tunnel was inundated by a flash flood with a deposition of 18 m during Typhoon Toraji in 2001 and up to 30 m during Typhoon There have been three major landslide episodes in the basin in Mindulle in 2004. Massive landslide debris also overwhelmed the recent years: the Chi-Chi earthquake induced shallow regolith hotel compound in the resort area. landslides, and typhoons Toraji and Mindulle in 2001 and 2004 induced landslides, debris flows, and sedimentation by flash floods. 3.4. Debris flows in the basin

The ML 7.3 Chi-Chi earthquake in 1999 triggered 2999 landslides and their total area in the basin was 31.4 km2 (SWCB, 2001, Fig. 2b). According to field investigations after Typhoon Toraji in 2001, there About 74% of the coseismic landslides occurred in regions with vertical were 73 debris-flow prone creeks in the basin (Council of Agriculture, ground motions greater than 200 gal, while about 81% occurred in 2003), primarily located between the Shihang Dam and the Guguan regions with mean horizontal peak ground accelerations (PGA-H) Dam (Fig. 2c). Typhoon Mindulle produced further 36 debris flows greater than 150 gal (Khazai and Sitar, 2004). The basin suffered over distributed throughout the seismic-induced landslide area between 250 gal of mean PGA-H. After the strong seismic energy of the the Guguan Dam and the Techi Dam (Lee et al., 2004). There were 27 earthquake loosened the soil mantle, Typhoon Toraji in 2001 caused villages located in the middle and upper areas of the basin. These 359 reactivated and 237 new landslides (SWCB, 2001). Later Typhoon villages were isolated after the rainfall-induced landslides and debris Mindulle in 2004 triggered another 907 landslides in an area of flows because some bridges were destroyed. Fig. 5c,d shows a debris 32.2 km2 between the Tianlun Dam and the Techi Dam (Chi et al., flow originating from a reactivated coseismic landslide during 2004). The landslides were mainly distributed in the central, Typhoon Mindulle in the Songher Tribal area that buried more than mountainous areas of the basin (Fig. 2a). 40 houses.

3.1. No. 8 expressway 4. Analysis of factors controlling landslide distribution and magnitude

The No. 8 expressway is a historical trunk road between western Potential controls of landslide distribution and magnitude ana- and eastern Taiwan that crosses the Central Mountain Range. The lysed here include geological conditions, rainfall distribution, and upper part of the expressway is often closed after severe landslides, riverbed topography.

Fig. 1. Dams, elevation, and the river network of the Tachia River basin.

Please cite this article as: Chen, C.-Y., Sedimentary impacts from landslides in the Tachia River Basin, Taiwan, Geomorphology (2008), doi:10.1016/j.geomorph.2008.10.009 laect hsatcea:Ce,C-. eietr mat rmlnsie nteTci ie ai,Tia,Goopooy(2008), Geomorphology Taiwan, Basin, River Tachia the in landslides from impacts Sedimentary C.-Y., Chen, as: article doi: this cite Please 10.1016/j.geomorph.2008.10.009 RIL NPRESS IN ARTICLE .Y hn/Goopooyxx(08 xxx– (2008) xxx Geomorphology / Chen C.-Y. xxx

Fig. 2. Landslides and debris flows in the Tachia River basin. (a) Landslides in the basin from 1999 to 2004. (b) Distribution of coseismic landslides and mean horizontal peak ground acceleration for the main shocks of the Chi-Chi earthquake in 1999 (PGA source Central Weather Bureau in Taiwan, http://www.cwb.gov.tw). (c) Identified debris-flow prone creeks, and Typhoon Mindulle-induced debris flows in the basin (Council of Agriculture, 2003; Chi et al., 2004). (d) Geologic formations and landslides in the basin. 3 ARTICLE IN PRESS

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Fig. 3. Landslides and debris flows in the upper Tachia basin. (a) Coseismic landslides along the No. 8 expressway (August 20, 2002). (b) The Techi Reservoir before debris flow (August 20, 2002). (c) Debris flow deposition in the Techi Reservoir after Typhoon Mindulle (July 21, 2004).

