Efforts to predict sediment flux must identify and quantify sediment availability within a catchment, analyzing the caliber and volume of sediments in differing stores/sinks, their accessibility/ease of reworking, and the distribution of features that inhibit rates of sediment conveyance through landscapes (i.e. landscape connectivity). These relationships are greatly influenced by the catchment-specific pattern of accommodation space - those parts of landscapes where sediments are stored (over differing timeframes). Landscape memory exerts a primary control upon the differing forms and patterns of accommodation space within a catchment. This reflects arrange of geologic, climatic and anthropogenic considerations. The variable imprint of landscape memory results in marked differences in lagged and off-site response to disturbance events from catchment to catchment. Understanding of the combined role of these factors is a critical issue in the prediction of sediment disasters and in river conservation/rehabilitation initiatives.
There are many experimental approaches, field investigations and numerical calculations for movements of woods in a clear water and debris flow. However, treatments and evaluations for accumulated logs and interactions between a main flow and logs are not fully developed. In mountainous torrents, the logs yielding from slopes and stream beds depend on the condition of previous and present forest stands. Mitigations for woods need to be conducted taking into account tree species such as conifer and broad-leaf trees and shapes such as root swells and crown. In the present study, we focus on the differences in specific weight of conifer and broad-leaf trees with some moisture in a sediment-water mixture flow with narrow flow width. Conifer and broad-leaf tree are considered as floating and submerged solid phase, respectively. Flume tests are conducted in steady flow of clear and debris flow over a rigid bed in order to evaluate flow characteristics of clear and debris flow with woods. Debris flow is specified as flow with clear water and sediment mixture layers on a rigid bed, whose slope is 0.045. Preliminary experimental data, which suggest that influences of specific weight on logs movements are significant, has been obtained to evaluate those experimental results such as movements of floating and submerged woods and multi-layer consisted of woods and debris flow. However, detail considerations and collections of flume data are needed based on flow characteristics such as spatial eddies structures in depth-scale, diffusion/dispersion of woods, momentum transfer induced interactions between logs and mixture flow and bed conditions.
In physical models that evaluate the stability of hillslope surface soil masses, soil depth contributes directly to slope stability due to its weight and volume, as well as control of groundwater movement. Electrical resistivity imaging (ERI), a noninvasive geophysical technique, is a recently introduced method used to detect geological structures in landslide-prone areas. However, this method has not been well studied as a tool to detect the depth of the surface soil layer. Questions remain about the reliability of ERI, especially in surface zones of mountainous areas. Here, we present a case study of the use of ERI to detect the surface soil thickness of hillslopes in granitic and slate watersheds in central Japan. Like invasive penetration tests and geotechnical surveys using boreholes, ERI appears to be suitable for detecting soil-bedrock interfaces, due to the high contrast of resistivity values between surface soil and bedrock layers that we found in all of the watersheds. However, ER subsurface values vary over a relatively wide range, as well as from site to site. ERI also failed to clearly identify the soil-bedrock interface at some points along our measurement line. By referring to hydrological properties of bedrocks observed in a previous study, we presume that differences in the water retention characteristics of weathered granitic bedrock are a major factor in the variation in bedrock ER values that we observed.
To reconstruct the history of high-magnitude floods in a mountain gorge channel, botanical evidence from two vertically isolated strath terraces assumed to be affected only by large, infrequent floods was investigated. The maximum water level was estimated from peak and bankfull discharges during major storm events of the past 20 years. Injury of shrubs significantly indicated that overflow on the downstream terrace occurred in 1993 storm event, but no flood event overflowed both terrace surfaces according to the estimation of peak and bankfull discharges. We also found field evidences that a landslide on the slope opposite the downstream terrace generated a landslide dam in 1993 storm event. Formation of the landslide dam probably caused local overflow onto the downstream terrace during the 1993 event. Therefore, it is essential to investigate not only the hydrologic characteristics, magnitude, and frequency of discharge, but also the spatial and temporal patterns of lateral sediment input to the channel to understand flood behavior in mountain gorge river systems.
SAR imagery back scattering intensity data for two case studies, involving the small eruption of Sakurajima volcano, Japan in February 2008 and the large eruption of Chaitén volcano, Chile in May 2008 are used to detect volcanically disturbed areas. From this, a method of predicting or mitigating volcanically-induced sediment disasters is proposed. The results indicate the following points: 1) The areas disturbed by the volcanic eruption, such as by pyroclastic flow or lahars, can be identified from SAR images. 2) In both study areas, volcanic ash fall areas were not visible in the SAR images. 3) SAR imagery back scattering intensity data can be used to perform preliminary scans in emergency procedures, aiding the prediction and mitigation of volcanic eruption-induced sediment disasters.
