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# Dams overtopping scenarios from catastrophic landslides in mountains' headwaters Case study in Kobe City, Japan

Christopher Gomez

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Christopher Gomez. Dams overtopping scenarios from catastrophic landslides in mountains' headwaters Case study in Kobe City, Japan. Forum Geografi, 2024, 38, 10.23917/forgeo.v38i1.4080. [https://hal.science/hal-04936861v1](https://hal.science/hal-04936861v1) [Submitted on 9 Feb 2025]

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Research article

# Dams Overtopping Scenarios from Catastrophic Landslides in Mountains' Headwaters: Case Study in Kobe City, Japan

Christopher Gomez*

Faculty of Maritime Sciences, Kobe University, Kobe 658-0022, Japan

* Correspondence: k0128086@gsuite.kobe-u.ac.jp

**Citation:**

Gomez, C., (2024). Dams overtopping scenarios from catastrophic landslides in mountains' headwaters: case study in Kobe City, Japan. Forum Geografi. 38(1), 1-10.

**Article history:**

Received: 21 January 2024

Revised: 11 March 2024

Accepted: 11 March 2024

Published: 17 March 2024

## Abstract

As a Mw 8.0 Nankai Trough Earthquake is predicted with an 80% probability of occurrence within the next 30 years, the efficiency of check dams in the mountains above Kobe City is a crucial question when considering co-seismic landslide disaster risk management. In the present contribution, the author aimed to define which subsection of the Sumiyoshigawa watershed may be more prone to generate impacts downstream of Kobe City from catastrophic landslides in the headwaters. For this purpose, the present state of the check dams network was analysed from the 2018 LiDAR (Light Detection and Ranging) data, considering the progressive infilling of the structures. As a result, the capacity of the dams in 2018 dropped by about 1/10 of the original designed capacity, arguably because of the heavy rainfall events in 2014 and 2018. Despite this evolution, landslides > 20,000 m³ starting from only one tributary can fill all the dams and flow downstream. This data makes it possible to prioritise dam curation programs, especially because population ageing and shrinking are reducing the manpower and the funds available to maintain the check dams.

Keywords: LiDAR; Landslides; SABO dams; Long-term Sedimentation.

## 1. Introduction

In Japan, a narrow coastal plain separating the sea and steep mountain or volcanic slopes - which occupy 70% of the land surface - has resulted in the need to control sediment hazards originating from the mountains (Siccard *et al.*, 2022) using check open and closed-dams made of steel frame, slit, culvert and steel and concrete dams (Dumont *et al.*, 2023). These structures are mainly designed to mitigate debris flows, which are concentrated mixtures of debris and water-flowing downslopes (Hungr *et al.*, 2001). These events are triggered mainly by typhoons, heavy rainfalls (e.g., Guzzetti *et al.*, 2008), under the influence of earthquakes (Lin *et al.*, 2004), which in combination with heavy rainfalls can eventually lead to exceptionally long runouts (Gomez and Hotta, 2021).

The design of check dams (named Sabo dams when they were constructed by the Japanese Ministry of Land, Infrastructure, Transport, and Tourism) is also developed at the system level, notably using simulations such as the CASFPS or “Computer-Aided Sabo Facilities Planning System” (Mizuyama *et al.*, 1998), in which sediment fluxes are routed along the talwegs including the effects of filled and empty check dams, in order to assess hazard levels. Such an approach is particularly important, as check dams in typhoon-battered regions can fill up within a few years (Chiu *et al.*, 2021), and, under recurrent impacts, can become unstable (Liu *et al.*, 2022) and collapse (Zhang *et al.*, 2023) highlighting the importance of the network approach.

A check-dam system has been notably developed in the hills above Kobe City (Figure 1) because landslide and debris flow have been a common occurrence historically (Sokobiki *et al.*, 1996). As the Kansai Region (where Kobe City is located) is geographically close to the early power centre in Nara-city, there are records of hazards as early as year 799 referring to extensive floods (Table 1). Flooding from heavy rainfalls and typhoons have also been differentiated as early as 1728, showing an acute understanding of meteorological phenomena. Besides flooding, which dominates the record, references to what could be a landslide that blocked a road as early as year 836 are more clearly described in 1659, with the burial of temples in soil and mud. There is also, in 1820, the clear description of a landslide dam (Table 1), in the mountains above Kobe. Out of this record, at least two historical earthquakes were recorded, one in 1854 and a second in 1995: The Hyogo-Nanbu earthquake, which made Kobe City infamous (e.g. Yamaguchi and Yamazaki, 2001).