4.1. Geological conditions basin. In general, the total rainfall was mainly concentrated in the mid-basin area between the Maan Dam and the Guguan Dam, The coseismic landslides were mainly located between the Maan probably due to the highly dissected topography (Fig. 2a). In areas Dam and the Techi Dam in an area consisting of quartzite, slate, and with landslides triggered by the Chi-Chi earthquake, cumulative coal shale. There, 1621 failures occurred with a total area of 22.3 km2, rainfall during Typhoon Toraji from 29 to 31 July exceeded 450 mm comprising approximately 70% of the total landslide area (Fig. 6). The with a maximum intensity of 85 mm h− 1 (Fig. 7a). Rainfall during rest of the landslides are 1082 in number and 10.3 km2 in area, Typhoon Mindulle was much greater, with a cumulative rainfall of consisting of argillite, slate, and phyllite. Typhoon Mindulle induced up to 1600 mm from 1 to 5 July, and a maximum intensity of up to 531 landslides with a total area of 19.3 km2, underlain by quartzite, 100 mm h− 1 was recorded in the coseismic landslide area (Fig. 7b). slate, and coaly shale. Additional 376 landslides were located in a Fig. 8 shows the cumulative rainfall from Typhoon Mindulle 12.9 km2 area underlain by argillite, slate, and phyllite (Fig. 2d). In (Fig. 7b) versus landslide density. The landslide density is deﬁned other words, Typhoon Mindulle re-activated the seismic landslides as the total landslide area (including coseismic landslides and more often in the area of argillite, slate, and phyllite than the area of landslides induced by typhoons Toraji and Mindulle) divided by the quartzite, slate and coal shale. coverage area of a given range of cumulative rainfall. Fig. 8 shows that the landslide density in the basin is directly proportional to the 4.2. Rainfall distribution cumulative rainfall of 1100 to 1500 mm. The highest landslide density is 37% at about 1500 mm of cumulative rainfall. The high- Two rainfall events by Typhoon Toraji in 2001 and Typhoon intensity torrential rainfall caused new landslides, as well as reac- Mindulle in 2004 caused signiﬁcant sedimentary impacts in this tivations of past landslides.

Fig. 4. Debris ﬂows in the basin. (a) Building buried by debris during Typhoon Toraji (August 20, 2002). (b) Power plant impacted during Typhoon Mindulle (July 21, 2004). (c) Bridge destroyed by debris ﬂow during Typhoon Mindulle (August 5, 2004).

Please cite this article as: Chen, C.-Y., Sedimentary impacts from landslides in the Tachia River Basin, Taiwan, Geomorphology (2008), doi:10.1016/j.geomorph.2008.10.009 ARTICLE IN PRESS

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Fig. 5. Mid-stream sediment deposition and debris ﬂows in the Tachia River. (a) Channel aggradation at the drainage tunnel during Typhoon Toraji. (b) Channel aggradation during Typhoon Mindulle (taken by Taipower, TPC, 2006). (c) Debris ﬂow at the Songher Tribe after Typhoon Mindulle in 2004. (d) Like (c), after Typhoon Aere in 2005 (taken by the SWCB, http://www.swcb.gov.tw/).

