Sediment Disasters Research Articles

Spatial distribution of rainfall–runoff characteristics and peak lag time in high‐relief meso‐scale mountain catchments where observations are scarce

AbstractClarifying the relationship between rainfall and runoff characteristics in mountainous areas is crucial to improving flood and sediment disaster prediction. This study investigates that relationship, focusing on meso‐scale catchments of 1–10 km2 in high‐relief mountainous regions where conventional observational data are limited. Such research requires observations in multiple adjoining watersheds due to the variability of runoff patterns in mountainous river systems. However, observations are challenging in meso‐scale catchments due to equipment loss in rivers. This study included seven observation sites with catchment areas of 0.16–9.01 km2 and relative elevations exceeding 1000 m within the Higashigochi River basin, a sub‐branch of the Oi River basin in Shizuoka Prefecture, Japan. Water level gauges were strategically installed at each of these locations to prevent loss. Peak lag time (the time difference between the rainfall and water level peaks), which is crucial to flood hydrograph definition and disaster preparedness, was assessed. The results indicate a general trend of increasing peak lag time with larger catchment area, although spatial heterogeneity was observed at small sites. This finding aligns with prior studies in meso‐scale catchments within mountainous regions with relative elevations of 450 m or less, suggesting that in such meso‐scale catchments, larger catchment area tends to result in longer peak lag times, regardless of topographic undulation. Overall, the impact of topographic undulations on peak lag time appears surprisingly modest, indicating that catchment area plays a more significant role in determining peak lag time. The observed heterogeneity of peak lag time could be attributable to the influence of bedrock groundwater discharge.

An approach to assess sediment disaster risk using soil moisture deficit: A case study about sediment disasters occurred during 2010–2011 in Niigata prefecture, Japan

An approach to assess sediment disaster risk using soil moisture deficit: A case study about sediment disasters occurred during 2010–2011 in Niigata prefecture, Japan

Parameter Sensitivity Analysis of a Korean Debris Flow-Induced Rainfall Threshold Estimation Algorithm

With the increase in both rainfall and intensity due to climate change, the risk of debris flows is also increasing. In Korea, the increasing damage caused by debris flows has become a social issue, and research on debris-flow response is becoming increasingly important. Understanding the rainfall that induces debris flows is crucial for debris-flow response, and methods such as the I-D method have been used to evaluate and predict the risk of debris flows. However, previous studies on debris flow-induced rainfall analysis have been limited by the subjective decision of the researcher to select the impact meteorological stations, which greatly affects reliability. In this paper, in order to establish an objective standard, various maximum allowable distances between debris-flow disaster areas and meteorological stations were adjusted to 1, 3, 5, 7, 9, 11, 13, and 15 km using the CTRL-T automatic calculation algorithm, and the optimal maximum allowable distance suitable for Korean terrain was derived through parameter sensitivity analysis. Based on this, we developed a nomogram for sediment disaster risk prediction and warning in Gangwon-do, and applied it to past disaster cases, and found that, although the prediction time for each stage varies depending on the maximum allowable distance, on average, it is possible to predict the risk of sediment flows 4 to 5 h in advance. It is believed that the results of this study can be used to reduce sediment flow damage in advance.

Applicability Comparison of GIS-Based RUSLE and SEMMA for Risk Assessment of Soil Erosion in Wildfire Watersheds

The second-largest wildfire in the history of South Korea occurred in 2022 due to strong winds and dry climates. Quantitative evaluation of soil erosion is necessary to prevent subsequent sediment disasters in the wildfire areas. The erosion rates in two watersheds affected by the wildfires were assessed using the revised universal soil loss equation (RUSLE), a globally popular model, and the soil erosion model for mountain areas (SEMMA) developed in South Korea. The GIS-based models required the integration of maps of the erosivity factor, erodibility factor, length and slope factors, and cover and practice factors. The rainfall erosivity factor considering the 50-year and 80-year probability of rainfall increased from coastal to mountainous areas. For the LS factors, the traditional version (TV) was initially used, and the flow accumulation version (FAV) was additionally considered. The cover factor of the RUSLE and the vegetation index of the SEMMA were calculated using the normalized difference vegetation index (NDVI) extracted from Sentinel-2 images acquired before and after the wildfire. After one year following the wildfire, the NDVI increased compared to during the year of the wildfire. Although the RUSLE considered a low value of the P factor (0.28) for post-fire watersheds, it overestimated the erosion rate by from 3 to 15 times compared to the SEMMA. The erosion risk with the SEMMA simulation decreased with the elapsed time via the vegetation recovery and stabilization of topsoil. While the FAV of RUSLE oversimulated by 1.65~2.31 times compared to the TV, the FAV of SEMMA only increased by 1.03~1.19 times compared to the TV. The heavy rainfall of the 50-year probability due to Typhoon Khanun in 2023 generated rill and gully erosions, landslides, and sediment damage in the post-fire watershed on forest roads for transmission tower construction or logging. Both the RUSLE and SEMMA for the TV and FAV predicted high erosion risks for disturbed hillslopes; however, their accuracy varied in terms of the intensity and extent. According to a comparative analysis of the simulation results of the two models and the actual erosion situations caused by heavy rain, the FAV of SEMMA was found to simulate spatial heterogeneity and a reasonable erosion rate.