In this long record of environmental hazards, the 1995 Hyogo-Nanbu Earthquake stands out as the worst disaster in the region, with important building damages and loss of lives (e.g., Yamaguchi and Yamazaki, 2001). It has been depicted in great detail in terms of geophysical phenomena (Guo *et al.*, 2013) and hazard and disaster risk management (Shaw, 2013; Hashemi, 2015). In the hills of Mt. Maya and Mt. Rokko to the North of Kobe City, the earthquake triggered 2,615 landslides, of which 1,900 were directly connected to the stream network, and 715 were isolated in

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Copyright: © 2024 by the authors. Submitted for possible open access publication under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).
the mountains (Table 2). Landslides mobilized a total volume of 746,720 m³ in 1995, representing 236,720 m² that were impacted by erosion and deposition (Table 2).

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Location map of the Sumiyoshigawa Watershed North of Kobe City, and the western section used as the study area. The white dots represent each a check dam on the river network.</figcaption></figure>

<table><caption>**Table 1.** Recorded natural hazards in today's Kobe City and the surrounding region, coded in dark blue: typhoon, in light blue: other floods, in yellow: earthquakes; in orange: sediment hazards and in white coastal floods (source: adapted and completed from the Sabo Office in Kobe, personal communication).</caption><thead><tr><th>Year</th><th>Hazard reported</th><th>Year</th><th>Hazard reported</th></tr></thead><tbody><tr><td>9/4/799</td><td>Extensive flooding</td><td>11/7/1860</td><td>Coastal storm and strong winds damage stone walls;</td></tr><tr><td>18/5/836</td><td>Some natural event blocked a road (landslide?)</td><td>15/8/1865</td><td>Levee breach of the Minatogawa River (7 death), levee breach of the Tennogawa River;</td></tr><tr><td>13/8/988</td><td>Large-scale flooding</td><td>7/1866</td><td>Levee breach of the Tennogawa River;</td></tr><tr><td>8/7/1302</td><td>Flooding</td><td>16/8/1866</td><td>Typhoon related flooding;</td></tr><tr><td>9/8/1475</td><td>Amagasaki-City, Hyogo City, Ashiya City experienced flooding</td><td>11/1868</td><td>Levee breach of the Tennogawa River</td></tr><tr><td>9/7/1544</td><td>Large scale flooding</td><td>18/9/1870</td><td>Severe winds and heavy rain cause various damages</td></tr><tr><td>6/8/1557</td><td>Large scale flooding</td><td>13/7/1873</td><td>Flooding in Kobe due to typhoon rainfall,</td></tr><tr><td>1608</td><td>February and August: flooding</td><td>2-3/10/1873</td><td>Levee breach of the Ikuta River due to heavy rainfalls</td></tr><tr><td>6/1614</td><td>Flooding in Kinai area</td><td>28/7-3/8/1874</td><td>Ikuta River and Minatogawa River levee breach due to heavy rainfalls</td></tr><tr><td>6/1637</td><td>Heavy-rainfall flooding in Hyogo area</td><td>16-18/5/1877</td><td>Extensive flooding through Kobe City</td></tr><tr><td>22/5/1659</td><td>Great flooding and "temples were buried in soil and mud".</td><td>6/1896</td><td>Continuous heavy rainfalls;</td></tr><tr><td>11/4/1674</td><td>Flooding in Kinai area</td><td>30/9/1896</td><td>Minatogawa break its banks;</td></tr></tbody></table>
