10th Asian Regional Conference of IAEG (2015)

The processes and rates of knickpoint migration in western Taiwan
Ming-Wan HUANG(1), Yii-Wen PAN(2) and Jyh-Jong LIAO(2)(1) Disaster Prevention & Water Environment Research Center, National Chiao Tung University, Taiwan
E-mail: mwh@nctu.edu.tw(2) Department of Civil Engineering and Disaster Prevention & Water Environment Research Center, National
Chiao Tung University, Taiwan
Abstract
A knickpoint is a location with a sharp slope change in the longitudinal channel
profile. Knickpoint migration is often a result in response to an abrupt base-level fall or achange in bedrock resistance. Many knickpoints have formed in river channels intersectedby fault ruptures after the 1999 Chi-Chi earthquake. The migration processes and rates forthese earthquake induced knickpoints were significantly different with the knickpoints
formed in river junction or due to a difference in bedrock resistance. Based on the attitudeof rock strata and the flow direction, the knickpoints could be classified into dip, strike, or
anti-dip knickpoint. We carried out the site investigations for studying knickpoint
migration process in western Taiwan. Also, the averaged migration rates in 10 reaches arederived from a series of chronological aerial photographs; among them, 5 reaches wereverified with DEM data. The average rates of knickpoint migration were 57, 110, 83 myr-1
for dip, strike, and anti-dip knickpoint, respectively. Finally, the knickpoint migration
processes were interpreted based on the results of field study.
Keywords: knickpoint, migration processes, migration rate, western Taiwan
1.

Introduction

vertical knickpoints with resistant caprock. They
assumed the resistant caprock could only be removed
through cantilever. Their results suggested that the
knickpoint migration rate was closely related todownstream incision rate divided by channel gradient
under the condition of a critical height for the
sub-caprock failures.
The knickpoint migration rate is usually evaluatedfrom the total retreat distance over the elapsed time
period. Due to the scarce data of the average
migration rate, the landscape change is often too
small to realize. The knickpoint migration models
considers the drainage area as a primary factor, otherfactors taken into account in the models may include
channel slope, channel width, sediment transportation,
lithology, geology structure, etc. (Berlin andAnderson, 2007; Bishop et al., 2005; Crosby andWhipple, 2006; Hayakawa and Matsukura, 2003;
Loget and Van Den Driessche, 2009; Valla et al.,2010)
Loget and Van Den Driessche (2009)
compared average migration rate of knickpoints forvariety ranges of rate (up to 1000 my-1 in alluvial
rivers, and 0.001 my-1 in bedrock rivers), time scales(101 – 107 years), and drainage areas (10-2 – 107 km2),
and found that the longer the time scale, the lower the
migration rate.
From their data, the average
migration rate of the bedrock knickpoints ranges from0.001 to 30 my-1 and is mostly less than 1 my-1.

A knickpoint is a location with a sharp slopechange in the longitudinal channel profile, and thechannel is connected with an over-steepened reach.
A plunge pool is often formed in the bottom of the
knickpoint through toe scouring due to theaccelerating flow of the knickpoint elevation drop.
Knickpoint migration is an important process of
bedrock channels in response to a base-level fall.
The possible causes to induce a base-level fall include
active tectonics, climate change, sea-level fall, river
capture, resistant caprock, and differential incision intributary junction, etc. (Frankel et al., 2007; Seidl andDietrich, 1992; Whipple, 2004)
Several researchers have investigated the
behavior of knickpoint migration based on flume tests(Frankel et al., 2007; Gardner, 1983; Holland andPickup, 1976). Gardner (1983) proposed three typesof knickpoint evolution models: inclination, parallel
retreat, and replacement.
The processes ofknickpoint evolution are controlled by two primaryfactors: the actual shear stress and the critical shear
stress of the channel bed. Frankel et al. (2007)
proposed another knickpoint evolution model in a
vertically bedded substrate; the model elaborated theprocesses of replacement and parallel retreat. Haviv
et al. (2010) numerically simulated the evolution of1

10th Asian Regional Conference of IAEG (2015)

The 1999 Chi-Chi earthquake (Mw=7.6) took
place in central Taiwan; a 100 km surface rupture
along the Chelongpu fault was generated by thrustfaulting with fault scarps or pop-up deformations
across four major rivers in central Taiwan (including
the Taan River, the Tachia River, the Wu River, andthe Choshui River).
The earthquake induced
riverbed deformations and formed knickpoints inthese rivers and their tributaries. The knickpoint
migration rate of the Taan River derived frommultistage annual longitudinal profiles was up to afew hundred meters per year (Huang et al., 2013).
The knickpoint migration rates in the Tachia Riverand three main branches of the Wu River were from
3.3 to 220 meters per year (Hayakawa et al., 2012;
Hayakawa et al., 2009). The migration rates of the
Chi-Chi earthquake induced knickpoints wereconsiderably much higher than worldwide data.
Due to the complicated geological structures andthe meandering channels in central western Taiwan,
the relationship of the attitude of rock strata and theflow direction varies with the channel location.
From the field observation, the migration rate and themigration process are different for the different
knickpoints.
This study aims to investigate the influence ofrock orientation (relative to the flow direction) on the
migration processes and rates of different types ofknickpoints in western Taiwan. The channel erosionof the surface deformation reaches and the processesof knickpoints migration along the Chelongpu fault
are examined. The averaged migration rates were
derived from a series of chronological aerial
photographs. The types of knickpoints migration
are discussed based on their processes and migration
rates.
2.

