Knickpoint evolution across anticline structure: A case of uplifted reach in the Taan River, Taiwan

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1 ICSE6-145 Knickpoint evolution across anticline structure: A case of uplifted reach in the Taan River, Taiwan Ming-Wan HUANG 1, Yii-Wen PAN 2, Jyh-Jong LIAO 2, Meng-Hsiung CHENG 3 1 Research Engineer/ Department of Civil Engineering and Disaster Prevention & Water Environment Research Center, National Chiao-Tung University 1001 University Road, Hsinchu, Taiwan mingwan.cv93g@nctu.edu.tw 2 Professor/ Department of Civil Engineering and Disaster Prevention & Water Environment Research Center, National Chiao-Tung University 1001 University Road, Hsinchu, Taiwan ywpan@mail.nctu.edu.tw, jjliao@mail.nctu.edu.tw 3 Engineer/ Water Resources Planning Institute, Water Resources Agency, Ministry of Economic Affairs, Taiwan 1340 Jhong Jheng Road, Wufong Township, Taichung County, Taiwan chengms@wrap.gov.tw The behaviour of knickpoint migration is a key to understand how the river morphology responses to surface uplift or base-level drop. Annual rate of knickpoint migration in bedrock channel may govern the incision rate that a stream can return to equilibrium. Generally, the migration rate ranges from sub-meter to sub-centimeter. Several researchers have attempted to correlate knickpoint migration rate to a varieties of factors, including upstream drainage area, rock properties and geological structure, among others. Usually, the migration rate is determined from the elapsed time and the knickpoint retreat distance. Very often, the knickpoint migration process is hard to be realized due to incomplete data of long term longitudinal profiles. A case study on the knickpoint migration of an uplifted reach in the Taan River, Taiwan was carried out through the collection and analyses of multistage annual longitudinal profiles, discharges in major flood events, and geological data. The longitudinal distance of the reach is about 1 km. The uplifted reach contains a pop-up structure with 10 meters of the maximum vertical uplift formed by the tectonic deformation due to the thrust faulting in the 1999 Chi-Chi earthquake. Since then, incision developed rapidly into the soft bedrock. The knickpoint migrated upstream in an astonishing rate of tens meters annually. It took less than 10 years for the channel to cut through the pop-up structure. The changes in multistage annual longitudinal profiles enable the understanding of the processes of knickpoint evolution. The studied reach covers an anticline structure so that contains three distinct types of knickpoint evolution. The major factors affecting the knickpoint evolution in the studied reach was identified and characterized. Key words Knickpoint, migration rate, evolution, soft rock. I INTRODUCTION Knickpoint is a localized discontinuity zone in the longitudinal profile of a river, where disturbs the generally equilibrium riverbed profile from concave to convex near this point. The slope of riverbed adjacent to the knickpoint is generally steep in nature. Often, the steep riverbed will result in a noticeable waterfall. In general, the erosive power of the river flow will be significantly intensified due to the steep slope or elevation drop of the riverbed near the knickpoint. Then, the knickpoint may migrate upstream due to the rock erosive resistance less than the erosive power. The behaviour of knickpoint migration is one of the important processes in river bedrock incision. Knickpoint typically usually forms in bedrock channel in response to an abrupt base-level fall or a change in the resistance of bedrock. A rapid base-level fall may be resulted from climate change, sea-level fall, surface rupture due to earthquake, etc. [Crosby and Whipple, 2006; Frankel et al., 2007; Whipple, 2004]. Numerous studies have suggested that the knickpoint migration is the dominant mechanism of fluvial adjustment responding to the perturbation [Bishop et al., 2005; Hayakawa and Matsukura, 2003; Wohl et al., 1994]. The rate of knickpoint migration in bedrock channel generally ranges from sub-meter to subcentimeter scale per year, although some studies showed the rate could be up to few meters per year [Loget 503

