Characteristics of discharge and water chemistry in a sediment disaster area in Hofu, Yamaguchi Prefecture,
western Japan
Masamitsu FUJIMOTO,1,2,* Takahisa MIZUYAMA2 and Maki SEKIGUCHI3
1 Colleage of Science and Civil Engineering, Ritsumekan University (Nojihigashi, Kusatsu, Shiga 5258577, Japan) 2 Graduate School of Agriculture, Kyoto University (Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 6068502, Japan) 3Department of Agriculture, Kyoto University (Kitashirakawa, Sakyo-ku, Kyoto, Kyoto 6068502, Japan)
*Corresponding author. E-mail: m-fuji@fc.ritsumei.ac.jp
Hydrological and hydrochemical observations were conducted in Hofu in Yamaguchi Prefecture in October 2011 to determine differences between catchments that collapsed and those that did not suffer collapse. The results indicate that the specific discharges were higher in the former case than in the latter. The chemical characteristics of the springwater and streamwater in the observed catchments varied owing to differences in bedrock weathering. Na+ and Ca2+
concentrations were lower in catchments where collapses occurred compared with uncollapsed catchments. These results suggest that the specific discharge and chemical components of streamwater are good indicators of the level of risk from landslides. Our study demonstrates the possibility of predicting potential disaster areas using these parameters.
Key words: sediment discharge, specific discharge, water chemistry, bedrock groundwater, weathering
1. INTRODUCTION
Recent increases in total and peak rainfall amounts have resulted in several major disasters in various areas in Japan. To minimize the slope failure damage, accurate predictions of areas of high vulnerability to landslides and debris flows are required. However, such predictions have proven difficult because the landslide area is usually limited to a small percentage of the overall area even in larger scale disasters, and debris flows do not occur in all streams.
Some studies have found that groundwater exfiltration from the bedrock to soil layer affects the occurrence of landslides [e.g., Wilson and Dietrich, 1987; Montgomery et al., 1997; Jitousono and Shimokawa, 1998; Onda et al., 2001; Hayashi et al., 2004; Jitousono et al., 2004]. Several studies have reported the existence of bedrock springs on landslide scars [e.g., Jitousono and Shimokawa, 1998; Hayashi et al., 2004]. This suggests that areas where there is movement and concentration of bedrock groundwater may have greater potential for landslides. In consideration of this, Jitouzono [2014]
proposed a method to predict deep-seated landslides by using streamwater chemistry (electrical
conductivity) and demonstrated its effectiveness.
Thus, by evaluating the spatial characteristics of discharge and streamwater chemistry (mainly bedrock groundwater), it may be possible to identify hazardous locations. In this study, we evaluated the spatial characteristics of discharge and streamwater chemistry in areas that suffered landslides and debris flows and compared them with areas that did not suffer any landslides or debris flows.
2. METHODS
2.1 Study site
This study examined a sediment disaster at Hofu in Yamaguchi Prefecture, where a shallow landslides and debris flows occurred on July 21, 2009, causing extensive damage [Furukawa et al., 2009]. On 21 July 2009, heavy rainfall occurred throughout the day associated with an activate Baiu front. The maximum daily and hourly rainfall amounts observed by the Hofu Meteorological Agency were 275 mm and 72.5 mm, respectively, the largest values ever recorded in Hofu, resulting in catastrophic damage. There were 17 deaths and considerable material loss, including 33 houses that
were completely destroyed and more than 100 were damaged.
In the period 1981–2010, the average annual temperature in Hofu was 15.6°C, and the average rainfall was 1,632.2 mm. The geology of the study site is categorized as mainly granite.