2 4.3. Channel characteristics where, AL =area of landslide (km ), δNL/δAL =the number of landslides −2 with areas between AL and AL +δAL (km ), and p(AL)=probability About 95% (33 km2) of the coseismic landslides were originated from density function. steep slopes with a gradient of over 55% (NCDR, 2005a). In addition, based The three-parameter inverse-gamma probability distribution (Johnson on investigations of 436 field-studied landslides and 2,070 SPOT-identified and Kotz, 1970; Evans et al., 2000)usedbyMalamud et al. (2004) is landslides, about 90% of the landslides occurred on slopes with angles ρ 1 a +1 a greater than 45° (Lin et al., 2006). Topography is also an important factor pAðÞ; a; ρ; s = exp − ð3Þ L aCðÞρ A −s A −s for debris transport. Fig. 9 shows the longitudinal profile of the Tachia river. L L The channel slope is steep above the Guguan Dam, while a depression area where ρ represents the decay of large landslides in power–law form, a fl exist below the dam. This topographic characteristic caused ooding in the is a parameter related to the location of the maximum probability Guguan hotel area during the heavy rains. The channel slope drops below distribution, s represents the decay of small landslide area, and Γ(ρ)is the Guguan Dam, and the thickness of the sediment deposited between the gamma function of ρ. the Tianlun Dam and the Guguan Dam averaged 5 m with a mean annual The frequency-size distributions of the post-1999 landslides for −1 deposition of 1.7 m yr for years 2004 and 2005 (Fig. 10). Table 2 shows the basin are plotted in Fig. 11. The coseismic and Typhoon Toraji- the generated landslide area after the Chi-Chi earthquake in 1999 and after induced landslide inventories were provided by the SWCB (Soil and TyphoonAerein2004,betweentheGuguanDamandtheTechiDam Water Conservation Bureau, http://www.swcb.gov.tw), using SPOT 2 7 2 (Chiou et al., 2007). The total new landslide area is about 2.0×10 m and and SPOT 4 remote sensing images taken in 1999 and in 2001, with an the landslide volume derived from the digital terrain model (DTM) is effective mapping resolution of 6.25 m. The inventory of Typhoon 7 3 roughly 4.1 ×10 m . The mean annual sediment supply was about Mindulle-induced landslides in the basin was provided by the Central 6 3 −1 8×10 m yr from 1999 to 2004. Most of the sedimentary material Geology Survey using Formosat-2 remote sensing images (http:// fl remained on the valley oor causing in-channel aggredation above the www.nspo.org.tw/) taken in 2004 (Chi et al., 2004), with an effective Tianlun Dam. However, the riverbed was scored below the Shigang Dam mapping resolution of 2.0 m. These landslide inventories were also fl and in the oodplain further downstream. Fig. 10 shows that most of the supported by aerial photographs with a resolution of 0.25 m, and, in debris from the upland area was deposited on the riverbed. The steep part, field verification. The landslide frequency plotted against area slope, exceeding the critical channel gradient to support debris flow, provides a potential for the debris to flow downstream.

4.4. Landslide frequency-size analysis

Malamud et al. (2004) deﬁned the landslide magnitude (mL)ofan event based on the total number of landslides (NLT):

mL = logNLT ð1Þ

− 2 They also deﬁned landslide frequency density f(AL) (km )as

δNL fAðÞL = = NLTpAðÞL ð2Þ δAL Fig. 6. Coseismic and Typhoon Mindulle-induced landslides for different geologic formations.

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Fig. 7. Isohyets of cumulative rainfall and maximum rainfall intensity of (a) Typhoon Toraji in 2001 and induced landslides (July 29–31, 2001) and (b) Typhoon Mindulle in 2004 and induced landsides (July 1–5, 2004).

shows a power-law size distribution with a negative scaling exponent The magnitude of the coseismic landslides (mL)is≈3.3 for α=−1.8 for the seismic landslides, and −2.0 and −2.1 for landslides ρ =0.78, a=1.4×10− 3 km2,ands =−6×10− 5 km2. The magnitude of 2 induced by typhoons Toraji and Mindulle for AL >0.01 km : the Typhoon Mindulle- and Typhoon Toraji-induced landslides is ≈2.8 for ρ =1.3, a =1.5×10− 2 km2,ands = −1.2 × 10− 3 km2.The ÀÁ 2 lnf = −1:8lnAL +2:19 r =0:96 ; for the coseismic landslides of 1999 ð4Þ parameter values were back-analysed by trial and error from the inverse gamma function used by Malamud et al. (2004),forρ =1.4, ÀÁ − 3 2 − 4 2 2 a=1.28×10 km ,ands =−1.32 × 10 km . The transition between lnf = −2:0lnAL +1:37 r =0:97 ; for the Toraji−induced landslides of 2001 ð5Þ the two regimes referring to the inversion of the trends of the ÀÁ frequency-size distribution is known as rollover or cross-over. The − : : 2 : ; − lnf = 2 1lnAL +131 r =098 for the Mindulle induced landslides of 2004 location of the rollover was considered first for determining the ð6Þ parameter a; then the slope of large landslides for the parameter ρ, and the slope of small landslides for the parameter s to fitthe Stark and Hovius (2001) fitted a Double Pareto distribution to the landslide inventory to a power–law form. Further, the landslide landslide inventory for the Central Mountain Range in Taiwan and magnitude of Typhoon Mindulle was higher than that of Typhoon obtained the cumulative form of the negative scaling exponent β≈1.11 Toraji, because of the higher cumulative rainfall of Typhoon Mind- (about 2.1 in non-cumulative form) which is comparable to the case ulle. There is no obvious difference in frequency-size distribution herein. between the earthquake and rainfall-induced landslides for larger

