7th International Conference on Debris-Flow Hazards Mitigation Study of prediction methods of debris-flow peak discharge
Akihiko Ikedaa,*, Takahisa Mizuyamab, Takahiro Itohc
aSabo & Landslide Technical Center (STC), 2-7-5 Hirakawacho, Chiyoda-ku, Tokyo 102-0093, Japan bProfessor emeritus, Kyoto University, 3-14 Shinjocho, Ibaraki, Osaka 567-0884, Japan cResearch and Development Center, Nippon Koei Co., Ltd., 2304 Inarihara, Tsukuba, Ibaraki 300-1259, Japan
Abstract
Prediction of peak discharge of debris flow is one of the most important factors to mitigate debris-flow disasters. In general, peak discharge of debris flow has been either measured at the observation sites directly or estimated based on debris-flow velocity and cross-sections derived via field investigations (e.g., Mizuyama et al., 1992; Rickenmann, 1999, 2016). Based on these data, peak discharge of debris flow has been estimated from various aspects: (a) theoretical formulae, numerical simulations and laboratory flume experiments; (b) peak flood discharge using rational formulae; and (c) empirical methods based on the relationship between the peak discharge and total debris flow volume (magnitude) for many debris flow events (e.g., Takahashi, 1991; Mizuyama et al.; 1992, Ikeda et al., 2015). Peak discharge and magnitude of debris flow measured via direct observation are reliable. Although, while residual markings do not always accurately indicate riverbed and cross-sections during the debris flow, as revealed by field investigations. Also, no generally accepted prediction method of field investigation has been developed, therefore it is difficult to compare peak discharge and magnitude of debris flow among various sites. To date, to estimate peak discharge of debris flow, the focus has been on riverbed and cross-sections. The depth and length of riverbed deposits (riverbed sediment volume) between initiation zone of debris flow and downstream evaluation points cannot be used to accurately estimate magnitude. Here, we (i) improve the unified accurate prediction method of peak discharge and magnitude of debris flow via field investigations; (ii) compile and update data on numerous recent debris flows in Japan; (iii) analyze properties of peak discharge of debris flow in different occurrence type and the relationship between peak discharge and magnitude of debris flow; and (iv) offer the practical prediction method of peak discharge of debris flow.
Keywords: debris flow; peak discharge of debris flow; magnitude of debris flow; longitudinal changes of peak discharge
1. Introduction
Prediction of peak discharge of debris flow is one of the most important factors to mitigate debris-flow disasters. When designing debris-flow countermeasures, the peak discharge of debris flow determines the spillway sections of check dams and cross-sections of channel works, and is used to estimate check dam stability parameters such as overflow depth, and the impact and fluid dynamic forces. Also, data of peak discharge of debris flow is essential when planning non-structural countermeasures such as assessment of debris-flow hazard areas and warning and evacuation systems. In general, peak discharge of debris flow has been either measured at the observation sites directly or estimated based on debris-flow velocity and cross-sections derived via field investigations (e.g., Mizuyama et al., 1992; Rickenmann, 1999, 2016). Peak discharge of debris flow has been estimated using (a) theoretical formulae based on debris flow characteristics derived via observations and field investigations, numerical simulations and laboratory flume experiments; (b) peak flood discharge using a rational formulae based on rainfall and sediment concentrations; and (c) empirical methods based on the relationship between peak discharge and total volume (magnitude) of debris flow for many debris flow events (e.g., Takahashi, 1991; Mizuyama et al., 1992; Ikeda et al., 2015). Currently, method (c) is used by the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) of Japan to determine debris-flow ______
* Corresponding author e-mail address: [email protected] Ikeda / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
