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臺灣地區集水區多地勢陡峻，地質脆弱，且部分地表覆蓋因人為活動而遭破壞，致使每逢颱風暴雨侵襲，集水區土砂大量流失，常使河川形成濃度偏高的含砂水流。另外，為保護沿岸居民的安全、維持人車通行的便利性，相關單位在河道中施作的相關工程亦常會造成河道底床驟變的情況。高濃度的含砂水流再加上驟變的河床，常會對沿程河防設施的穩定造成很大的威脅。以頭前溪部分河段為例，上游尖石堤防於颱洪期間均會面臨高濃度含砂水流直衝的威脅，造成河道刷深，屢屢造成尖石堤防的毀壞；中正橋固床工下游面河床保護層消失，岩床裸露，長期軟岩沖蝕(rock erosion)造成局部河床驟變，嚴重威脅橋梁的安全。因此，釐清驟變河道沖淤趨勢對防洪設施之影響，實為頭前溪流域當前相當重要的研究課題。為能更合宜掌握驟變河道沖淤趨勢對河防設施穩定之影響，本計畫將以頭前溪河道變遷的特性為例，納入河道沖淤、流路變遷與軟岩沖蝕的機制。本計畫為兩年度計畫案，第一年度(本年度)計畫案之工作項目包含基本資料蒐集與彙整分析、歷年防洪工程資料與流路變遷資料彙整分析、控制點入砂量推估研析、流變特性與高灘地土體力學試驗分析、二維驟變河道沖淤數值模式之發展、模式參數檢定與驗證、含砂水流對河道水理動床條件之影響分析、河防設施脆弱段之檢討評估與脆弱段河防設施之災害潛勢分析等工作。於基本資料蒐集與彙整分析、歷年防洪工程資料與流路變遷資料彙整分析部分，完成頭前溪流域地理、水系、河段、地質、地形、氣象與水文、河川特性、河床質、河道大斷面與水利構造物資料與過往相關研究成果資料彙整工作。於歷年防洪工程資料與流路變遷資料彙整分析部分，除彙整防洪工程記載資料外，係透過載資料了解頭前溪河道內易受破壞之河段，並完成各河段可能的驟變因子彙整，其中計畫中擬訂的驟變因子包含堤前灘地長度、河床質變化、主深槽河寬、偏移變化率、曲率、剪力速度變化等因子進行關聯分析，並完成相關對應驟變因子之參考值供日後應用參考。控制點入砂量推估研析部分，係採用一維GSTARS3.0模式進行全流域的模擬，輔以歷年床形變化、流量-輸砂量等資料作為檢定的基礎，其中，頭前溪主流與油羅溪部分係採用92年底床作為初始底床並以97年底床進行驗證；上坪溪部分係組合87年與97年資料為初始底床，並以100年斷面資料進行驗證，藉以完成模式參數檢定與驗證工作。並由一維動床模式(GSATRS3.0)找出常態河川輸砂觀點之高流量下的流量-輸砂率定曲線趨勢，藉此趨勢與成果提供二維模式(RESED2D)做為模擬條件，完成崩塌沖蝕情況下之高含砂情況之流量-輸砂曲線。此外，進行頭前溪現地採樣並完成細顆粒漿體之流變特性分析工作與高灘地土體力學試驗分析。在含砂水流對河道水理動床條件之影響分析，係採用二維RESED2D水理輸砂模式進行相關模擬工作。在高含砂效應部分，其對剪力的影響最為顯著，其最重要無因次參數為 與 ，彎道效應部分，二次流強度因子為最重要的參數；流路變遷部分，岸壁坡度的影響最大，其次則為現在不易改變之土壤性質；軟岩效應中，水力沖蝕與泥砂磨蝕的關鍵因子分別為無因次剪力與無因次岩床參數。在河防設施脆弱段之檢討評估與脆弱段河防設施之災害潛勢分析部分係以規劃洪水量(100年重現期)之洪峰流量作為評估河防設施受高含砂水流剪力破壞之脆弱段分析工作。

Due to the extremely climate change, the huge rainfall and amounts of damage flood events occurred frequently. The watershed, riverbed and the conservation projects were damaged by high water-level, discharge and hyper-concentrated flow. Furthermore, for citizen and its safe, the local government would construct the protection project such as consolidation works around the bridge. Such works would make the suddenly changed and threaten the river stability. Here we called the suddenly changed as Transition Reaches.The purpose of this study this year is to straighten out the main problem and resolve the reaches in To-Chian River. For example, Jian-Shi embankment as well knows transition reaches. During the typhoon period, its face the hyper-concentrated flow problem. Zhong-Zheng bridge section, have soft-rock erosion and stream-way changed questions. Therefore, to figure out the transition reaches affections and its damage potential is an important issue here.In this project, first of all is to arrange, investigate and comprehend with sediment income, watershed hydrology, channel hydraulics, geological structure, erosion, deposition data, and the reasons. By synthesize useful information could figure out the reasons and simple identify factors such as riverbank length, grain size distribution, shear stress etc., on the topic of transition reaches.For the 2D model simulations, first of all, here use GSTARS3.0 model to simulate whole channel sediment income conditions. Otherwise, there are no correspondence sediment hydrograph under high discharge situation. The GSTARS3.0 could straighten out such problem and feedback its result to 2D model. Second, use the RESED2D model to simulate the hazards potential by different frequency rainfall periods (2yr、5yr、10yr、20yr、25yr、50yr and 100yr).By effective analysis, for hyper-concentrated Flow, the most important effect factor is shear stress. For curved channel is Secondary-Flow effect. For stream-way changed is slope of channel bank. For soft-rock affections are soft-rock parameters and dimensionless shear stress.