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A braided gravel-bed river provides essential habitats for various plants and animals. However, human regulation of rivers, such as dams, channelization, and other engineering works, alter flow and sediment regimes, which generally cause the degradation of river and riparian ecosystems. One of the prominent changes prevailing in Japanese rivers is forest expansion over gravel bars, and many native plants and animals that depend on gravel-bar habitat are now endangered. The Satsunai River is typical of rivers experiencing forest expansion, so the Japanese government launched a restoration project in 2012 to partially restore its riparian ecosystems. This is a large-scale experiment developed jointly by an interdisciplinary science team and river managers, who conduct monitoring and evaluation under an adaptive management scheme. The main measure to restore gravel bed habitat was an artificial flood regime, releasing a maximum water volume of 120 m3/s, which was a 2-year return period flood before dam construction. A unique feature of the project is that we considered the role of high-magnitude floods with recurrence intervals greater than 20 years after dam construction. We hypothesize that the artificial floods can be timed seasonally to create sites for regeneration and nesting of riparian species, and the high-magnitude floods contribute to maintaining a shifting mosaic structure of floodplain forest and unvegetated gravel-bar patches. We also used critical non-dimensional shear stress analysis to define “flood-disturbance areas” that can be disturbed under the artificial flow regime created by a dam. Artificial floods have been initiated once a year since 2012 at the end of June, synchronized with the seed dispersal period of Salix arbutifolia, which is endangered and a high conservation priority in the project. Thus far, the idea of setting floodplain-disturbance areas and the strategy of using both artificial and high-magnitude floods to restore a shifting mosaic of floodplain habitat patches is appropriate, as we found successful regeneration of S. arbutifolia and an exponential decay curve of the age distribution of floodplain forest patches. However, the sediment regime regulated by the dam was not addressed in this research, so future monitoring should track changes in river morphology associated with reduced sediment supply caused by the dam.

Due to a typhoon and a stationary rain front, record amounts of rain fell in September 2011, and the largest class of discharge in recorded history was observed in the Otofuke River of eastern Hokkaido in Japan, and extensive bank erosion occurred in various parts of the river channel. Damages were especially serious in the middle reaches, where part of a dike was washed out. The results of a post-flood survey suggested that the direct cause of the dike breach was lateral advance of the bank erosion associated with the development of meandering channels. As the related development mechanism and predominant factors have not yet been clarified, this remains a priority from the viewpoint of disaster prevention. A past study on the development of meandering channels was reported by Shimizu et al. (1996). In this study, the meandering channel development process was reproduced using a slope failure model that linked bank erosion with bed changes. The study attempted to clarify the meandering development mechanism in the disaster and its predominant factors by using this model. The analysis properly reproduced the characteristics of the post-flood meandering waveforms. Therefore, it is suggested that the development of meandering during the flood attributed to the propagation of meandering downstream, which is triggered by the meandering flow from the meandering channel in the upstream, which also suggests that this propagated meandering then caused a gradual increase of meandering amplitude accompanied by bank erosion in the recession period of the flood., c Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 License.

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.

The variation of the water level at Palangka Raya is well described by the Nearest Neighbor Method (NNM) and the prediction can be made with 1-month lead time. A rainfall runoff model, traditional Tank Model, is applied to analyze the rainfall runoff to predict the water level at Palangka Raya using the data of several rainfall gauging station. The water balance is evaluated at Palangka Raya from the rainfall, the discharge and the evaporation.

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.

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.

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.

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.

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.

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.

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.

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.

In light of the fact that river life is affected by sediment, nutrients and other factors on which river ecosystems are based, i t is equally importanto understandth e statuso f these factorsa s well as theirb ehaviorc hangest hat may result from river channel alterations. It has been pointed out that most of the nutrients that stored in river channels are absorbed by fine sands and transported in times of flood, before being deposited at their subsequent locations. However, the behavior during floods of such very fine sands that have absorbed nutrients has yet to be sufficiently clarified, and its earliest possible clarification is required to help further promote future river projects. This report considers the flow down process of suspended solids, based on data obtained from flood observation of the Mukawa Rivera nds ubsequents urveyo n riverbankd eposits. And the behavioro f suspendeds olidsa t the timc of a flood was reproducible using the unsteady-flow and bed variation calculation model.

The concentration of suspended solids was measured at the Mukawa Bridge located 2.6km upstream of the Mukawa River mouth, and at the Hobetsu Bridge located 40.7km upstream of the river mouth. The lateral diffusion of suspended solids was measured by comparing the concentration of suspended solid at the center of river course and shores. Suspended solids are transported toward the center from the shore in the early flood stage and afterward move to the shore from the center. Observations of sediments on the floodplain were conducted before and after flood at the Mukawa River to understand the relationship between suspended solids and sediment on the floodplain. The scouring of the sediment on the floodplain and the deposition of the particles, which have the same size as suspended solids on the floodplain, was verified. From these investigations, it is shown that suspended solids and the sediment are interchanged during floods.