peak discharge because magnitude can be readily estimated in the field investigations; method (b) has served as a reference. Direct measurements of peak discharge and magnitude of debris flow based on velocity and cross-section of debris flow at the observation sites are reliable, because that measured at the fixed cross-section. Although, while residual markings do not always accurately indicate riverbed and cross-sections during the debris flow and could possibly reflect the aftermath of several surges rather than a single surge of debris flow, as revealed by field investigations. Furthermore, few studies have used method (c) to consider the characteristics of catchments and debris flows when analyzing the relationship between peak discharge and magnitude of debris flow. For example, the magnitude of the landslide, sediment supply conditions, riverbed sediment volume between debris flow initiation zone and downstream evaluation points, and the occurrence type of the debris flow, or the slopes of the torrent channel and catchment. The occurrence type (initiation process) of debris flow are; i) debris flow generated by slope failure (landslide-type debris flow); ii) mobilization of riverbed deposits turning into debris flow (stream bed flow-type debris flow); and iii) debris flow generated by landslide dam-break failure (landslide dam-type debris flow). In addition, no generally accepted prediction method of field investigation has been developed, therefore it is difficult to compare peak discharge and magnitude of debris flow among various sites. To date, to estimate peak discharge and magnitude of debris flow based on field investigations, the focus has been on riverbed and cross-sections, and riverbed sediment volume between initiation zone of debris flow and downstream evaluation points. Here, to improve the accuracy of field investigation data and accumulate that is easy to obtain, we (i) improve the unified accurate prediction method of peak discharge and magnitude of debris flow via field investigations; (ii) compile and update data on numerous recent debris flows in Japan; (iii) analyze properties of peak discharge of debris flow in different occurrence type and relationship between peak discharge and magnitude of debris flow; and (iv) offer the practical prediction method of peak discharge of debris flow.
2. Debris-flow data and analysis
2.1. Debris-flow data
Table 1 lists the debris-flow data analyzed herein; the data were derived via observation and field investigations during or after a debris flow [Mizuyama et. al. (1992), Ikeda et. al. (2015), MLIT Report, National Institute for Land and Infrastructure Management (NILIM) Disaster Investigation Report, and Disaster Committee Report].
2.2. Analysis of field investigation data
We extracted field data, including location of sites, photographs, showing cross-sections, residual markings, the depth and length, and thus sediment volume, of riverbed deposits between debris flow initiation zone and evaluation points, and riverbed gradients. We focused on flow paths that lacked tributary points to ensure continuity between the initiation zone and evaluation points. To compare different types of peak discharge of debris flow, we improved the unified accurate prediction method of cross-section (including riverbed) and debris-flow velocity (mean velocity), as shown in Figure 1 and described below: We identified riverbeds, including those at the bottom of check dam spillways, and solid basement rock; We measured water levels by reference to residual markings on both riverbanks; We measured cross-sections of peak discharges (A) around the above points; We determined mean depth of debris flow (the difference between riverbeds and water levels); We calculated mean width of debris flow (B) based on mean depth and cross-section; We measured riverbed gradient 200m upstream of evaluation points; We calculated mean velocity (m3/s) using the above data and the Manning formula (n=0.10; as a guide of the main flow of debris flow).
(a) (b) residual mark of debris flow A h A h B
A: cross-section (m2), h: mean depth, (m) B: mean width (m) Fig. 1. (a) cross-section and mean depth of debris flow; (b) mean width of riverbed Ikeda / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
Also, to estimate magnitude of debris flow, we extracted and measured riverbed sediment volume between debris flow initiation zone and evaluation points in view of Figure 1 based on topographic map, longitudinal profiles, photographs and cross-section of the field data, and LiDAR data before and after the debris flow, where the data exists and can be analyzed.
