圖書典藏及影音數位平台

台灣西部多砂河川河道沖刷與穩定工法為近年有關單位持續關注之重要議題，特別是堰壩下游劇烈沖刷行為導致岩盤裸露，危及相關水工及防洪構造物安全，本所過去針對集集堰下游進行之二維數模與物模試驗研究，可作為穩定方案布置原則，惟礙於二維水深平均模式理論限制及模型比例效應，關於局部沖淤及堰區下游複雜流況，尚待進一步研究探討。國內目前常見之動床模式，多以一維或二維為主，對於複雜三維流場與局部沖刷問題，如橋墩、丁壩、固床工、水庫等，均採用一維或二維模式計算，因其對於水深方向之平均假設，成果常有其限制或不確定性，須藉由更精進之三維模式以獲得更客觀之評估成果。然而由於三維模式之開發與應用較具有技術門檻，且國內切入該領域時間較短，現有模式大多仍於研發階段，有鑑於目前亟需解決之河工治理問題頻仍，考量模式應用之迫切性與現地模擬之可靠度，仍須藉助於國外先進三維動床模式。CCHE3D模式為美國國家計算水科學及工程中心所研發，為一套基於有限元素法之三維模式，適用於複雜自然環境下之自由液面紊流、泥砂傳輸、河床變遷、汙染傳輸與水質等問題之探討，模式曾藉由多筆實驗室與現場案例完成檢定驗證工作，近年更對於模式時間尺度進行研發改良，使其更適用於模擬現場案例問題。本所曾於民國96年至98年間成立計畫與前該中心進行實質之技術交流，其引進之CCHE1D與CCHE2D動床數值模式現今於台灣水利界之獲廣泛應用，包括石門水庫、濁水溪、高屏溪、卑南溪等流域，有效提升國內數值模擬之技術能力，爰此，持續本委託計畫，進行CCHE3D三維動床數值模式引進及應用研究，以強化國內對河道局部沖淤及三維複雜流況分析能力，作為河川整體治理規劃及河道穩定工法分析之工具。

In recent years, the stability of the western Taiwan’s rivers is an important issue, especially in the downstream reach of weir or dam, where the severe erosion often endangers the safety of flood-prevention structures. Most of the mobile-bed models currently used in Taiwan are one-dimensional or two-dimensional. For complex three-dimensional flow fields and local scour problems, such as the vicinity of piers, groins, aprons, etc., the use of 1D or 2D model is inadequate because of the use of section-averaged or depth-averaged velocity. With the adoption of sophisticated 3D model, we can obtain a more objective assessment of local flow phenomenon. Since the development of 3D mobile-bed model is difficult, and the existing 3D models are still at the developing stage in Taiwan, the implement and application study of foreign advanced 3D mobile-bed model is needed.The National Center for Computational Hydroscience and Engineering (NCCHE), located in the University of Mississippi, is founded by several US federal agencies and US Congressional Appropriation. NCCHE has developed the 3D sediment transport and local scour model (CCHE3D). After rigorous tests and verifications, the CCHE3D model has been applied to river and coastal problems in the USA and other countries with significantly successful achievements.This project is carried out in a three-year term in cooperation with the NCCHE for the implement of CCHE3D in Taiwan. The work is executed among NCCHE, Water Resources Planning Institute, WRA, and Disaster Prevention & Water Environment Research Center, NCTU. The main purpose of this project is to study the hydrodynamics and morphodynamics between the hydraulic structures and riverbed.The bedrock module was used to evaluate the erosion reduction plans in the reach from Mingchu Bridge to JiJi weir, where serious erosion and channel incision problem exists. The bedrock module was based on the shear stress method proposed by Liao et al. (2014). Typhoon Matmo and Morakot were selected for the model calibration and validation. The simulations agreed well with the measured data.In this year’s study (2015), an optimal plan is proposed based on the idea that the headcut approaching the JiJi Weir needs to be controlled by a high weir structure to reduce the erosion power of the flow; and channel incision needs to be controlled by a series of lower weirs to reduce the flow erosion power. A straight drop structure near CS-116, four low-headed weirs at CS-115, CS-113, CS-111, and CS-109, and the excavation from CS116 to CS-109 are considered. The bed slope, S=0.0028 is used as a reference slope with which the channel may be stabilized because the lower segment of this river reach is controlled by this slope. The current incision part of the channel has a slope of S=0.0057; with these proposed weirs installed, the sediments may fill up the pools and further deposit between the weirs to form a new stable channel covered with sediments.