<table><caption>**Table 1.** Recorded natural hazards in today's Kobe City and the surrounding region, coded in dark blue: typhoon, in light blue: other floods, in yellow: earthquakes; in orange: sediment hazards and in white coastal floods (source: adapted and completed from the Sabo Office in Kobe, personal communication) (Continued).</caption><thead><tr><th>Year</th><th>Hazard reported</th><th>Year</th><th>Hazard reported</th></tr></thead><tbody><tr><td>8/5/1676</td><td>Flooding in Kinai area</td><td>7-9/7/1903</td><td>Uji-River embankment damaged, 4 dead (heavy rainfall?)</td></tr><tr><td>6/1706</td><td>Great waves reported to have eroded the coast (tsunami?)</td><td>26/8/1905</td><td>Heavy rainfall (73.3 mm/h & 39 mm/20min), 10,000 dwellings flooded;</td></tr><tr><td>2/7/1712</td><td>Large river flooding along the Ikuta river (Kobe)</td><td>6-8/9/1910</td><td>251.8mm of rainfall triggered "The Big Flood";</td></tr><tr><td>8/7/1728</td><td>Flooding due to typhoon-triggered heavy rainfall</td><td>23/9/1912</td><td>Large typhoon from Shikoku to Osaka and total rainfall 159.7mm lead to extensive flooding;</td></tr><tr><td>1740</td><td>Ikuta River great flooding</td><td>11-12/9/1924</td><td>Typhoon (10 death and 1495 buildings flooded);</td></tr><tr><td>3/6/1742</td><td>Typhoon rainfall triggered: Gomao Village levee breach and flood; landslides in mountains</td><td>21/9/1934</td><td>Typhoon (precipitation: 26.5mm/h), 6 depth, 7919 buildings flooded to floor level; 2547 buildings flooded above floor level;</td></tr><tr><td>1754</td><td>Myohoji River levee breach</td><td>3-5/7/1938</td><td>Flooding(precipitation: day 1:49.6mm, day 2:141.8mm, day 3:270.4 mm): 1497 houses washed off; 966 submerged; 2658 destroyed; 7878 partially destroyed; 31,643 houses partially flooded; 75,252 houses flooded to floor level; 671 deaths, 24 missing; 14 embankment breaches; 69 roads washed away, 57 bridges lost;</td></tr><tr><td>29/5/1756</td><td>Ikuta River flooding and inundation of buildings</td><td>1/8/1939</td><td>Heavy rainfall (87.7mm/h), 14,165 buildings flooded, 2 deaths, 23 embankments breaches;</td></tr><tr><td>27/5/1768</td><td>Extended flooding</td><td>8-11/10/1945</td><td>Typhoon with 49.6 mm/h), led to 87 embankments breaches, 141 roads damaged and 61 bridges washed away;</td></tr><tr><td>6/1774</td><td>Tsunami</td><td>18-19/6/1946</td><td>Heavy rainfalls led to 53 river breaches, 46 roads damaged and 12 bridges lost</td></tr><tr><td>12/6/1779</td><td>Flooding of the Kinai Region due to typhoon</td><td>3/7/1947</td><td>Flooding led to 3 houses submerged, 7 half flooded, 2 washed away, 228 flooded above floor level and 5862 houses flooded up to floor level;</td></tr><tr><td>23/7/1779</td><td>Flooding of the Kinai Region</td><td>21/7/1948</td><td>Flooding led to 8 deaths, 10 houses evacuated, 26 houses destroyed, 1077 flooded above floor level and 2,425 flooded below floor level;</td></tr><tr><td>12/8/1785</td><td>Flooding of the Kinai Region</td><td>18-19/6/1949</td><td>Typhoon led to the transport and accumulation of 21,778m³ of sediments (most likely landslides and/or debris flows)</td></tr><tr><td>4/1791</td><td>Waves (tsunami?) entering the village of Wakihama</td><td>3/9/1950</td><td>Typhoon with 161mm of rain led to 1067 houses destroyed, 39 washed away, 58 flooded above floor level, 2682 flooded above floor level, 44 embankment breaches, 70 