Inoue T.,Japan Civil Engineering Research Institute for Cold Region |
Parker G.,University of Illinois at Urbana - Champaign |
Stark C.P.,Lamont Doherty Earth Observatory
Earth Surface Processes and Landforms | Year: 2017
In meandering rivers cut into bedrock, erosion across a channel cross-section can be strongly asymmetric. At a meander apex, deep undercutting of the outer bank can result in the formation of a hanging cliff (which may drive hillslope failure), whereas the inner bank adjoins a slip-off slope that connects to the hillslope itself. Here we propose a physically-based model for predicting channel planform migration and incision, point bar and slip-off slope formation, bedrock abrasion, the spatial distribution of alluvial cover, and adaptation of channel width in a mixed bedrock-alluvial channel. We simplify the analysis by considering a numerical model of steady, uniform bend flow satisfying cyclic boundary conditions. Thus in our analysis, 'sediment supply', i.e. the total volume of alluvium in the system, is conserved. In our numerical simulations, the migration rate of the outer bank is a specified parameter. Our simulations demonstrate the existence of an approximate state of dynamic equilibrium corresponding to a near-solution of permanent form in which a bend of constant curvature, width, cross-sectional shape and alluvial cover distribution migrates diagonally downward at constant speed, leaving a bedrock equivalent of a point bar on the inside of the bend. Channel width is set internally by the processes of migration and incision. We find that equilibrium width increases with increasing sediment supply, but is insensitive to outer bank migration rate. The slope of the bedrock point bar varies inversely with both outer bank migration rate and sediment supply. Although the migration rate of the outer bank is externally imposed here, we discuss a model modification that would allow lateral side-wall abrasion to be treated in a manner similar to the process of bedrock incision. © 2017 John Wiley & Sons, Ltd.
Inoue T.,Japan Civil Engineering Research Institute for Cold Region |
Yamaguchi S.,Japan Civil Engineering Research Institute for Cold Region |
Nelson J.M.,U.S. Geological Survey
Geomorphology | Year: 2017
Previous work has shown that the bedrock erosion rate E because of collisions of saltating bedload can be expressed by E = βqb(1-Pc), where qb is the sediment transport rate, Pc is the extent of alluvial cover, and β is the abrasion coefficient. However, the dependence of the abrasion coefficient on the physical characteristics of the bedrock material is poorly known, and in particular, the effects of wet-dry weathering on the saltation-abrasion bedrock incision has not been specifically characterized. Observation suggests that the typical wet-dry cycling of exposed bedrock in river beds gives rise to cracks and voids that are likely to alter the incision rate of the material when subjected to impacts of moving sediment. In this study, flume experiments are performed to develop an understanding of how wet-dry cycling affects the rock tensile strength and the bedrock erosion rate. To represent the physical effects of weathering, boring cores taken from natural bedrock channel are exposed to artificial wet-dry cycles. The experimental results suggest the following: (1) the abrasion coefficient for fresh bedrock is estimated by β = 1.0 × 10− 4σT− 2(d/ksb)0.5, where σT is the tensile strength, d is the diameter of colliding gravel, and ksb is the hydraulic roughness height of bedrock; (2) the tensile strength of the bedrock decreases exponentially as a result of repeated wet-dry cycles, σT/σT0 = exp (-CTNWa0/σT0), where σT0 is the initial tensile strength, Wa0 is the initial normalized rate of water absorption., N is the number of wet-dry cycles, and CT is a constant; (3) the erosion rate of fresh bedrock depends on the inverse of the square of tensile strength, but the erosion rate of weathered bedrock depends on the − 1.5 power of tensile strength. © 2017 Elsevier B.V.
Hayashi H.,Japan Civil Engineering Research Institute for Cold Region |
Hayashi T.,Japan Civil Engineering Research Institute for Cold Region
Proceedings of the 5th International Conference on Geotechnical and Geophysical Site Characterisation, ISC 2016 | Year: 2016
Fibrous and high organic peat is distributed widely in Hokkaido, the northernmost island of Japan. Peat ground is extremely soft and has strange engineering properties. Since behavior of peat ground differs greatly from that of ordinary soft ground, it is classified as a problematic soil. Peat ground is a significant hindrance to infrastructure construction. Electric cone penetration tests (CPT) for peat deposit were conducted at several sites on peat ground in Hokkaido, Japan and K0 consolidated-undrained triaxial compression test on undisturbed peat collected from the same sites, to clarify the undrained shear strength (Su) of such ground. This paper presents that results of the tests, and proposes a method for determining Su for peat using CPT. © 2016 Australian Geomechanics Society.