2 and Van Den Driessche, 2009]. The knickpoint migration is a complex processes of fluvial bedrock incision includes weathering, abrasion, plucking, and cavitation [Howard et al., 1994; Sklar and Dietrich, 2004; Whipple, 2004]. Generally, it combines both the horizontal retreat and the vertical incision. Knickpoint recession rate is often modelled as a function of catchment area, which is a proxy of upstream drainage discharge, similar to the stream power erosion model [Howard and Kerby, 1983]. Many studies have successfully applied model of this type calibrated with the measured incision data [Bishop et al., 2005; Crosby and Whipple, 2006; Hayakawa and Matsukura, 2003]. The characteristics of knickpoint, including forms and migration rates, are highly dependent on the bedrock properties [Gardner, 1983; Wohl et al., 1994]; it is, however, not directly described by the stream power erosion model. To model knickpoint evolution, Gardner [1983] and Frankel et al. [2007] conducted flume experiments with various artificial rock materials and strata; they further proposed models to describe the processes and features of knickpoint evolution. Knickpoint migration is generally very slow; it is difficult to examine the continuous and gradual adjustment in the morphology of the river channel within a decade or so. The studied site in this present work is an uplifted reach in the Taan River in central Taiwan, in which the knickpoint migrated upstream in an astonishing high rate since it was uplifted by the thrust faulting in the 1999 Chi-Chi earthquake. Then, incision developed rapidly into the weak bedrock [Huang et al., 2008]. This extraordinary case offers a very good opportunity to investigate the processes of knickpoint evolution. Through the collection and analyses of multi-staged (annual) longitudinal profiles, discharges in major flood events, and geological data, we identify and characterize the major factors affecting the knickpoint evolution, and recognize three distinct types of knickpoint evolution models in this reach. II STUDY SITE The Taan River locates in the central-west of Taiwan (figure 1); it has a drainage area of 758 km 2 and a total length of 96 km. Most of its catchment is in mountainous or hilly areas. The river of the first 60 km channel from the headwater is confined in mountainous valleys with the elevation from 3,500 to 500m above the sea level; the channel slope is generally larger than 2%. Downstream of the mountainous area, the channel became wider underlain the poorly consolidated rocks of late Tertiary stratum with an average slope from 1.5% to less than 1% approaching the estuary. The uplifted reach of the studied river section is located within 27.7 km and 28.7 km upstream from the estuary (figure 1). Before the 1999 Chi-Chi earthquake, the average channel slope at the study site is 1.3%. Figure 1: Location of the Taan River basin. The study site, a 1-km-long reach which is between two scarps extending transversely across the river valley, locates in the north end of the Chelungpu fault. The Chi-Chi earthquake occurred on 21 September 1999 in central Taiwan with epicentre near the Chi-Chi town. Its moment magnitude is 7.6 with a focal depth of 8 km [Shin and Teng, 2001]. In this earthquake, surface ruptures (approximately 100 km) developed mainly along the Chelungpu fault (figure 1) in the northsouth direction. In the Taan River, two surface ruptures paralleling with the Tungshih anticline cut through the river valley (figure 2) and produced a pop-up structure with vertical uplift up to 10 meters. Chen et al. [2007] reconstructed the structure of subsurface ruptures and the topography across the Tungshih anticline; they showed the surface ruptures are likely the folding scraps. The scarps extend transversely across the Taan River valley, 1 km in width. The pop-up structure comprises of a flat top and two tilted limbs (Figure 3). 504

3 ICSE6 Paris - August 27-31, 2012 Figure 2: Aerial orthophotos of the uplifted reach in the Taan River. (a) One day after the reach uplifted in Chi-Chi earthquake, the lake formed in the reach upstream. (b) Current channel location, the channel is within the rock banks. The polygon enclosed by thin black dot lines is the DEM adopted area using in the longitudinal profiles analyses. The X-X line is the cross-section line in Figure 3. The red arrows show the photographing locations and directions in Figure 5. Figure 3: The geological cross-section of this reach. The stratum exposed in the study reach is the Pliocene Cholan Formation. It is composed of sandstone, siltstone, mudstone, and shale in a monotonous alternating sequence. III III.1 STUDY METHODS Field investigation The study of the knickpoint evolution in this work relies on field investigation and detailed interpretation of DEM data. The field investigation relevant to this project began in The studied reach belongs to the Western Foothills Geology category with strata of weak sedimentary rocks. The bedrock in this region is the Pliocene Cholan Formation composed of sandstone, siltstone, mudstone, and shale. The Tungshih 505

