京都
 大 学 防 災 研 究 所 年 報 第 50 号 C 平 成 19 年 4 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 50 C, 2007

Studies of High-resolution Morphodynamics with Special Reference to River Bank
ErosionRyoukei AZUMA*, Hideo SEKIGUCHI㩷 and Tetsu ONO*** COE Researcher, DPRI, Kyoto University** Gazokeisoku Lab. Inc.
Synopsis
Many alluvial rivers in Japan have recently undergone effects of degradation due to suchfactors as river training for flood control, excessive riverine sand mining, or reservoir
sedimentation. This study focuses on bank erosion in clayey sediments of the Uji River floodplainthat has occurred over a period of several decades or longer. It applies digital photo-theodolite
surveying to precisely measuring the rate and extent of erosion on a 250m-long section of the leftbank of the main channel of the Uji River. The maximum rate of erosion proved to be as high as
6m over a 9-month period. The volume of erosion (or sediment supply) over the 250m-long section
was estimated to be equal to 3000m3. Notably, the erosion took place even under relatively mild
stream conditions. An important consideration is the presence of intercalated granular soil layers in
an otherwise thick deposit of clayey sediments. Our field observations point to the following
progressive nature of erosion: 1) A granular soil layer is scoured by stream flows, forming caves; 2)
The overlying clayey sediment then collapses in blocks; 3) The fallen clay blocks stay as a kind of
talus at the foot of the bank wall for some time; 4) Stream flows drift such natural protections
downstream over a period of time, then process 1) resumes, allowing the river bank to retreat by anamount.
Keywords: river bank erosion, high-resolution geomorphometry, sedimentary environments
1.

Introduction

According to a recent report by Intergovernmental
Panel on Climate Change (IPCC, 2007), the annual mean
temperature on a global scale has risen at a noticeable
rate and global warming is largely due to humanactivities. Global warming is likely to increase thefrequency of occurrence of extreme meteorologicalevents. This trend will certainly intensify flood disasterrisks in floodplains that have witnessed extensive
urbanization.
Alluvial rivers in Japan have particular runoff
characteristics. A flood stage is featured by a significant
peak flow rate with a short duration, as compared withordinary much milder flow stages. Due to this rapid
flooding, it is one of the most important requirements tofully understand the performance of river banks throughcareful field observational studies. Floodplains of alluvial
rivers are mainly a product of flood-related sediments,
and bank materials in middle-to-lower reaches are
fine-grained and cohesive sediments. Note in this regard
that laboratory test results with reconstituted clayeymaterials are difficult to apply to field clayey deposits
that have stood several thousand years or longer. Thus, it
is fundamentally important to observe evolution of bankerosion for a prolonged period of time, if no immediate
safety problems to flood-control levees are involved.
Fujita et al. (1983) undertook invaluable field－ 199 －

observations of bank erosion in Uji River in early 1980s.
They discussed the correlation the extent and frequencyof floods and the eroding velocity of the bank. Theyevaluated the tractive force which acted on the river bankfrom the gradient of the water surface at the flood stage,
and concluded that the magnitude and duration of
tractive force during the flood stage were the major
factors that controlled with the eroding velocity of theriver bank.
The purpose of this study is to make a revisit to thebank erosion problem in Uji River, with the aim ofpresenting an overview of evolution of the rivermorphology over past 70 years or so. Also, the present
study focuses its discussion on the performance of a “hotsegment” regarding bank erosion, in terms of digital
photo-theodolite surveying with high-resolution.
In what follows, we will first outline the study area ofbank erosion. This is followed by a discussion of resultsfrom the digital photo-theodolite surveys that were
performed on January 17, 2006 and October 17, 2006.
Moreover, based on cross-sectional profiles of the riversegment over past, we will provide a perspective for thetrend in the degradation of the river bed associated, alongwith implications of sedimentary environments. Also, thedynamical nature of the bank erosion process will be

Fig. 1 A map showing the location of the study area
examined in light of the hydrograph available andsnapshots of the bank wall that underwent recession in a
progressive manner.
2.