This research aims at clarifying the vertical section river use characteristic of fishes, in order to maintain the environmental function of river. Authors are performing spot investigation about the habitation environment of fishes, and the longitudinal-section characteristic of river in Shiribeshitoshibetu River of the Hokkaidou southern part from 1996 to present. This report is described about this result.

Field observations were performed on the Mukawa Bridge located 2.6km upstream of the Mukawa River mouth during the 1998 snowmelt flood. Observations were made of water level, discharge, bed material and various components of water quality. The following became clear as a result of the investigations.There is strong correlation in the concentration of the Nitrogen and the Phosphorus, which is adsorbed to by the suspended solids, with the average grain size of the suspended solids. Some water-quality components increase with discharge, and other components do not change with discharge. The waterquality components, which increase in concentration at the time of the flood, are primarily the undissolved elements. It seems that the rise in concentration of undisolved solids at the time of the flood was due to the suspension of flood plain and near bank deposits.

When trees in river channels drift during flood, those that are stacked in rivers sometimes cause flooding of river water. In addition, when they lodge against river structures, such as bridge piers, the structures may be destroyed due to an increased pressure of the river flow. Therefore, the plans for felling and planting trees in river channels need to be made so that trees do not lodge or drift during flood. The purpose of this research is to understand how the critical lodging value calculated from the results of the general test on felling trees fluctuates during flood. The results of the examination on relevant factors affecting the critical lodging value are reported in this paper.

In river planning, it is necessary to study the influence of river construction to the river flow and the river bed. A flow affected by the river construction is a three-dimensional complicated flow.Sand deposition and scour around the construction is caused by this complicated flow which make it difficult to grasp the river change with the construction.To investigate the mechanism of flows induced by spur-dike, we need the field survey around a spur-dike.we are reporting new results obtained from the occurrence of secondary flow behind the spur-dike.

Volcanic ash soil is widely scattered in Hokkaido, and numerous river banks are formed by volcanic ash. Although volcanic ash is generally considered susceptible to erosion, there have been few studies on rivers with volcanic ash banks. As a result river channels with volocanic ash banks are maintained similarly to those with sand banks, which may result in poor safety and flood control. To study the mechanism of erosion in river channels with volcanic ash banks, a relation between the erosion process and the shear stress is investigated in a largescale straight flume.

It is very important in river management works to take into account the influences of the vegetation to the flow during the flood. Logarithmic or parabolic law can be a good assumption for the vertical profile of downstream velocity distribution in open channel flow without vegetation. However, these assumptions cannot be applied to the flow through the vegetation because of the strong effect of the resistance caused by trees.In this paper, the vertical distribution of flow through the vegetation is predicted theoretically. The bed drag coefficient has to be changed in order to calculate the depth averaged downstream velocity accurately.

It is one of the most important subject in river improvement works to predict bed erosion. However, there have been few theoretical studies and the qualitative mesurements on bed erosion especially in actual rivers. In this paper, the relationship between hydraulic factors and the bed erosion is studied using river data. It is found that the tendency of the bed erosion can be expressed by two factors, which are the hight of bar and degree of meander. Using this expression, the bed erosion can be predicted very easily even at local offices at the site.

Formation of bars indicates meandering of flow and deposition or erosion in river bed. It is necessary for river improvement and water utilization to clarify this phenomenon.Therefore many researches have been conducted on bar formation. In our previous study, it was found that the channel formation process was influenced by the form of hydrograph and duration of discharge recession period. The purpose of this study is to determine the influence given to channel formation by difference of discharge recession pattern in detail. Experiment conditions are case of constancy and variation in discharge recession speed.It is found that the channel formation process is strongly influenced by gradient of hydrograph at discharge recession period. That is, the relationship both discharge and gradient of hydrograph(discharge recession speed) is important for the channel formation process.channel width is able to estimated by the stable channel width theory proposed by Ikeda. In addition, it is found that the active.

It is important to clarify the formation, break and flow of river ice on frozen rivers. The aim of this study is to clarify ice break and flow phenomena during river ice breakup, as these phenomena are the initial stages of ice jam. With regard to the break of river ice, we developed a method of judging ice break by comparing the stress caused by the flowing water and flowing ice, and the allowable bending stress of the river ice. The ice flow is expressed using the continuity equation that considers the change in amount of ice, and the equation of motion that considers the effect of the flowing water. This one-dimensional mathematical model showed that although there is a problem with the reproducibility of ice jam, it is possible to reproduce ice break and flow phenomena by comparing the freezing conditions from the results of the calculations, with those of aerial images of the actual river.