4 anticline transversely cut through the valley; it is the main geological structure in the vicinity. Referring to Figure 3, the outcrop in this reach includes four layers of thin (centimeters to millimeters) inter-layered sandstone and shale, one layer of massive shale, two layers of massive sandstone, and one layer of massive sandstone with occasional thin shale. Their unconfined compressive strength is typically under 10MPa. Field investigation was carried out periodically. Significant changes in channel morphology often took place after the flood of major typhoons, e.g., 2008 SINLAKU, 2009 MORAKOT. Various types of rockerosion mechanism including weathering, abrasion, plucking and cavitation were noticeable. There were many dislodged rock blocks remained in the channel; plucking appears extremely active and likely to be the dominant process in this channel. It also indicates that plucking is an efficient way to change the morphology of channel. III.2 Analyses of longitudinal profile There are three cross-sections which were surveyed by the Water Resources Agency in this 1-km reach. These surveyed data, however, is not sufficient for the detail analyses of morphology changes. In addition, the data of digital elevation model (DEM) derived from aerial photographs and airborne LiDAR (light detection and ranging) was also adopted as the basis of topography analyses. Twelve multi-staged DEM data sets covering a span of 11 years were adopted for the analyses of multi-staged longitudinal profiles. Table 1 lists the dates of major flooding events and DEM. The data in bold font is for DEM only. The data in normal font is the information of major floods; the last column shows the duration of discharge larger than 100 cms. We analyse the cross-section profiles along the channel at 5m-interval in the channel area (i.e., the polygon enclosed by the thin dot lines in Figure 2). Longitudinal profile was derived by connecting the lowest elevation in each cross section. Figure 4 shows the longitudinal profiles from 1999 to The profiles for 2000 and 2002 are omitted in Figure 4 because there is no obvious change in this period. Holland and Pickup [1976] pointed out an idealized knickpoint should consist of the following elements: over-steepened reach, lip (connecting face and over-steepened reach), face (water free falls to plunge pool), undercut, plunge pool, and bar, in a sequence from upstream to downstream. However, face, undercut, and plunge pool can not be shown on the longitudinal profiles (Figure 4) since the topographic data under the overhang rock at knickpoints can not be interpreted by aerial photographs. El.(m) HD HU RU RD Distance(m) Legend 1999-Sep Dec Nov Aug Oct Oct Jan Jun Jul Sep 12 KP1 KP2 Note Points Figure 4: The longitudinal profiles of the studied reach from 1999 to The red circle marks knickpoint 1, the red diamond marks knickpoint 2. The black inverted triangle shows the following positions: RD- rupture downstream; RU- rupture upstream; HD- anticlinal hinge at downstream limb; HU- anticlinal hinge at upstream limb. 506

5 Table 1: Dates of flood events and DEMs (font in bold) used in this study. Time Flood event * ; DEM derived method Max. discharge (cms); GCPs RMS/max. error (m) Duration (hrs) ** Flying altitude (m) / Point density (pts/m 2 ) 1999-Sep 22 aerial photographs 1.3 / Dec 10 aerial photographs 1.6 / Jul 10 KAI-TAK Aug 23 BILIS Aug 30 PRAPIROON Nov 01 XANGSANE Nov 08 aerial photographs 1.6 / Jun 25 CHEBI Jul 25 YUTU Jul 30 TORAJI Sep 13 NARI Nov 12 aerial photographs 1.6 / Jul 04 RAMMASUN Jul 10 NAKRI Sep 16 aerial photographs 1.7 / Aug 24 KROVANH Aug 26 aerial photographs 0.4 / Jul 03 MINDULLE Aug 12 RANANIM Aug 25 AERE Sep 12 HAIMA Oct 25 NOCK-TEN Oct 03 aerial photographs 1.2 / Dec 04 NANMADOL Jul 19 HAITANG Aug 05 MATSA Sep 01 TALIM Oct 02 LONGWANG Oct 27 aerial photographs 1.9 / Jun 09 heavy rain Jul 14 BILIS Jan 31 aerial photographs 1.6 / Jun 08 heavy rain Aug 19 SEPAT Sep 18 WIPHA Oct 06 KROSA Jun 12 heavy rain Jun 10 airborne LiDAR / Jul 18 KALMAEGI Jul 28 FUNG-WONG Sep 14 SINLAKU Sep 29 JANGMI Jul 23 airborne LiDAR / Aug 09 MORAKOT Sep 12 airborne LiDAR / 1.2 Note: * name of typhoon in upper-case; ** count for the discharge > 100 cms 507