Study area of bank erosion

The Uji River runs through Uji City and southernpart of Kyoto City and drains into Osaka Bay as Yodo
River. Our study area is a segment several hundred
meters long and includes the 43.0km cross section as

Fig. 2 Satellite image of study area (adapted from Google earth)－ 200 －

(A) A section of natural bank that extends upstream of the section protected with revetment(B) Natural-bank section extending further upstream
Photo 1 Panoramic views of river bank taken from crown of right levee on January 17, 2006 in Survey 1

Fig. 3 Longitudinal profile of the deepest channel bottom in Uji River (adapted from survey results of Yodogawa River
office, MLIT)
indicated in Figs. 1 and 2. Note that the study area is apart of low-lying area that develops upstream of theconfluence point of the three major rivers (Katsura, Uji
and Kizu Rivers). The Ogura pond shown in Fig. 1
located to the south of this study area was reclaimed in1941.
A satellite image adapted from Google earth isshown in Fig. 2, with a few geographical features forreference. In the downstream section from 43.0km post,
revetment was constructed at the left bank of main
channel in order to protect the bridge pier at the time of
construction of the Route 1 bypass because Uji River hasa gentle bend at this site. Spur dykes on the right bank
was installed at the same time as above construction.
Note that a natural river bank extends from immediately
upstream the 43.0km post. A view of the natural riverbank slightly upstream of 43.0km post is presented in
Photo 1. In order to protect the bridge pier of the RouteNo. 1 bypass road, the revetment works were performed
from 43.0km to the downstream section (Photo 1 (A)).
The bank without revetment works was eroded and theoutcrop was formed (Photo 1 (B)). Note that erosion has

made a very steep face of the river bank.
Fig. 3 shows the longitudinal profile of deepest pointin the channel bottom of Uji River in which survey data
in 1971 and 2005 were adapted from Yodogawa River
office, MLIT. What has to be noticed is that degradationof channel bottom is significant in the section from
46.0km to 41.0km. The study area falls into this hot
section regarding erosion.
3.

Digital photo-theodolite surveys of bank erosion

3.1 Features of the digital photo-theodolite survey
The digital photo-theodolite survey has superiorfeatures, such as non-invasive, remote and threedimensional measurement and preservation as an imageof the field condition. These features are crucial for fieldsurvey in view of the fact that we can not approachclosely to erosional bank. This system is effective forhigh accuracy and resolution survey of flood stricken
topography (Azuma et al., 2006).
The digital photo-theodolite consisted of a digital
camera that was mounted on a theodolite (Fig. 4). The－ 201 －

Table 1 Classification of identified points in Survey 1
Targeted objects
Number of points

Fig. 4 Illustration of the digital photo-theodolite systemused, together with principle of determination of atargeted material point of three-dimensional coordinates
digital camera used (Canon EOS 20D) had 8.2 million
pixels (35042336), allowing a high spatial resolution.
The theodolite facilitated the measurement of two angles
of orientation on vertical and horizontal planes at every
photographing
position.
The
three-dimensional
coordinates of a targeted object may be determined basedon the method of spatial intersection (Ono and Hattori,
2002; Ono et al., 2004). In essence, a pair of two
photographs of the targeted point from two different
positions permits its three-dimensional coordinates to be
determined (Fig. 4).
3.2 Performance of the digital photo-theodolite
surveys(1) Survey 1 conducted on January 17, 2006
First topographical measurement of river bank using
the digital photo-theodolite survey was carried out over asection approximately 1000m long on January 17, 2006
(Fig.5). This is hereinafter referred to as Survey 1. In
Survey 1, we selected 8 pairs (16 positions) ofBank topography, Debris
Top edges of outcrop

3897
899

Waterfront

461

Rubbles of revetment

50

Total

5307

photographing on the crown of the right-hand levee. Theinterval between cameras was set at 20m and the intervalof pairs of camera was set at 100m. In the distance to theriver bank on which the camera focused, resolution was
evaluated from the image corresponded to approximately
1cm per 1 pixel. When the calibration was performed in
the analysis process, there was no error larger than 1
pixel. Thus it can be concluded that the measurement
accuracy of survey 1 is of the order of millimeter.
In the case of analyzing process, a key for theidentification of survey points was to seek objects withsharp corners (Fig.4). For this purpose, the boundary lineof sedimentary facies, pebbles, debris and waterfront
became a good target. Identified points in Survey 1 aresummarized in Table 1 and a total of 5307 points wereable to be identified. It follows from what has been
discussed thus far that high-resolution and high-accuracy
topographical data can be measured by digital-photo
theodolite survey, compared with the normal survey.
(2) Survey 2 conducted on October 17, 2006
The second topographical measurement (hereinafterreferred to as Survey 2) was carried out on October 17,
2006. Survey 2 focused on the 250m-long section of theleft bank where erosion was particularly significant. The
three camera positions were selected on the gravel barthat extended on the right bank of the main channel(Fig.6).

