Sediment Bypass Tunnels Research Articles

Abrasion prediction at Asahi sediment bypass tunnel based on Ishibashi’s formula

Abrasion in a concrete-lined sediment bypass tunnel is estimated using a Japanese state-of-the-art prediction model and validated by measured invert abrasion data at Asahi Reservoir, Japan. The model is described in detail, certain shortcomings are disclosed, and a revised version is proposed. The model consists of a kinetic energy term accounting for the impact by saltating particles, and a friction work term accounting for the grinding stress. It is found that the latter term yields concrete abrasion values being consistently a multiple compared to its kinetic term contradicting other research. Based on that, and a possible particle impact angle inconsistency, it is proposed to omit the friction work term. It is shown that the calculated abrasion is overestimated by 138% on average compared with that measured, if both terms are accounted for. However, promising results are obtained with only 30% overestimation by neglecting the friction work term.

メタバーコーディングを活用した排砂バイパスダム上下流間の河川底生動物の群集構造の評価

南東スイスアルプスにおいて，排砂バイパストンネルが設置されたダムを有する河川（バイパスダム河川），バイパスが無いダムを有する河川（ダムあり河川），ダムなし河川，の３タイプを対象に，河川内地点間の底生動物の群集構造の違い（Bray-Curtis非類似度指数）をメタバーコーディングで評価した．調査した３つのバイパスダム河川のうち，２つではダムあり河川よりも群集構造の地点間の乖離が緩和されたが，残り１つでは緩和されなかった．緩和が見られないバイパスの運用年数（2年）は，他の２つ（39，93年）よりも短く，今後の継続的観測が必要である．本研究のメタバーコーディングでは大量標本（7,369個体）の群集構造が迅速に評価されたが，少個体数の分類群が検出されにくい等の技術的課題も明らかにされた．

EVALUATION OF SEDIMENT BYPASS EFFICIENCY BY FLOW FIELD AND SEDIMENT CONCENTRATION MONITORING TECHNIQUES

This paper evaluates the performance of the sediment bypass tunnel that diverts the suspended sediment from Lake Miwa to downstream reaches below Miwa dam. In such a complex processes during bypassing, it is difficult to predict the sediment bypassed efficiency numerically. Therefore, the digital image techniques were developed and implemented to measure the flow pattern and suspended sediment concentration SSC for the first time in Miwa dam. Where, field measurements were conducted during flood seasons in June and July 2010. Surface flow velocity and SSC were analyzed to clarify the performance before and after the commencement bypassing operation mode. The measured sediment bypassed efficiency assists the decision making on optimization of bypassing performance and validates numerical prediction. The facility proved its effectiveness in mitigating reservoir sedimentation in Miwa dam and turbidity of river water contributing towards restoration of downstream environment.

ANALYSIS OF THE FLOW IN A SEDIMENT BYPASS TUNNEL WITH A LARGE FLUCTUATION IN SEDIMENT LOADING

A numerical method has been constructed that computes the flow and the sediment transport in a sediment bypass tunnel that is being planned for installation at various dam sites. In these tunnels, sediment loads change abruptly due to the mechanism at the intake area. Existing methods of predicting sediments in rivers cannot be applied directly and unsteadiness and variation along the tunnel must be taken into account. The present method that considers these aspects can predict the flow and the sediment transport well compared with the model experiments. In the calculation, the transport is somewhat slower near the upstream end of the sediment deposition area. This, however, is due to the low friction on the smooth bottom wall of the model tunnel and will not be a problem in the application to the real sediment bypass tunnel, which will have a rough bed.