京都大学防災研究所年報 第 58 号 B 平成 27 年 6 月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 58 B, 2015

Riverbed Management by Changing Reach Scale Channel Configuration

Mikyoung CHOI, Yasuhiro TAKEMON and Tetsuya SUMI
Synopsis
Many rivers experienced channel alternation, riverbed degradation and vegetation
expansion result from artificial disturbance. Thus, works of channel reconstruction
carried out as a river restoration such as channel widening, excavation or making
meandering. This study presents possibility of channel widening by changing reachscale channel configuration (RSCC). RSCC and their historical changes in the Kizu
River during 1961 and 2010 were plotted in biplot dimension of width/depth ratio andspecific stream power. And based on their historical changes, we predict resultant RSCC
after manipulation of channel width. According to results, most of recent channel types
were distributed in the range of low width/depth ratio. However, channel widening
about 1.8-2.0 times in upper and lower site could be changed to the range of highwidth/depth ratio having high potentials of biodiversity in the Kizu River.
Keywords: RSCC, channel widening, width/depth ratio, biodiversity, bifurcatedwandering channel

1.

Introduction

In the last decade, many rivers experienced
riverbed degradation and vegetation expansion in
response to reduction of peak discharge and flood
frequency (Williams and Wolman, 1984).
Alternation of channel geomorphology due toreduction of disturbance influenced degradation of
habitats and decrease of species diversity (Takemon,2010). According to these alternation, riverrestoration has emerged as an increasingly
important to enhance biodiversity and resilience.
Especially, channel reconstruction carried out as awork of river restoration; e.g., channel alignment(widening or narrowing; Brierley and Fryirs, 2005),
excavation of floodplain material to the riverbottom level (Jahnig et al., 2009) and making
meandering (Kondolf, 2006). However, someprojects without reflecting potential river
characteristics, experienced failures (Kondolf,2006). He said that meanders have been created inmany channel re-constructions on rivers that were
not historically meandering, and in many cases,
these meanders have subsequently washed out. For
sustainable river management, development andapplication of river management practices requires
knowledge of the natural range of morphologicaladjustment for different river types (Thomson et al.,2001). In order to establish a scheme of rivermanagement aiming at maintaining a target riverbed
geomorphology (Takemon, 2010), characteristics ofriver should be focused on historical changes.
In this respect, geomorphic reach scale channel
configuration (RSCC) could be used as a powerfuldevice for riverbed management if the linkage
among hydraulic, geomorphology and ecology isunderstood. Thus, this study tried to make methodsof riverbed management using historical changing
RSCC in terms of understanding relations between
geomorphology and hydraulic. RSCC can beclassified by hydraulic-geomorphic parameters suchas discharge and slope (Leopold and Wolman,― 497 ―1957), depth-grain size ratio and width/depth ratio(Muramoto and Fujita, 1977), sediment load andlateral stability (Schumm, 1985), width/depth ratio
and specific stream power (Burge, 2005). Amongthese various parameters, this study focused on
width/depth ratio and specific stream power (Burge,2005) in order to detect possibility of channel
widening and narrowing as manipulation of riverrestoration. The purpose of this study is to detect
historical changes of RSCC in the Kizu River basedon relations between width/depth ratio and specificstream power, and to predict the possibility ofchannel changes in the future. To achieve purpose,
RSCC during 1961 and 2010 was plotted on biplot
dimension of width/depth ratio and specific stream
power, and historical changes of RSCC wereinvestigated on the biplot dimension. Finally, thepossibility of channel changes in the future waspredicted by manipulation of channel width.
2.

