This paper described the result of synthesized understanding of river environment using Quantification theory Type III and Cluster analysis. The twenty one river environment factors consist of physical, chemical, biotic, and river aesthetic were collected from 49 investigation points situated in twenty rivers categorized as main rivers in Kyushu. The result of qantitative analysis showed the classifications based on three quantitative axes, namely: riparian longitudinal location-axis (first axis), migration-axis (second axis), and diversity-axis (third axis), while the result of cluster analysis showed that 49 investigation points were classified into six groups. This study clarified that the particle size of river bed, benthic organism, ASPT and transparency gave high contribution on the first axis; the migration of aquatic animal and existence of weir were on the second axis; while flow and habitat structure condition highly contributed on the third axis.

The survey on river water quality by new index, established by MLIT in 2005, has been done mainly in Japanese first-class river. The results from 2005 to 2007 show that from the viewpoint of contact between man and river, many rivers were not evaluated to be in good condition, even though the BOD levels were below 1mg/L. Also the results indicate that the causes which lowered the evaluation.

To find countermeasures for these causes is the next step toward better river water quality management.

The social concern with environmental target in river management has been growing in recent years. In order to set environmental target, it is necessary to show the environment by some clear standards. The river landscapes consist of physical background such as morphology and hydraulics, material cycle and biota. Through cooperated research among river hydraulics, ecology and limnology more than ten years, we have accumulated a lot of fragmentary information, and the problem is how to organize them properly to apply to the ecosystem assessment.

We focused on a segment with alternate sand bars. The infonnation about habitat of typical organisms with their life cycles and bioelement cycle are concentrated on the "sub-bar scale landscapes" as ecological function in a river with alternate bars. These landscapes are classified by frequency of inundation and surface some materials using aero photographs. Also ecological functions of each bar are calculated by WUA. The results indicated their temporal and spatial distribution in the segment. With their information, we can discuss and judge what bars are contributive in ecosystem conservation, and what functions have been changed during 30 years. This paper suggests a strategic approach for river management in a sand river.

In this paper, we proposed a method to evaluate the river vegetation. Based on the data of individual species (Ohaba 1983), we proposed community evaluation index (V), community indigenous value(CIV), and environmental evaluation index (E) conclusively. By applying these indexes to Kokai River, E was in the range of 65-90. The E value was tended to be lower on the upper left side of the river because of the existence of many communities with low CIV such as farmlands and weeds. These indexes will enable a river environmental planning to estimate quantitatively as well as a flood-control planning.

Using geographic information on cadastral maps in 1890s, we quantitatively estimated structures of the aquatic habitat networks (AHN), which were composed of a river, ditch networks and paddy fields, by Geographic Information System (GIS). Also we quantitatively calculated the current velocities and water depths of the channel networks by hydro accounting and then evaluated the influences of differences of AHN between present and past on the spawning behavior of catfishes. The results indicated that the past AHN had the following characteristics compared with the present one: (1) catfishes were able to move easily into paddy fields from ditches as fragmentation between the ditches and the paddy fields was small; and (2) the frequency of catfish migration into the ditches, which were disconnected to the paddy fields, was lower because there were fewer branch structures of irrigation canals.

Conservation of habitat networks is important to conserve and restore aquatic systems. Especially for migrating species, habitat fragmentation may cause extinction of local populations. In this study, we made a field investigation of the distribution of the amphidromous freshwater shrimp Caridina leucosticta in Aida River, Aichi, Japan, to clarify the effects of river weirs on the migration of this shrimp. Also we developed a method of habitat network assessment for this shrimp by applying graph theory. The distribution pattern of this shrimp simulated by our method was very similar to this shrimp distribution observed in the field investigation. Thus, we conclude that our assessment method can simulate habitat networks with high accuracy. We discuss the development potential of this assessment method for habitat networks by applying graph theory and the application of this method for conservation and restoration of habitat networks.

