京 都 大 学 防 災 研 究 所 年 報 第48号 C 平成17年3月
Annuals of Disas. Prev. Res. Inst., Kyoto Univ., No. 48C, 2005

Relationship between Sediment Supply and Transport Processesin Miyagawa Dam Catchment

Fumitoshi IMAIZUMI*, Roy C. SIDLE*COE Researcher, DPRI, Kyoto University
Synopsis
Because sediment supply affects sediment yield, knowledge of the relationship between
sediment supply and transport are needed to estimate catchment-scale sediment yield. This study
investigates timing and volume of sediment supply and transport based on aerial photointerpretation and volume of sediment deposits behind the dam in Miyagawa Dam catchment. We
discuss the factors that affect the relationship between sediment supply and transport processes.
Timing of sediment transport around the dam lake corresponds to large rainfall-runoff events.
Sediment supply affects sediment yields for more than several years; this influence depends on theposition of landslide/debris flow deposits in the catchment.
Keywords: sediment supply, sediment transport, dam deposits, Miyagawa Dam, channel deposits

1.

Introduction

Because sediment supply processes affect sedimentyield (Walling and Webb, 1982; Asselman, 1999), a
unified sediment supply/transport model is needed todevelop better mitigative measures for preventing
sediment disasters. The relationship between sedimentsupply and transport (sediment yield) has been discussed
in sediment budget studies. Slaymaker (2003) reviewed
many sediment budget studies and noted that both
temporal and spatial factors characterize sedimentbudgets. Factors inherent in sediment budgets (e.g.,
sediment delivery ratio and types of sediment movement
processes) are relatively clear for smaller catchments if
only chronic sediment supply processes (e.g., surface
erosion and gully erosion) dominate (Kasai et al., 2001).
In larger catchments, various sediment supply processes
operate on hillslopes, as well as many sediment transport
processes (including episodic landslides and debris
flows). Sediment supply from hillslope to the channel
affects the timing and volume of sediment yield at thecatchment on outlet. However, only long term studies

have been conducted in larger catchment because of thelack of short term data to estimate catchment scale
sediment yields (Benda and Dunne, 1997a; Benda andDunne, 1997b).
Volume of dam deposits behind almost all of big
dams is measured every year in Japan, and these large
catchment scale sediment yields are available. Miyazaki
and Onishi (1996) investigated timing of sedimentsupply and transport by analyzing the volume of dam
deposits and rainfall. Hiramatsu et al. (2002) evaluatedinfluence of forest management on the volume of dam
deposits. However, sediment storage on hillslopes and inchannels, which contributes to the time lag between
sediment supply and transport (Benda, 1990; Nakamura
et al. 1995; May and Gresswell, 2003), was notinvestigated.
The overall aim of this study is to clarify the
relationship between sediment supply and transport
processes at the large catchment scale in Miyagawa Damcatchment, central Japan. The specific objectives of thisstudy are: (1) to investigate sediment supply andtransport in the catchment using aerial photos andsediment deposition at the dam; (2) to clarify the
temporal and volume relationships between sedimentsupply and transport processes; and (3) to identify
processes that influence the sediment supply/transport
relationship.

Miya Riv.