Table 1. Data of peak discharge, magnitude and property of each debris flow
Magnitude Location Peak Type of Riverbed (Total volume of Type of Occurrence Geologocal Analysis Name (Occurrence year Discharge Flow Source gradient 3 Debris Flow) (Initiation process) Condition method of disaster) QP (m /s) 3 Property QT (m )
Ishihara Creek Yamaguchi, JAPAN (2009) 0.137 195 57,290 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Wada River Yamaguchi, JAPAN (2009) 0.251 336 3,427 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report 0.342 231 1,340 Hiiragi Creek Yamaguchi, JAPAN (2009) 0.268 78 11,388 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Metamorphic Matano River 0.270 263 7,787 Granular Landslide/Stream bed Flow-type FI NILIM Report Ehime, JAPAN (2004) Sedimentary rock Metamorphic Nishishirahama River 0.207 21 6,149 Granular Landslide/Stream bed Flow-type FI NILIM Report Ehime, JAPAN (2004) Sedimentary rock Metamorphic Higashikusuzaki River 0.179 178 5,676 Granular Landslide/Stream bed Flow-type FI NILIM Report Ehime, JAPAN (2004) Sedimentary rock
0.118 429 9,653
Metamorphic Kusuzaki River 0.332 90 409 Granular Landslide/Stream bed Flow-type FI NILIM Report Ehime, JAPAN (2004) Sedimentary rock
0.469 28 199
0.357 56 757
Metamorphic Moriyukiohtani River 0.226 260 23,436 Granular Landslide/Stream bed Flow-type FI NILIM Report Kagawa, JAPAN (2004) Sedimentary rock Hime River, Nagano/Niigata, Committee Gamaharasawa Creek 0.319 661 54,333 Granular Large scale landslide Sedimentary Rock M JAPAN (1996) REPORT, 1997 Committee Atsumari River Kumamoto, JAPAN (2003) 0.160 5037 296,173 Granular Large scale landslide Volcanic Rock FI REPORT, 2004 Committee Harihara River Kagoshima, JAPAN (1997) 0.197 7705 322,812 Granular Large scale landslide Volcanic Rock FI REPORT, 1998 Funaishi River A Kagoshima, JAPAN (2007) 0.378 1526 21,915 Muddy Large scale landslide Granite FI NILIM Report B 0.283 5066 40,419 Takesawa Creek Mt.Fuji, JAPAN (2000) 0.290 403 13,728 Muddy Stream bed Flow-type Volcanic Rock FI Hanaoka, 2001 0.273 239 18,035 0.194 325 33,229 0.195 190 32,378 Kiso River, Nagano, JAPAN Hiramatsu, 2014 Nashizawa Creek 0.061 543 635,376 Granular Landslide/Stream bed Flow-type Granite FI (2014) Kusano, 2016 0.061 735 861,117 Trentino Alto Adige, ITALY Stava River 0.163 5341 597,346 Muddy Dam Failure Sedimentary Rock FI Muramoto, 1986 (1985)
Nakakura River Hiroshima, JAPAN (1999) 0.250 271 9,619 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Oomoji River Hiroshima, JAPAN (1999) 0.285 110 3,541 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Yasu River Hiroshima, JAPAN (1999) 0.213 51 5,974 Granular Landslide-type Granite FI NILIM Report Sarutaki River Hiroshima, JAPAN (1999) 0.140 67 12,066 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Kono River Hiroshima, JAPAN (1999) 0.123 199 25,522 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Doogahara River Hiroshima, JAPAN (1999) 0.454 101 2,951 Granular Landslide-type Granite FI NILIM Report Shimogasako River Hiroshima, JAPAN (1999) 0.666 209 6,363 Granular Landslide/Stream bed Flow-type Granite FI NILIM Report Landslide/Stream bed Flow- Name River Kiso River, Nagano, JAPAN 0.178 83-1556 23,900-110,000 Granular Granite M Ikeda, 2003 type/Landslide dam failure 737 28,519 Mizuyama, 1992 Nojiri River Sakurajima Island, JAPAN 0.185 1-827 296-994,718 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Mochiki River Sakurajima Island, JAPAN 0.040 15.4-122.4 1,871-40,369 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Harumatsu River Sakurajima Island, JAPAN 0.040 5 2,165 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Furusato No.1 River