Flood disaster occurred along the River Iga, Okazaki, Japan, in August 29, 2008. Two people died by this disaster; one is by overflow into resident area and another is by destroying a house in river area between banks. Precipitation data and flood phenomena are investigated and analyzed by data collection and field survey after the event. Results shows; (1) maximum average hourly rainfall intensity is 108.8mm/hr and more than a half of quantile removed from river by overflow due to lack of cross-section area in upstream river channel. (2) A part longitudinal part has narrow cross-section, which has more than 60 houses in river area between river banks, rose up water levels upstream side, and heavy overflow disaster occurred due to water level propagation up to 400m upstream.

The sudden water level rising caused by the severe rainstorm at the Toga River in July 2008 claimed five people. To prevent the water accident, a distributed rainfall-runoff analysis is conducted to understand the sudden runoff phenomena. The rainfall-runoff analysis and observed camera images revealed that discharge which caused the sudden water level rising came from the residential area and contribution from the mountainous area was quite small. It is proposed that only to monitor the discharge at the upper reaches is insufficient to prevent the accident; detailed distributed rainfall-runoff analysis with radar rainfall is essential to predict the runoff phenomena at the urbanized small catchment; and to foster the common view to keep off from the river at the alert of heavy rainfall warning is indispensable to avoid the accident.

Present paper proposes a method to evaluate the effectiveness of several integrated countenneasures against flood haz.ards with special attention focused on lives of inhabitants in an urbanized area formed in an alluvial flood plain. Each integrated countermeasure is constituted by combining some of a single measure such as a flood control dam, strengthening embankment performances, flood plain regulation works, warning and evacuation system etc. Supposing that floods of various sizes with 10- to 200-year return periods take place, each of the integrated measures is applied to a target area of Shiga Prefecture to investigate its effectiveness from a view point of human loss as well as of houses’ damages, using the followings: potential human loss curves, evacuation rating curves and simulated rainfall nunoffs, river flows and flood flows due to bank breaches. These analyses provide a year-by-year change of the human loss risk corresponding to each integrated countenneasure and useful infonnation for choosing a best measure.

Because of recent frequent heavy rains beyond expectation, flood countermeasures are shifted from hard infrastructure development to non-structural measures, providing the public with better information on river condition and evacuation. This policy change leads many municipalities in Aichi Prefecture to release flood hazard maps. However, as some survey showed that most residents do not recognize the presence of the hazard map even after it was delivered, it becomes a problem that the information is not certainly reached to the public. Therefore, since 2007, Aichi Prefectural Government started fostering people who can understand river information and make a right decision upon a flood, by supporting community-based disaster preparedness. This report introduces achievements and problems of these challenges.

In the afternoon of July 28 in 2008, there occurred a flash flood in the Toga River due to localized torrential rain and tragically caused five fatalities including children. The water level allegedly rose about one meter in ten minutes and there seemed to be little time remained to escape from the site. In order to clarify the cause of this disaster, hydrological information such as time variations of local rainfall distribution and river discharge is indispensable. This research pays attention to the flow phenomena within the river channel by estimating the peak flow discharge from video images and performing a two-dimensional flow simulation utilizing laser scanning data for representing complicated river bed topography. It was made clear from the video analysis that the peak discharge is within the range between thirty and forty cubic meters per second. Moreover, the numerical simulation using the appropriate roughness coefficient can reproduce the detailed flow situation, from which it becomes possible to produce CG animations for educating people about the danger of flash flood.

Quantitative evaluation of the flood prevention approaches in Kumamoto castle town in early modern times was very important in order to examine present flood crisis management. The flood prevention approaches were hypothesized as a method in which the flood damage does not expand taking into account the morphological features in Kumamoto castle town, while the spill overtopping allowed. Numerical simulations using one dimensional unsteady flow equation were conducted to confirm the hypothesis. Constructing the discontinuous embankments with different crown levels in the castle town river side was useful for causing the inundation to the opposite banks. The existence of water-level controlling system of Tsuboi River, the catchments of the spills in the castle town, was clarified.