examined through historical aerial photographs
including channel geometry, sediment/bedrock
distribution, possible knickpoint locations (locationsof rushing flow or water spray) and knickpoint retreat
distances, etc. Then, the field investigations forknickpoints erosion processes and surface geology
were carried out based on the results of aerial
photographs interpretations.
The results arecompiled in Table 1. The unconfined compressive
strength of the bedrock tested via Schmidt hammer(using a Silver Schmidt type BN hammer with amushroom plunger) are typically less than 10 MPa.

Site backgroundFig. 1 Location of the surface ruptures of the
Chelongpu fault in the 1999 Chi-Chi earthquake.
The green triangles with site numbers indicate theinvestigation reaches with data listed in Table 1.

The Chelongpu fault in western Taiwan is a major
thrust fault with the north-south strike and the dip
toward east (Chen et al., 2003). The fault thrusts the
Pliocene-Pleistocene strata over modern sediments
approximately along the mountain front of theWestern Foothills (Fig. 1). Table 1 lists the 19
locations of fault scarps of pop-up deformations
across the mentioned four major rivers and their
tributaries, according to the geological survey reportof Taiwan Central Geological Survey (Central
Geological Survey, 1999). The channel bedrocks
are young sedimentary rocks mainly composed ofsandstone, shale, thin-interlayered sandstone andshale.
3.

No knickpoint retreatment was observed in 9
rivers which are crossed by the Chelongpu fault; thepossible reasons for no retreat could be the followingone.
1. Certain protection work was constructed in
the channel shortly after the earthquake sothat has prevented further erosion to occur,e.g. the Han River (Photo 1a), the Wuniulan
River, the Puzikeng River and the Ailiao
River.
2. The uplift height was small, and the armor
layer was thick enough to compensate theearthquake uplift, e.g. the Wu River (Photo

Aerial photo interpretation andfield investigation
The changes of the 19 knickpoints were firstly

2

10th Asian Regional Conference of IAEG (2015)1b), the Changping River, and the Maoluo
River.
3. The earthquake rupture was nearly parallel
across the channel and was unlikely to form aknickpoint, e.g. the Caohu River (Photo 1c)
and the Dongpurui River.
The knickpoints in the other 10 reaches triggeredsignificant bedrock erosion by knickpoint retreatment
noticeable from aerial photographs. The locationsof knickpoints were carefully identified on series ofaerial photographs (Fig. 2). We also used the DEM

data to identify the knickpoint locations in fivereaches including the Taan River, the Tachia River(Meizi Bridge), the Toubiankeng River, the Gan
River, and the Choshui River (Fig. 3). The average
recession rates for these knickpoint were evaluatedfrom the total retreat distance over the total time
period of the respecting photographs. Based on thestream direction and the rock bedding attitude, theknickpoints can be classified into three types, dip,
strike, and anti-dip.

Table 1 Compiled data of the investigated reaches along the Chelongpu fault

Taan

1

6

dip, ~strike, and anti-dip
knickpoint(migration ~ perpendicular 1999-2010
cross an anticline )Averagerecession
rate (m/yr)
dip: 53~strike: 93
Anti-dip: 294

Shalien b2

5anti-dip knickpoint~ perpendicular 1999~2007

17

3

6~7

anti-dip knickpoint~ perpendicular 1999~2007

6

4

6~7

strike knickpoint

oblique

2005f~2010

50

5

4

strike knickpoint~ perpendicular 1999~2006

186

Han c

6

4

anti-dip knickpoint~ perpendicular (protection works)g -Wuniulan c7

0.9

anti-dip knickpoint~ perpendicular (protection works)-

Puzikeng c

91

anti-dip knickpoint~ perpendicular (protection works)-

Dali c8

1.5~2anti-dip knickpoint~ perpendicular 1999~201051

Toubiankeng c

10

2~3

dip knickpoint~ perpendicular 1999-2004

60

c

11

2

anti-dip knickpoint

parallel

-

c

12

1

anti-dip knickpoint~ perpendicular 1999~2006

58

13

1.5

anti-dip knickpoint~ perpendicular 1999~2009

38

14

1

anti-dip knickpoint~ perpendicular -

-

15

3

anti-dip knickpoint~ perpendicular (protection works)