Rain gauge

Study area0

Miyagawa Dam catchment, upstream of Miyagawa
Dam, is located in southern Mie Prefecture, central Japan(Fig. 1). Miyagawa Dam (completed in May 1957;
capacity of water is 70,500,000 m3) is a concrete gravity
dam designed for power generation, flood control andwater supply. Catchment area above the dam is 125.6
km2 and channel length is 26 km. The lowest point of thecatchment is the Miyagawa Dam (270 m a.s.l.; northeast
location) and the highest point is the peak of Mt.
Hidegatake (1695 m a.s.l) at southwest end of thecatchment. The Miyagawa Dam catchment is suitable forinvestigation of sediment movement because little
human activity has occurred within the area, except for
timber harvesting.
The main geologic unit is Chichibu Paleozoic strata
comprised of sandstone and clay slate. Most of thecatchment is characterized by steep, sub-vertical slopes;
typical gradients of hillslopes are 30˚–50˚.
Miyagawa Dam catchment is known for high annual
rainfall, ranging 1600–4500 mm in the period from 1957
to 2003 (average 3300 mm). Heavy rainfall events (i.e.,
total rainfall > 100 mm) occur during the Baiu rainyseason (from June to July) and in the autumn typhoonseason (from late August to early October). Sedimentsupply processes are active because of high precipitationand sometimes cause sediment disaster around thecatchment. From September 28 to 29, 2004, Typhoon
Meari (T0421) brought heavy rainfall (maximum rainfall
per hour was 139 mm at Kuzu rainfall station, about 1.5
km southeast of Miyagawa Dam); six people were killed(one person missing) by landslides in the area (Hayashi
et al., 2004; Kondo et al., 2004). Winter snowfall
occurs at high elevation sites in the catchment, but
precipitation from December to February is only about
7 % of total annual precipitation.
3.

Methodology

3.1 Aerial photo analysis
Monochrome aerial photos taken during five years

Mt. Kunimi(1283.0m)

4km

Dam Lake2.

Mt. Ikekiya(1395.9m)

Miyagawa
Dam
Mt. Senchiyogamine(1099.7m)

Ohsugi Valley
Mt. Hidegatake(1694.9m)

Miyagawa

Ohdaigahara

Pacific Ocean

Fig. 1 Map of Miyagawa Dam catchment(1982, 1986–87, 1992, 1996–1987, 2001) were used toinvestigate the location and area of landslides and debris
flows in the catchment. Landslides and debris flows are
confirmed by stereograph and mapped on 1: 5000 forest
management maps. Most of aerial photos were taken in
March (before the Baiu season), thus almost all of thelandslides and debris flows confirmed by aerial photo up
through autumn of the previous year. New occurrencesof landslides and debris flows in each period (1982–86,
1987–1991, 1992–1996, 1997–2000) were confirmed bycomparing former and later aerial photos. Brardinoni and
Church (2004) noted a underestimation of landslides <4000 m3 in forested terrain in British Columbia based on
assessment of 1 : 12,000 to 1 : 15,000 aerial photos.
Landslides and debris flows mapped on the Miyagawa
forest management map include the initiation zone,transport zone and deposition zone. Individual zones are
not distinguished in this study because the main subjectof aerial photo analysis is not to identify local erosionand deposition, but to clarify sediment flux withincatchment.
3.2 Dam deposits
Changes in volume of channel deposits were
estimated by fixed cross section surveys on the dam lakefrom 1957 to 2003. Field investigations indicated that the
turbidity of stream water draining into the lake was low
shortly after heavy rainfall. Thus, the amount ofwashload transported downstream without deposition in
the dam lake should be small compared to the total

3.5 Field survey
Hillslope and fluvial processes (and attributes) that
influence the relationship between sediment supply andtransport were checked by field surveys. Storage on
hillslopes and in channels may affect the relationshipbetween sediment supply and transport (Benda, 1990;
Nakamura et al. 1995; May and Gresswell, 2003);
however, the ratio of storage and newly produced
sediment cannot be identified from aerial photos. Thus,
volume and position of landslide/debris flow deposits at
the foot of hillslope were measured.
3.6 GIS and topography analysis
Mapped landslides/debris flows were scanned andanalyzed their position and area using Arc GIS software.
Channel topography was also investigated using Arc GIS
and the TIN model constructed from a 50 m grid DEM.
In this study, we define the term “channel” as the place
where sediments and water accumulate, confirmed by aline which crosses slope contours at an angle < 90°continuously on the 1: 5000 forest management map.
According to this definition, zero order basins are also
included as channels.

