[in Japanese]

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To evaluate the uncertainty of a real-time river discharge prediction with a distributed rainfall-runoff model, an error structure of real-time rainfall prediction by a translation model is modeled as a spatial random field and predicted rainfall fields are simulated according to the characteristics of the prediction error structure. Characteristics of error structures of real-time predicted rainfall by a translation model are statistically analyzed, and a relative prediction error field is found to be modeled using a lognormal spatial random field. Then a method to generate rainfall fields with the uncertainty having lognormal spatial random field characteristics is realized with a factorization technique of covariance matrix decomposition into its square root matrix approximately by using the Chebyshev polynomials. The generated rainfall fields will be used to evaluate the uncertainty of real-time river discharge predictions with a distributed rainfall-runoff model on a Monte Carlo simulation framework.

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Characteristics of the flood flow, bed load and bed topography in compound rivers were investigated using laboratory tests and field data. Based on experimental resulting a series of non dimensional parameters were deduced to characterize flood flow. Plan shape features were showed by 14 Japanese compound rivers data analysis. Steady flow experiments were conducted in compound channels with different value of sinuosity and relative depth. As a result, for the case of bank full flow, maximum rate of bed load and scouring occurred caused by large secondary flow under plan shape conditions of b_mc/B_m<0.5 and S<1.10. From unsteady experimental results with a given hydrograph, we have showed that the possibility of occurring inner bed scouring of main channel was very low due to the short duration of compound meandering channel flow. Finally, the classification diagram of the flood flow chacteristics (simple & compound meandering channel flow) in compound channels was presented in term of representative parameters such as sinuosity S and relative depth Dr, allowing the estimation of flood flow characteristics.

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The purpose of this study is to clarify the relationship amongst tidal range, distribution of Chlorophyll a and phosphorus flux around the mouth of the Arakawa River. We observed the phytoplankton bloom occurred around the density front during the spring tide when a moderately mixed estuary appeared. Conversely, the phytoplankton bloom occurred thinly under the halocline during the neap tide when a fully stratified estuary appeared. A three-dimensional hydrostatic model was used to better understand the mechanism of the change in Chlorophyll a distribution and phosphorus flux due to tidal range. Additionally, the phosphorus flux around the mouth of the river during the neap tide was found to be twice as large as during the spring tide. Therefore, it appears that the mouth of the river where the stratification strength changes with tidal range has a role of buffer against the phosphorus load.

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Relationship between wave-driven coastal seepage and topographical changes in swash zone is investigated, through experiment and numerical modelling. Behavior of phreatic surface and moisture characteristic of unsaturated sand are measured experimentally. As the result, upward directed exfiltration flux show that, in case of eroded beach, unstabilization of beach face leads to increasing backwash transport. The numerical modelling simulates the interacting wave motion and groundwater flow. The present result indicates that, seaward and upward directed groundwater flow due to rising of phreatic surface induced fluidisation of sand layer of saturated region. These patterns of flow have implication for erosion process.

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A relationship between foraging behavior and energy budget was investigated for Pseudorasbora parva. Energetic costs of the fish while engaging in swimming and energetic gain by foraging were calculated using a mathematical model. High flowing speed, satiation level and integrated energy consumption increased the duration of stay in the cavity for shelter and the rate of capturing prey in front of the fish. When the flow condition was above 7cm/s, the fish activity and net energy gain by foraging decreased. Fish compensated for energy losses with increasing capture of prey below the flow condition of 7cm/s.

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A stratified flow is generated along an inclined bed, when density anomaly is produced by anaerobic release of dissolved substances such as non-organic nutrients and metal ions from a “water-sediment” interface. A hybrid analysis of turbulent transport in the ambient water and chemical and biochemical reactions in the sediment was performed in order to obtain solutions of release flux of phosphorus and iron, sediment oxygen demand, profiles of their concentration and velocity. It was discussed how the bed slope, dissolved oxygen amount and velocity in the ambient water, concentration of phosphorus and iron in the sediment affect the transport rate of dissolved substances and sediment oxygen demand. The present study shows that buoyancy effect plays an important role in water quality transport across an anaerobic “water-sediment” interface.

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Typha angustifolia, sometimes utilized to clarify water, were cut in June, July and August for investigating the effect of different cutting season. The cutting effects were evaluated quantitatively by regrowth characteristics of aboveground organ just after cutting and belowground biomass in winter. After cutting, the initial regrowth speed of the T. angustifolia was 2.5 times higher than the growth speed in spring season of the species. The belowground biomass in winter affected by summer cutting was 1-1.5 times higher than that in initial growing season (spring). However, it had decreased greatly about 60-80% compared with no cutting area at the same time. Therefore, the cutting in summer was found effective for controlling their expansion. Moreover, the regrowth-model analysis indicate; 1) Nutrient condition influences more the regrowth of T. angustifolia by earlier cutting treatment, 2) During June to August, June-cutting is the most effective to maintain their stands at low density.

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Amount of water inflow into a dam reservoir is estimated using information of time-varying of the hydrostatic water level. But, it is difficult to know the hydrostatic water level. Because, in data of water level, a signal of the hydrostatic water level is disturbed by the noise that is caused by water surface fluctuations. These noises cause too much estimation error of water inflow. In this study, the hydrostatic water level is attempted to estimate using the synchronous addition/subtraction processing (SAS). It is confirmed that the SAS could estimate water inflow which restrain influence of the noise. And in this study, estimation performance of the SAS is discussed. It is confirmed that the SAS had suitable performance as an inflow estimation method for water level data which have unknown information about noise.

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