Tomisawa K.,Japan Civil Engineering Research Institute for Cold Region |
Kimura M.,Kyoto University
Procedia Engineering | Year: 2017
Nowadays, based on the experiences in the recent severe earthquakes, a certain level of seismic performance is required for the foundation design of newly-built bridges. Moreover, there are so many existing pile foundations with serious deteriorations and damages. In this report, new technique to improve seismic performance of pile foundation, in which ground improvement is provided around the existing piles in very soft or liquefiable soil, is proposed. It enables existing bridges to be sustainable and long life structures. The patent to this technique named the Composite-Pile Method has been granted in Japan in 2012. Strength and deformation of improved soil test pieces having different levels of strength were fully investigated through various types of material tests. Seismic performance of pile foundation with improved soil subjected to Level 2 Earthquake wave was then verified by large-scale shaking table tests for model piles. The bending strain in the existing piles surrounded by improved soil with a uni-axial strength of greater than 300 kN/m2, when subjected to severe earthquake, was reduced by half of the strain generated in the piles without soil improvement, which leads to the reduction of response displacement of the piles. It was clarified that, based on the test results, seismic performance of pile foundation could be drastically improved by the proposed method. Practical application of the proposed method, when needed in the future, will be undertaken for higher safety of public structures. © 2017 The Authors.
Matsushita H.,Japan Civil Engineering Research Institute for Cold Region |
Matsuzawa M.,Japan Civil Engineering Research Institute for Cold Region |
Abe O.,Japan National Research Institute for Earth Science and Disaster Prevention
Annals of Glaciology | Year: 2012
An experiment using artificial snow was conducted to clarify the influences of temperature and normal load on temporal variations in the shear strength of snow. Artificial snow consisting of dendritic crystals was allowed to accumulate to ∼60 cm depth for the experiment. The shear strength, temperature and weight of the overlying snow were measured at three different depths in the accumulated snow. For the measurement of shear strength, the shear frame index (SFI) was found using a shear frame by placing weights with different masses on the snow contained within the frame. The measured SFI values were treated with the Mohr-Coulomb failure criterion to find the snow cohesion factor C and the internal friction factor tan φ. The results highlighted similar trends for SFI and C values by which their rate of increase over time was greater with higher snow temperatures and normal load caused by overlying snow. This indicates that C contributes significantly to increased SFI values. tan φdecreased over time with higher snow temperatures and increased with lower snow temperatures. In low-temperature conditions, in particular, it is likely that snow crystals are compacted but maintain their dendritic morphology. © 2013 Publishing Technology.
Kishi N.,Hokkaido University of Science |
Khasraghy S.G.,ETH Zurich |
Kon-No H.,Japan Civil Engineering Research Institute for Cold Region
ACI Structural Journal | Year: 2011
To establish a performance-based impact-resistant design procedure for reinforced concrete (RC) members, it is essential to establish verification methods for limit states-mainly, serviceability and ultimate limit states. These limit states should be evaluated for single and consecutive impact loadings. The best way to accomplish this is to correlate experimental studies with numerical simulations. From this point of view, a three-dimensional (3-D) elasto-plastic finite element (FE) analysis method using simple constitutive models is proposed herein. This study aims at numerically evaluating the accumulated damage and residual load-carrying capacity of RC beams under consecutive impact loading. The applicability of this method is confirmed by a comparison with the experimental results. From this study, it can be seen that the maximum deflection, the residual deflection, and the vibration characteristics after impacting can be accurately predicted. These may be the most appropriate indicators for the evaluation of the damage and residual loadcarrying capacity of the beams. © 2011, American Concrete Institute. AU rights reserved.
Mizugaki S.,Japan Civil Engineering Research Institute for Cold Region |
Nanko K.,University of Tsukuba |
Onda Y.,University of Tsukuba
Hydrological Processes | Year: 2010
A field investigation of the effects of rainfall and slope angle on splash detachment was conducted for 5 months, using 27 splash cups in a Japanese cypress plantation in southern Japan. The unit, kinetic energy of throughfall (J m-2 mm-1), was found to be constant and independent of rainfall intensity, indicating that splash detachment can be related to both throughfall intensity and soil slope. The linear correlation coefficient was highly significant for both the maximum throughfall intensity over 1 h (RI1h) and the average splash detachment of all splash cups, although individual cups varied widely. No consistent relationship was found between the splash detachment rate and slope angle for individual periods. This variation in the relationship between splash detachment and slope angle was attributed to the effect of soil crusting and ponding on splash detachment. Splash detachment on a gentler slope (14°) exhibited a strong relationship with the maximum throughfall intensity lasting for short periods of 10-30 min. In contrast, the splash detachment from slopes >35° highly correlated with the maximum throughfall intensity over 3 h. This suggests that a longer period is required to prevent the splash detachment due to ponded water on steeper slopes. On gentler slopes, prolonged rainfall may result in a higher ponding depth, thereby reducing raindrop impact and causing less splash detachment. Thus, under forest canopies, the effect of slope angle on the rainfall parameter should be incorporated into future splash erosion models. © 2010 John Wiley & Sons, Ltd.