Percentage

2,000

1.2

1,500

0.9

1,000

0.6

500

0.3

0

Percentage in total catchment area

1.5

Area

0
1982 1986-87 1992 1996-97 2001
year

Fig. 2 Area of confirmed landslides and debris flows

20,0001000

15,000750

10,000

500

5,000

250

800
0

0
Total rainfall

600

Maximum daily rainfall
in the period (mm)

3.4 Discharge into the dam lake
Changes in water level of the Miyagawa Dam lake
have been measured every 10 min since the completionof the Miyagawa Dam. Instantaneous discharge (m3/s) isderived from changes in volume of accumulated water inthe lake, which are estimated from changes in waterlevels and lake topography. Daily discharge can becalculated from average instantaneous discharge during
any given day multiplied by time, while yearly discharge
is discharge per day summed for the year.

Area of confirmed landslides/debris flows ( 103 m 2)

3.3 Rainfall
A rain gauge was installed near Miyagawa Dam in
1957, and three gauges were later installed to identify the
distribution of rainfall which could be affected byelevation and spatial factors (Fig. 1). Comparison of
rainfall data recorded at the four sites showed that noclear relationship exists between elevation and rainfall
amount. Thus, the Thiessen method was used to estimate
rainfall within the entire catchment.

2,500

Area of new and expanded Total rainfall in thelandslides/debris flows
period (mm)( 103 m2)

volume of sediment transported. Therefore, we treat
deposition behind the dam as sediment yield.

Maximum per day

400
200
0
19821987199219971986
1991
1996
2001
PeriodFig. 3 Area of new and expanded landslides/debris flows
4.

Results

4.1 Sediment supply processes
The areas of landslides and debris flows confirmed
from aerial photos occupy 0.7 ‒ 1.2 % of the total
Miyagawa Dam catchment (Fig. 2). An increase in areaof landslides and debris flows area can be seen in 1992;
the area of new and expanded landslides/debris flows
during 1987–1991 is high compared to the periods (Fig.
3). Initiation of landslide/debris flows might be affectedby total rainfall in the period rather than maximum daily
rainfall; however, influence of rainfall factors onlandslide/debris flow area is not clear (Fig. 3). Hiramatsu
et al. (2002) and Numamoto et al. (2004) noted that clear
cutting induced many landslides and debris flows in the
Miyagawa Dam catchment, thus, both rainfall factors
and forest management may contribute to these mass

5000

Volume of new deposits
300

4000

Cumulative volume of deposits
3000

200
2000
100

1000

Cummulative volume of deposits
in the dam lake (10 3 m3)

Volume of new deposits in the
dam lake (103 m3)

400

0

0
1956

1966

1976

1986

1996

year

Fig. 4 Changes in volume of dam deposits
wasting processes.
4.2 Sediment transport processes
Cumulative volume of dam deposits increased
gradually since the completion of Miyagawa Dam in1956 (Fig. 4). Volume of new deposits fluctuate fromyear to year; however, the long-term trend exhibits anincrease with time, particularly after 1990. This trend
may be affected by the activity of sediment supply,
because increases in sediment yield start correspond to
the period when sediment supply is high (1987–1991;
Fig. 3).
Factors that affect the timing of sediment transport
were investigated by cross correlation analysis (Table 1).
The yearly volume of new dam deposits was used as alagged variable, whereas the three discharge parameters(maximum instantaneous discharge, maximum daily
discharge, and yearly discharge) and the three rainfall
parameters (maximum daily rainfall, yearly rainfall, andmaximum total rainfall) are used as fixed variables. Allof the cross correlation functions were calculated bysubtracting the 5-yr moving average from annual
parameter volume, with the annual value “centered” inthe 5 yr moving average interval to remove the effect of
the active sediment supply in 1987–91. Maximum
instantaneous discharge yields the highest crosscorrelation coefficient of these six discharge/rainfall
parameters. Maximum daily rainfall was the most highly
correlated rainfall parameter. The highest crosscorrelation coefficient between the new dam deposit
volume and maximum instantaneous discharge occurred
for a lag of 0 yr; this correlation rapidly declines for a lagof 1 yr (Fig. 5a). These characteristics are also seen in the
cross correlation function between the dam deposit
volume and maximum daily rainfall (Fig. 5b). Thus, itappears that sediment reaches the lake within the same
year after large rainfall-runoff events, and the effects ofsuch large events do not continue substantially into thenext year. Miyazaki and Onishi (1996) also investigated
the relation between rainfall factors and dam deposit
volume more than 15 dam lakes in Japan; in most ofthese catchments, maximum daily rainfall was strongly
correlated with sediment volume behind the dams.
Because channel gradients near the dam lakes are usually
gentle (< 5˚) (e.g., Miyagawa Dam: about 3˚), fluvial
processes, which can be explained by hydraulicconditions, dominate sediment transport near the Damlake. Thus, the timing of sediment transport cancorrespond to high instantaneous discharge and/or highshort-term rainfall intensity, which is closely related to
instantaneous discharge.
Table 1 Maximum cross correlation coefficient between
volume of new dam deposits and discharge/rainfall
parameters
Fixed variable