roads damaged;</td></tr><tr><td>25/7/1800</td><td>Flooding of Minatogawa (today in Kobe City) and assignment of 500 laborers to water defenses</td><td>6/7/1953</td><td>Typhoon nb 2 took the lives of 2, flooded 673 houses, damaged 30 roads and breached embankments at 37 locations;</td></tr><tr><td>6/1815</td><td>Flooding of the Kinai region</td><td>5/9/1953</td><td>Typhoon 13 destroyed 689 houses, flooded 1047;</td></tr><tr><td>3/8/1816</td><td>Extensive flooding of the Kinai region</td><td>24-27/6/1961</td><td>Rainfall 472.1 mm: 28 dead and 3 missing; 140 houses were destroyed, 263 houses were partially destroyed, 3960 houses were flooded above flood level and 29,376 below floor level; riverbank damages occurred at 973 locations and 580 roads were damaged and 62 bridges washed away;</td></tr><tr><td>23/5/1820</td><td>Collapse of the Tenjin Mountain in Nagata (Kobe) and creation of the Namekura landslide dam ponding.</td><td>16/9/1961</td><td>Heavy rainfall: killed 10, washed away 2,555 houses, flooded above floor level 8,801 and 36,034 below floor level, damaging river embankments in 98 locations with various river impacts in 1,756 locations; a total of 1,044 road damages were recorded and 121 bridges damaged;</td></tr><tr><td>8/4/1821</td><td>Large scale flooding of the Kinai region</td><td>10/9/1965</td><td>Typhoon 23 (118.6 mm rain) led to 1,756 houses being destroyed, 2,603 flooded above floor level and 1,262 below floor level;</td></tr><tr><td>26/4/1829</td><td>Minatogawa at levees height, mobilization of people against the flood</td><td>9/7/1967</td><td>Heavy rainfall (total 317.7 mm over 4 days, with maximum 75.8 mm/h) killed 90 + 8 missing, destroyed 367 houses, damaged 390 and flooded above floor level 9,187, and 49,650 below floor level. It triggered 141 landslides, 168 rockslides and river flooded in 74 locations.</td></tr></tbody></table>
<table><caption>**Table 1.** Recorded natural hazards in today's Kobe City and the surrounding region, coded in dark blue: typhoon, in light blue: other floods, in yellow: earthquakes; in orange: sediment hazards and in white coastal floods (source: adapted and completed from the Sabo Office in Kobe, personal communication) (Continued).</caption><thead><tr><th>Year</th><th>Hazard reported</th><th>Year</th><th>Hazard reported</th></tr></thead><tbody><tr><td>19/3/1843</td><td>Flooding of the Minatogawa River "beyond expectations".</td><td>17/1/1995</td><td>Hanshin-Awaji "daishinsai" (Hyogo-Nanbu earthquake): at 5:46 a Mw 7.3 earthquake took the lives of 6,402 individuals, 104,004 buildings were destroyed, another 136,952 were heavily damaged, 7,035 were burnt down by fire; 89 partially burned.</td></tr><tr><td>4/11/1854</td><td>Large-scale earthquake in the morning;</td><td>5-8/7/2018</td><td>Heavy rainfall (506.5 mm) with 9 locations above 400 mm in Hyogo Prefecture led to 2 deaths and 12 missing, as well as 140 buildings damaged and 781 flooded to different levels, it resulted in 31 important landslides and rockfalls;</td></tr><tr><td>6-22/5/1857</td><td>Uninterrupted of rainfall;</td><td>3-5/9/2018</td><td>Typhoon with winds up to 46.5 m/s and storm surge of 2.3 m in Kobe City led to 11 deaths, 694 injured and extensive flooding;</td></tr><tr><td>7/1857</td><td>Flooding in the Kinai area;</td><td></td><td></td></tr></tbody></table>