-

16

0.5

anti-dip knickpoint~ perpendicular --17

0.2~0.3

anti-dip knickpoint

oblique

-

18

1.2

anti-dip knickpoint~ perpendicular 1999~2005

120

19

1.5

strike knickpoint

parallel-Site No. Uplift height Stratum dip and flow Fault strike and Recession
direction
flow direction duration e
in Fig. 1 a (m)River

Tachia(Pifong Bridge)
Tachia(Shigang Dam)
Tachia(Meizi Bridge)

Caohu

BeigouGanc

Wu
Ailiaoc
Changping
MaoluoccChoshui
Dongpuruid(protection works)-(protection works)

a: Data from Central Geological Survey (1999).
b: Tributary of the Tachia River.
c: Tributary of the Wu River.
d: Tributary of the Choshui River.
e: Recession duration is the interval of the data (DEM or aerial photographs) used for analysis.
f: Shigank Dam knickpoint is located upstream of the Pifong Bridge (site 3). The knickpoint was formed around 2005.
g: (protection works) indicates that the reach was protected by sill works.

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10th Asian Regional Conference of IAEG (2015)

a. Han River (Jun. 2015)
b. Wu River (Mar. 2010)
c. Caohu River (Nov. 2012)
Photo 1 The photographs of the reaches which did not exhibit features of knickpoint retreatment.
following discussion is on the geological conditions
at the knickpoints.
The average recession rates of knickpoint
migration were evaluated according to the geometry
relationship of stratum orientation and flow direction,
which could be simply classified into dip, strike, andanti-dip knickpoints. The average recession rates
range between 6 to 294 myr-1 which are significantly
higher than the published knickpoint migration rates
in bedrock channel all over the world (e.g., Hayakawa
and Matsukura, 2003; Loget and Van Den Driessche,2009). Our data show that the maximum rate was in
the anti-dip knickpoint of the Taan River and theminimum rate was in the anti-dip knickpoint of theTachia River. The knickpoints were further grouped
into dip, strike, and anti-dip types to calculate averagerate for each group regardless rivers. The grouped
average rates are 57, 110, 83 myr-1 for the dip, strike,and anti-dip knickpoint respectively. The groupaverage rate in the strike knickpoint is about 2 times
higher than that in the dip knickpoint.
The
difference in the grouped average rate of knickpoint
migration could be explained from the erosion
processes based on field observations.
The migration processes in the dip knickpoint of
the Taan River observed from Fig. 4a shows that thedischarge flows over the strata top and down along
the over-steepened reach of the knickpoint. The
major erosion occurred along the over-steepened face,which is the bedding plane of bedrocks in dip
knickpoint. The knickpoint could migrate once theunderneath rock layers were eroded away; hence themigration rate would be controlled by the resistanceof the rock layers sequentially. Once the strata
contained a competent rock layer, the knickpoint
migration rate could be lowered down significantly.
In the strike knickpoint of the Tachia River(downstream of the Shigang Dam, Fig. 4b), the major
bedrock erosion occurred along the relatively weakrock layer, and the discharge flowed in the confinednarrow channel. The erosive rate of the weak rocklayer would be high and could effectively accelerate
the knickpoint migration rate.

Fig. 2 The knickpoint locations are identified on aseries of aerial photographs (the Pifong Bridge of theTachia River).
4.

Migration processes and rates of
the knickpoints

In principle, both hydraulic and geological factors
will affect the migration processes of knickpoint in ariver. The drainage basins of the four rivers are
relatively adjacent; their precipitation trends are alsosimilar. In spite that the drainage areas (proxy ofdischarge) of various reaches are different andinevitably affect the maximum discharge, channel
width and unit-width discharge, the focus of the4

10th Asian Regional Conference of IAEG (2015)

Fig. 3 The longitudinal profiles derived from DEM data for knickpoint locations identification (the Taan River).
Figure 4c demonstrated the anti-dip knickpoint in
the Taan River. The knickpoint is located in acompetent layer of massive sandstone and formed awaterfall. The height of the knickpoint gradually
increased because of undercutting and resulted in an
unstable overhung rock. The process of toe scouring
enabled the knickpoint migration in case of a resistantrock layer.
5.

