Kohji Muraoka

Bio: Kohji Muraoka is an academic researcher from National Institute for Environmental Studies. The author has contributed to research in topics: Surface runoff & Sediment. The author has an hindex of 7, co-authored 41 publications receiving 271 citations.

Critical Shear Stress of Cohesive Bottom Sediments

Abstract: The main purpose of this study is to obtain a simple critical shear stress function for cohesive soft bottom sediments. A series of experiments concerning threshold levels of mud transport and two series of tests of their rheological and setting properties were conducted to find some correlation among these indices. We defined two threshold levels for mud transport and divided muds into two groups by their characteristics (settling type, flow curve, resuspension behavior, etc.). In discussing the factors determining the characteristics of the two groups, we found that natural muds usually belong to the first group, in which the two critical shear stresses are given as a function of yield value τy1 (defined in paper) independent of material, following a new definition of threshold for sediment detachment from the bed. Finally, the correlation between this yield value and the final settling volume ratio is also discussed.

Characteristics of autochthonous deposition and resuspension of sediments in the Takahamairi Bay of Lake Kasumigaura

Abstract: Deposition rates were measured by means of sediment traps at five stations in hypertrophic Takahamairi Bay of Lake Kasumigaura for the selected periods of 1981-1986. Separation of deposited fluxes into autochthonous and sediment resuspension components was attempted on the basis of chlorophyll a concentration for the tracer. Even at the central station of the bay, the average daily flux of autochthonous matter was 4g·m-2·d-1 compared with 46g·m-2·d-1 of resuspension. There was a time lag of one or two months between the peak of primary production and that of autochthonous deposited flux, which resulted in a surprising increase in chlorophyll a concentration of lake water in mid-summer. Calculated autochthonous deposited rates of carbon, nitrogen, and phosphorus in backwater regions and near the inflow river mouth were much higher than those at offshore due to the contamination of allochthonous matter. In regard to the resuspension rates, a logarithmic decrease against water depth and significant correlations with some meteorological conditions were observed. The sum of autochthonous and allochthonous deposited fluxes exceeded the accumulation rates in sediments estimated both by the mass balance method and the sediment dating method by a factor of 8-17 for nitrogen and 2-10 for phosphorus, both of which were much larger than the ratios observed in other lakes. The shallowness of this lake greatly influenced these ratios because of the large resuspension rate followed by rapid decomposition in lake water and high activity of benthos.

Eutrophication of freshwater and coastal marine ecosystems a global problem

Abstract: Humans now strongly influence almost every major aquatic ecosystem, and their activities have dramatically altered the fluxes of growth-limiting nutrients from the landscape to receiving waters. Unfortunately, these nutrient inputs have had profound negative effects upon the quality of surface waters worldwide. This review examines how eutrophication influences the biomass and species composition of algae in both freshwater and costal marine systems. An overview of recent advances in algae-related eutrophication research is presented. In freshwater systems, a summary is presented for lakes and reservoirs; streams and rivers; and wetlands. A brief summary is also presented for estuarine and coastal marine ecosystems. Eutrophication causes predictable increases in the biomass of algae in lakes and reservoirs; streams and rivers; wetlands; and coastal marine ecosystems. As in lakes, the response of suspended algae in large rivers to changes in nutrient loading may be hysteretic in some cases. The inhibitory effects of high concentrations of inorganic suspended solids on algal growth, which can be very evident in many reservoirs receiving high inputs of suspended soils, also potentially may occur in turbid rivers. Consistent and predictable eutrophication-caused increases in cyanobacterial dominance of phytoplankton have been reported worldwide for natural lakes, and similar trends are reported here both for phytoplankton in turbid reservoirs, and for suspended algae in a large river. A remarkable unity is evident in the global response of algal biomass to nitrogen and phosphorus availability in lakes and reservoirs; wetlands; streams and rivers; and coastal marine waters. The species composition of algal communities inhabiting the water column appears to respond similarly to nutrient loading, whether in lakes, reservoirs, or rivers. As is true of freshwater ecosystems, the recent literature suggests that coastal marine ecosystems will respond positively to nutrient loading control efforts. Our understanding of freshwater eutrophication and its effects on algal-related water quality is strong and is advancing rapidly. However, our understanding of the effects of eutrophication on estuarine and coastal marine ecosystems is much more limited, and this gap represents an important future research need. Although coastal systems can be hydrologically complex, the biomass of marine phytoplankton nonetheless appears to respond sensitively and predictably to changes in the external supplies of nitrogen and phosphorus. These responses suggest that efforts to manage nutrient inputs to the seas will result in significant improvements in coastal zone water quality. Additional new efforts should be made to develop models that quantitatively link ecosystem-level responses to nutrient loading in both freshwater and marine systems.

Mechanics of Sediment Transport

Abstract: Nearly 40 years of theoretical development and practical experience has been incorporated in this book, which has won numerous awards as a scientific and technical publication in China. Now updated and fully translated into English, this volume is the first attempt in the field to unify the movement of sediment and boundary conditions. Going far beyond any book of its kind, it presents a synthetic analysis and thorough coverage of many schools of thought and provides practical survey of this discipline of the science. The authors introduce a synthesis of information gleaned from more than 800 papers spanning the fields of hydraulic engineering, mathematics, physics, geology, rheology, and chemistry, giving the reader a profound understanding of the present status and direction of the industry's research efforts. The volume includes chapters dedicated to rate phenomena and topics such as hyperconcentrated flows, effect of sediment existence on water flow, large-scale hydraulic construction on sediment-laden streams, and specialized research not available outside of China.