2.2 Field observation, sampling, and analysis This investigation was conducted in the Sawagawa catchment in Hofu, Yamaguchi Prefecture, western Japan. Observations were conducted from 12 to 15 October, 2011. The total rainfall amount over the 20 days before the investigation was 19 mm, indicating that the investigation period occurred during baseflow conditions. In total, there were 47 sample points: 36
in the catchment where a landslide had occurred, nine in the catchment where there was no landslide, one in a spring on a hillslope that had collapsed, and one in a spring on a hillslope that had not collapsed (Fig. 1). Hereafter, the above four types of investigation points are referred to as “landslide catchment,” “non-landslide catchment,” “spring on landslide hillslope,” and “spring on non-landslide hillslope,” respectively. We collected springwater and streamwater samples for chemical analysis and measured the discharge rate under baseflow conditions at sampling points in catchments of various sizes. Stream flow velocities were measured using an electromagnetic velocity meter, and discharge was calculated by multiplying the cross-sectional area by the stream velocity. At
0 1000 2000m ｍ
：Collapsed point
：Non-collapsed point
：Spring from natural slope
：Spring from collapsed slope
Fig. 1 Location of sample sites
Na+(mg/L)
4 5 6 7 8 9 10
Si(mg/L)
4 5 6 7 8 9 10 11 12
Landslide catchment Non-landslide catchment Spring from landslide hillslope Spring from non-landslide hillslope
Ca2+(mg/L)
0 1 2 3 4 5 6 7 8
Si(mg/L)
4 5 6 7 8 9 10 11 12
Catchment area (ha)
0.1 1 10 100 1000
Specific discharge (mm/day)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Landslide catchment Non-landslide catchment Spring from landslide hillslope
low-discharge points (<1 L/s), discharge was measured directly by collecting water using a measuring cylinder and a stopwatch. During the investigation, there was a small rainfall event (total rainfall, 40 mm). To avoid a subsequent increase in discharge, we did not observe the discharge after the beginning of the rainfall. For the water sampling carried out on 14 October 2011, the water chemistry should not be influenced by the canopy interception because the accumulated rainfall was <5 mm.
Although water samples taken at two points in the morning of 15 October may be affected by rainfall, we analyzed these data because due to the scarcity of sampling data for that period.
Ion concentrations were measured by ion chromatography (ICS-90: Dionex, Sunnyvale, CA, USA), and SiO2 concentrations were determined by the molybdenum yellow method using a spectrophotometer (U-1000: Hitachi, Tokyo, Japan).
The δD and δ18O compositions of water samples were determined by using an isotopic water analyzer (L2120-i: Picarro, Santa Clara, CA, USA).
3. RESULTS AND DISCUSSION
3.1Hydrological and hydrochemical analysis There was no clear relationship between catchment area and specific discharge (Fig. 2). The specific discharge tended to be higher in landslide catchments than in non-landslide catchments. The baseflow discharge consisted mainly of groundwater, especially bedrock groundwater. Our results suggest that the contribution of bedrock groundwater was larger in landslide catchments than in non-landslide catchments.
As the Na+ concentration of springwater and streamwater increased, so did the SiO2 concentration (Fig. 3). Although the Ca2+ concentration of springwater and bedrock groundwater also increased with increasing SiO2 concentration, the relationship between the two constituents was weak. Generally, Na+, Ca2+, and SiO2 are produced by chemical weathering of bedrock. Although the elution of Na+ and SiO2 from the bedrock did not differ markedly among catchments, the variability in the Ca2+
concentration reflected differences in the degree of bedrock weathering. Ca2+ concentrations were lower in non-landslide catchments than in landslide catchments, indicating more extensive bedrock weathering in the landslide catchments.
The Ca2+ concentrations of streamwater clearly increased with the specific discharge (Fig. 4). This suggests that Ca2+ concentrations can be used to
clarify the extent of bedrock weathering and the contribution of bedrock groundwater.
Figure 5 shows the relation between catchment area and water chemistry. For all chemical
Fig. 2 Relation between catchments and specific discharge
Fig. 3 Characteristics of streamwater and streamwater chemistry
components, concentrations were highly variable when the catchment area was small. As catchment size increased, variability in water chemistry decreased in the larger catchments, although the limited number of observation points in the larger catchments must be taken into account. Although the Si concentration of non-landslide catchments was highly variable, the variability was smaller in non-landslide catchments than in landslide catchments. Na+ and Ca2+ concentrations were lower in non-landslide catchments than in landslide catchments. These results suggest that Na+ and Ca2+
concentrations can be used as indicators of the chemical weathering of bedrock to determine the degree of risk of collapse in a catchment.
The relationships between the δD and δ18O isotopic ratios of springwater and streamwater and the specific discharge were weak and variable (Fig.