Methods

A total of 90 reaches (one reach is 2 km) weredivided into 8 channel types in the Kizu River (0-26km) during 1961-2010 (Choi, 2014). RSCC in theKizu River was classified into single, slightly
wandering straight and sinuous, quite wanderingstraight and sinuous, bifurcated wandering straightand sinuous, and braided channels using
geomorphic parameters, e.g., number of channels,
sinuosity, channel width, slope, ratio of landscape.
And the classified channel types were evaluated byhabitat diversity and soundness (diversity x total
habitat abundance), and bifurcated wanderingchannels had maximum habitat diversity andsoundness. Habitat diversity and soundness
influence
highbiodiversity.
A‘habitat
heterogeneity hypothesis’ (Simpson, 1949) statesthat an increase in the number of habitats and/or anincrease in their structural complexity leads toincreased species diversity. Thus, bifurcatedwandering channel had high potential of species
diversity (Choi, 2014).
In this study, channel types were simplified,and thus, a total of 4 types were used (single andslightly wandering, quite wandering, bifurcatedwandering and braided channels). All reaches inthe Kizu River was plotted in dimension ofhydro-geomorphic
parametersconsistofwidth/depth ratio (B/h) and specific stream power(Burge, 2005; classified into braided, meandering,wandering, single, anastomosed channels) (Fig. 1).
Specific stream power is pgQaS/w, where p is thedensity of water, g is the acceleration due to gravity,
Qa is the mean annual flood discharge, S is channel
slope, and w is the bankfull channel width (Burge,2005). Data of discharge was used by Yodogawariver offices (2011). Bankfull discharge was used asmean of the annual maximum daily dischargeduring 30 years in Inooka observatory (15-16 km).
Bankfull width (B) was defined by the boundaryline where terrestrial vegetation begins along thestream margin at bankfull discharge using aerialphotos. Depth was used as mean depth frombankfull discharge by calculation of HEC-RAS.
Width and depth were calculated using 10 sections
in a one reach (2 km) and averaged. Bed slope wascalculated using cross-section data per 2 km unit.
A range of 4 channel types was investigatedbased on mean values of B/h and specific stream
power per each channel type and its standard
deviation (SD). And then, historical changes of allsite (0-26 km) were detected to understandhistoricalchanging
patterns.
However,
representatively some site of upper (24-26 km),middle 1 (14-16 km), middle 2 (8-10 km), lower
site (0-2 km) of the Kizu River were introduced.

Fig. 1 RSCC in the Kizu River plotted in dimension ofwidth/depth ratio and specific stream power (Burge,2005). Kizu River was shown in meandering channel
and transitional wandering channel.

― 498 ―

Fig. 2 Distribution of each channel type within the biplot dimension of width/depth ratio and specificstream power in the Kizu River. Ranges of channel types of single and slightly wandering is blue line,
quite wandering is green, bifurcated wandering is orange and braided channels is red.

Based on these historical trace, equations andcoefficient of determination of trend line per eachreach were estimated, and changing channel ranges
from current channel to bifurcated wanderingchannels were expected by changing (increases/decreases) channel width.
3.

Results

All channel types in the Kizu River wereplotted within a restricted range of meanderingchannels and transitional wandering channels in thedimensions of width/depth ratio and specific stream
power (Fig. 1). Fig. 2 indicated plotted channel
types and ranges of channel types of single andslightly wandering (blue line), quite wandering(green), bifurcated wandering (orange) and braided
channels (red). Range of single and slightly
wandering channels showed the lowest values ofB/h and the widest range of specific stream power.
Range of quite wandering channels included higher
values of B/h than single and slightly wanderingchannels, and range of bifurcated wanderingchannels included higher values than quite
wandering channels. Range of braided channels had
the highest values of B/h and was included in areaof transitional wandering channel type.

Historical changes of channel types weredetected. Only upper, middle 1, middle 2 and lower
reaches were shown representatively in Fig. 3.
The upper site (18-20 km; dotted line) (Fig. 3a)
moved from braided channel having the highest B/hto slightly wandering channel during 45 years. This
reach experienced continuous decreases of B/h andincreases of specific stream power. Middle 1 site
(14-16 km; full line) (Fig. 3a) showed significant
decreases of B/h in area of meandering channel type.
Middle 2 site (8-10 km; dotted line) (Fig. 3b)
showed decreases in values of B/h and changes
from a quite wandering channel to a slightly
wandering channel. On the other hands, lower site(0-2 km; full line) (Fig. 3b) moved from a quite
wandering channel to a slightly wandering channel
with only significant changes of specific stream
power.
Based on changes of reaches, equations andcoefficient of determination of trend were estimated.
Relations between channel width (B) and B/h had
equation, y (B/h) = 0.287B 1.1, R2=0.95. And
relations between channel width (B) and specificstream power had various equations per each reach.
Fig. 4 indicated that predictable channel changes bychanging channel width using the equations. Onlysome reaches having values of high coefficient of― 499 ―determination were shown (R2>0.7). A recent
channel type in middle site 1 (14-16 km) showedslightly wandering channel (mark of red cross inblack square) (Fig. 4a). This reach tended to moveto range of bifurcated wandering channel by
increases of channel width about 1.8 -2.2 times. A
recent channel type in middle 2 site (8-10 km)
showed quite wandering channel (Fig. 4b).
Although this reach showed quite, it was almost
close to range of bifurcated wandering channel.