Tabelin C.B.,Hokkaido University |
Igarashi T.,Hokkaido University |
Tamoto S.,Japan Civil Engineering Research Institute for Cold Region
Minerals Engineering | Year: 2010
This paper describes the factors affecting arsenic (As) mobility from hydrothermally altered rock under in situ conditions. Four impoundments were built on site with rectangular base, truncated-pyramid structures. Impoundment 1 was composed solely of the hydrothermally altered rock while impoundments 2-4 were covered with different types of silty covering soil in order to minimize O2 and water intrusion into the rock. The results indicate that seasonal variations in temperature, O2 concentration and volumetric water content in the impoundments strongly influenced As leaching. When the temperature was high and the water content low, oxidation of sulfide minerals in the rock was enhanced because of a higher air-water-rock interaction. Concentration of As in the porewater increased when it rained after a specific period of dry weather. The use of a silty covering soil influenced the concentrations of As and SO42 - in the porewater although it did not affect the pH and Eh significantly. In this case, O2 and water movement might be the rate controlling step of As leaching, that is, if there is no covering soil on the impoundment, more As will leach out from the rock. The use of a silty covering soil showed promise of reducing As leaching from the waste rock, but utilizing it alone was insufficient to effectively prevent As release from the rock. A combination of covering soil and bottom As-adsorption layer to incorporate any As released from the rock is therefore recommended. © 2009 Elsevier Ltd. All rights reserved.
Kakinuma T.,Japan Civil Engineering Research Institute for Cold Region |
Shimizu Y.,Hokkaido University
Journal of Hydraulic Engineering | Year: 2014
This study aims to clarify the mechanism of riverine levee breach and propose a new numerical model for that phenomenon. Large-scale experiments of overtopping breach were performed using an experimental flume located on the floodway of an actual river channel. By taking advantage of the scale of the flume, the levee breach process was monitored with state-of-the-art observation devices under highly precise hydraulic conditions. Four test cases were performed with variations of inflow rate, levee material, and levee shape, and the levee breach was monitored quantitatively using acceleration sensors installed in the levee body. From the results of the experiments, the breach process is categorized into four stages, focusing on the breach progress and hydraulic characteristics. It was determined that the correlation between the breached volume and the hydraulic quantities of velocity, water level, and Shields number can be expressed by an equation similar to that for bed load transport. Finally, a two-dimensional numerical model is proposed by integrating the experimental results into geomechanics, and a good reproduction result is obtained. © 2014 American Society of Civil Engineers.
Nagayama S.,Japan Civil Engineering Research Institute for Cold Region |
Nagayama S.,Japan Aqua Restoration Research Center |
Nakamura F.,Hokkaido University
Landscape and Ecological Engineering | Year: 2010
To provide river managers and researchers with practical knowledge about fish rehabilitation, various studies of fish habitat rehabilitation that used wood were reviewed. The review focuses on fish responses, wood installation methods, and geomorphic features of the rehabilitation sites. Most studies were conducted in moderately sized (small and medium) streams with relatively high bed gradients and aimed to improve the habitats of salmonid species. In this stream type, structures spanning the full (log dam) and partial (log deflector) width of the river were most common, and wood structures that created pools and covers were successful in improving fish habitat. Some projects were conducted in moderately sized low-gradient streams, in which wooden devices used to create instream cover were effective for fish assemblages. There were few studies in other aquatic ecosystems. However, well-designed large wood structures, known as engineered log jams, were used in rehabilitation projects for large rivers. In slack-water or lentic systems such as side-channels, estuaries, and reservoirs, small and large wood structures that created cover were used to improve habitat for many fish species. For successful fish habitat rehabilitation projects, the hydrogeomorphic conditions of rehabilitation sites should be carefully examined to avoid physical failure of wood structures. Although artificial wood structures can be used to improve fish habitat in various aquatic ecosystems, they should be considered to be a complementary or interim habitat enhancement technique. The recovery of natural dynamic processes at the watershed scale is the ultimate target of restoration programs. © 2009 International Consortium of Landscape and Ecological Engineering and Springer.