<table><caption>**Table 2.** Landslides and sediment volumes were displaced four years following the Hanshin-Kobe earthquake (data courtesy of the Kobe Sabo office).</caption><thead><tr><th rowspan="2">Surface</th><th rowspan="2">Year</th><th colspan="2">Landslide units</th><th rowspan="2">Landslide Surface [m²]</th><th colspan="2">Volume of Sediments</th></tr><tr><th>[Nb]</th><th>[Nb/km²]</th><th>[m³]</th><th>[m³/km²]</th></tr></thead><tbody><tr><td rowspan="5">120 km²</td><td>1995</td><td>2,553</td><td>15.9</td><td>263,720</td><td>746,720</td><td>5,040</td></tr><tr><td>1996</td><td>97</td><td>0.6</td><td>2,000</td><td>2,010</td><td>20</td></tr><tr><td>1997</td><td>31</td><td>0.2</td><td>1,012</td><td>2,879</td><td>20</td></tr><tr><td>1998</td><td>23</td><td>0.2</td><td>1,689</td><td>2,170</td><td>10</td></tr><tr><td>1999</td><td>111</td><td>0.7</td><td>9,483</td><td>17,615</td><td>120</td></tr></tbody></table>

From these premises, two elements motivated the research questions: (1) nearly 30 years after this catastrophic earthquake in Kobe, a number of the check dams must have been partially or fully filled (notably after the heavy rainfall event of 2018, for instance, cf. Figure 2); (2) as the Nankai Trough Earthquake is predicted to occur with a probability of 80% in the next coming 30 years, co-seismic landslides are likely to occur, and if the earthquake was not to occur during the cold and dry season but during the Spring to Autumn wet season, then the size of landslides may be much larger. In light of this issue, this paper aims to (1) find the level of infilling of the check dams in the West-section of the Sumiyoshigawa basin and (2) simulate how the network of check dams in its present state would react to different landslide scenarios in different portions of the watershed.

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Photographs of the 2018 heavy rainfall event that triggered (a) erosion in the hills, (b) destroyed a check dam in the Sumiyoshigawa River, (c) eventually flowing down to the dwellings of Kobe City by (d) overtopping protection walls and (e) check dams, eventually filling houses with sediments, blocks, and drift-wood (f).</figcaption></figure>
## 2. Research Methods

The Sumiyoshigawa (Sumiyoshi meaning old living area and gawa (or kawa) meaning river) has 71 check dams, 66 being closed dams, 4 with openings with a slit grid, and 1 being a slit dam. For the present contribution, the author investigated a watershed section, often referred as the West section, in opposition to the main river (Otani). The studied section comprises 28 dams distributed on a channel network with three small true-right tributaries and one longer tributary on the true left side (Figure 3).

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Network of dams used in the simulation, with the dams 11, 15, and 23 being the controlled dams from which the results are presented. They are “key-indicator” nodes in the network, showing the influence of different parts of the network on the sediment transfer.</figcaption></figure>

### 2.1 Material

The topography used to delineate the watershed, find the check dams, and determine their present infilling level was generated from the 2016 LiDAR dataset flown over the Kobe area on July 11th. The LiDAR used in the present survey is a Trimble Harrier56, which generated points at 180 kHz, with a scanning swath occurring at 49 Hz per swath. The LiDAR was mounted on a Robin R44II helicopter, which travelled at 20 m/s. It resulted in a LiDAR point cloud of 10 points/m². The LiDAR data was spatialized using the internal IMU, the onboard GNSS, and three GNSS stations spread around the survey area. Furthermore, 10 locations visible from the sky calibrated the point cloud. The Root Mean Square Error between the GNSS points and the LiDAR was on average 0.05 m with a maximum value of 0.07 m.

### 2.2 Data Processing

The LiDAR data was imported in CloudCompare®, and as true 3D data was not necessary, the ground level was converted to a topographic grid with a 1 m horizontal resolution. Then, using the open-source GIS software QGIS, the height of the dams, the surface of the sediment trapped in the dam, and the calculation of the capacity of each dam were performed using the profiler add-
on and the standard geometric tool associated with shapefiles. The GIS data generated a network of dams, including their capacity in 2018.

## 2.3 The network model

The network was programmed in Python using the standard Numpy library. Each dam was created as an object, and each dam object was associated with a set of attributes (capacity, level, upstream and downstream slopes, distances, and inflow and outflow). This structure was chosen to develop the model further and have a set of objects to redeploy over different configurations.

Over this network, inputs of sediments are added to the headwater and then routed through the system. In the present contribution, a simple marching algorithm was implemented as the result was to see the results at the exit of the network.