The presented work was supported by Ministry of
Science and Technology, Taiwan and by the Water
Resources Agency Ministry of Economic Affairs of
Taiwan. These supports are gratefully acknowledged.
References
Berlin, M.M. and Anderson, R.S. (2007): Modelingof knickpoint retreat on the Roan Plateau, western
Colorado, Journal of Geophysical Research-Earth
Surface, Vol. 112, F3S06.
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, Vol. 30,
No. 6, 767-778.
Central Geological Survey, C.G.S. (1999): Report of
the geological survey of the 1999 Chi-Chi
Earthquake, Taipei, Taiwan (in Chinese).
Chen, W.S., Chen, Y.G., Shih, R.C., Liu, T.K.,
Huang, N.W., Lin, C.C., Sung, S.H. and Lee, K.J.
(2003): Thrust-related river terrace development inrelation to the 1999 Chi-Chi earthquake rupture,
Western Foothills, central Taiwan, Journal of Asian
Earth Sciences, Vol. 21, No. 5, 473-480.
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, Vol. 82, No. 1-2,
16-38.
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 ofAmerica Bulletin, Vol. 119, No. 3-4, 476-486.
Gardner, T.W. (1983): Experimental study ofknickpoint and longnitudinal profile evolution incohesive, homogeneous material, GeologicalSociety of America Bulletin, Vol. 94, No. 5,664-672.

Discussion and conclusion

The rock strata near a knickpoint created by fault
rupture are generally fractured because of tectonic
stresses. The young sedimentary rocks of the
Pliocene-Pleistocene strata in the western foothills of
Taiwan belong to soft rock and are generally notresistant to flow erosion. The rate scale of the
knickpoint migration ranges in tens to a few hundred
meters annually that is astonishingly high compared
to worldwide data. The extremely high rate ofknickpoint migration causes significant landformchanges in the reaches adjacent to the co-seismic
Chelongpu fault in the 1999 Chi-Chi earthquake andimposes great impacts on river environments andnearby infrastructures.
This study adopted both chronological aerial
photographs and DEM data to assess the conditionsof knickpoints induced by the Chi-Chi earthquake andto evaluate the average rates of knickpoint migration
after the formation of these knickpoints. Among theformed knickpoints, some of them did not exhibit
backward migration due to (1) prompt installation ofprotection works, (2) very thick armor layer, and (3)relatively small angle between the fault and thechannel. In this study, the knickpoints were grouped
into dip, strike, and anti-dip types to calculate averagerate for each group. The average migration rates were57, 110, 83 myr-1 for the dip, strike, and anti-dip
knickpoint.
Finally, the knickpoint migration
processes were interpreted based on the migration
rates and the results of field study.
Acknowledgements

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10th Asian Regional Conference of IAEG (2015)

Haviv, I., Enzel, Y., Whipple, K.X., Zilberman, E.,
Matmon, A., Stone, J. and Fifield, K.L. (2010):
Evolution of vertical knickpoints (waterfalls) withresistant caprock: Insights from numerical modeling,
Journal of Geophysical Research-Earth Surface, Vol.
115.
Hayakawa, Y. and Matsukura, Y. (2003): Recession
rates of waterfalls in Boso Peninsula, Japan, and a
predictive equation, Earth Surface Processes andLandforms, Vol. 28, No. 6, 675-684.
Hayakawa, Y.S., Matsuta, N., Maekado, A. andMatsukura, Y. (2012): Fault-scarp knickpoint
recession in 10 years after Chi-Chi Earthquake incentral Taiwan, AGU Fall Meeting, San Francisco,
pp. EP13A-0811.
Hayakawa, Y.S., Matsuta, N. and Matsukura, Y.
(2009): Rapid recession of fault-scarp Waterfalls :Six-year changes following the 921 Chi-Chi
earthquake in Taiwan, Transactions, Japanese
Geomorphological Union, Vol. 30, No. 1, 1-13.
Holland, W.N. and Pickup, G. (1976): Flume study ofknickpoint development in stratified sediment,
Geological Society of America Bulletin, Vol. 87,
No. 1, 76-82.
Huang, M.-W., Pan, Y.-W. and Liao, J.-J. (2013): A
case of rapid rock riverbed incision in a coseismic
uplift reach and its implications, Geomorphology,
Vol. 184, 98-110.
Loget, N. and Van Den Driessche, J. (2009): Wave
train
model
for
knickpoint
migration,
Geomorphology, Vol. 106, No. 3-4, 376-382.
Seidl, M.A. and Dietrich, W.E. (1992): The problemof channel erosion into bedrock, Catena Supplement,
Vol. 23, 101-124.
Valla, P.G., van der Beek, P.A. and Lague, D. (2010):
Fluvial incision into bedrock: Insights frommorphometric analysis and numerical modeling ofgorges incising glacial hanging valleys (Western
Alps,
France),JournalofGeophysicalResearch-Earth Surface, Vol. 115, F02010.
Whipple, K.X. (2004): Bedrock rivers and thegeomorphology of active orogens, Annual Review
of Earth and Planetary Sciences, Vol. 32, 151-185.

a. The dip-knickpoint reaches of the Taan River in
2004.

b. The strike-knickpoint reaches of the Tachia River
in 2013.

c. The antidip-knickpoint reaches of the Taan River
in 2008.
Fig. 4 Conditions of channel bedrock erosion.

6