In Jinzu River, Oncorhynchus masou masou has decreased for about 50 years. The existence of large pool is very important, but, for O. masou masou spending most of the duration of survival in a river, O. masou masou habits information is not known enough. A movement course of O. masou masou was clarified by using the radio telemetry system. O. masou masou spent summer in large pool of the main stream and it went up at a branch (Kumano River) and laid eggs, but some barrages disturbed going up. Therefore, for the resources recovery of O. masou masou, restoration of large pool in main stream and the improvement of some barrages in a branch are important.

Before river improvement work, it is necessary to conduct the environmental assessment. One of the most famous environmental methods is PHABSIM. A lot of researchers used this model to evaluate the habitat suitability of fish in the present river. There are little studies which investigate on whether a new habitat area can be made or not. In this study, a numerical simulation using a horizontal 2-D model was carried out with changing shape of cross section in Gokasegawa River. The suitability of spawning for ayu is predicted. As a result, it was conformed that a new spawning beds are made in some cases. The most suitable cross section is chosen with considering both of flood control and suitability of spawning for ayu.

The application of studies on the spawning environments of masu salmon to river projects is expected to enable the monitoring and conservation of good spawning environments in river development for cold, snowy areas.

With the aim of finding a survey method suitable for monitoring and conserving favorable spawning environments in river development, this study considers survey scales for the classification of spawning environments and examines the characteristics of masu salmon spawning grounds using bed topography and bed material composition as variables.

The results clarify that, when a river channel was classified using a survey scale similar to the river width, it was possible to identify pool-tail landforms that are easy for masu salmon to use for spawning. The characteristics of masu salmon spawning grounds with relative bed slope and bed material composition as variables were also clarified.

These results may contribute to monitoring and conserving favorable spawning environments of masu salmon.

The ayu, Plecoglossus altivelis altivelis, migrates from the sea to the river in spring. Horita investigated on the water temperature of Mikawa Bay and Umedagawa River and pointed out that the reason why the ayu migrates is coincidence of water temperatures in the river and sea. Wada pointed out that the water level at the sea greatly affects on the migration number. However, a collective view has not been obtained. In this study, the water level at upstream and downstream of the estuary barrage, water temperature, DO, COD, electric conductively and turbidity when the ayu migrates and those when the ayu does not migrate were compared. The ayu wants to migrate when discharge of Guide-flow Fishway, time variation of downstream water level and water temperature are high. On the other hand, the ayu does not want to migrate when the turbidity and COD are high. If the discharge of Guide-flow fishway is large, sometimes difference of water level is high. If time variation of downstream water level is high, time variation of electric conductively becomes high. Besides, if turbidity is high, electric conductively becomes high.

This study aims to propose a new approach of river flow management to sustain healthy riverbed condition by utilizing the grazing effects of freshwater fauna. We performed experiment to investigate the functions of grazing of ayu (Plecoglossus altivelis) and pale chub (Zacco platypus) on the composition of periphyton. The results of this study are as follows: i) The grazing of ayu restrains and reduces deposition of fine sediment and blooming of filamentous green algae, which were pointed out the inferior quality of riverbed condition. However, the grazing of pale chub cannot reduce them. ii) Feeding activities by not only ayu (Plecoglossus altivelis) but also pale chub (Zacco platypus) contributed to a decrease in the Autotrophic Index and an increase in the AFDM (%) and the ratio of living algae for the periphyton assemblages which were dominated by diatom.

From the sampling and survey work in Azamenose area, we found that there are three kinds of fresh water mussels. It was believed that, these mussels was transfer or brought by floods into this area. However, this presumption content be proved as the water flow, water fluctuation was not measured. In order to clarify the assumption of this process we performed a movement limit tachometry experiment for these mussels. From this experiment, the result showed that the Anodonta sp. can be transfer by a slow current velocity. The same result also exhibit by two other mussels Unio douglasiae nipponensis and Lanceolaria grayana. In addition Anodonta sp. showed a significant correlation between shell length and movement velocity limits. As a conclusion, we conclude that, the main factor that control the movement velocity are the form of the mussel.