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Table 2 Newly generated landslide areas and debris volumes between the Guguan Dam and the Techi Dam in the basin studied (after Chiou et al., 2007)

Stage Before After After After After Total Chi-Chi Chi-Chi Typhoon Typhoon Typhoon Earthquake Earthquake Toraji Mindulle Aere Period of SPOT 04/1999 10/1999 11/2001 07/2004 10/2004 – image acquisition New landslide – 1.6 × 107 3.0×106 3.7×106 1.9 × 106 2.5×107 area (m2) Landslide – 2.6×107 6.3×106 6.0×106 2.8×106 4.1×107 volume (m3)

landslides are associated with their surface morphology. Analysis shows that the average location of the rollover for available in- ventoriesis1.1×10− 2 ±1.8×10− 2 km2 (Guzzetti et al., 2002). There are two rollovers (Fig. 11) in the landslide frequency-size distribution at about 10− 3 and 10− 4 km2 for the coseismic landslides. The rollovers for the rainfall-induced landslides are clearer at about Fig. 8. Cumulative rainfall brought by Typhoon Mindulle and landslide density. 5×10− 3 and 4×10− 4 km2 (Fig. 11), and differ from those for the coseismic landslides. Similar rollovers of landslide frequency-size distribution were also found in the distributions studied by Malamud et al. (2004). However, the mapped landslide areas are strongly affected by the image resolution and the under- sampling of small landslides (Stark and Hovius, 2001). Landslide inventory statistics show that 62% of the coseismic slides fall into the size range of 10− 3–10− 2 km2, while roughly 20% are smaller than 10− 3 km2 (Fig. 12), The rollover in relatively small sizes may be a characteristic of the power–law distribution. The lower image resolution, however, may limit the delineation of smaller landslides and the determination of boundaries of larger landslides. In such a case, a landslide size distribution model (e.g. Stark and Hovius, Fig. 9. Longitudinal proﬁle of the Tachia river derived from 40-m of DTM, with vertical 2001, Malamud et al., 2004, Van den Eeckhaut et al., 2007) cannot exaggeration. provide sufﬁcient information to estimate the location of rollover for relatively small landslides. 2 − 2 sizes (AL >0.01km ). For smaller landslides with areas between 10 The highest landslide frequency density is found at an area of and 10− 4 km2, however, the coseismic landslides show a higher 10− 3 km2 for the coseismic landslides, but 5×10− 3 km2 for the frequency density than the rainfall-induced landslides. typhoon-induced landslides, though the characteristics of the two landslide groups are comparable for larger landslides. The rainfall- 5. Discussion induced landslides had a lower frequency density than the coseismic landslides for areas between 10− 2 and 10− 4 km2.Thismay The landslide frequency-size distribution can be modeled with be attributed to the further activity and extension of small an inverse gamma function as suggested by Malamud et al. (2004). coseismic landslides and their natural stabilisation. As shown in Guzzetti et al. (2002) explained that the locations of the rollover are Fig. 12, the coseismic landslides were mainly of medium size (62% controlled by friction for larger, deep-seated landslides, but by were 10− 3–10− 2 km2), while the Typhoon Toraji-induced landslides cohesion for small, shallow landslides. The rollovers for small were mainly of larger size (58% were 10− 2–10− 1 km2).

Fig. 10. Estimated sediment volume and thickness along the Tachia River (source WRAP, 2005, and reference time 2004/10).

