以數值模型進行調整型防砂壩經營管理分析、操作矩陣應用與調整策略設計

傳統型防砂壩作為目前國內外常見之溪流、河川工法，主要目的在於降低土砂災害的風險及保護下游保全對象與居民生命財產的安全，但近年來氣候變遷使得極端降雨事件之發生頻率提高，也導致了傳統型防砂壩經常面臨壩體上游淤砂過量、壩體下游沖刷嚴重之問題。調整型防砂壩的出現，雖然有效地提供了解決之道，但是如何長期永續經營與使用調整型防砂壩這項有利的工具，是目前學界與業界最關心的研究議題。有鑑於調整型防砂壩目前缺乏一套完整的經營管理與操作策略，因此本研究將使用HEC-RAS 2D水理輸砂數值模型作為工具，並以台中市廍仔溪為案例，研擬出完整的調整型防砂壩經營管理與操作矩陣之應用規則及防砂壩調整策略，冀望提供使用者可以針對不同溪流現地狀況以及上、下游土砂狀態選擇適合的防砂壩調整策略配合矩陣進行應用，在確保各式保全對象安全之前提下，同時解決上游淤積過量、下游沖刷嚴重之問題，使溪流恢復平衡穩定之狀態，並且延長防砂壩之使用年限。本研究透過四階段的模擬試驗，最終提出兩套完整的調整型防砂壩經營管理與操作策略，分別為最終策略保守型與最終策略成效型。其中，最終策略保守型經過七步驟之壩體調整策略及洪水事件模擬，成功使防砂壩上游溪床坡度降低，由原先的5.4%改善至3.5%，且原先沖刷嚴重的保全對象5K+065固床工下游沖刷坑，其全斷面淤積總量共增加54.56平方公尺，該處河道平均沖刷深度由原始地形之2.85公尺降低至0.7公尺，本調整策略有效地延長了防砂壩之壽命，使其平均使用年限達195年。此外，最終策略成效型經過五步驟之壩體調整策略及洪水事件模擬，最終成功使防砂壩上游溪床坡度由原先的5.4%改善至3.8%，保全對象5K+065固床工下游沖刷坑之全斷面淤積總量共增加50.74平方公尺，該處河道平均沖刷深度則同樣由原先之2.85公尺降低至0.7公尺，且經過本調整策略操作，有效地延長了防砂壩之壽命，使其平均使用年限達70年。依據本研究模擬結果，結論如下:最終策略保守型適用於防砂壩下游有鄰近生命財產之保全對象時，此方案能夠最大化防砂壩攔阻土砂之功效，同時解決上游淤積過量、下游沖刷嚴重之問題，使溪流恢復平衡穩定之狀態。最終策略成效型適用於防砂壩下游鄰近區段僅存在構造物保全對象，並無生命財產保全對象，且須盡快處理壩體上游淤積嚴重、下游沖刷劇烈之問題時，此方案能夠使防砂壩在確保一定的安全前提下，有效率地解決土砂沖淤不平衡之問題，使溪流快速恢復穩定狀態。

關鍵字

永續；防砂壩；調整型；經營管理與操作； HEC-RAS 2D ；數值模型

並列摘要

Traditional check dams, as a common stream engineering method, are mainly designed to reduce the risk of soil and sand disasters and to protect downstream protection targets. However, in recent years, climate change has led to an increase in the frequency of extreme rainfall events, which has resulted in the traditional check dams often facing excessive sediment accumulation in upstream and severe scouring in downstream of the dams. Although the invention of the adjustable check dam is an effective solution to the problem, how to long-term sustainable management and use adjustable check dams as a beneficial tool is currently the most concerned research topic. Due to the lack of a complete sustainable management and operation strategy for adjustable dams, this study will use the HEC-RAS 2D numerical model as a tool, and the Buzi Stream in Taichung as the study area, to develop complete rules for the application of the " Adjustable check dam management and operation matrix" and dam adjustment strategies. It’s expected to provide users to select the suitable dam adjustment strategies for different streams and sand conditions in conjunction with the application of the matrix. Through the dam adjustment strategy, the problem of excessive sediment accumulation in upstream and severe scouring in downstream can be solved under the premise of ensuring the safety of the protection targets, restoring the balance and stability of the stream, and extending the service life of the dams. Through the four-stage simulation test, this study finally proposed two complete sets of management and operation strategies for adjustable check dams, namely, the conservative final strategy and the effective final strategy. After seven steps of dam adjustment and flood event simulation, the conservative final strategy successfully reduced the upstream slope from 5.4% to 3.5%, increased the cross-section siltation of the protection target, 5K+065 scouring pit, by 54.56 m2, and lowered the average scouring depth of the river from 2.85 m to 0.7 m. It also extended the average service life of the check dam to 195 years. In addition, after five steps of dam adjustment and flood event simulation, the effective final strategy successfully improved the upstream slope from 5.4% to 3.8%, increased the cross-section siltation of the 5K+065 scouring pit by 50.74 m2. The average scouring depth of the river was also reduced from 2.85 meters to 0.7 meters and the average service life of the dam was extended to 70 years. Based on the results of this study, the conservative final strategy is recommended to be used when there are life and property protection targets in the downstream of the river, which can effectively reduce the risk of sediment disasters while solving the problem of sediment imbalance, and restoring the stream to a balanced and stable condition. On the other hand, the effective final strategy is recommended to be used when there are only structural preservation targets in the downstream of the river, and at the same time, there are problems of excessive sediment accumulation in upstream and severe scouring in downstream of the dams, which can efficiently solve the problem of sediment imbalance under the premise of ensuring the safety and quickly restoring the stream to a stable condition.