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In Japan, great damage to abutment backfills caused by large-scale floods occurs every year. It is necessary to examine the fundamental collapse mechanism on as close to full scale as possible, in order to achieve a sufficient effect of the similarity laws in geotechnical engineering, river engineering, and bridge engineering. In this study, geotechnical evaluations were carried out, such as observation of pore water pressure behavior in an embankment, settlement of an embankment crest, and applicability of a reinforced soil wall as a countermeasure construction, under conditions affected by flowing water. The results show that the collapse behavior of an embankment surface differs depending on the soil properties of the embankment material. The erosion of the river bed progressed even in the footing lower surface of the abutment foundation and in the upstream side of the abutment, where the flow velocity was locally large and changes in the flow direction were confirmed. In addition, when a gabion-reinforced earth wall with water permeability was used for the wall surface construction as a countermeasure, washout of the embankment in the abutment to the extent that a pavement surface above would collapse did not occur, thereby confirming the gabion’s usefulness as a countermeasure.

結氷河川の解氷現象は，水位の急激な上昇を引き起こすアイスジャムの発生の可能性がある．河川結氷時の河道内工事や流量観測を実施する際には，作業計画および安全面から，解氷現象に関する知見および河道内の氷板厚を計算する技術が求められている．結氷河川における解氷現象を解明するために，氷板厚を連続的に測定し，熱フラックスを用いた氷板厚計算モデルを用いて，放射・対流，積雪，流水の3因子が氷板厚の変動に与える影響を評価した．本計算モデルを基にして，積雪と晶氷の影響を係数α，河氷の融解の影響を係数βに織り込み，気温，水温，有効水深から氷板厚を計算する実用的な計算式を開発した．さらに，気温から水温を計算し，有効水深を一定値とすることで，気温データのみから氷板厚を計算可能であることを示した．

Field surveys, experiments and numerical calculations were conducted to examine driftwood accumulation in a river channel, with respect to the existence of sandbars. We investigated the areas where driftwood accumulated and captured from river channel is observed in two rivers. It seemed that factors such as the existence of riparian woods, sandbar configurations, and sandbar locations relative to the river flow determined the points of driftwood accumulation. Based on the field survey results, hydraulic model experiments investigated the flow path of driftwood in model river with alternate sandbars. The analysis related to flood flow indicates that when the flow velocity and the water depth of the main current differ greatly from the flow velocity and the water depth of the flow that is away from the main current, the force of the main current acting on driftwood pushes the driftwood outward. And we clarified the relationship between discharge and the locations of driftwood capture, traps were built at different points on a sandbar.

Disasters causedby torrential downpours have recently been increasing throughout ournation. Inparticular, there have been many cases of flood damage on small and medium-sized rivers with relatively small design flood discharge in valley flats. Specific instances of this type damage include that which occurred in Tochigi Prefecture's Yosasa River in 1998, the Appetsu River in Hokkaido in 2003. These disasters are characterized by the flow of flood waters that were not confined to the river channel and caused damage to many houses, roads and bridges. In this study, we examined a method for adopting double Fourier analysis, which is used for shape analysis of the results of experiments on sediment hydraulics, to assess the flow channel geometry of an actual valley flat. The originof the terrain of the valley flat was estimated based on the results of sediment hydraulic studies, including experiments on compound meandering channel anddouble-row bar. The potential for utilizing the estimate results to predict the flow of flood waters and measures to reduce the impact of disasters at times of large-scale flooding in a valley flat was then indicated.

This paper presents investigations, performed to assess the effect of the initial bed perturbation on the sand bar formation and evolution process. Computations were conducted with two different kind of initial bed perturbations. In the series of first computations, the influence of the amplitude of random field of perturbations was tested. The time to reach an equilibrium state was found to be depending on the amplitude of the initial random bed perturbation. It was found that the larger the amplitude, the shorter the time to reach an equilibrium state. The second test was conducted with the initial bed perturbation in the form of a double row bar. During the first stage of the computation, double row bar type bed configurations were formed, however the modes of bed configurations appeared to be reduced and finally turned into single mode type bed configurations. Since the hydraulic condition of the numerical experiment is single-bar regime, this experiment corresponds to the transformation of bar conditions from the doublemodes to the single-mode during the flood in the natural rivers.

The ecosystem in the tidal region of a river tends to be diverse because of seawater intrusion. Such an ecosystem is observed in the Abashiri River in eastern Hokkaido. The downstream end of Lake Abashiri is about 7 kilometers from the Abashiri river mouth. The lower layer of the lake is saline water that has flowed upstream from the sea. Dissolved nutrient salts from the saline water layer make the lake a good fishing ground. However, in recent years, the damage to fishery of blue tide happens frequently because the fresh water depth of the lake became shallow. One solution is based on the comb-shaped weir traps that are used for catching fish in Abashiri River. Mesh structures that simulate weir traps were used in laboratory experiments to study the relationship between mesh porosity and salinity intrusion velocity. The following findings were obtained: The salinity intrusion experiment showed that when the mesh porosity is low, the flow velocity decreases before the water reaches the structure, and that the flow velocity does not differ greatly between before and after passing through the structure.

