E3S Web of Conferences 415, 05027 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341505027

Spatial Variation in Specific Sediment Discharge Volume fromFirst-Order Catchment due to Heavy Rainfall and its Factors
Misa Tsushima1*, and Taro Uchida1
1University of Tsukuba

Abstract. In this study, we evaluated the variations in sediment discharge between basins from the firstorder catchment of the Akatani River watershed, where sediment discharges as a result of heavy rainfall. Wetested the correlation between sediment discharge, topography, and rainfall magnitude using LiDAR datasets.
The results showed that the specific eroded sediment volume in the first-order catchment was sufficiently
greater than the specific deposited sediment volume, and the variation of the specific discharge sedimentvolume from the first-order catchment was determined by the variation of the specific eroded sedimentvolume. The spatial variations of specific discharged sediment volumes could not be evaluated solely by
topography and rainfall indices; even in the catchment, where the topographic condition and rainfall
magnitude were almost the same, the specific sediment discharge varied by more than one magnitude.

1 Introduction
Debris flow and sediment-filled floods cause serious
damage to properties and human lives. The magnitudeof damage caused by the debris flow and sediment-filled
floods is influenced by various factors, among which theamount of sediment deposited during the flood is one of
the most important factors in determining the extent of
the damage.
Numerous methods have been used to study
sediment yield data, including in situ observations,
LiDAR data analysis, measurements of sedimentvolumes deposited in sediment control dams, debris
basins, and sediment traps, and assessments based on
sediment lobes volume [1-3]. These studies have shownthat a single rainfall can cause differences in sediment
discharged from one watershed to another, and even
within the same watershed, differences in sediment
discharged can occur depending on the amount ofrainfall [1,4].
It might be possible to evaluate the extent to which
factors other than geology, vegetation, and rainfall
conditions contribute to differences in sediment
discharged when we focus on the spatial variation ofsediment discharge caused by a single heavy rainfall
event. However, studies evaluating the spatial variationin sediment discharge and its factors in a single rainfall
event in the same area are limited. Therefore, theobjective of this study was to quantitatively evaluate
inter-basin differences in sediment discharge in the firstorder catchment in an area where intensive sediment
runoff has occurred. Additionally, the relationshipbetween the topographic indices and discharged
sediment volume, which had been correlated with
discharged sediment volume by previous studies [1], andthe relationship between rainfall and discharged
sediment volume were examined to analyze the factorsthat produce spatial variation in sediment volume.

2 Methods
2.1 Study site
The Akatani River watershed is located in Asakura
City, Fukuoka Prefecture, and is a tributary of the
middle reaches of the Chikugo River. The watershedarea is approximately 20 km2, and the elevationdifference from the confluence with the mainstream of
the Chikugo River to the highest point (Mt. Hirokura) inthe Akatani River watershed is approximately 560 m.
The Otoishi River feeds the Akatani River from the westside, the Ogouchi River from the east side, and theTable 1. Specs of Data

DataMeasuring
institution
DateResolution

LiDAR
Before the disaster
After the disaster
Kyushu Regional Development Bureau, Ministryof Land, Infrastructure, Transport and Tourism
2017/1
2017/71m1m

Rainfall data (R/A)
Japan Meteorological Agency
2017/6/5 0:00～8/6 0:00
1km

* Corresponding author: s2130290@s.tsukuba.ac.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 05027 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341505027

2.3.2 Rainfall and topographic indices

Ooyama River from downstream of the east side. Thereis a valley bottom plain along the main channel withsediment supplied by the tributary rivers.
The west side of the Akatani River is mainlycomposed of granodiorite (plutonic rocks) of the Late
Mesozoic-Cretaceous age, whereas the east side ismainly composed of muddy schist (metamorphic rocks)of the Late Mesozoic-Triassic age [5].

Previous studies have shown that the amount ofsediment discharged from a watershed is influenced byrainfall and topography [1-4,7]. Therefore, this study
also tested the correlation between rainfall or
topographic indices and each sediment amount, such as
eroded, deposited, and discharged sediments, per unit
area.
Seven rainfall indicators were tested at maximum
conditions of 0.5, 1, 3, 6, 12, and 24 h. Thirteen
topographic indicators were used based on the previous
research [1], namely: perimeter (km), area (km2), relief(m), length (m), relief ratio, number of zero-ordervalleys, mean drainage area (m2), mean slope (%), mean
topographic wetness index (TWI) (m2), Melton ratio,
form factor, elongation ratio, circularity index.
Perimeter and area are calculated using GIS, and relief
is the difference between the highest and lowest
elevation of the main channel, length (m) is the lengthof the main channel, and relief ratio is the relief of themain channel divided by its length. Several zero-ordervalleys are the number of valleys with a catchment areaof less than 10,000 m2 connected to the channel. Mean
drainage area (m2), mean slope (%), and mean TWI are
calculated as the averaged value of the 10-m-grid in thefirst-order watersheds.