Maximum crosscorrelation coefficient

Discharge
3
Maximum instantaneous discharge (m /s)
3
Maximum daily discharge(m /day)3
Yearly discharge (m /yr)

0.44
0.39
0.24

RainfallMaximum daily rainfall(mm/day)Yearly rainfall (mm/yr)
Maximum total rainfall (mm)

0.40
0.25
0.20

*Volume of new dam deposits are used as lagged variable0.5
Cross correlation coefficient

Cross correlation coefficient

0.5

0

0

-0.5

-0.5
0

2

4

6
lag (yr)

8010(a) Cross correlation function between volume of new
dam deposits and maximum instantaneous discharge

246
lag (yr)

810(b) Cross correlation function between volume of
new dam deposits and maximum daily rainfall

Fig. 5 Cross correlation function

4.3 Position of landslide/debris flow deposits
A portion of the landslide/debris flow sediments aretransported directly into the channel, while the otherportion of the sediments accumulate on hillslopes and inchannels (Photo 1). Thus, the position of landslide/debris
flow deposits must be considered when discussing therelationship between sediment supply and transportbecause position can influence on the future movementof these deposits (Benda, 1990).
Landslides and debris flows can be classified by the

position of its deposits as shown in Fig. 7: (1) all
sediment deposits on the hillslope (Type-1); (2) some
portion of sediment reaches the channel while the otherportion remains on the hillslope (Type-2, Photo 1); and(3) almost all sediment reaches the channel (Type-3).
The terminus of new landslides/debris flows that
occurred between 1982 and 1986 were investigated using
GIS software. This period was selected because it is prior
to the rapid increase of unstable sediment produced bylandslides and debris flows during 1987 to 1991. The
area of landslides/debris flows was used as an index of
sediment volume. Based on these aerial estimates, about
15% of landslides and debris flows deposited completely
on hillslopes (Type-1); the other 75% reached thechannels (Type-2 and Type-3). We could not distinguish
Volume of new dam deposits (m3/yr)

The relationship between maximum instantaneousdischarge, which had the highest cross-correlation
coefficient, and volume of new dam deposits is different
for each assessment period (Fig. 6). The volume of dam
deposits in from 1987 to 1991 is higher than thepreceding period (1982–86); this trend corresponds to theactivity of sediment supply (Fig. 3), implying that
sediment supply from landslides and debris flows affects
sediment yields within a few years. Activity of sedimentsupply in the period from 1992 to 1996 is almost same asthat of 1987 to 1991; however, the volume of new dam
sediments is still high. Thus, the effect of sedimentsupply on sediment yield may be sustained for more thanseveral years. Contributions of chronic sediment supply
processes (e.g. surface erosion, bank erosion) might besmall compared to episodic sediment supply processes(e.g. landslide and debris flow) in the Miyagawa Damcatchment, because large differences in new dam deposit
volumes are evident for different periods determined by
the frequency of aerial photos.

