Muddy debris flow has been observed at the Hasegawa River and the Nojiri River of Sakurajima, Kagoshima Prefecture, Japan and other areas. These rivers' bed and banks are lined with concrete. Characteristics of this flow are a flow on gentle channel slope and some surges in a short period. Occurrence condition or flow mechanism of the flow has not been yet studied sufficiently. This paper explains the occurrence condition of muddy debris flow surges and a turbulent - particle collision flow model for the muddy debris flow.

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 Objective of the study is to clarify flow dynamics in a connected brackish lake and to present numerical flow simulation technique over a long period in order to establish water management technology of Lake Nakaumi and Shinji.
 We measured and clarified lake flow characteristics such as flow and salinity flux, saline behavior, formation, movement and disappearance of the density stratification. Unsteady irregular plane multi-layer density model, perpendicular curvilinear coordinate as the plane coordinate system and level coordinate and sigma coordinate as the vertical coordinate system were adopted to simulate characteristic flow dynamics caused by the change in flow and density in the lake and channel with complicated topography. Numerical simulation showed good performance of computation of brackish lake flow dynamics over one year in the practical computation time.

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 Wind-induced water waves observed in natural lakes and ocean cause secondary currents called as Langmuir circulation. Since these large-scale longitudinal circulations promote significant transfers of mass and momentum, high and low speed streaks appear on the free-surface accompanied with strong upward and downward flows. So, in order to deal with environmental topics under the water waves, it is necessary to reveal generation mechanisum of Langmuir circulation. In the presnt study, all three velocity components could be measured simultaneously by the stereo scopic PIV technique. Furthermore, space and time variations of air/watter interface were evaluated accurately by the high-speed camera synchroneized to the velocity measurements. The present multi-camera system allowd us to investigte the phase relation between the surface fluctuation and the velocity components, effects of aspect ratio on formation properties of Langmuir circulation and so on.

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 In order to enhance the emergency preparedness for large-scale floods of the Ara River, we categorized the inundation patterns and calculated fatality estimates. We devised an effective continuous embankment elevation estimation method employing light detection and ranging data analysis. Drainage pump capabilities, in terms of operatable inundation depth and operatable duration limited by fuel supply logistics, were modeled from pump station data of eac h site along the rivers. Fatality reduction effects due to the enhancement of the drainage capabilities were calculated. We found proper operations of the drainage facilities can decrease the number of estimat ed fatalities considerably in some cases. We also estimated the difference of risk between floods with 200 years return period and those with 1000 years return period. In some of the 1000 years return period cases, we found the estimated fatalities jumped up whereas the populations in inundated areas changed only a little.

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 In order to enhance the emergency preparedness for large-scale floods of the Ara River, we conducted flooded water surface calculation for many possible levee failure points an d categorized the flood patterns. We also conducted hearing survey from the National Police Agency, the Fire and Disaster Management Agency and the Defense Ministry about the number and the capacity of the rescue boats available as well as the estimated cycle time of the rescue operations. Employing these data, we constructed rescue simulation model and estimated the number of the stranded persons and the rescue waiting time. The damage mitigation effectiveness analysis of rescue operations, drainage pumps operations, and evacuation rate improving measures was conducted. The results of the analysis showed the proper operations of rescue and drainage pumps as well as achieving higher evacua tion rate can substantially decrease the number of the stranded persons 3 days, which is an important reference time for stranded persons' health, after the flooding.

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 This paper presents an application of a quasi-3D numerical model to a flood flow in a composite river channel. First, surface velocity distribution of a flood flow was measured in a reach of 20 km long of the lower Tone River by analyzing a series of aerial photos taken in a flood of September, 1998, and then the results were used for verification of the numerical flow model. Before the numerical simulation, hydraulic condition at the time of photo shooting was examined through comparison of published hydraulic data and the results of aerial photo analysis, which suggested that onsite measurement by floating rods overestimated the river discharge by 20-30% and that quasi-steady flow was developed when the photos were taken. Being based on the findings, numerical simulation was carried out under constant boundary conditions with a flow discharge estimated from the aerial photo analysis. Manning's roughness was determined for each surface condition of flood plain by referring to values published in literatures. Computation results fairly reproduced not only the distribution of observed surface velocity in cross sections but also the deviation of surface streamline from the channel centerline which may be caused by secondary circulation in the meandering river channel. In addition, wavy patterns of surface turbidity along the boundaries of low water channel and flood plains, which were observed in the photos, were corresponding to ordered motion of large horizontal eddies appearing in the computation result. Comparison of observed and calculated longitudinal profiles of transverse velocity at the boundaries showed that the scales and intensities of the eddies were well reproduced by the computation.

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