Tests on erosion of the cohesive sediments in the Chikugo estuary in Japan were conducted. The results indicate that the decrease in the rate of erosion at constant shear stresses is caused rapidly 20 to 30 minutes after the start of tests and that tue decrease is caused by the increase in the solid fraction and the yield value of the seaiment remaining uneroded due to shear stress. In order to simulate the erosion process of sediments, a model in which the increase in the yield value of sediment is assumed to be proportional to the integration value of the absolute value of the sediment velocity gradient with respect to time was developed. The simulation based on this model is shown to be available to estimate the amount of sediments eroded in the erosional process of cohesive sediments.

A row of two-dimensional vortices forms in an offshore zone when regular surface waves run up a sloping flat bed. This vortex row is called the offshore vortex train. The vortices begin to appear near the breaking point. Moving in the offshore direction, they develop and increase their horizontal lengthscale through vortex merging. After reaching a particular offshore location, however, they decay rapidly. The formation region of the vortex train has been investigated on the basis of visual experiments for three bed slopes. Its formation does not depend on the type of wave breaking but is observed when the steepness of deep-water waves is smaller than 4.2 × 10−2. The horizontal lengthscale of the vortices and the velocities of the vortex movement have also been evaluated empirically.

The fundamental behaviour of suspended sediments and muds was investigated using an annular flume with the condition of cyclically changed shear stress. The results from two series of experiments showed that particle sizes which participate in the variation of suspended sediments were restricted to smaller sizes of the material. It was also found that the cyclically steady state was finally attained with respect to the suspended sediments as the tidal cycle repeated. In that state the suspended sediments were subject to three phenomena: erosion; deposition with dispersed state; and deposition with flocculation. A simple model on the suspended sediments in an estuary was developed and its behaviour in a model estuary was simulated by using the fluxes of erosion and deposition in the experiment. That model well described the variation of the suspended sediments and muds.

An intermittent aeration method is available for nitrogen removal in a small oxidation ditch. The purpose of this study is to establish a basis for design and operation of the oxidation ditch with the intermittent aeration method. Discussions on the essential process parameters depending on cycle time tc, aerobic period ta, specific nitrification rate vn and specific denitrification rate vd were carried out by using a continuous-flow stirred-tank model. It is shown theoretically that an optimum range of the aerobic time ratio (ta/tc) for nitrogen removal exists in the region of 1/v n* to 1-1/vd*. From evaluating an amount of leaking nitrogen in the intermittent aeration, the cycle time is proved to be minimized. Experiments in a full scale test plant were conducted to affirm the validity of the proposed basis. It is confirmed that the total nitrogen removal efficiency of 90% is achieved and the estimated removal efficiencies by this basis agree well with the experimental data. The newly proposed basis and parameters for nitrogen removal using the intermittent aeration method are available for not only an oxidation ditch but also other nitrogen removal processes by single sludge systems.

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