Fig.5 Camera positions and measurement range of Survey 1 performed on January 17, 2006

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Fig.6 Camera positions and measurement range of Survey 2 performed on October 17, 2006
The appearance of the face of the left bank is typified
in Photo 2. One can notice caves standing just above thewaterfront. They were an evidence of scouring due tostream power, leading to instability of the overlying soil
mass. This interesting aspect of erosion will be discussed
later in this paper in more detail. It would suffice at thispoint to mention that such caves were formed in a clayeysoil layer and stood over a prolonged period of weeks or
so.
Survey 2 enabled identification of a total of 5092
points for which three-dimensional coordinates were
determined with high-resolution. Namely, the precisionof the coordinate determination is 0.65mm per pixel.
3.3 Evaluation of topographical changes and extent
of bank erosion(1) TIN representations of 3-D topographical data
The 6th plane rectangular coordinate system ofJapanese Geodetic Datum 2000 was referred whenanalyzing the datasets of Surveys 1 and 2 in terms ofGeophysical Information System (GIS; ArcGIS software(ESRI)). The three-dimensional topographical data withTIN (Triangulated Irregular Network) representations are

Table 2 Classification of identified points in Survey 2
Targeted objects
Number of points
Bank topography, Debris
Top edges of outcrop

4034
282

Waterfront

332

Rubbles of revetment

444

Total

5092

shown in Fig. 7 in terms of contour maps.
Comparison of Figs. 7(A) and (B) presents of bankerosion that occurred in the 9-month period between
surveys 1 and 2. For instance, one may note that the
sediment taluses at the foot of the river bank were goneby erosion. Also, one may note significant changes that
occurred to cross-sectional profiles B-B’ (Fig. 8). The
retreat of the bank amounted to approximately 6m.
(2) Retreat of bank crown in entire segment surveyed
In order to make apparent the recession distance, theedge lines of bank crown in Surveys 1 and 2 are plottedon to a horizontal plane with elevation of T.P. +10.78m
as indicated in Fig. 9. Clearly, the bank underwent
erosion at the most significant rates in the middle of thesurveyed segment.

Photo 2 Topographical features of the left bank face as of October 17, 2006

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(A) Survey 1 on January 17, 2006

(B) Survey 2 on October 17, 2006
Fig. 7 Contour maps showing the three-dimensional topographical data based on TIN representation

Table 3 Results of volume analysis
Volume evaluated (m3)Survey 1(Jan. 17, 2006)
Survey 2(Oct. 17, 2006)
Erosional volume

Fig. 8 Cross section profiles B-B’ through the bankFig. 9 Lines of bank crown plotted on the x-y coordinate system indicated－ 204 －

100,000
97,0003,000

Fig. 10 Evolution of cross-section profiles of channel bottom (43.0km post, adapted from survey results of Yodogawa
River office, MLIT)
was 150m long (x = -119,900 to -119,750m), 250m wide(y = -22,800 to -22,550m) and had elevations equal toand higher than T.P. +5.30m. The height of thishorizontal plane was selected in view of the highest pointof water surface in both surveys. The subaerial sedimentvolumes evaluated are listed in Table 3. Thus, subaerial
sediment volume lost due to bank erosion proved to beequal to 3000m3. Since the survey segment of river bank
was approximately 250m-long and 5m-high above theriver water stage, averaged recession of the river bankwas equal to 2.4m over the 9-month period.
Fig. 11 Evolution of channel bottom degradation
4.
(3) Volume of sediment released by bank erosion
In order to evaluate the released sediment volume bybank erosion, we compared the sediment volumes offloodplain in Surveys 1 and 2. The domain of calculation

Discussion of bank erosion processes

4.1 Long-term evolution of bank erosion
In order to provide a concrete idea about the secularchange of the bottom profile in the main channel, we(A) Sideway movement of(B) Sideway movement of
deepest location of channel bottom
left-hand waterfront of main channel
Fig. 12 Time-series data showing evolution of bank erosion－ 205 －