400
82-86
87-91
92-96

300

97-00
200

100

0
0

500
1000
1500
2000
2500
Maximum instantaneous discharge (m3/s)

Fig. 6 Comparison of maximum instantaneous discharge
and volume of new dam deposits

(a)

(b)

Fig. 7 Position of landslide/debris flow deposits. (a)Hillslope, (b) Hillslope and Channel, and (c) Channel.

between Type-2 and Type-3 deposition based on aerial
photo analysis. Thus, volumes of landslide/debris flow
scars and hillslope deposits were measured by fieldsurvey and the percentage of hillslope deposits of thetotal sediment produced was calculated. An average ofabout 5 % of total landslide/debris flow sedimentremained on the hillslope in the 30 failures that wereclassified as Type-2 and Type-3; >90% oflandslide/debris flow sediment was transported directly
into the stream. Thus, about 20% of total landslide/debris
flow sediment deposited on the hillslope and about 80%
reached the channel in the Miyagawa Dam catchment.
This percentage is lower than other catchments, e.g. 40%
in Saru River, Japan (Nakamura et al., 1995). The steep
topography in Miyagawa Dam catchment could reduce
the amount of sediment deposited on the hillslope; slope
gradients from the headcrop of landslides/debris flows to
the channel are generally >35˚ based on field surveys
conducted in the 30 Type-2 and Type-3 landslides/debris
flows (Fig. 8). The relationship between erosion andPhoto 1 Landslide scar and deposits at the foot of ahillslope; overall, 17% of the total sediment deposits
accumulated at the foot of hillslope.

deposition caused by landslides and slope gradient incoastal Alaska was examined; slopes whose gradientsexceeded 35˚ were considered erosion zones for all forest
types (Johnson et al. 2000). This gradient is almost thesame or greater than the internal friction angle of thesoils; thus, most of the sediments in the Miyagawa Damcatchment are entrained and transported into the channel.
4.4 Channel gradient of landslide/debris flow
deposits
10

Landslide frequency

(c)

8
6
4
2
0
<30

30-35 35-40 40-45 45-50>50

Slope gradient (° )Fig. 8 Frequency of landslides related to slope gradient(gradient is from the headcrop of landslides/ debris flows
to the channel)(a)Percent of total landslide/debris flow
deposits in channel

Catchment
Landslide

Deposits

Channel gradient: Steep

40

30

20

10

0
<10

Future transport: Debris flow(b)

Catchment

Landslide

Deposits
Channel gradient: Gentle

10-15 15-20 20-25 25-30>30Channel gradient (° )Fig. 10 Channel gradient of landslide/debris flow
deposits
zero-order basins in Miyagawa Dam catchment basedon field surveys). Surface flow might not occur in thesechannels because debris flows can run down inunsaturated conditions (Imaizumi et. al, 2005); thus,
fluvial processes in such conditions would be
insignificant.

Future transport: Fluvial processes

5.
Fig. 9 Position of landslide deposits and future transport
processes. (a) Landslide deposits in steep channel, and(b) Landslide deposits in gentle channel.
Processes which transport landslide/debris flow
deposits depend on the channel gradient near thedeposits. Debris flows entrain deposits in steep channels,
while fluvial processes entrain sediments in gentle
channel (Fig. 9). Benda (1990) indicated that thedrainage area above deposits influences the longevity ofdebris flow deposits; deposits below small drainage
areas remain in the channel for a long time while
deposits below large drainage areas (>20 km2) areeroded by fluvial processes within several years.
About 15% of Type-2 and Type-3 landslides/debris
flows deposited in channels with gradient < 10˚ (Fig.
10). Deposits in this zone are likely later entrained byfluvial processes because of large drainage areas andgentle channel gradients. About 45% of Type-2 andType-3 landslides/debris flows terminated in channels
steeper than 25. Many debris flows initiate in such
reaches (VanDine 1985; Pareschi et al. 2002), and roleof debris flows on the transport of these deposits might
be high. About 30% of the landslide/debris flow
sediments deposit in channels > 30˚ (corresponding to