2.2 Phenomenon
Heavy rain continued to fall from July 5 to 6, 2017,
mainly due to warm and humid air flowing toward therainy season, resulting in record heavy rainfall in theAkatani River watershed area. This disaster recorded thehighest rainfall ever observed at many stations fromAsakura City, Fukuoka Prefecture, to the northern partof the Hita City, Oita Prefecture. For example, the 12hour rainfall observed at the JMA Asakura observatory
was 511.5 mm. Human casualties were significant, with41 people dead or missing, 197 houses destroyed, and102 houses half destroyed, mainly in Asakura City.
2.3 Data collection and analysis
2.3.1 Data preparation

3 Results

In this study, LiDAR data (10 m grid DEM, createdfrom 1 m grid DEM) measured before and after thedisaster was used for topographic data (Table 1). TheRadar/Raingauge-Analyzer was used to analyze rainfall
data. This product is an estimate of precipitation in areas
without rain gauges based on radar rainfall intensity
measurements. The spatial resolution of the data is a 1km grid.
First, to divide the Akatani River watershed into subwatersheds, the gradient and catchment area at eachpoint within the watershed were calculated using GIS.
The starting point of the channel was determined as thecatchment area of 10,000 m2 [6]. The channel network
was created by searching for the steepest gradientdirection from each channel initiation point. However,in cases where the steepest gradient could not be
represented by searching toward the steepest gradientbecause of gentle slopes, topographic maps were used tocompensate for this. As a result, 259 first-ordercatchments were classified.
Next, an elevation difference analysis was conductedusing LiDAR data from two periods before and after thedisaster to evaluate the volume of erosion, deposition,and discharge sediment in each first-order catchment.
Using the orthophotographs, and a slope map createdfrom the post-disaster DEM, the extent of the change inelevation due to sediment transport was deciphered, andpolygons were mapped for the sediment transport area.
Then, for each first-order catchment, the amount oferoded sediment (the integral value of the negative resultof the elevation difference analysis), deposited sediment(the integral value of the positive result of the elevation

3.1 Cumulative frequency of sediment volume

Cumulative frequency

The cumulative frequency distribution of eroded,
deposited and discharged per unit was shown in Fig. 1.
The specific eroded, discharged, and deposited sedimentvolume varied greatly among the watersheds, although
the study area is around 20 km2. The specific eroded
sediment volume ranged from 103.2 to 106.1 m3/km2(excluding 24 watersheds with no erosion), and thespecific discharged sediment volume ranged from 103.2
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0 2.0 3.0 4.0 5.0 6.0 7.0
Sediment volume log10x (m3/km2)
Erodedsediment
volumedifference analysis), and discharged sediment (thedifference between the amount of eroded and deposited
sediment) within the polygon of the sediment transportarea were calculated.

Fig. 12Discharged
sediment
volumeDeposited
sediment
volumeCumulative frequency of sediment volume

E3S Web of Conferences 415, 05027 (2023)
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The standard deviations for each class are generally
distributed between 100.2 and 100.6 m3/km2 for specificeroded and discharged sediment volume, and between
100.5 and 101.3 m3/km2for specific deposited sedimentvolume at (a) maximum 30-minute rainfall and (b) total
rainfall. (c) The mean slope (%) shows that the specificerosion and discharge sediments are distributed between
100.2 and 100.9, and the specific deposition sediment isdistributed between 100.2 and 100.9 m3/km2. The standard
deviations for each layer ranged from 0.4 to 1.8 times
the values calculated from watershed-wide data for
erosion and sediment discharged and from 0.3 to 1.2
times for deposited sediment.
These indicate that even if the three indicators thatare relatively highly correlated with each sedimentvolume are compared in watersheds with somewhat
similar conditions for each indicator, the variability inthe magnitude of each sediment volume is not
significantly reduced.

Table 2. Average volume of each sediment and standard
deviation (log10x m3/km2)

AverageSD

Eroded volume

105.0

100.5

Discharged volume

105.0

100.5

Deposited volume

104.2

100.8

to 106.1 m3/km2 (excluding 30 watersheds with nosediment discharge outside the watershed). The specificdeposited sediment volume was distributed between
101.1 to 105.7 m3/km2 (excluding 25 watersheds with nosediment transport), which were about one order ofmagnitude smaller than the erosion/discharged sedimentloads. The mean and standard deviation for thesesediment volumes per unit area are shown in Table 2.
Watersheds in which sediment volume was less than 0
m3/km2 were excluded from the calculation. These
indicate that erosion dominates in sediment production
in first-order valley watersheds.

5 Conclusions
In this study, we quantitatively evaluated thedifferences in the amount of sediment produced fromfirst-order valley watersheds using LiDAR data in theAkatani River watershed, Japan, which experienced
intensive debris flow due to heavy rainfall, and analyzed
the effects of topographic and rainfall indicators on thespatial variation in the amount of sediment discharged
to determine the following.
 The volume of eroded sediment was sufficiently large
compared to the deposited sediment volume, and thevariation in discharged sediment volume in the firstorder valleys was determined by the variation ineroded sediment volume.
 Spatial changes in specific eroded sediment volume,
specific discharged sediment volume, and specificdeposited sediment volume from primary catchments
are not determined by rainfall or topographic indicesalone but must also include factors other than rainfall
and topography, such as geology and vegetation.