Hydraulic experiments were conducted to clarify a changing process of bar shape formed during flood at usual time. Double-row bar-like shape appeared and reaching a meandering style channel was observed. It became clear that the component which has wavenumber 1 and 0 in the transverse and longitudinal directions respectively with formation of a meandering style channel increases as a result of the wave number analysis of bed forms. Furthermore, although it was examination near the limit of theory, weak nonlinear analysis considering the interference between (1, 1) and (2, 2) components was carried out, and the same result as an experiment result was obtained.

We measured the cross-sectional flow velocities in the tree zone of the Otofuke River during a flood caused by Typhoon Etau in August 2003 and tried to clarify the flow conditions at that time. It was confirmed that the obtained flow pattern of cross sectional distribution sufficiently reproduced behavior of the flow velocity in the tree zone. The calculated average flow velocity in the tree zone was about 0.12 - 0.27 m/s, which was 1/15th that of the main stream, whose average velocity was 3 m/s without trees. Discharge from the vegetated section accounted for about 6% of total section. And it has become necessary to examine the existing methods of measuring tree density. Key Words: field observation, the flow velocity in tree zone, ADP, tree density, the Otofuke river

Riparian wood area is assumed to be dead water section on study for flowing ability. But, calculation results of water level might not correspond with observations on medium-small scale rivers. We surveyed about vertical formation of flood flow velocity on the cross section including riparian wood area in Bibai River on August 22, 2005. As a result, about 27% of total flow quantity flowed down through the riparian woods on this survey section. And, formation of flow velocity on woods area almost agrees with the theoretical value. But, formation of flow velocity outside the woods is lower than the theoretical one.

Various hydraulic test and numerical simulations have reported that under the condition in which doublerow bars initially form, the mode of bars is reduced and alternate bars are formed over time. In this study, weakly nonlinear analysis is performed in order to elucidate the interaction between the modes of double-row bars and alternate bars. A concept of the rate of linear amplification is introduced in to the weakly nonlinear analysis in order to enable a consistent expansion of alternate bars and double-row bars. This analysis succeeds in reproducing the characteristics of bar mode decrease process. According to this analysis, neither of both modes (alternate bars and double-row bars ) can grow up simultaneously.

The magnitude 9.0 2004 Indian Ocean Earthquake triggered a series of lethal tsunamis on December 26, 2004 that killed over 225, 000 people, making it the deadliest tsunami in recorded history. In order to investigate this tsunami and related damages from the hydrological and hydraulic aspects, the Committee on Hydroscience and Hydraulic Engineering, Japan Society of Civil Engineers (JSCE) delegated its members whose majors are coastal hydraulics, river engineering, hydraulic engineering and hydrology as a tsunami investigation team by JSCE. In this paper, results of filed investigation are shown mainly for disasters in rivers. The present survey results will be useful for future planning of tsunami disaster prevention in Sri Lanka, as well as in Japan, where near future occurrence of tsunami disaster is highly expected.

Weakly nonlinear analysis was adapted to the hydraulic experiments in order to clarify the mechanisms of mode reduction process of sand bars. Hydraulic conditions of this analysis were set large width-depth ratio. The following facts became clear by this research. Initial bar growth rate must be calculated precisely when the mode reduction process is considered. The bar wavelength which passes through the mode reduction process of bar formation can be calculate by the linear stability analysis. The weakly nonlinear analysis can predict the mode reduction process of bars when the perturbation parameter is less than 1. This analysis shows good agreement on the contribution of each bar mode element to the bed formation.Although the growth rate expansion method has a mathematical problem applying to the multiple bar formation process, this analysis shows good agreement on the hydraulic experimental results.

The typhoon No.10 hit the Pacific cost in Hokkaido on August 9 in 2003. The casualties were 10 killed and 3 wounded. The missing person has not been found yet. This typhoon gave the biggest damage by flood to Hokkaido in the last twenty years. In Saru River basin and Appetu River basin in Ilidaka region, serious damages, such as flooded house, house failure and bridge fall, occurred by this typhoon. Farmland and flood plain were covered for the large sediment.In this paper, we will report an outline of heavy rainfall disasters of Hidaka region in Hokkaido by the typhoon No.10. We hope that the result of this investigation helps to clarify the mechanisms of the heavy rainfall disasters with hydrologic and hydraulic knowledge.