Influence of altitude, slope, and waterway characteristics on the occurrence of slow-moving landslides in South Korea

Forest soil sediment disasters, categorized as landslides, slow-moving landslides (land creep), and debris flows, result in destruction of forests and properties, loss of life, and damage to infrastructures. Therefore, it is imperative to evaluate factors that cause slow-moving landslides and establish a national-level hazard information system. In this study, we used geographic information system (GIS) to investigate the difference between landslides and slow-moving landslides by overlapping and analyzing landslide hazard map with slow-moving landslide sites. Slow-moving landslide areas (SLAs) and non-SLAs were characterized to compare the effects of altitude and slope in slow-moving landslide events. Finally, we performed a hydrographical analysis of waterways to compare the SLAs and non-SLAs. Grade 3 slow-moving landslide sites were found to be the most prevalent among all landslide hazard grades (1–5), including those in regions outside the specified grades. The altitude and slope of SLAs were 5 m higher and 1° steeper than those of non-SLAs, respectively. The waterways in non-SLAs (205.4 m) were longer than those in SLAs (85.2 m). Our study highlights the importance of altitude, slope, and waterways as triggers for slow-moving landslides and provides an empirical basis for establishing national hazard information systems for these events.

Application of X-Band rainfall radar data-based on soil water index for sediment Disaster monitoring in Boyong River (BO-D6)

Disasters those related to rainfall, will always come unexpectedly which can make huge potential of risk to human and facilities. Rainfall contribute a benefit and disaster to us, so we have to provide some research about the relationsip of rainfall and disaster. This research produce the information related to application of x-band radar data which have combination from soil water index. It shows the calculation of water content in soil layer by using x-band radar data. The rainfall dataset is extracted from June 2019 until June 2020. SWI can be used as preliminary information and works as a database for monitoring and observation tools for Boyong drainage area. The record of occurence sediment movement on 20th October 2019 proves in that day, the condition of soil water index reach maximum values with 80,28 mm and there is a small occurence of sediment movement around that month. The subsequent conctancy of SWI needs to be paid attention for possibility of disaster. As rainfall intensity reach the highest values, soil water index will also increase. A higher value of SWI show a higher risk of sediment disaster initiation.

Geomorphometric characterization and sediment connectivity of the middle Brahmaputra River basin

Knowledge of geomorphic characteristics and sediment connectivity is critical in understanding basin-scale surface processes that include steep mountainous and flat lowlands, particularly if the mid-basin and downstream areas are densely populated and disaster-prone. However, the lack of fluvial and sediment connectivity studies in downstream portions of large rivers of South Asia (particularly Bangladesh), where upper basin morphodynamics, hydrology, and sediment flux highly influence the repeated sediment disasters, limits our understanding of the large-scale fluvial geomorphic processes and basin-scale sustainable river management. Therefore, this article attempts to characterize the morphometric and topographic features along with the spatial sediment connectivity pattern of the middle Brahmaputra River basin (Teesta, Torsa, and Manas basins) covering ~75,000 km2 that influences the hydro-geomorphic response of the deltaic landscape of lowland Bangladesh. This work considers a set of morphometric and topographic parameters including the stream network, longitudinal profiles, stream power index (SPI), and topographic wetness index (TWI) for geomorphometric characterization. The linkage between the sediment sources to downstream areas also has been evaluated with the sediment connectivity (IC) index to understand better the sediment dynamics of the middle Brahmaputra River basin, draining towards lowland Bangladesh from the basin's upstream countries of India and Bhutan. The result of this study demonstrates a highly potential hydro-geomorphic response of the downstream areas attributed to steep topography, steep channel longitudinal profiles, high rainfall, and high sediment connectivity in upstream regions. However, the low topography, presence of anthropogenic stressors, almost flat longitudinal channel profiles with limited change in elevation, and lowered sediment connectivity potential in the mid-to-downstream areas depicts sensitivity to the depositional processes therein, impacting the basin-scale geomorphic connectivity from the upstream to the downstream region. These results will be the basic information for exploring the large-scale hydro-geomorphic response and structural sediment dynamics to understand the complex geomorphological processes in the South Asia region, where the fluvial sedimentation process is not fully understood with limited accessibility for field-based investigations.