And according to increases of channel width,channel may be moved to area of transitional
wandering channel. A recent channel type in lower
site (0-2 km) showed slightly wandering channel(Fig. 4c).

Fig. 3 Historical changes in channel types within the biplot dimension of width/depth ratio and specificstream power in the Kizu River from 1961 to 2010. The plots of upper site (24- 26 km; dotted line) andmiddle 1 site (14-16 km; full line) (a), the plots of middle 2 site (8-10 km; dotted line) and lower site
(0-2km) (b) in study area

― 500 ―

Fig. 4 Predictable channel changes of middle 1 site (14-16 km) (a), middle 2 site (8-10 km)(b) and lower
site (0-2 km)(c) by manipulation (increases or decrease) of channel width. A mark of red cross in blacksquare indicate recent channel. The black circles means predictable channel changes by increases (x 1.2,1.5, 1.8, 2.0, 2.2) of channel width and decreases (x 0.9, 0.8, 0.7) of channel width. Orange line means
range of bifurcated wandering channels having high potential for biodiversity. Arrows mean their direction
by changes of channel width.

― 501 ―

4.

DiscussionAccording to the distribution of RSCC and their
historical changes in biplot dimension ofwidth/depth ratio and specific stream power, wecould predict resultant RSCC after manipulation ofchannel width (Fig. 4). Most of channel types in2010 were distributed in the range of single andslightly wandering channels. Increasing channel
width could be moved from range of single andslightly wandering channel to the range ofbifurcated wandering channels as target channel
types for high potential of biodiversity.
We could propose increasing channel width as ashort-term countermeasure for biodiversity forriverbed management (Fig. 4). The middle site(8-10 km) was close to the range of bifurcatedwandering channels having maximum habitat
diversity and soundness. In other words, this reachalready satisfied habitat diversity with highbiodiversity. Actually, according to Terada (2011),
the middle site (8-15 km) showed intermediate
floodplain vertical shape had the highest number ofactive ponds and terrace ponds than the upper orlower sites. In addition, Yodogawa River Bureau(2010) noted that bitterling and mussel were moredetected in the middle site (7-18 km) than others. If
we want to change the range of channel types fromsingle and slightly wandering to bifurcated channel
types, we should focus on lower or upper sites
required channel widening about 1.8-2.0 times.
Some studies showed the effects of channel
adjustment by comparison between restored sites
and natural channel. Rohde et al. (2004) assessed
the restoration success of river widening as alandscape approach. Restored site, near-natural,
regulated site were compared by riparian landscape(vegetation) mosaic. They noted that river widening
provide opportunities for re-establishing riparianlandscapes. The widening mainly promotes pioneer
stages and more complex mosaic than near-naturalsites due to the limited size of widenings. Jahnig etal. (2009) analyzed effects of re-braiding onhydromorphology, floodplain vegetation, beetles
and benthic invertebrates in mountain rivers basedon restored and non-restored site. The excavation offloodplain material to the river bottom level,

initiation of one or two secondary channel, ordevelopment by fallen tree in the absence of bankfixations were established in restored sites. They
said that restoration increases habitat diversity andavailability of biota in floodplain meshohabitat
while the effects on aquatic microhabitats (substratetypes, terrestrial plants, etc.) and assemblages wereless obvious. And species richness, but not thespecies diversity, of floodplain vegetation andbeetles increased following increased habitat
diversity with no effects on benthic invertebrates.
However, channel widening occurred sometimes
armoring or flat shallow riverbed in case of small
stream by simple river engineering works (Misa,1996), thus, these works of channel adjustment
should be considered with ecological function.
5.

Conclusion

Historical RSCC variations in the Kizu River
could be well explained by biplot dimensions ofwidth/depth ratio and specific stream power.
According to the distribution of channel types andtheir historical changes, we could predict resultant
RSCC after manipulation of channel width anddevelop method of riverbed management.
Acknowledgements
We would like to thank ASIA AIR Survey CO.
Ltd., Yodogawa River Office and MLIT for
providing historical aerial photos and hydraulic
data.
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(Received June 11, 2015)