The input of sediments was divided into scenarios with landslides ranging from 1,000 m³ to 20,000 m³. As this contribution focuses on landslides in headwaters, six triggering locations were chosen, and at each location, 15 sub-scenarios were run successively. The choice of scenario was based on the observations of sediment transfer volumes reported during the Hyogo-Nanbu earthquake (Figure 2) and the possibility of a catastrophic landslide that would exceed the precedent values like it has been observed in extreme cases (e.g., Shiaorin landslide in Taiwan). As the headwater surfaces above dams 2, 3, 9, 13, 19, and 28 are respectively 0.144 km², 0.153 km², 0.138 km², 0.046 km², 0.1 km², and 0.04 km², single landslides are not expected to exceed 800 m³ on average based on the previous earthquake data. However, the 1995 earthquake occurred in January, a dry season in Kobe. If the same event were to occur between May and November, the event would certainly change in magnitude. To consider potential underestimation due to the season, the present contribution tests scenario from 1,000 m³ to 20,000 m³, which can be compared to 1 m thick shallow landslides impacting between 20% and 40% of the headwater surface. Finally, the discrete results from the models linking landslides' volumes (x in eq. 1) to the different dams' infilling levels y was written in a continuous format using logistic models (Equation 1). where L is the limiting value (maximum value), and k represents the rate of change towards the maximum L value.

$$f(x) = \frac{L}{1 + e^{-k(x-x_0)}} \qquad (1)$$

# 3. Results and Discussion

## 3.1. Dam infilling levels in 2018

The free volume of each of the 28 dams was calculated to be, on average, 1749 m³, with a minimum of 0 m³ (check dam full) to 8050 m³, and a total volume retainable in the network of 49,000 m³ in 2018. These volumes are only a fraction of the originally designed retention volumes (Figure 4). For the dams of which the original designed volume was available, the smallest dams with originally 10,000 m³ only have between 950 m³ and < 8,000 m³ remaining. Larger dams have also shown a similar trend (Figure 4). This shows that the dams were initially well-calibrated to their environment, but the threat due to dam overtopping needs reassessment.

<figure><img src="image_7.png" /><figcaption>Figure 4. Comparison of the free volume in 2018 with the original designed volume.</figcaption></figure>
However, the infilling levels and the original design are not following a linear correlation and some dams have filled faster than others. This discrepancy is set to impact the location of landslides in the watershed. For landslides triggered at dams 2 or 3 in the headwaters, landslides up to 20,000 m³ are not expected to translate sediments in the lower areas of the watershed (Figure 5).

<figure><img src="image_8.png" /><figcaption>**Figure 5.** Reaction of the network at three control nodes (dams 11, 15 and 23) to the input of a landslide in each branch of the channel network (dams 2, 3, 9, 13, 19 and 28).</figcaption></figure>

Control dam 11 is filled for landslide volumes > 2,500 m³ to 5,000 m³, while control dam 15 is filled if the landslide > 12,000 m³. If dam 11 reaches 100% infilling. The half mark is reached for a landslide between 2,000 and 3,000 m³ at dam 11, while it takes a landslide of 10,000 m³ to 12,500 m³ to infill dam 15 (Figure 5). Triggers from any of the two short tributaries (in dam 3 and dam 9) show a similar pattern with a rapid infilling of dam 11, while dam 15 needs a landslide >18,000 m³ to be filled. From these two last scenarios, dam 23 downstream is unaffected. Only the scenario with a landslide starting in dam 19 with >20,000 m³ shows sediments that can travel past dam 23 (Figure 5).

Systematically comparing the scenarios that would start in the main section of the channel against the tributaries, it appears that the tributaries are eventually posing the highest risk (Figure 6) when starting from dam 9 or 13, as dams 11 and 15 are infilling faster. However, when comparing landslides triggered in the main stem (dam 2) against the tributary of dam 19, a slight lag in the opposite direction can be observed, and dam 15 is infilling for slightly smaller when the landslide is triggered in the main stem. Compared to these single landslide scenarios, however, if two landslides of the same size were to occur in two tributaries (for instance, tributary 9 and 13), all three control dams would infill for landslides <14,000 m³ each. If one single landslide > 20,000 m³ triggered above dam 19 can generate a flux of sediment at the exit of the research basin, it takes a combined total of 25,000 m³ from the tributaries 9 and 13 for a flow to pass dam 23 (Figure 6).
<figure><img src="image_9.png" /><figcaption>**Figure 6.** Comparison of the scenarios from the tributaries against scenarios from the main channel, as well as a scenario with two landslides together. The dotted lines represent the tributaries in (a, b,c), i.e. a landslide triggered in dam 9, 13, and 19.</figcaption></figure>

**3.2. Discussion**

From the present analysis, the network of check dams in the Western section of the Sumiyoshigawa is a safe network that can capture sediments potentially produced in the headwaters by any landslide < 20,000 m³ (please note that it assumes no dam collapse), except if the landslide occurs above dam 19. In such a case, sediments can flow downstream to dam 23. In any other case, two concomitant landslides are needed for sediment to overcome dam 23, with a combined volume of at least 25,000 m³ of material from dams 9 and 13.