Mount Merapi is one of the most active volcanoes in Indonesia. Its eruptions have produced large amounts of sediment that have caused disasters in downstream areas. Sediment disaster mitigation has been introduced in the last 30 years to provide a high level of safety for local residents. Even so, some problems such as riverbank cutting have occurred, which have negative impacts on ecology. As a positive aspect, the deposited sediment has been widely used as a natural resource by local inhabitants. People tend to use the sediment as much as possible to support regional development. However, sediment mining can also have negative impacts on ecosystems and reduce the safety of river regulation efforts. Sediment disaster management using sabo works and sediment resource management through sustainable sand mining management are proposed for the upper mountain slopes taking into account the socioeconomic conditions, regional development, disaster reduction, and ecological issues. Sediment management using consolidation works is proposed for the downstream area as a countermeasure for riverbed degradation.
We conducted intensive tensiometer measurements of soil pore water pressure to examine the spatial heterogeneity of hydrological properties at a mountainous hillslope at the Hodaka Sediment Observatory of Kyoto University, Gifu, central Japan. The detailed measurements revealed that pressure head at the soil-bedrock interface was highly variable in terms of waveform, value, and timing throughout the slope. On the basis of its variation, pressure head was classified into four general groups (types 1-4). Type 1 showed rapid peaks with near-zero pressure values coinciding with rainfall peaks and type 2 showed almost no responses to hyetographs. Compared to types 1 and 2, types 3 and 4 showed unique characteristics. That is, type 3 showed delayed peaks in comparison with hyetographs and gradual recession limbs, indicating the existence of groundwater seepage from the bedrock fracture. Moreover, it also represented the characteristic base flow discharge from whole basin area. Type 4 showed sharp pressure spikes coinciding with rainfall peaks, suggesting vertical infiltration within macropores and irregular preferential flows above the bedrock. In this study, we successfully detected various patterns of soil water behavior within the relatively narrow area that characterized the hydrological processes of the hillslope and the watershed.
The “5.12” Wenchuan earthquake not only had catastrophic primary effects, but also triggered many major secondary effects in mountainous regions including collapse (rock fail, slide, and so on), landslides, debris flows, and the formation of barrier lakes. These secondary disasters had a major influence on the areas affected by the earthquake, as they resulted in significant blocks to aid and seriously slowed down the rescue process. Furthermore, huge amounts of uncompacted debris created by collapse and landslides continue to pose a substantial long-term risk to the safety of the people and to their property as it can form powerful debris flows with strong rains. In this study, the distribution characteristics and physical status were investigated through field surveys and image interpretation. The features and distribution of future sediment disasters were estimated, and suggestions for corresponding mitigation measures were proposed. These will play an important role in protecting the safety of the people and in facilitating the reconstruction of disaster areas.
Landslide dams form in river channels due to heavy rains or earthquakes. The flood discharge generated by overflow erosion, a known common cause of dam bursts [Schaster et al., 1986], must be estimated promptly to establish warning and evacuation systems downstream area residents. Previous studies [Satofuka et al., 2007a ; Satofuka et al., 2007b ; Satofuka et al., 2007c ; Mizuyama et al., 2006] have shown that the “two-layer model” proposed by Takahama et al.  can be used to roughly estimate the peak discharge and hydrograph of a flood caused by overflow burst from a landslide dam at a given point in the downstream area. A landslide dam was formed in the Mimi River in Miyazaki Prefecture by local downpour due to the typhoon in September 2005, which burst later because of overflow erosion. The authors used “two-layer model” to simulate the flood discharge caused by the burst and worked out an approach to simple analysis of the risk associated with landslide dams when multiple landslide dams form simultaneously.
According to the principle of the minimum energy dissipation, a model for the critical diameter between solid phase of the coarse particles and liquid phase composed by both the fine particles and water in non-homogeneous flow was established. Furthermore, based on the modified manning's formula and mass conservation law, in combination with our experimental results, two-phase flowing velocity models for non-homogeneous debris flow were developed preliminarily. It has been indicated by verification by the models that the calculated values for two-phase flowing velocities are better corresponding to the measured data from Jiangjia Gully, Yunnan Province, China. These results above are significance for both the harnessing of control projects and subject development for debris flow.