Sedimentation in dam reservoirs is a serious matter of great concern with river management works. In general, sediment deposited in the reservoir is hard to move comparing with river bed materials, even if the diamete is equal. Therefore, hydraulic experiments were conducted on the fall velocity of sediment and the entrainment rate of suspended sediment using the material which is deposited in the dam reservoir. It is found that the fall velocity of sediment and the entrainment rate of suspended sediment are smallre then the value currently used conventionally. These results are applied to calculate bed variation of the 2003 flood in the Nibutani Dam resevoir. The calculated results mostly agree with field surveys.

Recent problems facing river management include a decrease in the water storage capacity of dam reservoirs due to sedimentation and a lack of continuity in sediment moment from the upper to lower reaches. To clarify the characteristics of substance transported during flooding, we made observations of and collected samples from the Saru River in Hokkaido, which has a reservoir in its middle reaches. In this area, a massive flood occurred between August 9 and 11, 2003. We also observed this particular flood. Since the scale of this flood was very large, we were unable to make observations throughout the duration of the flood. We were, however, able to conduct observations during the first half of the flood and at the end of the flood end. Here we report our observation results.

Field observation of bed configuration was conducted during the 2003 August flood at the Tokachi River. Radio controlled boat was used in this measurement. The aim of this observation is to clarify the change of bed topography during flood. It is found that the bar formation becomes unclear and the flow becomes straight along the channel at the neighborhood of peak water level in which the hydraulic condition becomes no bar region. The changing of observed bed configuration during the flood is compared with the result of stability analysis under unsteady flow condition. The general tendencies of bar formation change by the theoretical results agree with field data. However, the theoretical results cannot reproduce the observed bar wavelength.

Hydraulic experiments were conducted in order to clarify the mechanisms of mode reduction process of sand bars. Hydraulic conditions of these experiments were set large width-depth ratio. The following facts became clear by these experiments. Starting time of the mode reduction becomes late and time to spend on the mode reduction has the tendency which becomes long, when a width-depth ratio becomes large. One of two scoured area near side walls in double-row bars is buried and single-row bars are formed.Furthermore, the prominence wave number of the bed form is analyzed. It is possible that the single row bars formed by mode reduction from double row bars are the same with the single row bars which are formed from the early stages of experiments on the bed form characteristics. Time changes of amplification of (α21+α40) and (α11+α20) have contributed greatly to the mode reduction process.

Typhoon Na.10 attacked the Pacific side of Hokkaido on August 9, especially in Iburi and Hidaka sub prefectures. The flood generated large amounts of driftwood and caused bridge washout in the Saru River basin. Based on an onsite survey conducted immediately after the typhoon, we examined the driftwood in the Saru River. We studied the distribution of locations where driftwood beached in the river channel, the volume of such driftwood, the distribution of driftwood origin in the river channel, reasons for driftwood originating there. The influence of driftwood to the rivers structures (bridge piers etc) is also investigated. It became clear that the driftwood generated from the river channel and the mountain area occupied 20% and 40% of all driftwood, respectively as a result of investigation. The generating places of the driftwood from the river channel were the flood plain at the inner bend and the eroded outer bank shore.

A project for river-restoration on the Shibetsu River was launched in 2000. For meander restoration, test flow of water started in March 2002 in a bifurcated channel formed by connecting the pre-improved channel (oxbow) with the present straight channel. In the eighteen months that have elapsed, the riverbanks and riverbed in the test section have greatly changed.To analyze hydraulic phenomena under various conditions, this study aimed to develop a numerical calculation method that enables estimation of flow and riverbed evolution. We made calculations that reproduce riverbed evolution for the test section. The reproducibility was verified by comparing calculation results and measurements. We clarified the applicability and applicable range of the calculation method.

It is important to consider the bed load transportation for performing of river management works. Many researches on bed load transportation were conducted and many estimation methods about the amount of bed load transportation were proposed.However, it is very difficult to measure the bed load transportation in steep rivers and there are a lot of unknown portions on the behavior of large scale materials. Therefore, the experiments on behavior of large scale materials using high-speed hydraulic channel were conducted. Experimental measurements of bed load were compared with calculated values using several bed load equations (Meyer·Peter-Muller; Ashida and Michiue; Sato, Kikkawa and Ashida). It was found that the Sato-Kikkawa-Ashida equation produced calculations most closely in agreement with the experimental data.

In rivers that have sections of single-row bar and sections of double-row bar, the occurrence mechanisms of bank erosion, sediment accumulation, and other phenomena differ between these two types of section. To determine which of these sections, non-linear analysis of conditions over the border between the regions is needed.In this study, hydraulic experiments were conducted using a wide width channel in order to understand the bar formation process under unsteady flow conditions. Hydrograph of the experiments were set as the alternate bars and double-raw bars were generated. The results of experiments were compared with linear stability analysis under unsteady flow conditions. It was found that alternate bars and double-raw bars were generated under the influence of flow conditions process.