From the result of the present analysis, marginal weaknesses can be highlighted. However, the existing network approach in designing check dams (Mizuyama *et al.*, 1998) provides reliable protection to landslides up to 20,000 m³, even several decades after the construction of the structures (Figure 7). This adequacy of the system is further demonstrated by the infilling rate of the check dams, which is grossly correlated with the originally designed volume following a relation $y=0.0017x + 122.75$, with a R2 of 0.76 (Figure 7).

However, in comparison to punctual events, such as co-seismic landslides, one question that arises is the role of background erosion and how it impacts the network over long periods, especially because climate change is set to trigger more intense rainfall events and typhoons over the Japanese archipelago and East Asia (Chiang and Chang, 2011, Saito *et al.*, 2014; Wang, 2013), which in consequence will result in an acceleration of dam infilling, mainly because the soil of the Sumiyoshigawa catchment is a deep altered granite, which is unlikely to result in a sediment production limited regime. Using a set of erosion plots in the mountains between 250 m and 490 m a.s.l., with slope inclination of 15, 23, 30 and 31 degrees, recorded erosion ranged between 0.1 g/m²/month to < 800 g/m²/month, with the maximum being isolated peaks (Figure 8). During the period 2003 to 2006, when plot erosion surveys were conducted, an average monthly erosion of 105 g/m²/month was measured at plot number 2, which is denuded. Under vegetation cover, the erosion was recorded at the three other locations as 5.7 g/m²/month, 7.4 g/m²/month, and 0.3 g/m²/month. In light of these results, and considering a material density of 2.1 to 2.5 g/cm³, it is safe to rule out the progressive accumulation of sediments from background erosion as a threat to the safety and usability of check dams.
<figure><img src="image_10.png" /><figcaption>**Figure 7.** Correlation between the free volume in 2018 and the originally designed volume, as well as the designed volume and the infilling rate for 19 dams, for which original data were available.</figcaption></figure>

**Acknowledgements**

This research was funded by the Hyogo Science and Technologi-cal Association Research Grant. The authors are also in debt to the Kobe Sabo Office and the Kobe City Office for providing sets of hydrological and geo-morphological data.

**Author Contributions**

**Conceptualization:** Gomez, C.;
**methodology:** Gomez, C.; **investi-**
**gation:** Gomez, C.; **writing origi-**
**nal draft preparation:** Gomez, C.;
**writing review and editing:**
Gomez, C.; **visualization:** Gomez,
C. The author have read and agreed
to the published version of the
manuscript.

**Conflict of interest**

All authors declare that they have
no conflicts of interest.

**Data availability**

Data is available upon Request.

**Funding**

This research received no external
funding.

<figure><img src="image_11.png" /><figcaption>**Figure 8.** Background erosion of the slopes in managed forest, Japanese oak and in thick forest with slopes ranging from 15 to 31 degrees between 2003 and 2006, from measured forest plots.</figcaption></figure>

**4. Conclusion**

Check dams deployed in the network are a strong hard-engineering measure to protect assets and population when relocation is impossible. Moreover, good dam calibration also shows that the network can have long-lasting positive impacts even with minimum maintenance. However, it is a safe system for average hazards and disaster risk and do not protect against larger events. It is a general problem that remains in disaster risk management, where the management of small and middle-sized events is the focus of these policies, but it also gives a false sense of security as it is ineffective against extreme events. In the midst of this work, I also realized that hydrologists and geomorphologists had been focusing their attention on natural systems in order to fathom their mechanisms. However, there are comparatively very few studies that look at the impacts of check dams on water and sediment transfers (e.g., Norman et al., 2016; Yazdi, 2017), and how they influence the geometry as well as the evolution of the stream network, and in turn the generation of hazards such as landslides, and further work is necessary on this front as well.

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