A Study on Comprehensive Strategies for Sedimentation Problems in Tachia River Basin, Central Taiwan

The water resources of the Tachia River Basin in central Taiwan are intensively utilized for flood control, power generation, water supply and irrigation with Techi Dam, Chinshan Dam, Kukuan Dam, Tienlun Dam, Mann Dam and Shihkang Dam. The reservoirs created by these dams constitute a highly efficient utilization system of water resources. Six hydroelectric power stations were setup by Taiwan Power Company in the Tachia River Basin to supply the demand of voltage and frequency stabilities, the standby power during the peak load period and the spare power during the power system collapse. However, the 921 Earthquake in 1999 and subsequently heavy rain storms associated with typhoons during the May - November period had caused many serious landslides in the main stream and the tributaries of the upper reaches of Tachia River Basin. Major rainstorms in recent years included Typhoon Toraji in 2001, Typhoon Mindulle in 2004 and Typhoon Aere in 2004. Theses disastrous typhoon rainstorms triggered not only large scale of landslides but also debris flows and caused many serious roadbed destructions, river channel siltation and rising flood problems in the Tachia river system from Techi Dam to Maan Dam. Furthermore, the facilities of Tachia river power stations also suffered from various degrees of damages. Water conservancy facilities were also damaged. Many houses were destroyed and lives lost. This study first described and discussed the characteristics and severity of sedimentation related problems in Tachia River Basin. This study then made thorough investigations and analyses for the above-mentioned sedimentation problems soil and water disasters caused by natural factors (hydrology, meteorology, topography, geology, earthquake, etc.) and man-made factors (social, political, economic, etc.). Efforts were also made to quantitatively assess the sedimentation production for proper watershed conservation and management. In addition, effective conservation strategies and measures were proposed. These strategies were made on basis of comprehensive watershed management and conservation viewpoint for achieving sustainable development in the Tachia River Basin. The amounts of sediment production by soil erosion were estimated by using Win Grid with Universal Soil Loss Equation (USLE) modified for local use in this study. The estimated sedimentation amount contributed by landslides was calculated using different periods of aerial survey and satellite images to make a digital terrain model (DTM). The information of the total sediment output and spatial distribution were obtained through digital elevation model (DEM) and ArcGIS software. This represented a new and effective quantitative way for sedimentation problem evaluation. After careful evaluation of sedimentation problems related soil and water disaster of the Tachia River Basin the following conservation strategies and measures were proposed for implementation to protect, restore and enhance watershed stability and hydrologic characteristics. Firstly, the principle of natural restoration should be followed to coordinate with engineering measures