We have surveyed the distribution of benthic macroinvertebrates in the Toyo River estuary area. The area was environmentally classified by the distribution of macroinvertebrates. Salinity and depth are two most significant environmental factors which control the distribution of macroinvertebrates. Trench which had been excavated during a reclamation project seemed to have negative impact on the habitat of bivalves. Restoration by which this trench is buried with sand and gravel seems to be effective to conserve the benthic habitat. Since the restoration of these habitat can enhance both the biomass of commercially useful bivalves and water quality control function, it is highly recommendable to pursue it.

Restoration of submerged macrophytes by isolated water areas was examined in Lake Kasumigaura. In the isolated water area, the light condition was improved to compare with the Lake Kasumigaura because of the decrease of turbidity. The soil where submerged macrophytes grow are evaluated to be useful as a seed bank. In the water area where submerged macrophytes grew well, the water quality was improved.

We created An artificial pond in A reclaimed land of Lake Kita-Inbanuma and spread it seedbank at the bottom of pond to restore the submerged plants. Then we knew it submerged plants were re-colonized from the seed banks, and examined condition of the growth and appropriate method of managements of submerged plants.

As a result we recognized that even if we use subsurface water if there are the number of Paraemon paucidens to large crustracean zooplankton and increase of phytoplankton is suggested. Then we predict that submerged plants will be decline by low transparency. Therefore when in germination of submerged plants we should do it remove animals. And if it occurs A great increase of phytoplankton, we think we should do it hurry it up turnover pond water.

In recent years, Water lettuce (Pistia Stratiotes L.) explosively grows at Wando (riverside pool) in Yodo-river. Water lettuce grows and increases the population in Summer and Autumn and thickly covers the water surface, so it is concerned that it influences upon the native plants and the eco-system in Yodo-river. MLIT Yodogawa office removes Water lettuce every year using machines, but it cannot be a decisive solution to exterminate it. To achieve the extermination of Water lettuce in Yodo-river, authors studied the characteristics of Water lettuce investigating the condition of its growth.

In this study, it is cleared that (1)Water lettuce grows at the static water surface in such as Wando, (2)it prefers Brazilian elodea(Egeria densa) habitat, and (3)the characteristics of its growth is similar to that of Brazilian elodea. Furthermore, it is assumed that removing Water lettuce by man power using boats and nets before it grows and covers the water surface is the most efficient at small labor and cost.

Developing an estimation method for viability of dwarf bamboo (Sasa senanensis (Franchet et Savatier) Rehder var. senanensis) rhizomes, the absorbance as an activity of the rhizomes was measured using TTC (triphenyl tetrazolium chloride) dyeing. As a result, the absorbance peak in living rhizomes appeared at 480nm of wavelength, while that in dead ones did not. Moreover, through a series of tests on hydrological environment such as groundwater level, duration of existence below water level and a change of water level, it was clarified that dwarf bamboo loses the activity below water level more than 30 days. Based on these findings, the method expects to be used as the objective judgment for viability of dwarf bamboo, when we conduct the techniques to prevent dwarf bamboo from invading a wetland at Sarobetsu in northernmost Hokkaido by controlling the water level.

It is said that the formation of sandbars in river channels is closely related to the growth of trees invading onto them, and is also controlled by the flow rate. The purpose of this study is therefore to clarify how sandbar development is related to factors such as flow fluctuations and tree growth. At the Shibetsu River meander restoration test site, detailed studies of sandbar formation and the invasion of vegetation onto them are being conducted in a rectangular channel. A vegetation survey was conducted by setting three survey sections (A, B and C) from the sides to the center of the channel in the transversal direction on a sandbar formed on a curve at the test site. The sediment accumulation height was compared with temporal changes in the flow rate and water level to study the relationships between them. Most of the trees here were Japanese fantail willow and Salix kinuyanagi.

Bamboo expands its area by enlarging underground stem. However, its characteristic has not been well investigated in the environment of river bank. We have investigated the formation process of bamboo forest and the distribution of underground stem. Bamboo’s rhizome can enlarge and expand its area if the thickness of sand is more than 10cm and relatively low water content is at the surface of the river bank. These features of river bank have been affected mainly by the change of hunan use and floods. Bamboo expands its area rapidly if the environment is suitable. Once they settle in a relatively dry sand bar where surface sand deposition is thick, rapid expansion is very likely to occur. River managers should pay more prevent bamboo area expansion.

