A side-cavity zone is often observed in natural rivers. It is generally known that additional shear stress is generated, and it results in depletion of mainstream energy. A part of mean kinetic energy in a main-channel is used for production of a large-scale horizontal circulation in the cavity. However, the detail mechanism such as energy transport is poorly understood. Therefore, we conducted PIV measurements in a laboratory flume and compared space distributions of mean velocity components and Reynolds stress varying a cavity geometry. In particular, a practical calculation method of Reynolds stress was also developed and its accuracy was examined by comparison with the measured data. Furthermore, contributions of components in an energy transport equation were revealed quantitatively.

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 In the calculation of the water level of the swollen river which has weir(s) water level just upstream of the weir(s) should be given as the boundary condition, where insufficiency of the weir formula is pointed out as the problem. In order to overcome this situation, dimensionless parameters relating the discharge and water levels were obtained through the use of momentum theorem, then the expressions of the relation between these parameters were derived by using the existing experimental data, and the applicability is verified using the experimental data. Firstly unique relationship which seemed to be universal relation was found between the ratio of water level upstream of the weir to the weir height and ratio of critical depth to weir height ratio for freer fall type flow. Secondly, another unique relationship was obtained for the submerged flow which gives upstream water level by water level downstream of the weir and the discharge. Examination of the result using the existing experimental data showed that the proposed method has the better rationality and applicability than the previous formulas.

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