In a stony-bed river with a wide grain-size distribution, large stones that would not move during a flood act as a strong fluid-resistance element. In addition, sands and gravels often remain in unevenness of the bed surface formed by large stones of various size. A stony-bed river will be in a stable condition when the cross sectional form and grain size distributions corresponding to flow force are established. However, it is difficult to predict riverbed variations in stony-bed rivers by the conventional riverbed variation analysis methods, which do not account for these essential mechanisms and unevenness of the bed surface in stony-bed rivers.
 In this study, we developed a new model of two-dimensional riverbed variation by considering the mechanism of riverbed variation and characteristics of bed surface unevenness in stony bed rivers. We verified the applicability of the model with results obtained by the laboratory experiments and the field experiments carried out in the Jyoganji River. The model is capable of explaining water surface profile, discharge hydrograph, bed variation, sediment transport rate, grain size distribution and height of each bed surface particles in stony-bed rivers.

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 Technologies to estimate the timing of breakup and ice sheet thickness in advance help with ensuring safety and developing work plans for construction work in rivers and flow observation in wintry conditions. In this study, non-contact measurement of ice sheet thickness was conducted until the thawing season to clarify the breakup phenomenon. To determine the factors affecting measured ice sheet thickness, the influences of three factors on thickness variations were evaluated quantitatively. In addition, a practical equation for ice sheet calculation using the air temperature, water temperature and effective water depth as independent variables was developed by Coefficient α and β based on an ice sheet thickness calculation model using heat flux.

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 In this paper, first, we discuss the physical interpretation of prop agation and transformation of water level hydrographs and discharge hydrographs by one dimensional equations for flood flows. And, relationships between storage volume of flood flows and propagation and transformation of discharge hydrographs are discussed for rivers with longitudinal changes in cross sections. From above discussions, we define "retarding storage volume of flood" which is the storage volume of flood when average velocities are decreasing during rising period of water levels.
 By using unsteady two dimensional flow analysis, we estimate water level hydrographs and discharge hydrographs of large floods through the valleys of the Kitakami River and the Go River. From this analysis, propagation and transformation characteristics of water level hydrographs and discharge hydrographs are explained with relation to river morphology, storage volume and retarding storage volume of flood in the river valleys. Finally, we discuss the application of this study for river plan ning and river management.

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 In many Japanese gravel-bed rivers, during these 30 years, river morphology has changed from single channel to compound channel, and the black locust has been rapidly spreading its habitat in the flood channel. It is said that this change has been caused by past gravel mining and the construction of river-crossing structures. This study aims to reveal how these human impacts affected and altered the river configuration. Previous study pointed out that theriver slope is determined by the size of sediment and the flow condition. In the Tama River, however, 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 formed compound channel. The low flow channel width was narrowest just downstream of a river-crossing structure but increased in the flow direction and was largest upstream of the next structure. This situation was also seen in other gravel-bed rivers, and its ecosystem was strongly related to the height of the weir and the length between a structure and a structure. In the upstream area of the alluvial fan of the Tama river, in 1968, when gravel mining had finished, bedrock was exposed in a lot of places due to gravel mining. This bedrock was firstly eroded just downstream of each structure, and the erosion progressed in the flow direction. This erosion formed low flow channel, and in its flood channel, the suitable condition for the black locust, which was revealed in this paper, was formed during several heavy floods and caused sudden expansion of blacklocust. On the other hand, from the upstream of the next structure, deposited sediment has formed gravel-bed river toward upstream direction. As a result, boundary of eroded channel and gravel-bed channel was formed between the structures.

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 The Yangtze Estuary receives abundant sediment in flood season, which results in large scale siltation. On the other hand, the flow field in the estuarine channel is highly pulsating even in the flood season due to its mild channel slope and large tidal amplitude in the East China Sea. This paper discusses the characteristics of the unsteady flow field and its effect on sedimentation in the estuarine channel of 145km long by using a quasi-3D flow model for numerical simulation under the typical flood flow condition. The calculation result suggests that the unsteady velocity field is influenced by the uneven shallow topography profoundly. The pulsating flow in mainstream is almost simple oscillation, following the direction of thalweg. While the cross-sectional flow component is remarkable especially at the edges of sandbars, which may induce lateral sediment transport and side migration of the sandbars. A numerical tracer experiment suggests that the excursion distance (about 22km) of suspended sediment during one tide is about the same as the interval of deep spots in the channel, which corresponds to a half-wavelength of main stream meandering.

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 This paper presents a hybrid one-dimensional (1D) and depth-averaged two-dimensional (2D) numerical model that may be used to simulate river flows with reduced computational load such as that with a 1D model, and without losing much accuracy like that with a 2D model. The time interval for 2D calculations, Δt_2D, was set to be quite larger than that for 1D calculations, Δt_1D, by introducing a new technique with high numerical stability to appreciably reduce the computational time, while 1D and 2D calculations were done separately. The present model was applied to the computation of flood flows in the Edogawa River in Japan with various values for Δt_2D/Δt_1D. The present model could significantly reduce the computational time and maintain high numerical accuracy comparable to that with 2D calculations even where Δt_2D/Δt_1D=100. We also noted that Δt_2D/Δt_1D=50 ensured efficient and accurate computation of flood flows.

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