Kushiro Mire is designated wetland under the Ramsar convention and a national park. It is the largest area of its kind in Japan, and has a valuable natural enviromnent. However, enviromnental changes have been problematic in some parts of the marsh in recent years. For example, a decrease in the groundwater level and the inflow of sediment from influent rivers have caused drying and invasion by alder forests around the marsh. Meanwhile, in some parts of the lower reaches of the Kuchoro River (a branch of the Kushiro River flowing through Kushiro Mire), alder forests have disappeared and been replaced by reed and sedge communities. This paper clarifies these changes through the interpretation of existing aerial photos, and presents the results of factor analysis based on hydrological, field survey and other data.

For evaluating the difference of tree breaking phenomena due to floods, i.e. trunk breakage or overturning, and wash-out of trees, Breakage or Overturning Index(BOI)and Wash-Out Index (WOI) were derived respectively in this study. Based on these indices, the method for classifying the dynamics of forestation in rivers was proposed. This method was applied to the middle stream reach of the Arakawa River The situation of ’breaking’ or ’washing out’ on a gravel-bedbar after 2007 flood can be evaluated by using BOI and WOL In addition, in three locations where relative height from ordinary water level isdifferent, the relationships between flood return period and breaking mode of trees due to floods were demonstrated. At the location where relative height is the smallest, possibility of forestation should be low because wash-out of trees occurs by 5 to 10 years return period of floods. In contrast, possibility of forestation should be high at the location where relative heightis the biggest, because wash-out of trees doesn’t occur by 40 years return period of floods. This method is available to judge the area where artificial management of trees is required.

Fine sediment tends to deposit in and around trees on bars during small and medium scale floods. On the other hand, trees are flowed away by large scale floods and the deposited fine sediment in and around trees are transported again. The transported fine sediment deposits at water intakes, weirs, river mouth and so on. In order to manage trees in river considering disaster prevention, water utilization and environmental conservation, the spatiotemporal change of sediment transport characteristics due to invasion, growth and removal of trees should be clarified. In this study, uprooting of a tree is introduced in horizontal two dimensional bed deformation analysis model and reproduce the sediment transport characteristics during large scale floods. Furthermore, the management method of trees in rivers is discussed with focusing on sediment transport characteristics.

In Watarase reservoir, naturally grown random trees are considered to be worsening the landscape and getting maintenance roads or other river management facilities unstabl. Therefore, situation of the reservoir must be checked regularly and unnecessary trees should be cut down systematically. To get a better understanding of the situation in the reservoir, we have carried out the field survey around the reservoir whole area and classified those trees into some categories based on the growth area, elevation and tree diameters.

In order to rationalize maintenance cost by cutting and disposing those trees and get appropriate outcome from the reservoir considering landscape and river management, we applied the asset management method to the tree management plan. Our research showed that preventive maintenance of trees is more effective and economically profitable.

Recently, outflow of the driftwood which originates in bank erosion and slope failure occur frequently. At Miyagawa of Mie prefecture the flood of the highest flow in observation history occurred in October 2004. In this flood, the cedar forest which is afforested to the side bank flowed out with stream bank erosion, but, the zelkova tree which has grown in the side bank did not flow out withstanding the flood.

From this circumstance, as for this study, the judgment whether afforestation of the zelkova tree in boundary of the river in order to control the occurrence of the driftwood is effective, and, history of afforestation of the zelkova tree and propagation circumstance etc of the zelkova tree were investigated.