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is mainly associated with landslides in quartzite, slate, and coaly shale areas. That of the Typhoon Mindulle-induced landslides also appears to be associated with quartzite, slate, and coaly shale especially for medium-sized (10− 3–10− 2 km2) landslides. The higher frequency density for medium-sized landslides in the quartzite, slate, and coaly shale areas may be due to the more fractured characteristics of those rocks than argillite, slate, and phyllite, which enhance inﬁltration, giving rise to higher pore pressure. However, fractures may also favor better drainage of slopes. The best ﬁtted frequency-size distribution curves for larger landslides by geology are:

ÀÁ 2 lnf = −1:59lnAL +2:62 r =0:97 ; for coseismic landslides in quartzite ð7Þ

ÀÁ 2 lnf = −1:77lnAL +1:25 r =0:98 ; for coseismic landslides in argillite ð8Þ

ÀÁ 2 lnf = −1:92lnAL +1:84 r =0:95 ; for Mindulle landslides in quartzite ð9Þ

ÀÁ 2 lnf = −1:80lnAL +1:42 r =0:87 ; for Mindulle landslides in argillite ð10Þ

The locations of the rollovers in the frequency-size distribution appear to be less associated with the geologic formations where the coseismic landslides occurred. Ground motion by the Chi-Chi earthquake was found to be the most signiﬁcant factor in triggering shallow landslides, and it was not associated with particular geological units (Khazai and Sitar, 2004; Lin et al., 2006). Lin et al. (2006) also found that after the Chi-Chi earthquake the density of rainfall-induced landslides in Pleistocene–Holocene ﬂuvial deposits are slightly larger than that occurred in the rest of geological formations. Fig. 15 shows the percentage of rainfall-induced landslide area and the corresponding geologic conditions in the Shihmen Reservoir Basin in northern Taiwan. The landslides were mainly located in the sandstone, shale, and coaly shale areas; the argillite, sandy shale, and sandstone areas; and the argillite, slate, and phyllite areas than in the other areas. In contrast, the rollovers of frequency-

Fig. 11. Landslide frequency-size distribution and magnitude mL for (a) the coseismic landslides (ρ=0.78,a=1.4×10−3 km2,ands=−6×10−5 km2 of the inverse gamma function) and (b) the rainfall-induced landslides (ρ=1.3, a=1.5×10−2 km2,ands=−1.2 × 10−3 km2).

The differing image resolutions of the satellite photos may affect the interpretation of the landslide areas. The lower resolution imagery for the coseismic landslides may create difficulties in identifying small landslides and defining the boundaries of larger landslides. A field investigation was conducted in the Guguan area to further discuss the variation in the landslide areas (Fig. 1). Some parts of the coseismic landslides were re-vegetated in 2004 (zone I, Fig. 13). Some small coseismic landslides showed vegetation recovery after reactivation due to Typhoon Toraji in 2004 (zone II, Fig. 13). The analysis shows that the medium-sized (10− 3–10− 2 km2) coseismic landslides later became larger rainfall-induced landslides, affecting the percentage of landslides by size. The ratio of the medium-sized landslides was 67% after the earthquake but de- creased to 25% after Typhoon Toraji, while the percentage of larger landslides (10− 2–10− 1 km2) increased to 66%. The effect of rock types on the landslide frequency-size distribution is not simple. The landslides in the basin were primarily located in quartzite, slate, and coaly shale areas as well as argillite, slate, and Fig. 12. Percentage of landslide area for (a) coseismic landslides and (b) Typhoon Toraji- phyllite areas. At the same time, the smaller coseismic landslides − − induced landslides. The medium size (10 3–10 2 km2) of coseismic landslides varied − 3 2 b − 2 − 1 2 ( 10 km ) mainly occurred in areas with other geologic formations from 67% to 25%, and the percentage of large landslides (10 –10 km ) increased to (Fig. 14). The frequency-size distribution of the coseismic landslides 66% after Typhoon Toraji.