3.2
Correlation
betweenrainfallandtopographic indicators and each sedimentvolume
The correlations between the thirteen topographic
indicators and seven rainfall indicators and eroded,
deposited, and discharged sediment amounts are shownin Table 3 and Table 4, respectively.
No indicators with r ≥ 0.5 were found for either
rainfall or topography indices. For rainfall indices,
specific eroded and discharged sediment volume had thehighest correlation coefficients with maximum 0.5 hrainfall, r = 0.33 and r = 0.36 (both p < 0.01). For
specific deposited sediment volume, total rainfall had
the largest correlation coefficient with r = 0.29 (p <0.01). In terms of topographic indices, mean slope (%)
had the largest correlation coefficient with specificeroded discharged sediment volume, with r = 0.37 and0.38, respectively. For specific deposited sedimentvolume, the Melton ratio had the highest absolutecorrelation coefficient, r = −0.29 (p < 0.01).

References
1.

4 Discussion

2.

4.1 Variation in specific produced sedimentvolume3.

The result showed large variations in the specificdischarged, eroded, and deposited sediment (Fig. 1).Thiswill help us examine how much this variation is affectedby topography and rainfall magnitude. Themagnitude of
the variation that occurs is evaluated evenwhen only thebasins with similar topography and rainfall conditions
are focused. Here, the catchments were classified using
the indicators shown to be highly correlated withsediment volumes in Section 3.2 (Fig. 2).

4.

3T. de Haas, A. L. Densmore, Geology 47, 791–794(2019)
J. E. Gartner, S. H. Cannon, P. M. Santi, Eng. Geol. 176,45–56 (2014)
M. Lorenzo, D’A. Vincenzo, Earth Surf. Process.
Landforms 29, 207–220 (2004)
Y. Hong, J. P. Wang, D. Q. Li, Z. J. Cao, C. W. W. Ng,P. Cui, Eng. Geol. 187, 122–134 (2015)

5.

Geological Survey of Japan,
https://gbank.gsj.jp/seamless/v2/viewer/ (2019)

6.
7.

D. R. Montgomery, Am. J. Sci. 301, 432–454 (2001)
T. Uchiogi, JSECE 79, 5, 21–34 (1979)

E3S Web of Conferences 415, 05027 (2023)
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Table 3. Correlation between topographic indicators and sediment volume (log10x m3/km2)
Number Mean Mean Mean
Perimete
Area
Relief Length Relief
Melton Form Elongati Circulariof 0th drainage slope
TWIr2(km )
(m)
(m)
ratio
ratio
factor on ratio ty index(km)
valleys area (m2) (%)(m2)
Eroded−0.15⁺ −0.06 −0.08 −0.12
0.12−0.04 −0.11 0.37⁺−0.08 −0.02
0.14
0.15
0.26⁺⁺⁺⁺⁺⁺
Discharged −0.15⁺ −0.07
0−0.12 0.22⁺−0.05 −0.11 0.38⁺
−0.14
0.11
0.11
0.13
0.21⁺
⁺
⁺⁺⁺
⁺
Deposited −0.01
0.08
−0.14
0−0.19⁺
0.06
0.02
0.19⁺
0.16⁺ −0.29⁺
0.1
0.11
0.23⁺
⁺⁺
⁺
⁺⁺

⁺ ⁺ p < 0.01, ⁺ 0.01 < p < 0.05
Table 4. Correlation between rainfall indicators and sediment volume (log10x m3/km2)
Maximum rainfall
0.5 hours

1 hour3 hours

6 hours

12 hours

24 hours

0.33⁺
⁺0.36⁺
⁺
0.25⁺
⁺

0.23⁺ ⁺

0.1

0.2⁺ ⁺0.25⁺ ⁺

0.28⁺ ⁺

0.27⁺ ⁺

0.12

0.21⁺ ⁺

0.23⁺ ⁺

0.19⁺ ⁺

0.1

0.21⁺⁺
0.19⁺
⁺

0.25⁺ ⁺

0.28⁺ ⁺ErodedDischarged
Deposited

Total rainfall
0.29⁺⁺0.25⁺⁺
0.29⁺
⁺⁺ ⁺ p < 0.01, ⁺ 0.01 < p < 0.05

Deposited sediment volume

Standard deviation (log10x m3/km2)

Discharged sediment volume

Eroded sediment volume

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0

(a) Maximum 0.5 hours
rainfall(b) Total rainfall(c) Mean slope (%)

0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.4
1.2
1.0
0.8
0.6
0.4
SDSD (N = 295)

0.2
0.0

0.8

0.9

1.0

1.1

1.2 0.8

0.9 1.0 1.1
Ratio to median

1.2 0.5

0.75

1.0

1.25

1.5

Fig. 2 Relationship between median ratio of (a) maximum 0.5 hours rainfall, (b) total rainfall, and (c) mean slope (%) andsediment dispersion4