Sakurajima Island, JAPAN 0.149 6.8-59.9 3,232-7,358 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Arimura River Sakurajima Island, JAPAN 0.020 19.6-335.3 9,832-70,145 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Kurokami River Sakurajima Island, JAPAN 0.015 8.9 9,614 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Kamikamihorisawa Creek Mt.Yakedake, Nagano, JAPAN 0.125 15.1-100 224.6-6,062 Granular Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Mt.Usuzan Mt.Usuzan, Hokkaido, JAPAN 0.090 54.5-98.2 3,850-3,927 Muddy Stream bed Flow-type Volcanic Rock M Mizuyama, 1992 Mt.Tokachidake, Hokkaido, Mt.Tokachidake － 1272 13,877,780 Muddy Stream bed Flow-type Volcanic Rock FI Mizuyama, 1992 JAPAN Cháng Jiāng River, Yunnan, Jiangjia Creek － 50.4-1142 7,826-1,120,495 Muddy Stream bed Flow-type Sedimentary Rock M Mizuyama, 1992 CHINA Cháng Jiāng River, Yunnan, Hunshui Gully － 3.9-326.1 4,425-294,022 Muddy Stream bed Flow-type Granite M Mizuyama, 1992 CHINA Nojiri No.8 Sabo Dam Sakurajima Island, JAPAN 0.128 25.3-233.4 9,579-129,619 Muddy Stream bed Flow-type Volcanic Rock M MLIT Report, 2015 Arimura No.3 Sabo Dam Sakurajima Island, JAPAN 0.065 117 50,389 Muddy Stream bed Flow-type Volcanic Rock M MLIT Report, 2015 Mizunashi No.1 Sabo Dam Mt.Unzen, Nagasaki, JAPAN 0.128 8-420 14,000-450,000 Muddy Stream bed Flow-type Volcanic Rock M MLIT Report, 2015 Alberta Creek Alberta Province, CANADA < 0.175 87.2-580.7 9,550-43,915 Muddy Stream bed Flow-type Sedimentary Rock M Mizuyama, 1992 *) Analysis method FI: Field Investigation, M: Monitoring Ikeda / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
3. Longitudinal changes in the debris-flow peak discharge of different occurrence types
Peak discharge of debris flow to vary longitudinally with changes in topographical features such as river width, riverbed gradient and meander. We used the field investigation data of Figure 1 to define initiation zones and the occurrence types of debris flow. We then tracked peak discharge debris flow longitudinally for 1) large-scale landslide flow-type; 2) stream bed flow-type; and 3) landslide/stream bed flow-type. We focused on data from Obaradani creek [case 1); Fig. 2(a)], Takesawa creek [case 2); Fig. 2(b)], and Nashizawa creek [case 3); Fig. 2(c)]. Obaradani creek located in the left tributary of the Tostukawa River of Nara Prefecture, is 1.04 km in length, with an average riverbed gradient of 14.7°, and catchment area of 0.98 km2. The base rock is formed by shale with blocks of chert and greenstone, and alternation of sandstone as an accretionary wedge. The source of debris flow was a large-scale landslide with a volume of approximately 2,500,000 m3 of which 1,300,000 m3 of sediment formed a debris flow (Ikeda et al., 2014). Takesawa creek located in Mt. Fuji in Shizuoka Prefecture, is 8.50 km in length, with an average riverbed gradient of 11.3°, and catchment area of 4.08 km2. The base rock is formed by basaltic lava sheets and alternation of pyroclastic rock and lava. The source of debris flow was a riverbed deposit at an elevation of about 2,500 m where the boundary of lava sheet and pyroclastic rock. The debris flow continued for about 1,000 m (Hanaoka et al., 2001). Nashizawa creek located in the left tributary of the Kiso River of Nagano Prefecture, is 3.60 km in length, with an average riverbed gradient of 18.4°, and catchment area of 3.29 km2. The base rock is formed by granite and granodiorite. The source of debris flow was an upstream landslide/riverbed deposit. Four check dams and one steel open check dam (the most downstream dam) have been installed (Hiramatsu et al., 2014; Kusano et al., 2016).