Hydraulic model test on channel shifting and yielding woody debris on the fan after sediment disaster in the past

The re-movement of sediment and woody debris in torrents after huge sediment transport in the past could cause new flood and sediment disasters due to heavy rainfall events. However, it is not clear how re-movement of logs effects on bed variations such as bars and river channel divergence. In present study, hydraulic model tests were carried out referring to the magnitude of floods, that was over the plan size, in Tottabetsu River basin in August in 2016. These tests include different magnitudes of flood, suppling logs from bed/side bank erosion, and existence of sabo facilities. The key results were as follows: The presence of logs by bed/side bank erosions influences on the patterns of flows and sediment transport, because deposition of logs affects formation of bars. Difference of the magnitudes of the floods affects the activeness of the interaction between logs and sediment transport. In additions, the locations and slits of the sabo dam need to consider hydraulic conditions and characteristics of woody debris for appropriate control of sediment and woody debris.

Sediment control and logs capturing in sand pocket with combination of sabo dam with large conduit and iron bars

In the Otoishi River, a tributary on right side of the Akatani River in the Chikugo River basin, a large amount of sediment and woody debris were deposited on riverbed due to the sediment disaster caused by the heavy rainfall in northern part of Kyushu Region on 5th to 6th July in 2017. As one of countermeasures to control the outflow of sediment and woody debris to the Akatani River, a sand pocket is planned with the sabo dam with large conduit parts and the log broom works. In this study, hydraulic model test and flume test were carried out to obtain knowledges for design of the sand pocket. Iron bars are installed vertically at the upstream front of the large conduit parts for capturing sediment and logs, and those intervals are desirable to be set in consideration of sediment diameter and sediment movement without the dam. Besides, it is necessary to consider capturing function and flow characteristics such as shockwave when the log broom works is planned in setting of layout and dimensions.

Multitemporal UAV surveys of geomorphological changes caused by postfire heavy rain in Kamaishi city, northeast Japan

Unmanned aerial vehicles (UAVs) are widely utilized for surveying various sediment disasters, and the high-resolution imaging they provide reveals detailed topographic characteristics. However, the availability of multitemporal UAV observations, which reveal geomorphological changes before and after the disaster, is still limited. In this research, we performed multitemporal UAV observations for geomorphological changes before and after extreme postfire heavy rain in the Ozaki Peninsula wildfire area in Japan. Significant geomorphological changes were observed associated with eight landslides and stream shifting, with a spatial resolution of 0.30 cm obtained by structure from motion (SfM) processing. To extract the surface changes based on multitemporal UAV observations, in addition to elevation displacement, vertex normal vectors were calculated for each mesh. We found that the elevation displacement showed comprehensive alterations in the entire area and that the changes in vertex normal vectors effectively captured the shape of these alterations. Deterioration, deposition, and scarping were shown for landslides, and the dynamics of erosion and deposition in the cross-section were for stream shifting. In addition, the detected geomorphological changes were compared with the predisaster topographic characteristics. Landslides occurred with a catchment area of 500 to 2,000 m2 and a slope inclination of 30 to 40°, and stream shifting occurred with a catchment area of 10,000 m2 or more and a slope inclination of less than 20°. In both regions affected by landslides and stream shifting, the logarithmic stream power index (logSPI), a natural logarithmic representation of the conventional stream power index (SPI), could describe the characteristics roughly. A logSPI value of 4 to 5 represented landslides, and 5 and greater represented stream shifting, with the probability of landslides and stream shifting occurring in areas with these values exceeding 99.5% and 99.9%, respectively. Therefore, the logSPI was utilized to roughly classify the terrain and geomorphological condition and the points among the unburnt, unlogged-burnt, and logged-burnt areas were compared. Even when the logSPI was similar, the damage was much more severe in the burnt areas. On slopes with a logSPI of approximately 4, there were signs of erosion in the logged-burnt area, and the increase in fallen trees was significant in the unlogged-burnt area, while there were no substantial changes in the unburnt area. In valleys with a logSPI of 8 or greater, sediment and woody debris were significantly deposited in the burnt areas following the postfire rain, while there were few changes in the unburnt area.