Fig. 13 Cross-section profiles through 43.0km post line is overlain onto a geological profile along Route No.1 bypass(adapted from Kyoto office, MLIT, 1991)
took up the 43.0km section and compiled the survey
results available from 1938 to 2006, as shown in Fig. 10.
Those survey results were provided by Yodogawa River
Office as paper-based data. In addition, the deepest pointof channel bottom is presented in Fig. 11 in order toexamine the evolution of channel bottom degradation. It
is noted in Fig. 11 that the channel bottom in 1938 wasrelatively high with an elevation of T.P. +5.9m.
Significant river bed degradation took place in the period
between the late 1960s and early 1970s. After that theriver bed elevation on the 43.0km section stayed at the
averaged elevation of T.P. +1.6m. However, thisobservation does not mean the cease of erosion at theparticular river section. Degradation stopped since 1978,
instead, the bank retreat to the lateral direction has started
and is progressing still now.
In order to investigate evolution of the river bankretreat, we referred both the deepest location of thechannel bottom and left-hand waterfront of the main
channel to the 43.0km post on right-hand levee. The
results are shown in Figs. 12 (A) and (B). It is evident
from these two figures that the bank erosion became
significant in the late 1960s and thereafter has progressedat an approximately constant rate, i.e. nearly 2mrecession annually.
The question now arises as to what factors controlled
the erosion progressing laterally while the river bed
elevation stayed nearly constant. We consider that a sortof rock control may be an important factor. In order tomake this statement more specific, let us look at Fig.13
in which the cross-sectional profile through 43.0km postin 2006 and the geological profile along Route No.1
bypass (adapted from Kyoto office, MLIT, 1991) aresuperimposed. From Fig. 13, the following observations
can be made, namely; 1) The Uji River incised the clay
layer (Ac1). 2) After arriving at the lower end of the clay
layer (Ac1), no further degradation occurred. Then,evolution has developed sideways.
4.2 Dynamical nature of bank erosion in response to
runoff characteristics
A set of snapshots showing temporal development ofbank erosion are presented in photo 3. The correspondingtime series data of the water level of Uji River observedby the Mukaijima observatory (44.8km + 66.8m) is
shown in Fig. 14.
The following observations may be made from Photo3, with reference to Fig. 14 as well. First, vegetation on
the bank face visible in Photo 3(A) was gone in Photo
3(B). It is inferred that the flood lasting for four days
from April 11, 2006 was responsible for the erosion of
the vegetation. However, it should be noted that nodiscernible topographical changes took place by the
April 2006 flooding event.
Second, sediment lumps were visible in Photo 3(C),at the foot of the bank wall. The photograph was taken
15 days after 18 July flooding, the annual maximum
flood.
Third, in Photo 3 (D), the fallen sediment lumps weregone, and caves stood immediately above the waterline.
Note herein that such significant bank erosion took place
under relatively mild stream conditions that lasted for a－ 206 －㩷㩷㩷(A) January 17, 2006

(B) May 1, 2006

(C) August 10, 2006

(D) November 30, 2006

Photo 3 Snapshots showing development of a bank erosion process

Fig. 14 Time series data of the Uji River water level in the Mukaijima observatory (adapted from the database ofhydrology and water quality, MLIT; http://www1.river.go.jp/).
period from August 10, 2006 to November 30, 2006.
The bank erosion process based on our fieldobservations may be summarized as follows: 1) A
granular soil layer is scoured by stream flows, forming
caves; 2) The overlying clayey sediment then collapses
in blocks; 3) The fallen clay blocks stay for some time asa kind of talus at the foot of the bank wall; 4) Stream
flows may drift such natural protections downstream
over a period of time. Then process 1) resumes, allowing
the river bank to retreat by an amount.

5.

Conclusions

The topographical changes induced by bank erosion
have been discussed based on the digital photo-theodolite
surveys. The topographical datasets obtained were
processed and analyzed using GIS. The relations of bankerosion process with runoff characteristics andsedimentary environments were investigated based on
observation data and information adapted from
Yodogawa River office and Kyoto office, MLIT. The
principal conclusions drawn are as follows:－ 207 －

1㧕 The digital photo-theodolite survey proved to be acost-effective and hands-on methodology for
obtaining high-resolution topographical data on ariver segment scale.
2㧕 The 250m-long river segment underwent a bankrecession of 6m at maximum during only 9 months;
the averaged bank retreat in this duration was 2.4m.
The sediment volume released by bank erosion
amounted to 3000m3, i.e. 12m3/m along the riversegment.
3㧕 Although, bank erosion is commonly believed tooccur at the flood stage, this field study showed thatbank erosion progressed significantly even if theflow conditions were relatively mild.
4㧕 In order to evaluate the erosional velocity,
information of sedimentary environments in thefloodplain becomes important because layer-scale
sedimentary conditions such as the presence ofintercalated granular soils in otherwise uniform
clayey soil may control the rate and sequence of thebank erosion.
Acknowledgements
The authors wish to thank the Yodogawa River
Office, MLIT for the provision of valuable survey data.
The assistance of Mr. Norio Otani, Mr. Kriyo Sambodho
and Mr. Kazuyuki Yana in the field measurements anddata processing regarding the photo-theodolite surveys
would be greatly appreciated.

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