Tree roots are generally understood to reinforce soil by increasing soil shear strength. However, few studies have quantified soil reinforcement by tree roots. This study estimated soil reinforcement by larch [Larix kaempferi (Lamb.) Carrière] and Korean pine (Pinus koraiensis Siebold et Zucc.) roots using the perpendicular root reinforcement model. Root area ratio (RAR) and root tensile strength were measured for each species to apply to the model. RAR ranges were 0.02-0.27% for larch and 0.08-0.23% for Korean pine. The root tensile strengths of larch and Korean pine ranged from 5.8-38.8 MPa and 8.3-75.6 MPa, respectively. The model indicated that roots increased soil shear strengths by as much as 3.9-28.2 kPa for larch and 13.5-35.4 kPa for Korean pine, implying that roots have substantial effects on slope stability. Moreover, results from the perpendicular root reinforcement model can be practically applied with a relevant reduction factor, considering the simplicity and limitations of the model.
The Philippines is highly prone to sediment-related disasters because of its geographical location and natural conditions. The country has active volcanoes, mountainous terrain, numerous fault systems and a tropical climate. Disasters are caused by large production of sediments, weak slope resistance against erosion and landslide and large amount of sediment discharge. Moreover, deforestation and watershed activities, improper settlement of people in hazardous areas, insufficient rainfall and water level information and inadequate forecasting and warning system make people at high risk of disasters. The recent sediment-related disasters in Leyte and Quezon provinces which claimed many lives emphasized the need for intensified hazard mapping and awareness campaigns. Risk management is an emerging paradigm in disaster management, and the government has resorted to combine this concept with structural measures to mitigate disasters. The Laoag River Basin Project provided sabo dams and dikes to contain excessive sediment and overflow, respectively. Likewise, watershed management programs were pursued. Road slope disasters along national highways causing road closures are now a concern for risk management planning and mitigation. Data on collapses and road closures are being collected for prioritization and planning of counter measures. The various agencies comprising the disaster coordinating councils, despite budget constraints, are intensifying their efforts. This paper describes current sediment problems, the aggravating factors and the various approaches being adopted. The constraints and strengthening needs of disaster management agencies in hazard identification and mapping, vulnerability assessment, collapse prediction and risk management, as well as recommendations are discussed.
The climate variability and global climatic change has brought tremendous impact on the high mountainous glacial environment. About 6% of glacier area has been decreased in the Tamor and Dudh Koshi sub-basins of eastern Nepal from 1970's to 2000. The Himalayan glaciers are shrinking, retreating and lowering its surface. Consequently the lakes formed at the glacier snouts are expanding rapidly in most cases. The ICIMOD in 2001 mapped 2323 glacial lakes and out of it 20 lakes were identified as potentially dangerous glacial lakes in Nepal however, three lakes were removed from the list of dangerous glacial lakes. As an impact of global warming 50 lakes is growing and 22 new lakes have been formed after 2000. Almost all the glacial lakes are situated at high altitude of rugged terrain with harsh climatic condition. Hence to carry out the physical mitigation work on these lakes are impractical but the awareness and adaptation measures can be carried out to reduce the GLOF risk. As a pilot case study GLOF risk reduction activities were carried out in Everest region downstream of Imja Tsho, one of the fastest growing lakes in the Himalaya.
The environmental and socio-economic impact of erosion in Nigeria is huge and well-known to international organizations including the United Nations. Although gully erosion is a serious problem in south eastern Nigeria, it does not affect all the states in the region equally. While Abia, Anambra Enugu and Imo states are severely affected, Ebonyi state is mildly affected. It is shown in this paper that geology and nature of soils underlying the areas, rather than the level of human activity, are the main reasons for the difference in severity. It is emphasized that even the smectite-rich soils covering major parts of Ebonyi state are less prone to gully erosion than the loosely consolidated coastal sands covering the other states. Multiple potholes are a constant distinguishing feature on the smectite-rich roads but not gullies. Most adults do not have proper understanding of sediment disaster nor do they want a career in geo-disaster related discipline. This paper theorizes that apathy and ignorance will hinder any effort at erosion control; and perpetuate the negative impacts of erosion in the area for a long time to come.
In recent years water-related disasters are increasing and so are sediment-related disasters. A global outlook of sediment-related disaster is highlighted in this paper to delineate the status and trends in the bigger picture of water-related disasters. This provides background information for policymakers. The increasing trend of disasters is evident; slides (including landslide, debris-flow and other mass-movement) have increased more than four-fold in Asia from 1980 to 2006 accounting for 64% of global slide fatalities. On the other hand, fatalities of individual slide events from 1900 to 2009 show that Europe is most vulnerable but the events are most frequent in Asia followed by the Americas. In terms of slide affected people, Asia ranks worst followed by the Americas. The damage estimate shows that Europe incurs the greatest loss followed by the Americas. Expansion of cities into marginal areas to accommodate population explosion, pressure on land due to improper land-use, deforestation and increasing slum population have lead to increased vulnerability to sediment-related disasters. This must be tackled through policy changes based on scientific knowledge.