報告書番号

V08064

タイトル（和文）

津波による砂移動量実験と浮遊砂巻上量式の提案

タイトル（英文）

Experiments on sediment transport rate due to tsunami and proposal of pickup rate formula of suspended sand

概要 （図表や脚注は「報告書全文」に掲載しております）

非砕波の津波を対象にシールズ数が最大1までの砂移動実験を行った。粒径0.08mm, 0.2mmの砂を対象に、津波（波高3～6cmの孤立波を30s間造波）を1波造波させ、水平移動床上で水位, 流速, 浮遊砂濃度の鉛直分布を計測した。砂層終端でトラップにより水深全体の流砂量と海底面近傍の掃流砂量を捕捉した。浮遊砂フラックスの収支から海底砂の巻上量を算定した。既往のシールズ数のみをパラメターとした巻上量式では、砂の粒径依存の影響により推定値に偏りが生じる。そこで、次元解析により、シールズ数に加えて2つの支配パラメター（無次元粒径と無次元沈降速度）を導入し、砂の粒径の違いが考慮できる新たな巻上量式を提案した。芦田・道上の掃流砂量式は本トラップ実験結果とよく一致し津波による掃流砂量の算定に有効である。本流砂量算定式を海底地形変化予測モデルに適用し津波による港湾実験による地形変化量と比較したところ、従来のモデルよりも再現性が向上した。

概要 （英文）

Sediment transport experiment due to tsunami was performed using a wave flume with a piston type wave paddle. Horizontal fine sand layer was setup with the length of 10.5m. Still water depth was 9cm on the sand layer. The range of the Shield's number in this experiment corresponded to 0-1. The grain sizes of sand used were 0.08mm and 0.2mm. Generating one solitary wave with the wave height of 3 to 6 cm and the run-up time of 30s and without wave breaking, water surface elevation, suspended concentration and velocity were measured on the horizontal sand layer. The vertical distributions of them were obtained in detail. Sediment transport amount corresponding to one wave was also caught in the trap set up at the end of sand layer. Sediment transport rate passing through the total water depth and bed load transport rate passing near the seabed were measured. Pickup rate of seabed sand was estimated from the balance of suspended sediment flux. The conventional pickup rate formula using the Shield's number alone as a parameter included the factor depending on the sand grain size. So, the property of the no-dimensional pickup rate estimated by the conventional formula was significantly affected by the sand grain size. Using the dimensional analysis, new two no-dimensional dominant parameters such as 'no-dimensional sand grain diameter' and 'no-dimensional settling velocity', were introduced. And a new pickup rate formula of suspended sand was proposed by using their two parameters and the Shield's number. Generally, the new pickup rate formula could reproduce the experimental results of two sand grain sizes, and pickup rates of the recent tsunami experiment (Takahashi et al., 1999) better than the conventional formula. On the other hand, the bed load transport rate formula by Ashida and Michiue could well reproduce the bed load transport rate by the above trap experiments. So, we recommended the combination of the new pickup rate formula and the bed load transport rate formula by Ashida and Michiue as the sediment transport rate formula for tsunami. A seabed deformation numerical model using the above sediment transport rate formula was applied to the physical model test on seabed deformation around a harbor due to tsunami. It was showed that the above formula was able to reproduce the experimental seabed deformation better than the conventional formula.

報告者

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