(a) Obaradani Creek (b) Takesawa Creek 400 Large scale landslide 2500 Upstream end of erosion 350 2000 Junction of Totsukawa River 300 1500 Downstream end of debris flow deposition Elevation (m) (m) Elevation Elevation (m) 250 1000
7.5° 12.3° 17.9° 6.3° 8.0° 10.3° 14.0°17.9° 26.6° 200 500 0 100 200 300 400 500 600 700 800 900 0 1000 2000 3000 4000 5000 6000 7000 8000 9000
3 Debris Flow Peak Discharge Qp(m /s) Water Depth (m) 3 Debris Flow Peak Discharge Q p(m /s) Water Depth (m) 40000 40 800 12 Debris Flow Peak Discharge 30000 30 Debris Flow Peak Discharge 10 Water Depth 600 Water Depth 8 20000 20 400 6 10000 10 200 4 0 0 2 0 0 0 100 200 300 400 500 600 700 800 900 0 1000 2000 3000 4000 5000 6000 7000 8000 9000 Distance from main stream (m) Distance from main stream (m) (c) Nashizawa Creek
2000 130° 135° 140° 145°
1750 Junction of 45° Kiso River 1500 1250 1000
Elevation (m) 750 40° 500 4.5° 8.0° 7.7° 12.3° 32.0° 250 Nashizawa Creek 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 (Nagano Prefecture)
3 Debris Flow Peak Discharge Qp(m /s) Water Depth (m) 3000 12 Osaka JAPAN Debris Flow Peak Discharge 2500 Water Depth 10 35° Tokyo 2000 8 Nagoya 1500 6 Takesawa Creek 1000 4 (Shizuoka Prefecture) 500 2 Obaradani Creek 0 0 (Nara Prefecture) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Distance from main stream (m) Fig. 2. Location and profiles of (a) Obaradani creek; (b) Takesawa creek; (c) Nashizawa creek Ikeda / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
1.0E+05 0.833 Qp=αQT
1.0E+04
） /s 3 m （
p Q
1.0E+03
Discharge
Peak
1.0E+02
Flow
Debris
1.0E+01
1.0E+00 1.00E+01 1.00E+02 1.00E+03 1.00E+04 1.00E+05 1.00E+06 1.00E+07 1.00E+08
3 Magnitude of Debris Flow QT（m ） Nojiri River (Japan)★* Mochiki River (Japan)★* Harumatsu River (Japan)★* Furusato No.1 River (Japan)★* Arimura River (Japan)★* Kurokami River (Sakurajima)★* Kamikamihorisawa Creek (Yakedake)★* Name River (Japan)★* Mt.Tokachidake (Japan)★* Mt.Usuzan (Japan)★* Jiangjia Creek (China)★* Hunshui Gully (China)★* Albreta Creek (Canada)★* Mt.St.Helens (USA)★* Mt.Nevado Del Ruiz (Colombia)★* Nojiri No.8 Sabo Dam (Japan)★ Arimura No.3 Sabo Dam (Japan)★ Name River (Japan)★ Dohara River (Japan) Yasu River (Japan) Oomoji River (Japan) Narakura River (Japan) Shimogasako River (Japan) Kono River (Japan) Sarutaki River (Japan) Funaishi River (Japan) Nojiri No.5 Upstream Sabo Dam (Japan)★ Nojiri No.4 Sabo Dam (Japan)★ Nojiri Channel Works (Japan)★ Nojiri Bridge (Japan)★ Nojiri River Mouth (Japan)★ Mizunashi No.1 Sabo Dam (Japan)★ Ishihara Creek (Japan) Wada River (Japan) Hiiragi River (Japan) Gamahara Creek (Japan) Harihara River (Japan) Atsumari River (Japan) Funaishi River A (Japan) Takesawa Creek (Japan) Nashizawa Creek (Japan) Stava River (Italy) Matano River (Japan) Nishishirahama River (Japan) Higashikusuzaki River (Japan) Kusuzaki River (Japan) Moriyuki Ohtani River (Japan) ★: observation, Others: field investigation *) Mizuyama et.al., 1992
Fig. 3. Relationship between peak discharge and magnitude of debris flow Ikeda / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
The peak discharge at the foot of the large-scale landslide in Obaradani creek was approximately 33,000 m3/s and gradually decreased as flow progressed toward the Totsukawa River. On the other hand, in Takesawa creek, the debris flow source was a riverbed deposit; the peak discharge increased gradually from the source to an elevation of about 1,450 m (approximately 730m3/s) and then decreased gradually to an elevation of about 1,000 m. In Nashizawa creek, at the upstream end of the debris flow caused by a landslide and riverbed deposit, the debris-flow peak discharge was approximately 2,600 m3/s, and the discharge across the entire section was similar (approximately 500–1,000 m3/s). Thus, longitudinal changes in peak discharge depend on the supply amount from the source and the type of occurrence.
