Effects of Dams Improving on the Sediment Transport and Maximum Scour Depth of the Existing River

九二一大地震後，台灣中部山區地質處於不穩定的狀態，並在接下來數年的颱洪事件中，產生河床劇烈淤積與洪氾之狀況。其中大甲溪為中部重要的河川之一，沿河岸有多處壩體橫亙其中，而河道內壩體的完工擋住了大量泥砂的流通，分別對壩體上、下游產生嚴重的淤積與淘空現象。為了解河川中各種河砂沖淤的機制，並進一步探討壩體改善後對大甲溪整體泥砂沖淤之影響，本研究以上游馬鞍壩至下游出海口為研究模擬區段，利用WASH123D（集水區數值模式）佐以沖刷經驗公式，配合現地調查之資料回歸，進行歷史颱洪事件、不同情境計畫流量以及壩體改善前後之模擬，分析水位、流速、最大沖刷水深及河道整體輸砂量之變化，並探討地質脆弱河段發生崩坍後，河道整體輸砂量的情形。目的在於探討壩體改善後對於大甲溪泥砂整體沖淤之影響，以做為日後探討壩體調整之參考。 研究結果顯示，利用WASH123D進行大甲溪水文水理模擬時，所模擬之莫拉克與蘇力颱洪事件，水位分別有7.4%與1.0%以內之誤差，而最大沖刷水深與實際值之結果則有8.01%與0.19%，顯示本模式搭配Su&Lu（2013）公式與Blench（1969）公式，用於大甲溪進行河川水位、流速及最大沖刷水深計算，有相當合理之模擬結果。接著進行泥砂運移模擬，泥砂最大濃度之延時並非與最大水位同時發生，本研究發現相對於水位延時則有一定比值的稽延時間，大約是1.5倍左右。接著在壩體改善方面，調整了壩體構造後，本研究發現分別在洪水頻率20年至100年時，於石岡壩至后豐橋段，有減緩12.69%至20.69%最大沖刷深度的改善量之成效，后豐橋至出海口段亦有減緩10.06%最大沖刷深度；另外壩體調整後將影響石岡壩向上游達3500公尺，在石岡壩上游約500公尺處有最大沖刷水深，較調整前增加2.04公尺之沖刷水深量，增加幅度為49.85%，而500至3500公尺平均增加0.1公尺之最大沖刷水深，平均增加量為1.85%，懸浮載濃度方面，壩體的改善對於壩體與其上下游1,000公尺有較顯著之影響，壩體本身與其上游1,000公尺有濃度上升之情形，上升幅度約為原始濃度150%至750%，壩體下游1,000公尺有濃度下降之情況，下降幅度約為原始濃度80%至90%，但對上下游1,000公尺以外之河段影響則較為緩和。 關鍵字：大甲溪、壩體改善、輸砂模擬、最大刷深The Chichi earthquake occurred in 1999, since that the geomorphology on mountainous area of central Taiwan is getting unstable. During the coming next few years, the frequent flash floods attacked Taiwan, resulting in severe riverbed scoring and sediment transports on rivers. The Dajia river is one of most important rivers in central Taiwan. There are some important dams was spanned on the river, while some of them in the river tend to block huge sedimentations of flow. Consequences are leading to serious riverbed scouring and turbid flows in Dajia river. In order to better understand the mechanism of sediment transport and bed scouring, form Maan dam to the estuary of Dajia river was selected as our study site. And the WASH123D hydraulic model combined empirical formula with referring to the field investigation data, was integrated to apply this study. The research was focused on discussing the variabilities of river stage, flow velocity, riverbed scouring, and sedimentation transports along the Dajia river. And the scenario of side collapse on some fragile geology channels was studies. The improving of the structure of river dams was studied to understand the overall impacts of sediment transport and maximum scouring depth of the existing river. Based on the results of conducting WASH123D for flood routing, simulations revealed that the stages errors of channel showed 7.4 and 1.0 percent different to the measurements on the case of typhoons Morokat and Soulic, respectively. And errors of the maximum depth of riverbed scoring were 8.01 and 0.19 percent difference as compared to the field measurement. Simulation results implied that using WASH123D with coupling empirical formula of Su&Lu (2013) and Blench (1969) in Dajia river has good performance. On the aspect of sediment transport, it revealed that the maximum concentration of sediment and its corresponding water level was not occurred at the same time. The study founded that the travel time of maximum concentration of sediment with divide by peak flow was about 1.5 time of protracted delay time. As the Shihgang Dam was reshaped, discharges with 20 to 100 years return period designed floods was studied. Results indicated that the maximum scoring depth of riverbed can be eased 12.69 to 20.69 percent as compared to original bed of cross sections of Shihgang Dam to Houfeng Bridge. And it showed that the scoring depth had 10.66 percent decrease from Houfeng Bridge to estuary. The maximum scoring depth was 2.04 m and was upward about 500 m from Shihgang dam, which was almost 49.85 percent from original bed. The influence of scoring can be over 3500 m of upstream from Shihgang dam. However, the increment was about 0.1 m and average 1.85 percent of original beds. Regarding to sediment concentrations, the range of 1000 m from upstream and downstream of Shihgang dam was obviously identified. Concentrations showed 150 to 750 percent increase as compared to existing river beds performed. And out of this range, the influence revealed much minor. Keywords：Dajia river, dam improving, sediment transport, maximum scoring depth.目錄 摘要 I Abstract III 圖目錄 VII 表目錄 X 符號目錄 XI 第一章 緒論 1 1-1研究前言與目的 1 1-2文獻回顧 3 1-2-1沖刷經驗式相關文獻 3 1-2-2 輸砂模式相關文獻 4 1-3研究架構 6 第二章 理論分析 7 2-1 WASH123D集水區數值模式 7 2-2 河道泥砂運移 10 2-1-1 一般沖刷經驗公式 10 2-1-2 推移載運移公式 13 第三章 研究方法 16 3-1 研究區域概述 16 3-1-1 河道地文因子 18 3-1-2 河道水文因子 31 3-2模式建立 34 第四章 結果與討論 38 4-1一維河道模擬 38 4-1-1 模式檢定與驗證 38 4-1-2情境模擬系統之分析 50 4-2河道整體泥砂運移研究 58 4-2-1 事件模擬 58 4-2-2 特定斷面崩塌 68 4-3壩體改善對下游河道影響之分析 73 第五章 結論與建議 79 5-1 結論 79 5-2 建議 80 參考文獻 81 附錄 8