6 IV DISCUSSION IV.1 Phenomenon and factors of knickpoint evolution As noted from the longitudinal profile, there were two initial knickpoints formed right after the uplift that was resulted from the Chi-Chi earthquake; the first one was in the downstream side of the rupture scarp of the uplift (KP1), the other was adjacent to the Tungshih anticline axis with a clear change in topography (KP2). Large head drop near the knickpoints resulted in intensive channel incision and dominated the evolution of river morphology. Table 2 lists the retreat distance and incision magnitude for KP1 and KP2 during various periods from 1999 to The annual migration rate was in tens of meters, even reaches 355 m/year; the annual incision rate was in the order of meters, sometimes above 10 m/year. Both the migration rate and the incision rate are astonishing for rock riverbed; the case is indeed very interesting. The starting period of KP2 s retreat was a year behind KP1. The knickpoint migration of KP2 seems to be affected by the morphological disturbance due to KP1 s retreat. Notably, the evolution of river channel responded to the collected results of the knickpoint migration in KP1 and KP2. The intensive erosion due to knickpoint migration should be responsible for the rapid incision of this reach. The major periods of knickpoint retreats, in general, are in agreement with the major flood events. Typhoon RAMMASUN brought maximum discharge of CMS, but caused little knickpoint retreat on KP1, it only resulted in some vertical incision. Similar phenomenon occurred in typhoon MINDULLE for KP2. For the initial exposure of knickpoint of rock stratum, it is possible that the stream power tends to undercut or scour the rock layer in front of the knickpoint before significant retreat can be triggered. Once the migration moves on, both of the horizontal retreating rate and the vertical incision rate can accelerate. The outcrops in the study reach are mainly weak sedimentary rock with low resistance to erosion. In general, outcrops in this reach do not have sufficient resistance against erosion under strong flow. Among various outcrop rocks, the compressive strength of massive rock (usually a few meters in thickness) is relatively higher. However, the massive rocks in this reach are often fractured with spacing less than 1 m; thus, the jointed rock mass is often vulnerable to plucking. The mechanism of plucking, together with the instability of rock mass in the steep slope, may explain why the knickpoint retreat rate and incision rate are so high. Their role and process will be described in the following context. Table2: Retreat distance and vertical incision depth of two knickpoints in each period. knickpoint 1 knickpoint 2 time interval retreat distance(m) vertical incision(m) note retreat distance(m) vertical incision(m) note 1999-Sep 22 to 1999-Dec alluvial Dec 20 to 2001-Nov alluvial Nov 12 to 2003-Aug dipping downstream horizontal bedding 2003-Aug 26 to 2004-Oct dipping downstream horizontal bedding 2004-Oct 03 to 2005-Oct dipping downstream horizontal bedding 2005-Oct 27 to 2007-Jan dipping downstream horizontal bedding 2007-Jan 31 to horizontal bedding horizontal bedding 2008-Jun Jun 10 to 2009-Jul Jul 23 to 2010-Sep 12 IV.2 vanished sediment deposited alluvial dipping upstream - - alluvial dipping upstream Types of knickpoint retreat and slope instability This reach is roughly perpendicular to the Tungshih anticline axis. Relative to the bedding orientation and the stream flow direction, the reach can be divided into three sections: (1) downstream of the anticline axis: the dip direction of rock bed is parallel to the flow, i.e., a dip stream; (2) adjacent to the anticline axis: the 508

7 rock bedding is roughly horizontal; (3) upstream of the anticline axis: the dip direction of rock bed is against the flow, i.e., an anti-dip stream. Close field investigation in these three sections reveals the types of knickpoint retreat are different for various orientations of bedding and knickpoint slope. Figure 5 shows the schematic illustrations and photographs of three knickpoint types. Figure 5(a) describes the case for a knickpoint in a dip stream. In the case, water seeps into the bedding planes, and reduces shear resistance against sliding. In addition to plucking mechanism, sliding failure of large rock blocks in various scales may take place along the bedding plane. Figure 5(b) describes the instability occurred in the case for a knickpoint in a horizontal stratum. Seepage pressure may result in the extension of tensile cracks near the crest of knickpoint; once cuts down and intersects