Relationships between variations in river topography, such as riffles and pools, and fish, benthos, etc., have long been studied. The topographical classification of riffles and pools, however, is still conducted qualitatively. To take measures for the restoration or improvement of habitats, it is necessary to quantify these relationships and explain them in hydraulic terms. Meanwhile, studies on river -bed morphology have been conducted from the sediment hydraulics viewpoint, allowing for expression with physical indices. This paper examines the quantitative classification of riffles and pools of mountain streams, based on the findings of sediment hydraulics, to clarify their characteristics.

A project of river channel restoration has been promoted in the Shibetsu River, that attempts to restore the natural meander channel remained as a oxbow lake by letting the river water run through. In this project, it is intended that water streams into both the restored meandering channel and the straightened current river course by a small dam constructed downstream of the bifurcation point. But there are few papers treating the relationship between bifurcated flow ratio and the height of a dam in such complex channel system. This study is devoted to investigate the regulation function of a dam for bifurcated flow rate into an oxbow channel. Theoretical solutions for the flow rate are derived from the momentum balance of the several controlvolume sections of the bifurcated area. The results are compared with observed data in model tests and in field survey at a study section of the Shibetsu River to show good agreement.

Conditions of watershed and rivers are closely connected with coastal environments. Suspended sediment is important to water quality on the coast. In particular, at times of flooding a large quantity of suspended sediment is transported to the coast, and the majority of phosphorus is transported in particle form rather than dissolved form. But not all sediment phosphorus circulates to the coast, for example apatite phosphorus. For this reason, a quantitative analysis on the chemical form of phosphorus was conducted. Suspended sediment was sampled during flooding of the Ishikari, Shiribetsu and Mukawa rivers, and the phosphorus chemical combined forms of the samples were analyzed. The results show that more than 60% of the phosphorous in the suspended sediment is circulating phosphorous, and that the total phosphorous content is related to the suspended sediment diameter and is different for each river according to land use.

Mountain rivers are very steep and sediment transportation occurs frequently. It is very difficult to prevent natural disaster on natural mountain rivers, whose forms tend to change. Therefore river works which had an impact on the environment, for example channel work, have been constructed. But mountain rivers have many pools, riffle and ecological variety. So we believe we can restore, rehabilitate and protect the river eco-system by using natural hydraulics features of mountain rivers. This report describes classification of habitat by longitudinal section. And it also describes relation between medium-scale bedforms.

Large-scale scouring was generated by the 1998 flood of the Shokotsu River resulting in revetment damage. The main channel was filled up by deposition during the 1981 flood of the Toyohira River. The cause of this scouring of the floodplain and deposition in main channel was considered for the purpose of understanding the impact of flood to channels. It seemed that the scouring and deposition was caused by the formation of double-row bars. In general, the nondimensional shear stress at the floodplain is smaller than at the main channel. Therefore, it seems that the floodplain is hard to be scoured. However, when the elevations of floodplain surface and main channel bed are not so different, a lot of deposition and scouring in channel is caused by multi-row bars during large scale flood. This phenomenon is shown in this paper using actual river data.

In the previous researches on the sand bed stability with finite amplitude and growth of alternate bars, it is assumed that flow changes over time due to the unsteady flow do not affect the generation of alternate bars. Hydraulic experiments on the formation of alternate bars under unsteady flow conditions are conducted in this paper. The wavelength and wave height of alternate bars differ significantly between unsteady and steady flow conditions and between rising and falling stage of the water level due to the flood wave. Unsteadiness is typical of actual formative conditions for alternate bars, especially for gravel bed rivers. Results suggest that finite amplitude effects of the flood-wave on alternate bar formation are not negligible when investigating their behavior under unsteady flow conditions.

To control suspended material transport in rivers, we must study the effect of the suspended material on the coastal sea area near river mouth in the end. In April and May, 2000, we investigated the transport of suspended material at the snowmelt flood at Mukawa river and the coastal sea at the same time. Snowmelt flood is dominant in the suspended material transport in a year in Hokkaido. Once suspended material of the river flows into the sea, it is transported by the coastal current directly, or goes to the bottom and rises again due to the ocean waves and is transported by the current near the bottom. On this investigation we used the acoustic Doppler current profiler (ADCP), which is designed for measuring current velocity and scattered intensity at the same time. Scattering intensity is related to turbidity, and if turbidity is related to concentration of suspended material, we can calculate the mass of suspended material. In this way, we estimate the direction and the mass of suspended material transport.