大甲溪土砂災害評估與效益分析－以青山電廠復建為例

大甲溪流域在921重大震災後，又歷經桃芝颱風、敏督利颱風、艾利颱風、海棠颱風、泰利颱風、龍王颱風、六九豪雨與聖帕颱風等豪雨風災事件，其中以敏督利颱風造成之災損最為嚴重，敏督利颱風期間引發德基水庫以下至馬鞍調整池間大量邊坡崩塌及土石流，並釀成沖毀路基、淤積河道、抬高洪水位等現象，致使大甲溪發電廠之德基、青山、谷關、天輪、馬鞍等分廠均遭受災害，尤以青山分廠最為嚴重。然而，大甲溪發電廠青山分廠具有提供快速啓動之尖峰電源、調節系統頻率及提昇系統電力品質之發電優勢，實有必要探討各事件後大甲溪流域德基至谷關段之主河道與各支流之崩塌地、土石流以及河道沖淤狀況。本研究利用3S（GIS、GPS、Remote Sensing）技術，運用各豪雨事件前後期之遙測影像資料，搭配雨量與流量資料，評估大甲溪主河道與各支流崩塌地及土石流之土砂產量，並以HEC-6程式進行大甲溪主河道長期之沖淤趨勢模擬。本研究藉各項分析與現調結果提出地質逐漸趨於穩定，且無惡化跡象之事證，並以經濟效益分析輔助解答青山分廠復建之經濟價值與必要性，最後根據復建計畫所涵蓋的範圍未來可能面臨的致災問題提出因應對策，以提供青山分廠復建安全性參考。

關鍵字

崩塌地；土石流；河道沖淤； 3S技術；經濟效益分析

並列摘要

After September 21, 1999, Chi-Chi earthquake, the subsequent typhoons and follow-up rainfalls caused the heavy landslides and debris flows in Ta-Chia river watershed, the sediment yields from the landslides were transported into main rivers. There were lots of damages in the watershed including dams, power plants, bridges, villages, amusement parks, and other infrastructure. In order to assess the impact of sediment yields from landslides and debris flows and to investigate the strategies of mitigating the geohazards, quantitative assessment was conducted by using aerial photos and satellite images obtained at 6 stages of major earthquake and typhoon events, taking Ching-Shan Power Plant reconstruction as an example. In order not only to estimate the volume of the sediment yields from landslides and debris flows, but also to establish the relationships between the volumes of sediment yields, the rainfalls intensity, and the discharge. The HEC-6 program was applied to simulate the flushing and deposit of Ching-Shan Power Plant in the near future. The results show the highest level of riverbed around the Chin-Shan Power Plant would raise more than 20m in addition. Among the branch rivers of Ta-Chia main River, Ji-Ler River and Pi-Ya-Sun River brought the most sediment yields from landslides in the subwatershed. In conclusion, there are at least 40% of the total sediment yields from landslides still remain in the study area. Therefore, the sediment will transported out in the near future, and monitoring should be conducted continually to mitigate the hazards. Finally, through safety assessment and benefit analysis of geohazards to draw up the geohazards adaptation strategies of Ching-Shan Power Plant, reducing injury to persons, damage to property and disruption to normal life.