that are considered necessary after critical assessment. In other words, engineering structures should not be adopted without proper comparison with other non-structural alternatives for mitigating disasters such as landslides, debris flows and riverbed siltation. The buffer zone can be adopted to protect the slope and road embankment. In addition, the ecological dykes can be used to protect riparian communities. A stable transportation and access systems for Tachia River Basin should also be critically evaluated with regard to necessity. Unnecessary and unstable roads should be put to bed with suitable vegetation methods. Above all, proper use of every piece of land in the watershed should be planned and implemented after land use capability evaluation. It should also be pointed out that proper comprehensive watershed conservation plan with suitable strategies and measures can only be designed and effectively implemented with inputs, supports and participation of the watershed residents to achieve sustainable development goal.台灣中部大甲溪流域水資源蘊藏量豐富，由上游之德基壩、青山壩、谷關壩、天輪壩、馬鞍壩及石岡壩共有6座重要的水資源設施，其中之水庫、調整池、水路及發電廠等構成一水資源利用效率極高之串聯水資源系統。該等設施不但於颱洪期間可成為防洪之主要設施，且其水源更為大台中地區灌溉及民生等各標的用水之主要來源，而台電公司於大甲溪流域所設置之6座電廠除擔負穩定台灣電力系統電壓、頻率，提供尖峰用電，並扮演台灣電力系統全黑啟動備用電源之重要角色。惟1999年台灣發生921地震時造成大甲溪中上游主、支流沿岸之嚴重崩塌，土石堆積河岸邊坡甚或下移至主河床。2001年後又經歷桃芝、敏督利與艾利等颱風挾帶豪雨事件更是數十年僅見，並引發德基水庫以下至馬鞍調整池間大量山坡崩塌及土石流，並釀成路基沖毀、河道淤積、洪水位抬高等現象，致使大甲溪發電廠各分廠之設施遭受不同程度的災害，亦造成交通道路中斷、水利設施受損、多處民房遭土石流淹沒、沖毀與居民傷害及失蹤等重大災害。 為能對症下藥，宜針對前述引致災害之元凶「泥砂」作一徹底之分析與探討，期能針對影響大甲溪流域之水土災害的自然因子(水文、氣象、地形、地質、地震等)及人為因子(社會、政治、經濟等)，評估流域過去及目前保育治理工作之內涵。另外，從宏觀及全方位之角度，依據學理及過去之經驗，針對集水區之問題提出全方位而有效之集水區整體保育治理對策，以有助於未來提供本流域水土資源保育治理及永續利用對策研擬之參考。 本研究對泥砂生產量之推估除沖蝕量採用Win Grid配合通用土壤流失公式為國內一般採用者外，崩塌量之推估則採國內尚少應用之方式即以各期航測及衛星影像製作之數位地形(DTM)圖資再透過ArcGIS軟體及數位高程模型(DEM)資料計算而得各事件集水區之崩塌量，並據此推求土砂之總產出量及其空間分佈，可提供擬定工程復建策略之參考，另本研究針對泥砂問題提出「尊重自然，不與自然對抗」之自然復育原則而採結構物地下化方式以免去土石流災害及河床淤高等問題，另提出河岸緩衝帶以保護道路邊坡與生態大堤防以保護社區及覓適當位址以實施地形改造以供公眾設施用地等均為國內尚未具體實施而可供參考之建議。 就大甲溪流域而言，一個穩定的運輸及人員出入系統應謹慎評估其需要性，不需要及不穩定的道路應予停用並實施適當之植生復育。最重要的是集水區之每一吋土地應該實施土地可利用限度評估後方得予以規劃與應用，本文並指出惟有透過集水區民眾的加入、支持及參與且配合適當策略及方法的全方位集水區保育計畫方能被有效執行而達成永續發展的目標。摘要………………………………………………………………………i ABSTRACT………………………………………………………………iii 目次………………………………………………………………………v 第一章、前言……………………………………………………………1 第一節 研究動機………………………………………………………2 第二節 研究目的………………………………………………………6 第二章、前人研究………………………………………………………7 第一節 沖蝕、崩塌與泥砂生產………………………………………7 第二節 泥砂問題與水力發電…………………………………………8 第三節 影響泥砂生產的自然環境因子………………………………11 第四節 影響泥砂生產的社會、人文及土地利用因子………………14 第五節 泥砂生產與自然環境因子及土地利用因子之相關性………16 第六節 集水區土砂產量之計算………………………………………18 第七節 泥砂遞移率……………………………………………………22 第八節 泥砂問題消減之硬體措施及不利影響………………………23 第九節 水庫及河道清淤………………………………………………26 第十節 泥砂問題消減之對策…………………………………………32 第十一節 集水區於大地震後崩塌地變遷案例………………………34 第三章、研究方法……………………………………………………35 第一節 研究流程與步驟………………………………………………35 第二節 分析方法………………………………………………………37 第三節 研究區域………………………………………………………50 第四章、結果與討論…………………………………………………62 第一節 大甲溪流域沖蝕、崩塌、河道泥砂問題及自然影響因子…62 第二節 大甲溪流域沖蝕、崩塌及淤積的土地利用及社會經濟因子98 第三節 集水區泥砂生產、平衡及災害潛勢…………………………98 第四節 水庫及河道清淤對策………………………………………116 第五節 綜合討論……………………………………………………133 第五章、問題與對策………………………………………………135 第一節 集水區永續發展之SWOT分析………………………………135 第二節 集水區永續發展願景與對策………………………………164 第三節 泥砂消減之全方位對策……………………………………165 第六章、結論與建議…………………………………………………184 第一節 結論…………………………………………………………184 第二節 建議…………………………………………………………185 第七章、參考文獻……………………………………………………18