This study focuses on the driftwood trapping function of riverside forests, aiming to clarify the behavior of flowing driftwood and verify the positions at which riverside forests in sandbars can trap driftwood effectively from the viewpoint of riverside forest management. The river channel form between two points (KP: 15.2 and 16.2) on the Tokoro River and the riverside forest vegetation conditions were reproduced in an experimental channel to enable experiments on driftwood behavior at a variety of flow rates and the trapping of driftwood by riverside forests. The experiment on driftwood trapped by riverside forests was conducted for two cases. Most driftwood was trapped by a riverside forest upstream of a sandbar. The pieces that flowed down without being trapped were caught by another riverside forest upstream of the next sandbar. The locations where driftwood was captured varied by the time of flooding, and it was found that riverside forests situated upstream of sandbars were effective in trapping driftwood.

In recent years, endocrine disrupting chemicals(EDCs) were found in river water. There is a possibility that EDCs can damge the health of human and other livings. Hence, EDCs should be removed from river water. In order to remove hazardous chemical substances from enviromnents, it is available to use ability of organisms such as plants or microbes to accumulate, degrade or transform chemical substances. In this study, an ability of periphyton, which consists of algae and microbes on river beds, to remove BPA and E2 from river water was studied by laboratory experiments and field observations in an existing river, and a model was proposed to predeict the ability of removal based on these studies.

This study attempts to reveal the natural function of phosphorus removal in the Abukwma River mainstream by the inflow of a tributary stream, the Ara River, which originates from volcanic mountain regions. We conducted laboratory experiments to identify the ability of phosphorus removal by the mixture of the tributary and mainstream water. We found a significant amount of phosphate-phosphorus is removed by the suspended sediments (SS) contained in the tributary water. The reduction in phosphate-phosphorus was attributed to the adsorption to soil particles and/or flocculation. Using XRF and SEM-EDX, we identified the major component of the SS is aluminium oxides, thus indicating the SS plays important role in water quality control in the Abukuma River.

Water level of Lake Inba-numa has been managed to be higher and less fluctuated than before since 1968 because of the water resources development. Unfortunately lake water quality became worse at that time, and these changes are thought to cause the disappearance of submerged plants in the lake.

In this paper, we present the results of drawdown experiment of lake water level to improve water quality through the restoration of submerged plants.

This study is aimed to build a runoff and pollutant load model of the Ariake Sea catchment area using GIS, tank model and field investigation. The model simulates hydrological phenomena in good agreement with observed one by introducing GIS based data of upstream river basins where tidal effects do not exist. In order to estimate water quantity and water quality of the downstream river basins where the tidal effects exist, a typical test field has been chosen and investigated continuously in the east Saga Plain for 7 months. Non-point pollutant load from paddy fields are estimated by observing water quality and quantity of efflux water discharge. As a result, a balance of water and substances of the paddy fields is estimated for irrigation period and non-irrigation one. By considering observed data and the existing pollutant load per unit activity, the pollutant loads from the downstream river basins are estimated.

A selective withdrawal system (SWS) is essential for the conservation of water quality in a dam reservoir. In order to preserve the water quality of a reservoir, the operation of SWS must take into account not only physical hydrodynamics but also chemical and biological interactions. To assist a dam operator, we have developed a decision support system called ARMS (Aquatic Real-time Management System) for Tono dam reservoir. ARMS is designed to aid in the management of daily operation by supplying water quality forecasts during both normal conditions and flood events through real-time numerical simulations. We also applied a neural network technique called Seif-Organizing Map (SOM) to forecast the meteorological and hydrological data. ARMS supplies easy access to the database and allows the user to gennerate figures and animations to visualize the simulated results. This information can be useful for a dam reservoir operator when deciding the SWS operation.

The restoration of the various flow in which there are riffles and pools is one of the strategic policy in nature restoration projects on Yasumuro River. But the river shape in the regeneration sector of nature has been modified as the single cross section by river works, and it was difficult to set the low water channel which riffles and pools are foemed. So we verified the way of the formation of riffles and pools by the two dimensional river bed deformation analysis which activates the sediment regime by digging the river bed. As the result of this analysis, we clarified the effect that the activation of the sediment regime contributes the formation of riffles and pools and that the transformation to the loose slope shape in the low water channel contributes the degree of freedom of the meander in the low water channel.