Assessment of potential maximum erosion and sediment disaster risk of typhoon events

Assessment of potential maximum erosion and sediment disaster risk of typhoon events

Temporal Trend in Landslide Occurrences in Post-fire Areas over the Past Two Decades

Analyzing the effects of forest fires on sediment dynamics is essential to establish effective mitigation measures to reduce the risk of fire-related sediment disasters, such as landslides, debris flows, and hyperconcentrated flows. The objective of this study was to investigate the temporal trend in landslide occurrences after a forest fire based on the statistical data of forest fires and landslides recorded over the past two decades. The results suggested that the landslide occurrences in the post-fire area increased the most in 1 - 5 years after the fire. Typically, landslide occurrences in the post-fire area decreased as the burnt slopes were covered with grass and shrubs within five years after a fire. However, our data showed that the frequency of landslide occurrences revealed relatively higher value until 6 - 10 years after the fire. This result was considered owing to the decrease in relative resistance to landslides initiated by root decay in the burnt slopes. Overall, the risk of landslides after forest fires is presumed to be higher than the pre-fire condition for the period that the root system of burnt slopes is regrowth and recovery (10 - 20 years or more after the fire).

Recent and Historical Background and Current Challenges for Sediment Disaster Measures against Climate Change in Japan

Japan is a country with one of the highest incidences of sediment disasters, which will become more severe and more frequent as a result of climate change. This paper reviews the recent occurrence of sediment disasters caused by heavy rainfall affected by climate change in recent years, challenges in adaptation measures, and recent policies targeting such sediment disasters in Japan. The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) has been conducting non-structural and structural measures based on legislation. Recently, climate change has resulted in more severe and frequent disasters along with damage caused by sediment movement phenomena that are not covered by the present system for warning and evacuation. Efforts to establish assessment methods concerning the risk of these phenomena are shown as examples of current challenges of climate change.

Relationship between landslides and long-term rainfall trends

Evaluating how the changes in rainfall caused landslides is important to prevent sediment disasters. Using the soil water index (SWI) and a three-layer tank model, this study examined the relationship between the occurrence of landslides and long-term changes in rainfall in Miyagawa Village, Mie Prefecture, Japan, where shallow and deep-seated landslides have occurred at different times. The three-layer tank model detected the effects of long-term changes in rainfall on landslides. We educated that the change in the first storage tank layer indicates the potential risk of shallow landslides, while that in the second and third storage tanks indicates the potential risk of deep-seated landslides. Our method could be used to examine the effects of long-term changes in rainfall on landslides in the same region at different times, or over a wider area. Besides, these results are valid because the three-layer tank model used to calculate the SWI is applied throughout Japan, and its effectiveness has been verified.

Risk Assessment of Soil Erosion Using a GIS-Based SEMMA in Post-Fire and Managed Watershed

A large wildfire occurred due to strong winds and dry climates in the Gangwon province of South Korea. Thereafter, floods and sediment damage were caused by Typhoon Mitag in the burned areas. This study was an attempt to quantitatively evaluate the risk of soil erosion in wildfire areas using the Soil Erosion Model for Mountain Areas (SEMMA) based on GIS, which was developed in South Korea. The model required the integration of maps of the main factors involved, i.e., rainfall erosivity, vegetation index, soil erodibility, and slope length and steepness. According to the model simulation results, high erosion rates of over 100 t/ha were concentrated within the wildfire areas. Sediment yields from the study watershed, including the wildfire areas, were estimated to be 40.33 t/ha for the 30-year frequency of rainfall, which is similar to those of the typhoon. The high risk of erosion was predominantly observed in the upper mountains, which are characterized by steep slopes, silt loam, and shallow soil depths within the wildfire areas. Urgent and excessive logging of burned trees further increased the risk of erosion. However, various treatment strategies were implemented to control soil erosion and sediment transport from the post-fire watershed. This study confirmed that temporal and spatial BMPs should be selected and enforced to reduce sediment disasters in wildfire areas.