Discussion

Sediment supply from landslides and debris flows
affects sediment yield and the influence continues formore than several years in Miyagawa Dam catchment.
This indicates that a large portion of the sediments
produced from landslides and debris flows are stored
temporally in the catchment as hillslope and channel
deposits. Location of sediment deposits (both on thehillslope and in the channel) affects sediment mobility,
specifically, the timing and amount of sediment
distributed downstream.
Landslide deposits on hillslope can be entrained intochannel by surface erosion and soil creep; however, theseprocesses (especially creep) are slow. Thus, the timing
landslide deposition on hillslopes may lag sedimenttransport. The mobilization of sediment deposits tochannels depends on channel gradient; the frequency of
sediment transport in steep channels is low because of
the long intervals between debris flows (Benda andDunne, 1997; May and Gresswell, 2003), whereas
sediment can be entrained frequently by fluvial processes
in gentle channels.
The position of deposits varies from catchment tocatchment depending on hillslope and channel
topography. Only a small portion of landslide/debris flow

sediments (≈ 20%) deposit on hillslopes in Miyagawa
Dam catchment because of steep slopes. About 45% of
the total channel deposits produced from landslide/debris
flows are in debris flow initiation zones (>25˚); there
may be a significant lag time between sediment supply
and transport in the Miyagawa Dam catchment for suchdeposits. About 15% of total channel deposits are ingentle channels. Even though armoring of the channel
can effectively decrease sediment mobility, sediment canbe entrained during stormflows and rapidly betransported downstream.
6.

Conclusion

Although a clear relationship between sedimentsupply and transport processes at the large-catchment
scale is necessary to develop a unified sedimentsupply/transport unified model, field data concerning
this relationship are scarce. The following conclusions
are obtained based on aerial photo assessments,
sediment deposits above the dam, and field
investigation in the Miyagawa Dam catchment, central
Japan:(1) Timing of sediment transport near Miyagawa Damlake corresponds to large rainfall-runoff events because
sediment transport processes near the lake are fluvial
processes that can be explained by hydraulic factors.
(2) Sediment yield from entire catchment is affected byoccurrence of landslides and debris flows in steep areasof the catchment. Occurrence of landslides/debris flows
begin to affect catchment sediment yield quite rapidly,
and the influence is sustained for more than several
years.
(3) The position of landslide/debris deposits plays animportant role on the relationship of sediment supply andtransport. Sediments delivered into gentle channels areeasily transported downstream during high discharge
events, whereas the mobility of sediment delivered into
debris flow sites (primarily zero-order basins), is not highbecause debris flows occur infrequently at such sites.
(4) Of the landslide/debris flow sediments that were
initially classified as being deposited in channels, about
30% of the deposits terminated in steep channels (> 30˚)
where fluvial processes are minimal. Furthermore, about
20% of sediments are deposited in channel reaches
where they may later move as debris flows (gradients of
25–30˚). Because mobility of deposits in such debris
flow channels is not high, these deposits create long-term

sediment supplies that influence later sediment yields.
Thus, the position of deposits and frequency of
sediment transport processes (especially debris flows)
should be considered when developing a unified
sediment supply and transport model.
Acknowledgements
This study is supported by a JSPS grant (#16380102)
to R.C. Sidle. The River Division of Mie Prefecture
kindly provided us with data on dam deposits, rainfall,
and river discharge. We thank Professor Shinya
Hiramatsu for letting us use aerial photos of thiscatchment. Finally, we are grateful to the residents of
Miyagawa village, including people from the Ohsugi
Natural School, Forest Association, and foresters, forhelping our field survey and giving us important
information about the site in the wake of the severe
damage caused by Typhoon Meari (T0421).
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山地流域における土砂生産と土砂流出の量的・時間的な関係
今泉文寿*・Roy C. Sidle*京都大学防災研究所COE研究員
要旨
本研究では、三重県宮川ダム流域において、空中写真判読、ダム堆砂量の解析、現地調査等を行い、土砂生産と土砂流出の量的・時間的な関係、およびその関係に影響を及ぼす要因を検討した。ダム湖周辺における土砂流出のタイミン
グは大きな降雨流出イベントのあった年と一致する。また、土砂生産の影響は土砂流出に数年以上継続する。これらの特性は、崩壊・土石流による堆積土砂の堆積位置に影響を受けていると考えられる。