For preserving a comfortable river ecosystem, it is important to protect and restore the riflle and pools within engineered channels, where should play a role in the habitat diversity and complexity. It will be no easy task to restore and preserve the pool-riffle in flat rock-bed river which is lightly covered with gravels, as contrasted with in the gravel river. In the Abashiri River, the improvement by channel deepening projects with rock-bed has been carried out, in which was introduced to construct artificial pools.

We investigate the restoration of the riffle and pools by constructing artificial pools. By field survey in the Abashiri River, the state of sediment on the improved channel is indicated. By distorted model experiment, we investigate the validity of artificial pools arrangement. As a result, it shows that the artificial constructed pools are corresponded to the location of generated ones in a mobile bed.

In recent years, floods caused by short-term freshets due to localized torrential rain have inflicted enormous damage on areas of human habitation, including a huge flood in the Atsubetsu river basin area of Hokkaido, Japan. Double-row bars had formed on the floodplain as a result of previous major floods, and the damaged area was located in the lower part of the floodplain that had formed as double-row bars. A channel consisting of constructed embankments formed a water course in normal or medium-scale flood conditions. It is very important to understand the process of channel formation during normal or medium-scale discharge for river disaster prevention and river environment.

The purpose of this study was to examine the channel formation process (the scouring of thalweg process) in low flow conditions on bars created in high flow conditions. Two types of hydrograph were used, which differed only in the discharge reduction process (high flow to low flow). One of the hydrograph is a convex form and another is a concave form. It was found that the channel formation process was influenced by the form of hydrograph at discharge reduction period. In addition, the relationship between discharge and flow width should be clarified for understanding the channel formation process.

This study aims to understand the characteristics of river morphology on alluvial plain from viewpoint of flood control and environmental restoration. Firstly, middle reach of the Tama River was divided into several sub-segments based on the landscape pattern. Consequently, the boundaries of sub-segments corresponded to the location of weirs. Secondiy, the damage by the flood in 2007 was investigated in each sub-segment. In each sub-segment, the upper reach had the compound cross-sectional shape that made stream concentrate in low flow channel and damaged the embankment. On the other hand, the lower reach had flat cross-sectional shape that was not damaged. In the middle reach, however, the flood plain was damaged by overflowing of high current. The upper reach that has compound channel and is densely forested now had supported high diversity of ecosystem in the old days. In this paper, it was pointed out that the loss of cobbles and boulders due to gravel mining made the riverbed slope in low flow channel milder than before and caused the ecosystem degradation.

River bed structure tends to be completely different before and after confluence. However, the reason why such change happens is not understood. Due to sediments supply from two rivers, the river-bed morphology around confluence becomes complex and various. But there are several common features among almost all confluences. These are change of river width and inclination, deposition tendency of　large sediments around confluence. Through the field observations and channel experiments, following steps are clarified. First, large sediments supplied from upper stream stop at the confluence. Just below there, river bed degrades because of sediment supply decrease and it causes the inclination change. In conclusion, confluence has a function like screen, so large sediments are stuck there and only small sediments can go through. That means confluence has roll of the boundary　of sub-segment scale river structure.

The Chiyoda Experimental Channel, which is capable of generating artificial flooding, is the largest full-scale river experimental facility in Japan. Preliminary experiments have been conducted there since FY 2007 to clarify its basic hydraulic characteristics. In FY 2008, discharge measurements were taken using a range of methods, and data under a wider variety of conditions were obtained. The experimental results were used to confirm the feasibility of applying the sand-wave theory obtained in past laboratory experiments to the full-scale Chiyoda Experimental Channel. The coefficients of float used for high-water discharge observation were also verified through comparison with the values calculated based on the results of flow velocity observation using an ADCP. The calculated coefficients of float tended to be too small, which was considered partly due to the influence of sand waves.

The local scour and the bed degradation during floods are serious factors causing the damage of river structures. For a structure maintenance and management, it is important to understand bedform changes around structures. Recently, two problems concerning river bed variation have been revealed in the Tama River. One is a local scour around river structures. The local scour by the 2007 flood caused damage of bed protection works upstream of Nikaryo Syukugawara weir. Another is the shale bed erosion problems. The shale bed is known to be susceptible to bed degradation. The objectives of this study are to clarify the relation between sandbar shape changes and local scour under the influence of the weir and examine time changes in shale bed elevation by floods.