6). However, no clear differences were identified between landslide and non-landslide catchments. In high-altitude areas, the δD and δ18O isotopic ratios of rainwater tend to be low, owing to the effect of altitude [Clark and Fritz, 1997]. Based on this effect, groundwater recharge areas and altitude have been estimated by using the distribution of isotope ratios in springwater and streamwater. However, although the altitude effect was generally apparent in the
Specific discharge (mm/day)
0.01 0.1 1 10
delta D (‰)
-8.4 -8.2 -8.0 -7.8 -7.6 -7.4 -7.2
Specific discharge (mm/day)
0.01 0.1 1 10
delta18 O (‰)
-54 -52 -50 -48 -46 -44
Fig. 4 Relation between discharge and the chemistry of springwater and streamwater (the legend is the same as in Fig. 3)
Catchment area (ha)
0 200 400 600
Si (mg/L)
4 6 8 10 12
Catchment area (ha)
0 200 400 600
Na+ (mg/L)
4 5 6 7 8 9 10
Catchment area (ha)
0 200 400 600
Ca2+ (mg/L)
0 2 4 6 8
Fig. 5 Relation between the catchment area and the chemistry of springwater and streamwater (the legend is the same as in Fig. 3)
Fig. 6 Stable isotope ratios of springwater and streamwater discharge (the legend is the same as in Fig. 3)
Specific discharge (mm/day)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Si (mg/L)
4 6 8 10 12
Specific discharge (mm/day)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Na+ (mg/L)
4 5 6 7 8 9 10
Specific discharge (mm/day)
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Ca2+ (mg/L)
0 2 4 6 8
(a)
Eigenvector 1
0.0 0.1 0.2 0.3 0.4 0.5 0.6 Na+
K+
Mg2+
Ca2+
Si
(b)
Eigenvector 2
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 Na+
K+
Mg2+
Ca2+
Si
isotopic composition of streamwater, it was difficult to identify the recharge area or altitude of the groundwater because the differences in the altitudes of the sampling points in our study area (~150 m) were small.
3.2 Principal component analysis
To assess the weathering of bedrock and its effects on springwater and streamwater chemistry, we applied principal component analysis (PCA) using the cations (Na+, K+, Ca2+, Mg2+) and Si. The cumulative contribution ratios of the first and second main components were 49.1 and 81.9%, respectively, indicating that the information obtained for the five chemical components can be summarized in two dimensions. The results showing the eigenvectors of the first and second main components are shown in Figure 7. The values of the eigenvector of the first principal component were positive (Fig. 7a). Since the five chemical components were largely derived from chemical weathering of bedrock, the value of the first principal component can be used as an indicator of such chemical weathering. The eigenvectors of the second principal components were positive for Na+ and Si and negative for Ca2+, Mg2+, and K+ (Fig. 7b).
These results indicated that in the chemical weathering process of bedrock, although the Na+ and Si eluted similarly, the differences in elution degree of Ca2+, Mg2+, and K+ in the catchments affected the eigenvector of the second principle component.
The results of the PCA are shown in Figure 8.
The scores of the first and second principle components were highly variable. The springwater and streamwater data for non-landslide catchments were mostly located in the negative area of the first principal component and the positive area of the second principal component. The scores for the collapsed areas indicated the process of bedrock weathering, suggesting that the extent of chemical weathering can potentially identify catchments that may collapse.
4. CONCLUSIONS
Hydrological and hydrochemical observations were conducted in Hofu, Yamaguchi Prefecture where a landslide and debris flow occurred in July 2009. The specific discharge was larger in landslide catchments than in non-landslide catchments. The chemical characteristics of springwater and streamwater in the observed catchments varied due to differences in bedrock weathering. Na+ and Ca2+
concentrations were lower in landslide catchments than in non-landslide catchments, suggesting that the concentrations of these chemical components are good indicators of the level of risk of collapse. Our study suggests the possibility of predicting areas that are likely to collapse based on hydrological and hydrochemical investigations. However, our investigations did not cover the entire area due to the difficulties of access to some sites. An additional investigation will consider disaster prediction.
First Principal Component
-3 -2 -1 0 1 2 3 4 5
Second Principal Component
-6 -4 -2 0 2 4 6
Fig. 7 Eigenvectors of first and second principal components
Fig. 8 Results of the principal component analysis (the legend is the same as in Fig. 3)
ACKOWLWDGEMWNT: This research was supported by the Japan Society Erosion Control Engineering Aid for young researcher.
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