キーワード：土砂生産、土砂流出、ダム堆砂、宮川ダム、渓床堆積物

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山地流域における土砂生産と土砂流出の量的・時間的な関係〇今泉文寿・Roy C. Sidle

１．はじめに生層の粘板岩および砂岩が大部分を占めている。

山地で生産された土砂は土石流として人的被宮川ダムでは毎年，ダム湖の堆砂量が測量され害を与えるほか，浮遊砂・掃流砂として流下し，ており，これから流域からの流出土砂量を推定しダム湖の貯水容量の減少など経済的被害を与えた。また，流域内での崩壊・土石流の発生によるる可能性がある。これらの被害の抑制には，砂防

土砂生産状況把握するため，最近約 20 年間の白構造物等での流出土砂量の調整に加え，森林整備

黒空中写真（1982，1986-87，1992，1996-97，

等による生産土砂量の抑制が必要である。山腹で2001 年）を判読した。これらのデータおよび降生産された土砂が一時的に谷底に貯留した後に雨データを対比させることにより，土砂生産と土下流へ流下を開始するように，土砂生産と土砂流

砂流出の間に存在する関係を検討した。

出の間には量的・時間的な関係が存在する。しかしながらこの関係は未解明の部分が多く，従来土

３．結果および考察

砂生産と土砂流出は個別に研究されていた。そこ

ダム堆砂の増加量と降雨因子・流量因子の相互で本研究は，実際の山地で両者の間に存在する時相関関数を調べたところ，ダム堆砂量の増加量と間的・量的な関係を明らかにするため，三重県南最も相関の高いものは年間最大流量（瞬時値）で部の宮川ダム流域において現地調査・空中写真判あった。また，相関係数が最大となるタイムラグ

読等を行った。

は 0 年であった。このことから，ダム湖周辺での土砂流出現象は大きな降雨・流出イベントのある

２．研究対象地および研究方法

年に活発であるといえる。その理由として，ダム本研究対象地の宮川ダム流域は大台ケ原山系

湖周辺での土砂の移動形態は主に掃流砂・浮遊砂

日出ヶ岳から宮川ダムまでの流域面積 125.6 km2，

であり，これらの流出は水理条件で説明すること

流路長約 26 km である（図−1）。年降水量 1600が可能であることから，流量によって流砂量が決mm〜4500 mm の多雨地域であり，地質は秩父古定されることが考えられる。また，空中写真判読
結果とダム堆砂量の変化を対比させた結果，流域内での多数の崩壊・土石流の発生がみられるとそ

Mt. Ikekiya(1395.9m)

Miya Riv.

04km

砂量へ影響していること，さらには生産された土
Miyagawa
Dam

砂のすべてがすぐに下流へ流下するわけではな

Mt. Senchiyogamine(1099.7m)が存在することがわかる。宮川ダム流域では，河Miyagawa

川で堆積する崩壊・土石流土砂のうちの約 45％が勾配 25˚以上）に堆積する。土石流は頻度の低い
土砂移動現象であるため，ここでの堆積は土砂生

Ohdaigahara

Pacific Ocean

図−1

く，流域内に一時的に貯留した後に流下する土砂

土石流の発生する可能性がある急勾配区間（河床Ohsugi Valley
Mt. Hidegatake(1694.9m)の増加は数年以上持続することが明らかになった。これらの結果から，流域内での土砂生産が流

Dam Lake

Mt. Kunimi(1283.0m)

れに伴い流砂量が増加すること，さらには流砂量

宮川ダム流域

産と土砂流出の間に時間的なラグを生じさせる
原因のひとつであると考えられる。