The re-meandering project in the experimental section where straightened channel was connected to horseshoes laker as 2-way channel have been conducted in the Shibetsu river in the east-northern part of Hokkaido since 2002. A maintenance method of the 2-way channel with distributary weir have not been established because a process of channel change doesn’t well understood. In this study, discharge-variations to responses of channel change were investigated with cross section profiles, orthogonalized aerial photographs and hydrological data such as water level and discharge. It was found from the results that, a dependence of the weir under summer flood discharge condition is smaller than usual discharge conditions. It is important to keep discharge to generate satisfying tractive force in order to maintain the re-meandering section.

This paper deals with changes of river channel after ten years from disaster restoration works in Yosasa-river. The main construction was performed as to revetment, its foot protection and widening of the channel. These were carried out in a short term of three years. Unstable meandering flow figure in the channel was expected to change after disaster restoration works. Generally, river morphological regime is apt to restore to original river channel even by usual disaster restoration works. In this paper, the structure of jam-up deposit of large stones and morphological river channel are considered base on field investigation. Main causes of the change of morphological channel were founded to be influenced in the stability of graded large bed materials.

In the Kita River, several large-scale river improvement works including major bed excavation were enforced from 1998 to 2003. After the works, the rivers experienced several floods which involve river-channel changes. The authors investigated the actual conditions of river-channel changes using longitudinal distribution of mean river bed and cross sectional form of river bed. The inclinations of cross sectional form change after the works were different from before works, especially the section which reduce mean river bed and around the edge of improved section. And, then, the authors calculated the longitudinal distributions of friction velocity with quasi-2D non-uniform flow computation. As a result, the linear relationships of actual conditions of river-channel changes before and after the works, and the the incline using friction velocity and river surface width were acquired.

The sediment transports in stony-bed rivers is different from gravel-bed rivers or sand-bed rivers. Authors have investigated the mechanism of sediment transport and stable cross sectional form and clarified characteristics of grain size distributions on stony-bed rivers by performing field experiments in the Joganji River. Currently, stony-bed rivers have degraded their bed elevation, due to the past extraction of bed materials and lack of bed materials transported from the upper region. Then, hydraulic structures such as levee of stony-bed rivers are susceptible to break by large momentum of flood flows. It is important to know how much stone and gravel layer is needed to maintain river bed level properly.

In this study, we investigated the development of river bed level recovery method by using large stones. Secondly we showed the usefulness of 2D riverbed variation analysis for this technology development.

Recently, in a lot of gravel bed rivers, as results of vegetated and fixed bars and of concentration around the main channel of flood disturbance, the fluvial environment has been deteriorating. In such rivers, the gravel bar reproduction has been tackled by increasing the tractive force and flood frequency by the river-bed excavation etc. For example, on Watarase river, the effectiveness of river-bed excavation for flood disturbance was shown as a result of a flood on September 2007. However, this flood disturbance caused the river bed deformation around the gravel bar too.

In this paper, as a result of numerical study on the flood on September 2007 at Watarase river, the influences that the scale of the excavation and the gravel bar materials would exert on the flood disturbance and the water level and so on was evaluated. In addition, effectiveness and notes of gravel-bar-excavation for the main channel management were shown.

The Asa river which is the main tributary of the Tama River is not necessarily sound in view of the sediment movement and river bed management, because the large amount of gravel extraction in the past brought the degradation of the river bed and the present sediment supply from the upstream reach is not abundant.

The torrential downpour occurred in the end of August, 2008, caused extremely high flood water level with sharp hydrograph in the Asa river, the flash flood flow brought scouring of outcropping shale, and the extent of the exposureon the river bed. In this research. we clarify time change in scouring of shale bed by the flood and effects on river structures. Finally.we propose the concept of countenneasures against the shale erosion.