4. Relationship between peak discharge and magnitude of debris flow
Figure 3 shows the relationship between peak discharge and magnitude of debris flow shown in Table 1. The line indicates the quasi-theoretical line relationship represented by the following equation:
833.0 p QQ T (1)
3 3 where Qp is the debris-flow peak discharge [m /s], QT is the magnitude [m ], and α = 1.0–0.001. The data shown in Figure 3 varies widely over the range of α = 0.1–0.001, principally in the interval α = 0.1–0.01. Monitoring data obtained near the initiation zone of debris flows from Kamikamihorisawa creek, the Nojiri River, the Mizunashi River, Jiangjia creek, the Hunshui gully and Alberta creek approximate the line (1). The α values tend to approximate α = 0.01 if the debris flows are muddy (Nojiri River, Mizunashi River, Jiangjia creek and Hunshui gully) but approximate α = 0.1 if the flows are granular (Kamikamihorisawa creek, Name River). Also, field investigation data approximate line (1), although the debris-flow peak discharges tend to be larger for events of similar magnitude, which is especially the case for the 2004 disasters of Hiroshima and Ehime, field investigation data approximates line (1). Relating to geological conditions such as volcanic and granite zone that account for the whole has no clear tendency of the α value, while in Kamikamihorisawa creek (volcanic zone) and Name River (granite zone) are both approximate α = 0.1, it seems that type of debris flow property is dominant than geological conditions. Focus on slopes of torrent channel and catchment area, the α values tend to approximate α = 0.1 if the slope is steep (more than 10°) or the catchment area is relatively small but in the interval α = 0.1–0.01 if the slope is gentle (3°–10°) or the catchment area is relatively wide. In the upper/lower part of the same catchment (Nojiri River, Kamikamihori creek, Name River, Takesawa creek, and Nashizawa creek) that indicates the same α value in each catchment. The α values tend to in the interval α = 1.0–0.01 if the occurrence type of debris flow is landslide/stream bed flow type (Nashizawa creek, Name River) but in the interval α = 0.1–0.001 if the occurrence type of debris flow is stream bed flow type (Nojiri River, Kamikamihorisawa creek), this indicates the peak discharge of debris flow increase due to impact of landslide.
5. Conclusions
In conclusion, we have improved the unified accurate prediction method of peak discharge and magnitude of debris flow for field investigation, and updated (and enhanced the complication of) recent debris flow data from Japan. Depending on the occurrence type (initiation process) of debris flow, longitudinal changes in peak discharge depend on the supply amount of the source and type of occurrence. We confirmed that the relationships between peak discharge and magnitude of debris flow, and the α values of equation (1) depended heavily on catchment characteristics (sediment supply condition; riverbed sediment volume between debris flow initiation zone and evaluation points), type of debris flow property, and occurrence type of debris flow. More accumulating field investigations and analyzing along with observation data are required to validate our findings, that determining the α values of equation (1) and Manning’s coefficient of roughness that determining the flow velocity to estimate peak discharge of debris flow.
Acknowledgements
Authors should be thankful for Ministry of Land, Infrastructure, Transport and Tourism (MLIT) and National Institute for Land and Infrastructure Management (NILIM) for providing monitoring and field investigation data and useful advice. Ikeda / 7th International Conference on Debris-Flow Hazards Mitigation (2019)
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