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Fig. 13. Examples of reactivation and stabilisation of landslides. Parts of the large coseismic landslide in zone I were re-vegetated in 2004. The coseismic small landslides in zone II showed vegetation recovery after Typhoon Toraji, but reactivation in 2004. size distribution of the Typhoon Mindulle-induced landslides landslide frequency-size distribution varies across different cumu- changed across different geologic formations. This is because the lative rainfall-induced landslides. The landslide frequency-size geologic formation exerts less inﬂuence on the shallow regolith distribution under higher cumulative rainfall tends to result in a coseismic landslides than on the rainfall-induced landslides found smaller scaling exponent in the power–law scheme. The cumulative across a diversity of geological formations. rainfall of Typhoon Mindulle in the basin (1680 mm in Table 1)was Rainfall is one of the external forces that initiate landslides. A key higher than that of Typhoon Toraji (482 mm). The scaling exponent issue involving rainfall is the quantity analysis of the rainfall characteristics and its relationship with the shape of the landslide frequency-size distribution. The Typhoon Toraji-induced landslides can be separated into two groups, one in areas with a cumulative rainfall greater than 400 mm, and the other with a cumulative rainfall of less than 400 mm. The landslides primarily occurred in areas over 350 mm of cumulative rainfall, and 400 mm isohyet was a useful marker for dividing the landslide inventory (Fig. 7a). Typhoon Mindulle-induced landslides were not analyzed for the higher cumulative rainfall in the whole basin, since the new, larger landslides were mainly located between the Tianlun Dam and the Techi Dam. The maximum-likelihood ﬁt for larger landslides under different cumulative rainfall are represented by,

ÀÁ 2 lnf = −1:84lnAL +1:71 r =0:90 ; for Toraji rainfall < 400 mm ð11Þ

ÀÁ 2 lnf = −1:57lnAL +1:52 r =0:83 ; for Toraji rainfall > 400 mm ð12Þ

The frequency-size distribution and the corresponding cumulative rainfall show that the Typhoon Toraji-induced landslide distribution is comparable to the landslides under cumulative rainfalls of less than 400 mm (Fig. 16). The scaling exponent β in the Fig. 14. Landslide frequency-size distribution and the corresponding geologic conditions.

Please cite this article as: Chen, C.-Y., Sedimentary impacts from landslides in the Tachia River Basin, Taiwan, Geomorphology (2008), doi:10.1016/j.geomorph.2008.10.009 ARTICLE IN PRESS

10 C.-Y. Chen / Geomorphology xxx (2008) xxx–xxx

Fig. 15. Percentage of rainfall-induced landslide area and the corresponding geologic conditions in the Shihmen Reservoir Basin (NCDR, 2005b).

β showed no obvious change across the two different rainfall- induced landslide events (Fig. 11). This could be inherited from the landslides originally found in the coseismic data, with fewer subsequent new rainfall-induced landslides in the inventory (Table 1). The cumulative rainfall has a greater effect on the shape of the landslide frequency-size distribution herein than the type of geological formation. The sensitivity of the inverse gamma model to the controls of earthquakes, rainstorms, and rock types is schematically depicted using graphs (Fig. 17). The effects of subsequent rainstorms after coseismic landslides tend to shift the position of rollovers, while the scaling exponent tends to become critical. The scaling exponent and rollovers of the inverse gamma model shift across differing rock types, and the shifting of the rollover is more notable than that of the scaling exponent. Higher cumulative rainfalls tend to decrease the scaling exponent.

6. Conclusion

A mountain basin in Taiwan was studied for both coseismic and subsequent climate-triggered landslides and their sedimentary impacts. The impacts of the Chi-Chi earthquake included coseismic landslides, typhoon-induced landslides on hillslopes disturbed by the earthquake, and the transfer of landslide-derived sediments into debris flows, associated with downstream inundation by flash floods.

The landslide magnitude (mL) was 3.3 for the coseismic landslides and 2.8 for the rainfall-induced landslides. The analysis of landslide frequency-size distribution shows that the curve has two rollovers. The post-seismic landslides have a negative scaling exponent for larger

Fig. 17. Graphs showing the effects of an earthquake, rainstorms, and rock types on the inverse gamma model. (a) Shifting of the rollover for the rainfall-induced landslides after an earthquake. (b) Shifting of the scaling exponent and the rollover across different rock types, where the shifting in the rollover is greater than that in Fig. 16. Frequency-size distribution of landslides induced by Typhoon Toraji and the scaling exponent. (c) Decrease in the scaling exponent due to higher cumulative corresponding cumulative rainfall. rainfall.

Please cite this article as: Chen, C.-Y., Sedimentary impacts from landslides in the Tachia River Basin, Taiwan, Geomorphology (2008), doi:10.1016/j.geomorph.2008.10.009 ARTICLE IN PRESS

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Please cite this article as: Chen, C.-Y., Sedimentary impacts from landslides in the Tachia River Basin, Taiwan, Geomorphology (2008), doi:10.1016/j.geomorph.2008.10.009