ICSE6 Paris - August 27-31, 2012
- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
ICSE6-145
Knickpoint evolution across anticline structure: A case of uplifted
reach in the Taan River, Taiwan
Ming-Wan HUANG1, Yii-Wen PAN2, Jyh-Jong LIAO2, Meng-Hsiung CHENG3
1
Research Engineer/ Department of Civil Engineering and Disaster Prevention & Water Environment Research Center,
National Chiao-Tung University
1001 University Road, Hsinchu, Taiwan 300 - [email protected]
2
Professor/ Department of Civil Engineering and Disaster Prevention & Water Environment Research Center,
National Chiao-Tung University
1001 University Road, Hsinchu, Taiwan 300 - [email protected], [email protected]
3
Engineer/ Water Resources Planning Institute, Water Resources Agency, Ministry of Economic Affairs, Taiwan
1340 Jhong Jheng Road, Wufong Township, Taichung County, Taiwan 413 - [email protected]
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
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- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
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 km2 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).
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- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
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 2008. 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
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- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
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 2010. 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)
375
RU
370
HU
HD
365
360
355
RD
350
345
340
335
0
100
200
Legend
1999-Sep 22
1999-Dec 20
2001-Nov 12
2003-Aug 26
2004-Oct 03
2005-Oct 27
2007-Jan 31
2008-Jun 10
2009-Jul 23
2010-Sep 12
KP1
KP2
Note Points
300
400
500
600
700
Distance(m)
800
900
1000
1100
1200
1300
1400
Figure 4: The longitudinal profiles of the studied reach from 1999 to 2010. 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.
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- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
Table 1: Dates of flood events and DEMs (font in bold) used in this study.
Time
Flood event*;
Max. discharge (cms);
Duration (hrs)**
DEM derived method
GCPs RMS/max. error (m) Flying altitude (m) / Point density (pts/m2)
1999-Sep 22
aerial photographs
1.3 / 2.6
1999-Dec 10
aerial photographs
1.6 / 3.0
2000-Jul 10
KAI-TAK
207
20
2000-Aug 23
BILIS
334
35
2000-Aug 30
PRAPIROON
454
64
2000-Nov 01
XANGSANE
281
31
2000-Nov 08
aerial photographs
1.6 / 3.0
2001-Jun 25
CHEBI
246
16
2001-Jul 25
YUTU
322
13
2001-Jul 30
TORAJI
3 470
31
2001-Sep 13
NARI
2 116
65
2001-Nov 12
aerial photographs
1.6 / 3.0
2002-Jul 04
RAMMASUN
2 466
50
2002-Jul 10
NAKRI
513
26
2002-Sep 16
aerial photographs
1.7 / 2.9
2003-Aug 24
KROVANH
409
27
2003-Aug 26
aerial photographs
0.4 / 0.6
2004-Jul 03
MINDULLE
4 456
92
2004-Aug 12
RANANIM
1 058
35
2004-Aug 25
AERE
7 376
64
2004-Sep 12
HAIMA
294
24
2004-Oct 25
NOCK-TEN
300
20
2004-Oct 03
aerial photographs
1.2 / 2.2
2004-Dec 04
NANMADOL
286
25
2005-Jul 19
HAITANG
2 016
78
2005-Aug 05
MATSA
5 317
61
2005-Sep 01
TALIM
465
34
2005-Oct 02
LONGWANG
380
25
2005-Oct 27
aerial photographs
1.9 / 3.0
2006-Jun 09
heavy rain
3 120
91
2006-Jul 14
BILIS
1 744
51
2007-Jan 31
aerial photographs
1.6 / 3.0
2007-Jun 08
heavy rain
1 527
142
2007-Aug 19
SEPAT
599
69
2007-Sep 18
WIPHA
3 702
48
2007-Oct 06
KROSA
3 943
65
2008-Jun 12
heavy rain
742
58
2008-Jun 10
airborne LiDAR
1 600 / 1.6
2008-Jul 18
KALMAEGI
3 276
45
2008-Jul 28
FUNG-WONG
1 428
32
2008-Sep 14
SINLAKU
6 253
79
2008-Sep 29
JANGMI
2 634
51
2009- Jul 23
airborne LiDAR
2 500 / 1.2
2009-Aug 09
MORAKOT
3 769
101
2010-Sep 12
airborne LiDAR
2 500 / 1.2
Note: * name of typhoon in upper-case; ** count for the discharge > 100 cms
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IV
IV.1
- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
DISCUSSION
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 2010. 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 2 466 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
note
retreat
vertical
retreat
vertical
distance(m) incision(m)
distance(m) incision(m)
1999-Sep 22 to
40.5
4.7
alluvial
1999-Dec 20
1999-Dec 20 to
36.0
2.8
alluvial
2001-Nov 12
2001-Nov 12 to
0.0
2.2
dipping downstream
0.0
1.7
2003-Aug 26
2003-Aug 26 to
58.5
10.2
dipping downstream
0.0
2.7
2004-Oct 03
2004-Oct 03 to
58.5
14.4
dipping downstream
99.0
7.4
2005-Oct 27
2005-Oct 27 to
180.0
9.4
dipping downstream
63.0
10.6
2007-Jan 31
2007-Jan 31 to
49.5
6.2
horizontal bedding
162.0
11.2
2008-Jun 10
2008-Jun 10 to
sediment
vanished
alluvial
355.5
7.8
2009-Jul 23
deposited
2009-Jul 23 to
alluvial
232.0
8.1
2010-Sep 12
IV.2
note
horizontal bedding
horizontal bedding
horizontal bedding
horizontal bedding
horizontal bedding
dipping upstream
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
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- Ming-Wan HUANG, Yii-Wen Pan, Jyh-Jong LIAO, Meng-Hsiung CHENG
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
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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.
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