報告書番号

U03054

タイトル（和文）

放流に起因した音の特性とその経験的予測手法

タイトル（英文）

Characteristics of sound induced by discharged water flow and development of empirical prediction method

概要 （図表や脚注は「報告書全文」に掲載しております）

9地点のダムにおいて，洪水吐からの放流時に発生する超低周波音と可聴域音の測定を実施し，減勢工形式や流況等の諸条件と放流音特性の関係や放流音が周辺環境に与える影響について検討した結果，以下のことが明らかになった．放流エネルギー１Ｗあたりの可聴域の音響パワーレベルは，諸条件によらず５５ｄＢ前後であるが，超低周波数帯のそれは，４５～６５ｄＢと大きく異なり，ゲート開度が大きく，急激に流れが減勢される流況で大きくなる傾向がある．放流音は，なだらかな広帯域の周波数分布をもち，諸条件によって音響パワーレベルが最大となる周波数帯が異なるが，下流減勢池のボイド率の高い領域の水深方向長さが１／４波長に相当する周波数付近の音が選択的に強く放射されているものと推測される．洪水吐からの大規模な放流時には，超低周波音による建具のがたつき等の物的影響が最も懸念されるが，流況改善によって大きく改善できる可能性がある．

概要 （英文）

Flow-induced sound and infrasound are sometimes emitted from hydraulic structures such as outlet works, falling works and energy dissipaters around hydropower plants. Though the generation mechanism of these sounds is supposedly related to complicated behaviors of air bubbles entrained into the water, the mechanism has not been unrevealed in detail, and methods of their prediction and effective reduction have not been established. The purpose of this study is to develop the methods of prediction and effective reduction of these flow-induced sounds. In this report, characteristics of the sound including infrasound induced by the discharged flow from gate spillways were examined by field measurements at nine dams. The main results are as follows.The acoustic power level of these sounds per unit flow potential energy in infrasonic-frequency range varies from 45dB to 65dB depending on flow conditions such as energy dissipater type, gate opening and the depth of stilling basin, and tends to increase where the discharged flow is forcibly dissipated, while that in audio-frequency range is almost 55dB independently of the flow conditions.The sounds have broadband and convex spectrum. Their dominant frequency varies depending on the flow conditions, and it roughly agrees with the frequency corresponding to the acoustic wavelength which is almost 4 times as long as the depth of high-void-fraction region formed in stilling basin.Environmental impacts of these sounds on neighboring area were assessed. In case of large-scale discharged flow, it is possible that the rattles of windows and doors of houses are caused by the infrasound, however, they can be reduced by means of improvement of flow conditions.We proposed the octave-band acoustic power level curve per unit dissipated flow energy of these kinds of flow-induced sounds, with which we can predict roughly octave-band sound pressure level around energy dissipaters and falling works.

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