Risk evaluation and warning threshold of unstable slope using tilting sensor array

Slope monitoring and early warning systems are a promising approach toward mitigating landslide-induced disasters. Many large-scale sediment disasters result in the destruction of infrastructure and loss of human life. The mitigation of vulnerability to slope and landslide hazards will benefit significantly from early warning alerts. The authors have been developing monitoring technology that uses a micro-electro-mechanical systems tilt sensor array that detects the precursory movement of vulnerable slopes and informs the issuance of emergency caution and warning alerts. In this regard, the determination of alarm thresholds is very important. Although previous studies have investigated the recording of threshold values by an extensometer which installation of an extensometer at appropriate sites is also difficult. The authors prefer tilt sensors and have proposed a novel threshold for the tilt angle, which was validated in this study. This threshold has an interesting similarity to previously reported viscous models. Additionally, multi-point monitoring has recently emerged and allows for many sensors to be deployed at vulnerable slopes without disregarding the slope’s precursory local behavior. With this new technology, the detailed spatial and temporal variation of the behavior of vulnerable slopes can be determined as the displacement proceeds toward failure.

Inoue, K.: Visit and Watch the Historical Major Sediment Disasters (Part 3)

Inoue, K.: Visit and Watch the Historical Major Sediment Disasters (Part 3)

An efficient and economic desilitation strategy for reservoir sustainable development under the threat of extreme flooding threaten

Abstract Severe sediment disaster has already been observed locally and worldwide, and serious disasters will happen increasingly in the future due to climate change. Hence, developing a proper reservoir desilitation strategy is an urgent priority to ensure sustainable water resources development worldwide. This study aims to investigate the release efficiency of the bypass tunnel in the reservoir. Firstly, a 2D numerical model was adapted to reproduce the most severe sedimentation disaster due to Typhoon Aere and obtain a reliable result compared with the measured data. Then, the concept of climate change was embedded to understand the release capability of the bypass tunnel under slight to worst scenarios. Finally, the bypass tunnel is demonstrated to be effective in releasing sediment during typhoon periods and prolonging the reservoir lifespan. In conclusion, this study proposes a proper solution to reservoir sedimentation during extreme flooding events. The optimization of water resources and economic benefits can help reservoir management achieve the goal of sustainable development. The presented research can be promoted in the worldwide reservoir to face the possibly severe sediment disaster under the threat of climate change.

Damage Types and Deterioration Characteristics of Check Dams Built on Mountain Streams in Southeast Korea

Check dams are one of representative structural measures to reduce sediment disaster at a forest watershed. Therefore, it is essential to assess the structural vulnerability and functionality of check dams in the management of sediment disaster risk at watersheds. This study examined the main types of damages and deterioration characteristics in concrete and stone check dams, which comprise more than 80.0% of Korea’s check dam stock. This was based on the 4-step condition assessment, which is the modified version of condition assessment methodology for embankment dams to meet the characteristics of check dams. As a result, the damages and deterioration of concrete check dams were classified into ten representative types. And representative damage types of stone check dams were classified into nine types. The condition assessment results demonstrated that the spillway part frequently contacted with water and debris flows was deteriorated approximately 1.2 times faster than the wing parts for both concrete and stone check dams, and the maintenance demands of both types of check dams increased 15–20 years after construction. Thus, these deterioration characteristics of concrete and stone check dams should be considered in decision-making to determine maintenance priorities among the check dams stock. Furturemore, we analyzed the relationship between condition index and service time in the examined concrete and stone check dams. The degree of deterioration in concrete and stone check dams increased significantly as the service time increased. Based on the relationship between condition index and service time, we classified deterioration characteristics of check dams into two groups: 1) those that follow the average deterioration trends of their facility group (Group II) and 2) those that undergo an unexpected end-of-service life because of exceptional incidents or severe defects (Group I). We hope that our work will form a useful resource for engineers and decision-makers involved in planning, designing, constructing, and maintaining check dams.

Assessment of potential sediment disasters and resilience management of mountain roads using environmental indicators

In recent years, extreme rainfall events occur frequently, causing serious watershed sediment disasters, destroying mountain roads, and endangering the safety of residents' lives and property. This study aims to deal with the spatial change of potential sediment movement on the road slope pre-disaster and to screen disaster hot spots for early warning and control system. The conceptual model is used to simulate the distribution of primary and/or derived disasters on a watershed scale to assess the impact of sediment disasters caused by heavy rain event. Correlation analysis shows that the models in assessment of primary disaster and derived disaster are significantly correlated with the collapse ratio and disaster ratio, respectively. Since the primary disaster has been considered when calculating the derived disaster risk, the terrain subdivision along Provincial Highway 21 (Tai-21) is extracted to understand the derived sediment disaster on the road slope. The model can effectively evaluate the road sections prone to disasters. According to the risk level, the hot spot of road slope disasters and the management of disaster resilience are determined and can be the reference for disaster prevention and control.