the bedding plane, may enable the formation of rock block and the mechanism of plucking to take place. Figure 5(c) describes the case for a knickpoint in an anti-dip stream. A gradual widening and deepening scour hole, in this case, will enhance the overhanging rock blocks above the scour hole to form rock fall so that cause the slope to retreat. a b c Figure 5: Schematic illustrations and photographs of three knickpoint types. (a) knickpoint at beckrock dipping downstream. (b) knickpoint at horizontal bedrock. (c) knickpoint at bedrock dipping upstream. V CONCLUSIONS This paper presents a case study with exceptionally rapid change in river morphology within just a decade or so. The work made use of DEM to produce multi-staged longitudinal profiles of river channel. Both the migration rate and the incision rate are uncommonly high for rock riverbed. The annual migration rate was tens of meters even up to 355 m/year; the annual incision rate was in the order of meters even up to 14 m/year. The extremely high rates of knickpoint retreat and vertical incision were responsible for the rapid evolution of river morphology. The major periods of knickpoint retreats, in general, were in agreement with the major flood events. Cross examination of multi-staged data shows that the stream power tends to undercut the rock layer in front of the knickpoint before significant retreat starts. Thereafter, both of the 509

8 horizontal retreating rate and the vertical incision rate can accelerate. In addition to the mechanism of plucking, the instability of rock mass in the steep slope adjacent to the knickpoint may explain why the knickpoint retreat rate and incision rate are so high. Various mechanisms of slope instability associated with different types of knickpoint migration were identified. VI ACKNOWLEGMENTS AND THANKS The presented work was supported by the National Science Council, Taiwan and by the Water Resources Agency Ministry of Economic Affairs of Taiwan. These supports are gratefully acknowledged. VII REFERENCES AND CITATIONS Bishop, P., Hoey T. B., Jansen J. D., and Artza I. L. (2005). Knickpoint recession rate and catchment area: the case of uplifted rivers in Eastern Scotland. Earth Surface Processes and Landforms, 30: Chen, Y. G., Lai K. Y., Lee Y. H., Suppe J., Chen W. S., Lin Y. N. N., Wang Y., Hung J. H., and Kuo Y. T. (2007). Coseismic fold scarps and their kinematic behavior in the 1999 Chi-Chi earthquake Taiwan. Journal of Geophysical Research-Solid Earth, 112: B03S02. Crosby, B. T., and Whipple K. X. (2006). Knickpoint initiation and distribution within fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New Zealand, Geomorphology, 82: Frankel, K. L., Pazzaglia F. J., and Vaughn J. D. (2007). Knickpoint evolution in a vertically bedded substrate, upstream-dipping terraces, and Atlantic slope bedrock channels. Geological Society of America Bulletin, 119: Gardner, T. W. (1983) Experimental study of knickpoint and longnitudinal profile evolution in cohesive, homogeneous material. Geological Society of America Bulletin, 94: Hayakawa, Y., and Matsukura Y. (2003) Recession rates of waterfalls in Boso Peninsula, Japan, and a predictive equation. Earth Surface Processes and Landforms, 28: Holland, W. N., and Pickup G. (1976). Flume study of knickpoint development in stratified sediment. Geological Society of America Bulletin, 87: Howard, A. D., and Kerby G. (1983). Channel Changes in Badlands. Geological Society of America Bulletin, 94(6), Howard, A. D., Dietrich W. E., and Seidl M. A. (1994). Modeling Fluvial Erosion on Regional to Continental Scales, Journal of Geophysical Research-Solid Earth, 99: Huang, M.-W., Cheng M.-H., Liao J.-J., and Pan Y.-W. (2008). Rapid bedrock erosion in the Taan River, Taiwan. in Fourth International Conference on Scour and Erosion, , The Japanese Geotechnical Society, Tokyo, Japan. Loget, N., and Van Den Driessche J. (2009). Wave train model for knickpoint migration. Geomorphology, 106: Shin, T. C., and Teng T. L. (2001). An overview of the 1999 Chi-Chi, Taiwan, earthquake. Bulletin of the Seismological Society of America, 91: Sklar, L. S., and Dietrich W. E. (2004). A mechanistic model for river incision into bedrock by saltating bed load. Water Resources Research, 40: W Whipple, K. X. (2004). Bedrock rivers and the geomorphology of active orogens. Annu. Rev. Earth Planet. Sci., 32: Wohl, E. E., Greenbaum N., Schick A. P., and Baker V. R. (1994). Controls on Bedrock Channel Incision Along Nahal-Paran, Israel. Earth Surface Processes and Landforms, 19: